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

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 gamma-fcc -> epsilon-hcp phase transformation was characterized. The phase transformation during PBF-EB was analyzed depending on the thermal history that was eluci-dated by the numerical simulation. It revealed that isothermal gamma -> epsilon transformation occurred during the fabrication. Importantly, the difference in gamma/epsilon 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 (E-area = 2.6 J/mm(2)), the martensitic transformation was dominant. As the building height increased from the bottom, the e phase fraction decreased. On the other hand, in the sample with a higher energy input (E-area = 4.4 J/mm(2)), the e phase formed via diffusional-massive transformation and only appeared in a short range of the lower part away from the bottom. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.