Keyhole refilled friction stir spot welding of aluminum alloy to advanced high strength steel

A keyhole refilled Friction Stir Spot Welding (FSSW) process is developed, which consists of two steps. Regular FSSW is performed in the first step. After that, the tool travels along a circular path to refill the original keyhole. The process is applied for joining aluminum alloy 6061 to Transformation Induced Plasticity (TRIP) steel. It increases the joint strength by 56.33% compared with that of conventional FSSW joints. A ductile failure mode is observed. The process can be implemented in any universal CNC machine and needs only one simple FSSW tool. An additional pure circular path welding process shows the enhanced performance of the keyhole refilled FSSW joints relies on three bonding mechanisms: the refilled original keyhole, the increased bonding area between steel and aluminum as well as the hook structure generated from the regular FSSW process. Three distinct layers of grain structures can be observed on the hook, which reveal the material flow pattern during the process.