Evolutions of grain size and micro-hardness during chip formation and machined surface generation for Ti-6Al-4V in high-speed machining

Machined surface with ultrafine grained microstructure can be obtained through severe plastic deformation in high-speed machining (HSM) process. The aim of this paper is to investigate the evolution of grain size and micro-hardness during HSM of Ti-6Al-4V alloy using the finite element method (FEM) and user subroutine VUSDFLD of Abaqus/Explicit. Firstly, a FE model to simulate the cutting process of Ti-6Al-4V is proposed. The proposed cutting simulation model is verified by high-speed machining experiments in terms of cutting force and chip morphology. Secondly, a novel user subroutine VUSDFLD based on equations of Zener-Hollomon and Hall-Petch is developed to simulate the modifications of grain size and micro-hardness in chip formation and machined surface generation under different cutting speeds. Parameters in the equations of Zener-Hollomon and Hall-Petch are modified for Ti-6Al-4V for simulation of the change of grain size and micro-hardness during HSM. Lastly, the simulation results of the microstructure evolution in chips and machined surfaces are compared with experimental results obtained by optical microscopy, scanning electron microscopy (SEM), and measurement of micro-hardness. The comparison results show that the evolution of grain size and micro-hardness of Ti-6Al-4V in HSM can be accurately predicted by the modified Zener-Hollomon and Hall-Petch equations. This research indicates that smaller grain sizes are produced into both chips and machined surfaces due to more severe deformations with increasing of the cutting speed. The findings validate that HSM is a reliable approach to generate refined grains if proper machining parameters are selected. HSM can also be applied as a novel material test method to study the relationship between microstructure evolution and deformation parameters.