Effects of heat treatment on the microstructure and mechanical performance of Co-Cr alloy by selective laser melting

Heat treatment of laser additive manufactured materials cannot only eliminate thermal stress during the printing process, but also regulate their microstructure and mechanical properties. This article studies the changes in the microstructure of selective laser melted Co-Cr alloy under different heat treatment conditions and its corresponding mechanical properties. It was found that the alloy structure in the as-printed state was γ-austenite, with a large amount of dislocation, sub-grain boundaries, and a small amount of tetragonal σ-CoCr precipitation phase in the matrix. A γ-ε transformation occurred at 1 150 ℃ with a holding time of 1 h, and the volume fraction of ε-martensite was 10.4%. At the same time, the precipitation phase transformed from a tetragonal σ-phase to a hexagonal Co₃W(Mo)₂Si phase. After further treatment at 800 ℃ for 2 h, the martensite content increased to 15.5%, and the quantity and size of the precipitation phase increased. The influence of the precipitation phase on the mechanical properties after heat treatment was significant. The material's hardness increased from 31 HRC to 38 HRC, and the yield strength increased from 848 MPa to 1 119 MPa. This study can provide a reference for the microstructure and property regulation of Co-based alloys by selective laser melting.