Effect of heat-treatment temperature on microstructures and mechanical properties of Co–Cr–Mo alloys fabricated by selective laser melting

Selective laser melting (SLM) has attracted considerable attention as an advanced method for the fabrication of biomedical devices. However, SLM-manufactured parts easily accumulate large amounts of residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. In this study, Co–Cr–Mo alloy specimens were fabricated by SLM and then heat-treated at various temperatures (750, 900, 1050, or 1150 °C) to relieve the residual stress and improve their mechanical properties. The alloy microstructure was analyzed via confocal laser scanning microscopy, scanning electron microscopy combined with energy dispersive X-ray spectroscopy, electron backscattered diffraction, and X-ray diffraction techniques, whereas the mechanical properties of the produced specimens were evaluated by tensile and Vickers hardness tests. The results showed that increasing the heat-treatment temperature from 750 °C to 1150 °C increased the ductility of the alloy and decreased its 0.2% offset yield strength and Vickers hardness. Both γ and ε phases formed in all heat-treated specimens, and the volume fraction of the ε phase decreased with increasing heat-treatment temperature. After the specimens were heated to 750–1050 °C, a recovery process was initiated, which proceeded as the temperature increased; however, the residual stress in the studied specimens was not sufficiently relieved. In contrast, after heating to 1150 °C, the formation of equiaxed grains and the drastic relief of the residual stress were observed simultaneously, accompanied by an increase in the elongation of the specimen and a decrease in its strength (as compared to those of the other heat-treated specimens), indicating that the specimen completely recrystallized and that the residual stress was the driving force of this recrystallization. Thus, heat-treating at 1150 °C for 6 h is an effective method for eliminating the residual stress, leading to a homogenized microstructure and satisfactory ductility.