Effect of liquid metal distribution on the flow field and macrosegregation during direct chill casting
A fully transient, 2-D axisymmetric numerical model is used to investigate the effects of liquid metal distribution on the flow field and macrosegregation during direct chill (DC) casting. The model is applied to 500 mm diameter billets of aluminum alloy 7050 cast at 30 and 60 mm/min. The predicted flow fields and compositions are compared for three metal feeding systems; one containing only a submerged nozzle, another with the addition of a rigid combo bag, and a final with the addition of a flow diffuser plate. The effects of nozzle diameter, casting speed, and liquid metal distribution system type are studied. The results from the numerical model show that the flow field and macrosegregation are strongly influenced by casting speed and nozzle exit velocity. At a 60 mm/min casting speed, oscillations in the flow field and composition banding in the axial direction develop due to the competition between the nozzle flow and the thermo-solutal buoyancy. At nozzle to billet area ratios less than 5%, the nozzle exit velocity is large enough to jet down the centerline, resulting in higher levels of macrosegregation. The addition of either a low permeability combo bag or a flow diffuser plate is shown to prevent oscillations at 60 mm/min and jetting of the nozzle flow down the centerline.
Krane, Purdue University.
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