Modelling of air gap development and associated surface macrosegregation in DC casting of aluminium sheet ingots

The interaction between thermally induced deformation and heat transfer in DC casting of aluminium often results in the formation of an air gap between the mould and the ingot. The metallostatic head can then force interdendritic liquid through the mushy zone, past the original casting surface, and into the air gap. Such exudation of liquid leads to a solute rich surface layer. A two dimensional mathematical model that quantifies surface macrosegregation due to exudation and solidification shrinkage has been presented elsewhere. A critical parameter of this model is the position of the air gap, and a thermomechanical calculation is needed for an accurate prediction of the development of this gap. For this purpose, the heat transfer calculation being input to the surface segregation model is carried out with an advanced model in which the air gap development is also taken into account by a model addressing the thermally induced deformations in the ingot. Periodic variations in the surface macrosegregation along the casting direction have been reproduced by the models simulating the DC casting of an AA5182 sheet ingot. These variations reflect oscillations in measured mould temperatures that are related to the periodic solidification/melting of the surface shell.