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

cylinder head

fatigue

thermo-mechanical fatigue

aluminium

ageing

A356

constitutive models

mechanical behaviour

Virtual Development Laboratory (VDL)
Opponent: Dr. Svjetlana Stekovic, Linköping University, Sweden

Författare

Elanghovan Natesan

Chalmers, Industri- och materialvetenskap, Konstruktionsmaterial

The natural tendency of metals is to expand when they are heated and contract again upon cooling. But what happens if this free expansion and contraction of the material is restricted? Actually, we stress the material and induce permanent shape change in the structure by deforming it plastically! Furthermore, when such heating-cooling cycles are repeated in structures that are inhibited against free expansion or contraction, we induce damage in the metallic material. Every time we start our cars, such loading is experienced by certain parts of the car engine. In cold climates, temperatures range from sub-zero to over 200 °C, rapidly increasing during start-up and cooled back again upon engine turn off. The number of such start-stop cycles have been increased owing to recent trends in vehicle electrification. This places even higher demands on the engineers to design structures that can avoid premature failures. For a cost and time efficient product development process, computer aided design methods are used which rely on accurate mathematical models that can predict the material response to applied loads and foresee any damage caused in the material.

In this thesis, we study the various factors that affect the material deformation and fracture behaviour at the temperatures expected in vulnerable parts of combustion engines. This enables us to develop reliable numerical models that can aid the design and development process and minimize the need for expensive and time-consuming physical testing. The results of the study enable us to better understand how the material behaves under loading and will help us design products that have a predictable performance and thus contribute to a successful electrification experience!

Utveckling av analysmodeller för termomekanisk utmattning TMF i förbränningsmotorer

Energimyndigheten, 2013-10-01 -- 2018-12-31.

Ämneskategorier

Maskinteknik

Materialteknik

Teknisk mekanik

Farkostteknik

Metallurgi och metalliska material

Drivkrafter

Hållbar utveckling

Styrkeområden

Transport

Materialvetenskap

ISBN

978-91-7905-355-0

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4822

Utgivare

Chalmers tekniska högskola

Virtual Development Laboratory (VDL)

Opponent: Dr. Svjetlana Stekovic, Linköping University, Sweden

Mer information

Senast uppdaterat

2020-09-22