Coupled thermal-stress model of the start-up phase of the aluminum direct chill casting process: Predictions relating to hot tearing

A three dimensional coupled thermal-stress model has been formulated to describe the thermal and stress/strain fields which evolve during the start-up phase of the direct chill (DC) casting process for AA5182 ingots. This model includes industry-scale ingot and bottom block geometries. The thermal analysis, as reported earlier, includes the various stages of heat transfer: primary (mold) cooling, secondary cooling (chill water impingement and water ejection), and ingot base cooling. The model has been validated against temperature and displacement measurements obtained from two 711 × 1680 mm AA5182 ingots, cast using different start-up conditions. By refining the mesh resolution to ∼ 10 mm in z and 5 to 25 mm in x and y, there are sufficient number of integration points to capture the continuous variation in heat transfer in the different cooling regimes. To assess the hot tearing susceptibility of this process, the stress strain predictions made by the model for different casting recipes have been compared to mushy zone tensile yield stress and strain-to-failure data available in literature. It appears that in a critical region just above the ingot lip, the stress state is such that it exceeds the mushy zone failure stress of the alloy, depending on cooling conditions, making it susceptible to hot tearing. Furthermore, the tensile stress accumulated during the vulnerable stage of solidification (between a fraction solid of 0.9 and 0.98) may exceed the failure stress at a temperature of about 565°C.