The influence of porosity on the fatigue life for sand and permanent mould cast aluminium

The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied. Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R = −1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life. The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.