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

Hla Htoot Wai Cho,Takao Hanawa,Hein Linn Htat,Atsushi Takaichi,Noriyuki Wakabayashi,Yuka Kajima,Nuttaphon Kittikundecha

doi:10.3390/met11071005

Hla Htoot Wai Cho, Takao Hanawa + Show 5 more

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https://doi.org/10.3390/met11071005

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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.

Highlights

Cobalt chromium molybdenum (CoCrMo) alloys are widely used in the fabrication of biomedical devices, such as knee joints, orthopedic implants, and dental devices, because of their excellent mechanical properties and biocompatibility [1,2,3,4,5]
We investigated the influence of post-heat treatment cooling conditions on the fatigue behaviors of selective laser melting (SLM)-processed CoCrMo samples under tension–tension cyclic loads
The specimens were divided into two groups, and the heat treatment was performed in the following sequence: (1) heating the furnace from room temperature to 600 ◦ C, (2) maintaining the temperature for 30 min, (3) heating the furnace to 1150 ◦ C, (4) maintaining the temperature for 1 h, and (5) switching off the heating machine

Summary

Introduction

Cobalt chromium molybdenum (CoCrMo) alloys are widely used in the fabrication of biomedical devices, such as knee joints, orthopedic implants, and dental devices, because of their excellent mechanical properties and biocompatibility [1,2,3,4,5]. One of the main drawbacks of SLM is that a residual stress is generated within the printed parts because of the distinctive thermal cycle pattern of rapid melting and cooling during the fabrication process [6,13,14,15]. This residual stress leads to micro-cracking or distortion, thereby degrading the resulting mechanical properties of the products [11,16]. Post-heat treatments are essential for reducing such residual stresses and improving the mechanical properties of the produced parts [11,13,16]

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