Optimization of hot process parameters of Ti-6.7Al-2Sn-2.2Zr-2.1Mo-1W-0.2Si alloy with lamellar starting microstructure based on the processing map

Isothermal compression of BT25 titanium alloy with lamellar starting microstructure was carried out in the deformation temperatures ranging from 940 to 1000°C, strain rate between 0.01 and 10s^-1, and the height reduction of 60%. The hot deformation behavior of BT25 alloy was characterized based on the analysis of the stress-strain behavior, kinetics and processing map for obtaining optimum processing windows and achieving desired microstructure during hot working. The apparent activation energy of deformation was calculated to be 545.4kJ/mol and constitutive equation that describes the flow stress as a function of the strain rate and deformation temperature was proposed for high temperature deformation of the alloy. The processing map was constructed to evaluate the efficiency of the forging process in the temperatures and strain rates investigated and to recognize the instability regimes. The processing map exhibits two domains with the peak efficiencies of 36-40% occur at the strain rate of 0.01s^-1, the temperatures of 960 and 1000°C, respectively. The domain (at 960°C/0.01s^-1) is the one in which the mechanism of globularization mainly operates and the power is dissipated by the globularization of α lamella. The other domain (at 1000°C/0.01s^-1) is the one in which the mechanisms of globularization and α+β→β phase transformation work simultaneously. The both domains are all the optimum processing regions. Plasticity instability was expected in the regime of strain rate higher than 1.0s^-1 and entire temperature range.