Comparative analysis of machinability and microstructure in LPBF and conventionally processed M300 maraging steel

The heat treatment and machining can be considered as necessary post process operations to create functional components manufactured by means of laser powder bed fusion. Therefore, complex investigation of influence of microstructure on the machinability of maraging steel (MS) M300 parts produced by laser powder bed fusion (LPBF) and rolling was performed. The main evaluation was carried out in regard to hardness, milling forces, surface morphology, surface roughness, chip formation, and subsurface hardness and microstructure. The findings revealed that LPBF MS M300 after direct aging heat treatment, containing martensite with dispersed precipitates and reversed austenite, generated highest milling forces. Surface roughness of this material showed a lower surface roughness in comparison to as-built sample Due to the higher hardness, which caused a lower lateral plastic flow and most severe tool wear. The subsurface hardness under the milled area of direct aged samples increased by 24% (900 HV) in comparison to the bulk value, while in case of conventionally manufactured rolled sample the increase was only by 9%. More pronounced increase in subsurface hardness was measured on samples milled utilizing highest cutting speed (60 m.min^-1). Furthermore, milling induced strain hardening affecting the subsurface area up to approximately 40 μm in depth.