PSI - Issue 13

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

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%), while the SLM samples were manufactured from Co212-f ASTM F75 alloy (Co-65.2; Cr-28.3; Mo-5.48; Si-0.5; Fe-0.164 wt. %). In order to have a good repeatability of the samples, a basic bridge-model was firstly made, from which the mold for casting of wax patterns and the virtual 3D model for the CAD/CAM technology were made. Detailed information about the samples fabrication is given in the works of Simov et al. (2014) and Dikova et al. (2015-1).

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Fig. 1 (a), (b) Bending test of dental bridges and (c) bending appliance.

2.2. Bending test During the experiment, in order to be able to carry out the load on the bridges as close as possible to the actual, a special device is designed and manufactured (Fig. 1-c). In order to fix the contact points between the bridge construction and the punches of the bending device, a simulation with chewing loading was done (Vasilev et al. (2016). On this basis, using the CAD software and according to a specially developed methodology (Vasilev et al. (2017), the device itself was designed. The experiment was performed on a universal tensile/compression test machine Tira Test 2300 SE/50kN with loading rate 1.2 mm/min until complete destruction of the samples (Fig. 1). Five samples of each group were tested. Due to the complexity of the shape of the dental bridges, the average loadings to the cracking and complete destruction were evaluated. The fracture surfaces were examined by optical metallographic microscope. 3. Results and analysis The microstructure of cast and SLMCo-Cr dental bridges was investigated in our previous study (Dikova et al. (2015 1). It was established that the microstructure of cast Biosil-F alloy is coarse grained with dendritic morphology, inhomogeneous concerning to the chemical composition. The dendrites consist of γ-phase, while in the inter-dendritic spaces there is presence of micro-eutectic and primary carbides of the mixed type (Cr,Mo) 23 C 6 . The structure of the SLM Co212-f alloy is porous with presence of unmelted and partially melted powder particles as well as cracks, which are mostly at the boundaries between the separate tracks and layers. It was established in our previous investigation (Atapek et al. (2016) that the microstructure of the SLM samples characterized with layered morphology, while the microstructure is fine with homogeneous chemical composition. It has dendritic structure with cellular dendrites in the melted pool bottom and columnar in the central part. The dendrites consist of γ-phase, as there is presence of higher quantity of ε phase and carbides of the mixed type M 23 C 6 . All these features of the microstructure determine the behavior of Co-Cr dental alloys during bending, the mechanism and type of their fracture. 3.1. Bending test Our previous studies by simulation of four-part dental bladders with chewing load have shown that the connector between the two bridge bodies (5-6 teeth) is the most loaded (Vasilev et al. (2016). The equivalent von Mises stresses there are the largest - 162 MPa, followed by the connectors between the two premolars (4-5 teeth) – 147 MPa and the two molars (6-7 teeth) – 95 MPa. In addition, the deformation between the two bridge bodies is highest and can reach