High-strength 18Ni-300 maraging steel parts were fabricated by selective laser melting (SLM) additive manufacturing. Aging treatment at 490 ºC for 6 hours was applied for improving the mechanical properties of SLM fabricated parts. The microstructure and properties of the as-fabricated SLM 18Ni-300 parts were carefully characterized and analyzed. The results show that the relatively low surface roughness, with 4.16μm for horizontal surface and 4.79μm for vertical surface, certified the high accuracy of forming process. The almost fully dense microstructures of the as-fabricated SLM specimens were consisted of sub-micron dendritic cellular structures for horizontal cross-section and elongated acicular structures for vertical cross section. Due to precipitated-phase strengthening, the hardness increased from 35~36 HRC to 51~55 HRC after heat treatment. Meanwhile, the ultimate tensile strength (Rm) increases from 1165 MPa to 2014 MPa, which increased by about 73%; while the break elongation (ε) was reduced from 12.44% to 3.28%, owing to the increased embrittlement caused by the heat treatment. The substantial plastic deformation and dispersively distributed dimples in the fracture morphologies indicated a ductile fracture of as-fabricated tensile specimens, while the age-hardened tensile specimen encountered with a main fracture mode of brittle fracture.

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