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Hard particle effect on surface generation in nano-cutting
Accepted Manuscript Title: Hard particle effect on surface generation in nano-cutting Authors: Feifei Xu, Fengzhou Fang, Xiaodong Zhang PII: DOI: Reference:
S0169-4332(17)32079-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.089 APSUSC 36623
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-5-2017 9-7-2017 12-7-2017
Please cite this article as: Feifei Xu, Fengzhou Fang, Xiaodong Zhang, Hard particle effect on surface generation in nano-cutting, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hard particle effect on surface generation in nano-cutting
Feifei Xu1, 2, Fengzhou Fang1, 3*, Xiaodong Zhang1
1 State Key Laboratory of Precision Measuring Technology & Instruments, Centre of MicroNano Manufacturing Technology, Tianjin University, Tianjin 300072, China 2 Institute of Mechanical Manufacturing Technology, China Academy of Engineering Physics, Sichuan 621900, China 3 School of Mechanical & Materials Engineering, MNMT-Dublin, University College Dublin, Dublin, Ireland Author: Feifei Xu, email: [email protected]
*Corresponding author: Fengzhou Fang, email: [email protected] Author: Xiaodong Zhang, email: [email protected]
Graphical Abstract Cutting direction
Cutting direction Chip Hard particle
Diamond tool
Diamond tool
Roll up
TB Hard particle
TB
TB
Shockley dislocations Stair-rod dislocations
Hard particle
ISF Diamond tool
Diamond tool
Collapsed edge
TB
Cutting direction
Cutting direction
TB ESF
Shockley dislocations Stair-rod dislocations
TB
Separation region
Hard particle
Roll down
Chip
Highlights
Nano-cutting of material with hard particles are investigated.
Hard particles effects on surface generation are studied.
Elastic recovery of hard particle happens after cutting through.
Twin boundaries and dislocations initiate around hard particles.
Hard particles induced increase of processing force is observed.
Abstract The influence of the hard particle on the surface generation, plastic deformation and processing forces in nano-cutting of aluminum is investigated by means of molecular dynamics simulations. In this investigation, a hard particle which is simplified as a diamond ball is embedded under the free surface of workpiece with different depths. The influence of the position of the hard ball on the surface generation and other material removal mechanism, such as the movement of the ball under the action of cutting tool edge, is revealed. The results show that when the hard particle is removed, only a small shallow pit is left on the machined surface. Otherwise, it is pressed down to the subsurface of the workpiece left larger and deeper pit on the generated surface. Besides that, the hard particle in the workpiece would increase the processing force when the cutting tool edge or the plastic carriers interact with the hard particle. It is helpful to optimize the cutting parameters and material properties for obtaining better surface quality in nano-cutting of composites or other materials with micro/nanoscale hard particles in it.
Keywords: Nano-cutting, Hard particle, Surface generation, Plastic deformation, Cutting mechanism
1.
Introduction
During the last several decades, nano-cutting which is one of the most efficient machining technologies has been applied by industry to generate components with nanometric surface roughness and sub-micrometric form accuracy or feature size [1-3]. In nano-cutting process, the material is removed in micrometric or nanometric scale which is usually smaller than the material grain size [4]. In this scale, even a very small defect in the workpiece material, such as the grain boundary [5, 6], pore [7], second-phase particle [7] or hard particle [8, 9], could be an important factor in influencing the generated surface quality. Generally, the workpiece material could be chosen and optimized at the design stage according to their machinability and the specific requirements of the machined components, such as the extreme surface roughness. The nickel-phosphorous (NiP) electroless plating which is known as a high purity amorphous material [10] without grain boundary, hard particle, or other defects, is chosen as an important material on the mould surface for manufacturing optical
components with nanometric surface roughness [11]. However, the machinability is not the only factor to be considered in determining the material to be used. The strength, stiffness, resistance, thermal stability, weight and so on are the other factors should be taken into consideration. Therefore, materials with good mechanical or optical properties accompanied with difficult-to-machine properties arise in the nano-cutting process. Hard particle in workpiece which is one of the difficult-to-machine properties plays an important role in surface generation and attracts many researchers’ attention. Metal matrix composites (MMCs) whose mechanical properties are enhanced by the particles, is a typical kind of material [7, 12]. In 2008, the influence of SiCp particles on the surface generation of SiCp /Al composites in ultra-precision cutting process is experimentally investigated by Ge et al. [12]. The results show that better surface quality could be obtained when the SiCp particles are removed by pressed into or cut through mechanism. Otherwise, the surface quality would be deteriorated when the SiCp particles are pulled out or crushed in cutting process. The hard particles also exist in the aluminum alloy Al-6061, which is a widely used material to produce optical quality surface in nano-cutting process. In 2009, Ding et al [8] found that the hard particles in Al-6061 causes the formation of voids on the machined surface. Near the voids, the hard particles could be found under the machined surface when the top layer of the machined surface is removed. Further researches reveal that the hard particle could be sheared and fractured when the cutting tool encounters the hard particle. The voids are created due to the vacation of the broken parts of the hard particle and enlarged by the loose particles [9]. The formation of the voids could be suppressed by making the hard particle be machined in ductile mode, such as the reduction of UCT [9] or application of other assisted cutting technology. Ultrasonic vibration assisted cutting is found to be an effective method to achieve a ductile-mode cutting of hard particles and generate an improved surface quality [13]. In 2015, Wang et al. [14] found that the precipitates (Mg 2 Si) generating at the isothermal heat treatment of Al6061, would cause the scratch marks on the machined surface and deteriorate the machined surface quality. In 2016, Zong et al. [15] experimentally investigated the influence of hard particles on the surface generation from the aspect of feed rate. The results show that large feed rate makes the hard particles be pulled out and scratch on the machined surface leaving micro-grooves on the tool mark. Small feed rate (less than the size of hard particle) makes the hard particles be cut off partially and then be pressed into the matrix. The pressed hard particles will protrude from the machined surface due to the elastic recovery of the matrix. However, the experimental investigation on the influence of the hard particles in nano-cutting is high-cost and time consuming. Numerical methods, such as the finite element method (FEM) [16] and the molecular Dynamics (MD) simulation [17] could give a result more straightforward. In 2015, Zhang et al. [18] investigated the influence of SiCp particles on the surface generation of SiCp /Al composites by FEM and found that the relative location between the cutting tool and the particle determines the initiation and propagation of the cracks in SiCp and the final machined surface quality.
Molecular Dynamics (MD) simulation is a useful approach in investigating the plastic deformation mechanism in nanoscale. It was first applied by Lawrence Livermore National Laboratory (LLNL) and Precision Engineering Department at Osaka University to investigate the nano-cutting process [19]. After that, it has been used to investigate material removal mechanism and surface generation of micro/nano-cutting processes [17, 20-24]. Recently, Xu et al. [22] investigated the plastic deformation mechanism and surface generation of Aluminum by MD simulations and found the preferred cutting directions whose minimum uncut chip thickness is relatively small. The side flow which is an important factor in deteriorating the generated surface quality [25, 26] has also been investigated by the application of MD simulation and found that it could be suppressed by optimizing the cutting tool geometry or choosing a preferred crystallographic orientation [17]. In 2016, MD simulation is employed by Li et al. [7] to investigate the influence of pore and second-phase particles on the subsurface damage and surface integrity in cutting process. They found that the workpiece with soft particle is easily crushed by the tool, while workpiece containing the hard particle still has a stable structure. Therefore, the experimental and simulation methods have been widely introduced to investigate the effect of hard particle on cutting process. However, the experimental methods could not give a visualized material removal process and the mechanism of the effect of hard particle on the surface generation is derived from the machined results indirectly. The simulation methods could give a visualized process about the material removal and surface generation, but the previous researches are not enough to fully reveal the plastic deformation, surface generation, and processing force in nano-cutting of materials with hard particles. In this study, MD simulations are employed to investigate the influence of the hard particle on nano-cutting processes. The depth of the particle under the surface of the aluminum which could simulate the hard particle in aluminum alloys Al-6061, is taken into consideration. This study contributes to a better understanding of the surface generation for materials with hard particles in nano-cutting.
2.
Method
MD simulation is employed to investigate the effect of hard particles on the surface generation in nano-cutting of aluminum. As shown in Fig. 1, the molecular dynamics simulation model consists of a rigid diamond tool, an aluminum workpiece and a hard particle. Generally, many hard particles are dispersed over the workpiece materials and the shape of them are irregular. But for simplify the simulations, a single hard diamond particle is introduced to the workpiece and its shape is simplified as a ball. In the cutting process, the diamond ball will stay stable without the initiation of cracks. Therefore, the crack of the hard particle will not be discussed in this study and it will be investigated in the future. From the analysis of the influence of the hard ball on the surface generation, the influence of hard particle with other kind of shape could be derived. The diameter of the ball is 8 nm which is comparable to tool edge radius 𝑟𝛽 (5 nm). In this condition, the total
size of the simulation model could be limited to a reasonable value and the computation time is acceptable. The center of the hard ball is defined as 𝑂. Its depth 𝐷 (from the center of the ball 𝑂 to the free surface of the workpiece) and distance 𝐿 (from the center of the ball 𝑂 to the left edge of the workpiece) for model 1 and model 2 is displayed in Table 1. At different value of 𝐷, the influence of the position of the hard particle on the surface generation could be revealed. And at the value of the distance 𝐿, the cutting process will attain a stable stage when the cutting tool touch the hard ball.
The cutting tool applied in this study has an edge radius 𝑟𝛽 of 5 nm. The rake angle and clearance angle is respectively 0° and 12.5°. The size of workpiece is 50(60) nm × 16 nm × 20 nm and it contains almost 1,200,000 atoms. Atoms of workpiece are defined as three parts: boundary layer, thermostat layer and Newtonian layer. Atoms in boundary layer are fixed at space to prevent the unexpected movement under the action of cutting force. The thermostat layer adjacent to it is kept at a constant temperature of 293 K to imitate the heat dissipation in nano-cutting. The rest atoms that would under the cutting of tool are in the Newtonian layer obeying the Newton’s law. In cutting model 1 (as shown in Fig. 1(a)) when the boundary layer is not introduced at the entrance edge (the left edge of the workpiece), the edge tends to collapse under influence of the hard particle which will be discussed in the next section. In this modeling condition, the influence of the hard particle on the surface generation, such as the pits generated in the cutting process, could not be revealed well. Therefore, the boundary layer is introduced at cutting model 2 (as shown in Fig. 1(b)) to inhibit the collapse. Fig. 1(a) and (b) are the nano-cutting model sliced at median line in z direction. Periodic boundary condition is applied along the zdirection to reduce the size effect in the nano-cutting process. The cutting directions in this study is {110}<001>. The uncut chip thickness is set to 5 nm. The cutting speed is 100 m/s at the negative x-direction. The cutting distance of the model is about 40 nm. Initial temperature of the cutting model is equal to the constant temperature in thermostat layer.
The interaction among the aluminum atoms is described by the embedded atom method (EAM) potential [27]. The tool energy
E is E Fi i i
1 i, j rij 2 i , j , j i
where Fi i is the embedding energy to embed atom potential energy between atoms
(1)
i into the electron density i , and i , j rij is the pair
i and j . The electron density i can be calculated by the following form
i
where f j rij
f r
j , j i
is the electron density casing by atom
j
ij
(2)
j which has a distance of rij to the location of atom i .
The interaction between the carbon atoms in the cutting tool is ignored due to the diamond is much harder than aluminum and the diamond tool is thought as rigid. However, the interaction between the carbon atoms in hard particle is described by long-range carbon bond order potential (LCBOP) [28]. In the previous researches, the Morse potential is usually used to describe the interaction between rigid diamond tool (or indenter) and workpiece materials [29-31]. Therefore, in this study, the interaction between the rigid diamond tool and aluminum atoms is also depicted by the Morse potential:
E D0 e2 ( r r0 ) 2e ( r r0 ) where
r0
(3)
E is the pair potential energy, D0 is the cohesion energy, is a constant determined by material properties,
is the distance at equilibrium and
r
is the distance between two atoms.
The MD simulation is based on the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [32]. The workstations used to simulate the nano-cutting processes have dual Intel E5 CPUs with total 28 cores and 56 threads. The computation time for each procedure is about 48 hours which is acceptable in this study. The simulation results is analyzed based on common neighbor analysis (CNA) [33], and dislocation analysis using dislocation extraction algorithm (DXA) [34] with software OVITO [35]. The microstructure, such as face-centered cubic (FCC) and hexagonal close-packed (HCP) structures, and dislocation type, such as perfect dislocation, Shockley partial, and stair-rod dislocations, of the workpiece system could be identified. A single HCP layer denote a coherent twin boundary (TB). Two HCP layers with or without a FCC layer between them indicate intrinsic stacking fault (ISF) and extrinsic stacking fault (ESF) respectively [36-38] . The displacement vectors of the workpiece atoms are analyzed to reveal the material removal mechanism in nano-cutting process. In this study, the plastic deformation mechanism, the surface generation, as well as the processing force influenced by the hard particle are discussed.
3.
Results and discussion
In order to exhibit the influence of the hard particle on cutting-induced plastic deformation in workpiece, the snapshots of the microstructure evolution and the displacement vector of the workpiece atoms are shown in Fig. 2, Fig. 3, Fig. 5, and Fig. 6 respectively. The atoms in FCC structure is in green color and not display in the figure of microstructure evolution. The HCP structures are red and other type of atoms such as dislocation cores and surface atoms are white. Dislocation lines are colored according to their types: perfect dislocations (blue line), Shockley partial dislocations (green line), stair-rod
dislocations (purple line), and Frank partial dislocations (pale blue line). The red HCP layers on {111} crystal plane indicate the generation of TB, ISF or ESF. The red arrows in displacement vector figures indicate the material flow direction. 3.1 Plastic deformation mechanism When cutting the aluminum at the direction of {110}<001>, a TB in front of the cutting edge forms on the {111} plane, as shown in Fig. 2(a). It expands beneath the cutting edge and segments by Shockley partial dislocations into two or more parts. During the cutting process, the Shockley partial dislocations move along the TB and making the segmented TB move forward. Along the TB, displacement vectors of atoms change abruptly making the atoms flow up, as shown in Fig. 2(b). The TB could be seen as the primary deformation plane with an angle of 35° which is the included angle between the {111} plane and the cutting direction. A stagnation region forms when the flow-up atoms attain the cutting tool edge. In the stagnation region, the displacement vectors almost equal to zero [39]. It means that the atoms in the region are entrapped by the cutting edge. And more time and cutting distance they should take to determine whether to be a part of chip or machined surface. At the tip of the stagnation region, the atoms separate into two parts. The atoms above the stagnation region tend to be removed and form as chip. The atoms below the stagnation region are pressed down to form as machined surface. A triangular shearing zone ABC bounded by the tool edge profile, ISF and the deformation boundary change the displacement vector of atoms making the atoms flow down to the clearance face of the cutting tool, as shown in Fig. 2(b). Due to the TB expands beneath the cutting edge and has several nanometers distance to the tool edge, more atoms are removed compared to the uncut chip thickness [22].
When the workpiece has a 4 nm deep hard particle under the surface, the microstructure evolution and the displacement vector are shown in Fig. 3. At the beginning of the cutting process, the TB forms in front of the cutting edge and is not influenced by the hard particle. The plastic deformation mechanism of the material is similar as the results shown in Fig. 2. When the TB meets the hard particle, part of the TB could not further expands and is blocked by the hard particle, as shown in Fig. 3(a) and (b). As the cutting edge contacts with the hard particle, the cutting tool pushes the hard particle and makes it roll upward along the tool edge. Meanwhile, the hard particle is working as another cutting edge to participate in the cutting process. More TBs expand from the bottom and the forehead of the hard particle, as shown in Fig. 3(c-f). Different from the cutting process shown in Fig. 2(a), a large number of Shockley partial dislocations nucleate in front of the cutting edge and around the hard particle. The Shockley dislocations are at the edge of TB, ISF or ESF. Stair-rod dislocations are also found in the plastic deformation zone, which are the meet and reaction of Shockley partial dislocations on different {111} planes. Besides that, a little of perfect dislocations are generated which nucleate without stacking fault
as shown in Fig. 3(a, c, e). While the diamond cutting through the surface, a TB forms under the tool clearance face causing the collapsed entrance edge, as shown in Fig. 3(e). The phenomenon would deteriorate the machined surface quality especially when the entrance edge need keep sharp. During the cutting process, the hard particle rolls up to the rake face of the cutting tool. The motion of the center of the hard particle at y-direction is shown in Fig. 4(a). The hard particle contact with the tool edge in the cutting distance of 6 nm, at which the hard particle starts to roll upward. When the cutting distance changes from 6 nm to 12 nm, the velocity at y-direction almost stays constant and is larger than the velocity of the cutting distance larger than 12 nm, as shown in Fig. 4(a). Therefore, the cutting process could be divided into three stages: the noncontact stage (cutting distance < 6 nm), the initial contact stage (6 nm < cutting distance < 12 nm), and the stable contact stage (cutting distance > 12 nm). At the noncontact stage, the hard particle keeps still at its initial position. After that, the hard particle moves up quickly at the initial contact stage and attains a relatively stable stage with the velocity at y-direction is relatively small. The influence of the hard particle on processing force would be discussed in following section.
When the workpiece has a 6 nm deep hard particle under the surface, the microstructure evolution and the displacement vector are shown in Fig. 5. Similar to the results obtained in cutting process with 4 nm deep hard particle, TB forms in front of the cutting edge and would be influenced by the hard particle. However, in this condition, the hard particle could not be removed and rolls downward to the subsurface of the workpiece, when the cutting edge contacts with it, as shown in Fig. 5(b). At this point, the hard particle partly participates in the cutting process making more TBs initiate in front of it and more material be removed. The TBs are segmented by Shockley partial dislocations into several parts. Besides that, Stair-rod dislocations and perfect dislocation could also be seen around the hard particle. In front of the hard particle, the atoms separate into two parts: a part of them flow up to the cutting edge and tend to be removed as chip, another part of them flow down to the bottom of the hard particle to form as the machined surface. The region the atoms separate is the separation region as shown in Fig. 5(b). The collapsed edge accompanied with the generation of the TBs and ESFs could also be seen at the entrance edge in this cutting process.
When the hard particle has 9 nm depth under the workpiece surface, the distance from the top of the hard particle to the bottom of the cutting edge is 0 nm. Therefore, the hard particle has less influence on the plastic deformation, as shown in Fig. 6. The material deformation mechanism is similar to the results obtained in the cutting process without hard particle
in workpiece subsurface. During the cutting process, the hard particle keeps still at its initial position. Almost no TBs and dislocations initiate around the hard particle. Collapse does not happen at the entrance edge, although a small TB left in the edge. Therefore, when cutting the workpiece with a hard particle in the subsurface, the depth of the particle would determine the interaction between it and the tool edge. As the hard particle close to the free surface of the workpiece, it tends to be removed and forms as part of chip. When the depth of the hard particle is larger enough, the cutting process would almost not be influenced by the hard particle. However, when the depth of the hard particle in the middle of them, it would be pressed down to the subsurface of the workpiece. Collapse would happen at the entrance edge of the workpiece, when the interaction of the hard particle and tool edge takes place in the cutting process making more material be removed. Further influence of the hard particle on the surface generation would be discussed in the next section, in which the boundary layer is introduced at the entrance edge of the workpiece to inhibit the collapse.
3.2 Surface generation As the boundary layer is introduced at the entrance edge of the cutting process, the influence of the hard particle on cuttinginduced plastic deformation in workpiece is similar to the results exhibited in previous section. However, the surface generation is different from the former section in which the collapsed edge happens when the hard particle interacts with the tool edge. The surface generation in the cutting model with the boundary layer in the entrance side is shown in Fig. 7. When the hard particle has a 4 nm depth under the free surface, only a small shallow pit is left on the machined surface, as shown in Fig. 7(a). It is because the hard particle moves up quickly to the rake face of the tool edge causing small amount of extra material to be removed and other material tends to fill the hard particle induced pit during the interaction of the tool clearance face and the workpiece. However, when the depth of the hard particle slight increase to 5 nm, a large and deep pit is left on the generated surface, as shown in Fig. 7(b). In this condition, the hard particle is pressed down to the subsurface of the workpiece, and it is also pushed by the tool edge to move at the cutting direction which makes the hard particle scratch along the workpiece surface causing larger amount of material to be removed. Therefore, a large pit is left on the surface. As the depth of the hard particle further increases, the hard particle could be pressed down to the workpiece subsurface quickly. The distance of hard particle moving along the cutting direction is relatively small. Therefore, small amount of material is removed and small pits would left on the machined surface, as shown in Fig. 7(c-d). The shape of the pressed-down hard particle in the machined surface is marked in the Fig. 7(b-d). Its top is higher than the machined surface. It is because that, when the tool edge cutting through the hard particle, the pressed-down hard particle would
recovery back due to the elastic deformation of the workpiece substrate. This phenomenon would further deteriorate the machined surface quality.
3.3 Cutting force The processing forces in the cutting and feed direction derived from the interaction between the tool and workpiece atoms are shown in Fig. 8 and Fig. 9. At the beginning of the cutting process, the forces in the cutting and feed direction increase with the cutting distance and attain a relatively stable state when the cutting distance is about 6 nm. The average processing forces are calculated when the cutting process attains the stable state. Fig. 8(a) is the processing force obtained in the cutting process without hard particle in the workpiece. The feed force 𝐹𝑓 is smaller than the cutting force 𝐹𝑐 and even attains zero or negative value during the cutting process. It is because that large amount of the material is pile-up due to the TB expanding beneath the cutting edge. When the pile-up material attains the cutting tool edge, large part of it is pressed down to form the machined surface. This phenomenon makes the feed force 𝐹𝑓 fluctuate over a greater range, even attain zero and negative value. Therefore, the average feed force for {110}<001> cutting direction is relatively small, as shown in Fig. 9. The processing force for the workpiece has a 4 nm deep hard particle under the surface is shown in Fig. 4(b). At noncontact stage, the deformation carriers, such as the dislocations and TB etc., are affected by the hard particle causing an increase of feed force 𝐹𝑓 . When the feed force attains the peak value where the tool edge contacts with the hard particle at cutting distance of 6 nm, it decreases and becomes similar to the cutting process without hard particle in workpiece after the cutting distance of 12 nm. The cutting force 𝐹𝑐 attains a relatively stable value when the tool edge contact with the hard particle and is slight larger than that of the cutting process without hard particle, as shown in Fig. 9. When the hard particle has a 6 nm depth under the workpiece surface, the cutting force 𝐹𝑐 and the feed force 𝐹𝑓 increase quickly until the tool edge contacts with the hard particle at cutting distance of 8 nm. Due to the intensive influence of the hard particle on the material deformation process, the cutting force 𝐹𝑐 and the feed force 𝐹𝑓 are the largest compare to the other cutting conditions as shown in Fig. 9. During the cutting process, the feed force decreases because that the hard particle is pressed down to the workpiece subsurface. Fig. 8(c) is the processing force obtained in the cutting process with 9 nm deep hard particle in the workpiece. The curve of the cutting force with the cutting distance is similar to the cutting process without hard particle. And its average value in Fig. 9 is also similar to that of the cutting process without hard particle. However, the feed force of it is relatively larger. It is because the feed force is affected by the hard particle in the initial cutting process. After the tool edge passes through
the initial contact distance (14 nm), the feed force decreases and becomes similar to the cutting process without hard particle in workpiece.
4.
Conclusions
The effects of hard particle on the plastic deformation mechanism, the surface generation, as well as the processing force in nano-cutting are investigated employing MD simulations. The conclusions can be drawn as follows: (1) At cutting direction of {110}<001>, TB expands beneath the cutting edge making larger amount of material flow up. A stagnation region forms when the flow-up atoms attain the cutting tool edge. At the tip of the stagnation region, the atoms separate into two parts. The atoms above the stagnation region tend to be removed and form as chip. The atoms below the stagnation region are pressed down to form as machined surface. (2) The depth of the hard particle under the workpiece surface determines the particle to be removed or to be pressed down to the subsurface of the workpiece. As the hard particle close to the free surface of the workpiece, it tends to be removed and forms as a part of chip. When the depth of the hard particle is larger enough, the cutting process would almost not be influenced by the hard particle. However, when the hard particle is in the middle of them, it would be pressed down to the subsurface of the workpiece. (3) When the hard particle is removed, only a small shallow pit is left on the machined surface. Otherwise, larger and deeper pit is left on the generated surface and its size decreases with a further increase of the depth. After the tool edge cutting through the pressed-down hard particle, it recovers back making the top of it higher than the machined surface which would further deteriorate the machined surface quality. (4) When the hard particle is at the entrance edge of the workpiece or the boundary layer is not introduced, it causes the collapsed edge as the interaction of the hard particle takes places. (5) The hard particle in the workpiece increases the processing force when cutting tool edge or the plastic carriers, such as the TB, different kinds of dislocations etc., interact with the hard particle.
Acknowledgements The
authors thank the
supports of the
National
Natural
Science
Foundation (Grant No.
61635008,
51320105009&91423101), the National Key Research & Development Program (Grant No. 2016YFB1102200), and the ‘111’ project by the State Administration of Foreign Experts Affairs and the Ministry of Education of China (Grant No. B07014)。
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[29] M. Lai, X.D. Zhang, F.Z. Fang, Y. Wang, M. Feng, W. Tian, Study on nanometric cutting of germanium by molecular dynamics simulation, Nanoscale Research Letters, 8 (2013). [30] T.H. Fang, J.H. Wu, Molecular dynamics simulations on nanoindentation mechanisms of multilayered films, Computational Materials Science, 43 (2008) 785-790. [31] M. Lai, X.D. Zhang, F.Z. Fang, Study on critical rake angle in nanometric cutting, Applied Physics A, 108 (2012) 809-818. [32] S. Plimpton, Fast Parallel Algorithms for Short-Range Molecular Dynamics, Journal of Computational Physics, 117 (1995) 1-19. [33] J.D. Honeycutt, H.C. Andersen, Molecular dynamics study of melting and freezing of small Lennard-Jones clusters, The Journal of Physical Chemistry, 91 (1987) 4950-4963. [34] S. Alexander, V.B. Vasily, A. Athanasios, Automated identification and indexing of dislocations in crystal interfaces, Modelling and Simulation in Materials Science and Engineering, 20 (2012) 085007. [35] S. Alexander, Visualization and analysis of atomistic simulation data with OVITO–the Open Visualization Tool, Modelling and Simulation in Materials Science and Engineering, 18 (2010) 015012. [36] J. Li, B. Liu, H. Luo, Q.H. Fang, Y.W. Liu, Y. Liu, A molecular dynamics investigation into plastic deformation mechanism of nanocrystalline copper for different nanoscratching rates, Computational Materials Science, 118 (2016) 6676. [37] J.J. Zhang, T. Sun, Y.D. Yan, S. Dong, X.D. Li, Atomistic investigation of scratching-induced deformation twinning in nanocrystalline Cu, Journal of applied physics, 112 (2012) 073526. [38] J. Li, J.W. Guo, H. Luo, Q.H. Fang, H. Wu, L.C. Zhang, Y.W. Liu, Study of nanoindentation mechanical response of nanocrystalline structures using molecular dynamics simulations, Applied Surface Science, 364 (2016) 190-200. [39] F.F. Xu, J.S. Wang, F.Z. Fang, X.D. Zhang, A study on the tool edge geometry effect on nano-cutting, The International Journal of Advanced Manufacturing Technology, (2017) 1-11.
y
Cutting direction
x
Diamond tool
Newtonian atoms
z
D (a)
Cutting direction Newtonian atoms
D
O
Periodic B.C.
Diamond tool
Hard particle
L
Fixed boundary atoms layer Model 1
(b)
O
Periodic B.C.
L
Hard particle
Thermostat atoms layer Model 2
Fig. 1 Schematic description of nano-cutting model (a) without, or (b) with boundary layer at left edge of the workpiece.
TB
Cutting direction
Chip Cutting direction
Diamond tool
Diamond tool TB
B TB A
Shockley dislocations (a)
Stagnation region
C
B C
TB ISF
Stair-rod dislocations Workpiece
A (b)
Fig. 2 (a) Snapshots of the microstructure evolution without hard particle in workpiece subsurface, (b) displacement vector sliced at z-direction.
Cutting direction Cutting direction
Chip Diamond tool
Hard particle
Diamond tool
TB Hard particle
Shockley dislocations Stair-rod dislocations (a)
(b)
Cutting direction
Cutting direction Chip
Hard particle
Diamond tool
Diamond tool TB
Hard particle
TB
TB
Shockley dislocations Stair-rod dislocations (c)
(d)
Chip
Cutting direction Cutting direction
Hard particle
Collapsed edge Diamond tool TB
TB
Hard particle
TB
Shockley dislocations Stair-rod dislocations (e)
(f)
Fig. 3 Snapshots of the microstructure evolution with 4 nm deep hard particle under the surface and the displacement vector sliced in z-direction, at the cutting distance of (a-b) 6 nm, (c-d) 12 nm and (e-f) 25.5nm.
Center of HP at y-direction (nm)
(a)
19
18.5
18
Stable contact stage
17.5 Noncontact stage
17
16.5
Initial contact stage 6 nm, 16 nm
16 15.5
0
(b)
Processing force (nN)
12 nm, 17 nm
800
10 20 Cutting distance (nm)
30
Fc Ff
300
-200 0
10 20 Cutting distance (nm)
30
Fig. 4 (a) postion of the center of the hard particle at y-direction, and (b) processing force for the cutting process with 4 nm deep hard particle under the workpiece surface.
Chip
Cutting direction
Cutting direction ISF
Hard particle
Diamond tool
Diamond tool
Collapsed edge
TB
TB
TB
Hard particle
Separation region
ESF Shockley dislocations Stair-rod dislocations
(a)
(b)
Fig. 5 (a) Snapshots of the microstructure evolution with 6 nm deep hard particle under the surface, (b) displacement vector sliced in z-direction at the cutting distance of 18 nm.
Cutting direction
Cutting direction
Chip
Diamond tool
Diamond tool
TB
TB TB
Hard particle
Shockley dislocations Hard particle (a)
(b)
Cutting direction
Chip
Cutting direction
Diamond tool
Diamond tool TB
C
B
Hard particle A
B
TB
C
TB A
Shockley dislocations (c)
Hard particle
Stair-rod dislocations
(d)
Fig. 6 Snapshots of the microstructure evolution with 9 nm deep hard particle under the surface and the displacement vector sliced in z-direction, at the cutting distance of (a-b) 12 nm and (c-d) 21 nm.
o x
Cutting direction (a)
3 nm y Initial position
8 nm
(b)
8 nm
(c) Hard particle
8 nm (d)
8 nm
0 nm
Fig. 7 Surface generation with (a) 4 nm, (b) 5 nm, (c) 6 nm and (d) 8 nm deep hard particle under the workpiece surface.
Processing force (nN)
(a) 800 Fc Ff
600 400 200 0 -200 0
5
10 15 Cutting distance (nm)
Processing force (nN)
(b) 800
20
25
Fc Ff
600
400 200
Initial contact distance
0
-200 0
5
Processing force (nN)
(c) 800
10 15 Cutting distance (nm)
20
25
Fc Ff
600 400 200
Initial contact distance
0 -200 0
5
10 15 Cutting distance (nm)
20
25
Fig. 8 Processing force (a) without hard particle and with (b) 6 nm, (c) 9 nm deep hard particle under the workpiece surface.
Processing force (nN)
800
Fc_average
Ff_average
600 400
494
434
379
375 291 208
200
90
101
0 No particle 4 nm depth 6 nm depth 9 nm depth
Cutting conditions
Fig. 9 Average processing force at different cutting conditions.
Table 1 Computational parameters used in MD simulations Cutting model
Model 1
Model 2
Material of hard ball
Diamond
Diameter of hard ball
8 nm
D of hard ball
4, 6, 9 nm
4, 5, 6, 8 nm
L of hard ball
9 nm
10 nm
Material of workpiece
Aluminum
Dimension of workpiece
50 nm × 16 nm × 20 nm
Material of cutting tool
Diamond
Tool edge radius 𝑟𝛽
5 nm
Rake angle
0°
Clearance angle
12.5°
Cutting direction
{110}<001>, -x direction
Uncut chip thickness
5 nm
Cutting speed
100 m/s
Initial temperature
293 K
60 nm × 16 nm × 20 nm
S0169-4332(17)32079-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.089 APSUSC 36623
To appear in:
APSUSC
Received date: Revised date: Accepted date:
19-5-2017 9-7-2017 12-7-2017
Please cite this article as: Feifei Xu, Fengzhou Fang, Xiaodong Zhang, Hard particle effect on surface generation in nano-cutting, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hard particle effect on surface generation in nano-cutting
Feifei Xu1, 2, Fengzhou Fang1, 3*, Xiaodong Zhang1
1 State Key Laboratory of Precision Measuring Technology & Instruments, Centre of MicroNano Manufacturing Technology, Tianjin University, Tianjin 300072, China 2 Institute of Mechanical Manufacturing Technology, China Academy of Engineering Physics, Sichuan 621900, China 3 School of Mechanical & Materials Engineering, MNMT-Dublin, University College Dublin, Dublin, Ireland Author: Feifei Xu, email: [email protected]
*Corresponding author: Fengzhou Fang, email: [email protected] Author: Xiaodong Zhang, email: [email protected]
Graphical Abstract Cutting direction
Cutting direction Chip Hard particle
Diamond tool
Diamond tool
Roll up
TB Hard particle
TB
TB
Shockley dislocations Stair-rod dislocations
Hard particle
ISF Diamond tool
Diamond tool
Collapsed edge
TB
Cutting direction
Cutting direction
TB ESF
Shockley dislocations Stair-rod dislocations
TB
Separation region
Hard particle
Roll down
Chip
Highlights
Nano-cutting of material with hard particles are investigated.
Hard particles effects on surface generation are studied.
Elastic recovery of hard particle happens after cutting through.
Twin boundaries and dislocations initiate around hard particles.
Hard particles induced increase of processing force is observed.
Abstract The influence of the hard particle on the surface generation, plastic deformation and processing forces in nano-cutting of aluminum is investigated by means of molecular dynamics simulations. In this investigation, a hard particle which is simplified as a diamond ball is embedded under the free surface of workpiece with different depths. The influence of the position of the hard ball on the surface generation and other material removal mechanism, such as the movement of the ball under the action of cutting tool edge, is revealed. The results show that when the hard particle is removed, only a small shallow pit is left on the machined surface. Otherwise, it is pressed down to the subsurface of the workpiece left larger and deeper pit on the generated surface. Besides that, the hard particle in the workpiece would increase the processing force when the cutting tool edge or the plastic carriers interact with the hard particle. It is helpful to optimize the cutting parameters and material properties for obtaining better surface quality in nano-cutting of composites or other materials with micro/nanoscale hard particles in it.
Keywords: Nano-cutting, Hard particle, Surface generation, Plastic deformation, Cutting mechanism
1.
Introduction
During the last several decades, nano-cutting which is one of the most efficient machining technologies has been applied by industry to generate components with nanometric surface roughness and sub-micrometric form accuracy or feature size [1-3]. In nano-cutting process, the material is removed in micrometric or nanometric scale which is usually smaller than the material grain size [4]. In this scale, even a very small defect in the workpiece material, such as the grain boundary [5, 6], pore [7], second-phase particle [7] or hard particle [8, 9], could be an important factor in influencing the generated surface quality. Generally, the workpiece material could be chosen and optimized at the design stage according to their machinability and the specific requirements of the machined components, such as the extreme surface roughness. The nickel-phosphorous (NiP) electroless plating which is known as a high purity amorphous material [10] without grain boundary, hard particle, or other defects, is chosen as an important material on the mould surface for manufacturing optical
components with nanometric surface roughness [11]. However, the machinability is not the only factor to be considered in determining the material to be used. The strength, stiffness, resistance, thermal stability, weight and so on are the other factors should be taken into consideration. Therefore, materials with good mechanical or optical properties accompanied with difficult-to-machine properties arise in the nano-cutting process. Hard particle in workpiece which is one of the difficult-to-machine properties plays an important role in surface generation and attracts many researchers’ attention. Metal matrix composites (MMCs) whose mechanical properties are enhanced by the particles, is a typical kind of material [7, 12]. In 2008, the influence of SiCp particles on the surface generation of SiCp /Al composites in ultra-precision cutting process is experimentally investigated by Ge et al. [12]. The results show that better surface quality could be obtained when the SiCp particles are removed by pressed into or cut through mechanism. Otherwise, the surface quality would be deteriorated when the SiCp particles are pulled out or crushed in cutting process. The hard particles also exist in the aluminum alloy Al-6061, which is a widely used material to produce optical quality surface in nano-cutting process. In 2009, Ding et al [8] found that the hard particles in Al-6061 causes the formation of voids on the machined surface. Near the voids, the hard particles could be found under the machined surface when the top layer of the machined surface is removed. Further researches reveal that the hard particle could be sheared and fractured when the cutting tool encounters the hard particle. The voids are created due to the vacation of the broken parts of the hard particle and enlarged by the loose particles [9]. The formation of the voids could be suppressed by making the hard particle be machined in ductile mode, such as the reduction of UCT [9] or application of other assisted cutting technology. Ultrasonic vibration assisted cutting is found to be an effective method to achieve a ductile-mode cutting of hard particles and generate an improved surface quality [13]. In 2015, Wang et al. [14] found that the precipitates (Mg 2 Si) generating at the isothermal heat treatment of Al6061, would cause the scratch marks on the machined surface and deteriorate the machined surface quality. In 2016, Zong et al. [15] experimentally investigated the influence of hard particles on the surface generation from the aspect of feed rate. The results show that large feed rate makes the hard particles be pulled out and scratch on the machined surface leaving micro-grooves on the tool mark. Small feed rate (less than the size of hard particle) makes the hard particles be cut off partially and then be pressed into the matrix. The pressed hard particles will protrude from the machined surface due to the elastic recovery of the matrix. However, the experimental investigation on the influence of the hard particles in nano-cutting is high-cost and time consuming. Numerical methods, such as the finite element method (FEM) [16] and the molecular Dynamics (MD) simulation [17] could give a result more straightforward. In 2015, Zhang et al. [18] investigated the influence of SiCp particles on the surface generation of SiCp /Al composites by FEM and found that the relative location between the cutting tool and the particle determines the initiation and propagation of the cracks in SiCp and the final machined surface quality.
Molecular Dynamics (MD) simulation is a useful approach in investigating the plastic deformation mechanism in nanoscale. It was first applied by Lawrence Livermore National Laboratory (LLNL) and Precision Engineering Department at Osaka University to investigate the nano-cutting process [19]. After that, it has been used to investigate material removal mechanism and surface generation of micro/nano-cutting processes [17, 20-24]. Recently, Xu et al. [22] investigated the plastic deformation mechanism and surface generation of Aluminum by MD simulations and found the preferred cutting directions whose minimum uncut chip thickness is relatively small. The side flow which is an important factor in deteriorating the generated surface quality [25, 26] has also been investigated by the application of MD simulation and found that it could be suppressed by optimizing the cutting tool geometry or choosing a preferred crystallographic orientation [17]. In 2016, MD simulation is employed by Li et al. [7] to investigate the influence of pore and second-phase particles on the subsurface damage and surface integrity in cutting process. They found that the workpiece with soft particle is easily crushed by the tool, while workpiece containing the hard particle still has a stable structure. Therefore, the experimental and simulation methods have been widely introduced to investigate the effect of hard particle on cutting process. However, the experimental methods could not give a visualized material removal process and the mechanism of the effect of hard particle on the surface generation is derived from the machined results indirectly. The simulation methods could give a visualized process about the material removal and surface generation, but the previous researches are not enough to fully reveal the plastic deformation, surface generation, and processing force in nano-cutting of materials with hard particles. In this study, MD simulations are employed to investigate the influence of the hard particle on nano-cutting processes. The depth of the particle under the surface of the aluminum which could simulate the hard particle in aluminum alloys Al-6061, is taken into consideration. This study contributes to a better understanding of the surface generation for materials with hard particles in nano-cutting.
2.
Method
MD simulation is employed to investigate the effect of hard particles on the surface generation in nano-cutting of aluminum. As shown in Fig. 1, the molecular dynamics simulation model consists of a rigid diamond tool, an aluminum workpiece and a hard particle. Generally, many hard particles are dispersed over the workpiece materials and the shape of them are irregular. But for simplify the simulations, a single hard diamond particle is introduced to the workpiece and its shape is simplified as a ball. In the cutting process, the diamond ball will stay stable without the initiation of cracks. Therefore, the crack of the hard particle will not be discussed in this study and it will be investigated in the future. From the analysis of the influence of the hard ball on the surface generation, the influence of hard particle with other kind of shape could be derived. The diameter of the ball is 8 nm which is comparable to tool edge radius 𝑟𝛽 (5 nm). In this condition, the total
size of the simulation model could be limited to a reasonable value and the computation time is acceptable. The center of the hard ball is defined as 𝑂. Its depth 𝐷 (from the center of the ball 𝑂 to the free surface of the workpiece) and distance 𝐿 (from the center of the ball 𝑂 to the left edge of the workpiece) for model 1 and model 2 is displayed in Table 1. At different value of 𝐷, the influence of the position of the hard particle on the surface generation could be revealed. And at the value of the distance 𝐿, the cutting process will attain a stable stage when the cutting tool touch the hard ball.
The cutting tool applied in this study has an edge radius 𝑟𝛽 of 5 nm. The rake angle and clearance angle is respectively 0° and 12.5°. The size of workpiece is 50(60) nm × 16 nm × 20 nm and it contains almost 1,200,000 atoms. Atoms of workpiece are defined as three parts: boundary layer, thermostat layer and Newtonian layer. Atoms in boundary layer are fixed at space to prevent the unexpected movement under the action of cutting force. The thermostat layer adjacent to it is kept at a constant temperature of 293 K to imitate the heat dissipation in nano-cutting. The rest atoms that would under the cutting of tool are in the Newtonian layer obeying the Newton’s law. In cutting model 1 (as shown in Fig. 1(a)) when the boundary layer is not introduced at the entrance edge (the left edge of the workpiece), the edge tends to collapse under influence of the hard particle which will be discussed in the next section. In this modeling condition, the influence of the hard particle on the surface generation, such as the pits generated in the cutting process, could not be revealed well. Therefore, the boundary layer is introduced at cutting model 2 (as shown in Fig. 1(b)) to inhibit the collapse. Fig. 1(a) and (b) are the nano-cutting model sliced at median line in z direction. Periodic boundary condition is applied along the zdirection to reduce the size effect in the nano-cutting process. The cutting directions in this study is {110}<001>. The uncut chip thickness is set to 5 nm. The cutting speed is 100 m/s at the negative x-direction. The cutting distance of the model is about 40 nm. Initial temperature of the cutting model is equal to the constant temperature in thermostat layer.
The interaction among the aluminum atoms is described by the embedded atom method (EAM) potential [27]. The tool energy
E is E Fi i i
1 i, j rij 2 i , j , j i
where Fi i is the embedding energy to embed atom potential energy between atoms
(1)
i into the electron density i , and i , j rij is the pair
i and j . The electron density i can be calculated by the following form
i
where f j rij
f r
j , j i
is the electron density casing by atom
j
ij
(2)
j which has a distance of rij to the location of atom i .
The interaction between the carbon atoms in the cutting tool is ignored due to the diamond is much harder than aluminum and the diamond tool is thought as rigid. However, the interaction between the carbon atoms in hard particle is described by long-range carbon bond order potential (LCBOP) [28]. In the previous researches, the Morse potential is usually used to describe the interaction between rigid diamond tool (or indenter) and workpiece materials [29-31]. Therefore, in this study, the interaction between the rigid diamond tool and aluminum atoms is also depicted by the Morse potential:
E D0 e2 ( r r0 ) 2e ( r r0 ) where
r0
(3)
E is the pair potential energy, D0 is the cohesion energy, is a constant determined by material properties,
is the distance at equilibrium and
r
is the distance between two atoms.
The MD simulation is based on the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) [32]. The workstations used to simulate the nano-cutting processes have dual Intel E5 CPUs with total 28 cores and 56 threads. The computation time for each procedure is about 48 hours which is acceptable in this study. The simulation results is analyzed based on common neighbor analysis (CNA) [33], and dislocation analysis using dislocation extraction algorithm (DXA) [34] with software OVITO [35]. The microstructure, such as face-centered cubic (FCC) and hexagonal close-packed (HCP) structures, and dislocation type, such as perfect dislocation, Shockley partial, and stair-rod dislocations, of the workpiece system could be identified. A single HCP layer denote a coherent twin boundary (TB). Two HCP layers with or without a FCC layer between them indicate intrinsic stacking fault (ISF) and extrinsic stacking fault (ESF) respectively [36-38] . The displacement vectors of the workpiece atoms are analyzed to reveal the material removal mechanism in nano-cutting process. In this study, the plastic deformation mechanism, the surface generation, as well as the processing force influenced by the hard particle are discussed.
3.
Results and discussion
In order to exhibit the influence of the hard particle on cutting-induced plastic deformation in workpiece, the snapshots of the microstructure evolution and the displacement vector of the workpiece atoms are shown in Fig. 2, Fig. 3, Fig. 5, and Fig. 6 respectively. The atoms in FCC structure is in green color and not display in the figure of microstructure evolution. The HCP structures are red and other type of atoms such as dislocation cores and surface atoms are white. Dislocation lines are colored according to their types: perfect dislocations (blue line), Shockley partial dislocations (green line), stair-rod
dislocations (purple line), and Frank partial dislocations (pale blue line). The red HCP layers on {111} crystal plane indicate the generation of TB, ISF or ESF. The red arrows in displacement vector figures indicate the material flow direction. 3.1 Plastic deformation mechanism When cutting the aluminum at the direction of {110}<001>, a TB in front of the cutting edge forms on the {111} plane, as shown in Fig. 2(a). It expands beneath the cutting edge and segments by Shockley partial dislocations into two or more parts. During the cutting process, the Shockley partial dislocations move along the TB and making the segmented TB move forward. Along the TB, displacement vectors of atoms change abruptly making the atoms flow up, as shown in Fig. 2(b). The TB could be seen as the primary deformation plane with an angle of 35° which is the included angle between the {111} plane and the cutting direction. A stagnation region forms when the flow-up atoms attain the cutting tool edge. In the stagnation region, the displacement vectors almost equal to zero [39]. It means that the atoms in the region are entrapped by the cutting edge. And more time and cutting distance they should take to determine whether to be a part of chip or machined surface. At the tip of the stagnation region, the atoms separate into two parts. The atoms above the stagnation region tend to be removed and form as chip. The atoms below the stagnation region are pressed down to form as machined surface. A triangular shearing zone ABC bounded by the tool edge profile, ISF and the deformation boundary change the displacement vector of atoms making the atoms flow down to the clearance face of the cutting tool, as shown in Fig. 2(b). Due to the TB expands beneath the cutting edge and has several nanometers distance to the tool edge, more atoms are removed compared to the uncut chip thickness [22].
When the workpiece has a 4 nm deep hard particle under the surface, the microstructure evolution and the displacement vector are shown in Fig. 3. At the beginning of the cutting process, the TB forms in front of the cutting edge and is not influenced by the hard particle. The plastic deformation mechanism of the material is similar as the results shown in Fig. 2. When the TB meets the hard particle, part of the TB could not further expands and is blocked by the hard particle, as shown in Fig. 3(a) and (b). As the cutting edge contacts with the hard particle, the cutting tool pushes the hard particle and makes it roll upward along the tool edge. Meanwhile, the hard particle is working as another cutting edge to participate in the cutting process. More TBs expand from the bottom and the forehead of the hard particle, as shown in Fig. 3(c-f). Different from the cutting process shown in Fig. 2(a), a large number of Shockley partial dislocations nucleate in front of the cutting edge and around the hard particle. The Shockley dislocations are at the edge of TB, ISF or ESF. Stair-rod dislocations are also found in the plastic deformation zone, which are the meet and reaction of Shockley partial dislocations on different {111} planes. Besides that, a little of perfect dislocations are generated which nucleate without stacking fault
as shown in Fig. 3(a, c, e). While the diamond cutting through the surface, a TB forms under the tool clearance face causing the collapsed entrance edge, as shown in Fig. 3(e). The phenomenon would deteriorate the machined surface quality especially when the entrance edge need keep sharp. During the cutting process, the hard particle rolls up to the rake face of the cutting tool. The motion of the center of the hard particle at y-direction is shown in Fig. 4(a). The hard particle contact with the tool edge in the cutting distance of 6 nm, at which the hard particle starts to roll upward. When the cutting distance changes from 6 nm to 12 nm, the velocity at y-direction almost stays constant and is larger than the velocity of the cutting distance larger than 12 nm, as shown in Fig. 4(a). Therefore, the cutting process could be divided into three stages: the noncontact stage (cutting distance < 6 nm), the initial contact stage (6 nm < cutting distance < 12 nm), and the stable contact stage (cutting distance > 12 nm). At the noncontact stage, the hard particle keeps still at its initial position. After that, the hard particle moves up quickly at the initial contact stage and attains a relatively stable stage with the velocity at y-direction is relatively small. The influence of the hard particle on processing force would be discussed in following section.
When the workpiece has a 6 nm deep hard particle under the surface, the microstructure evolution and the displacement vector are shown in Fig. 5. Similar to the results obtained in cutting process with 4 nm deep hard particle, TB forms in front of the cutting edge and would be influenced by the hard particle. However, in this condition, the hard particle could not be removed and rolls downward to the subsurface of the workpiece, when the cutting edge contacts with it, as shown in Fig. 5(b). At this point, the hard particle partly participates in the cutting process making more TBs initiate in front of it and more material be removed. The TBs are segmented by Shockley partial dislocations into several parts. Besides that, Stair-rod dislocations and perfect dislocation could also be seen around the hard particle. In front of the hard particle, the atoms separate into two parts: a part of them flow up to the cutting edge and tend to be removed as chip, another part of them flow down to the bottom of the hard particle to form as the machined surface. The region the atoms separate is the separation region as shown in Fig. 5(b). The collapsed edge accompanied with the generation of the TBs and ESFs could also be seen at the entrance edge in this cutting process.
When the hard particle has 9 nm depth under the workpiece surface, the distance from the top of the hard particle to the bottom of the cutting edge is 0 nm. Therefore, the hard particle has less influence on the plastic deformation, as shown in Fig. 6. The material deformation mechanism is similar to the results obtained in the cutting process without hard particle
in workpiece subsurface. During the cutting process, the hard particle keeps still at its initial position. Almost no TBs and dislocations initiate around the hard particle. Collapse does not happen at the entrance edge, although a small TB left in the edge. Therefore, when cutting the workpiece with a hard particle in the subsurface, the depth of the particle would determine the interaction between it and the tool edge. As the hard particle close to the free surface of the workpiece, it tends to be removed and forms as part of chip. When the depth of the hard particle is larger enough, the cutting process would almost not be influenced by the hard particle. However, when the depth of the hard particle in the middle of them, it would be pressed down to the subsurface of the workpiece. Collapse would happen at the entrance edge of the workpiece, when the interaction of the hard particle and tool edge takes place in the cutting process making more material be removed. Further influence of the hard particle on the surface generation would be discussed in the next section, in which the boundary layer is introduced at the entrance edge of the workpiece to inhibit the collapse.
3.2 Surface generation As the boundary layer is introduced at the entrance edge of the cutting process, the influence of the hard particle on cuttinginduced plastic deformation in workpiece is similar to the results exhibited in previous section. However, the surface generation is different from the former section in which the collapsed edge happens when the hard particle interacts with the tool edge. The surface generation in the cutting model with the boundary layer in the entrance side is shown in Fig. 7. When the hard particle has a 4 nm depth under the free surface, only a small shallow pit is left on the machined surface, as shown in Fig. 7(a). It is because the hard particle moves up quickly to the rake face of the tool edge causing small amount of extra material to be removed and other material tends to fill the hard particle induced pit during the interaction of the tool clearance face and the workpiece. However, when the depth of the hard particle slight increase to 5 nm, a large and deep pit is left on the generated surface, as shown in Fig. 7(b). In this condition, the hard particle is pressed down to the subsurface of the workpiece, and it is also pushed by the tool edge to move at the cutting direction which makes the hard particle scratch along the workpiece surface causing larger amount of material to be removed. Therefore, a large pit is left on the surface. As the depth of the hard particle further increases, the hard particle could be pressed down to the workpiece subsurface quickly. The distance of hard particle moving along the cutting direction is relatively small. Therefore, small amount of material is removed and small pits would left on the machined surface, as shown in Fig. 7(c-d). The shape of the pressed-down hard particle in the machined surface is marked in the Fig. 7(b-d). Its top is higher than the machined surface. It is because that, when the tool edge cutting through the hard particle, the pressed-down hard particle would
recovery back due to the elastic deformation of the workpiece substrate. This phenomenon would further deteriorate the machined surface quality.
3.3 Cutting force The processing forces in the cutting and feed direction derived from the interaction between the tool and workpiece atoms are shown in Fig. 8 and Fig. 9. At the beginning of the cutting process, the forces in the cutting and feed direction increase with the cutting distance and attain a relatively stable state when the cutting distance is about 6 nm. The average processing forces are calculated when the cutting process attains the stable state. Fig. 8(a) is the processing force obtained in the cutting process without hard particle in the workpiece. The feed force 𝐹𝑓 is smaller than the cutting force 𝐹𝑐 and even attains zero or negative value during the cutting process. It is because that large amount of the material is pile-up due to the TB expanding beneath the cutting edge. When the pile-up material attains the cutting tool edge, large part of it is pressed down to form the machined surface. This phenomenon makes the feed force 𝐹𝑓 fluctuate over a greater range, even attain zero and negative value. Therefore, the average feed force for {110}<001> cutting direction is relatively small, as shown in Fig. 9. The processing force for the workpiece has a 4 nm deep hard particle under the surface is shown in Fig. 4(b). At noncontact stage, the deformation carriers, such as the dislocations and TB etc., are affected by the hard particle causing an increase of feed force 𝐹𝑓 . When the feed force attains the peak value where the tool edge contacts with the hard particle at cutting distance of 6 nm, it decreases and becomes similar to the cutting process without hard particle in workpiece after the cutting distance of 12 nm. The cutting force 𝐹𝑐 attains a relatively stable value when the tool edge contact with the hard particle and is slight larger than that of the cutting process without hard particle, as shown in Fig. 9. When the hard particle has a 6 nm depth under the workpiece surface, the cutting force 𝐹𝑐 and the feed force 𝐹𝑓 increase quickly until the tool edge contacts with the hard particle at cutting distance of 8 nm. Due to the intensive influence of the hard particle on the material deformation process, the cutting force 𝐹𝑐 and the feed force 𝐹𝑓 are the largest compare to the other cutting conditions as shown in Fig. 9. During the cutting process, the feed force decreases because that the hard particle is pressed down to the workpiece subsurface. Fig. 8(c) is the processing force obtained in the cutting process with 9 nm deep hard particle in the workpiece. The curve of the cutting force with the cutting distance is similar to the cutting process without hard particle. And its average value in Fig. 9 is also similar to that of the cutting process without hard particle. However, the feed force of it is relatively larger. It is because the feed force is affected by the hard particle in the initial cutting process. After the tool edge passes through
the initial contact distance (14 nm), the feed force decreases and becomes similar to the cutting process without hard particle in workpiece.
4.
Conclusions
The effects of hard particle on the plastic deformation mechanism, the surface generation, as well as the processing force in nano-cutting are investigated employing MD simulations. The conclusions can be drawn as follows: (1) At cutting direction of {110}<001>, TB expands beneath the cutting edge making larger amount of material flow up. A stagnation region forms when the flow-up atoms attain the cutting tool edge. At the tip of the stagnation region, the atoms separate into two parts. The atoms above the stagnation region tend to be removed and form as chip. The atoms below the stagnation region are pressed down to form as machined surface. (2) The depth of the hard particle under the workpiece surface determines the particle to be removed or to be pressed down to the subsurface of the workpiece. As the hard particle close to the free surface of the workpiece, it tends to be removed and forms as a part of chip. When the depth of the hard particle is larger enough, the cutting process would almost not be influenced by the hard particle. However, when the hard particle is in the middle of them, it would be pressed down to the subsurface of the workpiece. (3) When the hard particle is removed, only a small shallow pit is left on the machined surface. Otherwise, larger and deeper pit is left on the generated surface and its size decreases with a further increase of the depth. After the tool edge cutting through the pressed-down hard particle, it recovers back making the top of it higher than the machined surface which would further deteriorate the machined surface quality. (4) When the hard particle is at the entrance edge of the workpiece or the boundary layer is not introduced, it causes the collapsed edge as the interaction of the hard particle takes places. (5) The hard particle in the workpiece increases the processing force when cutting tool edge or the plastic carriers, such as the TB, different kinds of dislocations etc., interact with the hard particle.
Acknowledgements The
authors thank the
supports of the
National
Natural
Science
Foundation (Grant No.
61635008,
51320105009&91423101), the National Key Research & Development Program (Grant No. 2016YFB1102200), and the ‘111’ project by the State Administration of Foreign Experts Affairs and the Ministry of Education of China (Grant No. B07014)。
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y
Cutting direction
x
Diamond tool
Newtonian atoms
z
D (a)
Cutting direction Newtonian atoms
D
O
Periodic B.C.
Diamond tool
Hard particle
L
Fixed boundary atoms layer Model 1
(b)
O
Periodic B.C.
L
Hard particle
Thermostat atoms layer Model 2
Fig. 1 Schematic description of nano-cutting model (a) without, or (b) with boundary layer at left edge of the workpiece.
TB
Cutting direction
Chip Cutting direction
Diamond tool
Diamond tool TB
B TB A
Shockley dislocations (a)
Stagnation region
C
B C
TB ISF
Stair-rod dislocations Workpiece
A (b)
Fig. 2 (a) Snapshots of the microstructure evolution without hard particle in workpiece subsurface, (b) displacement vector sliced at z-direction.
Cutting direction Cutting direction
Chip Diamond tool
Hard particle
Diamond tool
TB Hard particle
Shockley dislocations Stair-rod dislocations (a)
(b)
Cutting direction
Cutting direction Chip
Hard particle
Diamond tool
Diamond tool TB
Hard particle
TB
TB
Shockley dislocations Stair-rod dislocations (c)
(d)
Chip
Cutting direction Cutting direction
Hard particle
Collapsed edge Diamond tool TB
TB
Hard particle
TB
Shockley dislocations Stair-rod dislocations (e)
(f)
Fig. 3 Snapshots of the microstructure evolution with 4 nm deep hard particle under the surface and the displacement vector sliced in z-direction, at the cutting distance of (a-b) 6 nm, (c-d) 12 nm and (e-f) 25.5nm.
Center of HP at y-direction (nm)
(a)
19
18.5
18
Stable contact stage
17.5 Noncontact stage
17
16.5
Initial contact stage 6 nm, 16 nm
16 15.5
0
(b)
Processing force (nN)
12 nm, 17 nm
800
10 20 Cutting distance (nm)
30
Fc Ff
300
-200 0
10 20 Cutting distance (nm)
30
Fig. 4 (a) postion of the center of the hard particle at y-direction, and (b) processing force for the cutting process with 4 nm deep hard particle under the workpiece surface.
Chip
Cutting direction
Cutting direction ISF
Hard particle
Diamond tool
Diamond tool
Collapsed edge
TB
TB
TB
Hard particle
Separation region
ESF Shockley dislocations Stair-rod dislocations
(a)
(b)
Fig. 5 (a) Snapshots of the microstructure evolution with 6 nm deep hard particle under the surface, (b) displacement vector sliced in z-direction at the cutting distance of 18 nm.
Cutting direction
Cutting direction
Chip
Diamond tool
Diamond tool
TB
TB TB
Hard particle
Shockley dislocations Hard particle (a)
(b)
Cutting direction
Chip
Cutting direction
Diamond tool
Diamond tool TB
C
B
Hard particle A
B
TB
C
TB A
Shockley dislocations (c)
Hard particle
Stair-rod dislocations
(d)
Fig. 6 Snapshots of the microstructure evolution with 9 nm deep hard particle under the surface and the displacement vector sliced in z-direction, at the cutting distance of (a-b) 12 nm and (c-d) 21 nm.
o x
Cutting direction (a)
3 nm y Initial position
8 nm
(b)
8 nm
(c) Hard particle
8 nm (d)
8 nm
0 nm
Fig. 7 Surface generation with (a) 4 nm, (b) 5 nm, (c) 6 nm and (d) 8 nm deep hard particle under the workpiece surface.
Processing force (nN)
(a) 800 Fc Ff
600 400 200 0 -200 0
5
10 15 Cutting distance (nm)
Processing force (nN)
(b) 800
20
25
Fc Ff
600
400 200
Initial contact distance
0
-200 0
5
Processing force (nN)
(c) 800
10 15 Cutting distance (nm)
20
25
Fc Ff
600 400 200
Initial contact distance
0 -200 0
5
10 15 Cutting distance (nm)
20
25
Fig. 8 Processing force (a) without hard particle and with (b) 6 nm, (c) 9 nm deep hard particle under the workpiece surface.
Processing force (nN)
800
Fc_average
Ff_average
600 400
494
434
379
375 291 208
200
90
101
0 No particle 4 nm depth 6 nm depth 9 nm depth
Cutting conditions
Fig. 9 Average processing force at different cutting conditions.
Table 1 Computational parameters used in MD simulations Cutting model
Model 1
Model 2
Material of hard ball
Diamond
Diameter of hard ball
8 nm
D of hard ball
4, 6, 9 nm
4, 5, 6, 8 nm
L of hard ball
9 nm
10 nm
Material of workpiece
Aluminum
Dimension of workpiece
50 nm × 16 nm × 20 nm
Material of cutting tool
Diamond
Tool edge radius 𝑟𝛽
5 nm
Rake angle
0°
Clearance angle
12.5°
Cutting direction
{110}<001>, -x direction
Uncut chip thickness
5 nm
Cutting speed
100 m/s
Initial temperature
293 K
60 nm × 16 nm × 20 nm