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Jet electrochemical machining of micro dimples with conductive mask
Accepted Manuscript Title: Jet electrochemical machining of micro dimples with conductive mask Authors: X.L. Chen, B.Y. Dong, C.Y. Zhang, M. Wu, Z.N. Guo PII: DOI: Reference:
S0924-0136(18)30091-8 https://doi.org/10.1016/j.jmatprotec.2018.02.035 PROTEC 15662
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
6-11-2017 29-1-2018 22-2-2018
Please cite this article as: Chen XL, Dong BY, Zhang CY, Wu M, Guo ZN, Jet electrochemical machining of micro dimples with conductive mask, Journal of Materials Processing Technology (2010), https://doi.org/10.1016/j.jmatprotec.2018.02.035 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.
Jet electrochemical machining of micro dimples with conductive mask
X.L. Chen a,b *, B.Y. Dong a, C.Y. Zhang a, M. Wu a, Z.N. Guo a,b
a
School of Electro-mechanical Engineering, Guangdong University of Technology, Guangzhou 510016,
b
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RP China Guangzhou Key Laboratory of Nontraditional Machining and Equipment, Guangzhou 510006, RP
China Corresponding author: X.L. Chen
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*
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Tel: 86-20-39322412; Fax: 86-20-39322412; E-mail: [email protected] )
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Abstract Micro dimples as a typical surface texture has been used in many fields for enhancing the functionality and performance. Electrochemical machining (ECM) is a promising approach for generating micro dimple. However, due to the isotropy of metal dissolution, the lateral undercutting of micro dimple is inevitable in ECM, which reduces the machining localization. This paper proposed a method of conductive mask jet electrochemical machining to reduce the
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undercutting of micro dimple and improve the machining localization. In this method, a
conductive patterned mask instead of insulated patterned mask was covered on the workpiece
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directly during machining, which could decrease the undercutting of micro dimple by reducing the electric field intensity at the edge of micro dimple. In addition, a metallic nozzle (inner diameter of 2 mm) was employed to provide a stable columnar jet flow for enhancing the attachment
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between the mask and workpiece as well as the renewal of electrolyte in machining area, which
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was useful for generating deep micro dimple. Simulated results showed that the conductive mask
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could reduce the electric field identity at the edge of micro dimple effectively, and the undercutting of the profile was evidently reduced compared to that generated with insulated mask.
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Experimental results indicated that with conductive mask JEM, the undercutting of micro dimple was just 9 μm when the depth increased to 55 μm, the etch factor (EF) reached to 6.11, and it was
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four times greater than that with insulated mask. With the depth increased from 45 μm to 85 μm, the undercutting of micro dimple enlarged from 7 μm to 15 μm. The material removal rate in
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depth was evidently faster than that in diameter, which showed a low undercutting and high machining localization. In addition, compared with pulse current, direct current was more
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appropriate for generating deep micro dimple in conductive mask JEM. Keywords: surface texture; micro dimple; jet electrochemical machining; conductive mask; machining localization
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1. Introduction
Surface textures, particularly in micro and nanometer scales, have represented an advanced
technology in many fields for enhancing the functionality and performance. Micro dimples as a typical surface texture has been widely used for improving the properties in tribology and heat transfer (Bruzzone et al., 2008). Micro dimple prepared on tool rake face could increase the tool’s lifetime, because it can improve the wear resistance and anti-adhesion as well as reduce the cutting 2
force, cutting temperature, and friction coefficient (Su et al., 2014). Li et al. (2016) reported that micro dimples in the tube could improve the turbulence level and enhanced the heat transfer. Electrochemical machining (ECM) is a promising machining technique for generating micro dimples on metallic surface, which has advantages of no tool wear and burrs, a lack of heat-affected layer, independence of material hardness and toughness, and low production cost. Literatures have reported several ECM methods for preparing micro dimples. Byun et al. (2010)
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successfully fabricated a micro-dimple array with 300 μm in diameter and 5 μm in depth by micro-ECM using a tool electrode with 275 μm in diameter. While, compared with the increase in
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depth, diameter showed a greater degree of enlargement, and the undercutting (defined as the difference of radius between the micro dimple and tool) was serious. In addition, due to the small
machining gap, the renewal of electrolyte was weak, which made it difficult to generate deep
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micro dimple. Jet electrochemical machining (JEM) is an effective approach for generating deep
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micro dimple. The unique characteristic of this technology is that the electrolyte is ejected to the
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workpiece with high velocity from the metallic nozzle, which is helpful for preparing deep micro dimple as the electrolyte in the micro dimple can be renewed rapidly. However, because the
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workpicec surface is exposed with no insulated protection in JEM, it could lead to undercutting
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and secondary corrosion at the edge of micro dimple, and the phenomenon would become severer with the depth increased, which reduces the machining accuracy and surface quality. Hackert et al. (2012) employed JEM for generating micro dimple by using a metallic nozzle with inner diameter
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of 100 μm. With the depth increased from 37 μm to 90 μm, diameter of micro dimple enlarged from 173 μm to 220 μm, the undercutting increased from 36.5 μm to 60 μm, and the machining
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localization was reduced. They also showed that there was a secondary dissolution at the edge of micro dimple in JEM (Hackert et al., 2015). Through mask electrochemical micromachining (TMEMM) is also a popular method for
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generating micro dimple. In this method, patterned photoresist is covered on the workpiece as a mask, and the exposed region is dissolved through ECM when the electrolyte flows into the patterned mask, then micro dimples can be generated. While, the patterned photoresist is a single-use mask and each workpiece must be treated with lithography process, which shows a complex procedure. Several modified TMEMM methods have proposed to simplify the process. Nouraei and Roy (2008) employed the patterned photoresist on the metal as cathode, and 3
workpiece was placed close proximity to the cathode, then the micro dimples could be obtained using ECM. Researchers have also introduced some modified masks to TMEMM. Qian et al. (2010) employed an insulation layer coated with metal as mask to generate micro dimples. In this method, the insulation layer was covered on the workpiece, and the coated metal was used as cathode. Qu et al., 2014 prepared a PDMS mask instead of the photoresist mask. As these masks could be re-used during machining, the production efficiency was improved. However, due to the
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isotropy of metal dissolution (Lu and Leng, 2005), the undercutting in TMEMM is inevitable. Generally, the undercutting is enlarged with the depth increased, and then the machining
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localization is reduced. Chen et al. (2015) reduced the undercutting of micro dimple by using
oxygen accumulated at the edge of micro dimple. With this method, micro dimple with 56 μm in diameter and 16 μm in depth was generated using a high-aspect-ratio (5) mask with micro through
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hole of 50 μm in diameter. Zhang et al. (2016) proposed sandwich-like electrochemical
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micromachining for reducing undercutting and enhancing the dimensional uniformity, and micro
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dimple with 109 μm in diameter and 11 μm in depth was successfully generated. While, the oxygen bubbles and insoluble products accumulated in the machining region, and the removal of
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products was difficult, which made it a challenge to generate deep dimple. Li et al. (2011)
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introduced an inert metal mask with through holes to ECM for preparing micro structures, which could improve the machining accuracy by changing the electrical field in the inter-electrode gap, and the structures with the minimum size of about 400 μm were prepared. While, the electrolyte
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with lateral flow was applied in this method, with the diameter of micro holes in the mask decreased, it became difficult for the electrolyte to flow into the micro holes, which weakened the
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renewal of electrolyte and made it a challenge to generate deep structure with smaller size. Ming et al. (2015) employed inert metal perforation plate to fabricate biconically shaped through-hole by using electroforming and electrochemical machining. In the process, the conductive inert metal
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was mainly employed as mandrel to deposit metal on its surface, and then the ECM was performed to dissolve superfluous deposited metal for forming biconically shaped through-hole. In this method, the inert metal was employed as a carrier to prepare special shape but not to improve the machining accuracy of the structure. For electrochemical machining of micro dimple, reducing the undercutting could improve the machining localization, and enhancing electrolyte renewal is helpful to increase the machining 4
depth. It would be an effective approach to prepare micro dimples with high aspect ratio by combining the two points. For the purpose, this paper proposed a method of conductive mask JEM for generating micro dimple (Fig.1). A conductive patterned mask was introduced to be covered on the workpiece directly during machining, which could decrease the undercutting of micro dimple by reducing the electric field intensity at the edge of micro dimple. In addition, a metallic nozzle (inner diameter of 2 mm) was employed to provide a stable columnar and high speed jet flow for
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enhancing the attachment between the mask and workpiece as well as the renewal of electrolyte in
the small machining area, which was useful for generating deep micro dimple. In this paper, a
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mathematical model was built to analyze the electric field distribution in machining area with
different masks, and the evolution processes of the micro dimple profile were predicted. Experiments were also conducted to demonstrate the feasibility of the method.
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2. Description of the method and numerical simulation
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2.1 Description of the method
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As shown in Fig.1, the workpiece is covered with a conductive mask in this method. The columnar pressured electrolyte in the metallic nozzle is ejected toward conductive mask, which
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could ensure the conductive mask covered on workpiece closely. Meanwhile, some high speed
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electrolyte reaches the exposed workpiece through the micro hole, and then the micro dimple could be generated when a sufficient voltage is applied between the metallic nozzle and
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workpiece.
Fig.1 The schematic diagram of conductive mask JEM.
2.2 Numerical simulation In this paper, both a conductive mask and a large size metallic nozzle (compared with that
used in traditional JEM) were introduced, and the electric field distribution would be quite different from traditional TMEMM and JEM. Therefore, a mathematical model was built to investigate the electric field distribution on the workpiece as well as predict the evolution 5
processes of the micro dimple profile. 2.2.1 Model building The 2D model diagram of this method is shown in Fig.2, where H denotes the length of the nozzle, Rout and Rin denote the outside and inside radius of the nozzle, respectively, r denotes the radius of micro through holes in mask, t denotes the mask thickness, T is the thickness of the
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workpiece, and other parameters with the model are listed Table 1 in details. Rout Rin
H U Insulated layer
r t
Workpiece
T
t1
t2
U
Mask
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Nozzle
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Fig.2 The 2D model diagram of conductive mask JEM.
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Descriptions of the parameters
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Table 1. The parameters of the built model. Value 2 mm
Rin, Inside radius of the nozzle
1 mm
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Rout, Outside radius of the nozzle,
4 mm
t2-t1, Thickness of the insulated layer
0.1 mm
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H, Length of the nozzle
0.1 mm
r, Radius of the micro through holes in mask
0.1 mm
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t, Thickness of the mask
T, Thickness of the workpiece
0.5 mm
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According to Ohm’s law, the current density, i, can be described as follows:
i E
(1)
where σ is the electrolyte conductivity, and E is the electric field intensity on the workpicec. According to Faraday’s Law, the metal removal rate v can be expressed as follows:
v i
(2)
where η denotes the current efficiency and ω denotes the volumetric electrochemical equivalent of the material. 6
Then, it can be obtained as
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(3)
During machining, the electrolytic products and joule heat can be removed from the machining area rapidly by high velocity of electrolyte. Hence several assumptions are made during simulation as follows:
2) The temperature of the electrolyte, T, is constant. 3) The concentration gradient in the bulk electrolyte is negligible. 4) The current efficiency, η, is 100%.
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All the models were solved by COMSOL Multiphasics software version 5.2.
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1) The electrolyte conductivity, σ, is constant.
Figure 3 shows the scheme of the model built in the software (symmetry with the red line). There are five domains in the model. Domain I is the metallic nozzle (cathode). Domain II is the
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insulated layer on the surface of nozzle. Domain III is the mask covered on the workpiece.
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Domain IV is the workpiece (anode). And domain V is the zone of electrolyte. Both boundaries 1
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and 4 have an electric potential of 0 V (ground), and boundaries 2 and 3 are the cathode reaction
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surfaces. Boundary 5 has an electric potential of U, and boundary 9 is the anode reaction surface (workpiece surface) which is dissolved during machining. With insulated mask, boundaries 6, 7
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and 8 are set with electric insulation. On the contrary, they are set with an electric potential of U with conductive mask. And other uncharted boundaries are all set with electric insulation. 4.5 (mm)
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4
Ground
3.5
0.2 0.18 0.16
Ground
0.14
Rin
Ⅰ
3 2.5
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1
2
2
Rout
0.08
3
4
H
0.06
0.02 0
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-2.5
Ⅳ
-2
-1.5
-0.04
r
5
-0.5
-3
8
-0.02
Ⅱ
Ⅲ
Ⅲ
7
t0
0.04
1
0
r
6
0.1
1.5
0.5
Ⅴ
0.12
Ⅴ
-1
-0.5
t t1 t2 T
Electrical potential
-0.06
Ⅳ
9 Workpiece surface
-0.08 -0.1
0
0.5
1
1.5
2
2.5
-0.15
3
-0.1
-0.05
0
0.05
0.1
0.15
Fig.3 Simulation model with the definition of domains and boundaries. As the electrode polarization has a pronounced effect on the electrode reaction, the Secondary
Current Distribution (siec) module is employed in the simulation. Cathodic Tafel equation and Anodic Tafel equation are chosen as the electrode kinetics type in boundaries 2, 3 and boundary 9, respectively. And all the parameters for simulation are set in Table 2. Meanwhile, the evolution 7
process of micro dimple is simulated by using Deformed Geometry (dg) module (Hackert et al., 2015). The deformation of boundary 9 is quantified by Eq. (2).
vn i n
(4)
And in the simulation, the normal mesh velocity of boundary 9 is set as follows:
vn (siec.IIMag )
(5)
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In this equation, the siec.IIMag represents the normal current density value of boundary 9.
As the mask has no deformation in machining, a virtual gap (t0) with 10 μm is set between
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mask and workpiece to ensure the boundary 9 move properly (shown in Fig. 3). Figure 4 shows the model generated with free triangular mesh, and the deformed region is refined to improve the
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calculation accuracy.
Fig.4 Generated Meshes.
Table 2. The parameters set for simulation. Value
σc, Conductivity in domain I (cathode)
4.032×106 S/m
σi, Conductivity in domain II (insulated layer)
1.8×10-15 S/m
σm, Conductivity in domain III (insulated mask)
1.8×10-15 S/m
σm, Conductivity in domain III (conductive mask)
4.17×107 S/m
σw, Conductivity in domain IV (workpiece)
4.032×106 S/m
σe, Conductivity in domain V (electrolyte)
15 S/m
ω, Volumetric electrochemical equivalent in domain IV
0.035 mm3/(A·s)
i0, Exchange current density of boundaries 2, 3 and 9
8.38×10-5 A/m2
Eeq, Equilibrium Potential of boundaries 2, 3 and 9
-0.136 V
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Description of the parameters
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U, Electric potential of boundary 5
30 V
Aa, Anodic Tafel Slope of boundary 9
0.0466 V
Ac, Cathodic Tafel Slope of boundaries 2 and 3
-0.0652 V
2.2.2 Numerical simulation results Figure 5 shows the electric field intensity on the workpiece with different masks. It can be
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seen that with insulated mask, the electric field intensity is decreased from the edge to the center of exposed workpiece, and there is maximum electric field intensity at the edge of exposed
workpiece. On the contrary, with conductive mask, the electric field intensity is increased from the
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edge to center, and there is minimum electric field intensity at the edge of exposed workpiece (close to 0). From Eq. (3), it can be obtained that the material removal rate is proportional to the electric field intensity. Hence, with insulated mask, there is a maximum material removal rate at
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the edge of machining area, and the material removal at the edge would lead to a undercutting in
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micro dimple. On the contrary, with conductive mask, the electric field intensity at the edge is
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close to 0, there would be little dissolution at the edge of micro dimple, and then the machining
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localization could be improved.
Fig.5 Electric field intensity over the workpiece surface with different masks.
Figure 6 shows the simulation results with different masks. It shows that in the same depth of
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50 μm, there is an obvious undercutting in micro dimple with insulated mask. On the contrary, the undercutting of micro dimple is significantly reduced with conductive mask.
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(mm) V V
0.25 0.2
2525
Electrolyte
0.15 2020
0.1 0.05
Conductive mask
Conductive mask
1515
0 -0.05
1010
Undercutting
-0.1
55
Workpiece
-0.15 -0.15
-0.1
-0.05
0
0.05
0.1
0.15
-0.2
(a) Using insulated mask (b) Using conductive mask Fig.6 The simulation results with different masks.
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-0.2
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In order to further investigate the influence of different masks on the undercutting of micro
dimple, micro dimples in different depths (20 μm, 30 μm, 40 μm and 50 μm) generated with insulated mask and conductive mask are compared, as shown in Fig. 7. It can be seen that in the
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same depth, the diameter of micro dimple generated with conductive mask is evidently smaller
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than that generated with insulated mask. With the depth increased, there is an obvious increasing
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in the undercutting of micro dimple generated with insulated mask, and then the diameter is enlarged. On the contrary, with conductive mask, there is no significant increase in undercutting
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with the depth increased, and the machining localization is improved.
Fig.7 Comparison of evolution processes of the micro dimple profile with different masks. The mechanism of reducing undercutting by conductive mask EJM can be explained as
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follows:
With an insulated mask, all of the electric field is constrained in the machining area by the
mask, and because of marginal effect of electric field, the electric field intensity is even higher than that in the center (Fig.5 and Fig.8 a). The dissolution of material at the edge of micro dimple leads to the undercutting, which enlarges the diameter of micro dimple. Meanwhile, due to the isotropic of metal dissolution, the material dissolution at the edge is continued with the depth 10
increased, and then the undercutting becomes serious. On the contrary, when a conductive mask is introduced, as the mask is covered on the workpiece directly, it has the same potential with workpiece, and then the electric field can be formed between the conductive mask and cathode, which reduces the electric field intensity at the edge of exposed workpiece closely to zero (Fig.5 and Fig.8 b). Thus, the material dissolution at the edge of micro dimple is restrained, and then the
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7.87×105 V/m
1.4
7
1.2
6
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Y/mm
1 0.8
5
0.6
4
0.4
Conductive mask
0
Workpiece
-0.2 -1
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3
0.2
-1.2
U
(a) Using insulated mask. Nozzle
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undercutting is reduced, which improves the machining localization during machining.
-0.8 -0.6 -0.4 -0.2
0
0.2
0.4
0.6
0.8
2 1 1
1.71×10-3
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X/mm
(b) Using conductive mask. Fig.8 Electric field distribution at the edge of the workpiece using different masks.
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3. Experimental procedure
Figure 9 shows a schematic of the conductive mask JEM system for generating micro dimple.
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The conductive mask was bonded to the surface of metallic nozzle by using an insulated epoxy plate with the thickness of 100 μm (inter-electrode gap). Firstly, the metallic nozzle with mask was moved toward workpiece by controlling Z axis until the mask was contacted with workpiece. Then,
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the electrolyte was ejected toward the workpiece through the micro through holes in the mask. At last, the micro dimple could be generated with a sufficient voltage applied.
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Fig.9 Schematic of the conductive mask JEM system.
In this experiment, platinum sheet with a thickness of 100 μm was introduced as the
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conductive mask, which showed a good chemical inertness and could not be dissolved during machining. The micro through hole in the mask was machined by micro drilling with the drill
diameter of 200 μm (shown in Fig.10). The main experiment parameters were listed in Table 3.
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The profiles of micro dimples were examined using a confocal laser scanning microscope (CSLM,
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Olympus LEXT OLS4000, Japan).
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Fig.10 Micro through holes prepared by micro drilling. Table 3. Experimental machining parameters.
Parameter
Value
Electrolyte concentration
12%(wt.%), NaNO3
Electrolyte temperature
30℃
Electrolyte pressure
0.6 MPa
Diameter of the micro through-hole
0.2 mm
Distance of the micro through-holes
0.6 mm
Thickness of the mask
0.1 mm
Inter-electrode gap
0.1 mm 12
Applied voltage (V)
30
Workpiece material
Stainless steel 304
Metallic nozzle material
Stainless steel 304
Generally, in electrochemical machining of micro dimple, the metal is dissolved in directions of diameter and depth, and the dissolution in diameter leads to undercutting. Etch factor (EF),
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representing the ratio of material removal rate in depth and diameter, is usually used for evaluate the machining localization of micro dimple (shown in Fig.11). EF=2h/(d-d0)
(6)
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where h is the machining depth of micro dimple, d0 is the diameter of micro through hole in
conductive mask, and d is the diameter of micro dimple. Higher EF represents a better machining
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localization.
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4. Results and discussion
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Fig.11 Schematic diagram in calculation of EF.
4.1 Comparison of micro dimples generated with insulated and conductive mask
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In order to compare the etch factor (EF) of micro dimples generated with insulated and conductive mask, the two type of masks were designed with the same scale, the diameter of micro
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through hole was 200 μm, and the thickness was 100 μm. The experiments were performed with a DC voltage of 30 V, and the micro dimples were generated in the same depth with different masks.
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Figure 12a and b show profiles of micro dimples generated using insulated and conductive
mask, respectively. It can be seen that in machining depth of 54 μm, the diameter of micro dimple was about 273.6 μm by using insulated mask (Fig.12 a). However, a surprising phenomenon was
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observed when the conductive mask was introduced. As shown in Fig.12 b), in machining depth of 55.3 μm, the diameter of micro dimple was about 218.1 μm, and the diameter was reduced significantly compared with that in insulated mask. In addition, it can be seen that the simulated profile and experimental profile of micro dimple show a good agreement, verifying the rationality of the established model and the feasibility of the method. The values of EF for micro dimples are shown in Fig.13. The EF was improved from 1.47 to 6.11 when the insulated mask was replaced 13
with conductive mask, indicating that the undercutting was evidently restrained. And Figure 14 shows a micro-dimple array generated with this method. The comparative experiments indicated that the conductive mask could reduce the undercutting and improve the machining localization effectively. In order to further investigate the
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effect of conductive mask on the EJM, the following experiments were conducted.
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(a) Using insulated mask (b) Using conductive mask Fig.12 Profiles of micro dimple in same depth generated using different masks.
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Fig.13 EF of micro dimples in same depth generated using different masks.
Fig.14 Image of micro-dimple array generated using conductive mask.
4.2 Micro dimples generated with direct current Figure 15 shows the effect of machining time on the diameter and depth of micro dimples
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prepared with a DC voltage of 30 V. It can be seen that by using insulated mask, the diameter of micro dimple enlarged from 247 μm to 317 μm with the machining time increased from 1 s to 4 s (Fig.15 a). At the same time, the depth of micro dimple increased from 40 μm to 90 μm. While, by using conductive mask, the diameter increased from 214 μm to 230 μm with the time prolonged from 30 s to 120 s, the depth of micro dimple increased from 45 μm to 85 μm (Fig.15 b).
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(a) Using insulated mask. (b) Using conductive mask. Fig.15 Diameter and depth of micro dimple generated with different time.
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Figure 16 shows the etch factor (EF) of micro dimple with different depths. By using
insulated mask, the EF was about 1.7 with the depth in 40 μm, and the EF ranged from 1.48 to 1.6 with the depth further increased (Fig.16 a). In conductive mask, the EF was about 6.43 with the
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depth in 45 μm, and with the depth further increased, the EF was maintained about 5.5 (Fig.16 b).
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The results indicate that for the same depth of micro dimple, it took less machining time with
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insulated mask than that with conductive mask. But compared with EF in insulated mask, EF
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increased significantly in conductive mask, which showed a high machining localization.
(a) Using insulated mask. (b) Using conductive mask. Fig.16 EF of micro dimple with different depths.
In the previous literatures, researchers have also investigated the change of micro dimple size
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during machining. Qian et al. (2010) showed that by using TMEMM with micro through holes of 200 μm diameter in the mask, as the depth increased from 6 μm to 16 μm, the diameter was enlarged from 225 μm to 310 μm. The EF decreased from 0.48 to 0.29, and there was a serious undercutting. Madore and Landolt (1997) reported a decrease in EF from 1.7 to 1.3 as the depth increased from 7 to 25 μm with an insulated photoresist mask. Hackert et al. (2012) showed that by using JEM with a metallic nozzle of 100 μm inner diameter, the diameter of micro dimple 15
enlarged from 173 μm to 220 μm with the depth increased from 37 μm to 90 μm. The EF was about 1.5 with the depth of 90 μm. The results of the above literatures were similar to the result of experiments performed with insulated mask in this paper, and all of the EF was less than 2. While, by using the conductive mask, the increment of diameter was not obvious with the increasing depth, and the EF had a significant increase, showing a high machining localization. As show in Fig.17, when the depth of
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the micro dimple increased to 46 μm, the undercutting was just about 7 μm. And with the depth
increased to 82 μm, the undercutting enlarged to 14 μm. The material removal rate in depth was
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evidently faster than that in diameter, and it was useful for reducing the undercutting and
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improving the machining localization.
Fig.17 The profile of micro dimple generated in different mchining time.
4.3 Micro dimples generated with pulse current Pulse current is usually employed in ECM to improve the machining accuracy, as it is helpful
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to refresh the electrolyte. In this subsection, to investigate the influence of pulse current on conductive mask JEM, experiments with different pulse duty cycles and frequencies were performed at a voltage of 30 V. The pulse parameters are listed in Table 4. Table 4. Pulse parameters. Parameter
Value
Pulse duty cycle (%)
20, 40, 60 16
Frequency (kHz)
5, 10, 15
Figure 18 shows dimensions of micro dimples generated with different pulse duty cycles and frequencies at the same machining time (ton=60 s). It can be seen that there was little change in diameter with increasing pulse duty cycle and frequency, which was maintained at about 220 μm (Fig.18 a). However, there was a significant change in depth (Fig.18 b), especially in the pulse duty cycle of 20%, the depth of micro dimple was reduced from about 41 μm to 27 μm with the
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pulse frequency increasing from 5 kHz to 15 kHz (Fig.19 a). When the pulse duty cycle was 40%,
the change of depth with different frequencies became not obvious, ranging from 44 μm to 51 μm
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(Fig.19 b). With the pulse duty cycle increased to 60%, there was little change in depth as the
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pulse frequency increased, which was maintained at about 53 μm (Fig.19 c).
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(a) Diameter (b) Depth Fig.18 Diameter and depth of micro dimple generated with pulse current.
Fig.19 Comparison of the profiles of micro dimples generated with different pulse parameters. In order to investigate the reason for the phenomenon, the dissolution characteristic of 17
stainless steel 304 in NaNO3 solution was analyzed with a polarization curve, as shown in Fig.20. It can be seen that the current density had little increase with the potential between −0.5 V to 1.5 V, which indicated a passive region. In this region, there was a compact oxidation film on the metal surface, preventing the dissolution of the metal. With the further increase of potential, the metal surface was into the transpassive region, the oxidation film was broken and the metal began to be dissolved. According to the polarization curve, it can be obtained that oxidation film could be
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formed in electrochemical machining of stainless steel 304, especially in low current density. In
the section 2 (Fig.5), the numerical simulation indicated that although the voltage was high (30 V),
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the current density was quite low in conductive mask, and it was easy to form the oxidation film
during the machining. In pulse current, there were rising edge and falling edge at the beginning and end of each pulse width, respectively. In both of the stages, the current density was lower than
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that in steady stage, and the time was mainly used to form oxidation film. Then the oxidation film
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was broken and metal began to be dissolved in the steady stage of pulse width. With the frequency
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increased, the pulse width was reduced, and the proportion of the time in rising and falling edge was enlarged in every pulse width. Hence, the total time for the metal dissolution was reduced,
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which decreased the depth of micro dimple. The phenomenon would be more dramatic in low
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pulse duty cycle with high frequency. As shown in Fig.19 a, in the same pulse duty cycle of 20%, with the frequency increasing from 5 kHz to 15 kHz, the total time for dissolution of metal was reduced, and the depth of micro dimple decreased from 41 μm to 27 μm correspondingly. With the
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pulse duty cycle increasing, the proportion of time in steady stage was increased in one pulse width, the total time for metal dissolution was prolonged, and then the machining depth was
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increased, which weakened the difference of depth in different frequencies. Hence, in pulse duty cycle of 40%, the difference was reduced, and there was little difference when the pulse duty cycle
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increased to 60%.
18
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Fig.20 Polarization curve of stainless steel 304 in NaNO3 solution.
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According to the experimental results in pulse current, it can be obtained that with conductive
mask, the diameter of micro dimple had little change in different pulse duty cycles and frequencies. However, the depth of micro dimple had significant change in pulse duty cycle of 20% with
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different frequencies. With the pulse duty cycle increased, the difference of depth was weakened. Compared with pulse current, direct current was more appropriate for generating deep micro
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dimple with conductive mask.
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4.4 The analysis of current efficiency
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The experiment results have proved that conductive mask was helpful for improving the machining localization. In other side, current efficiency should be also considered during
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machining, and it means the fraction of the current consumed for the anodic dissolution of the workpiece. In this section, the current efficiencies of micro dimples generated with different
PT
masks were analyzed.
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According to Eq. (2), the rate of removal of material can be obtained as follows:
v i
I A
(7)
where I is the current detected during machining, and A is the machining area on the
workpiece. In the present machining process, the depth was proportional to machining time.
A
Hence, in the Y direction, the removal depth h can be obtained as follows:
h v t
It A
(8)
In this experiment, nine micro dimples were generated at one time, and the machining area on the workpiece is given by
A n
D2 4 19
(9)
where n is the number of the micro dimples, and D is the diameter of the micro dimple. The current efficiency can be obtained as follows:
nh D 2 100% 4 It
(10)
During the machining, the current was 1.5 A with conductive mask, and 0.9 A with insulated mask.
calculated as follows:
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n D 2 hinsulated mask 100% 48.8% insulated mask 4 ( It )insulated mask n D 2 hconductive mask 100% 0.5% conductive mask 4 ( It )conductive mask
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At last, according to the machining results in Fig.15a and b, the current efficiency can be
(11)
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The results showed that in insulated mask, the current efficiency was about 48.8%, while the current efficiency was just about 0.5% in conductive mask. The low current efficiency can be
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explained that along with the dissolution of workpiece material, electrochemical reactions
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(producing oxygen) also proceeded on the surface of conductive mask, as the whole conductive
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mask surface was exposed, it consumed considerable fraction of the current leading to a low current efficiency.
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From the results it can be recognized that although conductive mask was helpful for preparing micro dimple with high machining localization, the current efficiency was considerably lower than
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that in insulated mask, which raised the energy consumption of the process and reduced its productivity. Hence, in the future work, it is necessary to insulate the top surface of conductive
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mask to avoid the electrochemical reactions on the surface, which would improve the machining localization as well as the current efficiency. 5. Conclusion
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This paper proposed a method of conductive mask JEM for generating micro dimple. In this
method, a conductive mask instead of insulated mask was introduced to JEM for reducing the undercutting of micro dimple. Based on the simulation and experimental results, the following conclusions can be drawn: 1. Simulation results indicated that conductive mask used in JEM weakened the electric field intensity at the edge of micro dimple, and the undercutting of micro dimple was reduced 20
significantly. By machining micro dimple with the same depth of 55 μm, the etch factor (EF) of micro dimple increased from 1.47 to 6.11 when the insulated mask was replaced with conductive mask, and the machining localization was improved. 2. In conductive mask JEM, with the depth increased from 45 μm to 85 μm, the undercutting of micro dimple enlarged from 7 μm to 15 μm. The material removal rate in depth was evidently faster than that in diameter, which showed a low undercutting and high machining localization.
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3. With pulse current, the diameter of micro dimple had little change in different pulse duty
cycles and frequencies. However, the depth of micro dimple had significant decrease in pulse duty
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cycle of 20% with increasing frequency. Compared with pulse current, direct current was more appropriate for generating deep micro dimple in conductive mask JEM.
4. The conductive mask could improve the machining localization, but the current efficiency
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was lower than that in insulated mask.
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Acknowledgements：The work described in this study was supported by National Natural Science
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Foundation of China (Grant No. 51705089), Joint Funds of the National Natural Science Foundation of China and Guangdong Province (Grant No. U1601201), Jiangsu Key Laboratory of
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Precision and Micro-Manufacturing Technology, and the Guangzhou Key Laboratory of
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Nontraditional Machining and Equipment (No. 201605030007). Reference
Bruzzone, A.A.G., Costa, H.L., Lonardo, P.M., Lucca, D.A., 2008. Advances in engineered
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surfaces for functional performance. CIRP Ann. Manuf. Technol. 57, 750–769. Byun, J.W., Shin, H.S., Kwon, M.H., Kim, B.H., Chu, C.N., 2010. Surface texturing by micro
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ECM for friction reduction. Int. J. Precis. Eng. Manuf. 11, 747–753. Chen, X., Qu, N., Fang, X., Zhu, D., 2015. Reduction of undercutting in electrochemical micro-machining of micro-dimple arrays by utilizing oxygen produced at the anode. Surf. Coat.
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Tech. 277, 44–51.
Hackert-Oschätzchen, M., Meichsner, G., Zinecker, M., Martin, A., Schubert, A., 2012. Micro machining with continuous electrolytic free jet. Precis. Eng. 36, 612–619. Hackert-Oschätzchen, M., Paul, R., Martin, A., Meichsner, G., Lehnert, N., Schubert, A., 2015. Study on the dynamic generation of the jet shape in jet electrochemical machining. J. Mater. Process. Technol. 223, 240–251. 21
Li, D., Zhu, D., Li, H., 2011). Microstructure of electrochemical micromachining using inert metal mask. Int. J. Adv. Manuf. Technol. 55, 189–194. Li, M., Khan, T.S., Al-Hajri, E., Ayub, Z.H., 2016. Single phase heat transfer and pressure drop analysis of a dimpled enhanced tube. Appl. Therm. Eng. 101, 38–46. Lu, X., Leng, Y., 2005. Electrochemical micromachining of titanium surfaces for biomedical applications. J. Mater. Process. Technol. 169, 173–178.
titanium for biological applications. J.Micromech. Microeng. 7, 270–275.
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Madore, C., Landolt, D., 1997. Electrochemical micromachining of controlled topographies on
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Ming, P., Bao, X., Hao, Q., Wang, J., 2015. Fabrication of through-hole with biconically shaped cross sections by using electroforming and inert metal mask electrochemical machining. Int. J. Adv. Manuf. Technol. 76, 501–512.
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Nouraei, S., Roy, S., 2008. Electrochemical process for micropattern transfer without
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photolithography: a modeling analysis. J. Electrochem. Soc. 155, D97–D103.
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Qian, S.Q., Zhu, D., Qu, N.S., Li, H.S., Yan, D.S., 2010. Generating micro-dimples array on the
Adv. Manuf. Technol. 47, 1121–1127.
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hard chrome-coated surface by modified through mask electrochemical micromachining. Int. J.
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Qu, N.S., Chen, X.L., Li, H.S., Zhu, D., 2014. Fabrication of PDMS micro through-holes for electrochemical micromachining. Int. J. Adv. Manuf. Technol. 72, 487–494. Su, Y.S., Li, L., He, N., Zhao, W., 2014. Experimental study of fiber laser surface texturing of
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polycrystalline diamond tools. Int. J. Refract. Met. Hard. 45, 117–124. Zhang, X., Qu, N., Chen, X., 2016. Sandwich-like electrochemical micromachining of
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micro-dimples. Surf. Coat. Tech. 302, 438–447.
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S0924-0136(18)30091-8 https://doi.org/10.1016/j.jmatprotec.2018.02.035 PROTEC 15662
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
6-11-2017 29-1-2018 22-2-2018
Please cite this article as: Chen XL, Dong BY, Zhang CY, Wu M, Guo ZN, Jet electrochemical machining of micro dimples with conductive mask, Journal of Materials Processing Technology (2010), https://doi.org/10.1016/j.jmatprotec.2018.02.035 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.
Jet electrochemical machining of micro dimples with conductive mask
X.L. Chen a,b *, B.Y. Dong a, C.Y. Zhang a, M. Wu a, Z.N. Guo a,b
a
School of Electro-mechanical Engineering, Guangdong University of Technology, Guangzhou 510016,
b
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RP China Guangzhou Key Laboratory of Nontraditional Machining and Equipment, Guangzhou 510006, RP
China Corresponding author: X.L. Chen
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*
A
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M
A
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Tel: 86-20-39322412; Fax: 86-20-39322412; E-mail: [email protected] )
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Abstract Micro dimples as a typical surface texture has been used in many fields for enhancing the functionality and performance. Electrochemical machining (ECM) is a promising approach for generating micro dimple. However, due to the isotropy of metal dissolution, the lateral undercutting of micro dimple is inevitable in ECM, which reduces the machining localization. This paper proposed a method of conductive mask jet electrochemical machining to reduce the
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undercutting of micro dimple and improve the machining localization. In this method, a
conductive patterned mask instead of insulated patterned mask was covered on the workpiece
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directly during machining, which could decrease the undercutting of micro dimple by reducing the electric field intensity at the edge of micro dimple. In addition, a metallic nozzle (inner diameter of 2 mm) was employed to provide a stable columnar jet flow for enhancing the attachment
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between the mask and workpiece as well as the renewal of electrolyte in machining area, which
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was useful for generating deep micro dimple. Simulated results showed that the conductive mask
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could reduce the electric field identity at the edge of micro dimple effectively, and the undercutting of the profile was evidently reduced compared to that generated with insulated mask.
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Experimental results indicated that with conductive mask JEM, the undercutting of micro dimple was just 9 μm when the depth increased to 55 μm, the etch factor (EF) reached to 6.11, and it was
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four times greater than that with insulated mask. With the depth increased from 45 μm to 85 μm, the undercutting of micro dimple enlarged from 7 μm to 15 μm. The material removal rate in
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depth was evidently faster than that in diameter, which showed a low undercutting and high machining localization. In addition, compared with pulse current, direct current was more
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appropriate for generating deep micro dimple in conductive mask JEM. Keywords: surface texture; micro dimple; jet electrochemical machining; conductive mask; machining localization
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1. Introduction
Surface textures, particularly in micro and nanometer scales, have represented an advanced
technology in many fields for enhancing the functionality and performance. Micro dimples as a typical surface texture has been widely used for improving the properties in tribology and heat transfer (Bruzzone et al., 2008). Micro dimple prepared on tool rake face could increase the tool’s lifetime, because it can improve the wear resistance and anti-adhesion as well as reduce the cutting 2
force, cutting temperature, and friction coefficient (Su et al., 2014). Li et al. (2016) reported that micro dimples in the tube could improve the turbulence level and enhanced the heat transfer. Electrochemical machining (ECM) is a promising machining technique for generating micro dimples on metallic surface, which has advantages of no tool wear and burrs, a lack of heat-affected layer, independence of material hardness and toughness, and low production cost. Literatures have reported several ECM methods for preparing micro dimples. Byun et al. (2010)
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successfully fabricated a micro-dimple array with 300 μm in diameter and 5 μm in depth by micro-ECM using a tool electrode with 275 μm in diameter. While, compared with the increase in
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depth, diameter showed a greater degree of enlargement, and the undercutting (defined as the difference of radius between the micro dimple and tool) was serious. In addition, due to the small
machining gap, the renewal of electrolyte was weak, which made it difficult to generate deep
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micro dimple. Jet electrochemical machining (JEM) is an effective approach for generating deep
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micro dimple. The unique characteristic of this technology is that the electrolyte is ejected to the
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workpiece with high velocity from the metallic nozzle, which is helpful for preparing deep micro dimple as the electrolyte in the micro dimple can be renewed rapidly. However, because the
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workpicec surface is exposed with no insulated protection in JEM, it could lead to undercutting
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and secondary corrosion at the edge of micro dimple, and the phenomenon would become severer with the depth increased, which reduces the machining accuracy and surface quality. Hackert et al. (2012) employed JEM for generating micro dimple by using a metallic nozzle with inner diameter
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of 100 μm. With the depth increased from 37 μm to 90 μm, diameter of micro dimple enlarged from 173 μm to 220 μm, the undercutting increased from 36.5 μm to 60 μm, and the machining
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localization was reduced. They also showed that there was a secondary dissolution at the edge of micro dimple in JEM (Hackert et al., 2015). Through mask electrochemical micromachining (TMEMM) is also a popular method for
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generating micro dimple. In this method, patterned photoresist is covered on the workpiece as a mask, and the exposed region is dissolved through ECM when the electrolyte flows into the patterned mask, then micro dimples can be generated. While, the patterned photoresist is a single-use mask and each workpiece must be treated with lithography process, which shows a complex procedure. Several modified TMEMM methods have proposed to simplify the process. Nouraei and Roy (2008) employed the patterned photoresist on the metal as cathode, and 3
workpiece was placed close proximity to the cathode, then the micro dimples could be obtained using ECM. Researchers have also introduced some modified masks to TMEMM. Qian et al. (2010) employed an insulation layer coated with metal as mask to generate micro dimples. In this method, the insulation layer was covered on the workpiece, and the coated metal was used as cathode. Qu et al., 2014 prepared a PDMS mask instead of the photoresist mask. As these masks could be re-used during machining, the production efficiency was improved. However, due to the
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isotropy of metal dissolution (Lu and Leng, 2005), the undercutting in TMEMM is inevitable. Generally, the undercutting is enlarged with the depth increased, and then the machining
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localization is reduced. Chen et al. (2015) reduced the undercutting of micro dimple by using
oxygen accumulated at the edge of micro dimple. With this method, micro dimple with 56 μm in diameter and 16 μm in depth was generated using a high-aspect-ratio (5) mask with micro through
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hole of 50 μm in diameter. Zhang et al. (2016) proposed sandwich-like electrochemical
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micromachining for reducing undercutting and enhancing the dimensional uniformity, and micro
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dimple with 109 μm in diameter and 11 μm in depth was successfully generated. While, the oxygen bubbles and insoluble products accumulated in the machining region, and the removal of
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products was difficult, which made it a challenge to generate deep dimple. Li et al. (2011)
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introduced an inert metal mask with through holes to ECM for preparing micro structures, which could improve the machining accuracy by changing the electrical field in the inter-electrode gap, and the structures with the minimum size of about 400 μm were prepared. While, the electrolyte
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with lateral flow was applied in this method, with the diameter of micro holes in the mask decreased, it became difficult for the electrolyte to flow into the micro holes, which weakened the
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renewal of electrolyte and made it a challenge to generate deep structure with smaller size. Ming et al. (2015) employed inert metal perforation plate to fabricate biconically shaped through-hole by using electroforming and electrochemical machining. In the process, the conductive inert metal
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was mainly employed as mandrel to deposit metal on its surface, and then the ECM was performed to dissolve superfluous deposited metal for forming biconically shaped through-hole. In this method, the inert metal was employed as a carrier to prepare special shape but not to improve the machining accuracy of the structure. For electrochemical machining of micro dimple, reducing the undercutting could improve the machining localization, and enhancing electrolyte renewal is helpful to increase the machining 4
depth. It would be an effective approach to prepare micro dimples with high aspect ratio by combining the two points. For the purpose, this paper proposed a method of conductive mask JEM for generating micro dimple (Fig.1). A conductive patterned mask was introduced to be covered on the workpiece directly during machining, which could decrease the undercutting of micro dimple by reducing the electric field intensity at the edge of micro dimple. In addition, a metallic nozzle (inner diameter of 2 mm) was employed to provide a stable columnar and high speed jet flow for
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enhancing the attachment between the mask and workpiece as well as the renewal of electrolyte in
the small machining area, which was useful for generating deep micro dimple. In this paper, a
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mathematical model was built to analyze the electric field distribution in machining area with
different masks, and the evolution processes of the micro dimple profile were predicted. Experiments were also conducted to demonstrate the feasibility of the method.
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2. Description of the method and numerical simulation
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2.1 Description of the method
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As shown in Fig.1, the workpiece is covered with a conductive mask in this method. The columnar pressured electrolyte in the metallic nozzle is ejected toward conductive mask, which
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could ensure the conductive mask covered on workpiece closely. Meanwhile, some high speed
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electrolyte reaches the exposed workpiece through the micro hole, and then the micro dimple could be generated when a sufficient voltage is applied between the metallic nozzle and
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workpiece.
Fig.1 The schematic diagram of conductive mask JEM.
2.2 Numerical simulation In this paper, both a conductive mask and a large size metallic nozzle (compared with that
used in traditional JEM) were introduced, and the electric field distribution would be quite different from traditional TMEMM and JEM. Therefore, a mathematical model was built to investigate the electric field distribution on the workpiece as well as predict the evolution 5
processes of the micro dimple profile. 2.2.1 Model building The 2D model diagram of this method is shown in Fig.2, where H denotes the length of the nozzle, Rout and Rin denote the outside and inside radius of the nozzle, respectively, r denotes the radius of micro through holes in mask, t denotes the mask thickness, T is the thickness of the
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workpiece, and other parameters with the model are listed Table 1 in details. Rout Rin
H U Insulated layer
r t
Workpiece
T
t1
t2
U
Mask
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Nozzle
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Fig.2 The 2D model diagram of conductive mask JEM.
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Descriptions of the parameters
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Table 1. The parameters of the built model. Value 2 mm
Rin, Inside radius of the nozzle
1 mm
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Rout, Outside radius of the nozzle,
4 mm
t2-t1, Thickness of the insulated layer
0.1 mm
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H, Length of the nozzle
0.1 mm
r, Radius of the micro through holes in mask
0.1 mm
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t, Thickness of the mask
T, Thickness of the workpiece
0.5 mm
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According to Ohm’s law, the current density, i, can be described as follows:
i E
(1)
where σ is the electrolyte conductivity, and E is the electric field intensity on the workpicec. According to Faraday’s Law, the metal removal rate v can be expressed as follows:
v i
(2)
where η denotes the current efficiency and ω denotes the volumetric electrochemical equivalent of the material. 6
Then, it can be obtained as
v E
(3)
During machining, the electrolytic products and joule heat can be removed from the machining area rapidly by high velocity of electrolyte. Hence several assumptions are made during simulation as follows:
2) The temperature of the electrolyte, T, is constant. 3) The concentration gradient in the bulk electrolyte is negligible. 4) The current efficiency, η, is 100%.
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All the models were solved by COMSOL Multiphasics software version 5.2.
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1) The electrolyte conductivity, σ, is constant.
Figure 3 shows the scheme of the model built in the software (symmetry with the red line). There are five domains in the model. Domain I is the metallic nozzle (cathode). Domain II is the
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insulated layer on the surface of nozzle. Domain III is the mask covered on the workpiece.
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Domain IV is the workpiece (anode). And domain V is the zone of electrolyte. Both boundaries 1
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and 4 have an electric potential of 0 V (ground), and boundaries 2 and 3 are the cathode reaction
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surfaces. Boundary 5 has an electric potential of U, and boundary 9 is the anode reaction surface (workpiece surface) which is dissolved during machining. With insulated mask, boundaries 6, 7
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and 8 are set with electric insulation. On the contrary, they are set with an electric potential of U with conductive mask. And other uncharted boundaries are all set with electric insulation. 4.5 (mm)
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4
Ground
3.5
0.2 0.18 0.16
Ground
0.14
Rin
Ⅰ
3 2.5
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1
2
2
Rout
0.08
3
4
H
0.06
0.02 0
A
-2.5
Ⅳ
-2
-1.5
-0.04
r
5
-0.5
-3
8
-0.02
Ⅱ
Ⅲ
Ⅲ
7
t0
0.04
1
0
r
6
0.1
1.5
0.5
Ⅴ
0.12
Ⅴ
-1
-0.5
t t1 t2 T
Electrical potential
-0.06
Ⅳ
9 Workpiece surface
-0.08 -0.1
0
0.5
1
1.5
2
2.5
-0.15
3
-0.1
-0.05
0
0.05
0.1
0.15
Fig.3 Simulation model with the definition of domains and boundaries. As the electrode polarization has a pronounced effect on the electrode reaction, the Secondary
Current Distribution (siec) module is employed in the simulation. Cathodic Tafel equation and Anodic Tafel equation are chosen as the electrode kinetics type in boundaries 2, 3 and boundary 9, respectively. And all the parameters for simulation are set in Table 2. Meanwhile, the evolution 7
process of micro dimple is simulated by using Deformed Geometry (dg) module (Hackert et al., 2015). The deformation of boundary 9 is quantified by Eq. (2).
vn i n
(4)
And in the simulation, the normal mesh velocity of boundary 9 is set as follows:
vn (siec.IIMag )
(5)
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In this equation, the siec.IIMag represents the normal current density value of boundary 9.
As the mask has no deformation in machining, a virtual gap (t0) with 10 μm is set between
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mask and workpiece to ensure the boundary 9 move properly (shown in Fig. 3). Figure 4 shows the model generated with free triangular mesh, and the deformed region is refined to improve the
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calculation accuracy.
Fig.4 Generated Meshes.
Table 2. The parameters set for simulation. Value
σc, Conductivity in domain I (cathode)
4.032×106 S/m
σi, Conductivity in domain II (insulated layer)
1.8×10-15 S/m
σm, Conductivity in domain III (insulated mask)
1.8×10-15 S/m
σm, Conductivity in domain III (conductive mask)
4.17×107 S/m
σw, Conductivity in domain IV (workpiece)
4.032×106 S/m
σe, Conductivity in domain V (electrolyte)
15 S/m
ω, Volumetric electrochemical equivalent in domain IV
0.035 mm3/(A·s)
i0, Exchange current density of boundaries 2, 3 and 9
8.38×10-5 A/m2
Eeq, Equilibrium Potential of boundaries 2, 3 and 9
-0.136 V
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Description of the parameters
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U, Electric potential of boundary 5
30 V
Aa, Anodic Tafel Slope of boundary 9
0.0466 V
Ac, Cathodic Tafel Slope of boundaries 2 and 3
-0.0652 V
2.2.2 Numerical simulation results Figure 5 shows the electric field intensity on the workpiece with different masks. It can be
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seen that with insulated mask, the electric field intensity is decreased from the edge to the center of exposed workpiece, and there is maximum electric field intensity at the edge of exposed
workpiece. On the contrary, with conductive mask, the electric field intensity is increased from the
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edge to center, and there is minimum electric field intensity at the edge of exposed workpiece (close to 0). From Eq. (3), it can be obtained that the material removal rate is proportional to the electric field intensity. Hence, with insulated mask, there is a maximum material removal rate at
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the edge of machining area, and the material removal at the edge would lead to a undercutting in
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micro dimple. On the contrary, with conductive mask, the electric field intensity at the edge is
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close to 0, there would be little dissolution at the edge of micro dimple, and then the machining
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localization could be improved.
Fig.5 Electric field intensity over the workpiece surface with different masks.
Figure 6 shows the simulation results with different masks. It shows that in the same depth of
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50 μm, there is an obvious undercutting in micro dimple with insulated mask. On the contrary, the undercutting of micro dimple is significantly reduced with conductive mask.
9
(mm) V V
0.25 0.2
2525
Electrolyte
0.15 2020
0.1 0.05
Conductive mask
Conductive mask
1515
0 -0.05
1010
Undercutting
-0.1
55
Workpiece
-0.15 -0.15
-0.1
-0.05
0
0.05
0.1
0.15
-0.2
(a) Using insulated mask (b) Using conductive mask Fig.6 The simulation results with different masks.
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-0.2
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In order to further investigate the influence of different masks on the undercutting of micro
dimple, micro dimples in different depths (20 μm, 30 μm, 40 μm and 50 μm) generated with insulated mask and conductive mask are compared, as shown in Fig. 7. It can be seen that in the
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same depth, the diameter of micro dimple generated with conductive mask is evidently smaller
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than that generated with insulated mask. With the depth increased, there is an obvious increasing
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in the undercutting of micro dimple generated with insulated mask, and then the diameter is enlarged. On the contrary, with conductive mask, there is no significant increase in undercutting
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with the depth increased, and the machining localization is improved.
Fig.7 Comparison of evolution processes of the micro dimple profile with different masks. The mechanism of reducing undercutting by conductive mask EJM can be explained as
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follows:
With an insulated mask, all of the electric field is constrained in the machining area by the
mask, and because of marginal effect of electric field, the electric field intensity is even higher than that in the center (Fig.5 and Fig.8 a). The dissolution of material at the edge of micro dimple leads to the undercutting, which enlarges the diameter of micro dimple. Meanwhile, due to the isotropic of metal dissolution, the material dissolution at the edge is continued with the depth 10
increased, and then the undercutting becomes serious. On the contrary, when a conductive mask is introduced, as the mask is covered on the workpiece directly, it has the same potential with workpiece, and then the electric field can be formed between the conductive mask and cathode, which reduces the electric field intensity at the edge of exposed workpiece closely to zero (Fig.5 and Fig.8 b). Thus, the material dissolution at the edge of micro dimple is restrained, and then the
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7.87×105 V/m
1.4
7
1.2
6
A
Y/mm
1 0.8
5
0.6
4
0.4
Conductive mask
0
Workpiece
-0.2 -1
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3
0.2
-1.2
U
(a) Using insulated mask. Nozzle
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undercutting is reduced, which improves the machining localization during machining.
-0.8 -0.6 -0.4 -0.2
0
0.2
0.4
0.6
0.8
2 1 1
1.71×10-3
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X/mm
(b) Using conductive mask. Fig.8 Electric field distribution at the edge of the workpiece using different masks.
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3. Experimental procedure
Figure 9 shows a schematic of the conductive mask JEM system for generating micro dimple.
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The conductive mask was bonded to the surface of metallic nozzle by using an insulated epoxy plate with the thickness of 100 μm (inter-electrode gap). Firstly, the metallic nozzle with mask was moved toward workpiece by controlling Z axis until the mask was contacted with workpiece. Then,
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the electrolyte was ejected toward the workpiece through the micro through holes in the mask. At last, the micro dimple could be generated with a sufficient voltage applied.
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Fig.9 Schematic of the conductive mask JEM system.
In this experiment, platinum sheet with a thickness of 100 μm was introduced as the
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conductive mask, which showed a good chemical inertness and could not be dissolved during machining. The micro through hole in the mask was machined by micro drilling with the drill
diameter of 200 μm (shown in Fig.10). The main experiment parameters were listed in Table 3.
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The profiles of micro dimples were examined using a confocal laser scanning microscope (CSLM,
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A
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Olympus LEXT OLS4000, Japan).
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Fig.10 Micro through holes prepared by micro drilling. Table 3. Experimental machining parameters.
Parameter
Value
Electrolyte concentration
12%(wt.%), NaNO3
Electrolyte temperature
30℃
Electrolyte pressure
0.6 MPa
Diameter of the micro through-hole
0.2 mm
Distance of the micro through-holes
0.6 mm
Thickness of the mask
0.1 mm
Inter-electrode gap
0.1 mm 12
Applied voltage (V)
30
Workpiece material
Stainless steel 304
Metallic nozzle material
Stainless steel 304
Generally, in electrochemical machining of micro dimple, the metal is dissolved in directions of diameter and depth, and the dissolution in diameter leads to undercutting. Etch factor (EF),
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representing the ratio of material removal rate in depth and diameter, is usually used for evaluate the machining localization of micro dimple (shown in Fig.11). EF=2h/(d-d0)
(6)
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where h is the machining depth of micro dimple, d0 is the diameter of micro through hole in
conductive mask, and d is the diameter of micro dimple. Higher EF represents a better machining
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localization.
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4. Results and discussion
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Fig.11 Schematic diagram in calculation of EF.
4.1 Comparison of micro dimples generated with insulated and conductive mask
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In order to compare the etch factor (EF) of micro dimples generated with insulated and conductive mask, the two type of masks were designed with the same scale, the diameter of micro
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through hole was 200 μm, and the thickness was 100 μm. The experiments were performed with a DC voltage of 30 V, and the micro dimples were generated in the same depth with different masks.
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Figure 12a and b show profiles of micro dimples generated using insulated and conductive
mask, respectively. It can be seen that in machining depth of 54 μm, the diameter of micro dimple was about 273.6 μm by using insulated mask (Fig.12 a). However, a surprising phenomenon was
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observed when the conductive mask was introduced. As shown in Fig.12 b), in machining depth of 55.3 μm, the diameter of micro dimple was about 218.1 μm, and the diameter was reduced significantly compared with that in insulated mask. In addition, it can be seen that the simulated profile and experimental profile of micro dimple show a good agreement, verifying the rationality of the established model and the feasibility of the method. The values of EF for micro dimples are shown in Fig.13. The EF was improved from 1.47 to 6.11 when the insulated mask was replaced 13
with conductive mask, indicating that the undercutting was evidently restrained. And Figure 14 shows a micro-dimple array generated with this method. The comparative experiments indicated that the conductive mask could reduce the undercutting and improve the machining localization effectively. In order to further investigate the
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IP T
effect of conductive mask on the EJM, the following experiments were conducted.
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(a) Using insulated mask (b) Using conductive mask Fig.12 Profiles of micro dimple in same depth generated using different masks.
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Fig.13 EF of micro dimples in same depth generated using different masks.
Fig.14 Image of micro-dimple array generated using conductive mask.
4.2 Micro dimples generated with direct current Figure 15 shows the effect of machining time on the diameter and depth of micro dimples
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prepared with a DC voltage of 30 V. It can be seen that by using insulated mask, the diameter of micro dimple enlarged from 247 μm to 317 μm with the machining time increased from 1 s to 4 s (Fig.15 a). At the same time, the depth of micro dimple increased from 40 μm to 90 μm. While, by using conductive mask, the diameter increased from 214 μm to 230 μm with the time prolonged from 30 s to 120 s, the depth of micro dimple increased from 45 μm to 85 μm (Fig.15 b).
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(a) Using insulated mask. (b) Using conductive mask. Fig.15 Diameter and depth of micro dimple generated with different time.
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Figure 16 shows the etch factor (EF) of micro dimple with different depths. By using
insulated mask, the EF was about 1.7 with the depth in 40 μm, and the EF ranged from 1.48 to 1.6 with the depth further increased (Fig.16 a). In conductive mask, the EF was about 6.43 with the
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depth in 45 μm, and with the depth further increased, the EF was maintained about 5.5 (Fig.16 b).
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The results indicate that for the same depth of micro dimple, it took less machining time with
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insulated mask than that with conductive mask. But compared with EF in insulated mask, EF
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increased significantly in conductive mask, which showed a high machining localization.
(a) Using insulated mask. (b) Using conductive mask. Fig.16 EF of micro dimple with different depths.
In the previous literatures, researchers have also investigated the change of micro dimple size
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during machining. Qian et al. (2010) showed that by using TMEMM with micro through holes of 200 μm diameter in the mask, as the depth increased from 6 μm to 16 μm, the diameter was enlarged from 225 μm to 310 μm. The EF decreased from 0.48 to 0.29, and there was a serious undercutting. Madore and Landolt (1997) reported a decrease in EF from 1.7 to 1.3 as the depth increased from 7 to 25 μm with an insulated photoresist mask. Hackert et al. (2012) showed that by using JEM with a metallic nozzle of 100 μm inner diameter, the diameter of micro dimple 15
enlarged from 173 μm to 220 μm with the depth increased from 37 μm to 90 μm. The EF was about 1.5 with the depth of 90 μm. The results of the above literatures were similar to the result of experiments performed with insulated mask in this paper, and all of the EF was less than 2. While, by using the conductive mask, the increment of diameter was not obvious with the increasing depth, and the EF had a significant increase, showing a high machining localization. As show in Fig.17, when the depth of
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the micro dimple increased to 46 μm, the undercutting was just about 7 μm. And with the depth
increased to 82 μm, the undercutting enlarged to 14 μm. The material removal rate in depth was
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evidently faster than that in diameter, and it was useful for reducing the undercutting and
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improving the machining localization.
Fig.17 The profile of micro dimple generated in different mchining time.
4.3 Micro dimples generated with pulse current Pulse current is usually employed in ECM to improve the machining accuracy, as it is helpful
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to refresh the electrolyte. In this subsection, to investigate the influence of pulse current on conductive mask JEM, experiments with different pulse duty cycles and frequencies were performed at a voltage of 30 V. The pulse parameters are listed in Table 4. Table 4. Pulse parameters. Parameter
Value
Pulse duty cycle (%)
20, 40, 60 16
Frequency (kHz)
5, 10, 15
Figure 18 shows dimensions of micro dimples generated with different pulse duty cycles and frequencies at the same machining time (ton=60 s). It can be seen that there was little change in diameter with increasing pulse duty cycle and frequency, which was maintained at about 220 μm (Fig.18 a). However, there was a significant change in depth (Fig.18 b), especially in the pulse duty cycle of 20%, the depth of micro dimple was reduced from about 41 μm to 27 μm with the
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pulse frequency increasing from 5 kHz to 15 kHz (Fig.19 a). When the pulse duty cycle was 40%,
the change of depth with different frequencies became not obvious, ranging from 44 μm to 51 μm
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(Fig.19 b). With the pulse duty cycle increased to 60%, there was little change in depth as the
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pulse frequency increased, which was maintained at about 53 μm (Fig.19 c).
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(a) Diameter (b) Depth Fig.18 Diameter and depth of micro dimple generated with pulse current.
Fig.19 Comparison of the profiles of micro dimples generated with different pulse parameters. In order to investigate the reason for the phenomenon, the dissolution characteristic of 17
stainless steel 304 in NaNO3 solution was analyzed with a polarization curve, as shown in Fig.20. It can be seen that the current density had little increase with the potential between −0.5 V to 1.5 V, which indicated a passive region. In this region, there was a compact oxidation film on the metal surface, preventing the dissolution of the metal. With the further increase of potential, the metal surface was into the transpassive region, the oxidation film was broken and the metal began to be dissolved. According to the polarization curve, it can be obtained that oxidation film could be
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formed in electrochemical machining of stainless steel 304, especially in low current density. In
the section 2 (Fig.5), the numerical simulation indicated that although the voltage was high (30 V),
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the current density was quite low in conductive mask, and it was easy to form the oxidation film
during the machining. In pulse current, there were rising edge and falling edge at the beginning and end of each pulse width, respectively. In both of the stages, the current density was lower than
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that in steady stage, and the time was mainly used to form oxidation film. Then the oxidation film
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was broken and metal began to be dissolved in the steady stage of pulse width. With the frequency
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increased, the pulse width was reduced, and the proportion of the time in rising and falling edge was enlarged in every pulse width. Hence, the total time for the metal dissolution was reduced,
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which decreased the depth of micro dimple. The phenomenon would be more dramatic in low
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pulse duty cycle with high frequency. As shown in Fig.19 a, in the same pulse duty cycle of 20%, with the frequency increasing from 5 kHz to 15 kHz, the total time for dissolution of metal was reduced, and the depth of micro dimple decreased from 41 μm to 27 μm correspondingly. With the
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pulse duty cycle increasing, the proportion of time in steady stage was increased in one pulse width, the total time for metal dissolution was prolonged, and then the machining depth was
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increased, which weakened the difference of depth in different frequencies. Hence, in pulse duty cycle of 40%, the difference was reduced, and there was little difference when the pulse duty cycle
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increased to 60%.
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Fig.20 Polarization curve of stainless steel 304 in NaNO3 solution.
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According to the experimental results in pulse current, it can be obtained that with conductive
mask, the diameter of micro dimple had little change in different pulse duty cycles and frequencies. However, the depth of micro dimple had significant change in pulse duty cycle of 20% with
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different frequencies. With the pulse duty cycle increased, the difference of depth was weakened. Compared with pulse current, direct current was more appropriate for generating deep micro
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dimple with conductive mask.
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4.4 The analysis of current efficiency
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The experiment results have proved that conductive mask was helpful for improving the machining localization. In other side, current efficiency should be also considered during
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machining, and it means the fraction of the current consumed for the anodic dissolution of the workpiece. In this section, the current efficiencies of micro dimples generated with different
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masks were analyzed.
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According to Eq. (2), the rate of removal of material can be obtained as follows:
v i
I A
(7)
where I is the current detected during machining, and A is the machining area on the
workpiece. In the present machining process, the depth was proportional to machining time.
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Hence, in the Y direction, the removal depth h can be obtained as follows:
h v t
It A
(8)
In this experiment, nine micro dimples were generated at one time, and the machining area on the workpiece is given by
A n
D2 4 19
(9)
where n is the number of the micro dimples, and D is the diameter of the micro dimple. The current efficiency can be obtained as follows:
nh D 2 100% 4 It
(10)
During the machining, the current was 1.5 A with conductive mask, and 0.9 A with insulated mask.
calculated as follows:
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n D 2 hinsulated mask 100% 48.8% insulated mask 4 ( It )insulated mask n D 2 hconductive mask 100% 0.5% conductive mask 4 ( It )conductive mask
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At last, according to the machining results in Fig.15a and b, the current efficiency can be
(11)
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The results showed that in insulated mask, the current efficiency was about 48.8%, while the current efficiency was just about 0.5% in conductive mask. The low current efficiency can be
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explained that along with the dissolution of workpiece material, electrochemical reactions
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(producing oxygen) also proceeded on the surface of conductive mask, as the whole conductive
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mask surface was exposed, it consumed considerable fraction of the current leading to a low current efficiency.
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From the results it can be recognized that although conductive mask was helpful for preparing micro dimple with high machining localization, the current efficiency was considerably lower than
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that in insulated mask, which raised the energy consumption of the process and reduced its productivity. Hence, in the future work, it is necessary to insulate the top surface of conductive
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mask to avoid the electrochemical reactions on the surface, which would improve the machining localization as well as the current efficiency. 5. Conclusion
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This paper proposed a method of conductive mask JEM for generating micro dimple. In this
method, a conductive mask instead of insulated mask was introduced to JEM for reducing the undercutting of micro dimple. Based on the simulation and experimental results, the following conclusions can be drawn: 1. Simulation results indicated that conductive mask used in JEM weakened the electric field intensity at the edge of micro dimple, and the undercutting of micro dimple was reduced 20
significantly. By machining micro dimple with the same depth of 55 μm, the etch factor (EF) of micro dimple increased from 1.47 to 6.11 when the insulated mask was replaced with conductive mask, and the machining localization was improved. 2. In conductive mask JEM, with the depth increased from 45 μm to 85 μm, the undercutting of micro dimple enlarged from 7 μm to 15 μm. The material removal rate in depth was evidently faster than that in diameter, which showed a low undercutting and high machining localization.
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3. With pulse current, the diameter of micro dimple had little change in different pulse duty
cycles and frequencies. However, the depth of micro dimple had significant decrease in pulse duty
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cycle of 20% with increasing frequency. Compared with pulse current, direct current was more appropriate for generating deep micro dimple in conductive mask JEM.
4. The conductive mask could improve the machining localization, but the current efficiency
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was lower than that in insulated mask.
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Acknowledgements：The work described in this study was supported by National Natural Science
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Foundation of China (Grant No. 51705089), Joint Funds of the National Natural Science Foundation of China and Guangdong Province (Grant No. U1601201), Jiangsu Key Laboratory of
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Precision and Micro-Manufacturing Technology, and the Guangzhou Key Laboratory of
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Nontraditional Machining and Equipment (No. 201605030007). Reference
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