Study On Subsurface Damage Of Nano-cutting Single Crystal Tungsten Based On Molecular Dynamics

Tungsten is widely used in key components because of a series of excellent properties,for example,high strength,high hardness and large elastic modulus,which properties cause problems in ultra-precision machining such as low machining accuracy,severe damage in subsurface,and low machining efficiency.However,only the micron-scale surface morphology and microstructure of tungsten materials are clearly understood,so it is particularly important to discuss the removal mechanism and subsurface damage mechanism of tungsten materials at the micro scale.In this paper,the molecular dynamics(MD)models of common cutting and ultrasonic assisted cutting of single crystal tungsten are established,and the simulation results are verified by ultra precision cutting experiments.Through the MD simulation of nano-cutting of single crystal tungsten,it is found that there is a certain depth of amorphous layer is formed on the machined surface during the cutting process.Dislocation slip caused the change of crystal structure in the deeper layer on the surface of the workpiece.After the cutting has completed,the elastic recovery occurred in the high pressure phase transformation zone,the dislocation and other defects developed further and the damage layer decreased.The dislocation nucleation and movement inside the workpiece lead to the defects such as atomic clusters,stair-rod dislocation and “V-shaped” dislocation.By cutting along <110>,the best surface quality,the minimum subsurface defect layer depth and the greatest elastic recovery of defect layer can be obtained.During the different cutting process,there are two kinds of dislocation lines in the subsurface layer: 1/2<111> and <100>.At the cutting direction of [100],1/2<111> dislocation lines continuously merge into stable <100>dislocation lines,resulting in a sharp fluctuation of dislocation density.When the cutting directions are [110] and [111],1/2<111> dislocation lines play an important role in plastic deformation,and the proportion of dislocation merging reaction is very small.The simulation of ultrasonic elliptical vibration cutting(UEVC)shows the characteristics of intermittent cutting lead to higher cutting force when the tool cuts materials,and the stress value of sub-surface layer atom is larger than that of common cutting.Meanwhile,the UEVC can obtain better surface quality,but there is little difference in the depth of subsurface damage layer between them.This is because higher cutting force leads to greater deformation of workpiece and higher dislocation density in the subsurface layer,but when the tool and workpiece are separated,the material will no longer be affected by the continuous force of the tool,The energy from subsurface defects will be released to the surface of the workpiece to reduce dislocation propagation to the interior of the crystal.The experiment shows that the surface accuracy of UEVC is much better than that of common cutting,the surface roughness of common cutting is 398 nm,and that of UEVC is 40 nm.Through the observation of the subsurface of the workpiece,it is found that there are obvious dislocation defects on the subsurface of the workpiece in the two processing methods,but the UEVC of tungsten phase has higher dislocation density,which is in good agreement with the MD simulation.