Modeling of ingot development during the start-up phase of direct chill casting
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Modeling of Ingot Development during the Start-Up Phase of Direct Chill Casting A.J. WILLIAMS, T.N. CROFT, and M. CROSS Direct chill (DC) casting is a core primary process in the production of aluminum ingots. However, its operational optimization is still under investigation with regard to a number of features, one of which is the issue of curvature at the base of the ingot. Analysis of these features requires a computational model of the process that accounts for the fluid flow, heat transfer, solidification phase change, and thermomechanical analysis. This article describes an integrated approach to the modeling of all the preceding phenomena and their interactions
I. INTRODUCTION
DIRECT chill (DC) casting is a well-established process in the production of aluminum ingots and is used worldwide by the aluminum producing industry. Although straightforward to conceive, DC casting involves a range of coupled physical phenomena that must be represented appropriately if any process model is to be sufficiently comprehensive. A schematic of the DC casting geometry is given in Figure 1. At the start of the process, liquid metal is poured into an open rectangular mold over a movable bottom block/drawing block. As this block is moved downward, the metal is cooled first by contact with the mold (primary cooling) and second by water cooling sprays (secondary cooling). During processing, the ingot is subject to many distortions that arise as a consequence of combined thermal and mechanical effects. As the ingot cools and solidifies at the base and along the walls, it begins to develop residual stresses and deforms. The deformation of the ingot walls may result in gap formation with the mold and this will impact adversely upon the cooling of the ingot as it forms. One of the main concerns of DC casting engineers is the stability of the base of the ingot as the bottom block is drawn downward. At the start of the process, the base of the ingot is rapidly cooled, by both the bottom block and water sprays, and large thermal stresses are generated, which cause the base to curl upward from the center toward the sides. This phenomenon is known as “butt curl” and is highly undesirable because it results in a lack of contact between the ingot and the bottom block, affecting further cooling of the ingot and possible leading to remelting. In addition, the generation of large stresses and strains may lead to the formation of cracks and tears in the ingot. The process is governed by a series of interacting phenomena, which includes 1. 2. 3. 4.
Navier–Stokes flow in the liquid region of the ingot, heat transfer from the metal to the mold, solidification of the alloy, and the development of stress and deformation in the cooling ingot.
A.J. WILLIAMS and T.N. CROFT, Senior Research Fellows, and M. CROSS, Pro Vice Chancellor, are with the Centre for Numerical Modelling and Process Analysis, University of Greenwich, London, SE10 9LS, United Kingdom. Contact e-mail: [email protected] Manuscript submitted Janua
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