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.