Effects of heat treatment on the microstructure, residual stress, and mechanical properties of Co-Cr alloy fabricated by selective laser melting

Comparison of tensile properties and porcelain bond strength in metal frameworks fabricated by selective laser melting using three different Co-Cr alloy powders

STATEMENT OF PROBLEM

The selective laser melting (SLM) manufacturing technique has been widely employed to produce Co-Cr dental metal frameworks. The selection of Co-Cr alloy powders has the potential to influence the microstructure and tensile properties, consequently impacting the bond strength of the metal-porcelain. However, limited information is available regarding the effect of Co-Cr alloy powder on these properties when all other factors remain consistent.

PURPOSE

The purpose of this in vitro study was to assess how the choice of Co-Cr alloys during SLM manufacturing influences the microstructure, tensile properties, and bond strength of metal-ceramic combinations.

MATERIAL AND METHODS

Three different Co-Cr alloy powders, Co-Cr-Mo, Co-Cr-Mo-W, and Co-Cr-W were selected in this study. The powder characteristics and chemical compositions were analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis, respectively. Subsequently, 12×12×15-mm cube specimens, cylindrical tensile test specimens, and 25×3×0.5-mm metal strips were fabricated using the SLM technique. Microstructural investigations of the cube specimens were conducted after metallographic preparation using SEM. The cylindrical tensile specimens (n=8) from each composition were subjected to tensile tests at a deformation rate of 0.5 mm/min. Following the application of ceramic to the metal specimens (n=10) in each group, the strength of the metal-ceramic bond was evaluated through a 3-point bend test conducted at a crosshead speed of 1 mm/min. Mechanical properties obtained from the tensile tests and bond strength values were statistically analyzed using a one-way ANOVA and Tukey post hoc comparison tests (α=.05).

RESULTS

Melt pool boundaries, columnar and equiaxed grains, and precipitates were observed in the microstructures of 3 different alloys produced by SLM. The Co-Cr-Mo-W alloy had more uniformly dispersed and finely distributed precipitates compared with other alloy compositions. The Co-Cr-Mo-W alloy had exhibited the highest yield strength (1068.0 ±41.2 MPa) and ultimate tensile strength (1263.4 ±10.7 MPa) while showing the lowest ductility under the tensile tests (6.1 ±0.9%) among all 3 alloys. Significant differences in the tensile mechanical properties were observed in the alloys except between the yield strength of the Co-Cr-Mo and Co-Cr-W alloys. The highest elongation (8.9 ±1.2%) was seen in the Co-Cr-Mo alloy. However, no significant differences were detected regarding the bond strength of all 3 groups (P>.05). The mean bond strength values were approximately 42 MPa for all the alloys.

CONCLUSIONS

The results indicate that the selection of different Co-Cr alloy powders used in SLM production may influence both microstructure and tensile properties. However, the strength of the metal-ceramic bond of Co-Cr alloys remained unaffected by this selection.

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.

The effect of repeated baking of porcelain on its bonding strength to a Co-Cr alloy 3D-printed by selective laser melting

Objectives

This study aimed to evaluate the effect of multiple baking cycles of porcelain on its shear bond strength to a cobalt-chromium (Co-Cr) alloy that is three-dimensionally printed using Selective Laser Melting (SLM) technique.

Materials and methods

The research sample comprised forty-eight discs measuring 5 mm × 3 mm, divided into four groups according to: the manufacturing method (SLM, casting) and the number of porcelain baking cycles (1, 3) as follows: Group A: Co-Cr alloy by SLM with one baking cycle; Group B: Co-Cr alloy by SLM with three baking cycles; Group C: Ni-Cr alloy by casting with one baking cycle; Group D: Ni-Cr alloy by casting with three baking cycles. Then, porcelain was melted on disks, shear testing was performed and the values of the Shear Bond Strength (SBS) in MegaPascals (MPa) were calculated.

Results

The mean SBS values for each group were (A: 25.69 - B: 19.51 - C: 35.72 - D: 28.67 MPa). Statistical analysis showed that the manufacturing method and the number of porcelain baking cycles had a significant influence on shear bond durability (P > 0.05): the strength of this bond decreased when baking cycles increased. The Co-Cr samples manufactured by SLM also showed a decrease in binding strength compared to the Ni-Cr samples made by casting.

Conclusion

Repeated baking of porcelain reduces the strength of the porcelain bond with the Co-Cr alloy made by Selective Laser Melting (SLM) technique.

Mechanical and electrochemical characterization of 3D printed orthodontic metallic appliances after in vivo ageing

OBJECTIVES

Three-dimensional (3D) printing technology is a promising technique for fabricating custom orthodontic metallic appliances. Aim of this study was to assess the effect of intraoral aging on the mechanical / electrochemical properties of 3D-printed orthodontic metallic appliances.

METHODS

Twelve molar orthodontic distalization appliances 3D-printed from cobalt chromium (Co-Cr) alloy were retrieved after intraoral use and twenty blocks fabricated under similar conditions were used as control. The samples' microstructural / elemental composition assessment was assessed with SEM/EDS, while their mechanical properties (modulus of elasticity [E_IT], Martens hardness [HM] and the elastic index [η_IT]) were measured by instrumented indentation testing. Finally, the samples' electrochemical features were assessed with a potentiostat-connected cell arrangement in terms of open circuit potential (OCP), corrosion potential (E_z), current density (I₃₀₀) and breaking potential (E_pit). Results were analyzed by t-test / Mann-Whitney test (α = 0.05).

RESULTS

The used Co-Cr alloy was found to have a highly homogenous structure with no significant differences between retrieved and new specimens in HM (4037.7 ± 215.6 vs 4090.9 ± 259.8 N/mm²), E_IT (120.0 ± 13.2 vs 123.8 ± 12.9 GPa), or n_IT (28.4 ± 2.6 vs 28.6 ± 2.9 %) (P > 0.05 in all instances). Metallic surfaces retained the same oxidation tendency and oxide dissolution rate in passive region in both groups (P > 0.05 for OCP, E_z, and I₃₀₀). However, intraorally-aged specimens had a significantly lower breakdown potential due to degraded protection efficacy of surface oxide (P = 0.003 for E_pit).

SIGNIFICANCE

The tested 3D-printed Co-Cr orthodontic appliances present clinically-acceptable mechanical properties that remained unaffected by intraoral ageing, which however degraded the protection of surface oxide against pitting corrosion.

Combination of Scanning Strategies and Optimization Experiments for Laser Beam Powder Bed Fusion of Ti-6Al-4V Titanium Alloys

This paper studies the effects of different combinations of scanning strategies between layers on the surface quality, tensile properties, and microstructure of samples in a laser beam powder bed fusion (L-PBF) formation experiment of Ti-6Al-4V titanium alloy. The purpose of this experiment was to improve the comprehensive performance of the piece by selecting the optimal combination of scanning strategies. The results show that the surface roughness of the L-PBF specimen was the lowest under the combination of the CHESS scanning strategy, reaching 14 μm. The surface hardness of the samples was generally higher with the LINE scanning strategy and the angle offset of 90°, reaching 409 HV. The overall density of the samples was higher under the combination of CHESS scanning strategies, reaching 99.88%. Among them, the CHESS&45° sample had the best comprehensive properties, with a density of 99.85%, a tensile strength of up to 1125 MPa, a yield strength of 912 MPa, and an elongation of 8.2%. The fractured form was a ductile fracture, with many dimple structures. Compared with the CHESS scanning strategy, the tensile properties of the CHESS&45° samples were improved by 12.8%. The microstructure of the L-PBF sample was mainly composed of the primary β phase and α’ martensite phase. The upper surface of the CHESS scanning strategy combination sample had a clear melt channel, and the distribution of each phase was uniform. A certain number of columnar β crystals were distributed in the longitudinal section of the sample, which was paralleled to the build direction. The columnar β crystals of CHESS&45° were relatively coarse, which enhanced the tensile properties of the sample.