Experimental investigation of thermomechanical effects during direct chill and electromagnetic casting of aluminum alloy
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I.
INTRODUCTION
DURING direct chill casting ( D C C ) [1] or electromagnetic casting (EMC)[ 21 of large rectangular rolling sheet ingots of aluminum alloys, the metal deforms in different ways. During the start-up phase, butt curl is critical, especially for EMC, because the meniscus can be destabilized in the electromagnetic field, leading to a prema~re stop of the casting process. In the steady-state regime of casting, the solidified shell contracts toward the liquid pool, resulting in a nonrectangular shape of the cross section of the ingot. If the alloy is cast in a rectangular mold or inductor, the larger lateral faces of the final ingot are concave ("bone" shape). To compensate for this effect, the corresponding sides of the mold/inductor are designed with a convex shape, usually with two or three linear segments. Figure 1 shows a schematic top view of a typical mold shape used to produce nearly fiat rolling ingots. At present, the geometry of the mold/inductor is designed mainly by trial-and-error castings for given alloy, casting speed, and ingot size conditions. However, as the sheet ingot dimensional tolerances have become tighter because of scalping and rolling mill demands, the need for casting equipment capable of producing precise ingot geometry has increased accordingly. A general understanding of the mechanisms involved in the deformation of the metal is therefore highly desirable in order to reduce the time and cost associated with the mold/inductor bow design. In this respect, thermomechanical calculations of the start-up and of the steady-state regime have become an increasingly used tool. t3,4,51However, because of the many parameters involved in direct
J.-M. DREZET, Graduate Student, and M. RAPPAZ, Professor, are with the Department des Materiaux, Ecole Polytechnique Federale de Lausanne, MX-G Ecublens, 1015 Lausanne, Switzerland. B. CARRUPT and M. PLATA, Research Engineers, are with the Alusuisse-Lonza Company, CH-3965 Chippis, Switzerland. Manuscript submitted June 10, 1994. METALLURGICALAND MATERIALSTRANSACTIONS B
chill/electromagnetic (DC/EM) casting, these simulations heavily rely on experimental measurements. Besides, the thermomechanical properties of the cast alloys must be known up to or even above the solidus temperature,t61 and the thermal boundary conditions associated with the lateral water jet have to be estimated through inverse modeling techniquesY 1 Once these values are determined, the results of the simulation must be compared with the experiments. Very often, such a comparison is made only on the "cold" ingot, and the deformation during the casting process is not measured. The purpose of the present study is to describe a simple technique for the in situ measurement of the deformation and temperature histories during DCC or EMC. In addition to the total deformation of the main body after complete cooling to room temperature, the steady-state temperature profile of the metal during casting, the corresponding shape and depth of the melt pool ("sump"), and the contraction o
