Effects of vibration frequency on vibration assisted nano scratch process of mono crystalline copper via molecular dynamics simulation Effects of vibration frequency on vibration assisted nano scratch[.]
Effects of vibration frequency on vibration-assisted nano-scratch process of monocrystalline copper via molecular dynamics simulation , , Bo Zhu, Hongwei Zhao , Dan Zhao, Peng Zhang, Yihan Yang, Lei Han, and Hailin Kui Citation: AIP Advances 6, 035015 (2016); doi: 10.1063/1.4944760 View online: http://dx.doi.org/10.1063/1.4944760 View Table of Contents: http://aip.scitation.org/toc/adv/6/3 Published by the American Institute of Physics AIP ADVANCES 6, 035015 (2016) Effects of vibration frequency on vibration-assisted nano-scratch process of mono-crystalline copper via molecular dynamics simulation Bo Zhu,1 Hongwei Zhao,1,a Dan Zhao,1 Peng Zhang,1 Yihan Yang,1 Lei Han,1 and Hailin Kui2,a School of Mechanical Science and Engineering, Jilin University, 5988 Renmin Street, Changchun, Jilin 130025, China School of Transportation, Jilin University, 5988 Renmin Street, Changchun, Jilin 130025, China (Received January 2016; accepted March 2016; published online 21 March 2016) It has always been a critical issue to understand the material removal behavior of Vibration-Assisted Machining (VAM), especially on atomic level To find out the effects of vibration frequency on material removal response, a three-dimensional molecular dynamics (MD) model has been established in this research to investigate the effects of scratched groove, crystal defects on the surface quality, comparing with the Von Mises shear strain and tangential force in simulations during nano-scratching process Comparisons are made among the results of simulations from different vibration frequency with the same scratching feed, depth, amplitude and crystal orientation Copper potential in this simulation is Embedded-Atom Method (EAM) potential Interaction between copper and carbon atoms is Morse potential Simulational results show that higher frequency can make groove smoother Simulation with high frequency creates more dislocations to improve the machinability of copper specimen The changing frequency does not have evident effects on Von Mises shear strain Higher frequency can decrease the tangential force to reduce the consumption of cutting energy and tool wear In conclusion, higher vibration frequency in VAM on mono-crystalline copper has positive effects on surface finish, machinablility and tool wear reduction C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4944760] I INTRODUCTION Vibration-assisted machining (VAM) has gained popularity in recent years It involves a number of processing areas including turning, drilling, grinding and polishing.1–3 There are many benefits to utilize VAM Various experiments have been conducted to investigate the advantages of VAM in surface finish and tool wear However, it is still unclear what the material removal mechanisms and tool wear should be in atomic scale Scratching test, which can be applied to study deformation behaviors and surface quality, is a proper method to analyze machining of materials This method can also provide fundamental information on deformation patterns generating from specimen surface to study material removal behavior Molecular dynamic (MD) simulation method can provide the performance of atom behavior during simulation process Therefore, vibration-assisted nano-scratch process via MD simulation is used to solve the problem.4,5 Nano-scratch based on vibration-assisted technique has been adopted to study the deformation mechanism in nano-scratching process Zhang et al conducted ultrasonic vibration-assisted a Corresponding author Tel.: +86 431 85094594, 13504464796 E-mail address: hwzhao@jlu.edu.cn (H Zhao), khl69@163 com 2158-3226/2016/6(3)/035015/8 6, 035015-1 © Author(s) 2016 035015-2 Zhu et al AIP Advances 6, 035015 (2016) scratch (UVAS) experiments to study the characteristic of C-plane sapphire.6 They concluded that scratch load can be reduced through UVAS process compared with traditional scratching process Xiao et al used carbide tools to cut stainless steel and Inconel.7,8 They found that tools were not likely to wear through VAM compared to conventional cutting method Ma et al.9 turned aluminum workpiece with a carbide tool using VAM to compare with conventional method They found that the burr heights in VAM were much smaller Although VAM is widely applied in various experiments, there is rare MD analysis to investigate the effect of vibration frequency in vibration-assisted scratching process, especially for soft metal like mono-crystalline copper Therefore, a new kind of MD simulation of soft metal vibration-assisted scratching is proposed In this paper, the vibration frequency of MD simulations is changeable The MD simulations investigate the roughness of grooves, changes of dislocations, Von Mises shear strain and tangential force at different vibration frequency II SIMULATION METHOD A Initial physical model There are two parts in the MD simulation modal, including a diamond spherical tool and a mono-crystalline copper specimen, as shown in Fig The diameter of the spherical tool is 6nm It can be regarded as a rigid body in the simulations because diamond is evidently much harder than copper The specimen is rectangular with the size of 27.16nm×9.235nm×16.29nm along the X, Y and Z directions, which consists of 357,266 copper atoms Its surface is scratched along X-direction Thus, in X-Y plane the surface of the specimen is built with free surface The structure of Copper atoms in the specimen are initialized as face-centered cubic (FCC) and they vibrate around the perfect lattice structure position In the specimen, Copper atoms are categorized into three part, including Newtonian atoms, thermostatic atoms and boundary atoms The function of boundary atoms is to get rid of the boundary effects and maintain the proper symmetry of the lattice Thermostat atoms are used here to make sure reasonable outward heat conduction The motion of these two kinds of atoms is governed by Newton’s second law Moreover, along Z-direction, the periodic boundary condition is set in this simulation model to reduce the simulation scale.10 The purpose of the initial MD simulation is to make the structure of the specimen stable The diamond spherical tool started to process with the velocity of 50 m/s along X direction To match with scratching velocity, the period of vibration (Tp ) defined in the simulations is 12.5ps, 25ps and 50ps, respectively The velocity and frequency presented in the simulations are much higher compared with vibration-assisted nano-scratch process However, previous evidence confirmed that higher velocity had little effect in MD simulations,11 and the simple harmonic motion could be regarded as resultant motion in X and Y directions Parameters in simulation model are shown in Table I FIG Three-Dimensional MD simulation model 035015-3 Zhu et al AIP Advances 6, 035015 (2016) TABLE I Parameters used in this simulation model Dimensions of copper specimen Numbers of copper atoms in the specimen Numbers of carbon atoms in the diamond spherical tool Diamond abrasive radius Scratching depth Specimen machining surface Abrasive orientation and scratching direction Equilibration temperature Scratching velocity Scratching distance Time step 27.16nm × 9.235nm × 16.29nm 357,266 8394 3.000 nm 1.673 nm (010) Cubic and [1¯ 0] 296 K v = 50 m/s 20.0 nm fs The scratching simulations are performed by a diamond spherical tool which has been regarded as a rigid body Fig shows the procedure of the simulation The scratching depth is 1.673nm This tool scratches on the copper substrate with moving velocity The tool also does harmonic vibration in the Y-direction The trajectory of the spherical tool is a sine curve B Selection of potential energy function The key to make simulation reasonable is to use proper potential There are two kinds of atoms in the process simulations, Cu atoms and C atoms Therefore, three kinds of potentials should be FIG (a) Kinematic characteristic of spherical tool (b) Schematic of Vibration-assisted scratch simulation 035015-4 Zhu et al AIP Advances 6, 035015 (2016) included First is the interactions between mono-crystalline copper atoms (Cu-Cu); second is the interactions between diamond carbon atoms (C-C); the last is the interaction between mono-crystalline copper atoms and diamond carbon atoms (Cu-C) The Embedded-Atom Method (EAM) potential can be used here to describe the interaction between mono-crystalline copper atoms.12 The EAM potential evolves from the density function theory It is based on the recognition that the cohesive energy of a metal is governed not only by the pair-wise potential of the nearest neighbor atoms, but also by embedding energy related to the “electron sea” in which the atoms are embedded.13 The following formula can express this potential: 1 Etot = φi j (r i j ) + Fi (ρi ) (1) ij i Where Etot represent the total potential energy of the whole system; φi j represents the pair potential between the two atoms i and j; r i j represents the distance between atom i and atom j, and Fi (ρi ) represents the embedded energy of atom i; ρi represents the host electron density at atom i induced by all the other atoms in the system, as following: ρi = ρi (r i j ) (2) j,i Where ρi (r i j ) makes the contribution to the electronic density at the site of atom i and r i j represents the distance between atom i and atom j As is demonstrated above, diamond has been regarded as rigid body in this model, the wear and deformation can be neglected Additionally, the atoms in the spherical tool are fixed relatively, and there’s no need to identify potential between diamond atoms (C-C) As for the interactions in the specimen between mono-crystalline copper atoms and diamond carbon atoms in spherical tool are identified by the Morse potential,14 which can be expressed as following: V (r i j ) = D(e−2α(r i j −r0) − 2e−α(r i j −r0)) (3) Where V (r i j ) represents a pair-potential function and D, α and r are respectively related to the cohesion energy, the elastic modulus as well as the atomic distance at equilibrium D = 0.087 eV, α = 51.4nm−1, r = 0.205nm.5,10 The combination of EAM and Morse potential has been supported by previous simulations It matches the experimental data well with the simulation results.15 C Initial MD simulation This model is equilibrated to 296 K It’s under the microcanonical ensemble (NVE) and the atoms’ initial velocities accord with a Maxwell-Boltzmann distribution The direct velocity scaling method is used here to keep the total kinetic energy constant Large-scale Atomic/Molecular Massively Parallel simulator (LAMMPS), developed by Plimpton is used for the three-dimension molecular dynamic simulations.16 AtomEye is also used here to visualize results of the simulations.17 III SIMULATION RESULTS AND DISCUSSION A Effects on surface quality at different vibration frequency Fig and Fig show the surface quality of scratching-induced Cu (010) in different vibration frequency Fig is mainly based on X-Z plane To make a clear illustration, the machining-induced specimens have been rotated 36 degrees by Z axis The scratch depth of the specimen is 1.673nm and scratch distance is 20nm Grooves and ridges are generated during scratching process The surface quality basically depends on the vibration frequency of spherical tool The grooves get rougher 035015-5 Zhu et al AIP Advances 6, 035015 (2016) FIG AtomEye snapshots of machining-induced surface Cu (010) at different period of vibration (a) T p = 12.5ps, (b) T p = 25ps, (c) T p = 50ps at the scratch depth 1.673nm and the scratch distance 20nm when frequency decreases Small pile-ups can be clearly indicated when Tp = 50ps Fig gives the front view of grooves at different frequency It can be illustrated more clearly that surface finish of the groove is better when the frequency increases As is shown in Cross-sectional view in Fig 4, the spherical tool presses on specimen more times when frequency is high, and this can decrease the small pile-ups to reduce waviness of groove As a result, the surface quality of the groove is much smoother when the frequency increases On both sides of the grooves, the piled up material removed by scratch processing are also more irregular with vibration frequency decreasing, which also have a negative effect on the surface quality of scratch processing FIG Cross-sectional view of machining-induced surface at different period of vibration at the scratch depth 1.673nm and the scratch distance 20nm (a) T p = 12.5ps, (b) T p = 25ps, (c) T p = 50ps 035015-6 Zhu et al AIP Advances 6, 035015 (2016) FIG Cross-sectional views of the dislocations created in the specimen at different period of vibration (a) T p = 12.5ps, (b) T p = 25ps, (c) T p = 50ps (d) Traditional scratching at the scratch depth 1.673nm and the scratch distance 20nm B Effects on dislocation at different vibration frequency Fig is the cross-section views of the scratching process The atomic coordination numbers of Cu atoms in the simulations is represented by different color The number designates single vacancies and vacancy clusters They are generated from dislocations interacting with each other This kind of defects is immobile during the scratching process.18 They are generated at uncertain locations and there are no evident differences between single vacancies and vacancy clusters at different frequencies The number denotes the fixed dislocation loops These defects can be generated from composition and decomposition of dislocations These dislocations are immobile and independent Only the changing of temperature and pressure can destruct them They have little effect on the surface deformations during simulation process Bulk dislocation loops and surface dislocation loops are indicated by number This kind of dislocation loop is emitted from the around-the-tool region and glide through specimen thickness to the bottom surface The region circled by green line marked represents affected layer The layer is not obvious because the radius of tool is very small Plastic deformation occurs during scratching process on Cu specimen and almost all the copper atoms being processed are piled up to become the ridge Few atoms flow to the groove to form the affected layer It can be concluded through Fig that the specimen with higher vibration frequency can generate more dislocation loops The vibration-assisted scratching process can be regarded as traditional scratching and indentation As vibration frequency grows, more dislocations are generated through the indentation process As a result, defects generated from scratching simulations get larger both on quantity and volume, especially for the surface glide and bulk glide They are important because these defects correspond to the plastic deformation The yellow dotting line in Fig can demonstrate the changes The existence of dislocations can make deformation of Cu specimen much easier Material being processed moves along the deformation, the more dislocations emit during scratching process, the easier the deformation will be C Analysis of Von Mises shear strain at different vibration frequency Fig is the cross-section views of residual Von Mises shear strain distribution after scratching in the specimen, which is abstracted from software AtomEye The strain can describe the corresponding equivalent strain at the onset of plastic yielding and beyond.19 The stress and strain are interchangeable, so the strain state can also illustrate the stress state Fig shows that the 035015-7 Zhu et al AIP Advances 6, 035015 (2016) FIG Cross-section views of residual Von Mises shear strain distribution after scratching in the specimen (a) T p = 12.5ps, (b) T p = 25ps, (c) T p = 50ps at the scratch depth 1.673nm and the scratch distance 20nm FIG Scratching force-displacement in MD simulations with different vibration frequency (a) The calculated curve; (b) the filtered curve strain at the scratched area is larger than that at the unscratched specimen in a single simulation However, there are no significant changes both on stress distributions and values among simulations at different vibration frequency Therefore, there is no direct evidence to support that the change of vibration frequency has effect on Von Mises shear strain and stress of Cu specimen in the simulations D Analysis of the tangential force at different vibration frequency The force in MD simulation is the interatomic force between the tool and the specimen Fig presents the tangential force Fx at different vibration frequency Fig 7(a) displays the calculated force curve and Fig 7(b) shows the smoothed force curve The energy consumed during simulation process can be calculated through Fig 7(b) more clearly The area under tangential force curve can be regarded as cutting work during scratching process It is obvious that the cutting work consumed at the period of vibration 12.5ps and 25ps is less than that at 50ps, and traditional scratch consumes most cutting energy The tangential force can also result in tool wear during scratching process A larger Fx is present for a lower vibration frequency It can be concluded through Fig that the tangential force of vibration-assisted scratch is smaller than that in traditional scratch To a certain extent, higher vibration frequency can also reduce the tangential force This can provide the evidence that higher vibration frequency in vibration-assisted scratching processes can cause lower tangential force to reduce tool wear IV CONCLUSION (1) During vibration-assisted scratching simulation, vibration frequency of spherical tool has significant effect on the surface quality of groove being scratched, the roughness of the groove decreases when vibration frequency of spherical tool increases 035015-8 Zhu et al AIP Advances 6, 035015 (2016) (2) With the vibration frequency of tool increasing in the MD simulations, more dislocations have been established The growth of dislocations can make deformation of Cu specimen easier (3) The changing vibration frequency has no evident effect on Von Mises shear strain of Cu specimen during MD simulations (4) The average tangential force decreases when vibration frequency of spherical tool becomes higher during scratching simulations Smaller force leads to a smaller energy consumption and reduces the tool wear which can prolong the tool life ACKNOWLEDGMENT This research is funded by Special Projects for Development of National Major Scientific Instruments and Equipments (GrantNo.2012YQ030075), the National Natural Science Funds for Excellent Young Scholar (GrantNo.51422503), the National Natural Science Foundation of China (GrantNo.51275198, 51505180), Program for New Century Excellent Talents in University of Ministry of Education of China (GrantNo.NCET-12-0238) and Fund Guiding on Strategic Adjustment of Jilin Provincial Economic Structure Project (GrantNo.2014Z045), Major project of Jilin province science and technology development plan (20150203014GX) D E Brehl and T A Dow, Precis Eng 32, 153 (2008) D E Brehl and T A Dow, Proc ASPE 40 (2007) C.X Ma, E Shamoto, and T Moriwaki, Key Eng Mater 291-292, 443 (2005) A Chatterjee, A.A Polycarpou, J.R Abelson, and P Bellon, Wear 268, 677 (2010) P Zhang, H Zhao, C Shi, L Zhang, H Huang, and L Ren, Appl Surf Sci 280, 751 (2013) C Zhang, P Feng, and J Zhang, Int J Mach Tools Manuf 64, 38 (2013) M Xiao, S Karube, T Soutome, and K Sato, Int J Mach Tools Manuf 42, 1677 (2002) M Xiao, K Sato, S Karube, and T Soutome, Int J Mach Tools Manuf 43, 1375 (2003) C Ma, E Shamoto, T Moriwaki, Y Zhang, and L Wang, Int J Mach Tools Manuf 45, 1295 (2005) 10 Y Yang, H Zhao, L Zhang, M Shao, H Liu, and H Huang, AIP Adv 3, 102106 (2013) 11 Z.-C Lin and J.-C Huang, J Mater Process Technol 201, 477 (2008) 12 S.M Foiles, M.I Baskes, and M.S Daw, Phys Rev B 33, 7983 (1986) 13 Q.X Pei, C Lu, H.P Lee, and Y.W Zhang, Nanoscale Res Lett 4, 444 (2009) 14 S Plimpton, J Comput Phys 117, (1995) 15 L Zhang and H Tanaka, Wear 211, 44 (1997) 16 S Plimpton, J Comput Phys 117, (1995) 17 J Li, Model Simul Mater Sci Eng 11, 173 (2003) 18 S Jun, Y Lee, S.Y Kim, and S Im, Nanotechnology 15, 1169 (2004) 19 J.H.L Pang, 65 (2012) ... ADVANCES 6, 035015 (2016) Effects of vibration frequency on vibration- assisted nano- scratch process of mono- crystalline copper via molecular dynamics simulation Bo Zhu,1 Hongwei Zhao,1,a Dan Zhao,1... vibration frequency in vibration- assisted scratching process, especially for soft metal like mono- crystalline copper Therefore, a new kind of MD simulation of soft metal vibration- assisted scratching... behavior during simulation process Therefore, vibration- assisted nano- scratch process via MD simulation is used to solve the problem.4,5 Nano- scratch based on vibration- assisted technique has