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Secondary cooling in the direct-chill casting of light metals Etienne, Caron


The Direct-Chill (DC) casting process is used in the non-ferrous metals industry to produce ingots, blooms and cylindrical billets. During DC casting, primary cooling in the mould is followed by secondary cooling, in which the cast product surface is directly cooled by water jets. The formation of defects during the direct-chill casting process can be reduced by controlling the heat extraction in the secondary cooling zone during the start-up phase. The control and optimization of this process requires an accurate knowledge of the boundary conditions and their relationship with casting parameters. This research project studied the effect of different parameters on the heat transfer in the secondary cooling zone of the direct-chill casting process. This process was simulated by quenching instrumented samples of industrial DC-cast aluminum AA5 182 and magnesium AZ3 1 with water jets and recording the thermal history within the sample using sub-surface thermocouples. An inverse heat conduction algorithm specifically developed for this research project converted this thermal history into surface heat fluxes and surface temperatures. The relationship between heat flux and surface temperature was expressed by a boiling curve. Cooling experiments showed the influence of the cooling water flow rate on characteristic features of the boiling curve. The effect of thermophysical properties, initial sample temperature and water temperature on high temperature boiling regimes was also quantified. The influence of other parameters such as the water jet velocity and the surface roughness was determined in a qualitative fashion. Results from the quench tests were used as boundary conditions in a finite element model for the direct-chill casting of AZ3 1 billets. Simulations of the process start-up phase showed the critical role played by stable film boiling and water film ejection in determining the thermal history within the billet.

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