Study On Diffusion Mechanism At Tool-Workpiece Interface In Cutting Nickel-based Alloy Using CBN Tool

Nickel-based alloy GH4169 has been widely used in aerospace because of its good strength and oxidation resistance at high temperature.However,nickel-based alloy is a typical difficult-to-cut material,and it needs high machining accuracy.Researchers have always been committed to the research of surface machining quality and tool wear mechanism in the process of nickel-based alloy machining.GH4169 has various internal components and complex structure,so it is difficult to study its processing process more microcosmic and in-depth by macro experiment or simulation.Molecular dynamics?MD?method can more easily observe and study the microstructure evolution of GH4169 from the atomic scale,and then reveal various mechanisms,which is a good way to study the ultra precision machining of nickel-based alloy.In this paper,a simplified nano cutting model for cutting nickel-based alloy with cubic boron nitride?CBN?tool is established by using molecular dynamics method.The work-piece model is divided into Ni-Fe-Cr alloy model and multiphase model containing Ni-Fe-Cr alloy??phase material?and Ni₃Al??'phase material?.All kinds of atomic mass ratios refer to nickel-based alloy GH4169.In the model,three potential functions of EAM,Morse and Tersoff are used to endow the interaction between different kinds of atoms,and LAMMPS software was used to simulate the cutting process.Furthermore,the work hardening mechanism in workpiece surface and the atomic diffusion phenomenon on the tool surface at the tool-workpiece interface in cutting nickel-based alloy are studied.By tracing the microstructure changes of single crystal Ni-Fe-Cr model during cutting,the micro mechanism that causes work hardening,such as Lomer-Cottrell dislocation,dislocation pile-up and dislocation tangle,can be clearly observed.The nano cutting model of Ni-Fe-Cr alloy and single crystal Ni under the same conditions was constructed,and then the stacking fault energy of the two materials was calculated,and the internal stacking fault changes were observed and compared.The results show that the decrease of stacking fault energy leads to the increase of the probability of stacking fault in the process of cutting,which indirectly leads to the formation of more surface angle dislocations in the process of plastic deformation.It is found that the atoms of Ni,Fe and Cr in the workpiece diffuse into the tool through the milling experiment of Inconel 718 with CBN tool.The influence of atom diffusion on the crystal properties of CBN tool was studied by molecular dynamics method.The results show that the diffusion of workpiece atoms into CBN single crystal tool reduced the compressive strength of CBN crystal,and the compressive strength of CBN crystal decreased with the increase of temperature.The impact toughness of CBN tool before and after diffusion is obtained by establishing molecular dynamics impact model.Workpiece atoms diffuse into CBN tool crystal,which makes the impact toughness of tool crystal decrease obviously.Through the establishment of multi-phase cutting model,it is found that during the cutting process,the workpiece atoms diffuse into the CBN tool,and more workpiece atoms enter into the tool boundary,which indicates that the atoms are easier to diffuse into the CBN grain boundary and diffuse in the grain boundary.In the work piece,the diffusion activation energy of the same atom in the crystal lattice of the?phase or the?'phase is greater than that of the same atom in the crystal boundary of the?phase or the phase boundary of the two phases.The workpiece atoms are more easily diffused to the tool chip interface through the grain boundary or phase boundary,and then diffused into the tool.And the diffusion difficulty of different kinds of atoms in the same region is different.By calculating the formation energy of atom doping at CBN lattice,the formation energy of tool atom vacancy and the formation energy of self gap,it is found that compared with the normal lattice position of CBN lattice,the workpiece atom is easier to diffuse into the lattice gap position.