Microstructural evolution and strengthening mechanisms in multi-pass friction stir additive manufacturing of Al6061-Al5083 dissimilar builds: A study of overlapping thermo-mechanical zones

Abstract

This study investigates the microstructural evolution and mechanical properties of a five-layer friction stir additive manufacturing (FSAM) build composed of alternating layers of Al6061 and Al5083. A significant microstructural gradient along the build direction is observed along the build direction. Fine, equiaxed grains and dynamic precipitate transformations collectively influence the mechanical strengths observed throughout the build. EBSD analysis reveals a pronounced grain size variation, with the finest grains located in the pin-affected zone(PAZ) + PAZ regions where intense deformation and dynamic recrystallization occur, and the coarsest grains found in the bottom PAZ, which undergoes a lower degree of deformation upon the initial passes. HRTEM reveals fine, coherent β″ precipitates in regions subjected to severe shear, while larger β precipitates form in areas exposed to prolonged thermal exposure. Repeated heating cycles, driven by incremental increases in peak and baseline temperatures with each pass, further contribute to local transformations in both precipitate and dispersoid populations. Tensile testing of interfacial samples (T1 to T4) shows yield strengths (YS) ranging from 105 to 129 MPa and ultimate tensile strengths (UTS) from 229 to 256 MPa, which are consistently lower than those of the base materials. These reduced values are a result of localized deformation and repeated thermal cycles. A combination of grain boundary strengthening, dislocation strengthening, and precipitation hardening mechanisms contributes to the anisotropic mechanical properties observed in these multi-material FSAM builds. This study advances fundamental understanding of importance of repeated overlapping thermo-mechanical cycles on dynamic recrystallization, precipitation kinetics, dislocation evolution, and strengthening behaviour in multi-layer FSAM builds.