Numerical Simulation of Microstructure Evolution During the Solid Phase Transformation of Ti-6Al-4V Alloy in Investment Casting

Investment casting is widely used in producting complex thin-wall titanium alloy components. In this process, the beta ->alpha phase transformation decides the final microstructures of these components. However most of present studies on phase transformation of titanium alloys focus on the microstructure evolution in heat treatment process or after deformation rather than in casting process now. It is a main reason only this work aims at the solid phase transformation of Ti-6Al-4V alloy in investment casting. In this work, the growth model of edge of alpha phase plates based on multi component Zener-Hiller model, and the growth model of broad face of alpha phase plates based on diffusion and conservation of multi components were established. The growth competition of different colonies, which consist of alpha phase plates in same orientation, was simulated and the microstructures and their evolution with temperature were obtained. The comparison between simulated microstructures and their evolution with temperature and experimental data indicated that the proportion of undercooling degree caused by impurities in the alloy is about 45% of the total undercooling degree in broad face of alpha phase plates and a much smaller portion in edge of alpha phase plates. The comparison also showed that the enthalpy change of solid phase transformation of titanium alloy is about 70 kJ/kg. The simulated and experimental morphologies look like similar and the simulated growth rate is also in good accordance with experiment inferred growth rate.