Microstructure and properties of probeless friction stir spot welding of AZ31 magnesium alloy joints

The AZ31 magnesium alloy with a thickness of 1.8 mm was welded by the probeless friction stir spot welding process without Zn interlayer. The influence of process parameters on joint microstructure and mechanical properties was investigated by using different rotating speeds and dwell time. Microstructure of joints is divided into three regions: stir zone, thermomechanically-affected zone and heat-affected zone. With the increase of rotation speed and dwell time, the depth of stir zone gradually increases, and hook defects extend from the interface of two plates to the surface of the upper plate. The tensile shear strength of joints and two fracture modes (shear fracture and plug fracture) are closely related to hook defects. The maximum tensile shear strength of the joint is 4.22 kN when rotation speed and dwell time are 1180 r/min and 9 s, respectively. Microhardness value and its fluctuation in upper sheet are evidently higher than those of the lower sheet.