Modeling of ingot distortions during direct chill casting of aluminum alloys
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I.
INTRODUCTION
A schematic representation of the direct chill (DC) semicontinuous casting process is shown in Figure 1. For reasons of symmetry, only one quarter of the casting is shown. At the start of the process, liquid metal is poured in an open rectangular mold over a movable bottom block. As this block is moved downward, the metal is cooled first by contact with the mold (primary cooling) and second by the water-cooling sprays (secondary cooling). Direct chill casting proceeds in three main stages:t1] the start phase, during which the temperature field, the solidification front, the ingot shape, and the liquid metal-air interface (meniscus) change with time (nonstationary phase); the pseudostationary phase; and the end phase, during which liquid pouring is stopped and the ingot cools down. During processing, the ingot is subject to many distortions that arise as a consequence of combined thermal and mechanical effects.
A. Butt Curl and Swell During the start phase, the ingot experiences a higher cooling rate than during the steady-state regime. Indeed, at the start of the drop, the ingot butt is chilled both by the bottom block and by the mold. As the block is moved down, the ingot is also cooled by the lateral water spray. This rapid chilling generates excessive thermal stresses that result in the bow of the first section of the ingot. This phenomenon is called "butt curl."t2.3J Butt curl is a major problem1"1 because it reduces the rigid standing of the ingot on the starter block and causes a portion of the ingot to lose contact with the bottom block, leading to possible shell remelting. Moreover, water can enter the gap between the starting block and the underside of the ingot; its rapid vaporization may lead to a "bumping" of the ingot. Even-
J.-M. DREZET, Graduate Student, and M. RAPPAZ, Professor, are with the Laboratoire de Metallurgle Physique, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland. Manuscript submitted December 8, 1995. 3214~VOLUME 27A, OCTOBER 1996
tually, the thermally induced stresses and strains can form cracks and hot tears. Carrupt and Moulinf5,6] demonstrated that the decisive criterion in controlling butt curl is not its final value but its rate of evolution. Several solutions, such as injecting liquid CO 2 into the cooling water systemt3] or using pulsed water to diminish the cooling during startup,t4] were suggested to reduce butt curl. However, the start phase of DC casting of aluminum alloys still remains the most critical period of the process. "Butt swell" is associated with a thicker section of the butt of the ingot as compared with the rest of the slab. During the start phase, the depth of the liquid pool is much smaller than during the stationary phase, as a result of both the reduced casting speed and the partial cooling by the bottom block. Thus, the ingot contraction is also reduced and the slab has nearly the same nominal dimensions as the mold. This situation is in fact similar to casting a metal in a permanent mold. However, once the star
