Accepted Manuscript Thermal, mechanical and chemical material removal mechanism of carbon fiber reinforced polymers in electrical discharge machining Xiaoming Yue, Xiaodong Yang, Jing Tian, Zhenfeng He, Yunqing Fan PII:
S0890-6955(18)30098-1
DOI:
10.1016/j.ijmachtools.2018.05.004
Reference:
MTM 3349
To appear in:
International Journal of Machine Tools and Manufacture
Received Date: 2 March 2018 Revised Date:
4 May 2018
Accepted Date: 17 May 2018
Please cite this article as: X. Yue, X. Yang, J. Tian, Z. He, Y. Fan, Thermal, mechanical and chemical material removal mechanism of carbon fiber reinforced polymers in electrical discharge machining, International Journal of Machine Tools and Manufacture (2018), doi: 10.1016/j.ijmachtools.2018.05.004. 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.
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Thermal, mechanical and chemical material removal mechanism of carbon fiber reinforced polymers in electrical discharge machining Xiaoming Yue1, 2, Xiaodong Yang1, 2*, Jing Tian1, 2, Zhenfeng He1, 2, Yunqing Fan1, 2
Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of
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Education, Harbin Institute of Technology, Harbin, 150001, China
Department of Mechanical Engineering and Automation, Harbin Institute of Technology, Harbin, 150001, China
Corresponding author: Xiaodong Yang
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E-mail:
[email protected]
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Thermal, mechanical and chemical material removal mechanism of carbon fiber reinforced polymers in electrical discharge machining Abstract:
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Electrical discharge machining (EDM) has been proved to be a feasible way to machine the carbon fiber reinforced polymers (CFRPs); therefore, understanding the material removal mechanism of CFRPs by EDM is significantly important for optimizing the machining process and improving the machining surface quality. However, very few researches to reveal the above mechanism exist. To address this
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issue, this paper investigated on the material removal mechanism of CFRPs by EDM from thermal, mechanical and chemical respects. The heat conduction analysis
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(thermal aspect) was carried out to study the temperature distribution during the discharge process of CFRPs, which shows that CFRPs could be removed through the thermal decomposition and vaporization of the epoxy resin and the sublimation of the carbon fibers under ultra-high temperature caused by the plasma heat and Joule heat. On the other hand, a high-speed camera with a laser bandpass filter added in front of the camera lens to filter out the plasma was used to directly observe the discharge
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process of CFRPs without the interfere of the plasma, which shows that massive high-speed jets were emitted into the gap in the discharge process, producing the jetting force (mechanical aspect). Specifically, when the discharge occurred on the parallel surface of CFRPs, the carbon fibers were crashed by the jets whose motion
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direction was vertical to the arrangement direction of the carbon fibers, then removed by the jetting force. However, when the discharge occurred on the vertical surface, the jetting force was not the main way of the material removal of CFRPs. Moreover, the
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experiments were carried out in both deionized water and EDM oil to investigate the chemical removal mechanism. It was found that in the discharge of CFRPs with the oxygen environment, the oxidation reaction could not only generate extra heat but also enhanced the jetting force, thus, reaching a much higher material removal rate. Keywords: Carbon fiber reinforced polymers (CFRPs), Electrical discharge machining (EDM), Material removal mechanism, Heat conduction, Joule heat, Jetting force, Oxidation reaction.
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1. Introduction With the excellent material properties (e.g. high strength and stiffness to weight ratio, high corrosion and fatigue resistances, etc.), carbon fiber reinforced polymers (CFRPs) have been widely used in many fields, such as aerospace, automotive, civil engineering, sports goods. However, as an inhomogeneous and anisotropic material,
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the polymer matrix of CFRPs is soft while the carbon fibers are hard and of high strength, which makes it difficult to machine with conventional cutting methods. Thus, it meets great challenge to machine CFRPs with a low manufacturing cost and high quality.
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There are mainly two categories of methods to machine the CFRPs, including the conventional processes (e.g. milling, drilling, turning, etc.) and unconventional processes (e.g. water-jet machining, EDM, etc.). Milling is always used to machine
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complex shapes and surfaces of CFRPs components, however, the interaction between the end mill and CFRPs would cause the generation of the delamination and burrs with uncut fiber yarns [1]. Although different methods, such as an inclination milling with high helix angle end mill [2], were proposed to improve the surface integrity of machined surface of CFRPs, this problem still could not be solved. Drilling and turning are the principal operation for making holes and cylindrical surfaces in CFRPs
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components, respectively. But, they also faces two major unsolved problems: severe tool wear [3, 4] and poor surface quality [5, 6]. In summary, traditional machining techniques inevitably bring significant surface integrity problems and severe tool wear. Compared with traditional mechanical machining methods, non-traditional machining
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methods like water-jet machining have received considerable attentions due to their unique advantages in the machining of CFRPs, such as higher cutting speeds, smaller
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cutting force, lower tool wear and machining costs. However, although the water-jet machining has demonstrated high potential to effectively machine CFRPs, it still has some drawbacks, such as the delamination caused by the moisture absorption or the high-velocity jets impact, especially when cutting deep features like holes and slots [7, 8]. Another disadvantage of the water-jet cutting is that it cannot be applied for machining CFRPs components with complex shapes and freeform surfaces. Moreover, in the new applications of CFRPs, such as precision electrical components, surgical tools and biomedical devices which are always micro components, the water-jet machining cannot meet the dimensional requirements due to its natural limitation of the jet and abrasive size. 2
ACCEPTED MANUSCRIPT Electrical discharge machining (EDM) is another non-traditional machining method which can machine any electrically conductive materials regardless of their hardness with its removal mechanism of melting and evaporating of workpiece materials. Furthermore, by using EDM, the materials can be machined precisely into complicated shapes with low cutting force. Thus, EDM shows great potential to
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machine CFRPs with higher precision, lower costs and tool wear. Since Lau et al. [9] verified the feasibility of using EDM as a means of machining carbon fiber composite materials, several researches [10-12] on the machining characteristics of CFRPs using EDM have been carried out in the past. Furthermore, dry EDM technique [13, 14] was innovatively applied for the removal of burrs in drilled holes in CFRPs. Recently,
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researches on micro EDM of CFRPs [15, 16] were also carried out. However, until now, most literatures about the EDM of CFRPs focused on the research of processing
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tests, whereas the material removal mechanism of CFRPs in EDM has not been comprehensively investigated, which seriously hinders its further application and development in the EDM of CFRPs.
Therefore, this paper aims to clarify the material removal mechanism of CFRPs in EDM. To achieve the above goal, a high-speed camera with a laser bandpass filter added in front of the camera lens to filter out the arc plasma is used to directly
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observe the discharge process of CFRPs without the interfere of bright light from plasma, then, combined with the heat conduction simulation of discharge process of CFRPs, three mechanisms are proposed (i.e. thermal removal mechanism, mechanical removal mechanism, chemical removal mechanism). Specifically, section 2 analyzed
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the temperature distribution of CFRPs in the discharge process to reveal the thermal removal mechanism in the EDM of CFRPs and the influence of the anisotropy of CFRPs on the machining characteristics of CFRPs. Section 3 showed the discharge
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process of CFRPs through a high-speed camera, which verified that the jetting force served as the mechanical removal mechanism in the EDM of CFRPs. Section 4 compared the EDM of CFRPs in the deionized water and EDM oil to validate the chemical removal mechanism in the EDM of CFRPs.
2. Thermal analysis of CFRPs in discharge process CFRPs are composite materials consisting of two parts: a reinforcement and a matrix. In CFRPs, the reinforcement is the carbon fibers, which provides the strength. The matrix is usually the polymer resin, such as the epoxy resin, to bind the reinforcement together. Manufacturing of CFRPs involves two distinct processes: the 3
ACCEPTED MANUSCRIPT first is the process whereby the carbon fibers are manufactured into the fabric preforms through the textile processing techniques like the plain weave, as shown in Fig. 1(a), by which a fabric that contains the carbon fibers oriented with angles other than 0° and 90° to each other respectively is produced, as shown in Fig. 1(b); the second is the process whereby the fabric preforms are bonded with the matrix during
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moulding, as shown in Fig.1(c). EDM is a thermal machining process whose material removal is based on melting and/or evaporating the material. However, the carbon fibers are highly anisotropic. The thermal properties of the carbon fibers parallel to the fiber axis and perpendicular to the fiber axis are significantly different according to [17]. In the EDM of CFRPs, discharges occurring on the surface vertical to the
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carbon fibers, as shown in Fig. 1(d) will be significantly different from the discharges on the surface perpendicular to the carbon fibers, as shown in Fig. 1(e). Thus, in this
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section, discharges on the surfaces parallel and vertical to the carbon fibers, which are referred to as parallel surface and vertical surface respectively in the followings, were investigated by the finite element method (FEM) simulation and experiments to reveal the influence of the anisotropy of the carbon fibers on the EDM of CFRPs. In addition, the influence of the Joule heat generated inside CFRPs due to the discharge current on the temperature distribution around the discharge spot was also
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investigated through the simulation and experiment.
Fig. 1. Schematic diagram of CFRPs manufacturing and definition of discharge 4
ACCEPTED MANUSCRIPT surface. (a) Illustration of plain weave; (b) photograph of carbon fiber fabric; (c) illustration of CFRPs; (d) surface vertical to carbon fibers (vertical surface); (e) surface parallel to carbon fibers (parallel surface). 2.1 Computational algorithm and details 2.1.1 Simulation model and boundary conditions
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Fig. 2 shows the simulation model of CFRPs consisting of the carbon fibers and epoxy resin whose material properties are listed in Table 1. Theoretically, CFRPs are an insulator due to the nonconductive matrix. However, practically, CFRPs show a certain electric conductivity, which proves that the carbon fibers inside CFRPs are not totally isolated electrically by the nonconductive epoxy resin but contact each other to
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provide the electric conductivity verified by Hirano [18]. Thus, for simplicity, the carbon fibers with the diameter of 8 µm in CFRPs are considered to be distributed
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uniformly and contact each other to provide the electric conductivity. The simulation model with the discharge on the parallel surface is 400 µm long (Y axis) and 200 µm wide (X axis) and tall (Z axis), as shown in Fig. (2)a, and the size of the model with the discharge on the vertical surface is 200 µm × 200 µm × 200 µm (X × Y × Z axis), as shown in Fig. (2)b. The circular area with a time-dependent diameter d(t) at the center of the discharge surface of the workpiece is the energy input region where the
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current density j and heat flux q are exerted, as shown in Fig. 2(c). Except for the energy input region, all the other surfaces are considered to exchange heat with air (air temperature T=300K, convective heat exchange coefficient hc=15 W/(m2·K)). The electrical potential V on the bottomed surface of the workpiece is set as zero (ground
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level).
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Fig. 2. Schematic diagram of simulation model. (a) simulation model with discharge on parallel surface, (b) simulation model with discharge on vertical surface, (c)
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boundary conditions applied in model. Table 1. Material properties used for simulation according to [17, 19-22].
Electricity resistivity [Ω cm] Thermal conductivity [W m-1 K-1]
Carbon fiber 1.53×10-3 (along fiber axis) -2
1.53×10 (vertical to fiber axis) 50 (along fiber axis) 5 (vertical to fiber axis)
Specific heat [J/(Kg K)]
Epoxy resin 1016~1017 0.2
1884
710
Density [Kg m ]
1850
1250
Decomposition temperature [K]
----
693
Boiling point [K]
----
800
Sublimation point [K]
3923
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-3
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(2) In the simulation of discharge process of CFRPs, only the heat conduction process of CFRPs is considered, while the material removal process of CFRPs caused by the melting and vaporization is not taken into account.
(3) In practice, the contact of carbon fibers in CFRPs is very complex, and it is difficult to know the heat transfer coefficient between carbon fibers, because it
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depends significantly on the contact conditions, such as contact pressure, surface geometry. Thus, in this simulation, the contact of the carbon fibers is simplified as
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one node, as shown in Fig. 3.
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ideal line contact. That is, in the element meshing, the adjacent carbon fibers share
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Fig. 3. Element meshing of simulation model 2.1.3 Calculating equations and conditions for discharge energy input In this simulation, both the Joule heat effect and plasma heat source caused by the
discharge were considered. Thus, in the followings, the calculating equations for the Joule heat effect and plasma heat source were explained: (1) Measurement of diameter of arc plasma d ( t ) and discharge current ie ( t ) In the simulation of discharge process, the diameter of the arc plasma and discharge current are the most important input parameters. Therefore, before introduction of the calculating equations of the Joule heat and plasma heat source, experimental measurement of the diameter of arc plasma d ( t ) and discharge current ie ( t ) were explained firstly, as shown in the followings. In the past, several researchers proposed different mathematic models of the 7
ACCEPTED MANUSCRIPT expanding arc plasma. For example, Saito et al. [23] gave a plasma model related to the discharge current and time, as shown in [24]. Ikai et al. [25] presented a mathematic model of the arc plasma only related to the time, as shown in [26].
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However, there is a great difference among the above mathematic models. This is because the plasma diameter d(t) depends on the discharge conditions, such as the gap width, pulse conditions, discharge current, dielectric liquid and electrode materials, etc., and the above equations were obtained under different discharge conditions. Thereby, so far, there is not yet a totally correct model to describe the expanding arc plasma. To measure the plasma diameter, the plasma in a single pulse discharge was observed using a high-speed camera under the discharge conditions shown in Table 2. In order to ignite a single discharge easily, a very small amount of deionized water
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was placed in the gap. Limited by the observation capacity of the high-speed camera in our laboratory, the expanding process of the plasma cannot be observed totally, but the observation result shows that the plasma diameter was about 120 µm at t=20 µs, as shown in Fig. 4(a). Thus, for simplicity, in this simulation, a linear expanding arc plasma with the initial diameter of 5 µm at t=0 µs and the ending diamter of 120 µm
observation. Tool electrode
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at t=20 µs was used as the time-dependent diameter of the arc plasma d ( t ) . Fig. 4(b) shows the measured discharge current ie ( t ) . The measured d ( t ) and ie ( t ) will be used as the input parameters in the simulation. Table 2. Experimental conditions used in current measurement and plasma Copper (1 mm in diameter)
Workpiece
CFRPs
Discharge surface
110
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Open voltage [V]
Parallel surface of CFRPs
Discharge peak current [A] 9
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Gap distance [µm] Polarity
40 Workpiece (+)
Fig. 4. (a) Observation of plasma at t=20 µs (exposure time=3 µs, aperture=2.8, frame 8
ACCEPTED MANUSCRIPT rate=50000 fps) and (b) discharge current. (2) Joule heat effect The Joule heat is produced due to the passage of an electric current through a conductor. To simulate the above process, a discharge current ie was applied on the discharge surface of the workpiece in the form of the current density j, which was
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explained in simulation model and conditions in section 2.1.1. For simplicity, the current density j inside the arc column was assumed to be uniform in the whole discharge area according to the research by Zhao [24]. The theoretical calculation for the current density j was given by:
ie (t) π (d (t ) / 2)2
(1)
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j=
where ie ( t ) and d ( t ) are the discharge current and the time-dependent diameter of
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the arc plasma respectively measured in single pulse discharge experiment. (3) Plasma heat source
In the simulation of discharge, a circular Gaussian heat source was widely used to imitate the heat flux from the arc column [27, 28]. Thus, in this paper, a Gaussian heat source with a time-changing diameter was also used, which was described by the following equation:
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−kr 2 q( r, t ) = qm (t ) exp( 2 ) d (t ) 4
(2)
where r is the distance from the center of the arc column, q(r, t) is the heat flux
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density at radius r and time t, k=4.5 is the heat source concentration factor, qm (t) is the heat flux density at the center of the arc column at time t. The calculating process
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of the qm (t) was shown as follows. In Eq. (2), the heat flux density qm (t) at the center of the arc column can be
obtained by calculating the discharge power Q(t), as shown in Eq. (3). Q ( t ) = η u e ( t ) ie ( t ) =
∫
d ( t )/2
0
q ( r , t )2π rdr
(3)
where η = 0.4 is the ratio of the discharge energy distributed into the anode based on the research by Xia [29], ue (t ) is the discharge voltage, which equals to 18 V, Therefore, based on the Eq. (3), the heat flux density qm (t) at the center of the arc column can be expressed in the following equation: 9
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4.57ηue (t )ie (t ) π d 2 (t ) 4
(4)
2.1.4 Calculation of Joule heat Generally, in the EDM simulation of metal materials [30], such as copper and steel, etc., the Joule heat effect is ignored due to their significant small resistance. However,
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in the EDM of the high-resistivity materials, the Joule heat effect can be comparable to the energy input from the plasma during the discharge process according to the research by Rich et al. [31] and Zhao et al. [24]. In CFRPs, the epoxy resin is an insulator while the carbon fiber is the high-resistivity material according to the
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electric resistivity shown in Table 1. Moreover, the contact of the carbon fibers in CFRPs can cause significant higher contact resistance [18]. Thereby, in the EDM of CFRPs, the Joule heat due to the high electric resistivity may have significant
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influence on the temperature distribution around the discharge spot and the material removal. Thus, In order to reveal the above influences, the Joule heat caused by the discharge current was also calculated in the EDM simulation of CFRPs. The calculating process of the Joule heat generated inside the workpiece per unit time due to the electric current can be expressed by the following equation:
P= J ⋅E
(5)
intensity.
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where J is the current density (current per unit area) and E is the electric field In Eq. (5), the current density and electric field intensity can be obtained based on the Maxwell’s equation of the charge conservation described by Eq. (6) and Ohm’s
∫ J ⋅ ndS = ∫ S
V
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law shown in Eq. (7) rc dV
(6)
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where V is any control volume whose surface is S , n is the outward normal to
S , rc is the internal volumetric current source per unit volume.
J =σE ⋅ E
(7)
E where σ is the electrical conductivity matrix.
2.1.5 Calculation of temperature distribution The temperature distribution inside the workpiece can be described by calculating the heat conduction equation, as shown in the following:
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ρ cp
∂T − ∇ ( k∇T ) = qv + qs ∂t
(8)
where T is the temperature of the material; ρ is the density of the material; c p is the heat capacity of the material; k is the thermal conductivity matrix; q v is the Joule
flowing into the body. 2.2 Thermal analysis results 2.2.1 Influence of Joule heat on temperature distribution
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heat generated within the body; and q s is the heat flux per unit area of the body,
In order to investigate the influence of the Joule heat caused by the discharge
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current on the material removal and temperature distribution around the discharge spot, a heat conduction analysis was performed, in which only the current density j
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was applied on the discharge surface of the workpiece, as described in section 2.1. Fig. 5 shows the calculation result of the temperature distribution of 1/4 model at t=20 µs when the current density j was applied on the parallel surface of CFRPs. From the figure, it can be found that the highest temperature at the discharge spot increased to more than 4000 K at t=20 µs. According to the thermogravimetric and differential thermal analysis by Ohkubo et al. [19], when the temperature is above 800K, the
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epoxy resin can be oxidized (in the oxygen environment), thermally decomposed (420K) and vaporized (800K). Furthermore, according to the thermogravimetric analysis by Negarestani et al. [20], when the CFRPs were heated in air under the temperature of more than 1150K, 96% weight of CFRPs was lost due to the oxidation
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of the carbon fibers, and the carbon fibers can be sublimated with the temperature more than 3923K. Thus, it can be guessed that, in the practical discharge process of
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CFRPs, the epoxy resin and carbon fibers around the discharge spot can be removed due to the heat contributed by the Joule heat. In order to verify the material removal caused by the Joule heat, an observation experiment was conducted by using a high-speed camera. A copper tool electrode with the tip diameter of 50 µm touched tightly on the parallel surface of CFRPs, as shown in Fig. 6(a). Then, a 10 A current with the pulse duration of 0.5 ms passed through the tip tool electrode and CFRPs. A large amount of smog around the contact region was observed, as shown in Fig. 6(b). Thus, it can be concluded that in the practical discharge process, the Joule heat generated by the discharge current can cause the material removal.
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Fig. 5. Temperature distribution of 1/4 model caused by Joule heat with current density j applied on parallel surface (t=20 µs). Dotted rectangular shows temperature
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distribution on surface of y=0.
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Fig. 6. Observation of smog caused by Joule heat (exposure time=300 µs, aperture=6.7, frame rate=3000 fps).
2.2.2 Influence of CFRPs anisotropy on temperature distribution Given that the carbon fiber is highly anisotropic, the heat generated in the discharge
process will have great influence on the topography of the discharge crater. Thus, in this section, a heat conduction analysis was carried out to investigate the influence of the CFRPs anisotropy on the temperature distribution, in which both the plasma heat source and Joule heat effect were taken into consideration. Fig. 7 shows the calculation results of the temperature distribution of 1/4 model at t=20 µs with discharges on the parallel surface (a) and vertical surface (b). From the figure, it can 12
ACCEPTED MANUSCRIPT be found that compared with the discharge on the parallel surface, when the discharge was ignited on the vertical surface, the heat was transferred much more deeply along the depth direction (Z axis) while the range of the heat transfer along X and Y axis was much smaller. This is because the heat can be transferred much more easily along the axial direction of the carbon fiber than the radial direction due to much larger axial
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thermal conductivity of the carbon fiber, as shown in Table 1. As a result, the temperature distribution caused by the discharge on the parallel surface presented an elliptical shape while the temperature distribution on the vertical surface was circular, as shown in Fig. 8. In CFRPs, both the thermal conductivity of the epoxy resin and the radial thermal conductivity of the carbon fiber are quite small, as shown in Table 1.
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Thereby, in the discharge process, the heat at the discharge spot cannot be transferred inside the electrode easily, causing the heat concentration and temperature rise around
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the discharge spot. The simulation results show that the highest temperature at the discharge spot far exceeded the sublimation point of the carbon fiber. In the practical machining process, under such extremely high temperature, CFRPs can be removed through the oxidation (in the oxygen environment), thermal decomposition, vaporization of the epoxy resin and the oxidation, sublimation of the carbon fiber. In order to verify the above simulation results, single discharges on the parallel surface
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and vertical surface of CFRPs were ignited with the discharge conditions shown in Table 3. In order to ignite a single discharge easily, a very small amount of deionized water was placed in the gap. The experimental results show that the discharge crater generated on the parallel surface of CFRPs presented an elliptical shape while the
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discharge crater generated on the vertical surface of CFRPs was circular, as shown in Fig. 9, which were consistent with the simulation results shown in Fig. 8. From the above analysis, it can be known that since the anisotropy of CFRPs has influence on
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the single discharge crater, it must also have influence on the machining characteristics of CFRPs in the continuous discharges. Thus, the machining of CFRPs on different surfaces were investigated in wire EDM (WEDM) in the followings.
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Fig. 7. Temperature distribution of 1/4 model with discharges on parallel surface (a)
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and vertical surface (b) (t=20 µs). Dotted rectangular shows temperature distribution on surface of y=0.
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surface (a) and vertical surface (b) (t=20 µs).
Fig. 9. Photograph of discharge craters generated by single discharges on parallel surface (a) and vertical surface (b).
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Table 3. Experimental conditions used to verify simulation results. Copper (1 mm in diameter)
Workpiece
CFRPs
Discharge duration [µs]
20
Open voltage [V]
110
Discharge peak current [A]
5
Polarity
Workpiece (+)
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Tool electrode
2.3 Influence of CFRPs anisotropy on machining characteristics The influence of CFRPs anisotropy on the machining characteristics in the continuous discharges was investigated by WEDM, as shown in Fig. 10, under the
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experimental conditions shown in Table 4. Fig. 11 shows the comparison of the machining speed of CFRPs with WEDM on the parallel surface and vertical surface.
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It was found that the machining speed of CFRPs on the vertical surface was significantly higher than that on the parallel surface, which can be explained as follows. When the machining direction is along the axis of the carbon fiber (machining on the vertical surface), the heat can be transferred along the machining direction easily, as shown in Fig. 10(b), causing higher machining speed on the vertical surface. Furthermore, the influence of the CFRPs anisotropy on the kerf was also investigated with WEDM on the mixed surface, as shown in Fig. 10(c). Fig. 12 shows the 3D topography of the kerf measured by the VHX-5000 3D microscope. The 3D topography results shows that the side face of the kerf presented a wavy shape. The width of the kerf generated on the parallel surface, such as the kerf width d1, was 15
ACCEPTED MANUSCRIPT larger than that generated on the vertical surface, such as the kerf width d2. This is also because when the width direction of the kerf is along the carbon fiber (machining on the parallel surface), the heat can be transferred along the width direction of the kerf easily, causing much larger kerf width. Table 4. Experimental conditions used in WEDM. Brass wire (200 µm in diameter)
Workpiece
CFRPs (2 mm in thickness)
Pulse width [µs]
0.6~2.8
Open voltage [V]
80
Discharge peak current [A]
7.5
Servo reference voltage [V]
35
Dielectric fluid
Deionized water
Polarity
Workpiece (+)
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Wire electrode
Fig. 10. Schematic diagram of cutting directions with WEDM on parallel surface (a), vertical surface (b) and mixed surface (c).
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Fig. 11. Comparison of machining speeds of CFRPs on parallel surface and vertical
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surface.
Fig. 12. Topography of kerf machined by WEDM on mixed surface.
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3. Observation of material removal of CFRPs caused by jetting force 3.1 Experimental equipment and conditions In this section, the gap phenomena and material removal process of CFRPs in
single pulse discharge process were observed by a black and white high-speed camera to reveal another material removal mechanism of CFRPs in EDM. Fig. 13 shows the experimental device used to observe the discharge process of CFRPs. A single pulse discharge under the experimental conditions shown in Table 5 was ignited between the CFRPs and a rod tool electrode of diameter 1 mm. The end surface of the tool electrode was polished to obtain a spherical surface of diameter 1mm to generate a discharge at the center of the rod. In order to ignite the discharge easily, a very small 17
ACCEPTED MANUSCRIPT amount of deionized water was placed in the gap. After the discharge was ignited, the water was blown away totally, then the discharge happened in the air. Generally, discharge phenomena between electrodes during the discharge process cannot be directly observed by the high-speed camera due to the extremely bright plasma in the gap. Thereby, in this observation device, a laser bandpass filter with the bandwidth of
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810 ± 10 nm was added in front of the camera lens to filter out this extremely bright plasma which is visible light with the wavelength of 380~780 nm. Then, a pulsed high power diode laser (CAVILUX HF, Cavitar Ltd.) which can emit invisible laser light at 810 ± 10 nm was used together with the laser bandpass filter to provide the illumination for the camera. Thus, the discharge phenomena inside the electrodes gap
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can be observed clearly. On the other hand, the bubble generated in the single pulse discharge process was also observed using the experimental device shown in Fig. 14.
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A transparent plate whose position was adjusted by a feeler gauge (60 µm) was placed above the CFRPs. The single pulse discharge was ignited between the CFRPs and tool electrode with the same experimental conditions presented in Table 5. Then, the bubble generated in deionized water in the single pulse discharge process between the transparent plate and CFRPs was overlooked by a high-speed camera. It should be noted that limited by the frame rate and resolution of the high-speed camera, in these
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observation experiments, much longer discharge time and larger discharge current were used so that the discharge phenomena, such as the removed debris, could be
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observed easily and clearly.
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Fig. 13. Schematic diagram of experimental device used to observe discharge process.
Fig. 14. Schematic diagram of experimental device used to observe bubbles.
Table 5. Experimental conditions used in the observation experiments. Tool electrode
Copper (1 mm in diameter)
Workpiece
CFRPs
Dielectric
Air
Open voltage [V]
110 19
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15
Polarity
Workpiece (+)
3.2 Observation results Fig. 15 shows the direct observation of the discharge process of CFRPs on parallel surface without using the laser bandpass filter and laser illumination. The discharge
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conditions were shown in Table 5. From the figure, it can be found that in the discharge process of CFRPs, a large amount of materials were emitted from the discharge spot of CFRPs into the gap at a very high speed. According to the previous research in section 2.2, in the discharge process of CFRPs, CFRPs can be removed through the oxidation, thermal decomposition, vaporization of the epoxy resin and the
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oxidation, sublimation of the carbon fibers under the extremely high temperature. Thus, it was guessed that the above observed materials were the gaseous mixtures
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from the oxidation, thermal decomposition and vaporization of the epoxy resin and the oxidation, sublimation of the carbon fibers. These emitted materials, which looked like a funnel shown by the red arrows, were called jets in the following sections. In order to verify the above guess, the bubble generated in the deionized water in the discharge process of CFRPs was observed. This is because if the observed jets were massive gaseous materials, it must have significant influence on the formation of
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bubble with the discharge in dielectric liquid.
Fig. 15. Observation of discharge process of CFRPs on parallel surface without using laser bandpass filter and laser illumination (exposure time=3 µs, aperture=4.8, frame rate=50000 fps). 20
ACCEPTED MANUSCRIPT Generally, in the discharge process of metallic materials, the bubble is mainly generated from the vaporization and dissociation of the dielectric liquid. According to the research by Hayakawa et al. [32], in the discharge of metallic materials, at first, the largest diameter of the bubble increased with increasing the discharge duration because much more discharge energy was used to vaporize and dissociate the
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dielectric liquid. But, when the discharge duration increased to a certain value, such as 100 µs, the largest diameter of the bubble didn’t increase with increasing the discharge duration because there was no dielectric liquid except the bubble around the arc plasma when the discharge duration increased to a certain value, as shown in Fig. 16.
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Fig. 17 shows the forming process of bubble in the discharge of CFRPs in deionized water. The experimental conditions were the same with these in Table 5
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except the gap dielectric. From the figure, it can be found that the bubble diameter increased along with the discharge time. Moreover, the largest diameter of the bubble always increased with increasing the discharge duration, even to several milliseconds, as shown in Fig. 18. The fact that the bubble expanded continuously even if the discharge happened in the bubble without the dielectric liquid around the plasma, as shown in Fig. 17, verified that in the discharge of CFRPs, a large amount of gaseous
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materials from CFRPs were being emitted into the gap continuously, causing the continuous expansion of the bubble. Thus, in the discharge of CFRPs, the bubble is generated not only from the vaporization and dissociation of the dielectric liquid, but also from these massive jets which take up most of the bubble volume. Thus, the
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observation of bubble verified that in the discharge of CFRPs, a large amount of gaseous jets were emitted into the gap. Since in the discharge process of CFRPs, massive gaseous jets are emitted into the gap at a very high speed, which may have
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great influence on the material removal of CFRPs. Thereby, next, the influence of these jets on the material removal of CFRPs was analyzed.
Fig. 16. Comparison of diameter of bubble and plasma with long discharge time
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Fig. 17. Observation of bubble generated in deionized water in discharge of CFRPs
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(discharge duration=1 ms, exposure time=300 µs, aperture=6.7, frame rate=3000 fps).
Fig. 18. Bubble diameter variation with discharge durations in discharge of CFRPs. Fig. 19 shows the material removal of CFRPs with the discharge on the parallel surface, in which the extremely bright plasma was filtered out by the laser bandpass filter so that the material removal process could be observed. The discharge conditions were shown in Table 5. From the figure, it can be found that in the discharge of CFRPs on the parallel surface, lots of long and broken carbon fibers segments were flying from the CFRPs. It was guessed that two possible reasons 22
ACCEPTED MANUSCRIPT caused the removal of carbon fiber debris: (ⅰ) the epoxy was removed first due to its low melting and boiling point, then, the exposed carbon fiber was blown out by the extremely high temperature; (ⅱ) due to the extremely high temperature, massive gaseous materials (jets) were generated in a very short time and very narrow space, causing the sharp rise of the pressure in the discharge spot. As a result, massive jets exposed carbon fiber was removed by the jetting force.
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were emitted into the gap at a very high speed, generating the jetting force. Thus, the In order to reveal the removal mechanism of the carbon fiber debris, the discharge experiment on the parallel surface and vertical surface of CFRPs was conducted. This is because if the removal mechanism is (ⅰ), there will not be significant difference
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between the removal of carbon fiber with the discharge on the parallel surface and vertical surface of CFRPs; but, if the removal mechanism is (ⅱ), due to the
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significant difference of tensile strength and bending strength of the carbon fiber, the removal of carbon fiber caused by the jetting force with the discharge on the parallel surface and vertical surface of CFRP will be significantly different. The comparison of removal of carbon fiber with the discharge on the parallel surface and vertical surface of CFRPs was shown in the followings.
Figs. 19 and 20 show the removal of the carbon fiber debris with the discharge on
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the parallel surface and vertical surface respectively. From these figures, it can be found that there is a significant difference between the removal of carbon fibers with the discharge on the parallel surface and vertical surface of CFRPs. Specifically, there was a large amount of carbon fiber debris removed from CFRPs with discharge on the
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parallel surface of CFRPs, while just a little carbon fiber debris with the discharge on the vertical surface of CFRPs. Thus, the above observed result verifies that the jetting force serves as the mechanical removal mechanism in the discharge of CFRP, which
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can be explained as follows.
In the discharge of CFRPs on the parallel surface, a large amount of epoxy resin
around the discharge spot is removed first, leaving the uncovered carbon fibers. Then, these uncovered carbon fibers are crashed by the high-speed jets whose motion direction is vertical to the arrangement direction of the carbon fibers. Because the bending strength of the carbon fiber is extremely low, the carbon fibers are very easy to fracture and break under the impact blow of the high-speed jets, and broken carbon fiber segments are blew away along the jetting direction. Thus, it is thought that the jetting force serves as the mechanical removal mechanism of CFRPs when the discharge occurs on the parallel surface, which can be described by Fig. 21. When the 23
ACCEPTED MANUSCRIPT discharge is ignited on the vertical surface of CFRPs, the motion direction of the high-speed jets is parallel to the arrangement direction of the carbon fibers, as shown in Fig. 22. Because the tensile strength of the carbon fiber is extremely high, the carbon fibers are not easy to fracture and break under the jetting force along the axial direction of the carbon fibers and there is no large amounts of carbon fiber debris
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removed from CFRPs. Thus, it’s thought that when the discharge occurs on the vertical surface of CFRPs, the jetting force generated by the jets is not the main way of the material removal of CFRPs.
To sum up, the above evidence can prove that the removal of carbon fiber in the
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discharge of CFRPs is caused by the jetting force.
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Fig. 19. Observation of discharge process of CFRPs on parallel surface using laser bandpass filter and laser illumination (exposure time=0.3 µs, aperture=4.8, frame
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rate=50000 fps).
Fig. 20. Observation of discharge process of CFRPs on vertical surface using laser 24
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rate=50000 fps).
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Fig. 21. Schematic diagram of jets distribution in gap and carbon fiber removed by
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jetting force with discharge on parallel surface.
Fig. 22. Schematic diagram of jets distribution in gap with discharge on vertical surface.
4. Oxidation reaction in discharge of CFRPs EDM is undeniably a thermal process. However, it is also acknowledged that EDM is a chemical process, because the extremely high temperature of the arc plasma in the discharge process can not only vaporize the electrode materials and dielectric liquid but dissociate and ionize their molecules and atoms [33]. Also, some publications showed higher material removal rate of water-based dielectrics [34], because water is 25
ACCEPTED MANUSCRIPT dissociated by discharge to hydrogen and oxygen, resulting in oxidation of workpiece materials. Furthermore, CFRPs are mainly composed of the carbon, hydrogen and oxygen elements, thus, under the extremely high temperature, CFRPs can be oxidized easily in the oxygen environment. Thereby, in this section, the influence of the oxidation reaction on the material removal of CFRPs in the single pulse discharge and 4.1 Experimental equipment and conditions
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continuous discharges was investigated. Fig. 23 shows the experimental setup used to compare the machining speed of CFRPs in EDM oil and deionized water. The electrostatic induction power supply developed by Kunieda [35] was used due to its excellent machining performance
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compared with the conventional relaxation pulse generators. The drilling hole experiment was carried out with the experimental conditions shown in Table 6. In
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each machining experiment, the machining time was 5 minutes. The mass of the CFRPs workpiece before and after machining were weighed by a high-precise electronic weigher whose accuracy was 1 mg, by which the machining speed of
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CFRPs was calculated.
Fig. 23. Schematic diagram of experimental equipment used to compare machining speed of CFRPs in deionized water and EDM oil.
Table 6. Experimental conditions used to compare machining speed of CFRPs Amplitude [V]
Power supply
Frequency [kHz] Duty factor [%]
200 5, 15, 30 20
Tool electrode
Copper (2 mm in diameter)
Workpiece
CFRPs (+) 26
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1
Resistance R [Ω]
150
Dielectric fluid
Deionized water, EDM oil
Servo reference voltage [V]
20
Copper (1 mm in diameter)
Workpiece
CFRPs
Discharge surface
Parallel surface of CFRPs
Open voltage [V]
110
Discharge duration [ms]
1
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Tool electrode
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Table 7. Experimental conditions used to compare topography of discharge crater.
Discharge peak current [A] 7
Deionized water, EDM oil
Gap distance [µm]
40
Polarity
Workpiece (+)
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Dielectric
4.2 Influence of oxidation reaction on material removal process In the discharge process, deionized water is dissociated into the oxygen gas and hydrogen gas while the EDM oil is dissociated into the gases without the oxygen gas.
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Thus, the discharges in deionized water and EDM oil were compared to investigate the influence of the oxygen gas on the material removal of CFRPs. Fig. 24 shows the comparison of the topography of the discharge craters generated in deionized water and EDM oil in single pulse discharges ignited on the parallel surface of CFRPs under
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the experimental conditions shown in Table 7. It was found that both the major axis and depth of the elliptic discharge craters generated in deionized water were larger than those generated in EDM oil. Fig. 25 shows the comparison of the machining
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speed of CFRPs in deionized water and EDM oil. The result shows that the machining speed of CFRPs in deionized water was significantly larger than that in EDM oil under both short and long pulse durations. The above results can be explained as follows. After the discharge is ignited, the oxygen gas is generated due to the dissociation of deionized water. Under the extremely high temperature of the arc plasma, the epoxy resin and carbon fiber, which are composed of the carbon, hydrogen and oxygen elements, can be burned by the oxygen gas easily and directly. The above oxidation reaction can not only generate a large amount of extra heat but also enhance the jetting force, which improves the material removal rate. Therefore, the oxidation reaction in the discharge of CFRPs is considered to be another material 27
ACCEPTED MANUSCRIPT removal mechanism due to its significant influence on the material removal of CFRPs. The above research result also indicates that in the practical machining, applying the oxygen gas into the discharge gap in the EDM of CFRPs can significantly improve
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the material removal rate.
Fig. 24. Comparison of topography of discharge craters generated in deionized water
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and EDM oil in single pulse discharges.
Fig. 25. Comparison of machining speed of CFRPs in deionized water and EDM oil.
5. Conclusions This study reveals the thermal, mechanical and chemical material removal mechanisms in the discharge of CFRPs through the simulation and experiments. The results are summarized below: (1) The results of the heat conduction analysis show that in the discharge of CFRPs, 28
ACCEPTED MANUSCRIPT the extremely high temperature generated around the discharge spot by the Joule heat and plasma heat can remove CFRPs through the oxidation, thermal decomposition, vaporization of the epoxy resin and oxidation, sublimation of the carbon fibers, generating massive high-speed jets emitted into the gap. (2) The anisotropy of CFRPs has great influence on the machining characteristics
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of CFRPs. Specifically, the discharge craters generated on the parallel and vertical surface presented the elliptical and circular shape, respectively. Moreover, the machining speed of CFRPs on the vertical surface in WEDM was higher than that on the parallel surface. The kef width machined on the mixed surface of CFRPs showed a wavy shape due to significant difference of thermal conductivity of the carbon fibers
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in different directions.
(3) In the discharge of CFRPs, large amounts of jets were emitted into the gap at a
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very high speed, generating the jetting force. As a result, when the discharge occurred on the parallel surface, the carbon fibers at the discharge spot were crashed by the high-speed jets whose motion direction was vertical to the arrangement direction of the carbon fibers, then removed and blew away by the jetting force along the jetting direction. Thus, the jetting force is thought as the mechanical removal mechanism of CFRPs when the discharge occurs on the parallel surface. However, when the
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discharge occurred on the vertical surface, the carbon fibers were not easy to fracture and break under the jetting force along the axial direction of the carbon fibers, thus, in this case, the jetting force is not the main way of the material removal of CFRPs. (4) The machining speed of CFRPs in deionized water was significantly higher than
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that in EDM oil. The oxygen gas generated from the dissociation of deionized water could burn CFRPs directly, which not only generated a large amount of extra heat but enhanced the jetting force, indicating that the oxidation reaction caused by the oxygen
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gas serves as another important material removal mechanism in the EDM of CFRPs. Thus, applying the oxygen gas into the discharge gap in the EDM of CFRPs can contribute to improve the material removal rate.
References
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Acknowledgement The authors would like to thank the National Natural Science Foundation of China (General Program, No. 51575136), the Key Laboratory of Micro-systems and
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Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology (No. 2017KM002) and the Key Project of Natural Science Foundation of Heilongjiang Province (No. ZD2015009) for providing financial support for this
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research.
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Highlights 1.
The discharge process and material removal process of CFRPs were observed for the first time using a high speed camera with a laser bandpass filter added in front
from plasma.
The material removal mechanism of CFRPs by EDM was clarified from thermal, mechanical and chemical respects.
3.
The acting mechanism of jetting force caused by jets was proposed for the first
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time.
The influences of anisotropy of CFRPs on the machining characteristics of CFRPs
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were revealed.
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4.
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2.
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of the camera lens to filter out the arc plasma without the interfere of bright light
