Prediction of chip morphology and segmentation during the machining of titanium alloys

Chip morphology and segmentation play a predominant role in determining machinability and tool wear during the machining of titanium alloys. At lower cutting speeds the chip is often discontinuous, while the chip becomes serrated as the cutting speeds are increased. In this paper a new interpretation of chip segmentation in the cutting of Ti–6Al–4V is presented. It is based on an implicit, Lagrangian, non-isothermal rigid–viscoplastic finite element simulation of orthogonal machining of Ti–6Al–4V in which a dynamic flow stress model based on high strain rate and high temperature, and a ductile fracture criterion based on the strain energy are applied to the crack initiation during the chip segmentation. This model is verified by comparison with experimental results. It is shown from the simulation results that as the cutting speeds are increased the stress state near the tool tip changes, leading to the crack propagation shifting from the tool tip to the free surface of the deformed chip in the shear zone. This change in crack initiation and propagation is the primary reason for the chip changing from the discontinuous to a segregated continuous morphology.