Effect of rotation speed on microstructure and mechanical properties of self-reacting friction stir welded Al-Mg-Si alloy

The effect of tool rotation speed on microstructure and mechanical properties of self-reacting friction stir welded joints was investigated for Al-Mg-Si alloy. Joints were produced by a newly designed pin tool with rotation speeds ranging from 300 to 600 rpm at a constant welding speed of 350 mm/min. Cavity defect was observed in the weld obtained at the lowest rotation speed, and defect-free joints were formed with increasing of rotation speed. Self-reacting friction stir welding produced much refined microstructure accompanied by dispersion and dissolution of precipitated phases. There was little change of grain size in the weld nugget zone with increasing rotation speed, but more coarse microstructure was found in the thermomechanically affected zone and heat affected zone. The hardness of the weld was obviously lower than that of the base metal, and the lowest hardness location was in the heat affected zone. Results of transverse tensile tests indicated that the defective joint fractured in the weld nugget zone with the lowest tensile strength, while the fracture location of the defect-free joints changed to the heat affected zone.