Influence of crack mechanics on the chip segmentation in the machining of Ti-6Al-4V

Chip segmentation plays a predominant role in determining machinability and tool wear during 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 cutting Ti-6Al-4V is presented, which is based on an implicit, Lagrangian, non-isothermal rigid-viscoplastic finite element simulation in which a dynamic flow stress model is used on the basis of high strain rate and high temperature during cutting this alloy. A ductile fracture criterion based on the strain energy is applied to the crack initiation process during the chip segmentation. The model is verified by comparing with experimental results. It is shown that the change of the stress state near the tool tip caused by the variation of cutting speed which influences the crack initiation and propagation is the primary reason for the chip changing from the discontinuous to a segregated continuous morphology.