Influence of cutting parameters on the depth of subsurface deformed layer in nano-cutting process of single crystal copper
© Wang et al. 2015
Received: 27 August 2015
Accepted: 21 September 2015
Published: 9 October 2015
Large-scale molecular dynamics simulation is performed to study the nano-cutting process of single crystal copper realized by single-point diamond cutting tool in this paper. The centro-symmetry parameter is adopted to characterize the subsurface deformed layers and the distribution and evolution of the subsurface defect structures. Three-dimensional visualization and measurement technology are used to measure the depth of the subsurface deformed layers. The influence of cutting speed, cutting depth, cutting direction, and crystallographic orientation on the depth of subsurface deformed layers is systematically investigated. The results show that a lot of defect structures are formed in the subsurface of workpiece during nano-cutting process, for instance, stair-rod dislocations, stacking fault tetrahedron, atomic clusters, vacancy defects, point defects. In the process of nano-cutting, the depth of subsurface deformed layers increases with the cutting distance at the beginning, then decreases at stable cutting process, and basically remains unchanged when the cutting distance reaches up to 24 nm. The depth of subsurface deformed layers decreases with the increase in cutting speed between 50 and 300 m/s. The depth of subsurface deformed layer increases with cutting depth, proportionally, and basically remains unchanged when the cutting depth reaches over 6 nm.
In nano-cutting, the subsurface deformed (SSD) layers have great influence on machining dimensional accuracy, surface shape accuracy, and surface roughness, which even affect the mechanical performance and lifetime of machined components. The SSD layers are basically caused by the following three factors: foreign body embedment caused by chemical or physical adsorption , stress injury caused by residual stress , and the variation of material local crystal structure . The depth of SSD layers is a vital parameter on predicating the quality of SSD layers, the value of which has an important effect on the subsequent technical machining, mechanical property, and lifetime of machined component. The subsurface damage layer in nano-cutting is so difficult to observe that it cannot be verified by using experimental method. However, the atomistic computer simulation provides an effective and promising method to examine the subsurface defect and measure the depth of SSD layers.
Based on the molecular dynamics simulation, a large number of scholars do a lot of research on nano-cutting process. Shimada and Ikawa et al. [4–6] performed molecular dynamics (MD) simulation of micro-cutting of free machining materials under perfect motion of a machine tool. Based on the radial distribution function, they found that the ultimate depth of the deformed layer of a specimen is 5.0 nm. Luo  demonstrated the shape transferability by using nanoscale multi-tip diamond tools in the diamond turning for scale-up manufacturing of nanostructures. Based on the change of atomic potential energy, Zhang  realized the quantitative characterization of subsurface damage layer’s depth in nano-cutting process of a single crystal copper. Zhu  studied the deformation-induced formation mechanism of stacking fault tetrahedron occurring in the deformation of single crystal gold nanowires. Uezakia  designed a cutting tool to generate a localized compressive stress to suppress unnecessary plastic flow and to improve the surface integrity of workpiece, which is verified by the MD simulation. Guenole  investigated the plastic deformation of Si nanowires controlled by native interface defects and analyzed the inner stress influence on the yield strain. Ma  studied the plastic deformation of nanowires and analyzed the surface-induced structural transformation in the deformation process. They found two mechanisms involved in the deformation, twinning and detwinning, and stress-induced martensitic phase transformation. Zhao  preformed the nanoindentation process via the single-point diamond turning surface of single crystal copper. Fang  studied the nanometric cutting of germanium by MD simulation and discussed the phase transformation process during nano-cutting process. Wang  exploited the numerical experiments to study the evaporation and explosive boiling of ultra-thin liquid argon film on aluminum nanostructure substrate.
In this paper, a series of simulations on nano-cutting process of single crystal copper are implemented by using MD method. Theoretical analysis and investigation on the properties and depth of SSD layers in nano-cutting process will provide much information on the mechanisms of the plastic deformation in the workpiece material. Firstly, the centro-symmetry parameter (CSP) is adopted to characterize the distribution and evolution of the subsurface defect structure. Secondly, the measure method of the SSD layers’ depth is introduced and the depth of the SSD layer variation with the cutting distance is investigated. At last, the effect of the cutting parameter on the depth of the SSD layers is studied by information statistics. The research will give a distinct understanding for the formation of the subsurface deformed layers and the effect of cutting parameter on the evolution of the SSD layers, and underpin the scientific development of nano-cutting.
At the beginning of the simulation, a conjugate gradient method is used to carry out energy minimization for eliminating the initial unreasonable factors during modeling process. Then, the molecular dynamics relaxation is calculated for 100 ps by using the Nosé-Hoover thermostat method, which makes the system temperature up to 293 K. Thereafter, the cutting tool moves along Z  direction at a certain speed of 50 m/s to start the nano-cutting process.
MD simulation conditions in 3D nano-machining
Tersoff, Morse, EAM
Single crystal copper
40 nm × 30 nm × 22 nm
Tool rake angle α
Tool clearance angle β
Tool edge radius R
Interatomic potential functions
Parameters value in Morse potential
r 0 (Ả)
where, f c (r ij ) is truncation function between atoms, f A (r ij ) is the dual potential of absorption term, f R (r ij ) is the dual potential of repulsion term, r ij is atomic distance between atom i and atom j.
The range of CSP values for typical crystal structure.
Range of CSP value
CSP ≤ 3
3 < CSP ≤ 7
7 < CSP ≤ 9
9 < CSP ≤ 20
Surface defect atoms
CSP > 20
Results and discussion
Subsurface defect nucleation and evolution
atomic details information list
Effect of the cutting distance on the depth of SSD layers
Effect of the cutting depth on the depth of SSD layers
Effect of the cutting speed on the depth of the SSD layers
In the process of nano-cutting, the dislocation nucleation, motion, and annihilation result in a large number of defect structures existing in the subsurface of the workpiece. For instance, stair-rod dislocations, stacking fault tetrahedron, atomic clusters, vacancy defects, and point defects are formed in the workpiece. Finally, the subsurface deformed layers are formed.
When the cutting depth remains constant, the depth of SSD layers increases when the cutting distance increases to be less than 8 nm, and then decreases when the cutting distance is between 8 and 24 nm. When cutting distance is greater than 24 nm, the depth of SSD layers reaches a stable value.
The depth of SSD layers increases proportionally with the cutting depth when the cutting depth is less than 6 nm. When the cutting depth is greater than 6 nm, the depth of SSD layers remain unchanged at about 7 nm.
The depth of SSD layers significantly decreases with the increase in cutting speed. The quality of the workpiece subsurface becomes better with the cutting speed getting larger within certain realms.
The authors appreciate the supports of the National Natural Science Foundation of China (grant no. 51475108). The authors would like to thank the valuable inputs from anonymous reviewers for improving the quality of this manuscript.
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