Effect of rotation speed on microstructure and mechanical properties of refill friction stir spot welded 6061-T6 aluminum alloy

Refill friction stir spot welding (RFSSW) has been used to weld 6061-T6 aluminum alloy, and keyhole free joints were successfully obtained. Effect of tool rotation speed on microstructure and mechanical properties of joint was investigated. The joint was divided into four zones, i.e., the base material (BM), heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and stir zone (SZ) according to microstructural evolution. Defect was not found on the surface of welded joint, but inner defects of partial bonding, bonding ligament, hook, and voids were observed within the welded joint. The microhardness of HAZ and TMAZ was lower than that of the BM, while the maximum microhardness was obtained in the SZ of welded joints. With increasing tool rotation speed from 1100 to 1700 rpm, microhardness decreased with increasing grain size in the weld. The maximum tensile shear failure load of 7522 N was obtained for the joint under tool rotation speed of 1500 rpm. Two different failure modes of plug fracture and tensile-shear mixed fracture were observed during the tensile shear tests.