Microstructure evolution and mechanical properties improvement of selective laser melted Co-Cr biomedical alloys during subsequent heat treatments

Selective laser melting (SLM) technology has garnered considerable attention as an advanced manufacturing method that produces biomedical Co-Cr devices specifically developed for dental restoration applications. However, SLM technology inevitably causes unstable microstructures and excessive residual stress due to rapid heating and cooling, which negatively affects their mechanical properties. Also, information on the phase transformation and precipitation behavior during subsequent heat treatments is limited. In this work, SLM-manufactured Co-Cr specimens were initially subjected to stress-relieving annealing (SRA) at various temperatures (850, 950, 1050, and 1150 °C) and then porcelain firing (PF) treatments, and the microstructure evolution and mechanical properties improvement during subsequent heat treatments was investigated in detail. Scanning electron microscopy (SEM) combined with energy-dispersive x-ray spectroscopy (EDS), x-ray diffractometer, transmission electron microscopy, and Image Pro Plus 6.0 (IPP) software were used to analyze alloy microstructures, whereas tensile tests were performed to evaluate corresponding performance. Results showed that after SRA and/or PF treatments, a large number of nanoscale precipitates were observed, and acicular or lamellar ε phase appeared. For SRA-treated specimens, as annealing temperatures increased, the volume fraction of γ phase decreased, especially after annealing at 950 °C, the γ phase was completely transformed into the ε phase. In contrast, for (SRA + PF)-treated specimens, the volume fraction of γ phase first decreased and then increased with the increase of annealing temperatures. IPP analysis found that the amount, size, and morphology of the precipitates varied with the change in annealing temperature. During annealing, isothermal martensite transformation (γ →ε) dominated due to the long duration between 600 and 900 °C, while the amount of athermal martensite transformation (γ →ε) gradually increased when porcelain firing. Changes in the above-mentioned microstructures affected corresponding mechanical properties. From 850 °C to 1050 °C, tensile strength and elongation continue to improve. At 1150 °C, both tensile strength and 0.2% yield strength decreased, but the elongation was further improved. The variation in hardness (HV10) with temperature was the same as that of the volume fraction of ε phase, that is, the higher the volume fraction of ε phase, the higher the hardness. SEM/EDX analysis of tensile fracture surfaces revealed a quasi-cleavage fracture. In conclusion, 1150°C-SRA and PF treatments are an effective method for homogenizing microstructure and improving mechanical properties.