Elsevier

Computational Materials Science

Volume 126, January 2017, Pages 418-425
Computational Materials Science

Molecular dynamics simulation on formation mechanism of grain boundary steps in micro-cutting of polycrystalline copper

https://doi.org/10.1016/j.commatsci.2016.10.001Get rights and content

Highlights

  • Molecular Dynamics model was built to investigate the formation of grain boundary steps in micro-cutting of polycrystalline copper.

  • Sub-grains with transitional crystal orientations formed at the grain boundary due to plowing of the cutting edge and crystal rotation.

  • The misalignment in the slip directions between sub-grains and original grain resulted in the grain boundary step.

  • A peak of cutting force appeared at the grain boundary because of the geometrical hardening effect.

Abstract

Three-dimensional molecular dynamics (MD) simulations were performed to investigate the formation mechanism of grain boundary (GB) steps in the micro-cutting of polycrystalline copper. The effects of the GB and misorientation angle on the surface quality were studied. Based on the simulation results, the surface maximum peak-to-valley height of polycrystalline copper was greater than that of single crystal copper owing to the formation of grain boundary steps. The dislocations continuously nucleated on the tool-workpiece interface were stopped and piled up at the GB. As the dislocations piled up at the GB, the dislocations became aligned and formed the sub-grain boundary to minimize the total system energy. Sub-grains with transitional crystal orientations formed at the GB during the micro-cutting of polycrystalline copper for the plowing of the cutting edge and crystal rotation. The misalignment in the slip directions between sub-grains and original grain resulted in the grain boundary step. A peak cutting force appeared at the GB in the cutting of polycrystalline owing to the geometrical hardening effects. It is revealed that the GB has a strong effect on the surface quality of a workpiece during the micro-cutting process.

Introduction

In recent years, a highly efficient machining method called micro-cutting has been developed to produce high-accuracy miniaturized components, which are widely used in various industries, including electronics, aerospace, automotive, biomedical engineering and communications [1]. The cutting mechanism of micro-cutting is very different from that of conventional cutting because of size effects [2]. The schematic diagram of micro-cutting is shown in Fig. 1. Many materials cannot be regarded as homogeneous and isotropic in machining when the depth of cut is on the same order as the tool edge radius [3]. In addition, unlike single crystals, polycrystalline materials are composed of grains with different crystallographic orientations and grain boundaries, and the microstructure of a workpiece containing interstitial atoms, voids and dislocations has a strong effect on the cutting mechanism [4], [5].

The properties of grain boundaries have been extensively researched because of their significant effects on the mechanical behavior of materials [6], [7]. The classical Hall-Petch relationship is commonly used to describe the hardening effect of grain boundaries in single-phase polycrystalline materials. At the micrometer scale, it is a challenge to research the mechanical behavior of grain boundaries. It was found that the nanohardness in the vicinity of grain boundaries increased by a factor of 1.5 compared with nanohardness inside the grains via nanoindentation tests of annealed and electropolished high-purity copper [8]. Therefore, the elasticity modulus of grain boundaries could be higher than that of the grain. The frictional coefficient at grain boundaries is higher than that inside the grains based on molecular dynamics (MD) simulations of nanoscratching of bi-crystal copper [9].

The difference in mechanical properties between grain boundaries and grains has a significant influence on the mechanical behavior of materials during micro-cutting. The formation of grain boundary (GB) steps in micro-cutting of polycrystalline materials has been found by a few researchers. In the micro-orthogonal fly cutting of annealed polycrystalline oxygen-free copper with single-crystal diamond tools, grain boundaries became increasingly obvious on the free surface with increasing depth of cut. However, grain boundaries disappeared when the depth of cut was reduced to the order of 0.1 μm [10]. It was observed that the grain boundary step was approximately 20 nm high during the micro-cutting of polycrystalline germanium, which was attributed to the elastic deformation [11]. It was found that the height of grain boundary steps increased with increasing cutting speed in the ultra-precise cutting of beta titanium alloy [12]. Annealing treatment can magnify the difference in mechanical properties between grain boundaries and grains, thus contributing to the formation of grain boundary steps. During the micro-cutting of original polycrystalline oxygen-free copper and annealed copper with single-crystal diamond tools, the grain boundary step appeared on the machined surface of annealed copper, whereas it was absent on that of original copper, as shown in Fig. 2(a) and (b) [13]. It is notable that the twin boundary step was observed in partial enlarged view Fig. 2(c) using white light interferometer.

However, few efforts have been made to reveal the formation mechanism of the grain boundary step. Because it is difficult to observe the dynamic formation process of grain boundary steps in micro-cutting in situ using current experimental methods, simulations provide insights into the potential cutting mechanism and formation of grain boundary steps. Komanduri made a review of the MD simulations of machining at the atomic scale [14]. Ye et al. has performed MD simulations of the nanometric cutting of single-crystal copper were performed with the embedded atom method (EAM) potential [15]. Pei et al. performed large-scale MD simulations with the model size up to 10 million atoms to study materials deformation in nanometric cutting of single crystal copper [16]. The mechanics of nanometric cutting were investigated with the aid of MD modelling and simulations, which were compared with the results of cutting trials of single crystal silicon on an AFM [17], [18]. During cutting of single crystal and polycrystalline copper with a rigid diamond tool through MD simulations, smaller cutting forces were required to machine the polycrystalline structure than the single crystal structure [19]. Based on MD simulations results of scratching polycrystalline Cu for investigating plastic deformation mechanism, both dislocations and GB play a significant role in affecting plastic deformation [20]. Stress-induced crystallization of amorphous was observed in nano-cutting process of crystalline copper through MD simulations [21]. The effects of pore and second phase particle on the subsurface damage and surface integrity during machining pore copper materials were also investigated using MD simulations [22]. However, no obvious grain boundary steps were found in the simulation on cutting of polycrystalline copper. Shimada used MD simulations to study the ultimate surface quality of a workpiece in diamond micro-cutting of copper. In the cutting of polycrystalline copper, the dislocations were stopped and piled up at the GB; meanwhile, a sub-grain was generated by the rotation of the grain plane. A tiny grain boundary step was observed on the surface of polycrystalline copper; however, the height of the step was at the same level as that of single crystal copper [23]. MD simulations of micro-cutting of polycrystalline silicon carbide were performed to investigate the microstructure effects on cutting forces. The grain boundary step and variations of cutting forces were observed in the simulation, which could be attributed to elastic deformation [24]. However, elastic deformation cannot account for the peak at the GB as shown in Fig. 2(b). The real formation mechanism of the grain boundary step has not been fully revealed.

In this work, MD simulations were used to probe into the formation mechanism of the grain boundary step in micro-cutting of polycrystalline copper. This study will help explain the mechanical behavior and accommodation of grain and grain boundaries in microscale plastic deformation, especially the change of surface integrity in micro-cutting.

Section snippets

Methodology

Compared with other simulation techniques, such as the finite element method (FEM), MD simulation is an effective and promising tool to address many machining problems at the atomic scale [14]. MD simulations are performed using the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) developed by Plimpton [25]. Fig. 3 presents the MD simulation model of micro-cutting of polycrystalline copper. The model comprises a face-centered cubic (FCC) pure copper workpiece and a rigid

Micro-cutting of polycrystalline copper

A MD simulation of polycrystalline copper was performed to investigate the effects of grain boundaries on the surface quality and cutting forces. The model of the workpiece contained two grains and a symmetrical tilt grain boundary (STGB) of 29(520)43.60°/[001]. Because 29(520) STGB has a bigger misorientation angle, it helps to illustrate the effect of GB on micro-cutting of polycrystalline copper. The GB was far from the left end of the workpiece to reduce the interference of the fixed

Discussions

The experiments adopted Single Point Diamond Turning (SPDT) technique while the depth of cut was at the micrometer scale. However, due to the limitation of high computational cost, the model in this work is not as realistic as actual experiments. The depth of cut in simulations is at the nanometer scale (∼nm) rather than the micrometer scale (∼μm). The size of the workpiece in simulations was enlarged to approach the actual conditions in experiments. The formation of grain boundary steps is

Conclusions

In this paper, Molecular Dynamics (MD) simulations were conducted to investigate the formation process of grain boundary steps in micro-cutting of annealed polycrystalline copper. This study provides insights into the formation mechanism of grain boundary steps in micro-cutting of polycrystalline material. It also helps explain the mechanical behavior and accommodation of grains and grain boundaries in microscale plastic deformation, especially the change in surface integrity in micro-cutting.

Acknowledgements

This work was supported by National Natural Science Foundation of China (Project 51575305) and Beijing Natural Science Foundation (Project 3152013).

References (35)

  • J. Chae et al.

    Int. J. Mach. Tools Manuf.

    (2006)
  • D. Dornfeld et al.

    CIRP Ann. – Manuf. Technol.

    (2006)
  • Y. Furukawa et al.

    CIRP Ann. – Manuf. Technol.

    (1988)
  • Y. Soifer et al.

    Scr. Mater.

    (2002)
  • K. Sun et al.

    Wear

    (2013)
  • T. Moriwaki

    CIRP Ann. – Manuf. Technol.

    (1989)
  • J. Yu et al.

    Procedia Manuf.

    (2015)
  • K. Cheng et al.

    Wear

    (2003)
  • J. Li et al.

    Comput. Mater. Sci.

    (2016)
  • Y. Zhao et al.

    J. Non. Cryst. Solids

    (2016)
  • J. Li et al.

    Appl. Surf. Sci.

    (2016)
  • S. Shimada et al.

    CIRP Ann. – Manuf. Technol.

    (1994)
  • R. Komanduri et al.

    Wear

    (2000)
  • D. Brandon

    Acta Metall.

    (1966)
  • Q.X. Pei et al.

    Comput. Mater. Sci.

    (2007)
  • Q.X. Pei et al.

    Comput. Mater. Sci.

    (2006)
  • N.R. Tao et al.

    Acta Mater.

    (2002)
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