Dependence of Microstructure, Relative Density and Hardness of 18Ni-300 Maraging Steel Fabricated by Selective Laser Melting on the Energy Density

18Ni-300 maraging steel samples were fabricated by Selective Laser Melting technology (SLM). Microstructure and phase transformation before and after aging treatment were investigated by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). Meanwhile, hardness and relative density of samples were also tested. The results showed that there was a positive correlation relationship between energy density and relative density. Samples fabricated with high energy density of 6.0 J/mm2 could lead to extremely high relative density of 99.34%; After aging heat treatment (763 K for 4 h), the microhardness increased from 370 to 629 HV because of second phases such as Ni3Mo and Ni3Ti precipitated in the martensite matrix. When aging temperature reached to 793 K, the hardness was affected by both precipitation of strengthening phase and austenite reversion which had the opposite effect. The XRD results showed the appearance of austenite and the decrease of martensite.