design & construction of a spot welding machine
DESIGN AND CONSTRUCTION OF A SPOT WELDING
MACHINE
A project report submitted to the Department of Mechanical Engineering, Khulna University of
Engineering & Technology in partial fulfillment of the requirements for the degree of
―Bachelor of Science in Mechanical Engineering‖
SUPERVISED BY:
SUBMITTED BY:
Dr. Naseem Ahmed
Professor
Department of Mechanical Engineering
KUET
Md. Musaddiq Reza
Roll no. 0605078
Raihan kabir
Roll no. 0605084
DEPARTMENT OF MECHANICAL ENGINEERING
KHULNA UNIVERSITY OF ENGINEERING & TECHNOLOGY
KHULNA 9203, BANGLADESH
MAY, 2011 i ACKNOWLEDGEMENT
Authors want to express their gratefulness and humbleness to Almighty Allah for His immense blessing on them for the successful completion of undergraduate project work.
With deep sincerity, the authors express profound gratefulness to Dr. Nassem Ahmed, Professor,
Department of Mechanical Engineering, Khulna University of Engineering & Technology
(KUET), for his guidance and valuable counsel in execution and completion of the study without which it would be impossible to carry out the work.
They would like to extend their thanks to Professor Dr. Mihir Ranjan Halder, Head, Department of Mechanical Engineering, KUET, for giving permission to use lab facilities of the department.
Authors would like to complement their respective teachers of Mechanical Engineering
Department for their direct and indirect suggestions and opinions at different stages of work.
Authors are indebted to Professor Dr. Muhammed Alamgir, Vice Chancellor, KUET, for providing financial support of this project.
Authors would like to thank to all staff members of Wood Shop, Machine Shop and Welding
Shop. It would have been impossible to carry out the work without their help.
Authors wish to extend love and gratitude to their families who have always been a great support. Authors are fortunate to be a part of such families. Authors thank those parents who have sacrificed a lot for the prosperity and success of their children.
Finally sincere thanks to all well wisher.
Authors
ii
ABSTRACT
Welding is a process of joining two metal pieces. Resistance spot welding is one type of welding. Resistance spot welding is used to join two or more metal sheets together, and the technique is used widely in the automotive industry. Furthermore, other metal-to-metal connections, such as wire-to-wire joints in the electronics industry, are accomplished by resistance spot welding. Application-specific measures, such as the diameter of the welding spot, define the quality of the joint. The objective of the project work was to design and construct a spot welding machine. A direct pressure type stationary single spot welding machine has been constructed. The main factors influences on spot welding are pressure, current, time and proper alignment of the electrode. The pressure is applied on the upper arm by a lever and the pressure can be measured by a spring balance. AC current is supplied by a transformer to the two electrode tips and the current can be measured by an ammeter. Maximum available current is 200 amps. For this low current greater thickness sheet cannot be welded and maximum 0.7 mm thickness sheet will be welded. The performance of the project work was quite satisfactory. The constructed spot welding machine will given better performance if the limitations were eliminated. iii
TABLE OF CONTENTS
PAGE NO.
Acknowledgement
i
Abstract
ii
CHAPTER-1: INTRODUCTION
1.1 Introduction
1.2 Objectives
1
2
CHAPTER-2: LITERATURE REVIEW
PART-1: BASICS OF WELDING
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5
2.1.6
2.1.7
2.1.8
2.1.9
2.1.10
2.1.11
Introduction
History of welding
Metal joining processes
Advantages of welding over other joints
Welding as compared to riveting and casting
Commonly welded base metals
Classification of welding
Critical factors in welding
Heat affected zone (HAZ) in welding
Welding symbol
Application of welding
3
4
6
7
7
8
9
10
11
12
13
PART-2: ARC WELDING
2.2.1
2.2.2
2.2.3
2.2.4
2.2.5
2.2.6
2.2.7
2.2.8
2.2.9
2.2.10
Introduction
Different types of arc welding
Shielded metal arc welding (SMAW)
Advantages of SMAW
Submerged arc welding (SAW)
Advantages of SAW
Tungsten inert gas arc welding (TIG)
Advantages of TIG welding
Metal inert gas arc welding (MIG)
Advantages of MIG welding
14
15
15
16
16
17
17
19
19
20
iv
PART-3: GAS WELDING
2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
2.3.6
Introduction
Different types of gas welding
Oxyacetylene Welding
Advantages of oxyacetylene gas welding
Oxyhydrogen Welding
Pressure gas welding
21
22
22
24
25
26
PART-4: RESISTANCE WELDING
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
2.4.6
2.4.7
2.4.8
Introduction
Different types of resistance welding
Spot welding
Seam welding
Projection welding
Butt welding
Flash welding
Resistance welding benefits
27
28
28
30
31
33
34
34
CHAPTER-3: FUNDAMENTALS OF SPOT WELDING
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
3.18
3.19
Principle
Spot welding process
Circuit diagram of spot welding process
Sequence of events in resistance spot welding
Essential factors of spot welding
Heat generation
The time factor
Pressure
Electrode
Electrode tip size
Electrode coating
Factor Influence the life of electrodes
Influence of storage time on life of electrodes
Required condition for spot welding
Positive location of work piece
Advantages of spot welding
Disadvantages
Limitation
Practical uses of resistance spot welding v 35
35
37
37
38
38
39
43
44
45
46
47
47
48
49
50
50
51
51
CHAPTER-4: DESIGN & CONSTRUCTION
4.1 Design
4.1.1 Various types of spot welding machine
4.1.2 Design consideration for spot welding
4.1.3 Equipment required for spot welding machine
4.1.4 Electrode
4.1.5 Transformer
4.1.6 Spring Balance
4.1.7 Design of spot welding machine
4.2 Construction
4.2.1 Construction details
4.2.2 Operation procedure
4.3 Risks
54
54
55
56
56
57
58
59
61
61
CHAPTER-5: PERFORMANCE TEST & DISCUSSION
5.1
5.2
Performance test
Discussion
62
62
CHAPTER-6: CONCLUSION
6.1
6.2
Conclusion
Scope of future work
63
63
References
64
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LIST OF FIGURES
PAGE NO.
Figure 2.1
Flow chart of types of fusion welding
9
Figure 2.2
Flow chart of types of pressure welding
10
Figure 2.3
Welding symbol
12
Figure 2.4
Shielded metal arc welding
15
Figure 2.5
Submerged arc welding
17
Figure 2.6
Tungsten inert gas arc welding (TIG)
18
Figure 2.7
Metal inert gas welding (MIG)
20
Figure 2.8
Pressure gas welding
26
Figure 2.9
Spot welding
29
Figure 2.10
Seam welding
31
Figure 2.11
Projection welding
32
Figure 2.12
Butt welding
33
Figure 2.13
Flash welding
34
Figure 3.1
Resistance spot & TIG weld comparison
35
Figure 3.2
Spot welding assembly
36
Figure 3.3
Cross section of spot welding
36
Figure 3.4
Circuit diagram of Spot welding
37
Figure 3.5
Sequence of events in resistance spot welding
37
Figure 3.6
Graphs of resistance and temperature, as the result of Joule heating, as a function of location in a typical resistance welding arrangement
38
Figure 3.7
Spot welding Time cycle
40
Figure 3.8
Basic single-impulse welding cycle for Spot welding
41
Figure 3.9
Multi-impulse welding cycle for Spot welding
42
Figure 3.10
Nugget formation and heat dissipation into the surrounding base metal and electrodes during resistance spot welding
42
Figure 3.11
Weld strength as a function of current (a) & welding time (b)
43
Figure 3.12
Factor of influence the life of electrodes
47
Figure 3.13
Influence of storage time on life of electrodes
48
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Figure 3.14
Suitability of spot welding
48
Figure 3.15
Positive locations of workpiece
49
Figure 3.16
Resistance spot weld heat zones
52
Figure 4.1
Operation procedure
54
Figure 4.2
Electrode
56
Figure 4.3
Transformer
56
Figure 4.4
Spring balance
57
Figure 4.5
3-D view of spot welding machine
58
Figure 4.6
Constructed spot welding machine
60
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CHAPTER- I
INTRODUCTION
ix
1.1 INTRODUCTION
Welding is a process of joining two metal pieces as a result of significant diffusion of the atoms of the welded pieces into the joint (weld) region. Welding is carried out by heating the joined pieces to melting point and fusing them together (with or without filler material) or by applying pressure to the pieces in cold or heated state.
Now-a-days many processes of welding have been developed and probably there is no industry which is not using welding process in the fabrication of its products in some form or the other.
This is the most rapid and easiest way of fabrication and assembly of metal parts. The research carried out in this field has given various ways and methods to welds practically all metals.
Means have also been found out to weld dissimilar metals. One beauty of welding in comparison to others processes of joining metals is that by this process we can have more than 100% strength of joint and it is very easy process. We shall be dealing with all the various processes of welding in use these days, the equipment used for each process and the ways of preparation of joint and the various operations necessary.
Welding is now-a-days extensively used in the following fields: automobile industry, aircraft machine frames, structural work, tanks machine repair work, ship building, pipe-line fabrication in thermal power plants and refineries, fabrication of metal structures. There is a big competition between welding and casting process now-a-days. Many of the cast products are now-a-days being fabricated by welding various parts together. Such construction has th advantage that the products are lighter and stronger. Gas cutting is another field of application of welding process which is playing a very important role in industry.
In resistance welding a heavy electric current is passed through the metals to be joined over a limited area, causing them to be locally heated to plastic state and the weld is completed by the application of pressure for a prescribed period of time. No additional filler metal is required. In this process two copper electrodes are used and the metals to be welded are pressed between the electrodes. The current is passed through the electrodes which incorporate very low resistance in the circuit and the resistance at the joints of the metals is very high.
x
1.2 OBJECTIVES
The objectives of the project work were:
1.
To design a spot welding machine
2. To construct the spot welding machine according to the design.
3. To test the performance of the spot welding machine.
xi
CHAPTER- II
LITERATURE REVIEW
xii
PART-1: BASICS OF WELDING
2.1.1 INTRODUCTION
Welding is a joining process that produces coalescence of materials by heating to welding temperature with or without application of pressure or by application of pressure alone and with or without the use of filler metal. The term welding is used to cover a wide range of bonding technique. Broadly welding process could be classified as fusion welding and pressure welding.
Fusion welding is the process of joining two piece of metal by application of heat with or without application of pressure. Pressure welds are produced by bringing the clean face of component into intimate contact to produce a metallic bond with of without application heat but application of pressure.
Resistance spot welding is usually used in the fabrication of sheet metal assembly. The weld is made by a combination of heat, pressure, and time. Resistance welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localized heating in the joint, and the weld is made. The time is how long current flows in the joint, which is determined by the material thickness and type, amount of the current and cross-sectional area of the welding tips and contact surfaces. It is one of the oldest of the electric welding processes. It can be used to weld materials such as low carbon steel, nickel, aluminum, titanium, copper alloy, stainless steel
&high-strength low alloy steel. Resistance spot welding process is most applicable in the industrial fields of manufacture and maintenance.
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2.1.2 HISTORY OF WELDING
When driving a car or look at a light fixture in the street or open your microwave, chances are that there is something in any of those items that has been welded. These products and others have been a part of the process of welding for more years than might imagine.
Welding actually started a very long time ago during the Middle Ages. Many artifacts have been found that date back to the Bronze Age. These have been small boxes that were welded together with what is called lap joints; no one is exactly sure what these were used for, but this was important to that time.
The Egyptians also made a variety of tools by welding pieces of iron together. Perhaps this is where Maxwell's Hammer comes later? Who can say! Then came the rise of the Middle Ages and many people there were able to use blacksmithing for iron. Different modifications were made along the way until the welding that is used to day was developed.
There were several significant inventions in the 1800s that influenced welding included here:
The invention of acetylene by an Englishman named Edmund Davy.
Gas welding and cutting became known and a way to cement pieces of iron together.
Arc lighting was a very popular part of welding after the electric generator became known.
Arc and resistance welding become another popular aspect of welding.
Nikolai N. Benardos receives a patent for welding in 1885 and 1887 from America and
Britain.
C.L. Coffin receives an American patent for a arc welding process.
After the 1800s many more patents and inventions were made in order to create more ways of doing welding but one of the greatest needs would come much later during World War I because this process was needed to create arms. Because of the demand welding firms became a staple of
America and Europe because the war needed welding machines and electrodes to go with them.
During the war people really got a chance to look at how welding worked and it became a very
xiv
popular way of work. So much so that in 1919 the first American Welding Society was begun.
This nonprofit organization came directly out of through a group of men who called themselves the Wartime Welding Committee of the Emergency Fleet Corporation (Source: Miller Welds).
The 1950s and 1960s were also a significant time for welding because a welding process using
CO2 was discovered and a variation of this form of welding that used inert gas became very popular in the 1960s because it produced a different type of arc.
There have been a number of improvements in the welding trade over these years and today the process has added two areas, friction and laser welding. These two have created a more specialized field and therefore more opportunities for learning.
One interesting point about laser welding is that those people who use it have found that is a tremendous heat source so it can actually weld both metal and non-metal objects.
xv
2.1.3 METAL JOINING PROCESSES:
2.1.3.1 WELDING:
Welding is a Fusion or uniting of two metals by means of heat. Many welding process have been developed which differ widely in the manner in which the heat is applied and in the type of equipment used. Some process required hammering, rolling, or pressing the effect of weld. Other brings the metal to liquid state and required no pressure. Those process need pressure require bringing the surface of metal to a temperature enough that cohesion takes place. This is nearly always a sub fusion temperature. However the fusion temperature is reached metal must be confined by surrounding metal. No additional weld metal is required. Weld are also made by casting in which case the metal is heated to a high temperature and poured into a cavity between the two piece to be joined.
2.1.3.2 SOLDERING:
Soldering is the uniting of two piece of metal with a different metal which is applied between the two in a molten state and at a temperature not exceeding 800 F. In this process some alloying with base metal takes place and additional strength is obtained by mechanical bonding. The usual metals for soldering are low melting alloy of lead and tin.
2.1.3.3 BRAZING:
Brazing is a similar process in which the metal part are joined by non ferrous metals, such as copper-zinc and silver alloy, having melting point below that of parents metal but above 800 oF.
2.1.3.4 SOLDERING:
Soldering differ from brazing in that lower temperature filler metal (bellow 800 oF) are used in the joint. Lead and tin alloy having a melting range 300 to 700 oF are principally used in soldering. xvi
2.1.4 ADVANTAGES OF WELDING OVER OTHER JOINTS:
-
Buildings, bridges and structures can be built lighter and thus higher due to reduction in weight. There are cheap also due to reduction in weight and material cost. Additional joint strength can be obtained by using considerably smaller structural members. Joints are compact and to do not require additional plates as in case of riveted joints.
-
Welded joints have high corrosion resistance compared to bolted and riveted joints.
-
Welded joints are fluid tight for tanks and vessels.
-
Welded structures can be altered easily and economically.
-
Many different types of joint are possible in welded joints.
2.1.5 WELDING AS COMPARED TO RIVETING AND CASTING
-
Welding is more economical and is a much faster process as compared to both casting and riveting.
-
As compared to riveting and casting, fewer persons are involved in a fabrication process.
-
Welding involves less cost of handling.
-
Welding produces less noise as compared to riveting.
-
Welding designs involve less cost and are flexible also (we can change easily)
-
Cost of pattern making, strong, maintenance, repairs is eliminated.
-
Welding structures are comparatively lighter.
-
Welding can be carried out at any point on a structure, but riveting requires enough clearance. -
Drilling holes to accommodate rivets weakens a riveted structure.
-
Welding can produce a 100% efficient joint, which is difficult to make by riveting.
xvii
2.1.6 COMMONLY WELDED BASE METALS
Metals can be classified as:
1. Ferrous
2. Non-ferrous
Ferrous materials contain iron and these are the most important metals/alloys in the metallurgical and mechanical industries because of their extensive use.
Ferrous materials which can be used in welding application are:
-
Wrought iron
-
Cast iron
-
Cast steels
-
Alloy steels
-
Stainless steels
-
Carbon steels (low, medium and high carbon steels)
Non ferrous materials are those that are not iron based.
Non ferrous materials, which can be used in welding applications are:
-
Aluminum and its alloys
-
Copper and its alloys
-
Magnesium and its alloys
-
Zinc and its alloys
-
Nickel and its alloys
xviii
2.1.7 CLASSIFICATION OF WELDING
Mainly welding is generally classified as
1. Fusion Welding
2. Pressure Welding
2.1.7.1 FUSION WELDING
In this case, the metal at the joint is heated to a molten state and allowed to solidify. In this case filler material is used during welding process. This includes Arc welding, Gas welding and the thermit welding.
Figure 2.1: Flow chart of types of fusion welding
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2.1.7.2 PRESSURE WELDING
In it, the pieces of metal to be joined are heated to the plastic state and these forced together by external pressure without the adiitional of filler materials. Forge welding, Resistance welding and the thermit welding with pressure are the examples of this case.
Figure 2.2: Flow chart of Types of pressure welding
2.1.8 CRITICAL FACTORS IN WELDING
The critical factors involved in welding are the current, voltages, power, resistance and the transformer specifications, transmitting force, current density, throat dimensions and the electrodes. The current is usually measured in KA. A resistance weld cannot be made unless there is a sufficient weld current.
If current is the amount of electricity flowing, then Voltage is the pressure or force that‘s causing the flow.
Power is voltage measured by current and is in watts or KVA. This means the amount of current flowing times the pressure that‘s causing it to flow equals the amount of power generated.
Resistance is the opposition to flow of current. Since R to I is what generates the heat in the work piece, it is critically important that the area with the greatest resistance be at the interface xx between t he two parts being joined. The heat is where the resistance is, and the resistance is where the heat will be.
The transformer used is of high rating as is should be capable of supplying that much weld current. Current density describes how much current is being delivered to a specific area. It describes the concentration of the current in a small area of the work piece, viz., namely the area where the weld is.
Electrodes play a very important role in t he successful operation of a spot welding machine.
They must conduct the welding current to the work without overheating. They must at the same time conduct heat away from the surface of the sheet being welded and they must apply pressure to the work without deforming.
2.1.9 HEAT AFFECTED ZONE (HAZ) IN WELDING
The HAZ is the region that deforms AC, temperature to the temperature just below the melting temperature. It may be major matrix phase changes or precipitation process. Even n materials showing no phase change or precipitation during welding, recrystallisation and grain growth may occur. The HAZ has an important role to determine the weld cold cracking, notch toughness, hydrogen embrittlement, steel corrosion cracking etc. in severe environmental conditions of service.
Therefore a detailed study of HAZ is desirable. The width of the HAZ can be determined by the peak temperatures obtained at discrete points from the weld center lines by experiment. The variations of the micro-structure at different Jones of welding can be examined from photo macro and micro graphs.
xxi
2.1.10 WELDING SYMBOL:
A welding symbol has following basic elements:
1. Reference line
2. Arrow
3. Basic weld symbols (like fillet, butt joints etc.)
4. Dimensions
5. Supplementary symbols
6. Finish symbols
7. Tail
8. Specification processes.
These welding symbols are placed in standard locations (see figure below)
Figure 2.3: welding symbol
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2.1.10 APPLICATION OF WELDING
Automobile construction
Railroad equipment
Ships
Aircraft construction
Bridge construction
Pressure vessels
Storage tanks
Pipe and pipe lines
Fabrication of jigs, fixtures and machine tools
Repair of broken and damaged parts
Material handling equipments
Household furniture
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PART-2: ARC WELDING
2.2.1 INTRODUCTION
Arc welding is a welding process, in which heat is generated by an electric arc struck between an electrode and the work piece. Electric arc is luminous electrical discharge between two electrodes through ionized gas.
Any arc welding method is based on an electric circuit consisting of the following parts:
Power supply (AC or DC);
Welding electrode;
Work piece;
Welding leads (electric cables) connecting the electrode and work piece to the power supply. Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them.
When a long join is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.
When a filler metal is required for better bonding, filling rod (wire) is used either as outside material fed to the arc region or as consumable welding electrode, which melts and fills the weld pool. Chemical compositions of filler metal are similar to that of work piece.
Molten metal in the weld pool is chemically active and it reacts with the surrounding atmosphere. As a result weld may be contaminated by oxide and nitride inclusions deteriorating its mechanical properties.
Neutral shielding gases (argon, helium) and/or shielding fluxes are used for protection of the weld pool from atmospheric contamination. Shields are supplied to the weld zone in form of a flux coating of the electrode or in other forms.
xxiv
2.2.2 DIFFERENT TYPES OF ARC WELDING
1. Shielded metal arc welding
2. Submerged arc welding
3. Tungsten inert gas arc welding
4. Metal inert gas arc welding
2.2.3 SHIELDED METAL ARC WELDING (SMAW)
Shielded metal arc welding (fig. 2-4) is performed by striking an arc between a coated-metal electrode and the base metal. Once the arc has been established, the molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to forma sound joint. This process is known as fusion. The coating from the electrode forms a covering over the weld deposit, shielding it from contamination; therefore the process is called shielded metal arc welding. The main advantages of shielded metal arc welding are that high-quality welds are made rapidly at a low cost.
The electrodes are coated with a shielding flux of a suitable composition. The flux melts together with the electrode metallic core, forming a gas and a slag, shielding the arc and the weld pool.
The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation and stabilizes the arc.
SMAW can be performed both on AC and DC sources with dropping characteristics, highest current rating being about 600 amperes. Selection of proper electrode is essential for best results.
Figure 2.4: shielded metal arc welding xxv 2.2.4 ADVANTAGES OF SHIELDED METAL ARC WELDING
-
Simple, portable and inexpensive equipment;
-
Wide variety of metals, welding positions and electrodes are applicable;
-
Suitable for outdoor applications.
2.2.5 SUBMERGED ARC WELDING (SAW)
Submerged arc welding (SAW) is a common arc welding process. Originally developed by the Linde Union Carbide Company. It requires a continuously fed consumable solid or tubular (flux cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being
―submerged‖ under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
SAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and AC is supplied by a transformer. Current for a single wire ranges from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and
1200A.
In DC operation, the electrode is normally connected to the positive terminal. Electrode negative
(DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.
AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage xxvi and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burn off rate, i.e. the balance between the welding current setting and wire feed speed which is under feedback control.
Figure 2.5: submerged arc welding
2.2.6 ADVANTAGES OF SUBMERGED ARC WELDING
-
Very high welding rate;
-
The process is suitable for automation;
-
High quality weld structure.
2.2.7 TUNGSTEN INERT GAS ARC WELDING (TIG)
Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. TIG welding is a commonly used high quality welding process. TIG welding has become a popular choice of welding processes when high quality, precision welding is required.
Tungsten inert gas (TIG) welding is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding
xxvii
gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
TIG is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques.
In TIG welding an arc is created between a nonconsumable tungsten electrode and the metal being welded. The arc produces the heat needed to melt the work. The shielding gas keeps oxygen in the air away from the molten weld pool and the hot tungsten. Gas is fed through the torch in order to shield the electrode and the molten weld pool. The shielding gas used is pure argon. There may or may not be filler metal added to the molten weld pool during the process. Tungsten is used for the electrode because of its high melting temperature and good electrical characteristics.
Figure 2.6: Tungsten inert gas arc welding (TIG)
The main advantage of TIG welding is the wide range of materials that it can weld. TIG welding is used to a great extent for welding different kinds of alloys of aluminum and stainless steel, Specially when quality is of great importance. This technique is mainly used in aeronautical constructions and in the chemical and the nuclear power industry.
xxviii
2.2.8 ADVANTAGES OF TIG WELDING
-
Weld composition is close to that of the parent metal
-
High quality weld structure
-
Slag removal is not required (no slag)
-
Thermal distortions of work pieces are minimal due to concentration of heat in small zone. 2.2.9 METAL INERT GAS ARC WELDING (MIG)
Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in which the weld is shielded by an external gas (Argon, helium, CO2, argon + Oxygen or other gas mixtures).
Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms Joint.
Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed.
This is an arc welding process where hardfacing wire is fed continually from a spool through the welding torch into the arc, where it is melted and transferred to the work piece.
In the case of MIG/MAG welding, the weld pool is protected from the atmosphere with a stream of shielding gas. These MIG/MAG processes are very flexible, i.e. they can be partially or fully mechanised and they are suitable for a wide range of applications.
Wire is also used as the hardfacing consumable in the submerged arc process. In this process a mineral based fluxing powder flows around the consumable wire and is melted by the arc. It forms a gaseous shield around the arc and also forms a slag on top of the weld pool, thereby shielding the cooling weld pool from the atmosphere.
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Figure 2.7: Metal inert gas welding (MIG)
2.2.10 ADVANTAGES OF MIG WELDING
Continuous weld may be produced (no interruptions);
High level of operators skill is not required;
Slag removal is not required (no slag);
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PART-3: GAS WELDING
2.3.1 INTRODUCTION
One of the most popular welding methods uses a gas flame as a source of heat. Gas welding is widely used in maintenance and repair work because of the ease in transporting oxygen and fuel cylinders. Once you learn the basics of gas welding, you will find the oxyfuel process adaptable to brazing, cutting, and heat treating all types of metals.
Gas welding is a process in which the required heat to melt the surfaces is supplied by a high temperature flame obtained by a mixture of two gases. The gases are mixed in proper proportions in a welding blowpipe (torch). For controlling the welding flame, there are two regulators on the torch by which the quantity of either gas can be regulated. Usually the mixture of oxygen and acetylene is used for welding purposes. It produces temperature in the range of 3200-3300 oC.
However the mixture of hydrogen and other combustable gases can also be employed to some extent. Others gases used are MAPP (methyl acetate propediene) (2600-2900)oC, propylene
(2500-2850)oC, propane (2450-2775) oC, natural gas/methane (2350-2750)oC. In gas welding the two surfaces to be welded are properly prepared and placed near each other. The metal in the joint is brought to melting temperature by heat from the flame and then weld is completed by supplying additional metal as the filler metal obtained by a filler rod.
Filler rod is used when an additional supply of metal to weld is required. Shielding flux may be used if protection of weld pool is necessary. Most of commercial metals may be welded by Gas
Welding excluding reactive metals (titanium, zirconium) and refractory metals (tungsten, molybdenum). xxxi
2.3.2 DIFFERENT TYPES OF GAS WELDING
1. Oxyacetylene welding
2. Oxyhydrogen welding
3. Pressure welding
2.3.3 OXYACETYLENE GAS WELDING oxyacetylene welding, which uses acetylene gas as the fuel, is the most commonly used oxyfuel gas welding process due to its high flame temperature (i.e., intense source energy).
Oxyacetylene welding (OAW) derives the heat needed to cause melting of the substrates and, almost always, filler from two stages of combustion. In the first stage, known as primary combustion, the acetylene fuel gas partially reacts with oxygen provided from a pressurized gas cylinder to form carbon monoxide and hydrogen:
C2H2 + 02 (cylinder) = 2CO + H2
This reaction is exothermic and is responsible for about one-third of the total heat generated by the complete combustion of acetylene. The dissociation of acetylene to carbon and hydrogen releases 227 kJ/mol of acetylene at 15°C, while the partial combustion of the carbon to form carbon monoxide releases 221 kJ/mol of carbon. No combustion of the hydrogen takes place at
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this stage. The total heat released by the primary reaction is 448 kJ/mole (501 Btu/ft3) of acetylene. In the second stage of oxyacetylene or other fuel gas welding, known as secondary combustion, which occurs immediately after the primary combustion, the carbon monoxide resulting from partial combustion of the carbon dissociated from the acetylene (or other fuel gas) reacts further with oxygen, this time from the surrounding air, to form carbon dioxide, while the hydrogen from the primary combustion dissociation of acetylene (or other fuel gas) reacts with oxygen in the air to form water:
2CO + O2(air) = 2C02
H2 + 0.5O2 (air) = H2O
These reactions are also exothermic and are responsible for two-thirds of the total heat generated by burning the dissociation products of the acetylene completely. Burning of hydrogen to produce water vapor releases 242 kJ/mol of hydrogen, while further oxidation of carbon monoxide releases an additional 285 kJ/mol of carbon monoxide, or 570 kJ/mol for the reaction.
The total heat released by the second reaction is thus 812 kJ/mol (907 Btu/ft3) of acetylene.
The actual primary and secondary combustion reactions occur in the gas flame of an oxygenacetylene torch in two distinct regions, as shown in Figure. Primary combustion occurs in an inner cone, while secondary combustion occurs in an outer flame. Although only accounting for one-third of the total heat of the overall combustion reaction (448 kJ/mol out of 1260 kJ/mole), the inner cone tends to be more concentrated in volume, and so is hotter (i.e., the energy is more dense). Thus, the welder tends to work with the tip of the inner cone near the workpiece to cause melting, using the outer flame to provide a degree of shielding of the molten weld metal and hot, newly formed weld by the carbon dioxide, to provide preheating to aid in initial melting and to slow down cooling once the weld has been made (thereby sometimes avoiding adverse postsolidification or heat-affected zone transformations.
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Similar combustion reactions can be written and energy balances performed for other fuel gas mixtures with oxygen, with different amounts of energy being liberated and different flame temperatures being produced for each.
The exact chemical nature, or reactivity, of the flame in oxyfuel gas welding processes, such as oxyacetylene welding, can be adjusted to be chemically neutral, chemically reducing, or chemically oxidizing. 2.3.4 ADVANTAGES OF OXYACETYLENE GAS WELDING
It's easy to learn.
The equipment is cheaper than most other types of welding rigs (e.g. TIG welding)
The equipment is more portable than most other types of welding rigs (e.g. TIG welding)
OA equipment can also be used to "flame-cut" large pieces of material.
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2.3.5 OXYHYDROGEN GAS WELDING
Oxyhydrogen Welding is a Gas Welding process using a combustion mixture of Hydrogen (H2) and oxygen (O2) for producing gas welding flame.
Oxyhydrogen flame is used to weld and braze metals only with low melting points, e.g., aluminium, magnesium, lead etc. The temperature of the hottest part of an oxy-hydrogen flame suitable for welding is only about 25000C against 3200°C of an oxyacetylene flame.
In oxyhydrogen welding, if a higher temperature is obtained by increasing the oxygen supply, the flame becomes quite unsuitable for welding. Oxyhydrogen welding is therefore not used for welding steel. Hydrogen is available in compressed gas cylinders. Complete combustion of hydrogen requires an oxygen to hydrogen ratio of 1 to 2.
2H2 + O2= 2H20
This gas mixture produces a strongly oxidizing flame. The oxyhydrogen flame is scarcely visible and there are no combustion zones as in oxyacetylene flame. Therefore it is impossible to obtain a neutral oxyhydrogen flame by the visible methods of flame adjustment. To avoid an oxidizing flame, the pressure regulators must be set to provide an assured excess of hydrogen. In practice a ratio of 4:1 or 5:1 hydrogen: oxygen is required to avoid an oxidizing flame .
Since there is no carbon, the oxyhydrogen flame is only reducing (and never carburizing). The oxyhydrogen welding is similar to oxyacetylene welding with the difference that a special regulator is used for metering the hydrogen gas.
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2.3.6 PRESSURE GAS WELDING
Pressure Gas Welding is a Gas Welding, in which the welded parts are pressed to each other when heated by a gas flame.
In this process, a weld is made simultaneously over the entire area of abutting surfaces with gas flames obtained from the combustion of a fuel gas with oxygen and the application of pressure.
No filler metal is used. Acetylene is normally used as a fuel gas in pressure gas welding.
Pressure gas welding has limited uses because of its low flame temperature, but is extensively used for welding lead.
The process is similar to Resistance Butt Welding. Pressure Gas Welding does not require filler material. Pressure gas welding is used for joining pipes, rods, railroad rails.
Figure 2.8: pressure gas welding
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PART-4: RESISTANCE WELDING
2.4.1 INTRODUCTION
Resistance welding is a fusion welding process that requires the application of both heat and pressure to achieve a sound joint. The simplest form of the process is spot welding where the pressure is provided by clamping two or more overlapping sheets between two electrodes. A current is then passed between the electrodes, sufficient heat being generated at the interface by resistance to the flow of the current that melting occurs, a weld nugget is formed and an autogenous fusion weld is made between the plates. The heat generated depends upon the current, the time the current is passed and the resistance at the interface. The resistance is a function of the resistivity and surface condition of the parent material, the size, shape and material of the electrodes and the pressure applied by the electrodes.
There are a number of variants of the resistance welding process including spot, seam, projection and butt welding. It is an economical process ideally suited to producing large numbers of joints on a mass production basis. Spot welding in particular has been used extensively in the automotive industry, albeit mostly for the joining of steel and in the aerospace industry for airframe components in aluminium alloys. Seam welding is used in the production of thin sheet, leak-tight containers such as fuel tanks. Projection welding is generally used for welding items such as captive nuts onto plate. This variation is not normally used on aluminium and is not covered in this chapter. Flash welding, unlike spot and seam welding that require a lap joint, is capable of making butt welds. This is achieved by resistance heating the abutting faces and then forging them together.
Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current (100–100 000 A) is passed through the metal. In general, resistance xxxvii welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.
The welding cycle must first develop sufficient heat to raise a small volume of metal to the molten state. This metal then cools while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be sufficient to produce a weld nugget, but not so high as to expel molten metal from the weld zone.
2.4.2 DIFFERENT TYPES OF RESISTANCE WELDING
1. Spot welding
2. Seam welding
3. Projection welding
4. Butt/upset welding
5. Flash welding
2.4.3 SPOT WELDING
Spot welding is a resistance welding method used to join two to three overlapping metal sheets, studs, projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually power sources and welding equipment are sized, to the specific thickness and material being welded together. The thickness is limited by the output of the welding power source and thus the equipment range due to the current required for each application. Care is taken to eliminate, contaminates between the faying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes an heat build-up in the work between the copper electrodes, the rising temperature causes a rising resistance, and results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the workpiece in less than a second (controls are based in
AC cycles, or microseconds) the molten, or at least plastic, state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the weld, since copper is an excellent conductor. Some of the different currents used are Direct Current,
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Alternating Current, and Medium Frequency Half wave Direct Current, as well as high frequency half wave Direct current.
If excessive heat is applied, or applied too quickly, or if the force between the base materials is too low, or the coating too thick, or too conductive, the molten area may extend to the outside, and with its high pressure (typically 30,000 psi) will escape the containment force of the tips with a burst of molten metal called expulsion. When this occurs, the metal will be thinner and have less strength than a weld with no expulsion. The common method of checking a weld is a peel test. An alternative test is the restrained tensile test, which is much more difficult to perform, and requires calibrated equipment. Ultrasonic evaluation has been tried and is still in a "unapproved" state for many OEMs.
The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods, limiting the usefulness of the process. It is used extensively in the automotive industry— cars can have several thousand spot welds.
A specialized process, called shot welding, can be used to spot weld stainless steel.
Figure 2.9: spot welding process
There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In a solid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grain structure, xxxix e.g. molybdenum to tungsten, are joined using a very short heating time, high weld energy, and high force. There is little melting and minimum grain growth, but a definite bond and grain interface. Thus the materials actually bond while still in the solid state. The bonded materials typically exhibit excellent shear and tensile strength, but poor peel strength. In a fusion bond, either similar or dissimilar materials with similar grain structures are heated to the melting point (liquid state) of both. The subsequent cooling and combination of the materials forms a ―nugget‖ alloy of the two materials with larger grain growth.
Typically, high weld energies at either short or long weld times, depending on physical characteristics, are used to produce fusion bonds. The bonded materials usually exhibit excellent tensile, peel and shear strengths. In a reflow braze bond, a resistance heating of a low temperature brazing material, such as gold or solder, is used to join either dissimilar materials or widely varied thick/thin material combinations. The brazing material must ―wet‖ to each part and possess a lower melting point than the two workpieces. The resultant bond has definite interfaces with minimum grain growth. Typically the process requires a longer
(2 to 100 ms) heating time at low weld energy. The resultant bond exhibits excellent tensile strength, but poor peel and shear strength.
2.4.4 SEAM WELDING
Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material.
A transformer supplies energy to the weld joint in the form of low voltage, high current AC power. The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed
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Figure 2.10: seam welding process together by the welding pressure that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.
A common use of seam welding is during the manufacture of round or rectangular steel tubing.
Seam welding has been used to manufacture steel beverage cans but is no longer used for this as modern beverage cans are seamless aluminum.
2.4.5 PROJECTION WELDING
Projection welding(PW) is a variation of resistance welding in which current flow is concentrated at the point of contact with a local geometric extension of one (or both) of the parts being welded. These extensions, or projections, are used to concentrate heat generation at the point of contact. The process typically uses lower currents, lower forces, and shorter welding times than does a similar application without the projections. Projection welding is often used in the most difficult resistance-welding applications because a number of welds can be made at one xli time, which speeds up the manufacturing process. PW applications are generally categorized as being either embossed-projection welding or solid-projection welding. These variations are shown in Fig. and. Embossed-projection welding is generally a sheet-to-sheet joining process, in which a projection is stamped onto one of the sheets to be joined. Then, resistance welding is conducted on a stack of sheets. Heat initially concentrates at the contact point and in the walls of the projection during resistance welding. Early in the process, the projection itself collapses back into the original sheet. However, the initial heating raises the local resistivity of the joint area, allowing continued resistance heating at this location. Weld development then proceeds in the conventional manner, by forming a fused weld nugget. Solid-projection welding requires that the projection be forged onto one of the two components. Then, during resistance welding, the contact point and the projection itself experience preferential heating. In this case, the projection cannot simply collapse, as it does in embossed-projection welding. Rather, the projection collapses by penetrating the opposing material and by extrusion to the periphery. When compared with embossed-projection welding, the resulting joints are solid-state, rather than fusion, welds. The actual joints are caused by combination of material forging and diffusion bonding, much like they are in resistance butt and flash butt welding.
Figure 2.11: projection welding xlii 2.4.6 BUTT/ UPSET WELDING
Upset welding (UW) is a resistance welding process utilizing both heat and deformation to form a weld. The heat is produced by resistance to the flow of electrical current at the interface of the abutting surfaces to be joined. The deformation results from force on the joint in combination with softening from the electrical resistance heat. Upset welding typically results in solid-state welds (no melting at the joint). The deformation at the weld joint provides intimate contact between clean adjoining surfaces, allowing formation of strong metallurgical bonds. If any melting does occur during upset welding, the molten metal is typically extruded out of the weld joint area. A wide variety of shapes and materials can be joined using upset welding in either a single-pulse or continuous mode. Wire, bar, strip, and tubing can be joined end to end with a single pulse of welding current. Seams on pipe or tubing can be joined using continuous upset welding by feeding a coiled strip into a set of forming rolls, resistance heating the edges with wheel electrodes, and applying a force to upset the edges together. Equipment for single-pulse upset welding is relatively simple. It consists of a pneumatic or hydraulic system for force application, transformers or a bank of diodes as a source of electrical current, and a standard resistance-welding controller. A data acquisition system usually is employed to record the force, current, voltage, and motion of the weld head during welding. Equivalent welds have been made using both alternating and direct current. Upset welds have similar characteristics to inertia friction welds, which are also solid-state welds. The amount of deformation is usually less for upset welds, and the deformation can be more precisely controlled using upset welding. For example, a pipe butt weld made using inertia friction welding will have a large upset on both the inside and outside, whereas an upset weld can be controlled, through joint design and welding parameters, to have essentially no internal upset.
Figure 2.12: Butt welding process xliii 2.4.7 FLASH WELDING
Flash welding (FW) is a resistance welding process in which a butt joint weld is produced by a flashing action and by the application of pressure. In basic terms, it is a melting and a forging process. The process is capable of producing welded joints with strengths equal to those of the parent materials. Figure is a schematic representation of a typical flash welding operation. Flash welding is used to join metallic parts that have similar cross sections, in terms of size and shape.
The process lends itself to joining nearly all grades of steel, aluminum, brass, and copper parts, in addition to selected dissimilar materials.
Figure 2.13: Flash welding process
2.4.8 RESISTANCE WELDING BENEFITS
High speed welding
Easily automated
Suitable for high rate production
Economical
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CHAPTER- III
FUNDAMENTALS OF SPOT WELDING
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3.1 PRINCIPLE
Resistance spot welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localized heating in the joint, and the weld is made. The resistance spot weld is unique because the actual weld nugget is formed internally with relation to the surface of the base metal. Figure 3.1 shows a resistance spot weld nugget compared to a gas tungsten-arc (TIG) spot weld.
Figure 3.1: Resistance spot and TIG Weld Comparison
The gas tungsten-arc spot is made from one side only. The resistance spot weld is normally made with electrodes on each side of the workpiece. Resistance spot welds may be made with the workpiece in any position.
The resistance spot weld nugget is formed when the interface of the weld joint is heated due to the resistance of the joint surfaces to electrical current flow. In all cases, of course, the current must flow or the weld cannot be made. The pressure of the electrode tips on the workpiece holds the part in close and intimate contact during the making of the weld. Remember, however, that resistance spot welding machines are NOT designed as force clamps to pull the workpieces together for welding.
3.2 SPOT WELDING PROCESS
Spot welding is the most common form of resistance welding and is defined as a process in which a weld is produced at a spot in a work piece between the electrodes, the weld being of approximately the same areas as the electrode tips, or as the smaller tip where they differ in size.
Force is applied to the spot, usually through the electrodes and continuously throughout the xlvi process.
The term ‗spot‘ is an obvious one from the shape and appearance of the weld. The process is almost always applied to the lap joints. The basic assembly is shown in the Figure 3.2 below.
Fig 3.2: Spot Welding Assembly
Fig 3.3: cross-section of spot welding
The joint to be welded is placed between the two electrodes and the pressure is applied to ensure good contact. The electrodes are made of copper of high conductivity copper or lead and this ensures that a minimum amount of heat is produced where they make contact with the work piece. Heating of the weld area begins where the parts to be joined touch each other and is largely due to the contact resistance.
Any heat, which is generated near the electrodes, tends to be conducted away by the mass of the electrodes. The generation of the heat at the central interface continues until a slug of metal is in xlvii the molten condition and the current flow is terminated. This molten metal then cools and solidifies whilst still under the forging pressures of the electrodes. Success in spot welding depends on the correct balance between current, time and pressure and that these must suit the characteristics of the materials being welded.
3.3 CIRCUIT DIAGRAM OF SPOT WELDING PROCESS
Figure 3.4: Circuit diagram of spot welding
3.4 SEQUENCE OF EVENTS IN RESISTANCE SPOT WELDING
Figure 3.5: Sequence of events in resistance spot welding
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3.5 ESSENTIAL FACTORS OF SPOT WELDING
1. The welding current.
2. The time of current application.
3. The pressure of electrodes.
4. The shape or area of electrodes.
5. The surface condition of welded material.
3.6 HEAT GENERATION
A modification of Ohm‘s Law may be made when watts and heat are considered synonymous.
When current is passed through a conductor the electrical resistance of the conductor to current flow will cause heat to be generated.
The basic formula for heat generation may be stated:
H = I2R Where
H = Heat
I2 = Welding Current Squared
R = Resistance
Figure 3.6: Graphs of resistance and temperature, as the result of Joule heating, as a function of location in a typical resistance welding arrangement xlix The secondary portion of a resistance spot welding circuit, including the parts to be welded, is actually a series of resistances. The total additive value of this electrical resistance affects the current output of the resistance spot welding machine and the heat generation of the circuit.
The resistance spot welding machines are constructed so minimum resistance will be apparent in the transformer, flexible cables, tongs, and electrode tips. The resistance spot welding machines are designed to bring the welding current to the weldment in the most efficient manner. It is at the weldment that the greatest relative resistance is required. The term ―relative‖ means with relation to the rest of the actual welding circuit.
There are six major points of resistance in the work area. They are as follows:
1. The contact point between the electrode and top workpiece.
2. The top workpiece.
3. The interface of the top and bottom workpieces.
4. The bottom workpiece.
5. The contact point between the bottom workpiece and the electrode.
6. Resistance of electrode tips.
The resistances are in series, and each point of resistance will retard current flow. The amount of resistance at point 3, the interface of the workpieces, will depend on the heat transfer capabilities of the material, its electrical resistance, and the combined thickness of the materials at the weld joint. It is at this part of the circuit that the nugget of the weld is formed.
3.7 THE TIME FACTOR
Resistance spot welding depends on the resistance of the base metal and the amount of current flowing to produce the heat necessary to make the spot weld. Another important factor is time. In most cases several thousands of amperes are used in making the spot weld. Such amperage values, flowing through a relatively high resistance, will create a lot of heat in a short time. To make good resistance spot welds, it is necessary to have close control of the time the current is flowing. Actually, time is the only controllable variable in most single impulse resistance spot welding applications.
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Most resistance spot welds are made in very short time periods. Since alternating current is normally used for the welding process, procedures may be based on a 60 cycle time (sixty cycles
= 1 second). Figure 3.5 shows the resistance spot welding time cycle.
Previously, the formula for heat generation was used. With the addition of the time element, the formula is completed as follows:
H = I2RT
where H = Heat
I2= Current Squared
R = Resistance
T = Time
Control of time is important. If the time element is too long, the base metal in the joint may exceed the melting (and possibly the boiling) point of the material.
Figure 3.7: Spot Welding Time Cycle
1. Squeeze Time: The time interval between timer initiation and the first application of current needed to assure that electrodes contact the work and establish full force.
2. Weld time: The time for which welding current is applied (in single impulse welding) to the work to make a weld.
3. Hold Time: The time during which force is maintained on the work after the last impulse of welding current ends to allow the weld nugget to solidify and develop strength.
4. Off Time: The time during which the electrodes are off the work and the work is moved to the next weld location for repetitive welding.
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The full cycle or weld schedule is shown in Figure 3.8 This basic cycle can be enhanced to improve the physical and mechanical properties of a weld, as shown in Figure 3.8, where (1) precompression force is used to set electrodes and workpieces together; (2) preheat is applied to reduce thermal gradients at the start of weld time or to soften coatings such as zinc galvanize; (3) forging force is used to help consolidate the weld nugget; (4) quench and temper times are used to produce desired weld properties in hardenable steels; ( 5 ) postheat is used to refine weld nugget grain size and improve strength; and (6) current decay is used to retard cooling of aluminum alloys to help prevent cracking.
Figure 3.8 also shows multiple-impulse welding, as opposed to single-impulse welding shown in
Figure 3.9.
Figure 3.8: Basic single-impulse welding cycle for spot welding
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Figure 3.9: Multi-impulse welding cycle for spot welding
The strength of a weld nugget (shown schematically in Figure 3.9) made during resistance spot welding increases as current is applied for longer times, as the volume of melting and the size of the nugget formed increases accordingly. At some point, so much melting has occurred
Figure 3.10: Nugget formation and heat dissipation into the surrounding base metal and electrodes during resistance spot welding liii and the volumetric expansion of the molten metal compared to the solid metal from which it was produced becomes so great that the pressure applied by the electrodes cannot contain it. At this point expulsion or ―spitting‖ occurs. When this occurs, molten metal is lost from within the nugget, and the final, solidified weld inevitably contains a void, thereby reducing the strength, perhaps dramatically. For this reason, welding time should be limited to prevent expulsion. This is shown in Figure 3.11.
Figure 3.11: Weld strength as a function of current (a) and welding time (b).
3.8 PRESSURE
The effect of pressure on the resistance spot weld should be carefully considered. The primary purpose of pressure is to hold the parts to be welded in intimate contact at the joint interface.
This action assures consistent electrical resistance and conductivity at the point of weld. The tongs and electrode tips should NOT be used to pull the workpieces together. The resistance spot welding machine is not designed as an electrical ―C‖ clamp! The parts to be welded should be in intimate contact BEFORE pressure is applied.
Investigations have shown that high pressures exerted on the weld joint decrease the resistance at the point of contact between the electrode tip and the workpiece surface. The greater the pressure the lower the resistance factor.
Proper pressures, with intimate contact of the electrode tip and the base metal, will tend to conduct heat away from the weld. Higher currents are necessary with greater pressures and, conversely, lower pressures require less amperage from the resistance spot welding machine. liv This fact should be carefully noted particularly when using a heat control with the various resistance spot welding machines.
3.9 ELECTRODE
Copper is the base metal normally used for resistance spot welding tongs and tips. The purpose of the electrode tips is to conduct the welding current to the workpiece, to be the focal point of the pressure applied to the weld joint, to conduct heat from the work surface, and to maintain their integrity of shape and characteristics of thermal and electrical conductivity under working conditions. Electrode tips are made of copper alloys and other materials. The Resistance Welders
Manufacturing Association (RWMA) has classified electrode tips into two groups:
Group A − Copper based alloys
Group B − Refractory metal tips
The groups are further classified by number. Group A, Class I, II, III, IV, and V are made of copper alloys. Group B, Class 10, 11, 12, 13, and 14 are the refractory alloys.
Group A, Class I electrode tips are the closest in composition to pure copper. As the Class
Number goes higher, the hardness and annealing temperature values increase, while the thermal and electrical conductivity decreases.
Group B compositions are sintered mixtures of copper and tungsten, etc., designed for wear resistance and compressive strength at high temperatures. Group B, Class 10 alloys have about
40 percent the conductivity of copper with conductivity decreasing as the number value increases. Group B electrode tips are not normally used for applications in which resistance spot welding machines would be employed.
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3.10 ELECTRODE TIPS SIZE
When you consider that it is through the electrode that the welding current is permitted to flow into the workpiece, it is logical that the size of the electrode tip point controls the size of the resistance spot weld. Actually, the weld nugget diameter should be slightly less than the diameter of the electrode tip point. If the electrode tip diameter is too small for the application. The weld nugget will be small and weak. If, however, the electrode tip diameter is too large, there is danger of overheating the base metal and developing voids and gas pockets. In either instance, the appearance and quality of the finished weld would not be acceptable.
To determine electrode tip diameter will require some decisions on the part of the weldment designer. The resistance factors involved for different materials will certainly have some bearing on electrode tip diameter determination. A general formula has been developed for low carbon steel. It will provide electrode tip diameter values that are usable for most applications.
The formula generally used for low carbon steel is as follows:
Electrode tip diameter = 0.100‖ + 2t
Where ―t‖ is the thickness in inches of one thickness of the metal to be welded. This formula is applicable to the welding of metals of dissimilar thicknesses. The formula is applied to each thickness individually, and the proper electrode tip diameter selected for each size of the joint.
For example, if two pieces of 0.062‖ sheet metal are to be joined, the electrode tip diameter would be the same for both sides of the joint. The calculation would be as follows:
Electrode tip dia. = 0.100 + 2t
= 0.100 + 2 x 0.062‖
= 0.100 + 0.124‖
Electrode tip dia. = 0.224‖
If the two pieces were unequal in thickness, such a one piece 0.062‖ and the other 0.094‖, two calculations would have to be made. Each thickness would be treated as the basis for one electrode tip diameter determination, as follows:
Electrode tip dia. = 0.100 + 2t lvi = 0.100 + 2 x 0.062‖
= 0.100 + 0.124‖
Electrode tip dia. = 0.224‖ (one side only)
For the other side, the calculation is as follows:
Electrode tip dia. = 0.100 + 2t
= 0.100 + 2 x 0.094‖
= 0.100 + 0.188‖
Electrode tip dia. = 0.288‖ (one side only)
Remember that the formula is applicable to low carbon steels and may not be correct for other materials. 3.11 ELECTRODE COATING:
Electrode coating with slagging and fluxing materials are particularly necessary in the welding of alloys and nonferrous metals. Some of the elements of these alloys are not very stable and are lost if there is no protection against oxidation. Heavy coatings also permit the use of larger welding rods, higher current and greater welding speeds. In summary the coating do the following things:
1. Provide a protecting atmosphere.
2. Provide slag of suitable characteristics to protect the molten metal.
3. Facilitate overhead and position welding.
4. Stabilize the arc.
5. Add alloying element to weld metal.
6. Perform metallurgical refining operation.
7. Reduce spatter of weld metal.
8. Increase deposition efficiency.
9. Remove oxides and impurities.
10. Influence the depth of arc penetration.
11. Influence the shape of bed
12. Slow down the cooling rate of the weld.
lvii
3.12 FACTORS INFLUENCING THE LIFE OF ELECTRODES
The regulating and controlling possibilities, the welding machine and the parts to be welded have a strong influence on the life of the electrodes. Reducing the electrode metal pick-up is of great importance and can be influenced by having a uniform surface condition obtained through the use of mechanical and chemical pretreatments. This can be best controlled by measuring the contact resistance between electrode and sheet. The regulating and controlling possibilities of the welding machine, e.g., chronological programming of current and force, the resetting behavior of the electrodes as well as cooling of the electrode, affect the stroke frequency and consequently the number of possible weld spots. The higher stroke frequency of roll seam welding necessitates a direct external cooling of the electrodes.
Figure 3.12: Factors of influence the life of Electrodes
3.13 INFLUENCE OF STORAGE TIME ON LIFE OF ELECTRODES
With suitable chemical surface pretreatments, the sheets retain their suitability for welding over long periods of time, i.e., the attainable life of electrodes also remains unchanged. In spite of this, efforts must be undertaken to keep the storage time as low as possible in order to avoid problems caused by dust dirtying the surface of the sheets. lviii Figure 3.13: influence of storage time on life of electrodes
3.14 REQUIRED CONDITIONS FOR SPOT WELDING
The suitability of Spot Welding depends on 3 factors
1. Surface condition
2. Chemical composition
3. Metallurgical condition
Figure 3.14: Suitability of spot welding
lix
High affinity to oxygen, the free surface of metal is covered by a dense, tightly adhering oxide film. This film is a non-conductor of electricity and must, therefore, be removed prior to welding. Both electrical and thermal conductivity depend strongly on the chemical composition. The very good electrical conductivity of pure metal makes it necessary to use large currents.
Metallurgical changes occur due to heating and subsequent cooling of the weld and heat affected join of the parent metal, affecting the quality of the weld.
3.15 POSITIVE LOCATION OF WORKPIECES
Precise locations of spot welded parts are a cost-related process and should be considered during the design. Part positioning involves either extensive fixturing or, preferably, self alignment through built-in stamped features like holes and tabs. With the latter method, the location is predetermined by the accuracy of alignment features.
Figure 3.15: Positive Locations of Workpieces
The most preferred and most easily achieved method for accurately self-fixturing parts is the half sheared or extruded cylindrical button and matching hole in the mating part. (Figure 8). One mating hole should be 0.003 in. (0.08 mm) larger in diameter than the extrusion and the second hole should be slotted by 0.040 in. (1.02 mm) minimum to allow for normal fabrication tolerances as
lx
shown in the drawing. Another alternative is to produce a lanced tab in a punching process. Mating parts can then be brought up to it and located in position.
Knowledgeable designers recognize such cost-saving and quality-improvement methods and specify them in the manufacturing process. The consistency attainable with such methods surpasses that of sophisticated jigs and fixtures but the greatest value is the cost efficiency.
Additionally, these techniques can be used for fillet welding applications, and mechanical assemblies. 3.16 ADVANTAGES OF SPOT WELDING
1. Low cost
2.
High speed of welding
3. More general elimination of warping or distortion of parts
4. High uniformity of products
5.
No edge preparation is needed
6. No filler rod is needed
7. The process is normally free from smoke and spatter
8. Suitable for automation.
9. Efficient energy use.
10. Limited work piece deformation. Also, work piece is not melted to a larger extent. Heat is concentrated only at the spot to be welded.
3.17 DISADVANTAGES
1. The electrodes have to be able to reach both sides of the pieces of metal that are being joined together. A particular spot welding machine will be able to hold only a certain thickness of metal
(usually 5 to 50 inches) and although the position of the electrodes can be adjusted, there will be only a limited amount of movement in most electrode holders.
2. The size and shapes of the electrodes will determine the size and strength of the weld. The join forms only at the spot where the electrodes are in contact with the metal. If the current is not lxi strong enough, hot enough or the metal is not held together with enough force, the spot weld may be small or weak.
3. Warping and a loss of fatigue strength can occur around the point where metal has been spot welded. The appearance of the join is often rather ugly, and there can be cracks. The metal may also become less resistant to corrosion.
4. Silver and copper are difficult to weld because of their high thermal conductivity.
3.18 LIMITATION
Spot welding is useful in many circumstances, although there are certain limitations. It can create only localized joins, which may not be particularly strong. The strength of a spot weld depends on the force and temperature that has been applied and on the cleanliness of the electrodes and metal. The difficulty of attaching the electrodes to oddly shaped pieces of metal can be avoided by using a portable spot welder. This has electrodes that are attached to long cables so they can reach inaccessible places.
3.19 PRACTICAL USES OF RESISTANCE SPOT WELDING
Resistance spot welding techniques do not require extensive or elaborate safety precautions.
There are some common sense actions that can, however, prevent injury to the operator. Anytime work is being done in a shop, it is a wise rule to wear safety glasses. Resistance spot welding is no exception to the rule! Very often metal or oxides are expelled from the joint area. Protection of the face and especially of the eyes in necessary to prevent serious injury.
Another area of concern is ventilation. This can be a serious problem when resistance spot welding galvanized metals (zinc coated) or metals with other coatings such as lead. The fumes from the welding operation have a certain toxicity which will cause illness to the operator.
Proper ventilation can reduce the fume concentration in the welding area.
lxii
As explained in the preceding discussion on the fundamentals of resistance spot welding, there is a definite relationship between time, current, and pressure. Current and pressure help create the heat in the weld nugget.
If the weld current is too low for the application, current density is too weak to make the weld.
This condition will also overheat the electrode tips which can cause them to anneal, mushroom, and possibly be contaminated. Even though time is increased, the amount of heat generated is less than the losses due to radiation and conduction in the workpiece and thermal conduction of the electrodes. The result is the possibility, with long weld times at low currents, of overheating the entire base metal area between the electrodes. This could cause burning of the top and bottom surfaces of the workpiece as well as possibly imbedding the electrode tips in the workpiece surfaces. As current density is increased, the weld time is decreased proportionately. If, however, the current density becomes too high, there is the possibility of expelling molten metal from the interface of the joint thereby weakening the weld. The ideal time and current density condition is somewhere just below the level of causing metal to be expelled.
Figure 3.16: Resistance Spot Weld Heat Zones
It is apparent that the heat input cannot be greater than the total dissipation rate of the workpiece and the electrode without having metal expelled from the joint.
lxiii
An interesting discovery has been developed recently concerning the flow of current through the workpiece. Until recently, current was considered to flow in a straight line through the weld joint. This is not necessarily true when multiple thicknesses of material are being welded. The characteristic is for the current to ―fan out‖ thereby decreasing the current density at the point of weld the greatest distance from the electrode tips. The illustration (Figure 4-3) shows the resistance spot weld heat zones for several thicknesses of metal. We note that the uncontrollable variables (such as interface contamination) are multiplied when resistance spot welding several thicknesses of material. Quality levels will be much lower for ―stack‖ resistance spot welding, which explains why such welding practices are avoided whenever possible.
Disregarding the quality factor, it becomes apparent that the number of thicknesses of a material which may be successfully resistance spot welded at one time will depend on the material type and thickness as well as the KVA capacity of the resistance spot welding machine. KVA rating, duty cycle, and other pertinent information is shown on all resistance spot welding machine nameplates. The catalog literature and the operating manuals provide data on the maximum combined thicknesses of material that each unit can weld.
lxiv
CHAPTER- IV
DESIGN & CONSTRUCTION
lxv
4.1 DESIGN:
4.1.1 VARIOUS TYPES OF SPOT WELDING MACHINE:
Various spot welding machine are classified as:
1. Stationary single-spot machine
2. Portable single-spot machine
3. Multiple spot machine
Stationary single-spot machine may further classified as:
i.
Rocker arm type
ii.
Direct pressure type
A direct pressure type stationary single spot welding machine has been selected for the project work. 4.1.2 DESIGN CONSIDERATIONS FOR SPOT WELDING
Spot welding is often selected for joining sheet metal fabrications, stampings and assemblies because it is fast, reliable and economical. However, numerous design considerations can affect the quality and cost of the weld, among them: size of the spot weld, accessibility, positioning, materials and thicknesses being joined, and the number of spots needed to attain the desired strength. Figure 4.1: Operation procedure
lxvi
Thickness of the majority of parts joined by spot welding ranges up to 1/8 in. (3 mm) for each member, although parts up to 1/4 in. (6 mm) thick have been successfully spot welded.
Based on the experience of stampers and fabricators, certain general recommendations can facilitate spot welding of a sheet metal design, no matter what metal forming process is used to make it. It is always useful to consult with the metal former in the design stage when questions arise regarding the part design, application of spot welding or, control of spot welding cost for a particular design.
Knowledgeable designers avoid overspecifying the number of welds, weld size, and location. After evaluating strength requirements, it usually suffices to specify "a minimum number of spot welds equally spaced," thereby leaving the most economical positioning up to the metalformer.
Even though spot welding is a very cost effective way of fastening sheet materials, if other joining methods are also specified, it may be more economical to redesign so that one or the other method is eliminated.
Dimensional precision is often over specified, sometimes unintentionally. CAD systems, for example, specify three or four digits of precision unless instructed otherwise. Where possible, spot welding should be shown schematically without dimensions.
4.1.3 EQUIPMENT REQUIRED FOR SPOT WELDING MACHINE
1. Cap or electrode
2. Holder
3. Gun arm
4. Controller
5. Transformer
For Measurement:
1. Spring balance
2. Stopwatch
3. Ammeter
lxvii
Design of different parts of spot welding machine:
A proper design or plan must be needed for construct better spot welding machine. The available current of welding shop is low. For low current greater thickness sheet cannot be welded. Maximum 0.7 mm thickness sheet can be welded. For this reason electrode size is 10.3 cm length and 1.2 cm diameter. A bed is of 45, 4 X 21 cm size for hold the whole construction.
The size of the two arms 25, 5 X 5 cm is used to hold the electrode.
4.1.4 ELECTRODE
Figure 4.2: Electrode
Length of the Electrode-10.3 cm
Diameter of the Electrode-1.2 cm
lxviii
4.1.5 TRANSFORMER
Figure 4.3: transformer
4.1.6 SPRING BALANCE
Figure 4.4: Spring balance
lxix
4.1.7 DESIGN OF SPOT WELDING MACHINE
Figure 4.5: 3-D view of spot welding machine
lxx
4.2 CONSTRUCTION:
4.2.1 CONSTRUCTION DETAIL:
The spot welding machine will be constructed consist of mainly a bed, two arms, two electrodes and a transformer. Bed is of 45, 4 X 21 cm size. There are two tips, one is fixed and another is movable to pass current and apply pressure to region to be welded. Other materials included two
Copper welder tips, two 21 cm x 3 cm steel plated corner braces, drywall screws, and washers.
There are two wood arm used, where the electrodes are aligned. The size of the arms is 25, 5 X 5 cm. Then one of the arms is mounted with the bed by 3 screws. After the lower jaw was mounted, we also attached the corner braces. It was found that a spare piece of 5 X 5 cm as a shim worked well to align the upper and lower jaw. After the upper jaw was aligned we attached it to the steel braces with screws. This formed the hinged portion of the jaw. Fixed tip will be fixed with the bed. The copper electrodes to make sure they were tight. A loose connection will take heat away from the weld nugget. Then the electrode is mounted with the upper jaw, which is movable. Evenly aligning the welding electrodes, we were careful to keep the upper jaw in the natural position where it was mounted. This maintained a flat contact area for the welding electrodes. After we were sure that the electrodes had been properly aligned. Two end of a transformer will be connected to two tip to pass current through them. As a large amount of heat will generate. There are five zones of heat generation. One at the interference of two sheets, two at the contact point of electrodes with sheet and two at the metal of the sheets. It is very much desirable for obtaining good weld that proper heat balance should be there. A lever is attached in the upper arm and pressure is applied by this lever. One end of the spring balance is attached on the free end of the lever and the other end is fixed with top wall of the building.
lxxi
Figure 4.6: Constructed spot welding machine
lxxii
4.2.2 OPERATION PROCEDURE:
The surface to be welded should be thoroughly cleaned by picking or shot blasting to obtain strong welding. The two pieces to be joined by spot welding have to placed between two electrodes and then electrodes will be pressed against each other by hand lever so as to cause desired pressure on the sheet. A current of low voltage and sufficient current (120-300 amperes per mm2) will be passed between two electrodes causing the two parts to be brought to welding temperature and thus the metal under electrodes pressure will squeezed and welded. The duration of current must be between 0.3 to 2 second. This duration is known as welding time. After this the current will be switched off while the pressure will still act. The pressure will be applied till the weld cools and regain sufficient strength. This period is known as hold time. The pressure is then released and work is removed from machine. Material with higher thermal conductivity, like aluminum need higher current for shorter period than material with lower thermal conductivity. Some recommendation for different thickness of mild steel of different thickness are given bellow. Thickness(mm)
Time(sec)
Force(N)
Current(amp)
0.3
0.4
0.5
0.6
5
7
10
15
137
147
196
215
70
100
140
160
4.3 RISKS:
There are some limitations on material weldability but attention to correct setting up and good process control can solve most production problems. The main safety hazards are
(i)
The risk of crushing fingers or hands and
(ii)
Burns or eye damage from splash metal. Little fume is produced but may need attention when welding coated steels or when oils or organic materials are present.
lxxiii
CHAPTER- V
PERFORMANCE TEST & DISCUSSION
lxxiv
5.1 PERFORMANCE TEST
The performance of the resistance spot welding machine has been found quiet satisfactory. The main objective of this project is to make spot welding between thin sheets. A perfect pressure and current will make the weld smoothly.
5.2 DISCUSSION
A spot welding machine was designed and constructed under this project work. The main objectives of this project is to make a better spot weld between thin sheets. The actual weld is made at the interface of the parts to be joined. The electrical resistance of the material to be welded causes a localized heating at the interfaces of the metals to be joined. Welding procedures for each type of material must be developed for the most satisfactory results. The exact time, pressure, and current setting will depend on the specific application and the KVA rating of the resistance spot welding machine employed.
But for this three reasons such as Excessive tong pressure, Weld time too long, Misalignment of the electrode tips.
If the resistance spot weld does not have an even, concentric surface appearance, the problem could be misalignment of the electrode tips. Align electrode tips with the power off and a typical weld joint between the tip surfaces.
lxxv
CHAPTER- VI
CONCLUSION
lxxvi
6.1 CONCLUSION
After completing the project work, it may be concluded that,
1. The design of the spot welding machine was completed
2. The construction of various parts of spot welding machine according to design was completed. 3. The performance of the spot welding machine was satisfactory enough. But for various reasons there was a little bit variation in spot welding.
6.2 SCOPE OF FUTURE WORK
1. In future hydraulic system can be used for pressure applied.
2. In future for greater thickness spot welding a high range transformer can be used.
lxxvii
REFERENCES
1. R.k. jain; Production Technology 16th edition; khanna publishers 2-B, Nath market, Nai sarak, Delhi-11006(India)
2.
Myron L Begeman; Manufacturing process 8th edition; JOHN WILEY & SONS; New
York
3. Dave smith; Welding; 8th editions; Mc Grow international editions; Mechanical engineering Series
4. Little R.L; Welding and welding technology; Tata McGraw Hill publishing Company
Limited; New Delhi.
5. "Manufacturing Process"; Harold V. Johnson; 1984
6. "Manufacturing Processes Reference Guide"; Robert H. Todd, Dell K. Allen and Leo
Alting; 1994.
7. http://www.Olympusndt.com
8. http://www.wikipedia.com
9. Handbook for Resistance Spot Welding- Miller Electric Mfg. Co.
10. The welding handbook- Barwil Unitor Ships Service, http://www.barwilunitor.com
11. http://www.welderworld.com
12. http://www.cmwinc.com
13. Introduction to welding
14. Resistance welding- Talat 4500
15. Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and
Steven R. Schmid.
16. The Welding Journal, May 2003
17. Journal of Applied Sciences Research, 3(11): 1494-1499, 2007
18. Soldering and Welding Machines and Apparatus August 2008
lxxviii
MACHINE
A project report submitted to the Department of Mechanical Engineering, Khulna University of
Engineering & Technology in partial fulfillment of the requirements for the degree of
―Bachelor of Science in Mechanical Engineering‖
SUPERVISED BY:
SUBMITTED BY:
Dr. Naseem Ahmed
Professor
Department of Mechanical Engineering
KUET
Md. Musaddiq Reza
Roll no. 0605078
Raihan kabir
Roll no. 0605084
DEPARTMENT OF MECHANICAL ENGINEERING
KHULNA UNIVERSITY OF ENGINEERING & TECHNOLOGY
KHULNA 9203, BANGLADESH
MAY, 2011 i ACKNOWLEDGEMENT
Authors want to express their gratefulness and humbleness to Almighty Allah for His immense blessing on them for the successful completion of undergraduate project work.
With deep sincerity, the authors express profound gratefulness to Dr. Nassem Ahmed, Professor,
Department of Mechanical Engineering, Khulna University of Engineering & Technology
(KUET), for his guidance and valuable counsel in execution and completion of the study without which it would be impossible to carry out the work.
They would like to extend their thanks to Professor Dr. Mihir Ranjan Halder, Head, Department of Mechanical Engineering, KUET, for giving permission to use lab facilities of the department.
Authors would like to complement their respective teachers of Mechanical Engineering
Department for their direct and indirect suggestions and opinions at different stages of work.
Authors are indebted to Professor Dr. Muhammed Alamgir, Vice Chancellor, KUET, for providing financial support of this project.
Authors would like to thank to all staff members of Wood Shop, Machine Shop and Welding
Shop. It would have been impossible to carry out the work without their help.
Authors wish to extend love and gratitude to their families who have always been a great support. Authors are fortunate to be a part of such families. Authors thank those parents who have sacrificed a lot for the prosperity and success of their children.
Finally sincere thanks to all well wisher.
Authors
ii
ABSTRACT
Welding is a process of joining two metal pieces. Resistance spot welding is one type of welding. Resistance spot welding is used to join two or more metal sheets together, and the technique is used widely in the automotive industry. Furthermore, other metal-to-metal connections, such as wire-to-wire joints in the electronics industry, are accomplished by resistance spot welding. Application-specific measures, such as the diameter of the welding spot, define the quality of the joint. The objective of the project work was to design and construct a spot welding machine. A direct pressure type stationary single spot welding machine has been constructed. The main factors influences on spot welding are pressure, current, time and proper alignment of the electrode. The pressure is applied on the upper arm by a lever and the pressure can be measured by a spring balance. AC current is supplied by a transformer to the two electrode tips and the current can be measured by an ammeter. Maximum available current is 200 amps. For this low current greater thickness sheet cannot be welded and maximum 0.7 mm thickness sheet will be welded. The performance of the project work was quite satisfactory. The constructed spot welding machine will given better performance if the limitations were eliminated. iii
TABLE OF CONTENTS
PAGE NO.
Acknowledgement
i
Abstract
ii
CHAPTER-1: INTRODUCTION
1.1 Introduction
1.2 Objectives
1
2
CHAPTER-2: LITERATURE REVIEW
PART-1: BASICS OF WELDING
2.1.1
2.1.2
2.1.3
2.1.4
2.1.5
2.1.6
2.1.7
2.1.8
2.1.9
2.1.10
2.1.11
Introduction
History of welding
Metal joining processes
Advantages of welding over other joints
Welding as compared to riveting and casting
Commonly welded base metals
Classification of welding
Critical factors in welding
Heat affected zone (HAZ) in welding
Welding symbol
Application of welding
3
4
6
7
7
8
9
10
11
12
13
PART-2: ARC WELDING
2.2.1
2.2.2
2.2.3
2.2.4
2.2.5
2.2.6
2.2.7
2.2.8
2.2.9
2.2.10
Introduction
Different types of arc welding
Shielded metal arc welding (SMAW)
Advantages of SMAW
Submerged arc welding (SAW)
Advantages of SAW
Tungsten inert gas arc welding (TIG)
Advantages of TIG welding
Metal inert gas arc welding (MIG)
Advantages of MIG welding
14
15
15
16
16
17
17
19
19
20
iv
PART-3: GAS WELDING
2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
2.3.6
Introduction
Different types of gas welding
Oxyacetylene Welding
Advantages of oxyacetylene gas welding
Oxyhydrogen Welding
Pressure gas welding
21
22
22
24
25
26
PART-4: RESISTANCE WELDING
2.4.1
2.4.2
2.4.3
2.4.4
2.4.5
2.4.6
2.4.7
2.4.8
Introduction
Different types of resistance welding
Spot welding
Seam welding
Projection welding
Butt welding
Flash welding
Resistance welding benefits
27
28
28
30
31
33
34
34
CHAPTER-3: FUNDAMENTALS OF SPOT WELDING
3.1
3.2
3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
3.12
3.13
3.14
3.15
3.16
3.17
3.18
3.19
Principle
Spot welding process
Circuit diagram of spot welding process
Sequence of events in resistance spot welding
Essential factors of spot welding
Heat generation
The time factor
Pressure
Electrode
Electrode tip size
Electrode coating
Factor Influence the life of electrodes
Influence of storage time on life of electrodes
Required condition for spot welding
Positive location of work piece
Advantages of spot welding
Disadvantages
Limitation
Practical uses of resistance spot welding v 35
35
37
37
38
38
39
43
44
45
46
47
47
48
49
50
50
51
51
CHAPTER-4: DESIGN & CONSTRUCTION
4.1 Design
4.1.1 Various types of spot welding machine
4.1.2 Design consideration for spot welding
4.1.3 Equipment required for spot welding machine
4.1.4 Electrode
4.1.5 Transformer
4.1.6 Spring Balance
4.1.7 Design of spot welding machine
4.2 Construction
4.2.1 Construction details
4.2.2 Operation procedure
4.3 Risks
54
54
55
56
56
57
58
59
61
61
CHAPTER-5: PERFORMANCE TEST & DISCUSSION
5.1
5.2
Performance test
Discussion
62
62
CHAPTER-6: CONCLUSION
6.1
6.2
Conclusion
Scope of future work
63
63
References
64
vi
LIST OF FIGURES
PAGE NO.
Figure 2.1
Flow chart of types of fusion welding
9
Figure 2.2
Flow chart of types of pressure welding
10
Figure 2.3
Welding symbol
12
Figure 2.4
Shielded metal arc welding
15
Figure 2.5
Submerged arc welding
17
Figure 2.6
Tungsten inert gas arc welding (TIG)
18
Figure 2.7
Metal inert gas welding (MIG)
20
Figure 2.8
Pressure gas welding
26
Figure 2.9
Spot welding
29
Figure 2.10
Seam welding
31
Figure 2.11
Projection welding
32
Figure 2.12
Butt welding
33
Figure 2.13
Flash welding
34
Figure 3.1
Resistance spot & TIG weld comparison
35
Figure 3.2
Spot welding assembly
36
Figure 3.3
Cross section of spot welding
36
Figure 3.4
Circuit diagram of Spot welding
37
Figure 3.5
Sequence of events in resistance spot welding
37
Figure 3.6
Graphs of resistance and temperature, as the result of Joule heating, as a function of location in a typical resistance welding arrangement
38
Figure 3.7
Spot welding Time cycle
40
Figure 3.8
Basic single-impulse welding cycle for Spot welding
41
Figure 3.9
Multi-impulse welding cycle for Spot welding
42
Figure 3.10
Nugget formation and heat dissipation into the surrounding base metal and electrodes during resistance spot welding
42
Figure 3.11
Weld strength as a function of current (a) & welding time (b)
43
Figure 3.12
Factor of influence the life of electrodes
47
Figure 3.13
Influence of storage time on life of electrodes
48
vii
Figure 3.14
Suitability of spot welding
48
Figure 3.15
Positive locations of workpiece
49
Figure 3.16
Resistance spot weld heat zones
52
Figure 4.1
Operation procedure
54
Figure 4.2
Electrode
56
Figure 4.3
Transformer
56
Figure 4.4
Spring balance
57
Figure 4.5
3-D view of spot welding machine
58
Figure 4.6
Constructed spot welding machine
60
viii
CHAPTER- I
INTRODUCTION
ix
1.1 INTRODUCTION
Welding is a process of joining two metal pieces as a result of significant diffusion of the atoms of the welded pieces into the joint (weld) region. Welding is carried out by heating the joined pieces to melting point and fusing them together (with or without filler material) or by applying pressure to the pieces in cold or heated state.
Now-a-days many processes of welding have been developed and probably there is no industry which is not using welding process in the fabrication of its products in some form or the other.
This is the most rapid and easiest way of fabrication and assembly of metal parts. The research carried out in this field has given various ways and methods to welds practically all metals.
Means have also been found out to weld dissimilar metals. One beauty of welding in comparison to others processes of joining metals is that by this process we can have more than 100% strength of joint and it is very easy process. We shall be dealing with all the various processes of welding in use these days, the equipment used for each process and the ways of preparation of joint and the various operations necessary.
Welding is now-a-days extensively used in the following fields: automobile industry, aircraft machine frames, structural work, tanks machine repair work, ship building, pipe-line fabrication in thermal power plants and refineries, fabrication of metal structures. There is a big competition between welding and casting process now-a-days. Many of the cast products are now-a-days being fabricated by welding various parts together. Such construction has th advantage that the products are lighter and stronger. Gas cutting is another field of application of welding process which is playing a very important role in industry.
In resistance welding a heavy electric current is passed through the metals to be joined over a limited area, causing them to be locally heated to plastic state and the weld is completed by the application of pressure for a prescribed period of time. No additional filler metal is required. In this process two copper electrodes are used and the metals to be welded are pressed between the electrodes. The current is passed through the electrodes which incorporate very low resistance in the circuit and the resistance at the joints of the metals is very high.
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1.2 OBJECTIVES
The objectives of the project work were:
1.
To design a spot welding machine
2. To construct the spot welding machine according to the design.
3. To test the performance of the spot welding machine.
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CHAPTER- II
LITERATURE REVIEW
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PART-1: BASICS OF WELDING
2.1.1 INTRODUCTION
Welding is a joining process that produces coalescence of materials by heating to welding temperature with or without application of pressure or by application of pressure alone and with or without the use of filler metal. The term welding is used to cover a wide range of bonding technique. Broadly welding process could be classified as fusion welding and pressure welding.
Fusion welding is the process of joining two piece of metal by application of heat with or without application of pressure. Pressure welds are produced by bringing the clean face of component into intimate contact to produce a metallic bond with of without application heat but application of pressure.
Resistance spot welding is usually used in the fabrication of sheet metal assembly. The weld is made by a combination of heat, pressure, and time. Resistance welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localized heating in the joint, and the weld is made. The time is how long current flows in the joint, which is determined by the material thickness and type, amount of the current and cross-sectional area of the welding tips and contact surfaces. It is one of the oldest of the electric welding processes. It can be used to weld materials such as low carbon steel, nickel, aluminum, titanium, copper alloy, stainless steel
&high-strength low alloy steel. Resistance spot welding process is most applicable in the industrial fields of manufacture and maintenance.
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2.1.2 HISTORY OF WELDING
When driving a car or look at a light fixture in the street or open your microwave, chances are that there is something in any of those items that has been welded. These products and others have been a part of the process of welding for more years than might imagine.
Welding actually started a very long time ago during the Middle Ages. Many artifacts have been found that date back to the Bronze Age. These have been small boxes that were welded together with what is called lap joints; no one is exactly sure what these were used for, but this was important to that time.
The Egyptians also made a variety of tools by welding pieces of iron together. Perhaps this is where Maxwell's Hammer comes later? Who can say! Then came the rise of the Middle Ages and many people there were able to use blacksmithing for iron. Different modifications were made along the way until the welding that is used to day was developed.
There were several significant inventions in the 1800s that influenced welding included here:
The invention of acetylene by an Englishman named Edmund Davy.
Gas welding and cutting became known and a way to cement pieces of iron together.
Arc lighting was a very popular part of welding after the electric generator became known.
Arc and resistance welding become another popular aspect of welding.
Nikolai N. Benardos receives a patent for welding in 1885 and 1887 from America and
Britain.
C.L. Coffin receives an American patent for a arc welding process.
After the 1800s many more patents and inventions were made in order to create more ways of doing welding but one of the greatest needs would come much later during World War I because this process was needed to create arms. Because of the demand welding firms became a staple of
America and Europe because the war needed welding machines and electrodes to go with them.
During the war people really got a chance to look at how welding worked and it became a very
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popular way of work. So much so that in 1919 the first American Welding Society was begun.
This nonprofit organization came directly out of through a group of men who called themselves the Wartime Welding Committee of the Emergency Fleet Corporation (Source: Miller Welds).
The 1950s and 1960s were also a significant time for welding because a welding process using
CO2 was discovered and a variation of this form of welding that used inert gas became very popular in the 1960s because it produced a different type of arc.
There have been a number of improvements in the welding trade over these years and today the process has added two areas, friction and laser welding. These two have created a more specialized field and therefore more opportunities for learning.
One interesting point about laser welding is that those people who use it have found that is a tremendous heat source so it can actually weld both metal and non-metal objects.
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2.1.3 METAL JOINING PROCESSES:
2.1.3.1 WELDING:
Welding is a Fusion or uniting of two metals by means of heat. Many welding process have been developed which differ widely in the manner in which the heat is applied and in the type of equipment used. Some process required hammering, rolling, or pressing the effect of weld. Other brings the metal to liquid state and required no pressure. Those process need pressure require bringing the surface of metal to a temperature enough that cohesion takes place. This is nearly always a sub fusion temperature. However the fusion temperature is reached metal must be confined by surrounding metal. No additional weld metal is required. Weld are also made by casting in which case the metal is heated to a high temperature and poured into a cavity between the two piece to be joined.
2.1.3.2 SOLDERING:
Soldering is the uniting of two piece of metal with a different metal which is applied between the two in a molten state and at a temperature not exceeding 800 F. In this process some alloying with base metal takes place and additional strength is obtained by mechanical bonding. The usual metals for soldering are low melting alloy of lead and tin.
2.1.3.3 BRAZING:
Brazing is a similar process in which the metal part are joined by non ferrous metals, such as copper-zinc and silver alloy, having melting point below that of parents metal but above 800 oF.
2.1.3.4 SOLDERING:
Soldering differ from brazing in that lower temperature filler metal (bellow 800 oF) are used in the joint. Lead and tin alloy having a melting range 300 to 700 oF are principally used in soldering. xvi
2.1.4 ADVANTAGES OF WELDING OVER OTHER JOINTS:
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Buildings, bridges and structures can be built lighter and thus higher due to reduction in weight. There are cheap also due to reduction in weight and material cost. Additional joint strength can be obtained by using considerably smaller structural members. Joints are compact and to do not require additional plates as in case of riveted joints.
-
Welded joints have high corrosion resistance compared to bolted and riveted joints.
-
Welded joints are fluid tight for tanks and vessels.
-
Welded structures can be altered easily and economically.
-
Many different types of joint are possible in welded joints.
2.1.5 WELDING AS COMPARED TO RIVETING AND CASTING
-
Welding is more economical and is a much faster process as compared to both casting and riveting.
-
As compared to riveting and casting, fewer persons are involved in a fabrication process.
-
Welding involves less cost of handling.
-
Welding produces less noise as compared to riveting.
-
Welding designs involve less cost and are flexible also (we can change easily)
-
Cost of pattern making, strong, maintenance, repairs is eliminated.
-
Welding structures are comparatively lighter.
-
Welding can be carried out at any point on a structure, but riveting requires enough clearance. -
Drilling holes to accommodate rivets weakens a riveted structure.
-
Welding can produce a 100% efficient joint, which is difficult to make by riveting.
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2.1.6 COMMONLY WELDED BASE METALS
Metals can be classified as:
1. Ferrous
2. Non-ferrous
Ferrous materials contain iron and these are the most important metals/alloys in the metallurgical and mechanical industries because of their extensive use.
Ferrous materials which can be used in welding application are:
-
Wrought iron
-
Cast iron
-
Cast steels
-
Alloy steels
-
Stainless steels
-
Carbon steels (low, medium and high carbon steels)
Non ferrous materials are those that are not iron based.
Non ferrous materials, which can be used in welding applications are:
-
Aluminum and its alloys
-
Copper and its alloys
-
Magnesium and its alloys
-
Zinc and its alloys
-
Nickel and its alloys
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2.1.7 CLASSIFICATION OF WELDING
Mainly welding is generally classified as
1. Fusion Welding
2. Pressure Welding
2.1.7.1 FUSION WELDING
In this case, the metal at the joint is heated to a molten state and allowed to solidify. In this case filler material is used during welding process. This includes Arc welding, Gas welding and the thermit welding.
Figure 2.1: Flow chart of types of fusion welding
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2.1.7.2 PRESSURE WELDING
In it, the pieces of metal to be joined are heated to the plastic state and these forced together by external pressure without the adiitional of filler materials. Forge welding, Resistance welding and the thermit welding with pressure are the examples of this case.
Figure 2.2: Flow chart of Types of pressure welding
2.1.8 CRITICAL FACTORS IN WELDING
The critical factors involved in welding are the current, voltages, power, resistance and the transformer specifications, transmitting force, current density, throat dimensions and the electrodes. The current is usually measured in KA. A resistance weld cannot be made unless there is a sufficient weld current.
If current is the amount of electricity flowing, then Voltage is the pressure or force that‘s causing the flow.
Power is voltage measured by current and is in watts or KVA. This means the amount of current flowing times the pressure that‘s causing it to flow equals the amount of power generated.
Resistance is the opposition to flow of current. Since R to I is what generates the heat in the work piece, it is critically important that the area with the greatest resistance be at the interface xx between t he two parts being joined. The heat is where the resistance is, and the resistance is where the heat will be.
The transformer used is of high rating as is should be capable of supplying that much weld current. Current density describes how much current is being delivered to a specific area. It describes the concentration of the current in a small area of the work piece, viz., namely the area where the weld is.
Electrodes play a very important role in t he successful operation of a spot welding machine.
They must conduct the welding current to the work without overheating. They must at the same time conduct heat away from the surface of the sheet being welded and they must apply pressure to the work without deforming.
2.1.9 HEAT AFFECTED ZONE (HAZ) IN WELDING
The HAZ is the region that deforms AC, temperature to the temperature just below the melting temperature. It may be major matrix phase changes or precipitation process. Even n materials showing no phase change or precipitation during welding, recrystallisation and grain growth may occur. The HAZ has an important role to determine the weld cold cracking, notch toughness, hydrogen embrittlement, steel corrosion cracking etc. in severe environmental conditions of service.
Therefore a detailed study of HAZ is desirable. The width of the HAZ can be determined by the peak temperatures obtained at discrete points from the weld center lines by experiment. The variations of the micro-structure at different Jones of welding can be examined from photo macro and micro graphs.
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2.1.10 WELDING SYMBOL:
A welding symbol has following basic elements:
1. Reference line
2. Arrow
3. Basic weld symbols (like fillet, butt joints etc.)
4. Dimensions
5. Supplementary symbols
6. Finish symbols
7. Tail
8. Specification processes.
These welding symbols are placed in standard locations (see figure below)
Figure 2.3: welding symbol
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2.1.10 APPLICATION OF WELDING
Automobile construction
Railroad equipment
Ships
Aircraft construction
Bridge construction
Pressure vessels
Storage tanks
Pipe and pipe lines
Fabrication of jigs, fixtures and machine tools
Repair of broken and damaged parts
Material handling equipments
Household furniture
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PART-2: ARC WELDING
2.2.1 INTRODUCTION
Arc welding is a welding process, in which heat is generated by an electric arc struck between an electrode and the work piece. Electric arc is luminous electrical discharge between two electrodes through ionized gas.
Any arc welding method is based on an electric circuit consisting of the following parts:
Power supply (AC or DC);
Welding electrode;
Work piece;
Welding leads (electric cables) connecting the electrode and work piece to the power supply. Electric arc between the electrode and work piece closes the electric circuit. The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion the work piece edges and joining them.
When a long join is required the arc is moved along the joint line. The front edge of the weld pool melts the welded surfaces when the rear edge of the weld pool solidifies forming the joint.
When a filler metal is required for better bonding, filling rod (wire) is used either as outside material fed to the arc region or as consumable welding electrode, which melts and fills the weld pool. Chemical compositions of filler metal are similar to that of work piece.
Molten metal in the weld pool is chemically active and it reacts with the surrounding atmosphere. As a result weld may be contaminated by oxide and nitride inclusions deteriorating its mechanical properties.
Neutral shielding gases (argon, helium) and/or shielding fluxes are used for protection of the weld pool from atmospheric contamination. Shields are supplied to the weld zone in form of a flux coating of the electrode or in other forms.
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2.2.2 DIFFERENT TYPES OF ARC WELDING
1. Shielded metal arc welding
2. Submerged arc welding
3. Tungsten inert gas arc welding
4. Metal inert gas arc welding
2.2.3 SHIELDED METAL ARC WELDING (SMAW)
Shielded metal arc welding (fig. 2-4) is performed by striking an arc between a coated-metal electrode and the base metal. Once the arc has been established, the molten metal from the tip of the electrode flows together with the molten metal from the edges of the base metal to forma sound joint. This process is known as fusion. The coating from the electrode forms a covering over the weld deposit, shielding it from contamination; therefore the process is called shielded metal arc welding. The main advantages of shielded metal arc welding are that high-quality welds are made rapidly at a low cost.
The electrodes are coated with a shielding flux of a suitable composition. The flux melts together with the electrode metallic core, forming a gas and a slag, shielding the arc and the weld pool.
The flux cleans the metal surface, supplies some alloying elements to the weld, protects the molten metal from oxidation and stabilizes the arc.
SMAW can be performed both on AC and DC sources with dropping characteristics, highest current rating being about 600 amperes. Selection of proper electrode is essential for best results.
Figure 2.4: shielded metal arc welding xxv 2.2.4 ADVANTAGES OF SHIELDED METAL ARC WELDING
-
Simple, portable and inexpensive equipment;
-
Wide variety of metals, welding positions and electrodes are applicable;
-
Suitable for outdoor applications.
2.2.5 SUBMERGED ARC WELDING (SAW)
Submerged arc welding (SAW) is a common arc welding process. Originally developed by the Linde Union Carbide Company. It requires a continuously fed consumable solid or tubular (flux cored) electrode. The molten weld and the arc zone are protected from atmospheric contamination by being
―submerged‖ under a blanket of granular fusible flux consisting of lime, silica, manganese oxide, calcium fluoride, and other compounds. When molten, the flux becomes conductive, and provides a current path between the electrode and the work. This thick layer of flux completely covers the molten metal thus preventing spatter and sparks as well as suppressing the intense ultraviolet radiation and fumes that are a part of the shielded metal arc welding (SMAW) process.
SAW can be operated using either a DC or an AC power source. DC is supplied by a transformer-rectifier and AC is supplied by a transformer. Current for a single wire ranges from as low as 200A (1.6mm diameter wire) to as high as 1000A (6.0mm diameter wire). In practice, most welding is carried out on thick plate where a single wire (4.0mm diameter) is normally used over a more limited range of 600 to 900A, with a twin wire system operating between 800 and
1200A.
In DC operation, the electrode is normally connected to the positive terminal. Electrode negative
(DCEN) polarity can be used to increase deposition rate but depth of penetration is reduced by between 20 and 25%. For this reason, DCEN is used for surfacing applications where parent metal dilution is important. The DC power source has a 'constant voltage' output characteristic which produces a self-regulating arc. For a given diameter of wire, welding current is controlled by wire feed speed and arc length is determined by voltage setting.
AC power sources usually have a constant-current output characteristic and are therefore not self-regulating. The arc with this type of power source is controlled by sensing the arc voltage xxvi and using the signal to control wire feed speed. In practice, for a given welding current level, arc length is determined by wire burn off rate, i.e. the balance between the welding current setting and wire feed speed which is under feedback control.
Figure 2.5: submerged arc welding
2.2.6 ADVANTAGES OF SUBMERGED ARC WELDING
-
Very high welding rate;
-
The process is suitable for automation;
-
High quality weld structure.
2.2.7 TUNGSTEN INERT GAS ARC WELDING (TIG)
Gas Tungsten Arc Welding (GTAW) is frequently referred to as TIG welding. TIG welding is a commonly used high quality welding process. TIG welding has become a popular choice of welding processes when high quality, precision welding is required.
Tungsten inert gas (TIG) welding is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding
xxvii
gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.
TIG is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques.
In TIG welding an arc is created between a nonconsumable tungsten electrode and the metal being welded. The arc produces the heat needed to melt the work. The shielding gas keeps oxygen in the air away from the molten weld pool and the hot tungsten. Gas is fed through the torch in order to shield the electrode and the molten weld pool. The shielding gas used is pure argon. There may or may not be filler metal added to the molten weld pool during the process. Tungsten is used for the electrode because of its high melting temperature and good electrical characteristics.
Figure 2.6: Tungsten inert gas arc welding (TIG)
The main advantage of TIG welding is the wide range of materials that it can weld. TIG welding is used to a great extent for welding different kinds of alloys of aluminum and stainless steel, Specially when quality is of great importance. This technique is mainly used in aeronautical constructions and in the chemical and the nuclear power industry.
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2.2.8 ADVANTAGES OF TIG WELDING
-
Weld composition is close to that of the parent metal
-
High quality weld structure
-
Slag removal is not required (no slag)
-
Thermal distortions of work pieces are minimal due to concentration of heat in small zone. 2.2.9 METAL INERT GAS ARC WELDING (MIG)
Metal Inert Gas Welding (Gas Metal Arc Welding) is a arc welding process, in which the weld is shielded by an external gas (Argon, helium, CO2, argon + Oxygen or other gas mixtures).
Consumable electrode wire, having chemical composition similar to that of the parent material, is continuously fed from a spool to the arc zone. The arc heats and melts both the work pieces edges and the electrode wire. The fused electrode material is supplied to the surfaces of the work pieces, fills the weld pool and forms Joint.
Due to automatic feeding of the filling wire (electrode) the process is referred to as a semi-automatic. The operator controls only the torch positioning and speed.
This is an arc welding process where hardfacing wire is fed continually from a spool through the welding torch into the arc, where it is melted and transferred to the work piece.
In the case of MIG/MAG welding, the weld pool is protected from the atmosphere with a stream of shielding gas. These MIG/MAG processes are very flexible, i.e. they can be partially or fully mechanised and they are suitable for a wide range of applications.
Wire is also used as the hardfacing consumable in the submerged arc process. In this process a mineral based fluxing powder flows around the consumable wire and is melted by the arc. It forms a gaseous shield around the arc and also forms a slag on top of the weld pool, thereby shielding the cooling weld pool from the atmosphere.
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Figure 2.7: Metal inert gas welding (MIG)
2.2.10 ADVANTAGES OF MIG WELDING
Continuous weld may be produced (no interruptions);
High level of operators skill is not required;
Slag removal is not required (no slag);
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PART-3: GAS WELDING
2.3.1 INTRODUCTION
One of the most popular welding methods uses a gas flame as a source of heat. Gas welding is widely used in maintenance and repair work because of the ease in transporting oxygen and fuel cylinders. Once you learn the basics of gas welding, you will find the oxyfuel process adaptable to brazing, cutting, and heat treating all types of metals.
Gas welding is a process in which the required heat to melt the surfaces is supplied by a high temperature flame obtained by a mixture of two gases. The gases are mixed in proper proportions in a welding blowpipe (torch). For controlling the welding flame, there are two regulators on the torch by which the quantity of either gas can be regulated. Usually the mixture of oxygen and acetylene is used for welding purposes. It produces temperature in the range of 3200-3300 oC.
However the mixture of hydrogen and other combustable gases can also be employed to some extent. Others gases used are MAPP (methyl acetate propediene) (2600-2900)oC, propylene
(2500-2850)oC, propane (2450-2775) oC, natural gas/methane (2350-2750)oC. In gas welding the two surfaces to be welded are properly prepared and placed near each other. The metal in the joint is brought to melting temperature by heat from the flame and then weld is completed by supplying additional metal as the filler metal obtained by a filler rod.
Filler rod is used when an additional supply of metal to weld is required. Shielding flux may be used if protection of weld pool is necessary. Most of commercial metals may be welded by Gas
Welding excluding reactive metals (titanium, zirconium) and refractory metals (tungsten, molybdenum). xxxi
2.3.2 DIFFERENT TYPES OF GAS WELDING
1. Oxyacetylene welding
2. Oxyhydrogen welding
3. Pressure welding
2.3.3 OXYACETYLENE GAS WELDING oxyacetylene welding, which uses acetylene gas as the fuel, is the most commonly used oxyfuel gas welding process due to its high flame temperature (i.e., intense source energy).
Oxyacetylene welding (OAW) derives the heat needed to cause melting of the substrates and, almost always, filler from two stages of combustion. In the first stage, known as primary combustion, the acetylene fuel gas partially reacts with oxygen provided from a pressurized gas cylinder to form carbon monoxide and hydrogen:
C2H2 + 02 (cylinder) = 2CO + H2
This reaction is exothermic and is responsible for about one-third of the total heat generated by the complete combustion of acetylene. The dissociation of acetylene to carbon and hydrogen releases 227 kJ/mol of acetylene at 15°C, while the partial combustion of the carbon to form carbon monoxide releases 221 kJ/mol of carbon. No combustion of the hydrogen takes place at
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this stage. The total heat released by the primary reaction is 448 kJ/mole (501 Btu/ft3) of acetylene. In the second stage of oxyacetylene or other fuel gas welding, known as secondary combustion, which occurs immediately after the primary combustion, the carbon monoxide resulting from partial combustion of the carbon dissociated from the acetylene (or other fuel gas) reacts further with oxygen, this time from the surrounding air, to form carbon dioxide, while the hydrogen from the primary combustion dissociation of acetylene (or other fuel gas) reacts with oxygen in the air to form water:
2CO + O2(air) = 2C02
H2 + 0.5O2 (air) = H2O
These reactions are also exothermic and are responsible for two-thirds of the total heat generated by burning the dissociation products of the acetylene completely. Burning of hydrogen to produce water vapor releases 242 kJ/mol of hydrogen, while further oxidation of carbon monoxide releases an additional 285 kJ/mol of carbon monoxide, or 570 kJ/mol for the reaction.
The total heat released by the second reaction is thus 812 kJ/mol (907 Btu/ft3) of acetylene.
The actual primary and secondary combustion reactions occur in the gas flame of an oxygenacetylene torch in two distinct regions, as shown in Figure. Primary combustion occurs in an inner cone, while secondary combustion occurs in an outer flame. Although only accounting for one-third of the total heat of the overall combustion reaction (448 kJ/mol out of 1260 kJ/mole), the inner cone tends to be more concentrated in volume, and so is hotter (i.e., the energy is more dense). Thus, the welder tends to work with the tip of the inner cone near the workpiece to cause melting, using the outer flame to provide a degree of shielding of the molten weld metal and hot, newly formed weld by the carbon dioxide, to provide preheating to aid in initial melting and to slow down cooling once the weld has been made (thereby sometimes avoiding adverse postsolidification or heat-affected zone transformations.
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Similar combustion reactions can be written and energy balances performed for other fuel gas mixtures with oxygen, with different amounts of energy being liberated and different flame temperatures being produced for each.
The exact chemical nature, or reactivity, of the flame in oxyfuel gas welding processes, such as oxyacetylene welding, can be adjusted to be chemically neutral, chemically reducing, or chemically oxidizing. 2.3.4 ADVANTAGES OF OXYACETYLENE GAS WELDING
It's easy to learn.
The equipment is cheaper than most other types of welding rigs (e.g. TIG welding)
The equipment is more portable than most other types of welding rigs (e.g. TIG welding)
OA equipment can also be used to "flame-cut" large pieces of material.
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2.3.5 OXYHYDROGEN GAS WELDING
Oxyhydrogen Welding is a Gas Welding process using a combustion mixture of Hydrogen (H2) and oxygen (O2) for producing gas welding flame.
Oxyhydrogen flame is used to weld and braze metals only with low melting points, e.g., aluminium, magnesium, lead etc. The temperature of the hottest part of an oxy-hydrogen flame suitable for welding is only about 25000C against 3200°C of an oxyacetylene flame.
In oxyhydrogen welding, if a higher temperature is obtained by increasing the oxygen supply, the flame becomes quite unsuitable for welding. Oxyhydrogen welding is therefore not used for welding steel. Hydrogen is available in compressed gas cylinders. Complete combustion of hydrogen requires an oxygen to hydrogen ratio of 1 to 2.
2H2 + O2= 2H20
This gas mixture produces a strongly oxidizing flame. The oxyhydrogen flame is scarcely visible and there are no combustion zones as in oxyacetylene flame. Therefore it is impossible to obtain a neutral oxyhydrogen flame by the visible methods of flame adjustment. To avoid an oxidizing flame, the pressure regulators must be set to provide an assured excess of hydrogen. In practice a ratio of 4:1 or 5:1 hydrogen: oxygen is required to avoid an oxidizing flame .
Since there is no carbon, the oxyhydrogen flame is only reducing (and never carburizing). The oxyhydrogen welding is similar to oxyacetylene welding with the difference that a special regulator is used for metering the hydrogen gas.
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2.3.6 PRESSURE GAS WELDING
Pressure Gas Welding is a Gas Welding, in which the welded parts are pressed to each other when heated by a gas flame.
In this process, a weld is made simultaneously over the entire area of abutting surfaces with gas flames obtained from the combustion of a fuel gas with oxygen and the application of pressure.
No filler metal is used. Acetylene is normally used as a fuel gas in pressure gas welding.
Pressure gas welding has limited uses because of its low flame temperature, but is extensively used for welding lead.
The process is similar to Resistance Butt Welding. Pressure Gas Welding does not require filler material. Pressure gas welding is used for joining pipes, rods, railroad rails.
Figure 2.8: pressure gas welding
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PART-4: RESISTANCE WELDING
2.4.1 INTRODUCTION
Resistance welding is a fusion welding process that requires the application of both heat and pressure to achieve a sound joint. The simplest form of the process is spot welding where the pressure is provided by clamping two or more overlapping sheets between two electrodes. A current is then passed between the electrodes, sufficient heat being generated at the interface by resistance to the flow of the current that melting occurs, a weld nugget is formed and an autogenous fusion weld is made between the plates. The heat generated depends upon the current, the time the current is passed and the resistance at the interface. The resistance is a function of the resistivity and surface condition of the parent material, the size, shape and material of the electrodes and the pressure applied by the electrodes.
There are a number of variants of the resistance welding process including spot, seam, projection and butt welding. It is an economical process ideally suited to producing large numbers of joints on a mass production basis. Spot welding in particular has been used extensively in the automotive industry, albeit mostly for the joining of steel and in the aerospace industry for airframe components in aluminium alloys. Seam welding is used in the production of thin sheet, leak-tight containers such as fuel tanks. Projection welding is generally used for welding items such as captive nuts onto plate. This variation is not normally used on aluminium and is not covered in this chapter. Flash welding, unlike spot and seam welding that require a lap joint, is capable of making butt welds. This is achieved by resistance heating the abutting faces and then forging them together.
Small pools of molten metal are formed at the point of most electrical resistance (the connecting surfaces) as a high current (100–100 000 A) is passed through the metal. In general, resistance xxxvii welding methods are efficient and cause little pollution, but their applications are limited to relatively thin materials and the equipment cost can be high.
The welding cycle must first develop sufficient heat to raise a small volume of metal to the molten state. This metal then cools while under pressure until it has adequate strength to hold the parts together. The current density and pressure must be sufficient to produce a weld nugget, but not so high as to expel molten metal from the weld zone.
2.4.2 DIFFERENT TYPES OF RESISTANCE WELDING
1. Spot welding
2. Seam welding
3. Projection welding
4. Butt/upset welding
5. Flash welding
2.4.3 SPOT WELDING
Spot welding is a resistance welding method used to join two to three overlapping metal sheets, studs, projections, electrical wiring hangers, some heat exchanger fins, and some tubing. Usually power sources and welding equipment are sized, to the specific thickness and material being welded together. The thickness is limited by the output of the welding power source and thus the equipment range due to the current required for each application. Care is taken to eliminate, contaminates between the faying surfaces. Usually, two copper electrodes are simultaneously used to clamp the metal sheets together and to pass current through the sheets. When the current is passed through the electrodes to the sheets, heat is generated due to the higher electrical resistance where the surfaces contact each other. As the electrical resistance of the material causes an heat build-up in the work between the copper electrodes, the rising temperature causes a rising resistance, and results in a molten pool contained most of the time between the electrodes. As the heat dissipates throughout the workpiece in less than a second (controls are based in
AC cycles, or microseconds) the molten, or at least plastic, state grows to meet the welding tips. When the current is stopped the copper tips cool the spot weld, causing the metal to solidify under pressure. The water cooled copper electrodes remove the surface heat quickly, accelerating the solidification of the weld, since copper is an excellent conductor. Some of the different currents used are Direct Current,
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Alternating Current, and Medium Frequency Half wave Direct Current, as well as high frequency half wave Direct current.
If excessive heat is applied, or applied too quickly, or if the force between the base materials is too low, or the coating too thick, or too conductive, the molten area may extend to the outside, and with its high pressure (typically 30,000 psi) will escape the containment force of the tips with a burst of molten metal called expulsion. When this occurs, the metal will be thinner and have less strength than a weld with no expulsion. The common method of checking a weld is a peel test. An alternative test is the restrained tensile test, which is much more difficult to perform, and requires calibrated equipment. Ultrasonic evaluation has been tried and is still in a "unapproved" state for many OEMs.
The advantages of the method include efficient energy use, limited workpiece deformation, high production rates, easy automation, and no required filler materials. When high strength in shear is needed, spot welding is used in preference to more costly mechanical fastening, such as riveting. While the shear strength of each weld is high, the fact that the weld spots do not form a continuous seam means that the overall strength is often significantly lower than with other welding methods, limiting the usefulness of the process. It is used extensively in the automotive industry— cars can have several thousand spot welds.
A specialized process, called shot welding, can be used to spot weld stainless steel.
Figure 2.9: spot welding process
There are three basic types of resistance welding bonds: solid state, fusion, and reflow braze. In a solid state bond, also called a thermo-compression bond, dissimilar materials with dissimilar grain structure, xxxix e.g. molybdenum to tungsten, are joined using a very short heating time, high weld energy, and high force. There is little melting and minimum grain growth, but a definite bond and grain interface. Thus the materials actually bond while still in the solid state. The bonded materials typically exhibit excellent shear and tensile strength, but poor peel strength. In a fusion bond, either similar or dissimilar materials with similar grain structures are heated to the melting point (liquid state) of both. The subsequent cooling and combination of the materials forms a ―nugget‖ alloy of the two materials with larger grain growth.
Typically, high weld energies at either short or long weld times, depending on physical characteristics, are used to produce fusion bonds. The bonded materials usually exhibit excellent tensile, peel and shear strengths. In a reflow braze bond, a resistance heating of a low temperature brazing material, such as gold or solder, is used to join either dissimilar materials or widely varied thick/thin material combinations. The brazing material must ―wet‖ to each part and possess a lower melting point than the two workpieces. The resultant bond has definite interfaces with minimum grain growth. Typically the process requires a longer
(2 to 100 ms) heating time at low weld energy. The resultant bond exhibits excellent tensile strength, but poor peel and shear strength.
2.4.4 SEAM WELDING
Resistance seam welding is a process that produces a weld at the faying surfaces of two similar metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes, usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as the material passes between them. This allows the electrodes to stay in constant contact with the material to make long continuous welds. The electrodes may also move or assist the movement of the material.
A transformer supplies energy to the weld joint in the form of low voltage, high current AC power. The joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its melting point by the current. The semi-molten surfaces are pressed
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Figure 2.10: seam welding process together by the welding pressure that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders use water cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam welding produces an extremely durable weld because the joint is forged due to the heat and pressure applied. A properly welded joint formed by resistance welding is typically stronger than the material from which it is formed.
A common use of seam welding is during the manufacture of round or rectangular steel tubing.
Seam welding has been used to manufacture steel beverage cans but is no longer used for this as modern beverage cans are seamless aluminum.
2.4.5 PROJECTION WELDING
Projection welding(PW) is a variation of resistance welding in which current flow is concentrated at the point of contact with a local geometric extension of one (or both) of the parts being welded. These extensions, or projections, are used to concentrate heat generation at the point of contact. The process typically uses lower currents, lower forces, and shorter welding times than does a similar application without the projections. Projection welding is often used in the most difficult resistance-welding applications because a number of welds can be made at one xli time, which speeds up the manufacturing process. PW applications are generally categorized as being either embossed-projection welding or solid-projection welding. These variations are shown in Fig. and. Embossed-projection welding is generally a sheet-to-sheet joining process, in which a projection is stamped onto one of the sheets to be joined. Then, resistance welding is conducted on a stack of sheets. Heat initially concentrates at the contact point and in the walls of the projection during resistance welding. Early in the process, the projection itself collapses back into the original sheet. However, the initial heating raises the local resistivity of the joint area, allowing continued resistance heating at this location. Weld development then proceeds in the conventional manner, by forming a fused weld nugget. Solid-projection welding requires that the projection be forged onto one of the two components. Then, during resistance welding, the contact point and the projection itself experience preferential heating. In this case, the projection cannot simply collapse, as it does in embossed-projection welding. Rather, the projection collapses by penetrating the opposing material and by extrusion to the periphery. When compared with embossed-projection welding, the resulting joints are solid-state, rather than fusion, welds. The actual joints are caused by combination of material forging and diffusion bonding, much like they are in resistance butt and flash butt welding.
Figure 2.11: projection welding xlii 2.4.6 BUTT/ UPSET WELDING
Upset welding (UW) is a resistance welding process utilizing both heat and deformation to form a weld. The heat is produced by resistance to the flow of electrical current at the interface of the abutting surfaces to be joined. The deformation results from force on the joint in combination with softening from the electrical resistance heat. Upset welding typically results in solid-state welds (no melting at the joint). The deformation at the weld joint provides intimate contact between clean adjoining surfaces, allowing formation of strong metallurgical bonds. If any melting does occur during upset welding, the molten metal is typically extruded out of the weld joint area. A wide variety of shapes and materials can be joined using upset welding in either a single-pulse or continuous mode. Wire, bar, strip, and tubing can be joined end to end with a single pulse of welding current. Seams on pipe or tubing can be joined using continuous upset welding by feeding a coiled strip into a set of forming rolls, resistance heating the edges with wheel electrodes, and applying a force to upset the edges together. Equipment for single-pulse upset welding is relatively simple. It consists of a pneumatic or hydraulic system for force application, transformers or a bank of diodes as a source of electrical current, and a standard resistance-welding controller. A data acquisition system usually is employed to record the force, current, voltage, and motion of the weld head during welding. Equivalent welds have been made using both alternating and direct current. Upset welds have similar characteristics to inertia friction welds, which are also solid-state welds. The amount of deformation is usually less for upset welds, and the deformation can be more precisely controlled using upset welding. For example, a pipe butt weld made using inertia friction welding will have a large upset on both the inside and outside, whereas an upset weld can be controlled, through joint design and welding parameters, to have essentially no internal upset.
Figure 2.12: Butt welding process xliii 2.4.7 FLASH WELDING
Flash welding (FW) is a resistance welding process in which a butt joint weld is produced by a flashing action and by the application of pressure. In basic terms, it is a melting and a forging process. The process is capable of producing welded joints with strengths equal to those of the parent materials. Figure is a schematic representation of a typical flash welding operation. Flash welding is used to join metallic parts that have similar cross sections, in terms of size and shape.
The process lends itself to joining nearly all grades of steel, aluminum, brass, and copper parts, in addition to selected dissimilar materials.
Figure 2.13: Flash welding process
2.4.8 RESISTANCE WELDING BENEFITS
High speed welding
Easily automated
Suitable for high rate production
Economical
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CHAPTER- III
FUNDAMENTALS OF SPOT WELDING
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3.1 PRINCIPLE
Resistance spot welding is accomplished when current is caused to flow through electrode tips and the separate pieces of metal to be joined. The resistance of the base metal to electrical current flow causes localized heating in the joint, and the weld is made. The resistance spot weld is unique because the actual weld nugget is formed internally with relation to the surface of the base metal. Figure 3.1 shows a resistance spot weld nugget compared to a gas tungsten-arc (TIG) spot weld.
Figure 3.1: Resistance spot and TIG Weld Comparison
The gas tungsten-arc spot is made from one side only. The resistance spot weld is normally made with electrodes on each side of the workpiece. Resistance spot welds may be made with the workpiece in any position.
The resistance spot weld nugget is formed when the interface of the weld joint is heated due to the resistance of the joint surfaces to electrical current flow. In all cases, of course, the current must flow or the weld cannot be made. The pressure of the electrode tips on the workpiece holds the part in close and intimate contact during the making of the weld. Remember, however, that resistance spot welding machines are NOT designed as force clamps to pull the workpieces together for welding.
3.2 SPOT WELDING PROCESS
Spot welding is the most common form of resistance welding and is defined as a process in which a weld is produced at a spot in a work piece between the electrodes, the weld being of approximately the same areas as the electrode tips, or as the smaller tip where they differ in size.
Force is applied to the spot, usually through the electrodes and continuously throughout the xlvi process.
The term ‗spot‘ is an obvious one from the shape and appearance of the weld. The process is almost always applied to the lap joints. The basic assembly is shown in the Figure 3.2 below.
Fig 3.2: Spot Welding Assembly
Fig 3.3: cross-section of spot welding
The joint to be welded is placed between the two electrodes and the pressure is applied to ensure good contact. The electrodes are made of copper of high conductivity copper or lead and this ensures that a minimum amount of heat is produced where they make contact with the work piece. Heating of the weld area begins where the parts to be joined touch each other and is largely due to the contact resistance.
Any heat, which is generated near the electrodes, tends to be conducted away by the mass of the electrodes. The generation of the heat at the central interface continues until a slug of metal is in xlvii the molten condition and the current flow is terminated. This molten metal then cools and solidifies whilst still under the forging pressures of the electrodes. Success in spot welding depends on the correct balance between current, time and pressure and that these must suit the characteristics of the materials being welded.
3.3 CIRCUIT DIAGRAM OF SPOT WELDING PROCESS
Figure 3.4: Circuit diagram of spot welding
3.4 SEQUENCE OF EVENTS IN RESISTANCE SPOT WELDING
Figure 3.5: Sequence of events in resistance spot welding
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3.5 ESSENTIAL FACTORS OF SPOT WELDING
1. The welding current.
2. The time of current application.
3. The pressure of electrodes.
4. The shape or area of electrodes.
5. The surface condition of welded material.
3.6 HEAT GENERATION
A modification of Ohm‘s Law may be made when watts and heat are considered synonymous.
When current is passed through a conductor the electrical resistance of the conductor to current flow will cause heat to be generated.
The basic formula for heat generation may be stated:
H = I2R Where
H = Heat
I2 = Welding Current Squared
R = Resistance
Figure 3.6: Graphs of resistance and temperature, as the result of Joule heating, as a function of location in a typical resistance welding arrangement xlix The secondary portion of a resistance spot welding circuit, including the parts to be welded, is actually a series of resistances. The total additive value of this electrical resistance affects the current output of the resistance spot welding machine and the heat generation of the circuit.
The resistance spot welding machines are constructed so minimum resistance will be apparent in the transformer, flexible cables, tongs, and electrode tips. The resistance spot welding machines are designed to bring the welding current to the weldment in the most efficient manner. It is at the weldment that the greatest relative resistance is required. The term ―relative‖ means with relation to the rest of the actual welding circuit.
There are six major points of resistance in the work area. They are as follows:
1. The contact point between the electrode and top workpiece.
2. The top workpiece.
3. The interface of the top and bottom workpieces.
4. The bottom workpiece.
5. The contact point between the bottom workpiece and the electrode.
6. Resistance of electrode tips.
The resistances are in series, and each point of resistance will retard current flow. The amount of resistance at point 3, the interface of the workpieces, will depend on the heat transfer capabilities of the material, its electrical resistance, and the combined thickness of the materials at the weld joint. It is at this part of the circuit that the nugget of the weld is formed.
3.7 THE TIME FACTOR
Resistance spot welding depends on the resistance of the base metal and the amount of current flowing to produce the heat necessary to make the spot weld. Another important factor is time. In most cases several thousands of amperes are used in making the spot weld. Such amperage values, flowing through a relatively high resistance, will create a lot of heat in a short time. To make good resistance spot welds, it is necessary to have close control of the time the current is flowing. Actually, time is the only controllable variable in most single impulse resistance spot welding applications.
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Most resistance spot welds are made in very short time periods. Since alternating current is normally used for the welding process, procedures may be based on a 60 cycle time (sixty cycles
= 1 second). Figure 3.5 shows the resistance spot welding time cycle.
Previously, the formula for heat generation was used. With the addition of the time element, the formula is completed as follows:
H = I2RT
where H = Heat
I2= Current Squared
R = Resistance
T = Time
Control of time is important. If the time element is too long, the base metal in the joint may exceed the melting (and possibly the boiling) point of the material.
Figure 3.7: Spot Welding Time Cycle
1. Squeeze Time: The time interval between timer initiation and the first application of current needed to assure that electrodes contact the work and establish full force.
2. Weld time: The time for which welding current is applied (in single impulse welding) to the work to make a weld.
3. Hold Time: The time during which force is maintained on the work after the last impulse of welding current ends to allow the weld nugget to solidify and develop strength.
4. Off Time: The time during which the electrodes are off the work and the work is moved to the next weld location for repetitive welding.
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The full cycle or weld schedule is shown in Figure 3.8 This basic cycle can be enhanced to improve the physical and mechanical properties of a weld, as shown in Figure 3.8, where (1) precompression force is used to set electrodes and workpieces together; (2) preheat is applied to reduce thermal gradients at the start of weld time or to soften coatings such as zinc galvanize; (3) forging force is used to help consolidate the weld nugget; (4) quench and temper times are used to produce desired weld properties in hardenable steels; ( 5 ) postheat is used to refine weld nugget grain size and improve strength; and (6) current decay is used to retard cooling of aluminum alloys to help prevent cracking.
Figure 3.8 also shows multiple-impulse welding, as opposed to single-impulse welding shown in
Figure 3.9.
Figure 3.8: Basic single-impulse welding cycle for spot welding
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Figure 3.9: Multi-impulse welding cycle for spot welding
The strength of a weld nugget (shown schematically in Figure 3.9) made during resistance spot welding increases as current is applied for longer times, as the volume of melting and the size of the nugget formed increases accordingly. At some point, so much melting has occurred
Figure 3.10: Nugget formation and heat dissipation into the surrounding base metal and electrodes during resistance spot welding liii and the volumetric expansion of the molten metal compared to the solid metal from which it was produced becomes so great that the pressure applied by the electrodes cannot contain it. At this point expulsion or ―spitting‖ occurs. When this occurs, molten metal is lost from within the nugget, and the final, solidified weld inevitably contains a void, thereby reducing the strength, perhaps dramatically. For this reason, welding time should be limited to prevent expulsion. This is shown in Figure 3.11.
Figure 3.11: Weld strength as a function of current (a) and welding time (b).
3.8 PRESSURE
The effect of pressure on the resistance spot weld should be carefully considered. The primary purpose of pressure is to hold the parts to be welded in intimate contact at the joint interface.
This action assures consistent electrical resistance and conductivity at the point of weld. The tongs and electrode tips should NOT be used to pull the workpieces together. The resistance spot welding machine is not designed as an electrical ―C‖ clamp! The parts to be welded should be in intimate contact BEFORE pressure is applied.
Investigations have shown that high pressures exerted on the weld joint decrease the resistance at the point of contact between the electrode tip and the workpiece surface. The greater the pressure the lower the resistance factor.
Proper pressures, with intimate contact of the electrode tip and the base metal, will tend to conduct heat away from the weld. Higher currents are necessary with greater pressures and, conversely, lower pressures require less amperage from the resistance spot welding machine. liv This fact should be carefully noted particularly when using a heat control with the various resistance spot welding machines.
3.9 ELECTRODE
Copper is the base metal normally used for resistance spot welding tongs and tips. The purpose of the electrode tips is to conduct the welding current to the workpiece, to be the focal point of the pressure applied to the weld joint, to conduct heat from the work surface, and to maintain their integrity of shape and characteristics of thermal and electrical conductivity under working conditions. Electrode tips are made of copper alloys and other materials. The Resistance Welders
Manufacturing Association (RWMA) has classified electrode tips into two groups:
Group A − Copper based alloys
Group B − Refractory metal tips
The groups are further classified by number. Group A, Class I, II, III, IV, and V are made of copper alloys. Group B, Class 10, 11, 12, 13, and 14 are the refractory alloys.
Group A, Class I electrode tips are the closest in composition to pure copper. As the Class
Number goes higher, the hardness and annealing temperature values increase, while the thermal and electrical conductivity decreases.
Group B compositions are sintered mixtures of copper and tungsten, etc., designed for wear resistance and compressive strength at high temperatures. Group B, Class 10 alloys have about
40 percent the conductivity of copper with conductivity decreasing as the number value increases. Group B electrode tips are not normally used for applications in which resistance spot welding machines would be employed.
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3.10 ELECTRODE TIPS SIZE
When you consider that it is through the electrode that the welding current is permitted to flow into the workpiece, it is logical that the size of the electrode tip point controls the size of the resistance spot weld. Actually, the weld nugget diameter should be slightly less than the diameter of the electrode tip point. If the electrode tip diameter is too small for the application. The weld nugget will be small and weak. If, however, the electrode tip diameter is too large, there is danger of overheating the base metal and developing voids and gas pockets. In either instance, the appearance and quality of the finished weld would not be acceptable.
To determine electrode tip diameter will require some decisions on the part of the weldment designer. The resistance factors involved for different materials will certainly have some bearing on electrode tip diameter determination. A general formula has been developed for low carbon steel. It will provide electrode tip diameter values that are usable for most applications.
The formula generally used for low carbon steel is as follows:
Electrode tip diameter = 0.100‖ + 2t
Where ―t‖ is the thickness in inches of one thickness of the metal to be welded. This formula is applicable to the welding of metals of dissimilar thicknesses. The formula is applied to each thickness individually, and the proper electrode tip diameter selected for each size of the joint.
For example, if two pieces of 0.062‖ sheet metal are to be joined, the electrode tip diameter would be the same for both sides of the joint. The calculation would be as follows:
Electrode tip dia. = 0.100 + 2t
= 0.100 + 2 x 0.062‖
= 0.100 + 0.124‖
Electrode tip dia. = 0.224‖
If the two pieces were unequal in thickness, such a one piece 0.062‖ and the other 0.094‖, two calculations would have to be made. Each thickness would be treated as the basis for one electrode tip diameter determination, as follows:
Electrode tip dia. = 0.100 + 2t lvi = 0.100 + 2 x 0.062‖
= 0.100 + 0.124‖
Electrode tip dia. = 0.224‖ (one side only)
For the other side, the calculation is as follows:
Electrode tip dia. = 0.100 + 2t
= 0.100 + 2 x 0.094‖
= 0.100 + 0.188‖
Electrode tip dia. = 0.288‖ (one side only)
Remember that the formula is applicable to low carbon steels and may not be correct for other materials. 3.11 ELECTRODE COATING:
Electrode coating with slagging and fluxing materials are particularly necessary in the welding of alloys and nonferrous metals. Some of the elements of these alloys are not very stable and are lost if there is no protection against oxidation. Heavy coatings also permit the use of larger welding rods, higher current and greater welding speeds. In summary the coating do the following things:
1. Provide a protecting atmosphere.
2. Provide slag of suitable characteristics to protect the molten metal.
3. Facilitate overhead and position welding.
4. Stabilize the arc.
5. Add alloying element to weld metal.
6. Perform metallurgical refining operation.
7. Reduce spatter of weld metal.
8. Increase deposition efficiency.
9. Remove oxides and impurities.
10. Influence the depth of arc penetration.
11. Influence the shape of bed
12. Slow down the cooling rate of the weld.
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3.12 FACTORS INFLUENCING THE LIFE OF ELECTRODES
The regulating and controlling possibilities, the welding machine and the parts to be welded have a strong influence on the life of the electrodes. Reducing the electrode metal pick-up is of great importance and can be influenced by having a uniform surface condition obtained through the use of mechanical and chemical pretreatments. This can be best controlled by measuring the contact resistance between electrode and sheet. The regulating and controlling possibilities of the welding machine, e.g., chronological programming of current and force, the resetting behavior of the electrodes as well as cooling of the electrode, affect the stroke frequency and consequently the number of possible weld spots. The higher stroke frequency of roll seam welding necessitates a direct external cooling of the electrodes.
Figure 3.12: Factors of influence the life of Electrodes
3.13 INFLUENCE OF STORAGE TIME ON LIFE OF ELECTRODES
With suitable chemical surface pretreatments, the sheets retain their suitability for welding over long periods of time, i.e., the attainable life of electrodes also remains unchanged. In spite of this, efforts must be undertaken to keep the storage time as low as possible in order to avoid problems caused by dust dirtying the surface of the sheets. lviii Figure 3.13: influence of storage time on life of electrodes
3.14 REQUIRED CONDITIONS FOR SPOT WELDING
The suitability of Spot Welding depends on 3 factors
1. Surface condition
2. Chemical composition
3. Metallurgical condition
Figure 3.14: Suitability of spot welding
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High affinity to oxygen, the free surface of metal is covered by a dense, tightly adhering oxide film. This film is a non-conductor of electricity and must, therefore, be removed prior to welding. Both electrical and thermal conductivity depend strongly on the chemical composition. The very good electrical conductivity of pure metal makes it necessary to use large currents.
Metallurgical changes occur due to heating and subsequent cooling of the weld and heat affected join of the parent metal, affecting the quality of the weld.
3.15 POSITIVE LOCATION OF WORKPIECES
Precise locations of spot welded parts are a cost-related process and should be considered during the design. Part positioning involves either extensive fixturing or, preferably, self alignment through built-in stamped features like holes and tabs. With the latter method, the location is predetermined by the accuracy of alignment features.
Figure 3.15: Positive Locations of Workpieces
The most preferred and most easily achieved method for accurately self-fixturing parts is the half sheared or extruded cylindrical button and matching hole in the mating part. (Figure 8). One mating hole should be 0.003 in. (0.08 mm) larger in diameter than the extrusion and the second hole should be slotted by 0.040 in. (1.02 mm) minimum to allow for normal fabrication tolerances as
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shown in the drawing. Another alternative is to produce a lanced tab in a punching process. Mating parts can then be brought up to it and located in position.
Knowledgeable designers recognize such cost-saving and quality-improvement methods and specify them in the manufacturing process. The consistency attainable with such methods surpasses that of sophisticated jigs and fixtures but the greatest value is the cost efficiency.
Additionally, these techniques can be used for fillet welding applications, and mechanical assemblies. 3.16 ADVANTAGES OF SPOT WELDING
1. Low cost
2.
High speed of welding
3. More general elimination of warping or distortion of parts
4. High uniformity of products
5.
No edge preparation is needed
6. No filler rod is needed
7. The process is normally free from smoke and spatter
8. Suitable for automation.
9. Efficient energy use.
10. Limited work piece deformation. Also, work piece is not melted to a larger extent. Heat is concentrated only at the spot to be welded.
3.17 DISADVANTAGES
1. The electrodes have to be able to reach both sides of the pieces of metal that are being joined together. A particular spot welding machine will be able to hold only a certain thickness of metal
(usually 5 to 50 inches) and although the position of the electrodes can be adjusted, there will be only a limited amount of movement in most electrode holders.
2. The size and shapes of the electrodes will determine the size and strength of the weld. The join forms only at the spot where the electrodes are in contact with the metal. If the current is not lxi strong enough, hot enough or the metal is not held together with enough force, the spot weld may be small or weak.
3. Warping and a loss of fatigue strength can occur around the point where metal has been spot welded. The appearance of the join is often rather ugly, and there can be cracks. The metal may also become less resistant to corrosion.
4. Silver and copper are difficult to weld because of their high thermal conductivity.
3.18 LIMITATION
Spot welding is useful in many circumstances, although there are certain limitations. It can create only localized joins, which may not be particularly strong. The strength of a spot weld depends on the force and temperature that has been applied and on the cleanliness of the electrodes and metal. The difficulty of attaching the electrodes to oddly shaped pieces of metal can be avoided by using a portable spot welder. This has electrodes that are attached to long cables so they can reach inaccessible places.
3.19 PRACTICAL USES OF RESISTANCE SPOT WELDING
Resistance spot welding techniques do not require extensive or elaborate safety precautions.
There are some common sense actions that can, however, prevent injury to the operator. Anytime work is being done in a shop, it is a wise rule to wear safety glasses. Resistance spot welding is no exception to the rule! Very often metal or oxides are expelled from the joint area. Protection of the face and especially of the eyes in necessary to prevent serious injury.
Another area of concern is ventilation. This can be a serious problem when resistance spot welding galvanized metals (zinc coated) or metals with other coatings such as lead. The fumes from the welding operation have a certain toxicity which will cause illness to the operator.
Proper ventilation can reduce the fume concentration in the welding area.
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As explained in the preceding discussion on the fundamentals of resistance spot welding, there is a definite relationship between time, current, and pressure. Current and pressure help create the heat in the weld nugget.
If the weld current is too low for the application, current density is too weak to make the weld.
This condition will also overheat the electrode tips which can cause them to anneal, mushroom, and possibly be contaminated. Even though time is increased, the amount of heat generated is less than the losses due to radiation and conduction in the workpiece and thermal conduction of the electrodes. The result is the possibility, with long weld times at low currents, of overheating the entire base metal area between the electrodes. This could cause burning of the top and bottom surfaces of the workpiece as well as possibly imbedding the electrode tips in the workpiece surfaces. As current density is increased, the weld time is decreased proportionately. If, however, the current density becomes too high, there is the possibility of expelling molten metal from the interface of the joint thereby weakening the weld. The ideal time and current density condition is somewhere just below the level of causing metal to be expelled.
Figure 3.16: Resistance Spot Weld Heat Zones
It is apparent that the heat input cannot be greater than the total dissipation rate of the workpiece and the electrode without having metal expelled from the joint.
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An interesting discovery has been developed recently concerning the flow of current through the workpiece. Until recently, current was considered to flow in a straight line through the weld joint. This is not necessarily true when multiple thicknesses of material are being welded. The characteristic is for the current to ―fan out‖ thereby decreasing the current density at the point of weld the greatest distance from the electrode tips. The illustration (Figure 4-3) shows the resistance spot weld heat zones for several thicknesses of metal. We note that the uncontrollable variables (such as interface contamination) are multiplied when resistance spot welding several thicknesses of material. Quality levels will be much lower for ―stack‖ resistance spot welding, which explains why such welding practices are avoided whenever possible.
Disregarding the quality factor, it becomes apparent that the number of thicknesses of a material which may be successfully resistance spot welded at one time will depend on the material type and thickness as well as the KVA capacity of the resistance spot welding machine. KVA rating, duty cycle, and other pertinent information is shown on all resistance spot welding machine nameplates. The catalog literature and the operating manuals provide data on the maximum combined thicknesses of material that each unit can weld.
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CHAPTER- IV
DESIGN & CONSTRUCTION
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4.1 DESIGN:
4.1.1 VARIOUS TYPES OF SPOT WELDING MACHINE:
Various spot welding machine are classified as:
1. Stationary single-spot machine
2. Portable single-spot machine
3. Multiple spot machine
Stationary single-spot machine may further classified as:
i.
Rocker arm type
ii.
Direct pressure type
A direct pressure type stationary single spot welding machine has been selected for the project work. 4.1.2 DESIGN CONSIDERATIONS FOR SPOT WELDING
Spot welding is often selected for joining sheet metal fabrications, stampings and assemblies because it is fast, reliable and economical. However, numerous design considerations can affect the quality and cost of the weld, among them: size of the spot weld, accessibility, positioning, materials and thicknesses being joined, and the number of spots needed to attain the desired strength. Figure 4.1: Operation procedure
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Thickness of the majority of parts joined by spot welding ranges up to 1/8 in. (3 mm) for each member, although parts up to 1/4 in. (6 mm) thick have been successfully spot welded.
Based on the experience of stampers and fabricators, certain general recommendations can facilitate spot welding of a sheet metal design, no matter what metal forming process is used to make it. It is always useful to consult with the metal former in the design stage when questions arise regarding the part design, application of spot welding or, control of spot welding cost for a particular design.
Knowledgeable designers avoid overspecifying the number of welds, weld size, and location. After evaluating strength requirements, it usually suffices to specify "a minimum number of spot welds equally spaced," thereby leaving the most economical positioning up to the metalformer.
Even though spot welding is a very cost effective way of fastening sheet materials, if other joining methods are also specified, it may be more economical to redesign so that one or the other method is eliminated.
Dimensional precision is often over specified, sometimes unintentionally. CAD systems, for example, specify three or four digits of precision unless instructed otherwise. Where possible, spot welding should be shown schematically without dimensions.
4.1.3 EQUIPMENT REQUIRED FOR SPOT WELDING MACHINE
1. Cap or electrode
2. Holder
3. Gun arm
4. Controller
5. Transformer
For Measurement:
1. Spring balance
2. Stopwatch
3. Ammeter
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Design of different parts of spot welding machine:
A proper design or plan must be needed for construct better spot welding machine. The available current of welding shop is low. For low current greater thickness sheet cannot be welded. Maximum 0.7 mm thickness sheet can be welded. For this reason electrode size is 10.3 cm length and 1.2 cm diameter. A bed is of 45, 4 X 21 cm size for hold the whole construction.
The size of the two arms 25, 5 X 5 cm is used to hold the electrode.
4.1.4 ELECTRODE
Figure 4.2: Electrode
Length of the Electrode-10.3 cm
Diameter of the Electrode-1.2 cm
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4.1.5 TRANSFORMER
Figure 4.3: transformer
4.1.6 SPRING BALANCE
Figure 4.4: Spring balance
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4.1.7 DESIGN OF SPOT WELDING MACHINE
Figure 4.5: 3-D view of spot welding machine
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4.2 CONSTRUCTION:
4.2.1 CONSTRUCTION DETAIL:
The spot welding machine will be constructed consist of mainly a bed, two arms, two electrodes and a transformer. Bed is of 45, 4 X 21 cm size. There are two tips, one is fixed and another is movable to pass current and apply pressure to region to be welded. Other materials included two
Copper welder tips, two 21 cm x 3 cm steel plated corner braces, drywall screws, and washers.
There are two wood arm used, where the electrodes are aligned. The size of the arms is 25, 5 X 5 cm. Then one of the arms is mounted with the bed by 3 screws. After the lower jaw was mounted, we also attached the corner braces. It was found that a spare piece of 5 X 5 cm as a shim worked well to align the upper and lower jaw. After the upper jaw was aligned we attached it to the steel braces with screws. This formed the hinged portion of the jaw. Fixed tip will be fixed with the bed. The copper electrodes to make sure they were tight. A loose connection will take heat away from the weld nugget. Then the electrode is mounted with the upper jaw, which is movable. Evenly aligning the welding electrodes, we were careful to keep the upper jaw in the natural position where it was mounted. This maintained a flat contact area for the welding electrodes. After we were sure that the electrodes had been properly aligned. Two end of a transformer will be connected to two tip to pass current through them. As a large amount of heat will generate. There are five zones of heat generation. One at the interference of two sheets, two at the contact point of electrodes with sheet and two at the metal of the sheets. It is very much desirable for obtaining good weld that proper heat balance should be there. A lever is attached in the upper arm and pressure is applied by this lever. One end of the spring balance is attached on the free end of the lever and the other end is fixed with top wall of the building.
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Figure 4.6: Constructed spot welding machine
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4.2.2 OPERATION PROCEDURE:
The surface to be welded should be thoroughly cleaned by picking or shot blasting to obtain strong welding. The two pieces to be joined by spot welding have to placed between two electrodes and then electrodes will be pressed against each other by hand lever so as to cause desired pressure on the sheet. A current of low voltage and sufficient current (120-300 amperes per mm2) will be passed between two electrodes causing the two parts to be brought to welding temperature and thus the metal under electrodes pressure will squeezed and welded. The duration of current must be between 0.3 to 2 second. This duration is known as welding time. After this the current will be switched off while the pressure will still act. The pressure will be applied till the weld cools and regain sufficient strength. This period is known as hold time. The pressure is then released and work is removed from machine. Material with higher thermal conductivity, like aluminum need higher current for shorter period than material with lower thermal conductivity. Some recommendation for different thickness of mild steel of different thickness are given bellow. Thickness(mm)
Time(sec)
Force(N)
Current(amp)
0.3
0.4
0.5
0.6
5
7
10
15
137
147
196
215
70
100
140
160
4.3 RISKS:
There are some limitations on material weldability but attention to correct setting up and good process control can solve most production problems. The main safety hazards are
(i)
The risk of crushing fingers or hands and
(ii)
Burns or eye damage from splash metal. Little fume is produced but may need attention when welding coated steels or when oils or organic materials are present.
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CHAPTER- V
PERFORMANCE TEST & DISCUSSION
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5.1 PERFORMANCE TEST
The performance of the resistance spot welding machine has been found quiet satisfactory. The main objective of this project is to make spot welding between thin sheets. A perfect pressure and current will make the weld smoothly.
5.2 DISCUSSION
A spot welding machine was designed and constructed under this project work. The main objectives of this project is to make a better spot weld between thin sheets. The actual weld is made at the interface of the parts to be joined. The electrical resistance of the material to be welded causes a localized heating at the interfaces of the metals to be joined. Welding procedures for each type of material must be developed for the most satisfactory results. The exact time, pressure, and current setting will depend on the specific application and the KVA rating of the resistance spot welding machine employed.
But for this three reasons such as Excessive tong pressure, Weld time too long, Misalignment of the electrode tips.
If the resistance spot weld does not have an even, concentric surface appearance, the problem could be misalignment of the electrode tips. Align electrode tips with the power off and a typical weld joint between the tip surfaces.
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CHAPTER- VI
CONCLUSION
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6.1 CONCLUSION
After completing the project work, it may be concluded that,
1. The design of the spot welding machine was completed
2. The construction of various parts of spot welding machine according to design was completed. 3. The performance of the spot welding machine was satisfactory enough. But for various reasons there was a little bit variation in spot welding.
6.2 SCOPE OF FUTURE WORK
1. In future hydraulic system can be used for pressure applied.
2. In future for greater thickness spot welding a high range transformer can be used.
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REFERENCES
1. R.k. jain; Production Technology 16th edition; khanna publishers 2-B, Nath market, Nai sarak, Delhi-11006(India)
2.
Myron L Begeman; Manufacturing process 8th edition; JOHN WILEY & SONS; New
York
3. Dave smith; Welding; 8th editions; Mc Grow international editions; Mechanical engineering Series
4. Little R.L; Welding and welding technology; Tata McGraw Hill publishing Company
Limited; New Delhi.
5. "Manufacturing Process"; Harold V. Johnson; 1984
6. "Manufacturing Processes Reference Guide"; Robert H. Todd, Dell K. Allen and Leo
Alting; 1994.
7. http://www.Olympusndt.com
8. http://www.wikipedia.com
9. Handbook for Resistance Spot Welding- Miller Electric Mfg. Co.
10. The welding handbook- Barwil Unitor Ships Service, http://www.barwilunitor.com
11. http://www.welderworld.com
12. http://www.cmwinc.com
13. Introduction to welding
14. Resistance welding- Talat 4500
15. Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and
Steven R. Schmid.
16. The Welding Journal, May 2003
17. Journal of Applied Sciences Research, 3(11): 1494-1499, 2007
18. Soldering and Welding Machines and Apparatus August 2008
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