CATHODIC CLEANING AND HEAT INPUT IN VARIABLE POLARITY PLASMA-ARC WELDING OF ALUMINUM

For variable polarity plasma are welding (VPPAW) of 1100 Al, it was fo und that the net heat input to the aluminum workpiece did not decrease as independent changes in polarity balance enabled the tungsten elect rode to be come the predominant anode in the alternating current are. For the thin sheet edge welds made in this study, the independent para meters used to var); the are current polarity balance were very effect ive in delivering a wide range of actual are power polarity balance. T he ratio of electrode positive polarity are energy to the total are en ergy ranged from as little as 0.03 to as high as 0.99. Despite this pr onounced difference in are polarity, no significant variation in the a verage are efficiency (net heat input/arc energy) of 0.51 was found. S ubstantial heating of the workpiece during electrode positive polarity was attributed to field type emission of electrons from the low boili ng point aluminum cathode. Unlike thermionic emission at the tungsten, field emission electrons do not cool the cathode. While the actual ar e efficiencies were relatively constant, there were significant differ ences in the measured heat input, the weld size, and the effectiveness of the cathodic cleaning. Increases in the are voltage resulted in a gradual increase in are power as the aluminum workpiece became the pri mary cathode. This resulted in an increase in the measured heat input to the part and an increase in the melted volume. Despite variations i n melted volume, no correlation between are polarity and weld dimensio ns was observed. The effectiveness of positive polarity are power in r emoving the surface oxide was clearly apparent when examining the resu lting welds. Cleaning (i.e., removal of the oxide) can be effectively controlled and optimized through proper selection of the independent p arameters used to create the test weld matrix.