Effect of processing parameters on the microstructure and mechanical properties of Co–Cr–Mo alloy fabricated by selective laser melting

Abstract

Selective laser melting (SLM) is an additive manufacturing process that builds metal components according to computer-aided design (CAD), selectively fusing and consolidating metal powders one layer at a time. This study systematically investigates the influence of various laser conditions on the microstructure and mechanical properties of Co–Cr–Mo alloy fabricated by SLM. Hot isostatic pressing (HIP) was applied after SLM to further densify the fabricated components, and their microstructures and mechanical properties were compared with those of as-built samples. Parts with relative density of ∼97.17% were obtained with a high-energy laser density and double laser scanning, and subsequent HIP effectively eliminated the majority of pores. X-ray diffraction revealed that γ and ε phases co-existed owing to the high solidification rate during SLM. Directional columnar grains and fine cellular dendrites that were parallel and normal, respectively, to the build direction were observed. Although variations in microstructure resulting from different as-built samples were insignificant, the mechanical properties remarkably changed. The improved mechanical behavior was attributed mainly to increased densification and grain-boundary strengthening. However, the HIP treatment increased the tensile ductility and reduced the yield strength, as a result of the increase in grain size. The tension-fractured surface suggests that mostly quasi-cleavage brittle fracture took place in the SLM-processed parts.