A microscale cellular automaton method for solid-state phase transformation of directed energy deposited Ti6Al4V

Abstract

Directed energy deposition is a promising additive manufacturing technology that fabricates complex geometries by fusing feed material layer-by-layer. However, the formation mechanism of the directed energy deposited Ti6Al4V solid-state phase transformation process, which is crucial for understanding the process-structure–property relationship, has remained unclear. In this study, a microscale cellular automaton method is proposed to simulate the microstructure evolution process for Ti6Al4V, specifically the $\beta \to \alpha /{\alpha }^{\prime }$ phase transformation process within a few $\beta$ grains. The method is further integrated with the mesoscale cellular automaton method, which predicts the prior $\beta$ grain structure, the solid-state phase transformation kinetics model for the prediction of the phase volume fractions, and the finite volume method, which is used for the thermal-fluid flow modeling, providing a temperature field to the former. The integrated numerical framework not only links the thermal history with the phase volume fractions but also provides the columnar $\beta$ grain structures and acicular $\alpha /{\alpha }^{\prime }$ grain structures in satisfying agreement with the available experimental observation. Moreover, the predictions shed some light on the formation mechanism of the hierarchical $\alpha /{\alpha }^{\prime }$ structure and their particular clusters. The influence of the cooling rate on the $\alpha /{\alpha }^{\prime }$ grain formation is illustrated via the three-layer case simulation. The findings on the formation mechanism of $\alpha /{\alpha }^{\prime }$ are beneficial in tailoring the microstructure of Ti6Al4V for excellent mechanical properties in directed energy deposited Ti6Al4V.