Microstructures and mechanical properties of aluminum alloy joint produced by friction stir welding

The growing demand for lightweight materials in the automotive industry leads to the increased use of integrated die-cast aluminum (Al) alloys in structural components. However, the friction stir welding (FSW) behavior of these alloys remains inadequately understood. This study focuses on the non-heat-treatable THAS-1 Al alloy, investigating the influence of process parameters on thermal cycles, microstructural evolution, and joint performance. The results indicate that optimized parameters (1600 rpm, 100 mm/min) effectively reduce flash formation and promote the development of distinct onion-ring structures, which indicate improved material mixing. Microstructural analysis reveals gradient features across the joint: the stir zone (SZ) suffers complete dynamic recrystallization, significantly refining the eutectic silicon particles (1.5–2.0 μm), while the thermo-mechanically affected zone (TMAZ) exhibits partial recrystallization and shear-dominated textures ({111}〈112〉 on the retreating side and {111}〈110〉 on the advancing side). Increased heat input results in a widening of the TMAZ, grain coarsening in the SZ, and the growth of eutectic silicon particles. Mechanical properties exhibit strong correlations with these microstructural features: hardness follows a characteristic W-shaped profile, peaking in the TMAZ (125 HV) due to dislocation accumulation, while the SZ exhibits softening (<65 HV) due to recrystallization. Joints produced under 1600 rpm–100 mm/min achieve an optimal strength–ductility balance (UTS: 203.6 MPa, 82.1% of the base metal; elongation: 8.6%), with fracture occurring through ductile failure. These findings enhance the understanding of the process–microstructure–property relationships in FSW die-cast Al-Si alloys, offering insights for their application in integrated automotive structures.