Relationship between Phase Fractions and Mechanical Properties in Heat‐Treated Laser Powder‐Bed Fused Co‐Based Dental Alloys

Abstract

Metal additive manufacturing of dental prostheses consisting of cobalt−chromium−tungsten (Co−Cr−W) alloys poses an alternative to investment casting. However, metal additive manufacturing processes like Laser Powder‐Bed Fusion (LPBF) can impact the elastic constants and the mechanical anisotropy of the resulting material. To investigate the phase compositions of mechanically different specimens in dependence of their postprocessing steps (e. g. heat treatment to relieve stress), the current study uses X‐ray Diffraction (XRD), Electron BackScatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) for phase identification. Our studies connect plastic deformation of Remanium star CL alloy with the formation of the hexagonal ϵ‐phase and heat treatment with the formation of the D024‐phase, while partially explaining previously observed differences in Young's moduli.

In-Situ XRD Study of Phase Transformation Kinetics in a Co-Cr-W-Alloy Manufactured by Laser Powder-Bed Fusion

“…Regarding the Laves-phase precipitates, findings are in line with Yamanaka et al [8], but not with the results of von Kobylinski et al [18]. Hence, the electron diffraction and XRD patterns presented by the latter were reevaluated, and a fit could be achieved for Laves-phase also in these results.…”

“…Another open question is the link between the increase in Young's modulus on the macro-scale and the mechanisms active on the micro-scale during HTs. A γ-to-transformation was ruled out by von Kobylinski et al, as it was absent from their applied experimental conditions [18]. While taking the phase fractions into consideration, they furthermore concluded that precipitates can only account for a contribution to the observed increases, but cannot be the sole mechanism.…”

“…In contrast, the high cooling rates combined with the characteristic L-PBF buildprocess lead to a fine-grained, homogeneous, and largely dense microstructure resulting in mechanical properties generally superior to cast samples [17]. The AB condition is, however, also highly textured and bears residual stresses on the micro-scale [18,19]. On the macro-scale, this condition is anisotropic, especially regarding the Young's modulus, which not even reaches the ISO 22674 type 5 criteria of 150 GPa in certain directions [10].…”

“…The predominant phase in Co-Cr parts conventionally cast or as built (AB) with L-PBF is a metastable face-centered cubic (fcc) γ-phase with the constituent atoms of the alloy in solid solution [4,17]. This is known to transform to the equilibrium hexagonal close-packed (hcp) -phase during certain heat treatments (HTs) or via strain-induced transformation [8,18].…”

“…On the macro-scale, these result in a homogenization and an increase in mechanical properties, in particular, the Young's modulus [10]. On the micro-scale, multiple mechanisms like the γ-to-phase transformation, precipitation, recovery, residual stress relieve, recrystallization, and grain growth are activated by the HT [18][19][20]. Some of these mechanisms can affect the performance of dental frameworks adversely, which is the reason why the parameters of the HT have to be optimized with regard to the stated requirements.…”