Microstructural and mechanical gradient mappings across ND–TD planes of ultra-thick 6061-T651 FSW joints

Welding of thick Al alloy plates is among the critical technologies for advanced manufacturing, wherein the gradient evolution of microstructures and mechanical properties is a key factor determining joint performance. In this study, friction stir welding (FSW) of a 40 mm ultra-thick 6061-T651 Al alloy was investigated through a multiscale characterization approach involving OM, XRD, SEM, EDS and EBSD within the ND–TD planes, complemented by microhardness testing and layered/partitioned tensile measurements. The results indicated that the pin affected zone (PAZ) exhibited sufficient dynamic recrystallization, significant grain refinement, and the highest proportion of high-angle grain boundaries (HAGBs). In contrast, the heat-affected zone (HAZ) had the lowest hardness (47.8 HV) because of grain coarsening, making it the fracture initiation site. The thermo-mechanically affected zone (TMAZ) demonstrated notable microstructural heterogeneity, with a low-angle grain boundary (LAGB) fraction exceeding 80%. The distribution of average grain sizes across joint zones (an “M” shape) and the variation in the HAGB proportion (a “W” shape) correlated strongly with the “W”-shaped microhardness profile. The tensile properties exhibited significant layer dependence, wherein the strength of the top layer was the highest (198.3 MPa), whereas the strength of the middle-layer HAZ was the lowest (186.6 MPa), and the joint efficiency ranged from 65.6% to 76.0%. This study establishes a quantitative model of structure–property relationships and proposes a novel strategy for gradient-driven design and performance optimization in ultra-thick Al alloy FSW joints.