Modeling of Macrosegregation Formation and the Effect of Enhanced Cooling During Vacuum Arc Remelting Solidification of NbTi Alloy Ingot

Modeling vacuum arc remelting (VAR) is very challenging as this process involves a wide range of physical phenomena and complex interactions. The formations of macrosegregation and channel segregation during VAR are essential issues and require deep investigations. A two-dimensional transient model is developed to study the macrosegregation of NbTi alloy and predict the occurring tendency of channel segregation by dimensionless Rayleigh number. The most important feature is a full consideration of electromagnetic interaction, multi-force driven convection, heat as well as solute transfers, and time-variant characteristic values of microstructure. The simulation results reveal that the Lorentz force induced by the electromagnetic interaction mainly controls the macrosegregation distribution in the ingot. The effects of enhancing cooling on macrosegregation are studied by filling helium into an air gap formed between the inner wall of the crucible and the outer wall of the ingot. As the cooling efficiency is increased with filling pressure, there exists an optimized filling pressure to avoid the formation of a positive segregation in the central region. The homogenization of the ingot is improved. The simulated macrosegregation and channel segregation show fairly good qualitative agreement with the experimental results. This study elucidates the mechanism of enhanced cooling reducing channel segregation and highlights an economic and controllable method to produce homogenized ingots during VAR.