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INTRODUCTION

MAGNESIUM alloys offer the potential for weight and related energy savings in both the automotive and aerospace industries because they have the highest strength-to-weight ratio of common structural metals.[1] Despite higher cost, this potential benefit has led to a recent increase in demand for cast and wrought magnesium products. With this increase, the direct chill (DC) casting process, which is used to produce the starting material for these applications, is receiving significantly more attention from the standpoint of process optimization.[2] A schematic representation of a typical DC casting process is shown in Fig. 1. Initially, the dummy (starting) block is positioned inside the mold and liquid metal is poured into the open mold from the top. Once the liquid metal reaches a specified height in the mold, the dummy block is then lowered into a pit. As the partially solidified billet emerges from the mold, water impinges directly on the cast surface. Steady-state casting conditions are achieved when the sump profile ceases to evolve with time relative to the mold. The casting process stops when the desired cast length is obtained or the bottom of the pit is reached.[3] The process is similar in many respects to the DC casting process used to produce aluminum alloys. Referring to Fig. 1, during steady-state operation, heat is initially transferred from the billet to the mold (primary cooling), and secondarily to water in contact with the billet surface (secondary cooling). During cast startup, a third important mechanism is heat transport between the base of the billet and dummy block. It is typical for upward of 80 pct of the heat to be removed by secondary cooling during steady-state operation. Although the DC casting process has been used to produce aluminum ingots/billets since the 1930s, and more recently magnesium billets, there is still work necessary to optimize H. HAO, Postdoctoral Research Fellow, D.M. MAIJER and M.A. WELLS, Assistant Professors, and S.L. COCKCROFT, Associate Professor, are with the Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: daan@ cmpe.ubc.ca D. SEDIAKO, Senior Research Investigator, Research and Development Division, and S. HIBBINS, Superintendent, Technology Development & Metallurgical Services Department, are with Timminco Metals, Haley, ON, Canada K0J 1Y0. Manuscript submitted April 7, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

the design of the casting process from the standpoint of productivity, cost effectiveness, and final ingot quality. One of the challenges in optimization is the complex interaction between the casting parameters, such as withdrawal rate, water flow rate, dummy block design, and defect formation, which is difficult to rationalize experimentally. One approach to overcome this problem is to use fundamentally based mathematical models to analyze defect formation such as hot tearing, cold cracking, bleed outs, and cold shuts because most are directly related to heat flow