Why is TIG (GTAW) preferred for welding aluminum?

Tungsten Inert Gas (TIG) welding also known as Gas Tungsten Arc Welding (GTAW) is preferred when fabricating and welding thinner gauge aluminum alloys, complicated aluminum alloy joints and for repairing aluminum parts.

The reason why GTAW is preferred is its ability to remove surface oxides found on all aluminum alloys by applying Alternating Current with the process. The surface aluminum oxides that form very quickly are a secret of aluminums corrosion resistance but, they are so tightly adhering that they can cause issues with an electric arc during welding.

The way this works is that the alternating current moves from AC-electrode positive to AC-electrode negative and by alternating in this fashion breaks up the aluminum oxides from the surface. In actuality, during the electrode positive half cycle, electrons flow from the workpiece to the tungsten electrode and, in doing so, break up the surface oxide. This action exposes the bare metal for welding. During the electrode negative cycle, the electrons flow the other way, from the tungsten electrode to the workpiece. The consequent bombardment of the surface, during the electrode negative cycle increases the heat in the workpiece and this assists in overall penetration.

So, using AC current, the positive half cycle causes surface oxides to break up and during the negative half cycle penetration is enhanced.

Figure 1 illustrates how one can control the balance and imbalance of an AC square wave form by selecting more AC-positive for cleaning the surface (break up of oxides) or more AC-negative for more penetration.

Figure 1, AC Balance Control

The oxide layer on the aluminum surface has a melting point of 2072°C which is much higher than the base metal melting point of 660°C. This cleaning action, from the alternating current, along with the ability of directing the arc heat using the tungsten electrode in the welding torch allows better control of the arc and heat input.
When manually operating, the GTAW process allows greater control of arc heat input by using a foot control, similar to using the gas pedal in a car, the more you press down the more current and arc heat is generated, and the reverse reduces heat. This allows the welder to control the arc heat in welding from very thin material to thicker and it does this for all welding positions. This control is an added advantage in producing quality welds on aluminum over other arc welding processes.

Therefore, the advantage of using the GTAW process on welding aluminum and its alloys is to clean the surface of the tightly adhering oxides, to give precise heat control using the foot current control for welding thinner metals and overall higher weld bead quality with advanced bead shaping.



Bill Eccles
VP PPC and Associates