High- and low-cycle fatigue influence of silicon, copper, strontium and iron on hypo-eutectic Al-Si-Cu and Al-Si-Mg cast alloys used in cylinder heads

摘要

In this publication, ambient condition fatigue investigations with different types of Al-Si-Cu and Al-Si-Mg cast alloys in rotating-bending high-cycle fatigue (HCF) and push-pull low-cycle fatigue (LCF) regimes have been performed with varying Si, Cu, Fe and Sr contents. The cast alloys investigated here are common used in cylinder heads for automotive application. Because the cylinder head is one of the most fatigued parts in combustion chamber engines, the microstructural knowledge of the damage process provides a tool of construction and its material selection. The investigations were also supported with an in-situ microstructural crack observation in high plasticity rotating-bending regimes. The specimens were directly processed out of serial produced T79 heat-treated cylinder heads to provide the equal microstructure for testing as under operational conditions. The observations clearly identified the effects of the individual alloying elements both under low- and high-cycle fatigue. The crack propagation speed and the crack paths were majorly influenced by the eutectic silicon. Additional, the precipitation hardening due to copper affected significantly the fatigue endurance, too. In high plasticities the silicon's influence got almost lost and only the matrix strength was crucial. Thus, increased fatigue strength in high loaded LCF regimes was observed for alloys with less copper content, thus higher ductility. By contrast, improved HCF and low loaded LCF endurance was only achieved when the matrix strength was increased by copper's precipitation hardening. Crack branching and deflections strongly influenced the microstructural damage of the ductile AlSi7Mg(Sr) and hence, gained its fatigue strength. Iron phases could not identified as harmful inclusions, since the phases were similar in size of other hard phase elements like the other primary intermetallic phases like Al_2Cu and β-Si phases under notch stress aspects, by the well defined solidification process in the test section. Because the crack nucleation mainly occurred on Si particles, strontium as a refinement agent influenced the early crack onset and accordingly the fatigue in total. Thus, the AISi6Cu4(Sr) had increased lifetimes compared to AlSi6Cu4 both in HCF and LCF. Further, the presented results provide a modification of the Manson-Coffin approach to describe the relationship between plastic strain and lifetime, valid for all proposed alloys with only one set of parameters. Thus, it was possible to perform the fatigue calculation with a reduced range of scatter.