The dual role of Mg in LPBF-processed high-strength AlMnScZr alloys: strengthening and thermal softening mechanisms

The dual roles of Mg in the room-temperature strengthening and high-temperature softening of LPBF-Processed AlMnScZr alloys are systematically elucidated. The addition of Mg promotes compositional supercooling during solidification, transforming the as-built columnar grains of the Mg-free alloy into a refined bimodal structure comprising alternating coarse and fine equiaxed regions. Mg also accelerates the formation of primary Al6Mn precipitates, increasing their size from ∼43.7 nm in the Mg-free alloy to 54.0 and 114.6 nm in alloys containing 1.5 and 3.5 wt.% Mg, respectively. During aging, Mg addition significantly enhances the dispersion and number density of secondary Al6Mn particles, while exerting negligible influence on the formation of nanoscale Al3(Sc, Zr) precipitates (∼3 nm). The synergistic effects of solid-solution strengthening, precipitation hardening, and grain refinement yield superior room-temperature properties, achieving ultimate tensile strengths of 538 MPa (1.5 wt.% Mg) and 626 MPa (3.5 wt.% Mg). However, Mg addition also accelerates thermal softening in the AlMnScZr alloy. For instance, the yield strength at 300 °C drops to 108 MPa in the 3.5 wt.% Mg alloy, compared to 198 MPa in the base alloy. This strength loss is identified as being due to Mg-promoted coarsening of Al6Mn particles during thermal exposure.