Effect of Post-Welded Heat Treatments on Microstructure and Mechanical Properties of Friction Stir Welded Joints of 7A04-O Aluminum Alloy

Effect of post-welded heat treatment (PWHT) on the microstructure and mechanical properties of friction stir welded 7A04-O aluminum alloy was investigated. Solution heat treatment (ST) and artificial aging (T6) were given to specimens to improve the mechanical properties of the joints. The results show that defect-free joint was obtained via FSW and the microstructure of nugget zone was characterized by fine and equiaxed grains. After ST, most second phases dissolve in the matrix and the α-Al grains became coarse. Second phases precipitated in the α-Al matrix uniformly after T6 treatment. The Vickers hardness test shows that PWHT made the hardness distribution of the joints more evenly. The tensile test results indicate that the tensile strength of the joint increased from 228 MPa (in as-welded condition) to 440 MPa and 528 MPa after ST and T6 treatment respectively, but the elongation decreased to a certain extent. SEM fracture morphologies indicate that the joint failed through ductile fracture for the as-welded and ST states, and failed in a mixture mode of brittle fracture and ductile fracture for the T6 state.