Element Simulation And Experimental Study Of The Sawtooth Chips In High Speed Cutting

The high-speed cutting is one of the most effective ways to machine difficult-to-cut materials and the experimental study of serrated chips is holding important status in the material high-speed cutting domain. In this dissertation, the titanium alloy and ni-base superalloy are employed as research objects. Their serrated-chip’s forming process; evolution mechanism and influence of cutting parameter on geometric characteristics are studied by cutting test and finite element simulation. The serrated chip is a typical feature of chips in high-speed cutting. However, the formation of serrated chip tends to cause high frequency fluctuation of cutting force and deterioration of tool wear which shortens tool life and worsens the integrity of machined surface. Therefore, the comprehensive understanding of serrated chip is necessary not only for optimization of cutting technology, but also for improvement of surface integrity and tool life.First of all, the cutting experiments of titanium Ti6A4V and ni-base superalloy were carried out. The macro and micro morphology of chips and cutting forces in experiments were analyzed. The results show that the screw diameter increases with the increments of cutting speed and feed rate. What’s more, the cutting force increases with the rise of feed rate and back cutting depth, but decreases with the rise of cutting speed. The empirical formula of cutting force was established and the accuracy of the empirical formula was verified. The results show that the serrated degree and serrated pace increase with the growth of cutting parameters. On the other hand, the serrated frequency appears to enhance with the enlargement of cutting speed and feed rate, but decreases with the increase of feed rate. As the cutting speed increases, the adiabatic shear band’s morphology experiences three types in turn, which are the deformation band, the transformation band and the transformation with fracture band.Second, a finite element model for orthogonal cutting about ni-base superalloy was constructed using the software ABAQUS. Instead of separation line, shear damage area was applied to the geometric model. Johnson-Cook constitutive model was adopted to reflect the nature of material properties. The accuracy of the finite element model was verified through the contrast analysis of cutting force and chip shape.Finally, the serrated-chip’s forming process and evolution mechanism is studied through the contrast analysis of simulation and experiment. The results show that the evolution process of serrated chip is composed of three phases, which are the formation processes of the rudiment of sawtooth, and then the rudiment of sawtooth to the sawtooth segment, and finally the sawtooth segment to the serrated chip. The shear-slipping deformation begins to appear in lower cutting layer near the tool-tip region due to temperature weakening, and it extends in the direction from tool tip to top surface of cutting layer, which shows as "spikes". When the thermoplastic instability in tool-tip region happens, the shear-slipping extends to the top surface of cutting layer immediately, and the shear-slipping narrow-band is formed completely, and the rudiment of sawtooth is finished. Because the metal in narrow-band is in the state of thermoplastic instability, local shear-slipping deformation occurs along narrow-band, and the sawtooth segment is shaped. After the sawtooth segment comes into the second deformation zone, the metal in narrow-band has still been being in the state of thermoplastic instability. Because of pushing of the rake face, the local shear-slipping deformation has been continuing in the field of narrow-band until it left the second deformation zone, and the serrated chip is successful finally.