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

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. \ua0The 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 \ub0C. 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 \ub0C to establish the base material mechanical properties. \ua0The material has an elastic-plastic monotonic response with significant hardening exhibited at temperatures around and lower than 150 \ub0C and softening with plastic deformation at temperatures above 150 \ub0C. 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 \ub0C 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.\ua0The 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 \ub0C

Effect of Temperature on Deformation and Fatigue Behaviour of A356–T7 Cast Aluminium Alloys Used in High Specific Power IC Engine Cylinder Heads

Aggressive downsizing of the internal combustion engines used as part of electrified powertrains in recent years have resulted in increasing thermal loads on the cylinder heads and consequently, the susceptibility to premature thermo-mechanical fatigue failures. To enable a reliable computer aided engineering (CAE) prediction of the component lives, we need more reliable material deformation and fatigue performance data. Material for testing was extracted from the highly loaded valve bridge area of specially cast cylinder heads to study the monotonic and cyclic deformation behaviour of the A356–T7 + 0.5% Cu alloy at various temperatures. Monotonic tensile tests performed at different temperatures indicate decreasing strength from 211 MPa at room temperature to 73 MPa at 300 \ub0C and a corresponding increase in ductility. Completely reversed, strain controlled, uniaxial fatigue tests were carried out at 150, 200 and 250 \ub0C. A dilatometric study carried out to study the thermal expansion behaviour of the alloy in the temperature range 25–360 \ub0C shows a thermal expansion coefficient of (25–30) 7 10−6\ua0\ub0C−1. Under cyclic loading, increasing plastic strains are observed with increasing temperatures for similar load levels. The experimental data of the cyclic deformation behaviour are calibrated against a nonlinear combined kinematic–isotropic hardening model with both a linear and non-linear backstress

Effects of Temperature on the Evolution of Yield Surface and Stress Asymmetry in A356–T7 Cast Aluminium Alloy

As the electrification of vehicle powertrains takes prominence to meet stringent emission norms, parts of internal combustion engines like cylinder heads are subjected to an increased number of thermal load cycles. The cost-effective design of such structures subjected to cyclic thermo-mechanical loads relies on the development of accurate material models capable of describing the continuum deformation behaviour of the material. This study investigates the effect of temperature on the evolution of flow stress under cyclic loading in A356-T7 + 0.5% Cu cast aluminium alloy commonly used in modern internal combustion engine cylinder heads. The material exhibits peak stress and flow stress asymmetry with the stress response and flow stress of the material under compressive loading higher than under tension. This peak and flow stress asymmetry decrease with an increase in temperature. To compare this stress asymmetry against conventional steel, cyclic strain-controlled fatigue tests are run on fully pearlitic R260 railway steel material. To study the effect of mean strain on the cyclic mean stress evolution and fatigue behaviour of the alloy, tests with tensile and compressive mean strains of +0.2% and −0.2% are compared against fully reversed (Rε\ua0= −1) strain-controlled tests. The material exhibits greater stress asymmetry between the peak tensile and peak compressive stresses for the strain-controlled tests with a compressive mean strain than the tests with an identical magnitude tensile mean strain. The material exhibits mean stress relaxation at all temperatures. Reduced durability of the material is observed for the tests with tensile mean strains at lower test temperatures of up to 150 \ub0C. The tensile mean strains at elevated temperatures do not exhibit such a detrimental effect on the endurance limit of the material

Deformation and fatigue behaviour of A356-T7 cast aluminium alloys used in high specific power IC engines

The continuous drive towards higher specific power and lower displacement engines in recent years place increasingly higher loads on the internal combustion engine materials. This necessitates a more robust collection of reliable material data for computational fatigue life prediction to develop reliable engines and reduce developmental costs. Monotonic tensile testing and cyclic stress and strain-controlled testing of A356-T7 + 0.5 wt.% Cu cast aluminium alloys have been performed. The uniaxial tests were performed on polished test bars extracted from highly loaded areas of cast cylinder heads. The monotonic deformation tests indicate that the material has an elastic-plastic monotonic response with plastic hardening. The strain controlled uniaxial low cycle fatigue tests were run at multiple load levels to capture the cyclic deformation behaviour and the corresponding fatigue lives. The equivalent stress-controlled fatigue tests were performed to study the influence of the loading mode on the cyclic deformation and fatigue lives. The two types of tests exhibit similar fatigue lives and stress-strain responses indicating minimal influence of the mode of loading in fatigue testing of A356 + T7 alloys. The material exhibits a non-linear deformation behaviour with a mixed isotropic and kinematic hardening behaviour that saturates after the initial few cycles. There exists significant scatter in the tested replicas for both monotonic and cyclic loading

Increasing productivity of laser powder bed fusion manufactured Hastelloy X through modification of process parameters

One of the factors limiting the use of additive manufacturing, particularly powder bed processes, is their low productivity. An approach to increasing laser powder bed fusion (LPBF) build rate without costly hardware modifications is to alter process parameters. This study evaluates the possibilities to increase build rates through this route without compromising material quality. Equations for productivity are derived based on process parameters and build geometry, and applied on the process window for Hastelloy X in LPBF. It is demonstrated that virtually flaw-free parts can be printed at build rates that differ up to tenfold. To investigate potential variations in the microstructure and performance, Hastelloy X specimens manufactured at varying build rates were characterized. Electron backscattered diffraction (EBSD) analysis revealed that the specimen built at the lowest rate shows strong texture with columnar grains, while the specimen built at the highest rate presents significantly more random orientation and evident melt pool contours with pockets of very fine grains at the bottom. Despite the major differences in microstructure, the tensile properties do not necessarily vary substantially. Thus, the results indicate that the build rate of LPBF Hastelloy X can be significantly varied based on process parameters, still yielding consistent mechanical properties

Effect of Strain Rate on the Deformation Behaviour of A356-T7 Cast Aluminium Alloys at Elevated Temperatures

Internal combustion engine downsizing and powertrain electrification trends in recent years have led to higher thermal loads on the cylinder head materials with an increased number of engine start–stop thermal load cycles. This requires designing cylinder heads that are resilient against thermomechanical fatigue damage. To reduce the developmental costs, reliable numerical models for use in computer-aided design approaches are required. Thus, a comprehensive understanding of the material deformation behaviour under loads mimicking in-service conditions is desired to make better engineering decisions. This study examines the effect of strain rate on the cyclic deformation behaviour of the A356-T7 + 0.5% Cu aluminium alloy commonly used in modern internal combustion engine cylinder heads. Samples extracted from the valve bridge areas of the cylinder heads are tested in strain-controlled fatigue tests. Samples are tested at strain rates of 1% s−1\ua0and 10% s−1\ua0at room temperature, 150 and 200 \ub0C. The material exhibits increased isotropic hardening and softening rates and an increased number of cycles to failure at 10% s−1. The strain rate has a dramatic influence on the mean stress development at room temperature. The role of silicon particles in the fracture mechanism is investigated using electron microscopy techniques

Effects of Dwell Time on the Deformation and Fatigue Behaviour of A356-T7 Cast Aluminium Alloys Used in High Specific Power IC Engine Cylinder Heads

The electrification of automotive powertrains in recent years has been driving the development of internal combustion engines towards reduced volumes with higher power outputs. These changes place extreme demands on engine materials. Engineers employ the computer-aided engineering approach to design reliable and cost-effective engines. However, this approach relies on accurate knowledge of the material deformation and fatigue characteristics during service-like loading. The present study seeks to investigate the effect of dwell times on the deformation and fatigue behaviour of the A356-T7 + 0.5 wt.% Cu alloy used to cast cylinder heads. In particular, we study the effect of dwell time duration at various temperatures. A combined fatigue-dwell testing procedure, with the dwell at the maximum compressive strain, replicates the service conditions. It is found that the material exhibits a stress relaxation behaviour with a decreasing relaxation rate. At lower temperatures, the load level influences the relaxation more than at elevated temperatures. However, the dwell does not significantly affect the hardening behaviour or the life of the tested alloy. Finally, we model the time-dependent material behaviour numerically. The Chaboche model, combined with a Cowper–Symonds power-law, is found to capture the visco-plastic deformation behaviour accurately

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

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.\ua0 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 \ub0C. 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 \ub0C. 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 \ub0C.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 \ub0C. 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

Deformation and Fatigue Behaviour of A356-T7 Cast Aluminium Alloys Used in High Specific Power IC Engines

The continuous drive towards higher specific power and lower displacement engines in recent years place increasingly higher loads on the internal combustion engine materials. This necessitates a more robust collection of reliable material data for computational fatigue life prediction to develop reliable engines and reduce developmental costs. Monotonic tensile testing and cyclic stress and strain-controlled testing of A356-T7 + 0.5 wt.% Cu cast aluminium alloys have been performed. The uniaxial tests were performed on polished test bars extracted from highly loaded areas of cast cylinder heads. The monotonic deformation tests indicate that the material has an elastic-plastic monotonic response with plastic hardening. The strain controlled uniaxial low cycle fatigue tests were run at multiple load levels to capture the cyclic deformation behaviour and the corresponding fatigue lives. The equivalent stress-controlled fatigue tests were performed to study the influence of the loading mode on the cyclic deformation and fatigue lives. The two types of tests exhibit similar fatigue lives and stress-strain responses indicating minimal influence of the mode of loading in fatigue testing of A356 + T7 alloys. The material exhibits a non-linear deformation behaviour with a mixed isotropic and kinematic hardening behaviour that saturates after the initial few cycles. There exists significant scatter in the tested replicas for both monotonic and cyclic loading