Multiscale modelling of microstructure formation during vacuum arc remelting of titanium 6-4

The vacuum arc remelting of titanium 6-4 alloy is a complex process. Relatively high melt currents (≥30 kA) are used resulting in a very deep melt pool that is continually changing in size (first increasing and then decreasing). The process is further complicated due to the use of external stirring coils to control and steer the arc. A transient model is needed to adequately describe the process. A multiscale modelling approach was developed which combines an axisymmetric CFD model at the macroscale with a cellular automaton model at the mesoscale. The macro model is used to simulate the heat and mass transfer throughout the ingot and melt pool under the influence of the arc, including EMF. A decentred-square cellular automaton model is used to predict the nucleation and growth of grains. This multiscale model is applied to the initial phase of the process, and the predicted microstructures are compared with trial ingots. The effect of the model parameters such as stirring and grain nucleation on the morphology of the columnar zone is investigated.