Research On The Evolution Mechanism Of Subsurface Defect And Crystal Structure Of Crystal Copper In Nanometric Cutting Process

Nanometric machining technology is aimed at achieving nanoscale materials removal and obtaining the machined surface with nanoscale precision, which can provide support and assurance for the development of aerospace technology, electronic information engineering, computer technology and national defense. The dimensional accuracy and surface topography accuracy that nano machining technology can achieve represents the development level of national manufacturing industry. However, the further development of nano machining technology is restricted by the machining theory, processing technic and measuring technique, especially the lack of defect evolution mechanism of subsurface deformed layers during nano machining process. The formation and evolution of subsurface defects have a significantly effect on the materials removal and surface generation. It has important significance on improving the nano machining precision, surface quality and surface integrity for studying the evolution of subsurface defects and the change of crystal structure.Nowadays, molecular dynamics(MD) simulation method is widely used to carry out the research on nano machining mechanism, which has achieved fruitful results. Nevertheless, more attention is focused on nano-cutting process of single crystal materials under vacuum conditions. Few researches are carried out on studying polycrystalline materials and nano machining with medium conditions which more fulfills the actual machining process. Focused on the nanocrystal materials and nano machining under medium conditions, molecular dynamics simulations of the nano cutting process of monocrystalline and polycrystalline copper under medium environment are respectively carried out, according to materials mechanic, thermodynamics, statistical physics and crystallography theory. Further, the nano machining mechanism and the subsurface crystal structure evolution mechanism are explored.In this thesis, the nano cutting and nano indentation models for single crystal copper, polycrystalline copper under vacuum condition and aqueous medium are set up, respectively. The spherical harmonic function method is improved by building a calculation program of spherical harmonic function and calculating the basic data values of spherical harmonic function. Then, the MD simulation results are analyzed by spherical harmonic function method, which combines crystal defect analysis technology and crystal dislocation movement theory. And the materials deformation and subsurface defect structures are effectively identified, which lay the foundation for the research on the evolution mechanism of subsurface defect and crystal structure of crystal copper.On the basis of dislocation theory, friction and wear mechanism, crystallography and nanomechanics, MD simulations are performed to study the nano-cutting process of single crystal copper. Firstly, the nano-cutting mechanism of single crystal copper is deeply discussed by analyzing the materials removal and surface generation. And the mechanism of stress-induced dislocation nucleation and evolution is revealed by investigating the dislocation nucleation and evolution in nano-cutting process. Further, the paper emphatically investigated the deformation mechanism of workpiece subsurface in cutting process, revealing the evolution mechanism of crystal defects, residual stress and crystal structures of workpiece subsurface atoms. Finally, based on studying the change of the depths and crystal structures of workpiece subsurface under various cutting velocities and cutting depths, the influence of cutting parameters on the variation mechanism of the subsurface depths and crystal structures is revealed.The nano-cutting mechanism of polycrystalline copper is discussed, according to the research on the atomic migration and material crystal structures evolution in nano-cutting process of polycrystalline copper. Firstly, the nano-cutting mechanism of polycrystalline copper is investigated by researching the workpiece materials deformation and the change of cutting force in nano-cutting process. Secondly, the spherical harmonic function method is used to study the crystal structure change of subsurface deformed layers for polycrystalline copper, and the influences of grain parameters and cutting parameters on the change of subsurface crystal structure are studied. Finally, based on the research of dislocation defects evolution of polycrystalline copper subsurface during nano-cutting process, the "grain boundary-dislocation" transformation mechanism is revealed, which enriches the connotation of the deformation and nano-cutting mechanism of polycrystalline materials.Based on the established MD models of nano cutting and nano indentation under aqueous medium, the influence of aqueous medium on nano machining process of crystal copper is studied. The effect of aqueous medium on the materials deformation, dislocation nucleation and defect evolution in nano indentation process is detailly analyzed. And on this basis, the influence of water medium on nano processing mechanism, cutting forcechange,workpiece temperature distribution and subsurface defectsdistributionis analyzed,revealing the function of water medium in nano cutting process of crystal copper, while the influence of water medium on the crystal structure evolution of subsurface in nano-cutting process is investigated. Finally, the effect of water medium on nano-cutting process of polycrystalline copper is discussed and the evolution mechanism of defects and crystal structures of polycrystalline copper subsurface layers is deeply investigated.The evolution mechanism of the subsurface defect and crystal structure of nanocrystal materials is revealed in this research. This work has important theoretical guiding significance on optimizing the nano processing technology and improving the machining accuracy, surface and subsurface quality.