Microstructure evolution depended on welding tip geometry on AZ31B alloy joints applied ultrasonic spot welding

Our study systematically evaluated the microstructure evolution which depended on the shape of the welding tip. And then, it was revealed that the different microstructure was generated in the slope and flat regions of the welding tip. In the slope region corresponded to the end of the joints, the wavy behavior due to the intense plastic deformation was observed, and the grain refinement occurred near the weld interface in the slope region at the welding conditions. It was associated that the expansion of the weld diameter was led by the enhancement of the plastic deformation in the slope region, and the weld strength increased. On the other hand, as the welding process progresses, the grain growth occurred in both the slope and flat regions. It was revealed that the {10 1 2} deformation twin and the twin growth were the main deformation mechanism in the process which the grains became coarse during the high strain rate processing such as the ultrasonic spot welding.