Influence of base-material placement configuration on material flow, microstructure and failure behavior of friction stir welded THAS-1/AA6082-T6 joints

• Material placement configuration dominates thermal asymmetry and intermixing in dissimilar FSW. • AA6082-T6 undergoes CDRX, while THAS-1 shows recovery due to Si-particle pinning. • Placement controlled microstructure leads to distinct hardness and softening patterns. • Material placement configuration determines strain localization and ductile brittle transition in fracture. This study examines FSW of dissimilar aluminum alloys, i.e., high-pressure die-cast Al-Si THAS-1 and precipitation-hardenable AA6082-T6, to clarify how material placement and process parameters influence microstructural evolution and mechanical properties. Placing AA6082-T6 on the advancing side (AS) enhanced material mixing in the stir zone (SZ) and promoted well defined onion-ring banding, yielding the best strength ductility balance. At 1600 rpm and 250 mm·min⁻¹, the joint achieves a yield strength of 146.0 MPa, an ultimate tensile strength of 247.2 ± 6.87 MPa, and an elongation of 9 ± 0.35%. This ultimate tensile strength corresponds to 99.7% and 73.5% of the ultimate tensile strengths of the THAS-1 and AA6082-T6 base materials, respectively. Electron backscatter diffraction and transmission electron microscopy analyses reveal that the SZ on the AA6082-T6 side experiences continuous dynamic recrystallization, while the THAS-1 side is governed by dynamic recovery. The shoulder-affected zone (SAZ) features relatively coarser and more uniform grains, whereas the pin-affected zone (PAZ) shows finer but heterogeneous grains. Both thermo-mechanically affected zones (TMAZ) retain high dislocation densities and subgrain structures. Hardness profiles display through-thickness and transverse gradients, with the lowest values occurring in the AA6082-T6 heat-affected zone due to over-aging caused by dissolution and coarsening of β″/β′ precipitates. Digital image correlation demonstrates that material placement configuration dictates strain localization and fracture behavior: AA6082-T6 on the AS leads to banded strain concentration in the softened AS-HAZ and ductile fracture, while THAS-1 on the AS shifts localization toward the AS-TMAZ and induces brittle failure. These findings underscore the essential role of spatial configuration in achieving strength-ductility synergy in dissimilar aluminum alloy FSW joints.