Investigation of the grain refinement mechanism in machining Ti-6Al-4V: Experiments and simulations

The thermal and mechanical loading may induce the grain refinement in the deformation zone during machining, which has great impacts on the mechanical behavior of the workpiece. In order to investigate the grain refinement mechanism in the shear band of Ti-6Al-4V during machining, we quantitatively analyzed the final microstructural characteristics including the grain size, grain boundary misorientation, and geometrical necessary dislocation (GND) density distribution in shear bands by precession electron diffraction (PED). It is found that the grains are highly refined in the shear band, and obvious continuous dynamic recrystallization (CDRX) characteristics have been captured. Besides, discontinuous dynamic recrystallization (DDRX) is verified not happen in shear bands of Ti-6Al-4V during machining theoretically while DDRX is often considered as the main grain refinement mechanism to predict the grain size in shear bands in literature reports. Then, a physically-based CDRX model is applied in machining, which describes the formation and rotation of subgrain, as well as the migration of the grain boundary. Subsequently, the cross-scale simulation method combining finite element (FE) and cellular automata (CA) is used to reproduce the microstructure evolution process of the shear bands. And, the effectiveness of the proposed model is verified by comparing the microstructure morphology, grain size, and dislocation density obtained by simulation with those characteristics observed experimentally. Finally, the model was applied to analyze the microstructure evolution in machined surface.