Microstructural evolution and mechanical properties of deposit AA6061 aluminum alloy processed by additive friction stir deposition

Additive friction stir deposition (AFSD) is a solid-state additive manufacturing technique with potential for fabricating large-scale aluminum alloy structures while avoiding melting–solidification. In this study, a multifactorial design was used to identify an optimal parameter set for AA6061, and a ten-layer block was fabricated for microstructure/precipitate characterization and mechanical evaluation. Results indicate that the optimal parameters are 500 rpm and 120 mm/min. The deposit exhibits a fully refined equiaxed-grain structure with an average grain size of 5–8 μm, together with pronounced dislocation multiplication. Fine Al(MnCrFe)Si particles are dispersed in grain interiors, while coarsened β (Mg₂Si) precipitates are present along grain boundaries and within grains. The multilayer block shows a hardness range of 42.5–69.6 HV0.1 and near-isotropic tensile properties among BD/LD/TD, with UTS = 219.6 ± 5.0 MPa and elongation = 21.85 ± 2.71% along BD. These results provide a processing-window reference for dimensionally reliable AFSD build-up and repair of AA6061 components requiring low defect sensitivity and near-isotropic performance.