Einfluss von Defekten auf die Streuung der Lebensdauer in einer Lost Foam gegossenen Al-Si-Legierung unter thermomechanischer Ermüdung

In Al-Si cylinder heads, start-stop cycles lead to thermomechanical fatigue (TMF), which is accompanied by the initiation and growth of cracks in the critical web regions. A major issue is the high scatter of the resulting lifetime. The cylinder heads investigated in the framework of this study were manufactured by the lost foam casting process. Due to the low cooling rates caused by the process, the material contains a high number of large and sharp-edged pores. Under cyclic loading, these defects have a significant influence on crack initiation and growth and thus on the lifetime.
During the work, uniaxial TMF tests were performed on specimens extracted from the thermomechanically critically loaded region of the cylinder heads. For the determination of the scatter of the initial crack length a method based on the detection of pores using microcomputer tomography (μCT) was developed. In combination with scanning electron microscopy of the fracture surfaces, the pores in the primary crack origin and the relevant criteria for crack initiation were identified. A program developed in this work enables the computational localization of the primary crack origin from the μCT data. The program contains the identified crack initiation criteria. The scatter of the elastic-plastic crack propagation rate was determined by uniaxial TMF crack propagation tests.
On the basis of the experimentally obtained results, a new method was developed for the assessment of the lifetime scatter of thermomechanically loaded cylinder heads based on the scatter of defects within the material. For this purpose, a lifetime model based on fracture mechanics was used. This model estimates the lifetime scatter from the scatter of the initial crack length and the crack propagation rate. The deviation between the calculation and test results is quite small for both the lifetime and lifetime scatter. The combination of the μCT for the detection of pores within the material and the lifetime assessment method developed enables a non-destructive quality assurance. In the series process, negative effects of the material quality on the lifetime can be identified at an early stage.