Interface development and microstructure evolution during double-sided friction stir spot welding of magnesium alloy by adjustable probes and their effects on mechanical properties of the joint

Abstract

The flat friction stir spot welding using double side adjustable tools (Flat DSAT FSSW) was successfully applied to the joining of magnesium alloys in our previous research. In this study, the in-situ X-ray radiography observations using a high-intensity X-ray transmission device were carried out to investigate the interface development. In addition, the microstructure evolution at different welding stages was also studied. The mechanical properties of the welded joints were significantly affected by the minimum plate thickness of the joint, which was the distance from the tip of the unwelded interface to the bottom of the lower plate. The welding interface was well welded during the stage of the holding of probes, and the minimum plate thickness was eventually determined by the stage of raising the lower probe, which is the key stage affecting the joint properties. The in-situ X-ray radiography technique shows a significant advantage to reveal the welding process. In the tip area of the unwelded interface at low rotation speed welding condition, the grain growth was suppressed due to the limited heat generation, giving rise to refined microstructure. Moreover, a randomized texture was obtained owing to the weak material flow during the welding process and the introduced mechanical twins during the stage of raising the lower probe. The refined microstructure resulting from limited heat generation and randomized texture resulting from weak material flow contributed to the high strength welded joint.