PSI - Issue 13

Tsanka Dikova / Procedia Structural Integrity 13 (2018) 461–468 Tsanka Dikova / Structural Integrity Procedia 00 (2018) 000–000

467

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Therefore, the specific layered macrostructure, fine microstructure with dendritic morphology, phase composition – the presence of ε-phase and the typical defects of the SLM Co-Cr alloy Co212-f define the fracture mechanism during bending. In dental bridges, made by SLM, many pores with different sizes, poor welding between the separate layers and tracks as well as cracks between them were observed (Dikova et al. (2015-1). At larger magnification of the fracture surface, the cracks, connecting the individual pores, are clearly visible (Fig. 6-b, c). Consequently, the specific microstructure is a prerequisite for destroying the SLM bridges not by initiation and growth of single cracks but by generating and development of a network of cracks across the entire area of the high loaded section. Fig. 6-1 shows a scheme of the microstructure, while Fig. 6-2 and Fig. 6-3 - scheme of the fracture mechanism of dental bridge, made by SLM. During bending process, initially in the most loaded areas of the most loaded cross section between the two bridge bodies, the existing cracks at MPB between the individual tracks develop until they reach the adjacent pores (Fig.6-2, Fig.6-b). Since during bending test the tensile load acts in a direction parallel to the building layers, the cracks develop mainly along the MPB between the separate tracks (Lu et al. (2015). With increasing the load a moment reaches in which most of the area of the section is dotted with a network of cracks between the pores (Fig. 6-3), the section cannot withstand the load and the bridge breaks out suddenly by splitting. This mode of destruction is dangerous for constructions that work on a cyclical loading, such as fixed partial dentures. Therefore, prior to the introduction of the SLM process for production of dental constructions, the technological regimes have to be optimized in order to obtain a dense structure without pores and defects. 4. Conclusions In the present work an experimental study by bending test of three groups four-part dental bridges (1-st premolar to 2-nd molar) was conducted. The samples were made of Co-Cr alloys by conventional lost wax casting, casting with 3D printed patterns and selective laser melting. It has been found that bending fracture of dental bridges made of Co212-f alloy by SLM occurs at 9.255 kN load, which is commensurable with the cracking loads of the specimens cast from Biosil-F alloy: 9.820 kN in conventional casting and 10.171 kN in casting with 3D printed patterns. The fracture of dental bridges, cast from Biosil-F Co-Cr alloy, consists of three stages - a crack initiation, its development and final destruction. In contrast, the fracture of the SLM bridges from Co212-f alloy suddenly occurs by splitting as a result of generation and development of a network of cracks in the entire volume. The destruction type of the Co-Cr bridges, produced by SLM and casting, is identical - ductile, but the way that fracture occurs is different due to their structure. The fracture mechanism of SLM Co-Cr dental bridges is proposed. The specific layered macrostructure, the fine microstructure with dendritic morphology, the phase composition – the presence of ε-phase and the typical defects of the SLM Co-Cr alloy Co212-f define the fracture mechanism during bending. The fracture macro-geometry of the cast samples is heterogeneous with higher relief, characterizing with ductile component and fibrous-strip morphology with different dispersion in different directions. The fracture surface of the SLM bridges comprises a relatively homogeneous macro-geometry, fibrous and loose-textured macrostructure with ductile relief component and no presence of areas of fibrous-strip or fibrous-flaky morphology. Co-Cr dental bridges, produced by additive technologies, are destroyed at average loads that exceed many times the total chewing load of the four teeth of the studied constructions. Therefore, after optimization of the technological regimes, the additive technologies can be successfully used for manufacturing metal infrastructures of fixed partial dentures for metal-ceramic or covering with polymer or composite. Acknowledgements The cast samples were fabricated by D. Pavlova and M. Simov at Medical Colledge of Medical University of Varna. The 3D printed cast patterns and SLM specimens were produced in the Scientific Research Laboratory “CAD/CAM in Industry” at Technical University – Sofia with head Prof. Georgi Todorov. Bending test was done in “Multitest” JSK, Varna. References

Al Jabbari, Y.S., 2014. Physico-mechanical properties and prosthodontic applications of Co-Cr dental alloys: a review of the literature. The journal