Characterization of Fusion Behavior During Variable Polarity AC Welding of Aluminum

Aluminum alloys are commonly welded with the Gas Tungsten Arc Welding (GTAW) process utilizing variable polarity AC. Conventional literature and many power source panels indicate that “max penetration” (maximum fusion) will occur when greater than 50% of the AC cycle is spent on electrode negative (EN) polarity. “Max cleaning” is purported to occur when more than 50% of the cycle is on electrode positive (EP). This is based on two conventional assumptions:

1.   The EN heat distribution during GTAW is 30-40% at the electrode and 60-70% at the base metal.

2.    On AC, the thermal energy distribution between the cathode and anode during the EP cycle is identically opposite to the EN cycle.

<>Previous work by the authors⁽¹⁾ and later work by others⁽²⁾ demonstrated that the second assumption is incorrect and that weld fusion area increases with % EP. During the EP cycle, the primary source of the additional base metal fusion is the field emission mechanism where electrons are generated from the aluminum ‘cold cathode'. Second, the dielectric breakdown of surface oxide as electrons are emitted also contributes. In this study, an improved data acquisition system was employed over a wide range of %EP such that the complex voltage and current waveforms were assessed. The complex waveforms (Figure 1) has identified that this additional cathode energy is supplied via a voltage bias, where the EP cycle requires a higher voltage for stable operation as compared to the EN cycle. The lower voltage during EN is directly linked to thermionic electron emission at the tungsten electrode, which requires less energy (voltage) in comparison to the EP field emission mechanism. Comparison of this fusion behavior and the variable polarity waveforms with consumable-electrode welding processes has confirmed this trend of enhanced melting at the cold cathode during arc welding.