Microstructure evolution and fracture behaviour of friction stir welded 6061-T6 thin plate joints under high rotational speed

6061-T6 sheets with 0.8mm thickness were successfully welded using high-speed friction stir welding (FSW) technology. The microstructural evolution and fracture behaviour of the joints were studied. The results show that sound joints could be obtained at the investigated high rotational speed of 8000 rev min(-1) and welding speeds of 300-1200mm min(-1). Compared with conventional rotational speed, the grain size in the nugget zone (NZ) is obviously refined under high rotational speed. The Mg2Si, Al8Fe2Si and Al2CuMg precipitates reprecipitated adequately in the NZ during high-speed FSW, resulting in the number of the precipitates increased significantly, and further alleviating the weld softening. The difference in weld softening leads to different fracture characteristics during the tensile process. After artificial aging, the maximum welding softening in all joints is located in the heat affected zone, and the fracture is characterised by brittle fracture.