On the Mechanism of Surface Cracking in DC Cast 7xxx and 6xxx Extrusion Ingot Alloys

When applying the Hydro variant (Hycast Gas Cushion) of the Showa Denko gas slip technology for casting extrusion ingots of 7xxx alloys surface cracks occasionally occurred. Especially one alloy with 0.3 wt.% Cu caused problems. In order to identify the problem, the casting process for these alloys was simulated by a coupled stress, thermal and fluid flow model (ALSIM/ALSPEN). The simulations were designed as a factorial trial where casting speed, ramping of the speed, casting temperature, cone height of the starting block, cooling water efficiency and primary cooling were systematically varied. The hoop stress in the surface at the temperature when 97.5% of the material was solidified was used as a crack sensitivity indicator. Three stages were identified: (I) At the start a maximum hoop stress evolved, (II) then a minimum stress occurred before (III) the stress reached a stable level. For an AA6060 alloy the stress was found to be zero in the stable stage while the AA7108 alloy experienced tension stress also during the steady state regime. Based on the factorial analysis it was found that the stable stress increased most rapidly with increasing casting speed and decreased with an increased primary cooling and a reduced melt temperature.