Effect of Post-Heat Treatment Cooling Conditions on Microstructures and Fatigue Properties of Cobalt Chromium Molybdenum Alloy Fabricated through Selective Laser Melting

Abstract

Although post-heat treatment can improve the fatigue life of selective laser melting (SLM)-fabricated cobalt chromium molybdenum (CoCrMo) alloys, the effect of cooling conditions on the fatigue properties of such alloys remains unclear. In this study, we fabricated SLM CoCrMo alloy specimens and, after heat-treating them, cooled them either via furnace-cooling (FC) or air-cooling (AC). Subsequently, we analyzed their microstructures using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy, electron backscattered diffraction, confocal laser scanning microscopy, and X-ray diffraction. Tensile and Vickers hardness (HV) tests and axial-fatigue tests were also conducted to assess their mechanical and fatigue properties, respectively. The microstructures of all samples showed homogeneous equiaxed grains, with the grains and precipitates of the AC samples (grain size: 84.9 μm) smaller than those of the FC samples (grain size: 109.7 μm). The AC samples exhibited better ductility than the FC samples. However, we observed no significant differences in the 0.2% yield strength and HV tests. The S–N curve derived from the fatigue tests showed that the AC samples had greater fatigue life than the FC samples. Therefore, a high cooling rate during post-heat treatment is effective in reducing grain and precipitate sizes, resulting in improved ductility and fatigue life.