Abstract
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 °C and a corresponding increase in ductility. Completely reversed, strain controlled, uniaxial fatigue tests were carried out at 150, 200 and 250 °C. A dilatometric study carried out to study the thermal expansion behaviour of the alloy in the temperature range 25–360 °C shows a thermal expansion coefficient of (25–30) × 10−6 °C−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.
Highlights
Aluminium–silicon (Al-Si) cast alloys are often used in the automotive industry due to their castability, high thermal conductivity and good mechanical and corrosion properties [1,2]
A356 alloys with added Cu, like the material of interest in this study: A356 + 0.5% Cu, are used in structures like cylinder heads as they provide improved high temperature strength especially in the 200–250 ◦ C temperature range owing to the precipitation of Al2 Cu precipitates without significant reduction in ductility [6,7,8]
While a wide variety of chemical compositions of the A356 alloy are used and studied, there is dearth of relevant information on the deformation behaviour of A356 + 0.5% Cu–T7 group of alloys at high temperatures. Since these materials are often employed at such elevated temperatures, this study aims to establish the cyclic deformation and fatigue behaviour using completely reversed strain-controlled tests at total strain amplitudes of 0.2%, 0.3% and 0.4% at various temperatures
Summary
Aluminium–silicon (Al-Si) cast alloys are often used in the automotive industry due to their castability, high thermal conductivity and good mechanical and corrosion properties [1,2]. A356 cast aluminium alloys are often used owing to their good castability, high strength and toughness while having less susceptibility to casting defects in complex structures like cylinder heads [4,5]. A356 alloys with added Cu, like the material of interest in this study: A356 + 0.5% Cu, are used in structures like cylinder heads as they provide improved high temperature strength especially in the 200–250 ◦ C temperature range owing to the precipitation of Al2 Cu precipitates without significant reduction in ductility [6,7,8]. Other alloying elements are added in trace amounts to influence the shape, size and morphology of the secondary precipitates, to improve the strength and fatigue life of the material structure [9]
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