Research On The Mechanism Of Subsurface Defect And Mechanical Property Of Crystal Copper In Nano-machining

Products and workpieces of high precision and quality are now widely used in advanced research,national defense,and even some expensive household wares.However,the further development of nano-tech is under the constraint of the understanding about the mechanism of nano-machining.Molecular dynamics simulation,short as MD simulation,has been a helpful and important method for research on nanomachining.Furthermore,the mechanism of subsurface defects is a significant factor of the improvement of nano-machining,as one of the problems that needed to be solve.MD simulation and experiment of mono-crystal and poly-crystal to analyze the rule by which subsurface defects evolve during nano-cutting,and then discuss the influence caused by different parameters.First,MD simulation of nano-cutting on mono-crystal copper is conducted,and the evolution of different subsurface defects deconstructed by series of analyzing techniques aiming at crystal defects,including CNA,DXA,and other stress judging methods,in order to figure out their characteristics and forming rules.Then,the geometry of tools used in nano-cutting simulation is changed,including the rake angle,the clearance angle,and the edge radius,and the influence caused on the characteristics of subsurface is studied.MD simulation of nano-cutting on poly-crystal copper is followed.Through the approaches provided by cutting force and stress distribution,the rule by which the crystal structure and dislocations move is understood.It is found in the cutting that the grain boundary can cause the shielding phenomenon on stress spreading in poly-crystal copper,while blocking the dislocations from sliding through it.Besides,simulations with different cutting depth is performed to analyze its influence on subsurface characteristics of poly-crystal copper,and the linear relation between the cutting depth and the dislocation density in poly-crystal copper in then found.At the final part,MD simulation and real experiment are conducted together to perform nano-indentation on copper.On the one hand,the evolution of dislocations in copper during nano-indentation is studied,in which the “work hardening” phenomenon is found related to the subsurface defects.On the other hand,experiments of cutting and indentation compare the mechanical properties of workpieces from different cutting depth.The hardening degree of the machined workpiece rises with the cutting depth in a certain scale,which validates the simulation results qualitatively.