| Summary: | 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. === Applied Science, Faculty of === Materials Engineering, Department of === Graduate
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