Gas Tungsten Arc Welding

Gas Tungsten Arc Welding
The Process
Gas Tungsten Arc Welding, also known as Tungsten Inert Gas Welding, is an arc welding process that uses a non-consumable tungsten electrode to produce a weld. The weld area is protected from atmospheric contamination by an inert shielding gas, such as argon or helium, and filler metal is normally used, except in autogenous welds. 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.

GTAW 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, and allows 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.
How it works
Manual Gas Tungsten Arc Welding is considered the most difficult of all the welding processes commonly used in the industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. However, some welds combining thin materials (known as autogenous or fusion welds) can be accomplished without filler metal; most notably edge, corner, and butt joints.
Striking the Arc
To strike the welding arc, a high frequency generator (similar to a tesla coil) provides an electrical spark. The spark creates a path through the shielding gas and allows initiation of the arc while the electrode and the work piece are separated, typically about 1.5-3mm apart. An alternate way to initiate the arc is to scratch or touch the workpiece with the powered electrode which initiates the arc; this is known as “scratch starting”. Scratch starting, however, may cause contamination of the weld and electrode. Some GTAW equipment is capable of a mode called “touch start” or “lift arc”; here the equipment reduces the voltage on the electrode to only a few volts, with a current limit of one or two amps (well below the limit that causes metal to transfer and contamination of the weld or electrode). When the GTAW equipment detects that the electrode has left the surface and a spark is present, it immediately (within microseconds) increases power, converting the spark to a full arc. The electric arc can reach temperatures of at least 2500°C.
Welding
Once the arc is struck, the welder moves the torch in a small circle to create a welding pool. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld.
Tools Used
Power Supply
Gas tungsten arc welding uses a constant current power source, meaning that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of GTAW are manual or semiautomatic, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult.
Electrode
The electrode in GTAW is made of tungsten or a tungsten alloy because tungsten has the highest melting point among pure metals, at 3,422°C (6,192°F). As a result the electrode is not consumed during welding, though some erosion, called “burn-off”, may occur.
Shielding Gas
Shielding gases are necessary in GTAW to protect the welding area from atmospheric gases such as nitrogen and oxygen, which can cause fusion defects, porosity, and weld metal embrittlement if they come into contact with the electrode, the arc or the welding metal. The gas also transfers heat from the tungsten electrode to the metal, and it helps start and maintain a stable arc. Argon is the most commonly used shielding gas for GTAW. When used with alternating current, the use of argon results in high weld quality and good appearance. Another common shielding gas, helium, is most often used to increase the weld penetration in a joint, to increase the welding speed, and to weld metals with high heat conductivity, such as copper and aluminum. However, using Helium can make it difficult to strike the arc and decrease the weld quality. Argon-helium mixtures are also frequently utilized in GTAW because it combines the benefits of both. The mix is normally about 75% (or more) helium.
Safety Gear
Protective Clothing
Welders wear protective clothing, including light and thin leather gloves and protective long sleeve shirts with high neck collars, to avoid exposure to strong ultraviolet light.
Masks/Helmets
Because of the absence of smoke in GTAW, the electric arc light is not covered by fumes and particulate matter, and thus, is a great deal brighter, making operators especially susceptible to the effects of strong ultraviolet light exposure. Opaque full face and neck Helmets with dark eye lenses are worn to prevent this exposure to ultraviolet light, and in recent years, new helmets often feature a liquid crystal-type face plate that self-darkens upon exposure to the bright light of the struck arc.
Safety Precautions
Welding Curtains
Transparent welding curtains, made of a polyvinyl chloride plastic film, are often used to shield nearby workers and bystanders from exposure to the UV light from the electric arc.
Ventilation
Welders are also often exposed to dangerous gases and particulate matter. Although smoke is not produced, the brightness of the arc in GTAW can cause surrounding air to break down and form ozone which creates nitric oxides. Inside the lungs the ozone and nitric oxides react with the lung tissue and moisture to create nitric acid and ozone burn. Although the levels of ozone and nitric oxides are moderate, precautions, such as: limiting duration of exposure, limiting repeated exposure, correctly extracting fumes and ensuring air circulation, are required to keep relatively healthy lungs. If precautions are not taken welders may suffer emphasyemia and odema of the lungs which, as well as shortening life span, can result in death.
Cleaning Products
The brightness and heat from the arc can cause cleaning and degreasing agents to break down and react to form poisonous fumes. Cleaning operations using these agents should not be performed near the site of weld.