5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Melt conditioning, i.e. shearing of liquid metal using a rotor-stator device, refines the grain structure without the need for grain refiners. Melt conditioning as applied to direct-chill (DC) casting, forming the MC-DC process, has recently been demonstrated to be an effective method of producing high quality light alloy billets. The optimisation of melt conditioning parameters through experiments is expensive and difficult due to the large variability in DC casting and melt-conditioning parameters, diameters of billets, and compositions of the treated alloy melt. In this contribution, we present a new numerical model of melt conditioned direct-chill (MC-DC) casting that considers grain motion and use this model to determine the position of the mixer inside the sump that maximises the temperature gradient across the mushy zone. This numerical model is implemented using the OpenFOAM library. The model is based upon a continuum formulation that avoids the need for tracking phase interfaces, thereby making the model computationally affordable and attractive for optimization studies. The model is validated using temperature and sump profile measurements in conventional DC casting from the literature and in-house measurements in MC-DC cast billets. Turbulence is handled using a Large Eddy Simulation (LES) to accurately resolve the effect of turbulence on grain redistribution; this flow model has been validated using particle image velocimetry (PIV) measurements in a water tank. The optimization search is performed over the global design space using an Evolutionary Algorithm (EA). Melt shearing results in an increased temperature gradient across a shortened slurry zone and a shallower sump, consistent with previous experiments in both Mg and Al alloys. The consequent uniform cooling rate in the slurry zone contributes to a finer, more uniform grain structure in the as-cast billet. The optimization study will be extended to other design parameters (rotation speed, rotor-stator geometry, operating temperature, alloy composition etc.)