Hot deformation behavior of a novel alpha + beta titanium alloy TIMETAL®407

Abstract

The hot deformation behavior of a newly developed α + β titanium alloy TIMETAL®407 (Ti407) was characterized in starting transformed-β microstructure at different temperatures (T = 650–950 °C) and strain rates (ε̇ = 10−3 - 1 s−1) using hot-compression testing. The Arrhenius type constitutive equations are developed for Ti407 and the activation energy for deformation is determined as 333.1 and 179.8 kJ/mol in the α and β dominated phases, respectively. The processing and strain-rate sensitivity maps of Ti407 were generated at true strain, ε = 0.5 on the basis of modified dynamic materials model (DMM) proposed by Murty and Rao. Results obtained from the maps and microstructural analysis of the deformed specimens indicates three distinct stable domains (I-A, II-A and I-B) within the examined conditions. Out of the three stable domains, Domain I-A occurring in the α + β phase field (T = 800–860 °C and ε̇= 6.3 ×10−2 - 1 s−1) and Domain I-B in the β phase field (T = 900–950 °C and ε̇= 10−3 - 1 s−1) were optimum domains for possible hot working of Ti407. The peak in the efficiency of power dissipation of about 58% occurred in Domain I-A at 850 °C/1 s−1, where dynamic spherodization of α lamellae occurred. In Domain I-B, formation of fine dynamically recrystallized β grains was observed with peak in power dissipation efficiency of about 60% at 950 °C/10−3 s−1. Ti407 showed evidence of flow instabilities like flow localization and kinking of α lamellae in the predicted unstable zones identified from the processing map. The hot workability of Ti407 on the basis of expanding or shrinking stable regimes for processing is also compared with CP-Ti and Ti-6Al-4V alloys at identical strain rates and temperatures relative to the beta-transus. Finite element method (FEM) software DEFORM®− 3D was used for forging simulation at the peak in deformation efficiency condition in various stable domains to evaluate the flow response and distributions of stress/strain in the forged billets.