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

Driss El Khoukhi,Pierre Osmond,Nicolas Saintier,Daniel Bellett,Franck Morel,G Hénaff

doi:10.1051/matecconf/201816514006

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.

Highlights

1.1 Context and objectivesThis work focuses on cast Al-Si alloys, widely used in the automotive industry
These Microstructural Heterogeneities control the fatigue behaviour of these Al-Si alloys. These components are subjected to cyclic mechanical loads that can result in the appearance of cracks and lead to the failure of the structure
In [18,19,20] it was shown, that there is a continuous relationship between the fatigue strength expressed as Ktσa and the highly stressed volume

Summary

Introduction

1.1 Context and objectivesThis work focuses on cast Al-Si alloys, widely used in the automotive industry. In order to manufacture engine components, the PSA Groupe uses two foundry processes that result in components containing Microstructural Heterogeneities (MH) with different characteristics. These Microstructural Heterogeneities control the fatigue behaviour of these Al-Si alloys. A typical engineering design practice [4] involves the identification of the fatigue criteria, used to size and validate real components, based on tests carried out on laboratory specimens, which have a specific volume, under simple loading modes (tension, bending, and torsion). The industrial objective of this work is to develop predictive tools to ensure the passage from an elementary volume to the structure taking in account the size and stress gradient effects. The effect of microstructural heterogeneities, principally shrinkage porosity, on the statistical size effect in cast Al-Si alloys is investigated

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Results

Conclusion