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

The hot deformation characteristics of equiaxed (alpha + beta) preform of T i-6Al-4V have been evaluated in two oxygen grades viz., commercial grade an d extra-low interstitial (ELI) grade. Constant strain rate hot compression tests were conducted on cylindrical specimens in the temperature range 750- 1100 degreesC 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 alpha-beta region and beta -region, the kinetic parameters, and the processing maps obtained on the two grades have been compared with a view to evaluate the effect of oxy gen on the processing of Ti-6Al-4V with equiaxed (alpha + beta) preform. Th e shapes of the stress-strain curves are similar in the two oxygen grades a lthough the Row stress of the commercial grade is slightly higher than that of the ELI grade. While the values of the stress exponent are similar in b oth grades, the apparent activation energy for hot deformation of commercia l grade is higher than that of the ELI grade in bath alpha-beta and beta ra nges. The processing maps revealed that superplasticity occurs in the alpha -beta range while dynamic recrystallization (DRX) occurs in the beta 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 gr ade. On the other hand, the ductility peak in the ELI grade is narrow and m ay require closer temperature control during processing. The DRY of beta oc curs at 1100 degreesC in both grades but at a higher strain rate in the com mercial grade than in the ELI grade. Unlike the commercial grade, deformati on of the ELI grade close to the beta transus has to he avoided since a pos sibility 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 abou t l s(-1). (C) 2001 Elsevier Science B.V. All rights reserved.