Deformation Behaviour of A356-T7 Cast Aluminium Alloys Used in High Specific Power IC Engine Cylinder Heads

Licentiatavhandling, 2019

The constant development drive towards higher specific power and lower displacement engines in recent years to produce environmentally friendly high-performance cars has placed increasingly high thermal loads on the internal combustion engine materials. Further, the advent of hybrid power trains placing higher demands on quick starts and a rapid approach to maximum power necessitates the automotive industry to move towards a more robust computational thermo-mechanical fatigue life prediction methodology to develop reliable engines and reduce developmental costs. The overarching goal of the research project is to develop constitutive and lifetime prediction models with just the necessary and sufficient parameters to predict the thermo-mechanical fatigue life of the highly loaded engine cylinder heads. The cylinder heads of the internal combustion engines are often made with primary A356 cast aluminium alloys and are employed in a T7 overaged condition.

 

The present thesis aims at establishing the mechanical deformation and fracture behaviour of the material with test samples extracted from the highly loaded valve bridge regions of specially cast cylinder heads made of the said A356-T7 alloy. The deformation behaviour of the alloy is predominantly determined by the cast microstructure characterized by the dendritic arm spacing, the size of the secondary precipitates, the various defect distribution and by the temperature during deformation. The scope of this study covers uniaxial isothermal tests to establish the cyclic deformation behaviour and fatigue properties of the alloy at temperatures ranging from the ambient temperature to 250 °C. Completely reversed strain controlled uniaxial low cycle fatigue tests are run at three different total strain amplitude levels of 0.2, 0.3 and 0.4 % with multiple replicas at a constant strain rate of 1 % sec-1 to capture the cyclic deformation behaviour and the corresponding temperature dependent fatigue life curves. The model parameters of a suitable constitutive model are calibrated to predict the non-linear stress-strain response under thermal and mechanical load cycling. Monotonic deformation tests are also performed at the standard strain rate of 0.01% sec-1 at varying temperatures from the ambient to 300 °C to establish the base material mechanical properties.

 

The material has an elastic-plastic monotonic response with significant hardening exhibited at temperatures around and lower than 150 °C and softening with plastic deformation at temperatures above 150 °C. The strength of the material decreases with increasing temperatures with corresponding increase in ductility. The material exhibits cyclic hardening at room temperature and cyclic softening at and above 150 °C in strain controlled completely reversed cyclic tests. The material exhibits decreasing peak stress response and increased plastic strain amplitudes with increasing temperatures in cyclic loading. The experimental data is calibrated against the Chaboche model with multiple back stresses to capture the temperature dependent kinematic and isotropic hardening behaviour of the alloy. The material exhibits a non-linear deformation behaviour with a mixed isotropic and kinematic hardening behaviour that can be modelled using a linear and a nonlinear backstress.

 

The material exhibits significant scatter in measured mechanical properties at lower temperatures between replicas owing to the diverse microstructure obtained owing to material variability between the extracted samples. Increase in test temperatures shows a reduction in the said scatter indicating a waning influence of the microstructure on the deformation behaviour at elevated temperatures. A dilatometric study of the material using a cylindrical specimen indicates a stable coefficient of thermal expansion in the temperature range of 25 – 250 °C.