Microstructural Evolution and Mechanical Performance of Dissimilar AA7075-T651/AA2024-T3 Friction Stir Welds: Influence of Weld Configuration, Pass Number, and Post-Weld Heat Treatment

: This study investigates the effects of weld configuration (WC: single-sided and double-sided), welding pass number (WPN: single and two-pass), and post-weld heat treatment (PWHT) on the microstructure and mechanical performance of dissimilar AA7075-T651/AA2024-T3 friction stir welded (FSWed) joints produced under identical processing parameters. The double-sided (DS) configuration improved material flow and reduced internal defects due to enhanced stirring symmetry, but was less effective in achieving grain refinement in the stir zone compared with single-sided (SS) welding. SEM/EDS analysis revealed that the second pass (2P) was beneficial only for the SS configuration, whereas in DS friction stir welding (FSW) it caused excessive localized heating, leading to the formation of Cu-rich (θ-Al 2 Cu) intermetallic compounds (IMCs) and microstructural degradation. PWHT significantly improved microstructural homogeneity and grain refinement in single-pass (1P) conditions for both configurations, resulting in enhanced mechanical performance. The HT-1P-DSFSW joint exhibited the best performance, achieving an ultimate tensile strength (UTS) of 419.5 ± 17.4 MPa, elongation of 5.92%, and joint efficiency (JE) of 86.4% relative to AA2024-T3. Hardness mapping indicated that joint reliability correlates more strongly with hardness uniformity than with peak stir-zone hardness.