Improved dynamic recrystallization modeling in the high-speed machining of titanium alloy

The severe plastic deformation process induces dynamic recrystallization in the machined surface and the adiabatic shear band during high-speed machining. It is a desirable approach to directly improve the machined surface integrity by the ultra-fine grains generated from the finish machining. The grain size modification mechanism depends on the incorporated effects of high strain, strain rate, and temperature in machining. The Zener-Hollomon (Z-H) model has been a popular approach to predicting the dynamic recrystallization behavior in grains in processing. The model combines strain rate and temperature into a single parameter, Z, to achieve simplicity in prediction. However, it cannot achieve reliable prediction in high-speed machining involving large strain, high strain rate, and high temperature. Hence, based on the original Z-H model, a modified dynamic recrystallization model is proposed particularly for high-speed machining. In the modified model, the competitive roles of strain rate and temperature are emphasized since extremely high strain rate induced dynamic recrystallization and high temperature promoted dynamic recovery. Additionally, large plastic strain, neglected in the original Z-H model, is treated as a control variable to capture the behavior of grain dynamic recrystallization. Furthermore, an incremental formulation based on the modified dynamic recrystallization model is proposed to accurately explore the evolution of grain size to overcome the unpredictable abrupt changes in grain sizes generated from the original Z-H model. The results demonstrate the effective enhancement of predicted accuracy in terms of the evolution behavior of dynamic recrystallization in high-speed machining of titanium alloy.