PSI - Issue 13

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

466

6

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

occurs in the most loaded area of the four-part bridges - the lower part of the connector between the second premolar and the first molar, shown with arrow on Fig. 5. With the load increasing the crack develops along the boundaries of the grains or phases - carbides and γ-phase until the fracture occurs. The type of rupture is typical for cast construction - predominantly intergranular with clearly visible traces of individual grains and dendrites, which confirm the research of Al Jabbari (2014) and Choi et al. (2014). The fracture macro-geometry is heterogeneous. A higher relief of the fractured surface is observed, as the ductile component of the relief is characterized by a gray/dark-gray color. Fibrous strip morphology in two approximately orthogonal directions is recognized on the fracture surface of the specimen, as the dispersion is different in different directions. The metal gloss in the upper left corner is characteristic of trans crystal destruction. The fibrous-strip fracture shows anisotropy of the mechanical properties of the material. 3.3. Fracture of SLM Co-Cr dental bridges The study of the fracture surface of the SLM specimens showed a relatively homogeneous macro-geometry (Fig. 6-a) as opposed to that of the cast samples (Fig. 5). The fracture surface of the SLM bridges (Fig. 6-b, c) is fibrous with a relatively rough and loose-textured macro-structure, with no areas of fibrous-strip or fibrous-flaky fracture. Ductile component of the relief in gray/dark-gray color is also observed, which is evidence of the analogous - ductile type of fracture of the SLM samples with that of the cast. The destruction type of the Co-Cr bridges, produced by SLM and casting, is identical, but the way that fracture occurs is different due to their structure. It was found that in SLM samples, the melted pool boundaries (MPB) have a significant influence on the way of demolition, macro-plastic deformation and microscopic sliding during the load (Shifeng et al. (2014). Their destruction starts from cracks initiating at the MPB between the separate melted tracks. The macroscopic texture of MPB may lead to a change in the linear development of the cracks (Qian (2014) and they will continue to develop along the MPB between the layers (Shifeng et al. (2014). During the growth and development of the crack, besides the applied load, the high residual stresses in the SLM sample exert an additional impact (Kajima et al. (2016). For SLM specimens, destroyed in tension, ductile type of fracture is observed, as the zones with ductile fracture surround “quasi-cleavage” facets (Mengucci et al. (2016). The fracture surface of SLM Co-Cr dental alloys reveals homogeneously distributed dimples that are finer than that of the cast, with semi-cleavage morphology observed in some areas (Mergulhão et al. (2018). The dimple morphology of the fracture surface is determined by the cellular dendrites in the microstructure of the SLM Co-Cr dental alloys, while the cleavage fracture is characteristic for alloys with fcc crystal lattice and it is favored by the presence of ε-martensite (Lu et al. (2015).

a

b

c)

c

b

1 mm

1 mm

3)

1)

2)

Fi g. 6 (a), (b) and (c) Fractured surface of Co-Cr dental bridge, manufactured by SLM. (1) Scheme of the microstructure of SLM Co-Cr bridge and (2), (3) initiation and growth of multiple micro-cracks during bending test.