Precipitation behavior, microstructure and mechanical properties of Al-4.8Mg-0.82Sc-0.28Zr alloy fabricated by selective laser melting

In this study, Al-4.8Mg-0.82Sc-0.28Zr alloy was fabricated by selective laser melting. The formability was explored and the microstructure, precipitation behavior and mechanical properties were systematically investigated. The optimal scanning speeds and hatch spacing are in the range of 1000-1200 mm/s and 0.12-0.14 mm, respectively. The specimens exhibited different densification behaviors at the same laser volumetric energy density, and the forming quality was better at the relatively small scanning speed. The alloy showed a typical bimodal microstructure with fine equiaxed grains and coarse columnar grains distributed at the boundary and center of the melt pool, respectively. Benefited from the high Sc content, the grains were significantly refined with the average size of 0.86 and 2.34 mu m for coarse and fine grain regions. A significant number of Al-3(Sc, Zr) particles presented in fine grain region and were responsible for the formation of equiaxed grains and the improvement of the mechanical properties of the specimens. Besides, there were also high density of Al-Mg oxides, Fe-rich and Mn-rich phases uniformly distributed in the specimen. The tensile strength, yield strength and elongation of the specimen fabricated using the optimal process parameter were 344.20 MPa, 290.72 MPa and 24.5%. The bimodal microstructure, solid solution strengthening, grain boundary strengthening and precipitation strengthening enabled the specimens to achieve strength-ductility synergy.