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

Abstract

Cold cracking is a potentially catastrophic phenomenon in direct chill (DC) casting of 7xxx series aluminum alloys that leads to safety hazards and loss of production. The relatively low thermal conductivity and wide solidification temperature range in these alloys results in accumulation of residual thermal stress under nonuniform cooling conditions of the billets. In addition, such alloys show a severe loss in ductility below a critical temperature of 573 K (300 °C). This brittleness along with high stress concentration at the tips of voids and microcracks can lead to catastrophic failure. Casting process parameters affect the magnitude and distribution of stresses in the billet and increase the susceptibility of the material to cold cracking. In order to investigate the effect of casting process parameters such as casting speed, billet size, and water flow rate, thermomechanical simulations were applied using ALSIM5 casting simulation software. Among the studied casting process parameters, the increased billet size and high casting speed resulted in the most dramatic increase in residual stress level. Critical crack sizes that led to catastrophic failure were also calculated and are reported against process parameters.