Study On Mechanism And Influencing Factors Of Surface And Chip Generation In Nanomeric Machining Of Single Crystal Nickel

With the rapid development of material science and the increasing demand of special performance parts in modern science and technology,single crystal parts are more and more used in various industries of national production.In the process of obtaining the final single crystal parts by means of mechanical machining,it is found that the original machining mechanism of traditional material is no longer suitable for the machining of single crystal material.In order to study the machining mechanism of the single crystal material in essence,it is necessary to start with the atoms of the single crystal material.Nanometric machining technology,a good integration of the cutting-edge scientific and technological achievements in mechanical,computer,material as well as measurement,can achieve the removal of the nanometric surface material.It is an effective method to study the machining mechanism of single crystal material in essence.However,it is necessary to carry out a large number of nanometric machining experiments for the systematic study of material removal,surface formation and material microstructure state in the nanometric machining.But due to the limitation of the precision of current machining and testing equipments as well as experimental cost,it is very difficult to carry out omnidirectional dynamic observation and detection of the nanometric mechanical machining process in the experimental study of the micro-nanometric scale machining mechanism of single crystal material.At present,the molecular dynamics simulation method is widely used domestic and overseas in the study of nanometric cutting mechanism,and it has achieved fruitful results.Now more studies are focusing on the single crystal copper nanometric machining process,but less studies are focusing on the nickel-based single crystal that occupies a large proportion in aerospace single crystal parts.In this thesis,the nanometric machining simulation under different machining conditions of single crystal nickel was conducted.The nanometric machining process of single crystal nickel was simulated using molecular dynamics simulation method and combining with the basic theory of material mechanics,thermodynamics,statistical physics and crystallography.This thesis conducted further study mainly from the material deformation mechanism in machining process,the workpiece internal defect evolution,the temperature characteristics of the machining system,the comparison of the machining results under different machining conditions,the conditions of the chip formation and the performance of the machined surface.The main research work is reflected in the following aspects.(1)Molecular dynamics method was used to establish a simulation model of single crystal nickel nanometric machining and indentation.The crystal identification technology,as well as related analysis means and methods that reflected the microscopic changes in material were reasonablely applied,providing a technical support for the further analysis of single crystal nickel nanometric machining mechanism using molecular dynamics simulation.(2)The displacement of the internal defect atoms that guided the formation of the chip and machined surface was analyzed in the machining process.A comprehensive analysis was conducted combining with the size of the cutting force,the deformation of the workpiece and the internal defects.The relationship among the changes of the cutting force,the number of atoms with elastic displacement inside the material and the internal defect structure of the workpiece was studied.In addition,two main influencing factors of the material crystal transition were explained.Finally,the temperature characteristics of the system in the machining process were studied.The error influence of the time averaging method on the system temperature description was presented,and the causes of the generating error were analyzed.(3)The nanometric machining simulation model of different front angle and fillet tool was established.The nanometric mechanical machining molecular dynamics simulation of single crystal nickel was conducted under different machining conditions(tool front angle,fillet,machining speed and machining depth),and the machining results obtained under various machining conditions were analyzed in detail.Comparative studies of the cutting force under different tool front angle,fillet,machining depth,machining speed,and the friction coefficient,and the chip height that reflecting the chip shape,and the number of the chip atoms that reflecting the machining removal rate,and the intact atoms ratio in the chip that reflecting the tool wear degree,and the sub-surface damage extent of the workpiece,and the machined surface quality,and the temperature distribution condition of the internal workpiece were conducted.(4)The position of the tool diversion point and the relationship between it and the tool fillet in nanometric machining was studied.Through the theoretical calculation,the minimum machining thickness of different fillet was pre-estimated.The basis of judging chip production in nanometric machining was proposed,and the material internal deformation with different chip shapes and the microscopic characteristics of the chip were also studied.Starting from two main aspects of cutting force and machined surface,the change rule of cutting force and machined surface caused by the increase of the machining depth was investigated.(5)The nanometric indentation simulation of the complete single crystal nickel and the machined single crystal nickel with a defective surface was conducted.The atomic displacement deformation and internal defect evolution of the complete and machined single crystal nickel were analyzed and compared in the process of nanometric indentation.The main reasons that led to the two different deformations and the defect evolution method were found.The indentation-unloading curves of the complete and machined single crystal nickel were compared,and the hardness and elastic modulus values of different surface:conditions were obtained,and the softening effect of the machined surface due to internal defects was verified.In this thesis,the nanometric machining process of single crystal nickel was deeply explored and studied through the molecular dynamics simulation.It is helpful to understand the machining mechanism of the nickel-based single crystal parts,and provides an important theoretical guidance for optimizing the single crystal parts machining process and improving the machining precision and obtaining the high quality of the machined surface.