High Strength Al−Mg−Sc−Zr Alloy with Heterogeneous Grain Structure and Intragranular Precipitation Produced by Laser Powder Bed Fusion

In this study, the intrinsic heterogeneous grain structure derived from the special temperature gradient in additive manufacturing and the intragranular precipitation were utilized for constructing a high-strength Al−Mg−Sc−Zr alloy. The alternative distribution of the coarse-grained (CG) layer in the middle of the molten pool and ultrafine-grained (UFG) layer at the boundary between the adjacent two molten pools were produced by using laser powder bed fusion (L-PBF) technology. High contents of Sc and Zr elements were introduced into the Al−Mg alloy, which contributed to the stabilization of UFG boundaries in the form of sub-micro primary Al3(Sc, Zr) particles and resulted in the formation of supersaturated Sc and Zr solid solutions in CG. The afterward aging treatment or well-controlled hot isostatic pressing (HIP) induced uniform precipitation of the high-density nano-scale coherent Al3(Sc, Zr) phase in CG. The HIP also promoted a significant decrease in porosity of the alloy and resulted in a combination of high ultimate tensile strength of 560 MPa and elongation of 12 % of the L-PBF Al−Mg−Sc−Zr alloy. The high strength of the alloy was attributed to the hierarchical formation of the Sc-rich phase, exerting a “dragging effect” on intergranular boundary migration as well as a “pinning effect” on intragranular dislocation sliding.