In-situ Mg2Si phase and post-process Al3(Sc,Zr), Al6Mn phases for enhanced strength-ductility performance for laser powder bed fused Al-Mg-Sc-Mn-Zr alloy

This study systematically investigates microstructure evolution and mechanical properties in laser powder bed fused Al-Mg-Sc-Mn-Zr alloys through integrated thermodynamic analysis and experimental characterization. Scheil solidification calculations reveal that increasing Sc and Zr content elevates the primary Al3(Sc,Zr) formation temperature from 809.1 to 863.8 °C while Mn and Si additions promote Al6Mn and Mg2Si phase formation with volume fractions of 3.46 % and 17.5 %, respectively. The as-deposited condition exhibits a unique bimodal grain structure consisting of 0.64 µm fine grains along melt pool boundaries and 1.84 µm coarse grains in pool interiors. During direct aging treatment, numerous secondary Al3(Sc,Zr) precipitates with an average radius of 1.21 nm form. Thermal history simulations demonstrate that Mg2Si precipitates in-situ due to prolonged exposure above 130 °C, while Al6Mn requires post-process heat treatment for nucleation. The direct-aged alloy achieves significantly enhanced mechanical properties, with yield strength and ultimate tensile strength reaching 633 and 647 MPa, respectively, while maintaining an elongation of 13.8 %, representing 37.9 % and 33.7 % improvements in strength over the as-deposited condition. In-situ tensile testing reveals that cracks initiate at fine-coarse grain interfaces due to strain incompatibility and the presence of brittle Al6Mn phases. These findings provide fundamental insights into phase selection and precipitation kinetics during additive manufacturing, establishing a new pathway for developing high-performance aluminum alloys through controlled multi-phase engineering. The demonstrated approach of combining in-situ and ex-situ precipitation achieves an optimal balance between strength and ductility in additively manufactured aluminum components.