Understanding effect of Mg and Mn equivalents on columnar-to-equiaxed transition of high-strength Al alloy fabricated by laser powder bed fusion

In this work, a Mg-riched/Mn-riched high strength Al graded alloy was fabricated by high-throughput laser powder bed fusion process. The effect of variation from Mg to Mn on the microstructure and mechanical properties of high strength Al alloy was investigated. A control of columnar to equiaxed grain structure transition in high strength Al alloy was achieved by adjusting the ratio of Mn to Mg element. Based on the kinetics simulation, the increase of Mn solutes can shorten the incubation time of Al nucleation and generate sufficient undercooling, which favors the formation of the equiaxed grains. A significant increase in strength from 362.2 MPa to 495.4 MPa can be obtained by the substitution of Mn to Mg in printed Al alloy due to the microstructure refinement and the precipitation strengthening of Al6Mn. An ageing treatment can improve the overall strength because of secondary phase precipitation of Al3Sc and Al6Mn. A strength/ductile trade off mechanical properties of UTS 564.7 MPa and Elongation 8.5% can be found with the Al-3.8Mn-2.8 Mg-0.78Sc-0.57Zr component.