Microstructure evolution and mechanical properties of extruded Mg96Zn2Y2 alloy joints with ultrasonic spot welding

Our study evaluated the microstructure formation mechanism and weldability of Mg-Zn-Y alloy with a long-period stacking ordered (LPSO) structure phase and applied ultrasonic spot welding technology to the welding for the first time. The microstructure evolution of the joints depended on the macro shape of the welding tip with both flat and slope region. A band with fine grains was generated around the weld interface. In the slope region, a novel waved band was generated, and it was revealed that the plastic flow occurred in the entire band. Outside the band in the flat region, Zn and Y segregated at the grain boundaries of an Mg-matrix. The presence of temperature distribution along the weld interface was suggested from the segregation region. Furthermore, the maximum weld strength was obtained with the weld condition in which the novel band was generated, and it was revealed that the enhancement of the plastic flow in the slope region contributed to an increase in the weld strength. The fracture modes transmuted with the condition and the base metal plug fracture mode was exhibited.