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 patientcustomization 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 e phase fraction decreased.On the other hand,in the sample with a higher energy input(Earea=4.4 J/mm2),the ε phase fo rmed via diffusional-massive transformation and only appea red in a short range of the lower part away from the bottom.