Influence Mechanism of Process Parameters on Relative Density, Microstructure, and Mechanical Properties of Low Sc-Content Al-Mg-Sc-Zr Alloy Fabricated by Selective Laser Melting

Additive manufacturing of Al-Mg-Sc-Zr alloys is a promising technique for the fabrication of lightweight components with complex shapes. In this study, the effect of the process parameters of selective laser melting (SLM) on the surface morphology, relative density, microstructure, and mechanical properties of Al-Mg-Sc-Zr high-strength aluminum alloys with low Sc content was systematically investigated. The results show that the energy density has an important effect on the surface quality and densification behavior of the Al-Mg-Sc-Zr alloy during the SLM process. As the energy density increased, the surface quality and the number of internal pores increased. However, the area of the fine-grained region at the boundary of the molten pool gradually decreased. When the laser energy density was set to 151.52 J/mm3, a low-defect sample with a relative density of 99.2% was obtained. After heat treatment, the area of the fine grains at the boundary increased significantly, thereby contributing to the excellent mechanical properties. The microstructure was characterized by a unique “fan-shaped” heterogeneous structure. As the energy density increased, the microhardness first increased and then decreased, reaching a maximum value of 122 HV0.3. With the optimized process parameters, the yield strength (YS), ultimate tensile strength (UTS), and elongation of the as-built Al-Mg-Sc-Zr alloys were 346.8 ± 3.0 MPa, 451.1 ± 5.2 MPa, 14.6% ± 0.8%, respectively. After heat treatment at 325 °C for 8 h, the hardness increased by 38.5% to 169 HV0.3, and the YS and UTS increased by 41.3% and 18.1%, respectively, to 490.0 ± 9.0 MPa and 532.7 ± 7.8 MPa, respectively, while the elongation slightly decreased to 13.1% ± 0.7%.