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

摘要

The microstructural changes that occur during hot deformation of Ti-6A1-4V alloy with #beta# transformed (#beta#_t) and equiaxed #alpha#+#beta# ((#alpha#+#beta#)_e) starting microstructures have been modeled in the temperature range 750-1100 deg 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 #beta#_t preform exhibits continuous flow softening behavior below the transus (#alpha#-#beta# range) and is harder than the (#alpha#+#beta#)_e preform. In the #beta# range, the stress-strain curves were of steady-state type except at the highest strain rate where oscillations have been observed. In the #alpha#-#beta# range, the apparent activation energies for hot deformation are 455 and 330 kJ/mole for the #beta#_t and (#alpha#+#beta#)_e preforms respectively and the stress exponents are similar. In the #beta# range, the apparent activation energy is in the range 172-210 kJ/mole which is close to that for self diffusion in #beta# 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 #beta#_t preform undergoes a microstructural conversion by the process of globularization of lamellar colony structure. The optimum parameters for globularization are 960 deg 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 (#alpha#+#beta#)_e preform, on the other hand, deforms superplastically and the optimum processing parameters are 825 deg C/ 0.0003 s~(-1). In this domain, the variation of a grain size with Z is linear on a log-log scale. The processing windows are similar in both the preforms except at the lower temperature limit which is higher for the #beta#_t preform by about 100 deg C than the (#alpha#+#beta#)_e preform. The #beta# phase undergoes dynamic recrystallization irrespective of the preform structure and the resulting grain size is dependent on the Z parameter. Both the preforms exhibit flow instabilities when deformed in the two phase range at higher strain rates and these are manifested as adiabatic shear bands causing flow localization.