Effects of three different post-weld heat treatments on microstructure and properties of 7075/6061 friction stir welded joints

• Solution-aging treatment improved tensile strength of 7075/6061 FSW joints from 207 MPa to 290 MPa, with joint strength coefficient rising from 67 % to 94 %. • Post-weld heat treatment eliminates onion ring structure, improves hardness uniformity and causes abnormal grain growth in the stir zone (3 μm to 50 μm). • Solution-aging treatment enhances joint strength via Mg 2 Si precipitation, improves microstructure uniformity and reduces stress concentration of the joints. This study examines the effects of three post-weld heat treatment (PWHT) processes: solution treatment (ST), artificial aging (AA), and solution-aging treatment (STA) on the microstructure and mechanical properties of 7075/6061 dissimilar aluminum alloy friction stir welded joints. Compared to untreated joints (UP) with a tensile strength of 207 MPa, STA joints achieve 290 MPa, a 40.10 % improvement. The joint strength coefficient rises from 67 % to 94 %, and elongation increases from 2.49 % to 3.07 %. The ST joint exhibits the highest elongation (5.42 %), while the AA joint has the lowest (1.55 %). Fractures occur in the HAZ on the 6061 side for all joints. PWHT improves strength and microhardness while eliminating the “onion ring” structure. Grain coarsening occurs in the stir zone (SZ), increasing from 3 μm (UP) to 50 μm (STA). The strength increase of the STA joint is mainly attributed to the high-density precipitation behavior of Mg 2 Si phase in the 6061. Texture analysis shows that the SZ of the UP and STA joints have higher and lower shear strain microstructures, respectively. The STA treatment significantly improves the uniformity of the microstructure and hardness, thereby significantly reducing the degree of stress concentration and enhancing the joint strength.