Fluid-Like Properties Research Of Deformation In High-Speed Metal Cutting Process

The properties of chip flow under high cutting speed conditions are different from those of chip flow under low or conventional cutting speed conditions. Therefore, the use of conventional analytical methods to understand machining under high-speed conditions is not appropriate. Chip deformation in high-speed metal cutting needs to be studied from the fluid aspect for its exceedingly high strain rate and viscosity.In this thesis, theory analytical, computer simulation and experimental studies are conducted with three different metals to investigate the fluid characteristic of HSM. The main contents of the thesis include:shear localization in HSM; the analysis of comparability between fluid flow and chip flow in HSM process; the fluid model of high speed metal cutting process.Firstly, the properties of chip flow under high strain rate conditions are investigated, for most metals that are plastically deformed at extremely high strain rates, elastic deformation is negligible and the shear stress substantially increases as a linear function of shear strain rate, the behavior of the work material is more similar to viscous fluid than to normal solid metals because of linear dependence between shear stress and shear strain rate above a certain strain rate. In metal cutting, the deformation mechanism changes from thermal activation at low strain rates to a form of dislocations’ drag with increasing cutting speed. The first consideration in high-speed metal cutting is the effect of viscosity. Therefore, chip deformation in high-speed metal cutting needs to be studied from the fluid aspect.Secondly, a theoretical model is established to predict the appearance of the serrated chip in cutting process based on Johnson-Cook law, the critical speed is calculated to assess the sensitivity of material input parameters, Three material’s sensitivity, with different space lattice, has been calculated and compared. The results show that, titanium alloy-TC4is more sensitivity than other2materials. All J-C parameters have influence on adiabatic shear; Parameters "A"and "n"play significant influence on the serrated chip phenomena in all material. Thirdly, the temperature, the stress and the strain distributions of material7050and TC4are investigated by computer simulation from solid aspect, it is found that the temperature in continuous chip produced mostly by the frication between chip and rake face, but for saw-chip, the deformation in first deformation zone is the main cause for temperature. The point with the highest temperature is near the rake face. There are small changes in the direction of the shear plane, but in vertical direction of shear plane the temperature changes rapidly.Lastly, the model for high speed machining is established based on fluid mechanics. The velocity distribution, the pressure distribution and the strain rate distribution are calculated by solving the Navier-Stokes equation and energy equation, which provides a new method to study high speed machining. Analytical results show that a speed stagnation point is located at some distance from the tool tip on the tool rake face, on which the maximum value of the pressure occurs, with zero speed. Its location influences the life of the tool and the quality of the finished surface. The pressure decreases along the rake face and reaches zero at some point away from the tool tip, which is the point of separation of the chip from the tool. The value of the strain rate exhibits a rapid increase from the tool tip to the free surface corner, and then decreases outwards. The chip morphology of most ductile metal materials is continuous in a conventional cutting process but is serrated when the cutting speed falls within the HSM range. So the fluid stability theory was used to explain shear instability in high speed metal cutting process.