Study On Hot Deformation Behavior Of BT25 Titanium Alloy

Phase transformation kinetics of αâ†'β in BT25 titanium alloy during heating, effect of hot deformation on the continuous cooling transformation curve and hot deformation behavior of the alloy were investigated by high differential dilatometry B?hr DIL805A/D. Besides, more accurate flow stress curve was obtained by the modified temperature. Deformation activation energy and flow stress constitutive equation of the alloy were obtained in α+β phase zone and β single phase zone. Furthermore, processing maps of the BT25 alloy were established on the basis of dynamic material modeling(DMM) and the theory of power dissipation, the effect of the deformation thermal effects on the maps of the alloy were analyzed. The main conclusions of the study are as follows:(1) Heating rate has great effect on the αâ†'β phase transition temperature(Tβ) of BT25 alloy. The difference value between the highest and the lowest is 25 °C. The overall activation energy for the αâ†'β phase transformation in the BT25 alloy was evaluated on the basis of Kissinger equation, and the value was953.15KJ·mol-1.(2) Hot deformation has remarkable effect on continuous cooling transformation curve(CCT) of BT25 alloy.When cooling rate is less than or equal to 3 °C·S-1, starting and finishing temperatures of the phase transformation of dynamic CCT curve are lower than those of static. When cooling rate is higher than or equal to 10 °C·S-1, the starting temperatures of the phase transformation of a dynamic CCT curve are significantly higher than those ofstatic, however the finishing temperatures are basically identical.(3) The effect of the deformation thermal effects is remarkable under high strain rate. Deformation activation energy Q of BT25 alloy was calculated:950 ~ 1010 °C, Q = 763.51 KJ/mol, 1040 ~ 1100 °C, Q = 231.36KJ/mol. Flow stress constitutive equation of the alloy ware obtained :950~1010℃:)/763507.18exp()]0.0162622[sinh(2.9840685339.70=seRTe·1040~1100℃:)/85.231357exp()]0286237.0[sinh(2.7275278921.17=seRTe·(4) Processing maps of BT25 alloy were established on the basis of corrected flow stress curve. The results indicated that the alloy possesses good forgeability at the peak of power dissipation efficiency when the alloy deformed at 1000 ~ 1100℃ with the primary strain rate of 0.001 ~ 0.01 s-1.(5) Instability regions in the uncorrected processing maps of BT25 alloy are respectively greater than the corrected. At the same time, the power dissipation efficiency decrease obviously in these regions. Flow stress in higher strain rate condition must be corrected in order to get the accurate processing map during the isothermal constant strain rate compression process, particularly for titanium alloys with low thermal conductivity.