Effects of heat treatment on the microstructure, residual stress, and mechanical properties of Co–Cr alloy fabricated by selective laser melting
Graphical abstract
Introduction
Because Co–Cr–Mo (CCM) alloys are relatively inexpensive and exhibit superior mechanical properties and biocompatibility, they are widely used as metal substructures for dental prostheses (Al Jabbari, 2014; Hedberg et al., 2014; Suleiman and Vult von Steyern, 2013). Prostheses made of CCM-based materials are generally manufactured using a casting method, which, however, can cause solidification shrinkage, internal defects, increased porosity, and the formation of coarse microstructures; this can lead to reduced mechanical properties and the premature failure of the prosthesis (Bae et al., 2015; Zhou et al., 2018).
Recently, additive manufacturing has been employed to manufacture prostheses. Powder bed fusion is most often used when manufacturing metal structures due to its excellent surface quality and mechanical properties of the resultant specimen. Among the powder bed fusion methods, selective laser melting (SLM) is the most commonly used method for manufacturing metal structure of dental prosthesis (Tulga, 2018). The metal powder is locally heated by high energy laser beam, completely melted, and rapidly cooled before adding more powder. By repeating this process, the structure created with the CAD program can be fabricated (Konieczny et al., 2020; Koutsoukis et al., 2015). A cooling rate as high as 1 × 106 K/s can be achieved using SLM, creating a fine microstructure (Wang et al., 2019). Several studies have reported on the improved microstructure and mechanical properties of CCM alloys manufactured by the SLM method (Al Jabbari et al., 2014; Kim et al., 2016; Koutsoukis et al., 2015). Unlike casting, it is possible to produce CCM alloy with a high density, complex geometric shape, and low porosity through SLM (Li et al., 2015; Xiang et al., 2012; Zhou et al., 2020). In addition, the grains of the alloy are fine; the solid solution limit of the alloying element is increased, thus reducing precipitation and segregation (Zhou et al., 2018), and there are few secondary phases or intermetallic compounds (Suleiman and Vult von Steyern, 2013; Tonelli et al., 2020). In the case of most CCM alloys, the transformation rate of a face centered cubic (FCC) phase to a hexagonal close packed (HCP) phase is low, so it has a mixed structure of FCC and HCP phases at room temperature. However, in the case of CCM alloys manufactured by SLM, the FCC phase is present after the SLM process due to the rapid melting and cooling cycles. These microstructural features result in the improved mechanical properties of CCM alloys produced by SLM.
On the contrary, SLM causes the accumulation of a large amount of residual stress due to the layer-by-layer fabrication and rapid heating and cooling, which adversely affects mechanical properties (Edwards and Ramulu, 2014; Kajima et al., 2018; Mengucci et al., 2016). The accumulation of such a large amount of residual stress may cause distortion of specimen (Li et al., 2015). Therefore, heat treatment is required to improve their microstructures and mechanical properties, as well as to reduce residual stress (Lu et al., 2015; Mengucci et al., 2016). The heat treatment conditions affect the size, shape, and distribution of precipitates (Mengucci et al., 2016; Takaichi et al., 2020; Wei et al., 2020), size and shape of grains (Kittikundecha et al., 2019; Seki et al., 2019; Takaichi et al., 2020), and phase fraction of the FCC and HCP (Wei et al., 2020; Zhou et al., 2020), which also changes the mechanical properties of the specimen. Kajima et al. (2016) found that CCM alloy produced by SLM exhibited a higher fatigue strength than CCM alloy produced by the casting. Seki et al. (2019) and Kittikundecha et al. (2019) reported that the fatigue strength of CCM alloy manufactured by SLM was improved after heat treatment. However, it has been reported that a long heat treatment time promotes excessive grain growth (Kajima et al., 2018) and increases the size of the precipitate and thickness of the oxide layer, which adversely affects the mechanical properties (Kittikundecha et al., 2019). As such, studies on the effect of heat treatment on the microstructure and mechanical properties of CCM alloy manufactured by SLM have been reported; however, little has been reported on the resultant change in residual stress. Therefore, it is necessary to study heat treatment conditions that can improve the microstructure and mechanical properties of CCM alloy produced by SLM while also reducing the residual stress.
The purpose of this study was to compare and evaluate the microstructure and mechanical properties of CCM alloys produced by the casting and SLM. In addition, CCM alloy produced by SLM was further studied to determine the conditions for optimal residual stress removal and to evaluate the changes in microstructure and mechanical properties before and after heat treatment.
Section snippets
Specimen preparation
CCM alloy specimens were prepared by casting and SLM. Disc shaped CCM alloy specimens with a diameter of 10 mm and a thickness of 2 mm were fabricated for microstructure observations and dumbbell shaped specimens prepared according to ISO 22674 were used for tensile testing. Two disc shaped specimens and five dumbbell shaped specimens were prepared by casting (Cast Co–Cr) and SLM (SLM Co–Cr). The SLM Co–Cr specimens were tested under four heat treatment conditions (no heat treatment, 750 °C,
BSE image, SEM observation, and EDS analysis
The microstructures of the surface of the disc specimens before etching and after polishing were determined from the BSE images (Fig. 2). The Cast Co–Cr group consisted of solid solution matrix and dispersed precipitate (bright white area), and a large amount of porosity was observed owing to solidification shrinkage. Contrarily, a fine microstructure was observed in the SLM Co–Cr group compared to the Cast Co–Cr group, and almost no precipitate or porosity was observed.
In the case of the Cast
Discussion
The ultimate tensile strength, yield strength, and hardness of the As-SLM group were found to be significantly higher than those of the Cast Co–Cr group, as indicated from tensile and hardness tests. No significant difference in elongation between these groups was observed; however, the As-SLM group did exhibit a greater elongation. The difference in mechanical properties due to these manufacturing methods was related to the porosity, grain boundary strengthening, and solid solution
Conclusion
SLM is considered to be a superior manufacturing method to casting if appropriate heat treatment is included. Most of the residual stress, except the micro residual stress, was reduced during heat treatment at 750 °C. It is necessary to consider the heat treatment temperature according to the type of dental prosthesis since the mechanical properties of CCM alloy change according to the heat treatment temperature. However, considering the reduction of tensile residual stress and increase of
CRediT authorship contribution statement
Kyung-Ho Ko: Writing – review & editing, Writing – original draft, Visualization. Hyeon-Goo Kang: Formal analysis, Methodology. Yoon-Hyuk Huh: Validation, Investigation. Chan-Jin Park: Conceptualization. Lee-Ra Cho: Supervision, Project administration.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was supported by 2020 Scientific Research Program (SR2004) of Gangneung-Wonju National University Dental Hospital.
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