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UBC Theses and Dissertations

Mathematical modeling of the evolution of thermal field during start-up phase of the direct chill casting process for AA5182 sheet ingots Sengupta, Joydeep

Abstract

The control of the thermal cooling conditions at the start-up phase of the Direct Chill (DC) casting process for aluminum sheet ingots is difficult, and is critical from the standpoint of defect formation. Firstly, boiling water heat transfer governs the secondary cooling experienced by the ingot surfaces as they emerge from the mould. This results in varying rates of heat transfer from the ingot faces as the surface temperature of the ingot changes with time during the start-up phase. Moreover, if the ingot surface temperature at locations below the point of water impingement is high enough to promote film boiling, the water is ejected away from the surface. This can result in a sudden decrease in heat transfer and the formation of local hot spots. Also, the chill water may enter into the gap formed between the ingot base and the bottom block with the evolution of the butt curl. This process of water incursion alters the heat transfer from the base of the ingot, and in turn affects the surface temperature of the ingot faces. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the D.C. 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, impingement point temperature, 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 the process of water incursion between the ingot base and bottom block. The model has been extensively validated against temperature measurements obtained from two 711 x 1680 mm AA5182 ingots, cast under different start-up conditions (non typical "cold" practice and non-typical "hot" practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot base deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model that accounts for the processes of water ejection and water incursion, is capable of describing the flow of heat in the early stages of the casting process, satisfactorily. The research programme represents a significant improvement over existing thermal models that do not quantitatively describe the important phenomena related to the effects of water ejection and water incursion, which are specific to the transient start-up phase of the process. The thermal model, which has been extensively validated by the industrial data, not only provides an insight into the link between ingot base cooling and secondary water cooling heat transfer during the start-up phase, but also emerges as a basis for the development of thermomechanical models, based on fundamental principles, which can be used as a powerful tool for process optimization and quality control.

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