Micro/nanostructure evolution and deformation mechanisms in friction-stir-welded 7075 Al alloy: A comparative analysis of weld zones

We investigated the micro/nanostructure evolution and deformation behavior of each weld zone—base material (BM), heat-affected zone (HAZ), thermomechanically affected zone (TMAZ), and stir zone (SZ)—in friction-stir-welded (FSWed) 7075 Al alloys. The HAZ maintained a microstructure similar to the BM, whereas the TMAZ showed a high dislocation density due to severe plastic deformation and insufficient dynamic recrystallization (DRX). In contrast, the SZ exhibited significant grain refinement, driven by sufficient DRX from high frictional heat during welding. The yield strengths (YS) of the HAZ, TMAZ, and SZ were 248 MPa, 278 MPa, and 292 MPa, respectively—lower than that of the BM (515 MPa)—due to coarsening and re-dissolution of strengthening precipitates. However, the SZ demonstrated improved elongation (16.1 %) compared to the BM (14.0 %), indicating enhanced ductility. Fracture analysis revealed a mixed brittle-ductile failure mode in the BM, HAZ, and TMAZ, while the SZ showed predominantly ductile fracture, attributed to reduced dislocation density and precipitate re-dissolution from DRX. Among all zones, the precipitation strengthening contributed most significantly to overall strength (BM: ∼82.5 %, HAZ: ∼69.7 %, TMAZ: ∼61.5 %, SZ: ∼63.0 %). Dislocation strengthening was most pronounced in the TMAZ, while grain boundary strengthening was key in the SZ due to grain refinement. Strain hardenability was greater in the HAZ, TMAZ, and SZ than in the BM, with the SZ showing the highest hardenability due to its refined grain structure and lower dislocation density. These findings highlight the distinct strengthening mechanisms across weld zones and the importance of microstructural evolution on mechanical performance.