Effect of zinc interlayer on Hook defects of aluminum alloy ultrasonic assisted friction stir spot welding joint

Hook defect is one of the factors that restrict the quality of aluminum alloy friction stir spot welding (FSSW) joints. In this paper, a new process of ultrasonic friction stir spot welding (UAFSSW) with zinc interlayer was proposed. Ultrasonic vibration improved the fluidity of the material and promoted the elements diffusion. Meanwhile, by adding pure zinc interlayer, the rapid metallurgical bonding of Hook zone materials can be realized, therefore, the quality of UAFSSW joint can be improved. As the results shown, compared with conventional UAFSSW method, adding zinc interlayer had obvious positive effects on Hook forming of joint. Under different heat input conditions, tensile-shear strengths of UAFSSW joints increased by adding zinc interlayer. The tensile-shear failure load of the joint obtained by adding zinc interlayer method improves 21.36% and 12.79% than conventional UAFSSW process when the rotational speed is 600 and 1 200 r/min, respectively.