Effects of pre-weld 6061 base metal composition and post-weld heat treatment on microstructure and mechanical properties of 7075/6061 dissimilar aluminum alloy FSW joints

This study systematically investigates the influence of four different 6061 base metal compositions and post-weld heat treatment (PWHT) processes on the microstructure and mechanical properties of 7075/6061 dissimilar aluminum alloy friction stir welded (FSW) joints. The results indicate: Prior to PWHT, the position and size of the “elliptical onion ring” structures formed in the stir zone (SZ) of 7075/6061 FSW joints varied significantly depending on the 6061 base metal composition, with notable differences in material mixing and flow behavior. Joints made with 6061 base metals having either high Mg/Si ratio with low total alloy content or low Mg/Si ratio with high total alloy content exhibited superior metal flow characteristics. All as-welded joints developed fine equiaxed grains through dynamic recrystallization in the SZ, with no significant difference in average grain size. The joint made with high Mg/Si ratio and low alloy content 6061 base metal demonstrated tensile strength and elongation of 214 MPa and 6.11 %, respectively. After PWHT, abnormal grain growth (AGG) occurred in the SZ of all joints. Joints with high Mg/Si ratio 6061 developed grains exceeding 500 μm in maximum dimension, showing strong texture. PWHT significantly improved the mechanical properties of joints with low Mg/Si ratio 6061 base metal (BM). In particular, the joint with high alloy content showed reprecipitation of dissolved Si-rich phases and Mg₂Si phases, which were uniformly distributed in the Al matrix with weaker texture. Its tensile strength increased from 207 MPa to 290 MPa, and elongation improved from 2.49 % to 3.07 %, respectively. For 6061 BM with high Mg/Si ratio and low Si content, PWHT failed to enhance joint mechanical properties. The combination of pre-weld 6061 base metal with low Mg/Si ratio and high total alloy content, coupled with appropriate PWHT, can significantly improve the microstructural stability and mechanical properties of 7075/6061 FSW joints, demonstrating the greatest potential for performance enhancement.