Microstructural features of biomedical cobalt–chromium–molybdenum (CoCrMo) alloy from powder bed fusion to aging heat treatment

Abstract The design freedom of powder bed fusion process selective laser melting (SLM) enables flexibility to manufacture customized, geometrically complex medical implants directly from the CAD models. Co-based alloys have adequate wear and corrosion resistance, fatigue strength, and biocompatibility, which enables the alloys to be widely used in medical devices. This work aims to investigate the evolution of microstructures and their influence on tribological property of CoCrMo alloy processed by SLM and aging heat treatment. The results showed that very weak texture along the building direction and microsegregation along cellular boundaries were produced. The presence of high residual stress and fine cellular dendrite structure has a pronounced hardening effect on the as-SLM and aging-treated alloys at moderate temperatures. Furthermore, the hexagonal e phase transformed from the γ matrix during SLM became significant after subsequent aging at moderate temperatures, which further increased the nanohardness and scratch resistance. High temperature (1150 °C) heating caused homogenized recrystallization microstructure free of residual stress and e phase, which sharply decreased the hardness and scratch resistance. The material parallel to the building direction exhibited improved tribological property in both SLMed and aging-treated alloy than that of the material perpendicular to the building direction. The anisotropy in frictional performance may be considered when designing CoCrMo dental implants using laser additive manufacturing.