Thermomechanical Fatigue Life Predictions of Cast Aluminum Cylinder Heads Considering Defect Distribution

Semi-Permanent Mold (SPM) cast aluminum alloy cylinder heads are commonly used in gasoline and diesel internal combustion engines. The cast aluminum cylinder heads must withstand severe cyclic mechanical and thermal loads throughout their lifetime. The casting process is inherently prone to introducing casting defects and microstructural heterogeneity. Porosity, which is one of the most dominant volumetric defects in such castings, has a significant detrimental effect on the fatigue life of these components since it acts as a crack initiation site. A reliable analytical model for Thermo-Mechanical Fatigue (TMF) life prediction must take into account the presence of these defects. In previous publications, it has been shown that the mechanism-based TMF damage model (DTMF) is able to predict with good accuracy crack locations and the number of cycles to propagate an initial defect into a critical crack size in aluminum cylinder heads considering ageing effects. In the current work, the model has been extended to also include the effect of porosity which is treated as the initial defect size. It is shown that the model can explain the difference in the fatigue lives of Low-Cycle Fatigue (LCF) samples taken from chilled and non-chilled regions of the heads made of an A356-T6 alloy and tested at different temperatures. On the component level, a non-linear transient elasto-viscoplastic finite element analysis is performed to simulate the thermal cycle that the cylinder head experiences during engine testing including ageing effects. A casting simulation of the head is carried out to provide pore size distribution throughout the casting. The pore sizes are then treated as the initial crack sizes at each node in the mechanism-based short-crack growth model. The TMF life prediction results are compared with zone-based analysis and with engine dyno tests.