Modeling of microstructure formation of Ti-6Al-4V alloy in a cold crucible under electromagnetic field

An integrated macro/micro model is developed for simulating the microstructure evolution during solidification processes of Ti-6Al-4V alloy in a cold crucible under electromagnetic field, which combines the 3-D finite difference method (FDM) at the macroscale with a 2-D cellular automaton (CA) model at the microscale. Based on the FDM solution of momentum, thermal transport and Maxwell equations, the macro model is used to simulate the fluid flow and heat transfer throughout the casting under electromagnetic field. The micro model is used to predict the nucleation and growth of grains for the vertical central plane. Validity of the model is confirmed by comparison between the result from calculation and it from direct measurement. Numerical simulations are performed to investigate the influences of coil current, duration and heat transfer coefficient at the casting/crucible interface on fluid flow and microstructure formation. Calculated results reveal that the growth of coarser columnar structure is promoted for a longer duration. The variation of coil current has a minor effect on the formation of microstructure. Increasing the heat transfer coefficient generates a coarser cast structure. The main action of electromagnetic field is that a small vortex in the upper part rotates in opposite direction to the buoyancy convection. The underlying mechanisms responsible for those physical phenomena are discussed.