Recrystallization behavior of selective laser melted Co–Cr–Mo alloys with several heat treatment times

Abstract

For parts manufactured by selective laser melting (SLM), post-heat treatment is essential to relieve residual stress. However, recrystallization and grain growth processes upon the stress-relieving heat treatment in SLM-fabricated Co–Cr–Mo alloys are not well understood. In this study, Co–Cr–Mo alloy specimens were fabricated by SLM and then heat treated at 1150 °C for various time periods (10, 20, 30, 45, and 60 min) to evaluate the influence of heat treatment time on the recrystallization process. The alloy microstructure was analyzed via confocal laser scanning microscopy, scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) and X-ray diffraction analyses. The mechanical properties of the produced specimens were evaluated by tensile tests. When the heat treatment was conducted for up to 45 min, the 0.2% yield strength (YS) decreased, whereas elongation increased. In contrast, when heated for 60 min, both the 0.2% YS and elongation decreased. The EBSD images demonstrated that both columnar and equiaxed grains existed, and recrystallization partially initiated after heating for 10 min. When heated for 45 min or more, the samples consisted entirely of equiaxed grains; however, TEM images showed regions with remaining subgrain boundaries and dislocations. Fully recrystallized microstructures were not obtained even after heating for 60 min. In addition, heat treatments longer than 60 min induced coarse grains and precipitates along the grain boundaries, which decreased the strength and ductility of the alloys. Retaining the subgrains within the grains to some extent enables producing alloys with excellent strength and ductility.