Effects of Process Variables and Size Scale on Solidification Microstructure in Laser-Based Solid Freeform Fabrication of Ti-6Al-4V

Abstract

Abstract : This paper summarizes a combination of analytical and numerical modeling approaches which have been used to investigate the effects of process variables and size scale on solidification microstructure in laser-deposited Ti-6A1-4V. The analytical approach is based on the well-known Rosenthal solution for a moving point heat source, which provides dimensionless process maps for solidification cooling rate and thermal gradient (the key parameters controlling microstructure) as a function of laser deposition process variables (laser power and velocity). Based on these process maps, results for both 2-D thin-wall and bulky 3-D geometries are plotted on solidification maps for predicting grain morphology in laser-deposited Ti-6A1-4V. Although the Rosenthal results neglect the nonlinear effects of temperature-dependent material properties and latent heat of transformation, a comparison with 2-D and 3-D nonlinear thermal finite element results for both small-scale (LENS(TradeMark)) and large-scale (high power) processes suggests that they can provide reasonable estimates of trends in grain morphology. Finally, 3-D cellular automaton solidification modeling is used to provide direct predictions of solidification microstructure, and results are compared to experimental observations for both LENS(TradeMark) and a larger scale process under development at SDSM&T. The results of this work suggest that changes in process variables could potentially result in a grading of the microstructure throughout the depth of the deposit, and that a transition from columnar to equiaxed microstructure is possible at high laser powers.