Gas tungsten arc welding
Gas tungsten arc welding is a welding process where the heat for welding is generated by an electric arc between a non-consumable tungsten electrode and the work. Filler metal may or may not be used with the process. Shielding is obtained from an inert gas or inert gas mixture. Common and slang names for the process are TIG welding, Argonarc or Heliarc welding and Tungsten arc welding. A diagram of this process is shown below.
The GTAW process can be used to weld steel, stainless steel, aluminium, magnesium, copper, nickel, titanium, and others. The process can be used to weld a wide range of material thickness. However, due to the relatively low deposition rates associated with the process, thinner materials are most often welded. It is also often used for depositing the root pass on piping and tubing in the petrochemical and power generation industry where a radiographic quality weld is required and is also commonly used for the welding of dairy fabrications. Thin materials may also be welded autogenously i.e. no filler material is used.
Welding Positions and Application
The GTAW process can be used in all the welding positions (flat, horizontal, vertical and overhead) to produce quality welds on all metals used in industry. The GTAW process is normally applied using the manual or semi automatic method. The welder controls the torch with one hand and feeds filler metal with the other. In the manual method, a high degree of welding skill is required. The semi-automatic method is also sometimes used where filler metal is fed into the weld puddle by a wire feeder.
The GTAW process can be used in all the welding positions (flat, horizontal, vertical and overhead) to produce quality welds on all metals used in industry. The GTAW process is normally applied using the manual or semi automatic method. The welder controls the torch with one hand and feeds filler metal with the other. In the manual method, a high degree of welding skill is required. The semi-automatic method is also sometimes used where filler metal is fed into the weld puddle by a wire feeder.
Welding Power Source
In general, power sources of the constant current mode are used for gas tungsten arc welding using alternating current (AC) or direct (DC). The selection of alternating or direct current depends on the material being welded. Alternating current is recommended for welding aluminium and magnesium and their alloys. Direct current is recommended for welding stainless steel, carbon steels, copper and its alloys, nickel and its alloys, and precious metals.
In general, power sources of the constant current mode are used for gas tungsten arc welding using alternating current (AC) or direct (DC). The selection of alternating or direct current depends on the material being welded. Alternating current is recommended for welding aluminium and magnesium and their alloys. Direct current is recommended for welding stainless steel, carbon steels, copper and its alloys, nickel and its alloys, and precious metals.
Welding Torch
The welding torch houses the tungsten electrode and directs the shielding gas and the welding power to the arc. Torches come in various sizes and the larger sizes are usually water-cooled. The torches normally come equipped with a cable assembly that directs the gas, welding power current, cooling water (when used) from the machine to the torch.
The welding torch houses the tungsten electrode and directs the shielding gas and the welding power to the arc. Torches come in various sizes and the larger sizes are usually water-cooled. The torches normally come equipped with a cable assembly that directs the gas, welding power current, cooling water (when used) from the machine to the torch.
Shielding Gas
A shielding gas protects the weld puddle and tungsten electrode from oxidation during welding. The two most commonly used shielding gases with the gas tungsten arc welding process are argon and helium.
A shielding gas protects the weld puddle and tungsten electrode from oxidation during welding. The two most commonly used shielding gases with the gas tungsten arc welding process are argon and helium.
Tungsten Electrodes and Filler Metals
The electrodes used with the gas tungsten arc welding process are made of tungsten alloys. Tungsten has a high melting point of around 3400oC and is considered a non-consumable during welding. Electrodes are available in several alloys, e.g. Cerium, Lanthanum, Thorium, Zirconium and one of pure tungsten. Electrodes are colour-coded for ease of recognition and generally in diameters ranging from 0.5 mm up through 5.0 mm. The lengths of tungsten electrodes are normally 75 mm to 150 mm.
The electrodes used with the gas tungsten arc welding process are made of tungsten alloys. Tungsten has a high melting point of around 3400oC and is considered a non-consumable during welding. Electrodes are available in several alloys, e.g. Cerium, Lanthanum, Thorium, Zirconium and one of pure tungsten. Electrodes are colour-coded for ease of recognition and generally in diameters ranging from 0.5 mm up through 5.0 mm. The lengths of tungsten electrodes are normally 75 mm to 150 mm.
The filler metal for gas tungsten arc welding is a solid wire or rod. Filler metals are available in a wide range of sizes in an approximate range from 1.6, 2.4, 3.2 mm but can be obtained in larger diameters. Filler metals are manufactured in straight cut lengths (500mm to 1000mm) for manual welding and continuous spools for semi-automatic and automatic welding.
Filler metals for joining a wide variety of materials and alloys are available; these should be similar, although not necessarily identical, to the material being joined. Generally the filler metal composition is adjusted to match the properties of the base material in its welded (cast) condition. Filler metals for gas tungsten arc welding are classified using the same system for gas metal arc welding electrodes, such as ER70S-6. The only difference is gas metal arc wires carry electric current and are considered electrodes (E), while gas tungsten welding wires or rods do not carry current and are considered filler rods (R).
Advantages
• Capable of welding thin material
• Controls heat input extremely well because the heat source and the filler material are separately controlled.
• Welds can be made with or without adding filler material by fusing the base metals together.
• Full penetration welds that are welded from one side only can be made.
• Produces superior X-ray quality welds.
• Recommended for materials that form refractory oxides, like aluminium and magnesium.
• It can be used to weld almost all metals, including dissimilar metal joints.
• It allows for excellent control of root passes and penetration.
• Capable of welding thin material
• Controls heat input extremely well because the heat source and the filler material are separately controlled.
• Welds can be made with or without adding filler material by fusing the base metals together.
• Full penetration welds that are welded from one side only can be made.
• Produces superior X-ray quality welds.
• Recommended for materials that form refractory oxides, like aluminium and magnesium.
• It can be used to weld almost all metals, including dissimilar metal joints.
• It allows for excellent control of root passes and penetration.
Disadvantages
• Cost of equipment and shielding gas is high.
• Deposition rate is slow, therefore less economical than other processes.
• A high degree of welder skill is required to produce quality welds
• Fit-up tolerances are restrictive.
• Difficulty in shielding the weld zone properly in windy conditions.
• Low tolerance for contamination on filler or base metal.
• Tungsten inclusions can occur
• Cost of equipment and shielding gas is high.
• Deposition rate is slow, therefore less economical than other processes.
• A high degree of welder skill is required to produce quality welds
• Fit-up tolerances are restrictive.
• Difficulty in shielding the weld zone properly in windy conditions.
• Low tolerance for contamination on filler or base metal.
• Tungsten inclusions can occur
Discontinuities and Defects Common to GTAW
The AWS refers to discontinuities and defects of various types and sizes. Below some given acceptable level these are not considered harmful, however above that level they are considered defects. The following weld problems may occur; these are a collection of the more common types of flaws: Incomplete penetration, Incompletely filled groove, Excess penetration, Undercut, Craters, Crater pipes, Uneven profile, Uneven root penetration, Unequal leg lengths, Burn through, Wormholes, Arc strikes, Tungsten inclusions.
The AWS refers to discontinuities and defects of various types and sizes. Below some given acceptable level these are not considered harmful, however above that level they are considered defects. The following weld problems may occur; these are a collection of the more common types of flaws: Incomplete penetration, Incompletely filled groove, Excess penetration, Undercut, Craters, Crater pipes, Uneven profile, Uneven root penetration, Unequal leg lengths, Burn through, Wormholes, Arc strikes, Tungsten inclusions.
Lack of fusion may occur in the following forms: Lack of inter-run fusion, Lack of sidewall fusion, Lack of root fusion.
Porosity may occur as: Isolated, Group, Linear, Uniform or Stop-start, Oxidation in stainless steel welds
Cracking that may occur is Heat Affected Zone (HAZ) in low alloy and alloy materials. Other types of cracking can also occur.
