Study On Surface Quality Of Nickel-base Single Crystal Alloy ACS Cutting By Ceramic Tool

Nickel-based single crystal alloys are widely used in aerospace and other national defense fields due to their excellent properties such as heat resistance,corrosion resistance and creep resistance.At the same time,the work hardening effect of nickel-based single crystal alloy is strong,the surface phase transformation is serious,the cutting resistance is large,and high machining accuracy and surface quality are required,so it is a typical difficult to machine material.Ultra precision machining technology is an important means to ensure the surface quality of parts.Among them,atomic level and closed-to atomic level(ACS)cutting will become the mainstream direction of the third generation manufacturing technology,which will have an important impact on the future scientific and technological revolution and the upgrading of national equipment manufacturing industry.At present,there is a lack of research on the evolution of surface defects,phase transformation and work hardening in ACS cutting process of nickel-base single crystal alloy,and there is a lack of understanding of the material deformation mechanism in ACS cutting process.In this paper,theoretical derivation combined with molecular dynamics method is used.The mixed potential function is used to simulate.The Morse potential function is calculated and the EAM and Tersoff potential functions are selected to establish the molecular dynamics model of the ACS cutting nickel-base single crystal alloy with silicon nitride ceramic tool.The surface quality related problems are studied from many aspects.The nucleation,expansion and evolution of subsurface defects were analyzed.The variety,quantity and area of subsurface defects caused by the change of cutting speed,crystal orientation and cutting depth were discussed.The type,area,quantity and deformation area of defects on the machined surface of nickel-base single crystal superalloy were analyzed and the dislocation evolution in the workpiece was analyzed through cutting simulation and visualization processing.The results show that increasing the cutting speed can effectively reduce the number and area of subsurface defects and improve the quality of machined parts.With the increase of the cutting depth,the number of stacking faults increases.The anisotropy of single crystal materials has an important influence on the machined surface quality.When the crystal orientation(010)[00-1] is used for cutting,more defects will be generated in the subsurface area.The evolution mechanism of surface defects and the law of material removal were analyzed.The mechanism of phase transformation in cutting process was introduced.Radial distribution function(RDF),coordination number(CN)and common neighbor analysis(CNA)were used to study the effect of cutting speed on phase transformation.The experimental results and simulation results were compared.The results show that hydrostatic pressure and cutting temperature have important influence on phase transformation.In order to verify the universality of the research results,the orthogonal cutting model was modified to obtain the same strain rate as the cutting speed,and the nano compression process with equal strain rate was simulated.The change of crystal structure during nanocompression was studied.It is found that the strain rate has no effect on Young's modulus.The larger the strain rate,the greater the change of crystal structure.The work hardening effect on the surface of workpiece in ACS cutting process was studied.By analyzing the simulation of nano compression at the same strain rate,it was found that there is stage work hardening effect.It was found that the dislocation density of Ni-based single crystal alloy workpiece changes greatly with the change of cutting distance during ACS cutting.According to the change of dislocation density,a new stage hardening mechanism was proposed.The development of work hardening in the cutting process was divided into three stages,the different hardening mechanisms in each stage were studied,and the transition nodes in each stage were defined.At the same time,the hardening mechanism in different stages and the reasons for the change of work hardening mechanism in different stages were summarized.