A welding inspector is reviewing a procedure for welding a high-strength aluminum alloy. The WPS specifies Gas Tungsten Arc Welding (GTAW) with alternating current (AC). What is the primary reason for using AC in GTAW for aluminum and its alloys?
- ATo allow for faster travel speeds and higher deposition rates.
- BTo minimize heat input and reduce distortion.
- CTo provide a cleaning action that removes surface oxides.
- DTo maximize penetration into the base metal.
Show answer & explanationAnswer & explanation
Correct answer: C. To provide a cleaning action that removes surface oxides.
Aluminum naturally forms a tenacious oxide layer that melts at a much higher temperature than the base metal. The reverse polarity (electrode positive) half-cycle of AC current provides a 'cathodic cleaning' action, effectively breaking up and removing this oxide layer, which is crucial for achieving a sound weld.
Why the other options are wrong
- A. GTAW, in general, is known for precision and quality, not high travel speeds or deposition rates, and AC does not significantly enhance these aspects.
- B. While AC offers good heat control, its primary advantage for aluminum is cleaning, and distortion control is more related to overall heat input management.
- D. AC provides moderate penetration, but DCEN (Direct Current Electrode Negative) generally offers deeper penetration.
GTAW AC for Aluminum
In Gas Tungsten Arc Welding (GTAW), Alternating Current (AC) is predominantly used for welding aluminum and its alloys. The key benefit of AC is its cathodic cleaning action during the electrode positive (EP) half-cycle, which effectively removes the tenacious aluminum oxide layer, ensuring a clean and sound weld.
- Aluminum forms a tenacious oxide layer.
- Aluminum oxide has a higher melting point than aluminum.
- AC provides cathodic cleaning action.
- Electrode positive (EP) half-cycle performs cleaning.
- Essential for sound aluminum welds.
Memory trick: DC is for steel and deep, AC is for aluminum and clean.