Gas tungsten arc welding (GTAW) also known as tungsten inert gas (TIG) welding is a process that melts and joins metals and heating them with an arc established between a no consumable tungsten electrode and the metals. There are shielded gases which act as a protector for welded part from atmospheric contamination. Shielded gas commonly used is an inert gas such as argon, and a filler metal is normally used, even though, auto-genous 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 plasma
The GTAW is suitable to weld thin sections such as stainless steel and light metals for instances aluminum, magnesium, and copper alloys. The GTAW is comparatively more complex and difficult to master and it needs some skills or technique from welder and furthermore, it is significantly slower than other welding techniques.
Figure 3.0.1: GTAW weld area
Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. This is because the welder must use two hands and it’s very difficult to maintain a tungsten electrode and filler rod simultaneously in order to prevent contact between the electrode and the work piece. The welder has to feed a filler metal into the weld area manually with one hand while, the other act to manipulate welding torch. However not all welds need filler metal for example auto-genous welds which is combining thin materials most notably edge, corner and butt joints.
The separation between the electrode and the work piece is approximately 1.5-3 mm (0.06-0.12 in). Getting both into contact also serves to strike an arc, but this can cause contamination of the weld and electrode. Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, and depends on the size of the electrode and the current. In order to maintain a constant separation between the electrode and the work piece, the operator should moves the torch back slightly and tilts it backward about 10-15 degrees from vertical.
Welders often develop a technique of rapidly alternating between moving the torch forward and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the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. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to prevent the formation of a crater at the end of the weld.
Working Principle
In gas tungsten arc welding operation, the equipment required is includes, a constant-current welding power supply, welding torch utilizing a non-consumable tungsten electrode and also a shielding gas source.
a) Welding torch
Tungsten inert gas (TIG) welding torches are designed for both automatic or manual operation and it was attached with cooling systems whether air or water. The manual torch has a handle while the automatic torch comes with a mounting rack. But both have similar in construction. The angle between the centerline of the handle and the centerline of the tungsten electrode, known as the head angle, which can be varied on some manual torches according to the comfortable of the operator. Air cooling systems are usually used for low-current operations, while water cooling is needed for high-current welding (up to about 600 A). The torches are connected with cables to the power supply and shielding gas source hoses.
The internal metal parts of a torch are made from hard alloys which are copper or brass. This is because to transmit current and heat effectively. The tungsten electrode must be detained definitely in the center of the torch with properly sized collets, and ports around the electrode offer a constant flow of shielding gas. The body of the torch is made of heat-resistant, insulating plastics which casing the metal components, providing insulation from heat and electricity to protect the welder.
The size of the welding torch nozzle depends on the size of the preferred welding arc, and the inside diameter of the nozzle is usually at least three times the diameter of the electrode. The nozzle must be heat resistant and thus is normally made of alumina or a ceramic material, but fused quartz, a glass-like substance offers better visibility. Devices can be inserted into the nozzle for special appliance, such as gas lenses or valves to control shielding gas flow and switches to control welding current.
The filler metals are similar to the metals to be welded, and flux is not used. The shielding is usually argon or helium (or a mixture of the two). Welding with GTAW may be done without filler metals- for example, in the joining of close-fit-joints.
b) Shielding gas
As with other welding processes such as gas metal arc welding, 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 in 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 retain a stable arc.
The selection of a shielding gas depends on several factors. That’s the type of material being welded, joint design, and desired final weld form. Argon is a shielding gas that most normally used for GTAW, it will helps to prevent defects due to a varying arc length. Using argon with alternating current will produce high weld quality and good appearance. Another common shielding gas is helium, which is normally used to increase the weld penetration in a joint, to boost the welding speed, and to weld light metals such as copper and aluminum. A major disadvantage is the difficulty of striking an arc with helium gas, and the decreased weld quality related with a variation of arc length. Argon-helium mixtures are also commonly utilized in GTAW, since they can improve in controlling the heat input while maintaining the benefits of using argon.
Usually, the mixtures are made with primarily helium (often about 75% or higher) and a balance with argon. These mixtures will increase the speed and quality of the AC welding of aluminum, and also ease to strike an arc. Another shielding gas mixture, argon-hydrogen, is used in the mechanized welding of light gauge stainless steel, but it utilize are limited because hydrogen will cause porosity, it utilize are limited. In the same way, nitrogen can be added to argon to help stabilizing the austenite in austentitic stainless steels and increase penetration when welding copper. Due to porosity problems in ferritic steels and limited benefits, however, it is not a popular shielding gas additive.
Advantages
There are several advantages to GTAW as well. The welds created by the GTAW process are suitable for high quality and can be achieved in almost every alloy or metal. Secondly, the welds need little or no cleaning after they are finished. The precision of the welds is increased due to the fact that the welder can see the arc and the weld pool easily. The idea that no filler material is transferred during welding is there area minimal amount of spatter. Welding of this type can be done in numerous positions, and there is no slag produced to leave impurities in the weld
Limitation
Everything is not perfect including GTAW. The GTAW required greater welder deftness than MIG or stick welding and it’s also need lower deposition rates. And finally, for welding thick sections there need more cost.
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