Friction stir welding between 5052 aluminum alloy plates with a thickness of 2 mm was performed. The tool for welding was rotated at speeds ranging from 500 to 3000 r/min under a constant traverse speed of 100 mm/min. The results show that at all tool rotation speeds, defect-free welds are successfully obtained. Especially at 1000, 2000 and 3000 r/min, the welds exhibit very smooth surface morphologies. At 500, 1000, and 2000 r/min, onion ring structure is clearly observed in the friction-stir-welded zone (SZ). In addition, the onion ring structure region becomes wider as the tool rotation speed is increased. The gain size in the SZ is smaller than that in the base metal, and is decreased with a decrease of the tool rotation speed. In all tool rotation speeds, the SZ exhibits higher average hardness than the base metal. Especially at 500 r/min, the average hardness of the SZ reaches a level about 33% greater than that of the base metal. At 500, 1 000 and 2 000 r/min, the tensile strength of the friction stir welded (FSWed) plates is similar to that of the base metal (about 204 MPa). The elongation of the FSWed plates is lower than that of the base metal (about 22%). However, it is noticeable that the maximum elongation of about 21% is obtained at 1 000 r/min.
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