Modeling of microstructure refinement in Ti-6Al-4V alloy by hydrogen treatment

An integrated macro/meso/micro-model of microstructure refinement of Ti-6Al-4V alloy by hydrogen treatment is developed, which combines the 3-D finite difference method (FDM) at the macroscale with the 2-D cellular automaton (CA) models at the mesoscale and microscale. The macro-model is used to simulate the heat transfer and fluid flow throughout the casting. The meso-model is taken to predict the microstructure formation. The micro-model is used to depict the dendrite morphology. Important relationships between the number of dendritic fragments and processing variables (initial hydrogen content and melt superheat) are proposed, the validity of which is testified by the experimental measurements. With the developed model, numerical simulations are performed to investigate the influences of processing variables on microstructure refinement. Calculated results reveal that a finer equiaxed structure is obtained with increasing the initial hydrogen content. Changing the melt superheat is found to have a negligible influence on the microstructure refinement. The underlying mechanisms responsible for those physical phenomena are discussed in detail.