Abstract: 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.

Key words: Additive manufacturing, Aluminium alloy, Thermodynamic, Kinetic, In-situ tensile