On the development of a three-dimensional transient thermal model to predict ingot cooling behavior during the start-up phase of the direct chill-casting process for an AA5182 aluminum alloy ingot

Abstract

The control of the heat transfer during the start-up phase of the direct-chill (DC) casting process for aluminum sheet ingots is critical from the standpoint of defect formation. Process control is difficult because of the various inter-related phenomena occurring during the cast start-up. First, the transport of heat to the mold is altered as the ingot base deforms and the sides are pulled inward during the start-up phase. Second, the range of temperatures and water flow conditions occurring on the ingot surface as it emerges from the mold results in the full range of boiling-water heat-transfer conditions-e.g., film boiling, transition boiling, nucleate boiling, and convection-making the rate of transport highly variable. For example, points on the ingot surface below the point of water impingement can experience film boiling, resulting in the water being ejected from the surface, causing a dramatic decrease in heat transfer below the point of ejection. Finally, the water flowing down the ingot sides may enter the gap formed between the ingot base and the bottom block due to butt curl. This process alters the heat transfer from the base of the ingot and, in turn, affects the surface temperature on the ingot faces, due to the transport of heat within the ingot in the vertical direction. A comprehensive mathematical model has been developed to describe heat transfer during the start-up phase of the DC casting process. The model, based on the commercial finite-element package ABAQUS, includes primary cooling via the mold, secondary cooling via the chill water, and ingot-base cooling. The algorithm used to account for secondary cooling to the water includes boiling curves that are a function of ingot-surface temperature, water flow rate, impingement-point temperature, and position relative to the point of water impingement. In addition, a secondary cooling algorithm accounts for water ejection, which can occur at low water flow rates (low heat-extraction rates). The algorithm used to describe ingot-base cooling includes both the drop in contact heat transfer due to gap formation between the ingot base and bottom block (arising from butt curl) as well as the increase in heat transfer due to water incursion within the gap. The model has been validated against temperature measurements obtained from two 711 ϫ 1680 mm AA5182 ingots, cast under different start-up conditions (nontypical "cold" practice and nontypical "hot" practice). Temperature measurements were taken at various locations on the ingot rolling and narrow faces, ingot base, and top surface of the bottom block. Ingot-base deflection data were also obtained for the two test conditions. Comparison of the model predictions with the data collected from the cast/embedded thermocouples indicates that the model accounts for the processes of water ejection and water incursion and is capable of describing the flow of heat in the early stages of the casting process satisfactorily.

The use of water cooling during the continuous casting of steel and aluminum alloys

“…In the beginning at the meniscus, the solidifying metal is in close contact with the mold, and the heat-transfer rate is very high. Specifically, peak heat fluxes can exceed 10 MWm Ϫ2 in steel continuous casting [1,20] and 1 MWm -2 in aluminum DC casting [2] . The latter process has smaller values for several reasons.…”

“…In steel continuous casting, the newly formed shell remains in relatively good contact with most of the 700-mm-long mold, owing to pressure from the internal liquid pool pushing the weak shell against the mold walls and intentional tapering of the mold walls to match the solidification shrinkage. In DC casting, however, the duration of this initial contact stage is quite brief, ending within ϳ80 mm [2] (depending upon the casting speed, alloy composition, and ingot geometry) of mold-metal contact. Stage 1 ends with the formation of a significant air gap between the metal and mold as soon as the solid shell is strong enough to contract away from the mold faces.…”

“…The shell thickness predictions from a 2-D [46] and a 3-D [2] FE-based thermal model for casting steel and aluminum are shown in Figures 19 and 20, respectively. Profiles at the mold exit and in the secondary cooling are compared.…”

“…20-Temperature profiles and shell thickness predicted in cross sections through the ingot during secondary cooling taken ϳ375 s after startup for DC casting of aluminum. [2] gradients through the shell are linear at the mold exit. The shell thickness at the mold exit is ϳ20 mm for a typical case, as shown in Figure 19, and its surface temperature drops to 70 pct of the melting (liquidus) temperature, T m in absolute degrees (K).…”

“…For steel slabs, the casting speed increases with decreasing thickness from 0.01 ms Ϫ1 (for 300-mm blooms) to over 0.08 ms Ϫ1 (for 50-mm thin slabs). Owing to cracking difficulties during startup, aluminum alloy ingots and billets are cast at much lower speeds, increasing from ϳ0.00075 to 0.001 ms Ϫ1 [2] to steady speeds ranging from 0.001 to 0.003 ms Ϫ1 [3] .…”