Mathematical model for studies of metallurgical structures in aluminium DC castings

Abstract

Detailed studies have been carried out of the heat transfer within the mould in the direct-chill casting of aluminium blocks and billets, together with studies of the conditions within the region of secondary flood or spray cooling below the mould. These measured parameters have been applied to mathematical simulations of the casting process, which have enabled the temperature fields within the mould and the casting to be computed, and these in turn allow the cell size distribution within the casting to be predicted. Casting systems which have been investigated include:(i) 97 x 30 cm blocks cast through conventional open-head moulds(ii) 15 cm dia. billets cast through a closed-head system (iii) 97 x 30 cm blocks cast with reduced secondary cooling.The cell size distributions as predicted by the mathematical model are shown to be in good agreement with measurements from cell counts on alloys cast through these three systems. In conventional open-head block casting the peripheral zone structure can determine the depth of surface metal which has to be scalped off prior to rolling, and the model indicated that this is primarily dependent on the metal depth in the mould during casting. The model also showed that average cell size in any casting is dependent on casting speed, particularly in billet casting, where a wide range of speeds can be employed. In conventionally cast billets, increased speed reduces the average cell size,. on the other hand, in spray-cooled castings, where the reduced secondary cooling produces much larger cells, an increase in casting speedgives rise to an increase in average cell size. A successful system for producing spray-cooled blocks is described in which the model played a key role in aiding the design.