Reduction in anisotropic response of corrosion properties of selective laser melted Co–Cr–Mo alloys by post-heat treatment
Introduction
Selective laser melting (SLM) technology produces three-dimensional (3D) metal parts by layer-by-layer melting of metal powders using a scanning laser. This method enables rapid, semi-automatic, and precise fabrication of complex geometries; thus, SLM techniques have been widely applied to produce metal dental prostheses [1,2]. In particular, Co–Cr alloys prepared by SLM are increasingly used for metal copings [3], removable partial denture frameworks [4], and implant-supported frameworks [5] owing to their good mechanical properties [6] and biocompatibility [7]. However, SLM-fabricated parts usually undergo a complex cyclic thermal history consisting of directional heat extraction, repeated melting, and rapid solidification, which creates a variety of orientation-dependent properties (i.e., anisotropy) [8,9]. In Co–Cr alloys, anisotropic microstructures, such as the epitaxial growth of columnar grains and <001> fiber texture along the build direction (BD), and anisotropic mechanical properties, determined through tensile and fatigue tests, were reported in previous studies [6,[10], [11], [12]].
Anisotropic corrosion properties have also been observed, where different behavior was measured in the different planes of SLM-produced Al–12Si [13] and Ti–6Al–4 V [14] alloys. However, to the best of our knowledge, no previous studies investigated the anisotropy in the corrosion properties of SLM-processed Co–Cr alloys along different planes. The surfaces of dental prostheses prepared by SLM consist of various orientation planes with respect to the BD. Therefore, the corrosion properties of the surfaces should vary depending on the site because the microstructure of the alloy depends on the sample plane (i.e., the horizontal and vertical planes with respect to the BD). The oral environment is a harsh system that promotes the corrosion of dental alloys. Metal ions and by-products released in the corrosion process can be harmful to the patient [15]; thus, for clinical safety, the corrosion resistance of different planes with respect to the BD needs to be investigated.
As another well-known disadvantage of SLM, SLM-manufactured parts easily accumulate residual stress due to the rapid heating and cooling of the sample during fabrication, which negatively affects their dimensional accuracy and mechanical properties [[16], [17], [18]]. Therefore, post-fabrication heat treatments are normally required to relieve residual stresses and improve mechanical properties [[19], [20], [21], [22]]. However, such heat treatments induce microstructural changes, which can affect the corrosion resistance. A previous study investigated the corrosion properties of SLM-processed Co–Cr alloys before and after porcelain firing, and reported that these specimens exhibited similar corrosion behavior at both pH 5.0 and 2.5, and had significantly better corrosion resistance than traditional cast specimens [23]. However, the holding time at high temperature during porcelain firing is short and differs from that in post-fabrication heat treatment to relieve residual stress [19]. Conversely, another study performed immersion tests with SLM-processed Co–Cr alloys after solution treatments at 1323 K (1050 °C), 1423 K (1150 °C), and 1473 K (1200 °C) for 1 h followed by water quenching, and at 1423 K (1150 °C) for 1 h followed by furnace cooling [19]. The samples heated to 1423 K (1150 °C) and furnace cooled resulted in a larger amount of released metals due to the presence of coarse precipitates, as compared to the as-built samples and the water-quenched heat-treated samples. However, no studies exist that comprehensively evaluated the influence of various heat treatments followed by furnace cooling on the corrosion properties of SLM-processed Co–Cr alloys and compared their performance with traditional cast sample. The study of furnace cooling is important as it is used in dentistry after heat treatment to retain the dimensional accuracy of the prosthesis.
In this study, SLM-processed Co–Cr–Mo alloy samples were fabricated with either the horizontal or vertical plane parallel to the BD, and various heat treatments were performed, followed by furnace cooling. The microstructures and corrosion properties of the SLM-processed Co–Cr–Mo alloy samples were evaluated, focusing on the anisotropy with respect to the planes and the influence of various heat treatments. The results were evaluated through a comparison with traditional cast samples. The first null hypothesis of this study was that there would be no difference in corrosion response between the different planes (x-y or x-z planes) for the same heat-treatment conditions. The second null hypothesis was that the different heat treatment conditions would not influence the corrosion resistance.
Section snippets
Specimen preparation
Commercially available Co–Cr–Mo alloy powders (MP1, EOS, Krailling, Germany) composed of 60–65 wt.% Co, 26–30 wt.% Cr, 5–7 wt.% Mo, and <1 wt.% Si, Mn, Fe, C, and Ni, were used. Cubic samples (10 mm × 10 mm × 10 mm) were prepared using an SLM machine equipped with a fiber laser (EOSINT M280, EOS, Krailling, Germany). The SLM machine was operated using standard deposition parameters for MP1 under a laser power of 195 W and a nitrogen atmosphere. The laser scan pattern was rotated by an angle of
Microstructures
Fig. 2 depicts 3D microscopy images of the microstructures of the as-built SLM alloys. In the CLSM images, regular laser-melted tracks corresponding to the alternating 67° filling strategy were observed on the cross-section, as shown by the x-y cross-section (Fig. 2(a)). In the x-z/y-z cross-section, the layered half ellipses accumulated along the z-axis were identified as melt-pool traces, which were formed via the layer-by-layer melting of the metal powders. In the SEM images, numerous fine
Discussion
Corrosion of Co–Cr alloys is induced by the release of Co and Cr, which has cytotoxic, genotoxic, and metal sensitizing effects on the human body [23]. In this study, analysis of the anodic polarization curves indicated that the heat treatment process and specimen’s plane did not greatly affect the corrosion resistance of the SLM samples; the corrosion behavior of the SLM samples prepared under all conditions was comparable to that of the traditional cast samples. However, the higher mean
Conclusions
Under all conditions, the SLM samples exhibited a considerably lower metal ion release than the traditional cast samples, indicating that the corrosion properties of the SLM-processed Co–Cr–Mo alloy samples were superior to those of the traditional cast samples, irrespective of the post-fabrication heat treatment. These results were attributed to the formation of relatively small carbides in the SLM-processed Co–Cr–Mo alloy samples and the protective passive film. However, excessive heat
Acknowledgments
This work was partially supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan [grant numbers 17K17152 and 17K17157]; and the Amada Foundation [grant number AF-2016231].
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