Effects of melt temperature and casting speed on the structure and defect formation during direct-chill casting of an Al
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INTRODUCTION
DIRECT-CHILL (DC) casting is the major technology for producing aluminum billets for deformation. The process was invented in the 1930s and, since that time, major engineering developments have been implemented in the construction of the melt delivery system, mold, and the way in which the water is sprayed onto the billet surface.[1,2] However, the basics of the process remain unchanged—the melt is poured into the water-cooled mold, it starts to solidify and forms a solid shell (primary cooling), then the solid bottom part is withdrawn from the mold and the solid side surface is directly cooled by water jets (secondary cooling). The solidification occurs in a rather narrow layer of the billet and can be well controlled. At the steady-state stage of the process, the positions of the liquidus and solidus and the temperature and flow patterns in the liquid and semiliquid parts of the billet remain constant. The main process parameters that can be controlled and that affect to a great extent the quality of the billet and its structure are the following: the casting speed (speed at which the billet is withdrawn from the mold), water flow rate (rate at which cooling water goes through the mold), and melt temperature. The effects of the first two parameters on the structure and defect formation are the subjects of numerous publications and have been discussed in our previous articles.[3,4] The melt temperature, however, has not received much attention from the viewpoint of its influence on the D.G. ESKIN, Senior Scientist, and V.I. SAVRAN, Ph.D. Student, are with the Netherlands Institute for Metals Research, 2628 AL, Delft, The Netherlands. Contact e-mail: [email protected] L. KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, 2628AL, Delft, The Netherlands. Manuscript submitted July 21, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
structure and defect formation in DC cast billets. In a very good review of DC casting,[2] the authors mention the melt temperature only once, and then among other parameters that can also affect the structure and quality of a billet. Only few experimental results are available on the subject, which can be explained based on the technological difficulties accompanying high pouring temperatures during DC casting. The golden rule of the cast shop is “melt hot, cast cold.” Grün and Schneider[5] examined billets of grain-refined commercial aluminum obtained by level-pour DC casting at different pouring temperatures (700 °C to 740 °C). They concluded that the increase in melt temperature resulted in the development of feathery columnar grains and higher temperature gradients closer to the mold. Tarapore[6] reported the experimental and computer-simulation results on DC casting of a 2024 alloy. The melt temperature was varied from 660 °C to 715 °C in the trough (the resultant melt temperature in the hot top changed from 642 °C to 696 °C). The increased depth of the sump, the higher temperature gradients in the liq
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