Rheology of Ti-6Al-4V alloy under non equilibrium conditions during forging

During the "beta-forging" of Ti-6Al-4V structural aircraft components that follows pre -heating above the beta-transus, the skin zone of the blank actually cools down to below the beta-transus whilst being deformed, as a result of heat losses towards the atmosphere or the dies. The core areas of the bulkiest sections of the blank, however, can be deformed above the beta-transus during the whole processing time. This paper focuses on the development of an innovative rheological model to support the numerical modelling of the forging process, suitable for both the equilibrium conditions above the beta-transus and the non -equilibrium conditions below. For that purpose, smallscale hot compression tests were performed with the thermomechanical simulator Gleeble 3500, after preheating in the beta-field of Ti-6Al-4V at 1100 degrees C and rapid cooling to the compression temperature under the beta-transus, which was varied between 750 degrees C and 950 degrees C. In some cases, a lag time -or stabilization time- at constant temperature was introduced between the cooling step and the deformation step. Effects of lag time and deformation temperature on microstructural evolutions and rheological properties of Ti-6Al-4V were investigated. Results show that the lack of stabilization time leads to a significant reduction of the flow stress at low strain. Such a behavior is associated with non -equilibrium thermodynamic conditions at the beginning of deformation and the progress of phase transformation during deformation. Based on that observation, an analytical formulation for the targeted rheological model is proposed. It includes a sub -model for the kinetics of phase transformation derived from the literature.