Precipitation behavior and strengthening-toughening mechanism of additive friction stir-deposited Al–Mg–Si–Cu alloy

An 18-layer thick-walled Al–Mg–Si–Cu alloy part with a width of 32 mm and single-layer thickness of 4 mm was fabricated using force-controlled additive friction stir welding. The evolution of the grain structure and second-phase particles during multilayer deposition was investigated in detail. After being subjected to multi-pass thermal cycles and plastic deformation, the recrystallization fraction and grain size of the final deposits were almost unaffected. Interface grain refinement was more pronounced, with grain sizes reaching 3.4–4.2 µm, owing to greater recrystallization compared to the interlayer. The reprecipitated Q' and β' precipitates, which were coherent or semicoherent with the aluminum matrix and attached to the diffusely distributed spherical Al(MnCrFe)Si dispersoids, played a critical role in strengthening the final deposits. After repeated heat input and plastic deformation thrice or more, the hardness and tensile properties of the deposits remained unchanged because the quantity and distribution of the precipitates did not vary. The average tensile strength (Rm) and elongation (A) after fracture of the final deposits along the tool traverse direction could reach 182.4 MPa and 37.3%, respectively. The mean values of Rm and A along the build direction reached 92.2% and 50.5% of those along the tool traverse direction. Owing to the refined grains produced at the metallurgically bonded interface, fine diffusely distributed dispersoids, and coordinated deformation of alternating coarse and fine-grain regions, the tear toughness of the as-printed deposits could reach 132.4–208.7% of that of the wrought 6061-T6 Al alloy.