Why is AC used in GTAW for aluminum welding?

Using alternating current (AC) in Gas Tungsten Arc Welding (GTAW) for aluminum welding is an important practice primarily because it provides a cleaning action during the welding process. When welding aluminum, a layer of aluminum oxide naturally forms on the surface. This oxide layer has a higher melting point than aluminum itself, making it necessary to remove it for effective welding.

The AC current alternates between positive and negative phases. In the positive phase, the electrode acts as the anode, which helps to burn off the aluminum oxide layer due to the higher heat generated. This cleaning action ensures that the base metal is exposed and free from contaminants, allowing for a better-quality weld. The negative phase provides the necessary heat for melting the aluminum and forming the weld pool.

While factors such as reducing spatter and improving penetration are relevant to welding processes, they are not the primary reasons for using AC with aluminum. The main goal when utilizing AC is to achieve a clean weld by effectively removing the oxide layer that can impede the welding process.