A three-dimensional transient thermal-fluid flow-compositional study of ingot casting during electron beam remelting of Ti-6Al-4V

Electron beam cold hearth remelting (EBCHR) is a consolidation and refining process capable of consolidating titanium scrap and sponge material into high quality titanium alloy ingots. During the final stage of casting in the EBCHR process, operators must balance the potential to form large shrinkage voids, caused by turning off the electron beam heating, against the tendency to evaporate alloying additions, which occurs if the top surface remains molten. A comprehensive understanding of the evaporation and fluid flow conditions occurring during the final stage of EBCHR is required in order to optimize ingot production. This research focused on developing a coupled thermal, fluid flow and composition model, capable of predicting the temperature, fluid flow and composition fields within an EBCHR cast, Ti-6Al-4V ingot. The physical phenomena of thermal and compositional buoyancy, mushy zone flow attenuation and aluminum evaporation were incorporated in the model formulation. Industrial scale experiments were carried out at the production facilities of a leading industrial producer of titanium to provide data and measurements for model verification. Model predictions for liquid pool profile, last liquid to solidify and composition fields are in good agreement with the industrially measured results. A sensitivity analysis was performed with the model to study the effects of variation of electron beam power input and hot top time duration on the evaporative losses and position of solidification voids. For the cases examined, there was a strong correlation between electron beam power and alloying element losses, while hot top duration variation results indicated a stronger dependence on last liquid to solidify than on alloying element losses. Therefore a classic optimization problem arises between balancing hot top duration with alloying element losses.