Microstructure evolution and tensile deformation behavior of an ultrahigh-strength Al-Mg-Mn-Sc-Zr alloy fabricated by laser powder bed fusion

Abstract

In recent years, it has been proved to be an effective strategy to obtain high strength and ultrahigh-strength Al alloys by laser powder bed fusion (LPBF). However, these ultrahigh-strength LPBFed Al alloys are difficult to achieve good ductility. In this work, high content of Sc/Zr modified Al-Mg-Mn-Sc-Zr alloy was prepared by LPBF. The high content of Sc/Zr brought unique heterogeneous microstructure and further affected the tensile deformation behavior of the alloy. It was found that the precipitation of primary Al₃(Sc,Zr) phase significantly increased the number of nucleation sites at the front of the liquid-solid interface and promoted the columnar-to-equiaxed transition (CET), leading to a microstructure of near-full equiaxed grains with a grain size range from 0.55 μm to 1.10 μm. Tensile tests showed that the yield strength of the as-built sample was 442 MPa, and the strengthening effect brought by ultrafine grains plays a leading role, which is difficult to achieve for the Al-Mg-Mn-Sc-Zr alloy with low Sc/Zr content. After aging treatment, the yield strength of the LPBFed Al-Mg-Mn-Sc-Zr alloy reached up to 613 ± 5 MPa under the combined effects of grain-boundary strengthening and precipitation strengthening. The decrease of work-hardening capacity induced a longer Lüders elongation after aging, but the reliable dislocation accumulation ability of coarse grains still existed, which finally made the elongation of the as-aged sample reach 11.7 ± 0.5 %.