Microstructural and Mechanical Properties of Dissimilar AA7075 and AA2024 Rotary Friction Weldments

This study aims to explore the effects of various pre- and post-weld heat treatments (PWHTs) on the microstructural and mechanical properties of dissimilar aluminium alloys, namely AA7075 and AA2024, joined through rotary friction welding. The joints were rigorously evaluated through multiple characterization methods, revealing no signs of cracking or incomplete bonding. This study observed that dissimilar joints between AA7075 and AA2024 alloys showed increased flash formation on the AA7075 side due to its lower melting point relative to the AA2024 alloy. Various zones within the weld region were identified, such as the dynamic recrystallized zone (DRZ), the thermo-mechanically affected zone (TMAZ)—which includes TMAZ-1 with elongated grains and TMAZ-2 with compressed or distorted grains—the heat-affected zone (HAZ), and the base metal (BM) zone. Of all the welding conditions examined, the post-weld heat-treated (PWHT) AA2024/AA7075 joint produced by rotary friction welding showed the highest strength, with a yield strength (YS) of 305 ± 2 MPa and an ultimate tensile strength (UTS) of 477 ± 3 MPa. This improvement in strength can be attributed to the significant strengthening precipitates of MgZn2 (found on the AA7075 side), θ-Al2Cu, and S-Al2CuMg (found on the AA2204 side) formed during post-weld ageing. Notably, all dissimilar welds failed in the HAZ region on the AA2024 side due to coarse grain formation, identifying this as the weakest area.