Oriented face-centered cubic to hexagonal close-packed martensitic transition, grain morphology, and mechanical properties of Co-Cr alloy fabricated by selective laser melting

Abstract

Statement of problemRemovable partial dentures (RPDs) can be fabricated by selective laser melting (SLM) with different build orientations. How microstructures and mechanical properties of SLM Co-Cr alloy are affected by different build orientations is unclear. PurposeThe purpose of this in vitro study was to investigate the phase structures, grain morphology, and mechanical properties of SLM Co-Cr alloy with different build orientations. Material and methodsSLM Co-Cr tensile specimens were fabricated at orientation angles of 0, 45, and 90 degrees between the building and longitudinal direction named the T0, T45, and T90 design groups (n=14). Mechanical properties were obtained by tensile testing conducted by using a universal testing machine according to the International Organization for Standardization (ISO) 22674. The grains along the longitudinal direction of the specimens and phase structures were observed before and after tensile testing by electron backscatter diffraction. One-way analysis of variance followed by the Bonferroni post hoc test and Kruskal-Wallis test were used for statistical analysis (α=.05). ResultsThe 0.2% yield strengths in descending order were T90 (870 MPa)>T45 (840 MPa)>T0 (786 MPa) (P<.05); the elongations were T0 (21.8%)>T45 (15.6%)>T90 (8.7%) (P<.05); the ultimate tensile strengths were T45 (1226 MPa)>T90 (1200 MPa)>T0 (1149 MPa) (P<.05). The average grain sizes in the T0, T45, and T90 groups were 22 μm, 18 μm, and 14 μm, respectively. After the tensile test, a face-centered cubic (FCC) to hexagonal close-packed (HCP) martensitic transition was found in each group, and the phase transition area fractions were T0 (38.3%)>T45 (11.4%)>T90 (0.7%). ConclusionsThe FCC to HCP martensitic transition, grain morphology, and mechanical properties of SLM Co-Cr alloy depended on the build orientations. The oriented phase transition and grains affected the anisotropic mechanical properties of SLM Co-Cr alloy.