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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.

The 5-mm-thick AZ61 magnesium alloy was friction stir welded by using the specially designed bobbin tool with various rotation speeds. Defect-free welds were successfully obtained with rotation speed ranging from 550 to 600 rev min −1 . Grain size in different regions of the joints varied depending on the rotation speed. The hardness value of the joint is uniform. 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. The impact energy of weld nugget zone is higher than that of the heat affected zone, and the impact energy of the two zones in defect-free joint both decreased with increasing rotation speed.

AA2219-T87 alloy was self-reacting friction stir welded at different traverse speeds. Joints produced at low traverse speed exhibit tunnel defects. The tunnel defects can be avoided by higher traverse speed, but excessive increase in traverse speed leads to the joint line remnants presenting in a linear geometry which comprises strength of the joint by providing an easier path for crack propagation. In the weld nugget zone, grains in the lower layer are coarser than those in the upper layer due to smaller extent of heat dissipation through the bottom shoulder. Extensive softening occurs in the weld zone owing to dissolution of the θ precipitates. Tensile strength of the defect-free joint reaches 74% of that of the base metal.

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