Microstructural characteristics and mechanical properties of Al–Mg–Si alloy self-reacting friction stir welded joints

The 5 mm thick Al–Mg–Si alloy was self-reacting friction stir welded using the specially designed tool at a constant rotation speed of 400 rev min ⁻¹ with various welding speeds. Defect-free welds were successfully obtained with welding speeds ranging from 150 to 350 mm min ⁻¹ , while pore defects were formed in the weld nugget zone (WNZ) at a welding speed of 450 mm min ⁻¹ . Band patterns were observed at the advancing side of WNZ. Grain size and distribution of the precipitated phase in different regions of the joints varied depending on the welding speed. 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 (HAZ). Results of transverse tensile tests indicated that the defective joint fractured in the WNZ with the lowest tensile strength, while the fracture location of the defect-free joints changed to the HAZ.