The influence of surrounding environment on the fatigue properties for a high pressure die cast AlSi9Cu3 alloy

The influence of mechanical strength for aluminium castings is often correlated in the literature to the occurrence of cast defects like porosity. However, because aluminium castings in the automotive industry are often used without surface treatment such as painting etc., the influence of corrosion and its effect on fatigue properties are required. Basically a surrounding environment can affect fatigue strength either by enhancing initiation or by increasing fatigue crack propagation properties. In this study, the influence of pre-exposure prior to fatigue testing has been evaluated. This implies that environmental enhancement of fatigue initiation due to corrosion has been studied. Two different environments [seawater acidified test (SWAAT) and Volvo Indoor Corrosion Test (VICT)] often employed for assessment of corrosive properties in the automotive industry have been used for pre-exposure of specimens. Based on experimental results, it is shown that fatigue strength is reduced by approximately 50% for specimens pre-exposed to SWAAT environment, while insignificant influence was found for specimens pre-exposed to VICT environment. The degradation of fatigue strength was found to be due to localized corrosive attacks. Propagation of these corrosive attacks takes place preferably in the eutectic phase and especially at the borderline between primary aluminium dendrites and the eutectic phase.