Microstructural evolution and fatigue response of friction stir welded hybrid aluminum joints after T6 heat treatment

Build-volume constraints in additive manufacturing motivate hybrid assemblies that join additively manufactured features to conventionally produced parts, combining the complementary strengths of both fabrication routes. A comprehensive understanding of the mechanical behavior of such hybrid joints, particularly under cyclic loading conditions, is essential for a reliable application of such components. In this work, the impact of a post-welding T6 heat treatment on the microstructure and mechanical properties of a friction stir welded joint of additively manufactured AlSi10Mg and cast AlSi8MnMg was analyzed. Microstructurally, the heat treatment led to abnormal grain growth in the weld zone. While the silicon phases transformed into a globular morphology throughout most parts of the joint, they preserved their coarse, needle-like appearance in the cast region. Artificial aging yielded a mostly homogeneous hardness distribution throughout the joint. Compared to an as-welded condition, increased yield and tensile strengths and improved fatigue performance were observed. However, the macroscopic ductility remained limited due to the brittle cast zone. Failure under low-cycle fatigue loading was primarily governed by defects in the cast region, which served as the main sites for crack initiation. The results demonstrate that heat treatment can effectively reduce potentially undesirable heterogeneity, thereby enhancing joint performance for specific applications.