| Abstract Scope |
A key aspect of integrating sheet metals into automotive structures is joining. For the automotive industry, the dominant technology for joining such sheet materials is resistance spot welding. That technology has been dominant in the industry for nearly 100 years, primarily due to low costs of use and high manufacturing robustness. This is particularly the case for steel applications. The advantages of spot welding however have not translated as well for aluminum sheet. Resistance spot welding of aluminum sheet is challenged by high current demand as well as reduced electrode life. The former is driven by the high thermal and electrical conductivity of aluminum. In the latter case, the direct current power supplied by state of the art systems has exacerbated the problem. This has been related to Peltier voltages between the aluminum sheet and copper electrodes. When that voltage opposes the direction of direct current flow, excessive heating can result with damage to that electrode. To address the first issue, capacitor discharge welding (CD) has long been employed for spot welding aluminum. CD welding is characterized by a rapid energy discharge, achieving high currents in relatively short (milliseconds) times. Recently, that technology has been adapted to provide polarity switched pulses on discharge. In this work the potential benefits of short heating times and polarity switching that result from this new power supply have been investigated. Key metrics for evaluation included process robustness, internal metallurgical integrity, joint strengths, compatibility with adhesive, and electrode life. With respect to process robustness, it was found that polarity switching CD welding offered improved current ranges over medium frequency direct current (DC) power supplies. Further, spot welds made with the new power supply showed reduced internal porosity and cracking compared to those made with MFDC. With respect to mechanical performance, replicate cross-tension test 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 pre-pulse were done, and both showed excellent weldability. Significant improvements in electrode life were also noted with the polarity switching CD power supply. In production today, electrodes have total anticipated lives (including dressing) are on the order of 1000 welds. In this study if was found that duplicate electrode life tests were consistently extended out to 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 indeed possible. |