What are the main differences between AC and DC operation in GTAW, and which materials typically require AC?

• A. DC provides one-direction arc; AC alternates; aluminum commonly requires AC for oxide cleaning, while steel is typically welded on DC
• B. DC alternates; AC provides one-direction arc; aluminum requires DC; steel requires AC
• C. AC provides more heat; DC less heat; aluminum requires DC; steel requires AC
• D. DC produces pulsed arc; AC constant; aluminum requires DC; steel requires AC

Answer: (A)

The main idea here is how polarity in GTAW affects arc behavior and oxide removal, and which materials benefit from AC versus DC. Direct current flows in one direction, giving a stable, directional arc that’s ideal for steels and similar metals when welding on DC. Alternating current switches polarity, which helps cleanse the surface of oxides on metals like aluminum by repeatedly reversing the electrode’s role, aiding arc stability and weld cleanliness. Aluminum forms a tough oxide that doesn’t weld well unless it’s cleaned during the process, and AC provides that periodic cleaning action. Steel welds are typically done on DC because it gives predictable heat focusing and penetration, with polarity chosen to shape the weld and control electrode wear.

So the option that states DC is one-direction, AC alternates, aluminum commonly requires AC for oxide cleaning, and steel is typically welded on DC best captures the practical differences and typical material practices in GTAW. The other statements mix up which polarity alternates, which metal benefits from AC versus DC, or claim inherent heat differences that aren’t the defining factor.