Isothermal γ → ε phase transformation behavior in a Co-Cr-Mo alloy depending on thermal history during electron beam powder-bed additive manufacturing

Abstract

Powder bed fusion with electron beam (PBF-EB), allows Co-Cr-Mo (CCM) implants with patient-customization to be fabricated with high quality and complex geometry. However, the variability in the properties of PBF-EB-built CCM alloy, mainly due to the lack of understanding of the mechanisms that govern microstructural heterogeneity, brings limitations in extensive application. In this study, the microstructural heterogeneity regarding the γ-fcc → ε-hcp phase transformation was characterized. The phase transformation during PBF-EB was analyzed depending on the thermal history that was elucidated by the numerical simulation. It revealed that isothermal γ → ε transformation occurred during the fabrication. Importantly, the difference in γ/ε phase distribution was a result of the thermal history determining which method phase transformation was taking place, which can be influenced by the PBF-EB process parameters. In the sample with a low energy input (Earea = 2.6 J/mm2), the martensitic transformation was dominant. As the building height increased from the bottom, the ε phase fraction decreased. On the other hand, in the sample with a higher energy input (Earea = 4.4 J/mm2), the ε phase formed via diffusional-massive transformation and only appeared in a short range of the lower part away from the bottom.