Multiphysics modeling of phase transformation and microhardness evolution in laser direct deposited Ti6Al4V

Abstract

This paper investigates the phase transformations occurring in laser direct deposited Ti6Al4V and assesses the microhardness of the as-built components using predictive modeling tools. A three-dimensional laser direct deposition (LDD) model was utilized to predict the geometry and thermal history of the multi-track LDD process. The predicted free surface geometry, fusion zone (FZ) and heat affected zone (HAZ) boundaries were in good agreement with experimental results. Temperature profiles and cooling rates were extracted from the validated LDD model and incorporated into a two-dimensional phase prediction model to determine phase transformations in FZ and HAZ regions. In the FZ, the high peak temperature and high cooling rate produced a complete α’ martensitic microstructure, which was predicted with the phase transformation model and observed experimentally. Within the HAZ, a low peak temperature and correspondingly low heating and cooling rates led to a mixture of α’ martensite, transformed and untransformed α phase in the final microstructure. When depositing the subsequent tracks, the LDD process produces overlapped HAZ regions, which contain a higher volume fraction of α’ martensite due to the repeated heating cycles. Based on the volume fraction of α’ martensite and α phase in the final microstructure, the microhardness within different zones of a three-track Ti6Al4V deposit was accurately predicted.