Heterogeneous Distribution of Microstructure and Mechanical Properties in M2 High-Speed Steel Fabricated by Laser Powder Bed Fusion

In this study, crack-free M2 high-speed steel (HSS) was successfully fabricated by laser powder bed fusion (L-PBF) using a relatively high substrate preheating temperature of 260 °C. The influence of the resulting microstructure on the hardness and tensile properties along the build direction was thoroughly investigated. The results demonstrate that M2 HSS achieves a relative density of 99.6% when processed with a laser power of 280 W and a scanning speed of 0.8 m s−1. The microstructure predominantly consists of fine martensite, along with retained austenite, lower bainite, and a small quantity of eutectic carbide. With increasing build height (from 0 to 9 mm), the fraction of lower bainite decreases from 32.1 to 13.1%, while the austenite content increases from 0.9 to 29.1%. These microstructural changes lead to a gradual reduction in the material’s strength along the build direction. Specifically, the hardness and tensile strength decrease from 845 HV0.3 and 1520 MPa to 745 HV0.3 and 1251 MPa, respectively. Additionally, the elongation varies between 2.6% and 3.3% across the different build heights.