A Study of Beta Processing of Ti-6Al-4V: Is it Trivial?

Deformation of Ti-6Al-4V alloy in the \beta phase field is generally considered to be trivial since the material is highly workable at these temperatures and does not normally pose problems during processing. In view of this, studies on the hot deformation behavior of \beta are scanty compared to those on the $\alpha- \beta$deformation. This paper is focussed on understanding the \beta deformation characteristics in Ti-6Al-4V with a view to examine whether such studies help in optimizing the process design and achieving microstructural control. The emphasis has been on the two industrial grades, viz. commercial purity (CP) versus extra-low interstitial (ELI), and also on the effect of starting microstructure (transformed \beta versus equiaxed $\alpha+\beta$). The stress-strain curves obtained in compression exhibited steady-state behavior at strain rates lower than $1\hspace{2mm}s^{-1}$ and oscillatory/softening behavior at higher strain rates. Kinetic analysis of the flow stress data obtained at different temperatures and strain rates has shown that the stress exponent is about 3.6-3.8 and the apparent activation energy is in the range 150-287 kJ/mole, which is comparable to that of self-diffusion in \beta phase (150 kJ/mol). Dynamic recrystallization (DRX) of \beta is identified as the microstructural mechanism in all the cases except in ELI grade with transformed \beta starting structure. The prior \beta grain size in the DRX region exhibits a good correlation with the Zener-Hollomon parameter. In case of ELI grade, a mechanism of large grained superplasticity involving sliding of prior colony boundaries has been identified. However, deformation close to the transus in ELI grade causes nucleation of voids which may grow during soaking at the deformation temperature under the influence of tensile residual stress. The results clearly show that a study of \beta deformation mechanisms holds the key during hot working of Ti-6-4. More importantly, the processing schedule used for CP grade Ti-6-4 should not be used for ELI grade to achieve microstructural control and avoid defects