Effects of rapid infrared radiation heating on the warping deformation and mechanical properties of selective laser melted Co-Cr dental alloy

Statement of problem: Infrared radiation heating (IRH) technology has been innovatively applied to the annealing of selective laser melted (SLM) cobalt chromium (Co-Cr) frameworks. However, previous studies have not reported the effects of IRH on the warping deformation and mechanical properties of these frameworks.

Purpose: The purpose of this in vitro study was to investigate the effects of IRH on the warping deformation and mechanical properties of dental SLM Co-Cr alloy and to evaluate its potential applications in dental restorations.

Material and methods: Two types of specimens, bone-shaped tensile specimens and warping deformation specimens (cantilever beam structures), were fabricated by SLM, followed by annealing using IRH and general furnace heating (GFH). The specimens were divided into 4 groups (n=6) based on the applied heat treatment method and build direction (IRH-XY; IRH-Z; GFH-XY; GFH-Z). The cantilever beam support structure of the warping deformation specimen was cut using wire cutting, and the warping deformation was measured using a digital image correlation optical extensometer. Tensile tests were used to evaluate mechanical properties, with the fracture characteristics being observed using a scanning electron microscope (SEM). A micro-Vickers hardness tester was used to measure the microhardness. The microstructures were analyzed using a metallographic microscope. All data were analyzed using a 1-way ANOVA and the Tukey Honestly Significant Difference test (α=.05).

Results: The surfaces of the SLM Co-Cr warping deformation specimens treated with IRH showed slight oxidation, while those treated with GFH exhibited a dark black appearance. No significant difference was found in deformation between the IRH group (0.412 ±0.039 mm) and the GFH group (0.379 ±0.070 mm) (P>.05). The elongation of the longitudinally built specimens was significantly higher than that of their transversely built counterparts (P<.05), while the yield strength showed an opposite trend. The longitudinally built specimens demonstrated ductile fracture characteristics, while the transversely built specimens displayed quasi-cleavage fractures. The specimens treated with IRH exhibited comparable tensile strength and microhardness with the GFH-treated specimens, with no statistically significant differences (P>.05). IRH treatment resulted in finer grains in the SLM Co-Cr alloy. Many fine second-phase particles precipitated from the matrix in the longitudinally built specimens, while few were observed in the transversely built specimens.

Conclusions: The SLM Co-Cr specimens treated with IRH achieved comparable warping deformation and mechanical properties with those of the GFH-treated specimens. The IRH technology holds great potential for application to dental restorations.