Microstructures and Mechanical Properties of Dental Co-Cr-Mo-W Alloys Fabricated by Selective Laser Melting at Different Subsequent Heat Treatment Temperatures

Abstract

To apply the dental Co-Cr-Mo-W alloy restorations fabricated by selective laser melting to clinical use cases, it is necessary to perform a subsequent heat treatment (HT) after SLM processing. However, investigations regarding the effects of HT temperatures on microstructures and mechanical properties inherent in SLM-manufactured Co-Cr-Mo-W alloys are still insufficient. In this article, the microstructure variation during HT at different temperatures and its effect on mechanical properties were systematically investigated and discussed. The Co-Cr-Mo-W dental alloys fabricated by SLM were heat treated at 750 °C, 850 °C, 950 °C, 1050 °C, and 1150 °C for 1 hour and subsequently air cooled. Then, the microstructures were characterized and the mechanical properties evaluated. The results showed that, during HT, Co₃(Mo, W)₂Si intermetallic compounds with hcp crystal structures precipitated in specimen matrixes and at grain boundaries. With heat treatment temperatures increasing from 750 °C to 950 °C, the number of precipitates gradually increased, and the volume gradually grew. When the temperature increased to ≥ 1050 °C, the number of intermetallic compounds decreased. On the other hand, with increasing HT temperatures, the content of ε-Co phases increased initially but then dropped, reaching a maximum of 97.5 pct at 950 °C and a minimum of 4.4 pct at 1150 °C. Furthermore, there is a coherent relationship between hcp phases (ε-Co) and fcc phases (γ-Co), and the two lattice cells allow the relative orientation of {0001}ε//{111}γ, 〈11-20〉ε //〈110〉γ. Mechanical test results showed that, with increasing HT temperatures, the 0.2 pct yield strength (0.2 pctYS) and ultimate tensile strength (UTS) increased initially and then dropped, reaching a maximum value at 850 °C. Conversely, the ductility decreased initially and then rose, reaching a minimum at 850 °C and 950 °C. The mechanical properties depended primarily on the change of volume fractions of the ε-Co in mixture as well as the size, morphology, and distribution of precipitates. Additionally, specimens heat-treated at 1050 °C for 1 hour were superior to as-SLM (AS) specimens in both strength and ductility.