Analysis of microstructure and mechanical properties of refill friction stir spot welded aluminum alloy

7075-T6 aluminum alloy was successfully spot joined by refill friction stir spot welding process. The microstructure, microhardness,tensile/shear strength and cross-tension strength of the joints were tested. Experimental results indicate that the microstructure can be divided into the nugget zone,thermo-mechanically affected zone,heat-affected zone and base material, respectively. Defects such as hook,cavity,incomplete fusion,incomplete refill and bonding ligament were found in the joint. The microhardness profile in the welded zone exhibited Wshaped appearance,but that in the stir zone exhibited V-shaped appearance. When the rotational speed was 1 400 r/min,the highest tensile/shear strength reached 7 916.0 N,which was 39.6% of the base material. The change of the cross-tension strength with the processing parameters was complicated,and the highest cross-tension strength can reach 43.9 MPa.