Influence of oxygen content on the forging response of equiaxed (α+β) preform of Ti–6Al–4V: commercial vs. ELI grade

Abstract

The hot deformation characteristics of equiaxed (α+β) preform of Ti–6Al–4V have been evaluated in two oxygen grades viz., commercial grade and extra-low interstitial (ELI) grade. Constant strain rate hot compression tests were conducted on cylindrical specimens in the temperature range 750–1100°C and strain rate range 10⁻³-100 s⁻¹ up to a true strain of about 0.5. The shapes of stress–strain curves in the α–β region and β-region, the kinetic parameters, and the processing maps obtained on the two grades have been compared with a view to evaluate the effect of oxygen on the processing of Ti–6Al–4V with equiaxed (α+β) preform. The shapes of the stress–strain curves are similar in the two oxygen grades although the flow stress of the commercial grade is slightly higher than that of the ELI grade. While the values of the stress exponent are similar in both grades, the apparent activation energy for hot deformation of commercial grade is higher than that of the ELI grade in both α–β and β ranges. The processing maps revealed that superplasticity occurs in the α–β range while dynamic recrystallization (DRX) occurs in the β range. In the commercial grade, the superplasticity domain is spread over a wider temperature range and occurs at lower strain rates than that in the ELI grade. On the other hand, the ductility peak in the ELI grade is narrow and may require closer temperature control during processing. The DRX of β occurs at 1100°C in both grades but at a higher strain rate in the commercial grade than in the ELI grade. Unlike the commercial grade, deformation of the ELI grade close to the β transus has to be avoided since a possibility of void nucleation or wedge crack formation exists at slow strain rates. Both grades exhibit flow instabilities manifested as adiabatic shear bands and flow localization when deformed at strain rates higher than about 1 s⁻¹.