This article is dedicated to the high cycle fatigue (HCF) behaviour of cast Al-Si alloys. In particular, three similar alloys with different microstructural characteristics are investigated. The result of an experimental campaign are presented, in order to characterise the fatigue behaviour, and more specifically the fatigue damage mechanisms related to the different microstructural heterogeneities (i.e. casting porosity, dendrite size, SDAS, non-metallic inclusions and silicon particles), observed under different multiaxial loading conditions: pure tension, plane bending, pure torsion and combined tension-torsion with a load ratio R=-1. It is shown that casting porosity has a very detrimental influence on the uniaxial and combined tension-torsion fatigue strengths. However, a much lower influence is observed for the torsional fatigue strength. For the porosity-free alloy, it is observed that the formation of persistent slip bands (PSB) in the aluminium matrix is the major fatigue crack initiation mechanism regardless of the loading modes, at a load ratio of R=-1. It is also shown that the aluminium matrix has a large role in the formation of PSB and that the Si particles facilitate the formation of PSB.

Cast Al-Si alloys have been widely used in automotive applications with regard to their low density and excellent thermal conductivity. Many components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, for these materials the well know size effect in fatigue, whereby the fatigue strength is reduced when the size is increased, can be significant and need to be properly evaluated. This paper analyses the role of casting defects on the fatigue strength's size effect sensitivity. A uniaxial fatigue testing campaign (R=0.1) has been conducted using two cast aluminium alloys, fabricated by different casting processes (gravity die casting and lost foam casting), associated with the T7 heat treatment, and with different degrees of porosity. The fatigue response of different specimens (smooth and notched) with different stressed volumes has been investigated. The first part of this article is dedicated to the experimental characterization of the size effect in both alloys via the concept of the Highly Stressed Volume. The second part investigates the effect of the Highly Stressed Volume on the critical defect size via Kitagawa-Takahashi diagrams. The results show that the magnitude of the size effect and the experimental scatter are strongly linked to the characteristics of the defect population present in the alloy. It is revealed that the alloy B, with a high density of pore and a population of defects with relatively large size, shows non-significant size effect and less scatter in fatigue strength. In comparison, alloy A that exhibits a low density of pore and a population of defects of relatively small size manifests significant size effect and high scatter in fatigue strength.

Porosity generated by the casting process has a detrimental effect on the high cycle fatigue strength of cast aluminium alloys. The current study presents an investigation using the non-destructive X-ray micro-tomo-graphy technique of the effect of the size and the population of casting pores on the fatigue strength of the AlSi7Mg0.3 alloy. This alloy was obtained by the lost foam casting process, which leads to a "high" volume fraction of porosity. Observations by micro-tomography, realized on specimens containing fatigue cracks, highlight an important role of the pore distance to the specimen surface in addition to the pore size. In the second part, the local mechanical response around real 3D pores is investigated thanks to finite element models using an elasto-plastic material behaviour law for the aluminium matrix. A critical volume approach based on the Dang Van criterion to predict the fatigue limit is introduced and shows good agreement with the experimental data. The effect of the pore geometry on the Dang Van equivalent stress distribution around individual pores is also investigated.

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