Microstructural and mechanical characterization of six Co-Cr alloys made by conventional casting and selective laser...

Three Co-Cr alloy types (Co-Cr-Mo, Co-Cr-W, and Co-Cr-Mo-W) have been commonly used in the fabrication of dental prostheses. These alloys can be manufactured using either conventional casting or selective laser melting (SLM) techniques. Nevertheless, research that directly compares these materials and/or manufacturing processes in terms of their microstructural and mechanical characteristics is sparse.

The purpose of this in vitro study was to conduct microstructural and mechanical analysis via X-ray interpretation, optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), image analysis, X-ray diffraction (XRD), instrumented indentation testing (IIT), and 3-point bending testing to characterize Co-Cr-Mo, Co-Cr-W, and Co-Cr-Mo-W alloys produced through conventional casting and SLM.

Six Co-Cr-based alloys were analyzed and divided into 3 types based on their elemental composition (Co-Cr-Mo, Co-Cr-W, and Co-Cr-Mo-W). Additionally, each group was categorized based on the manufacturing process used (casting or SLM). X-ray scans were used to assess porosity. The microstructures of the specimens were assessed through SEM/EDS examination and XRD analysis. IIT was used to determine the Martens hardness (HM) and elastic index (η_IT), while the elastic modulus (E) was estimated through the 3-point bending test. The mechanical properties were statistically analyzed using 2-way analysis of variance (ANOVA) and the Tukey multiple comparison post hoc test, with alloy type and manufacturing process as discriminating variables (α=.05).

All cast groups exhibited gross porosity, while no pores or other flaws were found in the SLM groups. Based on the XRD results, the crystalline structure of all Co-Cr specimens consisted of the face-centered cubic γ phase (γ-fcc), along with the hexagonal close-packed ε phase (ε-hcp) and Cr₂₃C₆ carbide. Different microstructures were identified between the cast and SLM alloys. Significant differences were identified for the mean standard deviation HM (ranging from 2601 ±94 N/mm² to 3633 ±61 N/mm²) and mean ±standard deviation η_IT (ranging from 16.8 ±0.3% to 20.9 ±0.3%) among alloys prepared by the same manufacturing process, while all SLM alloys had statistically higher HM and η_IT results than their cast counterparts (P<.05). No statistically significant differences were identified for the mean ±standard deviation E_b (ranging from 170 ±25 GPa to 244 ±36 GPa) among the groups prepared with the same manufacturing process (P>.05), but the SLM alloys had significantly higher results (P<.05) than the cast alloys.

In general, the manufacturing procedure significantly affected the porosity, microstructure, and mechanical properties of the tested Co-Cr alloys. SLM decreased the internal porosity, provided a uniform microstructure, and improved the mechanical properties for all the tested alloy types.