The effect of microstructural heterogeneities on the High Cycle Fatigue scatter of cast aluminium alloys: from an elementary volume to the structure

Abstract

Cast Al-Si alloys have been widely used in automobile applications thanks to their low density and excellent thermal conductivity. A lot of components made of these alloys are subjected to cyclic loads which can lead to fatigue failure. Furthermore, the well know size effect in fatigue, whereby the fatigue strength is reduced in proportion to an increase in size, can be important. This is caused by a higher probability of initiating a crack in larger specimens (i.e. statistical size effect). This paper analyses the role of casting defects on the statistical size effect. For that, 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. Different specimens (smooth and notched) with different stressed volumes have been investigated. The first part of this article is dedicated to the experimental characterization of the statistical 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 diagram of Kitagawa-Takahashi. The results show that the presence of statistical size effect is strongly linked to the characteristics of the pore population present in the alloy. A numerical approach, linking the observed pore distribution to the volume of loaded material, is proposed and discussed.