Aluminum castings are widely used by automotive industry especially within the drive chain due to their excellent lightweight performance. When high power densities are required die castings are preferred, because of their excellent material properties. The fatigue design of aluminum die castings accounts for a large number of stress related factors, such as mean stress, stress gradient and stress state, as well as environmental and load sequence effects. On the other hand local differences of the material properties are neglected up to now, since appropriate material models are missing. Therefore, the aim of this thesis was to identify the essential microstructural parameters influencing the fatigue behavior of AlSi casting alloys and to provide appropriate material models to account for the influence of local microstructure, as well as to identify the limitations of the model’s applicability in terms of heat treatment, alloy composition and microstructure. For that purpose, fatigue tests using two different AlSi alloys (AlSi7MgCu0,5 and AlSi10Mg) were performed. The results show that within a component from serial production a difference of local endurance limit of up to 35% can be observed. The reasons for the variation of endurance limit were determined depending on failure mechanism. For failure from defects the defect size governs fatigue behavior, overruling effects from heat treatment and other microstructural characteristics, such as dendrite arm spacing and eutectic morphology. Pores and intermetallic compounds, as well as oxides and in some cases bifilms were the defects observed. On the other hand when it comes to slip band induced failure heat treatment, alloy composition and microstructural characteristics do effect the fatigue behavior significantly. In addition the interaction of microstructural effects with selected influencing factors (mean stress, notches and elevated temperature) of fatigue behavior was investigated based on the defect afflicted AlSi10Mg alloy. The results show that mean stress sensitivity is independent of local microstructure, when it comes to a defect induced failure mechanism and it can be estimated based on fracture mechanics. On the other hand the effect of notches is dependent on the defect size distribution. Therefore, a model was developed to determine the endurance limit within a notch for defect dominated material based on stress distribution and defect size. Furthermore, the effect of local microstructure on fatigue behavior at an elevated temperature was examined. It was shown, that at a temperature of 200°C the effect of local microstructure on fatigue behavior diminishes and the maximum difference of endurance limit decreases from about 35% within the investigated component at room temperature to about 7%.
|Translated title of the contribution||The influence of microstructure on the high cycle fatigue behavior of AlSi-casting alloys|
|Publication status||Published - 2014|