Microstructural Modeling and Process Control During Hot Working of Commercial Ti-6A1-4V: Response of Lamellar and Equiaxed Starting Microstructures

Abstract

The microstructural changes that occur during hot deformation of Ti-6A1-4V alloy with β transformed (β_t) and equiaxed α+β ((α+β)_e) starting microstructures have been modeled in the temperature range 750-1100°C and strain rate range 0.0003-10 s^-1. The stress-strain behavior, the kinetic parameters, and the processing maps have been compared for these two preforms with a view to evaluate the mechanisms of hot deformation and to establish correlations between the microstructural changes and the process parameters. The β_t preform exhibits continuous flow softening behavior below the transus (α-β range) and is harder than the (α+β)_e preform. In the β range, the stress-strain curves were of steady-state type except at the highest strain rate where oscillations have been observed. In the a-p range, the apparent activation energies for hot deformation are 455 and 330 kJ/mole for the β_t and (α+β)_e preforms respectively and the stress exponents are similar. In the β range, the apparent activation energy is in the range 172-210 kJ/mole which is close to that for self diffusion in β titanium. The grain size variation with temperature and strain rate could be correlated with the Zener-Hollomon (Z) parameter. Deformed at slow strain rates in the two phase range, the β_t preform undergoes a microstructural conversion by the process of globularization of lamellar colony structure. The optimum parameters for globularization are 960°C/0.0003 s^-1. The size of globules is not dependent on strain but on temperature and strain rate of deformation and could be correlated with Z. The (α+β)_e preform, on the other hand, deforms superplastically and the optimum processing parameters are 825°C/ 0.0003 s^-1. In this domain, the variation of α grain size with Z is linear on a log-log scale. The processing windows are similar in both the preforms except at the lower