Fatigue behavior of three-sheet aluminum-steel dissimilar resistance spot welds for automotive applications

Responding to the increasing demand of multi-material solutions for structural lightweighting in automotive industry, resistance spot welding (RSW) of two sheets, e.g. aluminum to aluminum, or aluminum to steel, has been successfully developed and implemented in production using novel multi-ring domed (MRD) electrodes and multiple solidification weld schedules. In this study, we report, for the first time, the successful RSW of three sheets, i.e. 1.2 mm thick AA6022 to 0.65 mm thick high strength low alloy (HSLA) steel and 1.4 mm thick CR780T steel sheets. The resultant RSWs formed a thin layer of intermetallic compound between the aluminum-steel interface and a much smaller but completely melted weld nugget between the two steels, achieving an acceptable joint strength com-pared to RSWs of 1.2 mm thick AA6022 to itself and 1.2 mm thick AA6022 to 2.0 mm thick HSLA. Fatigue test results show that the three-sheet RSWs reached comparable fatigue behavior to that of 1.2 mm AA6022 to 2.0 mm HSLA. Using the structural stress analysis, all the fatigue data fall onto one master curve indicating that the aluminum-steel weld nugget diameter dominates the fatigue life.