The effect of water ejection and water incursion on the evolution of thermal field during the start-up phase of the direct chill casting process

A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the direct chill casting process. The model, based on the commercial finite element package ABAQUS, includes primary cooling to the mould, secondary cooling to water and ingot base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of surface temperature, water flow rate and position relative to the point of water impingement. In addition, the secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat extraction rates). The algorithm used to describe ingot base cooling, includes the drop in contact heat transfer due to base deformation (butt curl) and also the increase in heat transfer due to water incursion between the ingot base and the bottom block. The model has been verified against temperature measurements obtained from two 711×1680 mm AA5182 ingots, cast under different conditions (non-typical “cold” practice and non-typical “hot” practice). Ingot base deflection data has also been obtained for the two test castings. Comparison of the model predictions with the data collected from the embedded thermocouples indicates that a 2-D longitudinal model is capable of describing the flow of heat in the early stages of the casting process in a region close to the centre of the rolling face. A sensitivity analysis completed with the model has clearly identified the link between ingot base cooling and secondary water-cooling heat transfer during the start-up phase.