Laser powder bed fusion of Al-Mg-Zn-Cu-Si-Sc-Zr alloy synergistically strengthened by Mg2Si, L12-Al3(Sc, Zr), and η′ multiple nano-particles

Abstract

During the additive manufacturing process, Al-Zn-Mg-Cu alloys are susceptible to cracking. A specific Al-6.2Mg-1.95Zn-0.6Cu-3Si-0.35Sc-0.15Zr-0.4Mn alloy was developed for laser powder bed fusion (LPBF) technology. The addition of Si, Sc, and Zr improved the printability of the Al-Mg-Zn-Cu alloy. An excellent tensile strength of 600 ​MPa and a ratio of yield stress to ultimate stress of 0.75 were obtained. The as-printed sample showed bimodal microstructures with fine equiaxed grains at the molten pool boundary and coarse columnar grains within the melt pool along the vertical direction. Various precipitates, including β-Mg₂Si, L1₂-Al₃(Sc, Zr), and η′ were observed, resulting in high-strength aluminum alloys. The mechanism of segregation of Sc, Zn and other alloying elements was revealed by first-principles calculations, and the binding energy of Si-Sc atomic pairs was found to be as high as 0.093 ​eV/pair at the nearest neighbor distance. The TEM and DFT results revealed that the η′-Mg (Cu_1.5Zn_0.5) structure, formed by three Cu atoms co-substituting Zn atoms, exhibited the lowest formation energy and represented the most thermodynamically stable η′ phase discussed in this study. This study provided the theoretical foundation for composition modification and precipitation of Al-Zn-Mg-Cu fabricated through laser powder bed fusion (LPBF).