Superior strength-ductility synergy in a selective-laser-melted Al-Zn-Mg-Cu-Sc-Zr alloy enabled by Zn/Mg ratio optimization and synergistic precipitation

• The 7 series aluminum alloy with balanced strength and plasticity (tensile strength 678 MPa, elongation 9 %)was obtained by adjusting the Zn / Mg ratio through composition design; The high zinc aluminum alloy is designed by composition, which makes the tensile strength up to 688 MPa. • Through microstructure analysis and strength calculation, it is revealed that its excellent mechanical properties are mainly provided by precipitation strengthening and deformation strengthening caused by dislocation density changes. The Zn/Mg ratio and precipitated phase is a critical in controlling hot cracking and mechanical properties of Al–Zn–Mg–Cu–Sc–Zr alloys fabricated by selective laser melting (SLM). This study systematically examines the effects of Zn/Mg ratios on the microstructure and mechanical performance of crack-free alloys after T6 heat treatment. Thermo-Calc simulations were employed to predict phase stability and interpret non-equilibrium precipitation behavior. A lower Zn/Mg ratio (2.67) promoted Mg segregation, while a higher ratio (3.67) caused Al–Zn eutectic formation. The alloy with a Zn/Mg ratio of 3 exhibited the best balance of strength and ductility, achieving a tensile strength of 678 ± 10 MPa, yield strength of 667 ± 8 MPa, and elongation of 9 ± 1.7 %. Quantitative analysis revealed that grain refinement and precipitation dominated the strengthening contributions. EBSD results showed average grain sizes of 4.16, 1.82, and 4.87 μm for the respective Zn/Mg ratios, indicating pronounced refinement and a non-monotonic size evolution. This work provides experimental and theoretical insights for designing high-performance SLM Al–Zn–Mg–Cu alloys with tunable microstructures and properties.