Thermo-mechanical modelling to predict cold cracking in 7xxx aluminium alloys during DC casting
Tutor / director / evaluadorEskin, Dmitry
Tipo de documentoProyecto/Trabajo final de carrera
Condiciones de accesoAcceso restringido por acuerdo de confidencialidad
During direct-chill (DC) casting of aluminium alloys high temperature gradients are developed across the ingot such that stresses are generated. These stresses may induce cracks or even lead to ingot fracture. There are two types of cracks to consider: hot tears which are formed at temperatures above the solidus and cold cracks which appear at temperatures below the solidus and can damage the entire ingot. High-strength aluminium alloys are more vulnerable to cold cracking mainly because of the poor thermal and mechanical properties in the genuine as-cast condition. Low thermal conductivities and large solidification temperature range are the main thermal properties which make the alloy more prone to cracking. The temperature range between rigidity temperature, the temperature below which the material starts holding thermal strains, and non-equilibrium solidus determines the solidification contraction range, and is the crucial factor for hot cracking susceptibility. In this Master’s thesis this temperature was defined for a group of AA7050 alloys with different chemical compositions and prepared under different casting process, to study the effect of grain refinement, impurities and cooling conditions. The temperatures when the tested alloys started to contract, Tth, were between 510-548 ºC. The solidification range over which the material changes from liquid state to the solid was in the range from 176ºC to 161ºC. Grain refinement decreased Tth and the solidification range. Numerical simulations have been run using the thermal properties of an AA7050 grain refined aluminium alloy to determine the stress and strain state of a DC-cast billet. Simulation results showed that material is prone to yielding at temperatures ranging from 400ºC to rigidity point (510ºC) and hot cracks can probably form, especially in the centre of the billet, where the residual thermal stresses are all tensile. Rapid propagation and catastrophic failure may occur later, if the formed micro-cracks get the chance to propagate in the residual stress fields.
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