PSI - Issue 2_A

M. Wicke et al. / Procedia Structural Integrity 2 (2016) 2643–2649

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M. Wicke et al./ Structural Integrity Procedia 00 (2016) 000–000

the mechanical properties of the final product but also to be deleterious to castability, resulting in an increased porosity content [Dinnis et al. (2006), Lu and Dahle (2005), Taylor et al. (1999)]. Porosity has been shown to be highly detrimental to the fatigue life of commercial Al-Si casting alloys [Couper et al. (1990), Gao et al. (2004), Skallerud et al. (1993), Wang et al. (2001)]. Previous studies have indicated a predominant initiation of fatigue cracks at large pores situated in close proximity to the specimen surface since these pores induce much higher stress concentration compared to those located in the volume. Consequently, the fracture mechanics assessment of porosity in Al-Si alloys requires the accurate reconstruction of pores (i.e. distribution, size and morphology) within a volume. While the traditional techniques for analyzing such defects are spatially limited, recent developments in X-ray computed tomography (x-CT), whose application to pore characterization in cast Al alloys was pioneered by Buffière and coworkers [Buffière et al. (2001), Ferrié et al. (2005)], have provided the possibility for exhaustively reconstructing the porosity of samples in detail. The morphological assessment of porosity coupled with finite element analysis (FEA) has been increasingly used in the past years to investigate the correlations between defect geometry, stress concentration and crack initiation. In [Li et al. (2006)] the influence of rather regularly-shaped pores on the evolution of fatigue damage was illustrated using a volumetric image obtained by X-ray CT as input for a FEA. A similar approach was applied in [Vanderesse et al. (2011)]. After finely reconstructing the entire porosity of fatigued Al-Si casting samples, the pore influence zone, which represents the matrix volume stressed above the yield stress, was quantified on the basis of elastic FEA. More recently in [Nicoletto et al. (2012)], the role of casting pore morphology and loading direction on stress concentration was investigated with FEA considering realistic pores obtained by high-resolution x-CT. In the present study, high-resolution X-ray CT is applied to cast Al-Si-Cu for determining the size and morphology of casting pores. Shrinkage pores, which are complex-shaped with multiple arms, are reconstructed and imported into commercial finite element software with the aim of assessing the structural influence of these defects. Special emphasis is put on an appropriate visualization of the results, allowing the identification of hot spots, i.e. regions of elevated stress on the contour of the pores investigated. 2. Material The material used in this work is a Fe-rich near-to-eutectic Al-Si-Cu alloy with the chemical composition given in Table 1. Al-25Fe (wt. %) master alloy was added to the melt in order to achieve 0.6 weight % of Fe content with a relationship Mn:Fe of 0.28. Although this Mn:Fe relationship can promote partial substitution from ß-Al 5 FeSi to the less detrimental α-Al 15 (Fe,Mn) 3 Si 2 [Ashtari et al. (2003)], a considerable amount of ß-phase is formed.

Table 1. Chemical Composition of the Al-Si-Cu alloy (wt. %). Si Cu Mg Fe

Zn

Mn

Ni

Ti

Al

12.96

1.52

0.68

0.6

0.48

0.17

0.05

0.04

rest

Sand casting was performed at 760°C ± 5°C to form sheets of 4 mm thickness in a sodium mold at room temperature, from which specimens of 20 mm length with a square cross section of 4x4 mm were machined out for high-resolution x-CT analysis. 3. Structural influence of casting pores on 3D stress concentration The basics of the x-CT technique and its application to the present Al-Si-Cu casting alloy are presented in this section, before the generation of 3D volumetric models by surface reconstruction is described. Finally, the 3D pore morphology in combination with finite element computation is proposed for investigating the stress concentration caused by casting pores.