Enhancing the mechanical property of laser powder bed fusion CoCrMo alloy by tailoring the microstructure and phase constituent

Abstract

Cellular structure with a high density of dislocations and elemental segregation at the cell boundaries is the characteristic microstructure of the laser powder bed fusion (LPBF) CoCrMo alloy. This structure is known to be merit to the alloy's strength, but this structure and elemental heterogeneity are likely to be detrimental to the ductility. In this study, the effect of solution treatment and aging on the microstructure, phase fraction, and mechanical behaviors of a CoCrMo alloy fabricated by laser powder bed fusion (LPBF) was investigated, in order to achieve a microstructure state with excellent synergy in strength and ductility. The results showed that solution treatment at 1150 °C for 1 h was sufficient to remove the cellular structure and eliminated the elemental segregation, but it resulted in a dramatic reduction in yield strength. Direct aging at 750 °C for 2 h led to a boost in the yield strength due to the precipitation of Cr-rich carbides, but the alloy ductility was significantly reduced and the elemental segregation was maintained. An excellent combination of strength and ductility was achieved via a two-step solution and aging treatment, by which both the strength and ductility were improved compared to that of the as-built CoCrMo alloy. Multiscale characterizations demonstrated that the synergetic strength and ductility were mainly attributed to the strengthening effect of the ε phase formed during aging and stress-induced γ to ε phase transformation under mechanical loading. In addition, the enhanced ductility was also evidently benefited from the eliminated cellular structure and elemental segregation, which reduces the tendency in crack nucleation.