Effects of Nonconstant Strain Rate on Forming Limit and Efficiency in High Pressure Pneumatic Forming of Ti-Alloy Components

Abstract

A process with gas pressure up to 70MPa is introduced, which is called High Pressure Pneumatic Forming (HPPF), comparing to superplastic forming (SPF) with pressure lower than 5MPa. HPPF process can be used to form tube blank at lower temperature with high energy efficiency and also at higher strain rate than SPF. With Ti-3Al-2.5V Ti-alloy tube, the potential of HPPF was studied through experiment in the temperature range of 700~850°C. To know the formability of the Ti-alloy tube, HPPF experiments of a large expansion tube and a square cross-section tube were carried out at different temperature and pressure. The limit expansion ratio and limit radius were measured to evaluate the forming limit of Ti-3Al-2.5V tube within HPPF. The results show that the lower the pressure, the better formability and the lower efficiency. At a constant pressure, the strain rate increases exponentially with bulging time during the free bulging procedure, but decreases exponentially during the small corner calibration. Through EBSD pictures, the deformation mechanism of the corner forming process in HPPF was analyzed. Because of a nonconstant strain rate deformation state and complicated stress and strain state during HPPF, the microstructure at the transition zone of the components are also nonhomogenous, but the grains are refined to a certain extent. Key words: HPPF, Ti-3Al-2.5V, limit expansion ratio, corner forming