Gas Tungsten Arc Welding (GTAW) is commonly known as Tungsten Inert Gas Welding (TIG), however since some process variations utilize active (other than inert) shielding gases, the correct technical term is GTAW. In this process a non-consumable tungsten electrode is used and the electric arc occurs between this electrode and the work. Filler metal usage is optional; a weld when no external filler metal is used it is known as an autogenous weld. The molten weld and the arc zone are protected from atmospheric contamination by a shielding gas (usually, but not always, an inert gas such as argon). Current is conducted across the arc through a column of highly ionized gas and metal vapours known as a plasma.

Key Process Variables

• Polarity and Current Type;
• Welding Current;
• Shielding Gas (composition and flowrate)
• Arc Voltage;
• Travel Speed;
• Wire Feed Speed (if applicable);
• Electrode Geometry.

For the welding current type and polarity, DCEN is widely used (e.g., for ferrous alloys). Here ~70% of the arc heat energy occurs in the base metal (~30% on the electrode). DCEP provides cathodic cleaning action which is important when welding aluminum and magnesium alloys, however ~70% of the heat is generated on the electrode (thus a large diameter electrode is required).

AC current provides a ~50/50 balance in heat, and the cleaning action of the DCEP ½ cycle and is widely used for alloys that develop a refactory oxide layer, namely aluminum and magnesium. Due to the recurring shutdown of voltage with the AC current type, High Frequency (HF) voltage is often utilized to stabilize the welding arc. Modern GTAW power sources provide square-wave AC output which can eliminate the need for contiuous HF. These machines may also provide additional controls to allow the operator to adjust the period, and frequency of the DCEN verses DCEP ½ cycles (within the AC output).

Because of its high melting point (3422 °C), the tungsten electrode is not consumed up by GTAW welding, though to prolong its life, tungsten is usually combined with thorium, lanthanum, cerium, or zirconium oxides (1-2%). A 2% thorium dioxide alloy is the most common, but is being phased out because of its radioactivity.

Gas Tungsten Arc Welding Electrodes

Welding Aluminium

Welding aluminium is done with AC current rather than DC and pure tungsten or zirconiated tungsten tips are preferred. The electrode is generally larger than that used with steel, and is often slightly blunted as the arc tends to wander around a sharp tip when using AC. Only inert gases can be used as shielding gas with aluminum, as other gases oxidize the tungsten electrode and the base metal. The most common shielding gas is argon, which is sometimes mixed with helium to get better penetration and/or higher welding speed. A high helium content makes it harder to ignite and maintain the arc.

When TIG-welding steel, you normally use DC. As the tungsten electrode is negative, electrons jump from the electrode to the steel. This causes more heat to develop in the material being welded than in the tungsten electrode, which of course is desirable. However, if you try to TIG-weld aluminium using DC, an oxide layer develops on the surface of the molten aluminium, making it hard for the aluminium to fuse. So, aluminium is usually TIG-welded using AC.

When using AC (for aluminium), electrons jump from the tungsten electrode to the aluminium during the electrode-negative part of the AC cycle, and from the aluminium to the electrode during the electrode-positive part of the cycle. The electrode positive part of the cycle causes more heat buildup in the electrode, which is not desirable. However, during the electrode-positive part of the cycle, the aluminium is negative and positive ions in the arc are attracted to the aluminium. When these ions smash into the oxide layer, it breaks up, making welding possible.

If TIG-welding is done using a sine wave welding machine, the arc has a tendency to die out during the zero voltage transitions. One way to deal with this problem is to apply continous HF pulse across the arc. Another way is to use a square wave instead of a sine wave, as the zero transitions in a square wave are so fast the arc doesn't have the time to die out.

Om many modern AC TIG machines it's possible to adjust the balance between the electrode-negative and the electode-positive half period. If more heat in the aluminium is desired, the electrode-negative half period is increased and if more oxide breakup is desired, the electrode-positive half period is increased.

References and Further Reading

American Welding Society, Welding Handbook, Vol 2 (9th ed.)

See also

• Gas Metal Arc Welding or MIG welding
• Shielded Metal Arc Welding or stick welding