Capacitor Discharge Spot Welding of Aluminum, Part 2: Electrode Life Assessments

A major challenge for high-volume resistance spot welding of aluminum sheet is durability of the electrodes themselves. In production today, electrodes have total anticipated lives (including dressing) on the order of 1000 welds. This is largely related to the use of medium-frequency direct current (MFDC) power. The single-polarity orientation of MFDC welding results in excessive heating of one electrode (anode) and accelerated wear rates. Recently, technology employing capacitor discharge (CD) welding in conjunction with polarity switching has been developed. This work is the first effort in examining the response of resistance spot welding on aluminum sheet using this power source. Part 1 of this research (Ref. 1) described basic process robustness in spot welding with CD power systems. Part 2 addresses electrode life response. Duplicate electrode life tests were completed for 2000 welds without failure. These results were related to the polarity switching and short time that produced balanced and minimized wear. Additional testing was done without the use of electrode-cooling water. A limited test (500 welds) largely paralleled the ones done with cooling, suggesting that long-term spot welding with polarity-switching CD power and no water was possible.

Capacitor Discharge Spot Welding of Aluminum, Part 1: Weldability Assessments

A key aspect of integrating automotive sheet into automotive production are the costs associated with joining. While the majority of sheet steel assembly is done with resistance spot welding, that has not readily translated to aluminum. Resistance spot welding of aluminum sheet is challenged by high current demand as well as reduced electrode life. In the latter case, direct current (DC) power supplied by state-of-the-art systems has exacerbated the problem. Recently, technology employing capacitor discharge (CD) welding in conjunction with polarity switching has been developed. This work is a first effort in examining the response of resistance spot welding on aluminum sheet to polarity-switching CD power. In this paper, the current range response between medium-frequency DC (MFDC) and polarity-switching CD was investigated. It was found that polarity-switching CD welding offered improved current ranges over MFDC. In addition, replicate mechanical testing cross-tension results were similar, but tensile shear strengths improved nominally 20–25%. Finally, some limited tests were done to assess the suitability of CD resistance spot welding in the presence of an adhesive. Current range tests with and without a prepulse were done, and both showed excellent weldability.