The influence of the heat treatment on the microstructure evolution and strengthening mechanism of the Al–Mg–Mn–Sc–Zr alloy via selective laser melting

In this study, Al-Mg-Mn-Sc-Zr alloy was prepared using selective laser melting (SLM). The research focused on the evolution of the microstructure and the changes in mechanical properties under various heat treatment temperatures. Additionally, the mechanisms behind the strength and toughness of both the as-printed and heat-treated samples were analyzed. The microstructure exhibits a bimodal microstructure characterized by alternating fine coarse regions. Sc/Zr elements preferentially segregate to grain boundaries to form Al 3 (Sc, Zr) phases, serving as potent heterogeneous nucleation sites, refining the α-Al matrix and contributing to precipitation strengthening. After aging heat treatment, the mixed crystal structure and the grain size in the equiaxed crystal region remained largely unchanged, while the grains in the columnar region showed signs of coarsening. Aging treatment facilitated the diffusion-mediated redistribution within the matrix, leading to the nanoscale precipitation of Al3(Sc, Zr) phases with an L1 2 structure. These phases contributed to significant precipitation strengthening, enhancing the overall mechanical properties. The ultimate tensile strength of 480 MPa was achieved at 330 °C/5h, representing a 34 % enhancement over the as-printed condition, The dislocation density decreased by solution aging treatment, reducing the grain boundary's hindering effect and significantly lowering toughness, with a reduction to just 9.3 %. Synergistic strengthening mechanisms are identified: (i) grain refinement from Al 3 (Sc,Zr) nucleants, (ii) solid solution hardening, and (iii) L1 2 nanoprecipitation.