The hardening effect of deformation twinning based on visco-plastic self consistent model and a multi-scale grain refinement prediction model during machining of titanium alloy

Deformation twins act as one of the deformation characteristics of hexagonal close packed (HCP) materials and occur during machining of Ti-6Al-4V alloy. To clarify the hardening mechanisms of deformation twins in Ti-6Al-4V machined surface, work-hardening effects induced by grain refinement in meso scale as well as deformation twins in micro scale were investigated. A 2D cutting finite element (FE) model was established to predict grain refinement in multi-scales and its hardening behavior during machining of Ti-6Al-4V. A visco-plastic self consistent model (VPSC) coupled with predominant twin reorientation (PTR) scheme was also used to simulate stress variation with considering the activations of twin systems. The results show that different grain refinement degrees in meso scale are observed with the increasing of cutting speeds (100 - 500 m/min). {101} compression nano twins generate at higher cutting speeds, and the twin volume fraction is relatively small (20%). The hardening behaviors of grain refinement in meso scale as well as nano twins predicted with FE model and VPSC model are similar, and the generation of nano twins cause inhomogeneous hardening in Ti-6Al-4V machined surface. Nano-indentation tests further prove the accuracy of the prediction results for the hardening effect induced by nano twins in Ti-6Al-4V machined surface. The results provide two explanations and quantitative evaluations for the hardening effects of deformation twins from the macroscopic stress and the microscopic deformation of slip and twinning modes.