Multiaxial high cycle fatigue damage mechanisms associated with the different microstructural heterogeneities of cast aluminium alloys

Abstract

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.