Aluminium castings of the lightweight driven automotive industry often possess complex geometries with strongly varying local cooling rates. In order to enhance the durability of these components, different subsequent heat treatment processes are realised. Generally, this results in differing microstructural formations in terms of secondary dendrite arm spacing and porosity.
Thus, the overall fatigue assessment needs to cover the microstructure respectively cooling rate dependent damage mechanisms as well as load level dependent ones.
Recent research activities within the cast aluminium alloy ENAC46200 revealed a change of the main failure mechanism with increasing load from porosity induced failure to slip band induced failure at positions with high cooling gradients through the use chills. This enabled a fine microstructure and therefore a low degree of porosity, resulting an enhanced material resistance. Additionally, it should be mentioned, that the work focusses on samples taken out of aluminium crankcases, facilitating different variations in terms of cooling rate and heat treatment conditions. As microstructural indicator, the secondary dendrite arm spacing values are within a range of 25 to 55 microns. To reduce porosity, several crankcases are hot isostatic pressed (HIP) and subsequently peak aged (T6) to compare identical T6 heat treatment conditions.
Porosity induced failure turned out to be the main failure mechanism for coarse and fine microstructural conditions in the high-cycle fatigue region, enabling a defect-based probabilistic fatigue strength assessment. In order to evaluate the cyclic material properties, low-cycle fatigue tests were conducted at the aforementioned positions, each of them with two different heat treatment conditions. Within this study, persistent slip band induced failure occurred mainly for HIP+T6 treated specimens inheriting the fine microstructure with directional solidification, resulting from the chills. In contrast, a coarser microstructure leads to porosity induced failure. Moreover, it turned out that HIP of positions with high cooling rates leads to almost no significant improvement of the low cycle fatigue strength, a coarse microstructure can be significantly improved with HIP.
Summing up, the study improves the understanding of changing failure mechanisms in complex modern cast parts, inheriting varying cooling rates and heat treatment conditions.