Effect of post-process heat treatment on the microstructure evolution of Additive Laser Powder Bed Fusion Processed Co-Cr-Mo alloy

The Co-Cr-Mo alloy with low carbon content is widely used for medical prosthetic implants due to its high strength, biocompatibility, excellent corrosion and wear resistance. Additive manufacturing is of interest to produce net-shape, customized implants. In this study, ingots of Co-Cr-Mo alloy were additively manufactured by laser powder bed fusion (LPBF) and then subjected to heat treatments involving either direct aging or solutionizing followed by aging at temperatures between 700-850°C. Microstructural changes were characterized by XRD, optical, SEM, EBSD, EDS, and TEM, together with hardness measurements. The as-built microstructure was composed of mainly 20-25 micron-size fcc grains with carbides along grain boundaries. Aging led to an accelerated massive transformation of the γ-fcc to ε-hcp structure long with appreciable hardening (400 to 700 HV). In contrast, aging of the solutionized samples produced sluggish phase transformation involving both competing massive and isothermal martensitic transformation modes. Reasons for this difference in phase transformation modes, kinetics, and mechanisms governing microstructure and property evolution will be discussed.