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.