Deformation and Fatigue Behaviour of Aluminium Alloys for High Specific Power IC Engine Applications

Doctoral thesis, 2020

The development towards higher specific power and lower displacement engines in recent years 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 cylinder heads of the internal combustion engines are often made with primary A356 cast aluminium alloys subjected to an ageing (T7) heat treatment. The overarching goal of the research work is to develop a deeper understanding of the continuum deformation and fatigue behaviour of the improved A356-T7+0.5 % Cu aluminium alloy. Understanding the influence of various factors on the mechanical properties of the cast aluminium alloy should enable improved thermo-mechanical fatigue prediction methodology of the highly loaded engine cylinder heads using computer aided design methods. 

Samples for testing are extracted from the highly loaded valve bridge regions of specially cast cylinder heads. The deformation and fatigue behaviour of the alloy is predominantly determined by the cast microstructure characterized by the dendritic arm spacing, the size of the secondary precipitates, the 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 ambient temperature up to 250 °C. The material exhibits decreasing strength and increasing ductility with increasing temperatures under monotonic loading. The material exhibits cyclic hardening at room temperature for all tested load levels and cyclic softening with strain load cycles at all the elevated test temperatures of 150, 200 and 250 °C. The material exhibits yield strength and peak stress asymmetry under cyclic loading with the stress response in compression higher than in tension under fully reversed strain controlled cyclic load cycles at all load levels. Mean stress relaxation is observed in the material for all test temperatures when run with a tensile or compressive mean strain. Tensile mean strain has a deleterious effect on the number of cycles to failure at temperatures below 200 °C.

Hold time effects mimicking the in-service loads (dwell in compression loading for 10 minutes or 1h) are examined to study their influence on the deformation and fatigue behaviour of the alloy. The material exhibits a significant stress relaxation during the dwell periods at all temperatures and load levels with a rapidly decreasing stress relaxation rate. The dwell times at constant compressive strains have no discernible influence on the following cyclic hardening behaviour or the fatigue life of the material, even at elevated temperatures. The visco-plastic deformation behaviour can be modelled using a combination of the Chaboche combined non-linear kinematic and isotropic mixed hardening model and the rate dependent Cowper-Symonds overstress power law model. The role of artificial and natural ageing is explored and both time and temperature associated changes in the microstructure result in transient states of both the continuum deformation and fatigue behaviour of the alloy. The effect of strain rate on the cyclic deformation behaviour of the alloy is studied by testing at strain rates of 1% s-1 and 10% s-1 at room temperature, 150 and 200 °C. The influence of the strain rate on the cyclic peak stress development is small, but it has a significant influence on the development of cyclic mean stress, especially at room temperature. Fractographic investigation of the fracture cross-section highlights the role of porosities, silicon rich phase in the eutectic region and the intermetallics on the crack initiation process. The larger precipitates are preferentially cracked highlighting the importance of refining the microstructure and minimizing the shrinkage porosity.