Effect of powder fabrication process on microstructure and mechanical properties of selective laser melted metals for biomedical applications

摘要

Introduction: Selective laser melting (SLM) process has been received much attention because this process enables us to fabricate complicated shape without mold or conventional cutting tools. By utiliing this process, it is expected to achieve custom-made implants for patients. In addition, some metals fabricated through SLM process shows different microstructure and mechanical properties compared to as-cast or forged metals. We have been focusing on cobalt-chromium-molybdenum (Co-Cr-Mo) alloys which have been widely used in orthopedics and dentistry. Co-Cr-Mo alloys show excellent strength and wear resistance but limited ductility owing to the formation of martensite (hcp) and the precipitation of carbides or sigma phase. Our group has previousely reported that elongation and strength of high Cr and N Co-Cr-Mo alloy showed higher strength compared to the as-cast alloy. This is mainly due to the suppression of martensite and sigma phase. Therefore, it can be concluded that the effects of high Cr and N with the combination of applying the SLM process works for enhancing the mechanical properties. In this study, we focused [on the processing of high Cr and N containing Co-Cr-Mo powder, because the SLM builds could be suffered from impurities from the powder. Threfore, the purpose of this study is to fabricate high Cr and N containing Co-Cr-Mo powder with different atomization process and investigate their microstructure and mechanical properties. Materials and Methods: Co-33Cr-5Mo-N alloy powders were prepared using gas atomization and water atomization processes. Ar gas was used in the gas atomization. Size distribution was evaluated by optical microscope. The surface of the powder was analysed by SEM-EDX. Chemical composition of these powders were determen by ICP-MS. The selective laser melting (SLM) process was carried out using the gas atomized and water atomized powders under the diameter of 45 microns. Tensile test specimens with the guage length of 18 mm were build to evaluate mechanical properties and microstructure. Results and Discussion: Fig. 1 shows SEM images of gas atomized and water atomized Co-33Cr-5Mo-N alloy powders. Spherical shape and smooth surface with dendritic morphology was observed in the gas atomized powders. On the other hand, relatively deformed sphere shape with gray contrast was observed in the water atomized powders. This deformation was caused by the higher impact enegy from water. SEM-EDX analyses revealed that Si and oxygen were enriched in the gray area. This suggests that the water atomized powder was oxidized during the process, although such area could not be seen at the gas atomized powder. Table 1 summarizes the mechanical properties of SLMed Co-Cr-Mo alloys using gas and water atomised powders. The SLMed high Cr and N Co-Cr-Mo alloys showed higher tensile properties compared to the as-cast alloy. Comparing the builds using gas and water atomized powders, almost the same properties was achierved. However, when the energy density, which was determined by the SLMed parameter, was decreased, the strength and elongation of the SLM build using water atomized powder showed higher values compared to those using gas atomized powder. Since the porosities in the builds using gas atomized powder was higher than those using water atomized powder at the low energy density, this may cause the mechanical properties difference in the SLMed builds. Figure 1. SEM images of high-Cr and -N Co-Cr-Mo alloy powders fabricated by (a) gas atomization process and (b) water atomization process. Table 1. Mechanical properties of SLMed Co-Cr-Mo alloys using gas and water atomized powders. Conclusion: The powder fabrication process gave influences on the porosity of the SLMed builds, which leads to the mechanical properties difference at the same energy density for the SLM process. Oxygen content in the powder could be one of the important factor, and it needs further investigation.