- Email: [email protected]
Wear-resist Electrodes for Micro-EDM
Chinese
Chinese Journal of Aeronautics 22(2009) 339-342
Journal of Aeronautics
www.elsevier.com/locate/cja
Wear-resist Electrodes for Micro-EDM Wang Yuanganga,b, Zhao Fulinga,*, Wang Jina a
Key Lab for Precision & Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China b School of Mechanical Engineering, Dalian University, Dalian 116622, China Received 10 June 2008; accepted 1 September 2008
Abstract
This article proposes a new type of electrode for micro-electro-discharge machining (micro-EDM) , which can produce ultra fine micro components from various kinds of materials including those that cannot be processed by the silicon or the Lithographie Galvanoformung Abformung (LIGA) processings. This electrode is made by way of the electrodeposition process on the basis of the difference between the discharging performance of the electrodeposited coating and that of the matrix to ensure uniform wear of electrode bottom faces. With this electrode, micro-holes on ANK80 steel can be successfully manufactured in a short period of time. Compared to the traditional electrodes, Cu-ZrB2 composite coating electrodes display better wear resistance on the same experimental conditions. The results disclose the prospects for this electrode to be used in producing micro components of high-aspect-ratio and 3D micro-cavities. Keywords: microelectrodes; wear; compensation; electrodeposition; composite coating electrode; micro-EDM
1. Introduction1 Micro-electro-discharge machining (micro-EDM) can be used to fabricate micro-holes and micro-shafts as small as 5 ȝm in diameter as well as a variety of complex shapes by minimizing the discharging energy to the 107 J level and by realizing the precise processes with submicron accuracy[1]. It is considered to be one of the excellent machining methods to produce three-dimensional (3D) complicated microstructures, which have found broad application in the aerospace industry. Wear that has occurred on electrodes in EDM process impairs the geometric accuracy of workpieces. Especially in micro-EDM, it is quite difficult and tedious to repeatedly achieve the desired 3D cavities in the same processing owing to quick changes of the electrode shape[2-3]. One solution is to repeat the processing several times using new or reground microelectrodes until the required profile of holes is obtained. Obviously, it is impracticable because of being rather time-consuming and difficulty in predicting the number of electrodes required. Application of the uniform *Corresponding author. Tel.: +86-411-84706535. E-mail address: [email protected] Foundation item: National Natural Science Foundation of China (50635040) 1000-9361/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi: 10.1016/S1000-9361(08)60108-9
wear method[4], the real-time wear compensation method and the method of enhancing loss speed of electrode[5] to ensure contour integrality is effective only to a certain extent. However, these methods rely on rules about the loss of the electrodes caused by wearing, which are very difficult to be precisely estimated beforehand as the processing conditions change irregularly all the time. On the other hand, the special electrode encased in a dielectric jacket[6] is far from being ready for use in commercial micro-EDM. Composite electrodeposition provides an effectual way to strengthen the electrode surface by depositing onto it a composite layer containing dispersed ceramic particles. In this study, ZrB2 selected as the second phase of the composite coating, has several properties that are superior to others, such as high melting point (3 040 °C), high electrical conductivity, thermal shock stability, anticorrosion, etc. It has already been used to make critical components in aircraft and rockets working at high temperatures[7]. This article investigates the wear characteristics of cylindrical electrodes and discusses the problems about the fabrication of the Cu-ZrB2 composite coating electrode and the strengthening effect of the coating on the micro-EDM. Experimental results from micro-EDM-drilling indicate the effectiveness of the proposed method in decreasing electrode wear and ensuring uniformity of bottom loss.
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Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
No.3
2. Compensation for Electrode Wear 2.1. Characteristics of electrode wear When wear occurs on both bottom and corner of a cylindrical micro-electrode during micro-EDM-drilling, the product will no longer retain the accuracy of its size and form. The experiments performed in this study reveal that the electrode’s form remains invariable after reaching a given shape during micro-EDM-drilling. Finite element analysis (FEM) can be used to simulate the three appearances of electrode wear during micro-EDM (see Fig.1) , from which it is found that the shape of the electrode changes in such a way as to achieve uniform intensity of the electric field eventually.
Fig.2
Structure of composite coating electrode.
matrix as well as increases the hardness of the coating. Therefore, the worn volume of the matrix is larger than that of the coating, thereby protecting the coating material of the bottom face, and the initial form of the electrode undergoes less alteration during micro-EDMdrilling. 3. Experimental Setup 3.1. Fabrication of Cu-ZrB2 composite coating Cu-ZrB2 composite coating was electrodeposited in a copper nitrate bath containing ZrB2 particles of an average size of 2.5 μm. The diameter of the copper tube electrode is 0.5 mm. Fig.3 depicts the experimental facilities and Table 1 presents its detailed information.
Fig.1
Intensity diagrams of electric field of electrode.
As the electrode protrudes into a workpiece, the sparking area undergoes changes, which will make the sparking conditions continuously alter and the electrode taper off because discharging takes place not only at the bottom, but also at the lateral face. This phenomenon dominates the behavior of the electrode wear.
Fig.3 Table 1
2.2. Methods to reduce electrode wear The principle of the composite electrodeposition method put forward in this article lies in taking advantage of the difference between the wear ratio of the matrix and that of the deposited coating so as to achieve an identical wear speed on the bottom face. Fig.2 illustrates the structure of the composite coating electrode. Consisting of Cu and ZrB2, the coating has the same electrical conductivity as the matrix[7], thus facilitating the transfer of thermal energy produced by discharging from the coating to the matrix. Furthermore, containing ZrB2 particles refines the microstructure of the copper
Experimental setup.
Bath and processing conditions
Item
Value
Cu (NO3)2· 3H2O/ (ggL–1) Cl– /10–6 ZrB2 / (ggL–1) pH Temperature/°C Current density/ ( Agdm–2) Duty cycle/%
400 30 20-50 3.0 25 5 10-90
Fig.4 shows the ZrB2 distribution in the Cu-ZrB2 composite coating. 3.2. EDM performance of Cu-ZrB2 Experiments were performed on SODICK EDM
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Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
· 341 ·
coating electrode after operation. The products demonstrate higher accuracy of holes could attain in micro-EDM-drilling by the Cu-ZrB2 composite coating electrode. 4. Discussions
Fig.4
Distribution of ZrB2.
machine (KIC) with workpieces made of NAK 80 in it. A pure copper electrode and a Cu-ZrB2 composite coating electrode served as tool electrodes, respectively. Electrolyte was pumped through the tube electrode. Table 2 lists the parameters in the experiments. Table 2
Experimental conditions
Process parameters
Value
High voltage peak current /A Low voltage peak current /A Supply voltage /V Pulses-on time /ȝs Pulses-off time /ȝs Polarity
4.0 2.5 170 10 20 Anode
Fig.5 depicts the photographs of the cross sections of two holes separately processed with the pure copper electrode and the Cu-ZrB2 composite coating electrode. Fig.6 shows the appearance of Cu-ZrB2 composite
The experimental results indicate that Cu-ZrB2 composite coating electrode has better discharging performance than the pure copper one. This may be ascribed to the fact that since copper has a considerably lower melting point than ZrB2, most of the heat energy generated during discharging is conducted by copper instead of ZrB2, which significantly decreases the amount of eroded ZrB2 particles. The molten and/or evaporated copper carries away most of the heat energy and reduces the surface temperature of the particles. Thus the molten copper tends to recrystallize rapidly onto ZrB2 particles during the sparking interval. Meanwhile, at the end of pulse, the molten material affected by the high temperature and high pressure splashes out, some as scraps falling into electrolyte, while some solidifying on the surfaces of electrode compensating for its wear to some degree. Fig.7 shows the wide yet uneven distribution of Fe on the surface of Cu-ZrB2 composite coating electrode after operation.
Fig.7
Fig.5
Holes processed by different electrodes.
Distribution of Fe on the surface of Cu-ZrB2 composite coating electrode after operation.
Thus, Cu-ZrB2 composite coating serves as a sheath to secure the side surface of electrode against wearing, which will undoubtedly improve the quality of the product. In addition, this proposed process can be integrated into a multi-functional micro-EDM machine in association with wire electrical discharge grinding (WEDG). The performance of coating can be adjusted to fit in with the matrix by modifying the components of the coating and the processing parameters. All this will noticeably enhance the machining efficiency and precision of micro-EDM. 5. Conclusions
Fig.6
Photo of Cu-ZrB2 composite coating electrode after operation.
A wear-resist electrode for micro-EDM has been theoretically analyzed and experimentally investigated. The results prove that Cu-ZrB2 composite coating electrodes have better corrosion resistance than pure copper electrodes. The main conclusions can be drawn as follows: 341
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Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
(1) It is feasible to use the wear compensation method on the basis of using the difference between the wear ratio of matrix and that of coating material to maintain electrode shape precision. (2) The proposed method can be integrated into a multi-functional micro fabrication equipment for online machining of micro electrodes in micro-EDM, which has superior maneuverability. References [1]
[2]
[3]
Masaki T, Kawata K, Masuzawa T. Micro electrodischarge machining and its applications. Proceedings of IEEE International workshop on Micro Electro Mechanical Systems. 1990;21-26. Fleischer J, Kotschenreuther J. The manufacturing of micro molds by conventional and energy-assisted processes. The International Journal of Advanced Manufacturing Technology 2006; 33(1-2): 75-85. Pham D T, Ivanov A, Bigot S, et al. An investigation of tube and rod electrode wear in micro EDM drilling. The International Journal of Advanced Manufacturing
[4] [5] [6]
[7]
No.3
Technology 2007; 33(1-2): 103-109. Yu Z Y, Masuzawa T, Fujino M. Micro-EDM for threedimensional cavities—development of uniform wear method. Annals of the CIRP 1998; 47(1): 169-172. Rajurkar K P, Yu Z Y. 3D micro-EDM using CAD/ CAM. Annals of the CIRP 2000; 49(1): 127-130. Kumagai S, Misawa N, Takeda K, et al. Plasma-applied machining of a narrow and deep hole in a metal using a dielectric-encased wire electrode. Thin Solid Films 2004; 457(1): 180-185. Guo D M, Zhang M, Jin Z J. Pulse plating of copper-ZrB2 composite coatings. Journal of Materials Science and Technology 2006; 22(4): 514-518.
Biography: Wang Yuangang Born in 1979, he received Ph. D. degree from Dalian University of Technology in 2008, and now he is a teacher of Dalian University. His main research interest is precision and non-traditional machining. E-mail: [email protected]
Chinese Journal of Aeronautics 22(2009) 339-342
Journal of Aeronautics
www.elsevier.com/locate/cja
Wear-resist Electrodes for Micro-EDM Wang Yuanganga,b, Zhao Fulinga,*, Wang Jina a
Key Lab for Precision & Non-traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China b School of Mechanical Engineering, Dalian University, Dalian 116622, China Received 10 June 2008; accepted 1 September 2008
Abstract
This article proposes a new type of electrode for micro-electro-discharge machining (micro-EDM) , which can produce ultra fine micro components from various kinds of materials including those that cannot be processed by the silicon or the Lithographie Galvanoformung Abformung (LIGA) processings. This electrode is made by way of the electrodeposition process on the basis of the difference between the discharging performance of the electrodeposited coating and that of the matrix to ensure uniform wear of electrode bottom faces. With this electrode, micro-holes on ANK80 steel can be successfully manufactured in a short period of time. Compared to the traditional electrodes, Cu-ZrB2 composite coating electrodes display better wear resistance on the same experimental conditions. The results disclose the prospects for this electrode to be used in producing micro components of high-aspect-ratio and 3D micro-cavities. Keywords: microelectrodes; wear; compensation; electrodeposition; composite coating electrode; micro-EDM
1. Introduction1 Micro-electro-discharge machining (micro-EDM) can be used to fabricate micro-holes and micro-shafts as small as 5 ȝm in diameter as well as a variety of complex shapes by minimizing the discharging energy to the 107 J level and by realizing the precise processes with submicron accuracy[1]. It is considered to be one of the excellent machining methods to produce three-dimensional (3D) complicated microstructures, which have found broad application in the aerospace industry. Wear that has occurred on electrodes in EDM process impairs the geometric accuracy of workpieces. Especially in micro-EDM, it is quite difficult and tedious to repeatedly achieve the desired 3D cavities in the same processing owing to quick changes of the electrode shape[2-3]. One solution is to repeat the processing several times using new or reground microelectrodes until the required profile of holes is obtained. Obviously, it is impracticable because of being rather time-consuming and difficulty in predicting the number of electrodes required. Application of the uniform *Corresponding author. Tel.: +86-411-84706535. E-mail address: [email protected] Foundation item: National Natural Science Foundation of China (50635040) 1000-9361/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi: 10.1016/S1000-9361(08)60108-9
wear method[4], the real-time wear compensation method and the method of enhancing loss speed of electrode[5] to ensure contour integrality is effective only to a certain extent. However, these methods rely on rules about the loss of the electrodes caused by wearing, which are very difficult to be precisely estimated beforehand as the processing conditions change irregularly all the time. On the other hand, the special electrode encased in a dielectric jacket[6] is far from being ready for use in commercial micro-EDM. Composite electrodeposition provides an effectual way to strengthen the electrode surface by depositing onto it a composite layer containing dispersed ceramic particles. In this study, ZrB2 selected as the second phase of the composite coating, has several properties that are superior to others, such as high melting point (3 040 °C), high electrical conductivity, thermal shock stability, anticorrosion, etc. It has already been used to make critical components in aircraft and rockets working at high temperatures[7]. This article investigates the wear characteristics of cylindrical electrodes and discusses the problems about the fabrication of the Cu-ZrB2 composite coating electrode and the strengthening effect of the coating on the micro-EDM. Experimental results from micro-EDM-drilling indicate the effectiveness of the proposed method in decreasing electrode wear and ensuring uniformity of bottom loss.
· 340 ·
Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
No.3
2. Compensation for Electrode Wear 2.1. Characteristics of electrode wear When wear occurs on both bottom and corner of a cylindrical micro-electrode during micro-EDM-drilling, the product will no longer retain the accuracy of its size and form. The experiments performed in this study reveal that the electrode’s form remains invariable after reaching a given shape during micro-EDM-drilling. Finite element analysis (FEM) can be used to simulate the three appearances of electrode wear during micro-EDM (see Fig.1) , from which it is found that the shape of the electrode changes in such a way as to achieve uniform intensity of the electric field eventually.
Fig.2
Structure of composite coating electrode.
matrix as well as increases the hardness of the coating. Therefore, the worn volume of the matrix is larger than that of the coating, thereby protecting the coating material of the bottom face, and the initial form of the electrode undergoes less alteration during micro-EDMdrilling. 3. Experimental Setup 3.1. Fabrication of Cu-ZrB2 composite coating Cu-ZrB2 composite coating was electrodeposited in a copper nitrate bath containing ZrB2 particles of an average size of 2.5 μm. The diameter of the copper tube electrode is 0.5 mm. Fig.3 depicts the experimental facilities and Table 1 presents its detailed information.
Fig.1
Intensity diagrams of electric field of electrode.
As the electrode protrudes into a workpiece, the sparking area undergoes changes, which will make the sparking conditions continuously alter and the electrode taper off because discharging takes place not only at the bottom, but also at the lateral face. This phenomenon dominates the behavior of the electrode wear.
Fig.3 Table 1
2.2. Methods to reduce electrode wear The principle of the composite electrodeposition method put forward in this article lies in taking advantage of the difference between the wear ratio of the matrix and that of the deposited coating so as to achieve an identical wear speed on the bottom face. Fig.2 illustrates the structure of the composite coating electrode. Consisting of Cu and ZrB2, the coating has the same electrical conductivity as the matrix[7], thus facilitating the transfer of thermal energy produced by discharging from the coating to the matrix. Furthermore, containing ZrB2 particles refines the microstructure of the copper
Experimental setup.
Bath and processing conditions
Item
Value
Cu (NO3)2· 3H2O/ (ggL–1) Cl– /10–6 ZrB2 / (ggL–1) pH Temperature/°C Current density/ ( Agdm–2) Duty cycle/%
400 30 20-50 3.0 25 5 10-90
Fig.4 shows the ZrB2 distribution in the Cu-ZrB2 composite coating. 3.2. EDM performance of Cu-ZrB2 Experiments were performed on SODICK EDM
No.3
Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
· 341 ·
coating electrode after operation. The products demonstrate higher accuracy of holes could attain in micro-EDM-drilling by the Cu-ZrB2 composite coating electrode. 4. Discussions
Fig.4
Distribution of ZrB2.
machine (KIC) with workpieces made of NAK 80 in it. A pure copper electrode and a Cu-ZrB2 composite coating electrode served as tool electrodes, respectively. Electrolyte was pumped through the tube electrode. Table 2 lists the parameters in the experiments. Table 2
Experimental conditions
Process parameters
Value
High voltage peak current /A Low voltage peak current /A Supply voltage /V Pulses-on time /ȝs Pulses-off time /ȝs Polarity
4.0 2.5 170 10 20 Anode
Fig.5 depicts the photographs of the cross sections of two holes separately processed with the pure copper electrode and the Cu-ZrB2 composite coating electrode. Fig.6 shows the appearance of Cu-ZrB2 composite
The experimental results indicate that Cu-ZrB2 composite coating electrode has better discharging performance than the pure copper one. This may be ascribed to the fact that since copper has a considerably lower melting point than ZrB2, most of the heat energy generated during discharging is conducted by copper instead of ZrB2, which significantly decreases the amount of eroded ZrB2 particles. The molten and/or evaporated copper carries away most of the heat energy and reduces the surface temperature of the particles. Thus the molten copper tends to recrystallize rapidly onto ZrB2 particles during the sparking interval. Meanwhile, at the end of pulse, the molten material affected by the high temperature and high pressure splashes out, some as scraps falling into electrolyte, while some solidifying on the surfaces of electrode compensating for its wear to some degree. Fig.7 shows the wide yet uneven distribution of Fe on the surface of Cu-ZrB2 composite coating electrode after operation.
Fig.7
Fig.5
Holes processed by different electrodes.
Distribution of Fe on the surface of Cu-ZrB2 composite coating electrode after operation.
Thus, Cu-ZrB2 composite coating serves as a sheath to secure the side surface of electrode against wearing, which will undoubtedly improve the quality of the product. In addition, this proposed process can be integrated into a multi-functional micro-EDM machine in association with wire electrical discharge grinding (WEDG). The performance of coating can be adjusted to fit in with the matrix by modifying the components of the coating and the processing parameters. All this will noticeably enhance the machining efficiency and precision of micro-EDM. 5. Conclusions
Fig.6
Photo of Cu-ZrB2 composite coating electrode after operation.
A wear-resist electrode for micro-EDM has been theoretically analyzed and experimentally investigated. The results prove that Cu-ZrB2 composite coating electrodes have better corrosion resistance than pure copper electrodes. The main conclusions can be drawn as follows: 341
· 342 ·
Wang Yuangang et al. / Chinese Journal of Aeronautics 22(2009) 339-342
(1) It is feasible to use the wear compensation method on the basis of using the difference between the wear ratio of matrix and that of coating material to maintain electrode shape precision. (2) The proposed method can be integrated into a multi-functional micro fabrication equipment for online machining of micro electrodes in micro-EDM, which has superior maneuverability. References [1]
[2]
[3]
Masaki T, Kawata K, Masuzawa T. Micro electrodischarge machining and its applications. Proceedings of IEEE International workshop on Micro Electro Mechanical Systems. 1990;21-26. Fleischer J, Kotschenreuther J. The manufacturing of micro molds by conventional and energy-assisted processes. The International Journal of Advanced Manufacturing Technology 2006; 33(1-2): 75-85. Pham D T, Ivanov A, Bigot S, et al. An investigation of tube and rod electrode wear in micro EDM drilling. The International Journal of Advanced Manufacturing
[4] [5] [6]
[7]
No.3
Technology 2007; 33(1-2): 103-109. Yu Z Y, Masuzawa T, Fujino M. Micro-EDM for threedimensional cavities—development of uniform wear method. Annals of the CIRP 1998; 47(1): 169-172. Rajurkar K P, Yu Z Y. 3D micro-EDM using CAD/ CAM. Annals of the CIRP 2000; 49(1): 127-130. Kumagai S, Misawa N, Takeda K, et al. Plasma-applied machining of a narrow and deep hole in a metal using a dielectric-encased wire electrode. Thin Solid Films 2004; 457(1): 180-185. Guo D M, Zhang M, Jin Z J. Pulse plating of copper-ZrB2 composite coatings. Journal of Materials Science and Technology 2006; 22(4): 514-518.
Biography: Wang Yuangang Born in 1979, he received Ph. D. degree from Dalian University of Technology in 2008, and now he is a teacher of Dalian University. His main research interest is precision and non-traditional machining. E-mail: [email protected]