Thermo-mechanical fatigue of cast aluminium alloys for engine applications under severe conditions

Abstract

The increase in target performance of engines and hence the loading of their structural materials has dictated the need for more information about the behaviour of cast aluminium alloys under severe conditions up to 400°C. This study was therefore conducted in order to determine how different cooling rates, different pre-treatments and different alloying elements can improve the performance of cast aluminium (Al) alloys under thermo-mechanical fatigue (TMF) loading compared to a reference alloy and condition, AlSi6Cu4-T6. An existing TMF test rig was modified to allow an investigation of temperature gradients corresponding to those prevailing in real cylinder heads. The measured data were implemented in a Chaboche damage model and in an FEM tool in order to simulate low cycle fatigue (LCF) and TMF behaviour. These models provide a possibility to simulate LCF and TMF behaviour taking into account microstructural changes. On metallographic examination, a dependence of crack initiation on secondary dendrite arm spacing (SDAS) and on porosity was observed. Here, a smaller SDAS and a HIP modified microstructure led to a longer lifetime. Furthermore, clusters of brittle Si particles, decohesion or intermetallic phases were also found to initiate cracks. Stage I crack behaviour was seen at low strain amplitudes, where the crack propagates along the interface between the Al matrix and the intermetallic phases. Stage II behaviour was observed for higher strain amplitudes with crack propagation taking place along intermetallic phase boundaries such as Al2Cu, α-phase and β-phase or through pores. An increasing proportion of matrix cracks was observed for low strain amplitudes. Investigation of the decohesion behaviour revealed decohesion under high strain amplitudes and in areas with a high particle fracture volume attributable to high notch stresses. A refinement of the microstructure, particularly the particles, was found with increased Si content, associated with an increase in particle density. Following all TMF tests, an orientation of particles dependent on the loading conditions was observed, and this seemed to have had an influence on crack behaviour. A drift of stress was also found after long term high temperature exposure for strain amplitudes of 0.2%, attributed to creep.