Optimizing Post Friction Stir Welding Heat Treatment in Dissimilar Friction Stir Welded 6061/AlSi10MgMn Joints for Electrical Vehicle Battery Housing

This study systematically investigates the microstructural features and Vickers microhardness evolution of friction stir welded dissimilar joints between rolled AA6061‐T6 and high‐pressure die‐cast AlSi10MgMn alloys. Optical microscopy revealed that the AA6061‐T6 base material contains elongated α‐Al grains (54 ± 6 µm), which recrystallized into fine, equiaxed grains within the stir zone due to dynamic recrystallization. The AlSi10MgMn alloy exhibited a characteristic layered architecture consisting of skin, eutectic band, and core regions, with α‐Al grain size decreasing from ∼6 µm in the core to ∼3 µm in the skin. Severe plastic deformation during friction stir welding (FSW) fragmented and spheroidized Si‐eutectic particles, yielding a more homogeneous phase distribution in the stir zone. Despite this grain refinement, precipitate dissolution resulted in a ∼30% reduction in Vickers microhardness relative to the base materials. Postweld direct‐aging treatments at 170°C–220°C were applied to restore mechanical uniformity, with the optimal condition of 200°C for 4 h minimizing hardness variation (ΔHV ≈ 47 HV _{0. 2} ) without inducing grain coarsening. These findings provide quantitative insight into microstructural evolution and postweld aging in dissimilar FSW joints, supporting improved property uniformity for electrical vehicle battery housing applications.