Numerical Simulation of Residual Thermal Stresses in AA7050 Alloy during DC-Casting Using ALSIM5

Abstract

Abstract. Non-homogenous cooling rates and solidification conditions during DC-casting of high strength aluminum alloys result in the formation and accumulation of residual thermal stresses with different signs and magnitudes in different locations of the billet. Rapid propagation of microcracks in the presence of thermal stresses can lead to catastrophic failure in the solid state, which is called cold cracking. Numerical models can simulate the thermomechanical behavior of an ingot during casting and after solidification and reveal the critical cooling conditions that result in catastrophic failure, provided that the constitutive parameters of the material represent genuine ascast properties. Simulation of residual thermal stresses of an AA7050 alloy during DC-casting by means of ALSIM5 showed that in the steady-state conditions large compressive stresses formed near the surface of the billet in the circumferential direction. Stresses changed sign on moving towards the centre of the billet and became tensile with high magnitudes in radial and transverse directions, which made the alloy prone to hot and cold cracking.

Cold Cracking Development in AA7050 Direct Chill–Cast Billets under Various Casting Conditions

“…For more details of stress evolution and contour maps of the stress components, one may refer to the previous articles. [22,34] Investigation of the simulation results showed that water temperature, water flow rate, and cast temperature had no noticeable effect on the state of residual thermal stresses. The threshold water flow rate of 80 L/min was found to be sufficient to prevent the remelting of the solidified shell at the surface of the billet (bleed-out) in the standard case.…”

Cold cracking in DC-cast high strength aluminum alloy ingots: An intrinsic problem intensified by casting process parameters

“…As can be seen in Fig. 3a, after 380 s of casting the temperature in the lower part of the billet is already below 80 • C. For the detailed explanation of the stress tensor components in the billet and their time evolution, one may refer to the previous works of present authors [8,16]. It is however worth to recall that under steady-state casting conditions, stresses are mainly tensile in the center of the billet and with moving towards the surface they either diminish or turn to compressive.…”

“…A wider solidification temperature range, a lower thermal conductivity, and a relatively higher coefficient of thermal expansion compared to other aluminum alloys [15] led to high temperature gradients and consequently high thermal stresses in 7xxx series aluminum ingots [16]. Therefore, high-strength aluminum alloys are intrinsically prone to cold cracking due to the poor thermo-physical properties and the special microstructure in the genuine as-cast condition.…”