PSI - Issue 7

A. Brueckner-Foit et al. / Procedia Structural Integrity 7 (2017) 36–43 A. Brückner-Foit / Structural Integrity Procedia 00 (2017) 000–000

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4. Discussion

The stress concentration factors (Kt) of gas and shrinkage pores were calculated by lineal-elastic FE-analysis converting the geometry of the 3D reconstructions to CAD models and inserting the pores as cavities in cylindrical volumes subjected to tensile loading in Abaqus [Dassault Systemes]. The results revealed that the highest stress concentration factors (ranging from 7 to 20) are located in several points of the complex dendritic network (see Fig. 10) that tend to be surrounded by gas pores, what implies the fact that multiple initiation is susceptible of occurring, as the observation of the fracture surfaces confirmed. The Kt factors on the gas pores are relatively smaller in comparison with the shrinkage pores, but their location near the surface can induce the fatal failure even in the presence of larger shrinkage pores. This is not the case of the Fe-rich inclusions, which tend to be located in the inner part of the specimen. Nevertheless, the agglomeration of these particles in large cluster promotes the formation of pores (see Fig.11) as a consequence of the reduction of permeability in the casting process. Indeed, an increasing porosity was measured in the specimens with higher Fe-content, what may significantly reduce the fatigue properties of the alloy.

a) b) Fig. 10 Stress concentration factors for typical pore geometries (maximum, average values and the index of the orientation at which the maximum value was obtained): a) gas pores; b) shrinkage pore.

Fig.11 Intermetallic network of ß-Al 5 FeSi inclusions in which shrinkage and gas pores are retained.

5. Conclusions Multiple crack initiation on the interaction between gas pores located at the inter-dendritic spaces and the shrinkage pores was identified as the main cause for fatal fatigue failure in the absence of gas pores located at the near surface. The Fe-rich inclusions did not have a direct effect on fatigue life, although the Fe-content promotes the formation of porosity due to larger Fe-rich inclusions that block the melt flow in the casting process.