Tensile Strength of Friction Stir Spot Welded Dissimilar AA5754-to-AZ31B Alloys

Friction stir welding (FSW) is a relatively new joining process. As a solid-state joining technique, FSW provides good potential for dissimilar materials, even those considered as “difficult” and "unweldable". As a variant, friction stir spot welding (FSSW) can have significant potential to replace riveting or resistant spot welding for aerospace and automotive applications. To date, limited results have been reported on the FSSW of dissimilar material combinations. In this work, the tensile shear strength obtained is reported for a dissimilar 2-mm thick AA5754-to-AZ31 alloy system. The main process parameters investigated include tool rotation speed, tool plunge rate, dwell time and work-piece placement (i.e. either Al or Mg alloy on the top of the lap spot welds). The tensile strength is also compared with that for similar AA5754-to-AA5754 and AZ31-to-AZ31 welds. Furthermore, the tensile shear strength is correlated with the joint geometrical dimensions and the welding defects such as hooking, kissing bond, and top plate thinning. The process window for FSSW of the 2-mm thick dissimilar Al-to-Mg alloy system was optimized based on the minimization of the welding defects and the maximization of the tensile shear strength.