Cathodic cleaning and heat input in variable polarity plasma arc welding of aluminum

Abstract

For variable polarity plasma arc welding (VPPAW) of 1,100 Al, it was found that the net heat input to the aluminum workpiece did not decrease as independent changes in polarity balance enabled the tungsten electrode to become the predominant anode in the alternating current arc. For the thin sheet edge welds made in this study, the independent parameters used to vary the arc current polarity balance were very effective in delivering a wide range of actual arc power polarity balance. The ratio of electrode positive polarity arc energy to the total arc energy ranged from as little as 0.03 to as high as 0.99. Despite this pronounced difference in arc polarity, no significant variation in the average arc efficiency (net heat input/arc energy) of 0.51 was found. Substantial heating of the workpiece during electrode positive polarity was attributed to field type emission of electrons from the low boiling point aluminum cathode. Unlike thermionic emission at the tungsten, field emission electrons do not cool the cathode. While the actual arc efficiency were relatively constant, there were significant differences in the measured heat input, the weld size, and the effectiveness of the cathodic cleaning.