利用选区激光熔化(SLM)增材制造技术成功制备了5CrNi4Mo 模具钢试件，研究了激光成形5CrNi4Mo 模具钢的相变过程及其机制，分析了激光线能量密度η(激光功率与扫描速度之比)对SLM 成形件致密度、显微组织和力学性能的影响规律。研究表明：过高的η(387.5 J/m)引起球化效应，使得成形件内部含有残留孔隙，成形致密度降低；过低的η(155.0 J/m)导致熔体润湿性较低，成形致密度较差。将η优化为258.3 J/m 时，成形试件加工缺陷减少，成形致密度提升到98.12%。激光加工的快速冷却作用易导致马氏体相变；原始粉末中的Mn、Ni、Cr等合金元素可以增加过冷奥氏体的稳定性，降低马氏体临界冷却速度，从而确保了马氏体转变的顺利进行；随着η的降低，马氏体组织发生明显的细化。当η=193.8 J/m，成形试件具有较高的显微硬度(689.5 HV0.2)、较低的摩擦系数(0.44)和磨损率[2.3×10^-5 mm³/(N?m)]。

The 5CrNi4Mo die steel is successfully prepared by selective laser melting (SLM) additive manufacturing technology. The mechanism of the phase transformation is investigated and the influence of the applied laser linear energy density h (the ratio of laser power to scan velocity) on densification, microstructure, and mechanical properties of SLM-processed parts is studied. It shows that a high h results in the formation of residual pores and lowers the densification level, caused by severe balling phenomenon. A low h causes the limited wetting ability and low densification level. When h is optimized at 258.3 J/m, the SLM-processed parts have an improved densification level of 98.12% without apparent process defects. The laser-induced large solidification rate leads to the martensitic transformation. Alloying elements such as Mn, Ni, and Cr in the original powder can stabilize the undercooling austenite and lower the critical cooling rate of martensite transformation, ensuring the successful process of martensite transformation. The microstructures of martensite are further refined with decreasing h. When h is 193.8 J/m , high microhardness of 689.5 HV0.2, low friction coefficient of 0.44 and wear rate of 2.3×10^-5 mm³/(N?m) are obtained for the SLM-processed parts.