What specific current type and polarity would an expert select for Gas Tungsten Arc Welding of aluminum, and precisely why does this choice minimize oxide contamination?

For Gas Tungsten Arc Welding (GTAW) of aluminum, an expert would specifically select Alternating Current (AC). Within this AC waveform, the polarity continuously switches between Electrode Negative (EN) and Electrode Positive (EP).

This choice minimizes oxide contamination primarily due to the Electrode Positive (EP) half-cycle, which provides a critical "cleaning action." During the EP phase, the tungsten electrode becomes positively charged, and the workpiece becomes negatively charged. Electrons flow from the workpiece to the electrode. Simultaneously, positively charged gas ions from the inert shielding gas, such as argon, are accelerated from the positively charged electrode towards the negatively charged workpiece. These high-velocity ions physically bombard the surface of the aluminum workpiece. This bombardment process is known as sputtering, where the impact of the ions effectively disrupts, dislodges, and removes the tenacious, high-melting-point aluminum oxide layer present on the aluminum surface. This exposes clean, underlying base metal, which is essential for achieving a sound weld fusion.

The Electrode Negative (EN) half-cycle, on the other hand, is crucial for efficient heat generation. During the EN phase, the tungsten electrode is negatively charged, and the workpiece is positively charged. Electrons flow from the negatively charged tungsten electrode to the positively charged workpiece. This concentrated flow of electrons delivers intense heat directly to the workpiece, efficiently melting the aluminum base metal and any filler material, while keeping the non-consumable tungsten electrode relatively cool, preventing its rapid degradation or contamination of the weld pool.

Aluminum inherently forms a hard, refractory (high-melting-point) oxide layer, aluminum oxide, which melts at approximately 3700°F (2038°C), significantly higher than aluminum's melting point of about 1220°F (660°C). If this oxide layer is not effectively removed, it will prevent proper fusion of the aluminum base metal, leading to poor weld quality, porosity, and lack of penetration. Using Alternating Current leverages the unique benefits of both polarities: the cleaning power of the EP half-cycle to break down the oxide and the efficient heat transfer of the EN half-cycle to melt the aluminum, thereby ensuring a clean, strong, and complete weld.