Which statement is true about aluminum and magnesium in GTAW?

• A. They require AC power
• B. They are best welded with DCEN
• C. They can't be welded with GTAW
• D. They require DC power only

Answer: (A)

Aluminum and magnesium form oxide films that quickly build up on the surface and inhibit the welding arc. Using alternating current in GTAW provides a cleaning action that continuously removes this oxide while still delivering heat to melt the metal. During the positive half of the cycle, the arc helps break up and wipe away the oxide; during the negative half, heat is focused into the workpiece to form a proper weld pool. This combination is what makes AC the best and most reliable choice for welding these alloys. Relying on direct current without that cleaning action leaves the oxide stubbornly in place, leading to poor fusion and defects. While there can be special cases, the standard and effective approach for GTAW of aluminum and magnesium is AC power.