Influence of stress-relieving heat treatments on the efficacy of Co-Cr-Mo-W alloy copings fabricated using selective laser melting.

Purpose This study aimed to evaluate the influence of stress-relieving heat treatments on the metal-ceramic bond strength and fitness accuracy of selective laser melting (SLM)-fabricated Co-Cr alloy copings.Methods SLM-manufactured Co-Cr samples were stress-relieved at 750 (Ht-750) and 1150 °C (Ht-1150). The microstructure, surface roughness, metal-ceramic bond strength, marginal and internal fit, Vickers hardness, and residual stress were then compared with those of the non-heat-treated group (As-built). The results were analyzed using one-way ANOVA and post-hoc tests (Tukey's or Student's t test) (P = 0.05).Results The microstructure of the Ht-1150 samples had a brittle oxide layer and lower surface roughness, resulting in significantly lower bond strength values than those of the other groups. The As-built group exhibited significantly lower marginal gap values than the Ht-750 and Ht-1150 groups. Therefore, the post-heat treatments degraded the marginal fitness. The surface residual stress in all sample groups were compressive because of the sandblasting effect. The compressive stresses were larger in Ht-1150 than in As-built and Ht-750 owing to their low hardness values.Conclusions Stress-relief annealing porcelain-fused-to-metal single crowns does not improve bond strength and degrades fitness accuracy because additional post-heat treatments induce thermal distortion. These findings are expected to facilitate the direct application of As-built SLM single crowns in dentistry to minimize post-manufacturing costs and time.

Effect of ultraviolet irradiation treatment on shear bond strength between polymethyl methacrylate and cobalt-chromium-molybdenum alloy.

We aimed to elucidate the effects of ultraviolet (UV) irradiation on the shear bond strength (SBS) between heat-cured polymethyl methacrylate (PMMA) and a Co-Cr-Mo alloy. Disk-shaped Co-Cr-Mo alloy prepared by casting were subjected to different UV treatment times (0, 15, and 30 min). To determine the effect of UV treatment on surface properties of the alloy, surface roughness, wettability, and chemical compositions were analyzed. To evaluate the SBS, cylindrical PMMA was bonded to the UV-treated alloy, and subsequently subjected to the SBS test after 24 h of storage at room temperature or following 10,000 thermal cycles (n=10/group). After the UV treatment, the surface roughness remained unchanged, but oxidation resulted in the surface exhibiting greater hydrophilic characteristics. The UV-treated group showed significantly higher SBS values than those of the non-treated group (p<0.001). These results suggested that UV treatment-mediated oxidation improved the bond strength between PMMA and Co-Cr-Mo alloy.

Physical Properties of Additively Manufactured Tooth-Colored Material Attached to Denture Base-Colored Material in a Printed Monolithic Unit.

Additive manufacturing is an emerging technology that has been successfully used in dentistry for denture fabrication. However, the conventional issue of tooth debonding exists in additively manufactured dentures. In this study, we investigated the physical properties of conventional teeth attached to a heat-cured denture base material compared to additively manufactured tooth-coloured materials attached to denture base-coloured materials in a printed monolithic unit. We designed a model consisting of a tooth attached to a cylindrical base to fabricate the additively manufactured group and the conventional group. All groups were tested for fracture load before and after thermocycling, water sorption, solubility, and shape accuracy. The Mann-Whitney U test was used for statistical analysis. The fracture load of the additively manufactured group was significantly higher than that of the conventional group after thermocycling (p = 0.019). The water sorption of the conventional group was significantly lower than that of the additively manufactured group (p = 0.000), whereas there was no significant difference in the water solubility between them (p = 0.192). The shape accuracy of the additively manufactured group was significantly better than that of the conventional group (p < 0.05). In conclusion, additive monolithic manufacturing technology may provide an alternative way to enhance the fracture load between the teeth and denture bases.

Bond strength of CAD/CAM denture teeth to a denture base resin in a milled monolithic unit.

Purpose Herein, the bond strength (BS) of denture teeth to a denture base resin in a milled monolithic unit was investigated and compared with those of 3D printed teeth to a 3D printed denture base and prefabricated teeth to a heat-cured acrylic resin before and after thermocycling.Methods Sixty specimens of a denture tooth attached to a cylindrical denture base were fabricated following ISO Standard 19736. Three fabrication techniques-3D printing, conventional compression molding, and milling using monolithic technology-were employed to mill teeth and denture base parts as a single unit. The BS was investigated before and after thermocycling. Data were statistically analyzed using the Kruskal-Wallis test with Bonferroni correction (α = 0.05).Results Before thermocycling, the mean BS of the milled group was significantly higher than that of the conventional group (P = 0.002). The 3D printed group showed no statistically significant difference from the milled (P = 0.051) and conventional (P = 0.824) groups. After thermocycling, although the mean BS values of the milled (P = 0.00) and 3D printed (P = 0.01) groups were significantly higher than that of the conventional group, there was no significant difference between them (P = 0.226). Only the BS of the conventional group was significantly reduced by thermocycling (P = 0.00).Conclusions The milled monolithic fabrication technique, which eliminates the need for a bonding step, offered a promising combination of high-precision digital fabrication and a significantly high BS. The BS of the conventional group significantly decreased after thermocycling.

Reduction in anisotropic response of corrosion properties of selective laser melted Co-Cr-Mo alloys by post-heat treatment.

OBJECTIVES: The application of selective laser melting (SLM) to dentistry has been rapidly expanding; however, SLM-processed parts possess orientation-dependent properties (i.e., anisotropy) that can affect the long-term reliability of the dental prostheses. This study aimed to evaluate the anisotropic corrosion response of SLM-processed Co-Cr-Mo alloys under various heat treatments. METHODS: Samples fabricated via SLM along the horizontal plane (x-y plane) and vertical plane (x-z plane), with respect to the build direction, were subjected to various heat treatments. The resulting microstructures of the samples were characterized, and their corrosion properties were evaluated using anodic polarization and immersion tests. RESULTS: All samples showed similar transpassive behavior of the polarization curves. However, the immersion tests showed that the as-built x-z plane samples released significantly more metal ions than those fabricated on the x-y plane because of the larger area of preferentially corroded molten pool boundaries (MPBs) in the x-z plane samples. Our results further demonstrated that the heat treatments eliminated the MPBs, resulting in isotropic corrosion properties. However, excessive heat treatment at high temperatures induces the formation of coarse precipitates, resulting in a less-protective passive film. SIGNIFICANCE: The post-build heat treatments at temperatures that eliminate the MPBs are effective in reducing anisotropic corrosion behavior, and the lowest possible temperature is suitable for reducing the amount of released metal ions. These findings are expected to facilitate the application of SLM in dentistry to allow fast and precise production of prosthetic devices.

Effect of heat treatment on the anisotropic microstructural and mechanical properties of Co-Cr-Mo alloys produced by selective laser melting.

The purpose of this study was to evaluate the effect of heat treatment on the anisotropy of the microstructure and mechanical properties of cobalt-chromium-molybdenum (Co-Cr-Mo) alloys fabricated by selective laser melting (SLM). Dumbbell samples were fabricated with the axes deviating from the build direction by 0° (0°-sample), 45° (45°-sample), or 90° (90°-sample) and were subjected to heat treatment at various temperatures (750, 900, 1050, or 1150 °C) for 6 h. In samples heat-treated at 750, 900, and 1050 °C, the microstructures exhibited columnar grains with a <001> fiber texture along the build direction, the same as in the as-built state. The mechanical properties showed anisotropy; the 0.2% offset yield strengths (YS) of the 0°-samples were lower than those of the 90°-samples, and the elongations of the 0°-samples were significantly higher than those of the 45°- and 90°-samples. By contrast, in samples heated to 1150 °C for 6 h, the anisotropic columnar grains completely disappeared, and equiaxed grains with random orientations were found in all samples, indicating that recrystallization had occurred. Moreover, the specific microstructures and texture generated during SLM disappeared. Regarding tensile properties, the initially strong anisotropy exhibited by the as-SLM samples was significantly reduced. Thus, heat treatment at the recrystallization temperature produced uniform equiaxed grains with random texture, which contributed to reducing the mechanical anisotropy of the SLMed Co-Cr-Mo alloys.

Influence of magnetic susceptibility and volume on MRI artifacts produced by low magnetic susceptibility Zr-14Nb alloy and dental alloys.

The artifact volume generated in magnetic resonance (MR) images was quantitatively evaluated to investigate how artifact behavior correlates to the magnetic susceptibility and volume of an implanted metal device. For this, a new low-magnetic-susceptibility Zr-14Nb alloy was compared with two conventional dental alloys, Ti-6Al-7Nb alloy and Co-Cr-Mo alloy, using spherical specimens of each alloy prepared with four different diameters. Then, MR images were recorded under fast spin echo and gradient echo conditions, from which the artifact volume was measured. The artifact volume decreased with the magnetic susceptibility, volume, and mass of the specimens, and significant linear correlations were observed. The artifact volume can be estimated by the equations presented here; nevertheless, further studies are necessary to interpret the influence of some important factors (e.g., imaging conditions, shape, and orientation) to predict the artifact volume more precisely.

Fatigue properties of removable partial denture clasps fabricated by selective laser melting followed by heat treatment.

The aim of the study was to investigate the effect of post-heat treatment on the microstructures and fatigue strengths of Co-Cr-Mo (CCM) clasps prepared by selective laser melting (SLM). Clasp specimens and rod-shaped specimens were fabricated by SLM using CCM powders with different angulations (0°, 45°, and 90°). Two heat treatment conditions were used: 1 and 6 h at 1150 °C in an argon atmosphere followed by cooling in a furnace. Subsequently, the fatigue strength and microstructure were investigated. The results revealed that for the control group, the mean fatigue life was different when building angulations were changed as manifested by the control-90 specimen, which exhibited the highest fatigue life, followed by control-45 and then control-0. One-hour heat treated samples showed higher fatigue strength in all axes than the 0°- and 45°-axes control samples and lower fatigue strength than the 90°-axes control samples. The survival rate after heat treatment for 1 h exhibited no significant difference in all the axes. In terms of microstructure, after heat treatment, the samples showed homogeneous equiaxed grain and randomized texture in all angulations. Therefore, using a post-heat treatment can reduce the anisotropy effect on the microstructure and fatigue strength due to homogenized microstructure.

Effect of adding support structures for overhanging part on fatigue strength in selective laser melting.

Selective laser melting (SLM) technology was recently introduced to fabricate dental prostheses. However, the fatigue strength of clasps in removable partial dentures prepared by SLM still requires improvement. In this study, we attempted to improve the fatigue strength of clasps by adding support structures for overhanging parts, which can generally be manufactured at an angle to be self-supporting. The results show that the fatigue strength of the supported specimens was more than twice that of unsupported specimens. Electron back-scattered diffraction analysis revealed that the supported specimens exhibited lower kernel average misorientation values than the unsupported specimens, which suggested that the support structure reduced the residual strain during the SLM process and helped to prevent micro-cracks led by thermal distortion. In addition, the supported specimens cooled more rapidly, thereby forming a finer grain size compared to that of the unsupported specimens, which contributed to improving the fatigue strength. The results of this study suggest that the fatigue strength of overhanging parts can be improved by intentionally adding support structures.

Fatigue strength of Co-Cr-Mo alloy clasps prepared by selective laser melting.

We aimed to investigate the fatigue strength of Co-Cr-Mo clasps for removable partial dentures prepared by selective laser melting (SLM). The Co-Cr-Mo alloy specimens for tensile tests (dumbbell specimens) and fatigue tests (clasp specimens) were prepared by SLM with varying angles between the building and longitudinal directions (i.e., 0° (TL0, FL0), 45° (TL45, FL45), and 90° (TL90, FL90)). The clasp specimens were subjected to cyclic deformations of 0.25mm and 0.50mm for 10(6) cycles. The SLM specimens showed no obvious mechanical anisotropy in tensile tests and exhibited significantly higher yield strength and ultimate tensile strength than the cast specimens under all conditions. In contrast, a high degree of anisotropy in fatigue performance associated with the build orientation was found. For specimens under the 0.50mm deflection, FL90 exhibited significantly longer fatigue life (205,418 cycles) than the cast specimens (112,770 cycles). In contrast, the fatigue lives of FL0 (28,484 cycles) and FL45 (43,465 cycles) were significantly shorter. The surface roughnesses of FL0 and FL45 were considerably higher than those of the cast specimens, whereas there were no significant differences between FL90 and the cast specimens. Electron backscatter diffraction (EBSD) analysis indicated the grains of FL0 showed preferential close to <001> orientation of the γ phase along the normal direction to the fracture surface. In contrast, the FL45 and FL90 grains showed no significant preferential orientation. Fatigue strength may therefore be affected by a number of factors, including surface roughness and crystal orientation. The SLM process is a promising candidate for preparing tough removable partial denture frameworks, as long as the appropriate build direction is adopted.

Evaluation of the shear bond strength of dental porcelain and the low magnetic susceptibility Zr-14Nb alloy.

The aim of this study was to investigate the bond strength of dental porcelain and the preheated Zr-14Nb alloy, and compare this strength with that of titanium. White oxide layers, which were predominantly composed of monoclinic zirconia, were formed on the preheated sample groups, and exhibited a greater roughness than the control samples. At the metal-ceramic interface, a greater Nb diffusion range was observed than in the control samples. The bond strengths of the samples subjected to 20min preheating treatment were the lowest (33.6 ± 3.2 MPa), which may be ascribed to the formation of a brittle thick oxide layer under excessive heat treatment. The samples subjected to this heat treatment for 5 min exhibited the highest mean bond strength (43.7 ± 5.9 MPa), which was significantly higher than that of titanium (35.3 ± 3.5 MPa). Thus, the Zr-14Nb alloy is a promising candidate for fixed dental prosthesis, as long as the appropriate treatment conditions are adopted.

Surface characteristics and castability of Zr-14Nb alloy dental castings.

The purpose of this study was to evaluate mechanical properties, surface characteristics and castability of Zr-14Nb dental castings. The mean 0.2% proof and ultimate tensile strengths of Zr-14Nb were approximately 68% and 76% those of Ti-6Al-7Nb, respectively, while they were comparable to Co-29Cr-6Mo. Elongation of Zr-14Nb was the highest of all alloys tested. The hardened reaction layer was formed on the surfaces of the Zr-14Nb castings. The layer consisted of oxygen and aluminum originating form investment. The castability of Zr-14Nb was comparable to that of Ti-6Al-7Nb. Dental castings of Zr-14Nb reveal mechanical properties that were within the range of the other dental alloys. Further improvements in castability and minimization of the surface reaction layer are needed for applications in dental prostheses.