- Email: [email protected]
Study on Kerosene Submerged Jet Electrolytic Micromachining
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 432 – 437
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Study on Kerosene Submerged Jet Electrolytic Micromachining Pingmei Ming*, Xinchao Li, Xinmin Zhang, Xudong Song, Jintao Cai, Ge Qin, Liang Yan, Xinshuai Zheng Institute of Non-traditionary Machining and Equipment, Henan Polytechnic University, Jiaozuo 454000, China
* Corresponding author. Tel.: +86-391-398-7530; fax: +86-391-398-7511. E-mail address: [email protected]
Abstract
This paper proposes a non-conventional jet electrolytic machining process to further improve localization of anodic dissolution by submerging both the nozzle and the workpiece in the kerosene, and verification experiments are conducted by fabricating micro-dimples. The experimental results show that, compared to the traditional unsubmerged Jet-ECM processes, the kerosene submerged Jet-ECM process shows a better localization if appropriately low voltages are chosen, and further, it exhibits both a better localization and a bigger removal rate when a suitable working gap is also satisfied. In addition, smoother surfaces can be observed, showing a smaller surface roughness, in the all micro-dimples machined under the kerosene. ©2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of thecscientific committee of the 19th CIRP Conference and Chemical Machining. Peer-review under responsibility of the scientifi committee of the 19th CIRP Conference on Electro Physical on andElectro ChemicalPhysical Machining Keywords: Jet Electrolytic Micromachining; Submerged Impinging Jet; Jet Electrochemical Machining; Kerosene; Jet-ECM
1. Introduction Jet electrolytic machining (Jet-ECM), which is based on the anodic dissolution process using an impinging free electrolytic jet ejected from a cathodic nozzle, has been closely studied in recent years for its significant potential applications as a high precision and fast micro-machining technique. In Jet-ECM, metallic workpiece can be machined selectively under a relatively low temperature, showing no residual stresses and cracks, burrs, and heat affected layers [1]. Up to now, Jet-ECM has been used as a variety of machining tools for pitting [2], surface texturing [3], perforating [4], cutting [4-6], milling [46], and polishing [7], grooving [4], turning [6], and making complicated structures [8,9], etc. In most cases, Jet-ECM processes are carried out in an unsubmerged condition where the impinging free jet is surrounded only by the ambient air, and interaction effects between the air and the liquid jet such as entraining effect are
usually neglected. In such a situation, the electrolyte flow film is generally formed on the workpiece surface surrounding the stagnation point of the impinging jet, extending in radial directions, and a hydraulic jump phenomenon can be frequently observed in the region close to the stagnation point during the flow film extending, which further thickens the electrolyte film. The electrolyte flow film inevitably delocalizes the current distribution of the electrolyte jet because it carries active ions and conducts stray current. This would lead to overcut, decreasing machining accuracy, and also a decrease in machining rate during Jet-ECM. Therefore, further confining the electrolytic jet current into the designated zone has become one of common attempts to improve capacities of the Jet-ECM processes. It has been verified that, the addition of laser beam to the electrolyte jet co-axially can achieve a higher dissolution rate and a better localization and surface finish [10,11]. Recently, the air jet has been additionally superimposed to the electrolyte jet to enhance the dissolution rate and machining
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.091
Pingmei Ming et al. / Procedia CIRP 68 (2018) 432 – 437
accuracies of Jet-ECM process by reducing the electrolyte film and localizing the electrolytic current [5]. This paper presents a non-traditional Jet-ECM process, which is carried out in an insulating liquid medium (such as kerosene) indissoluble with the electrolyte, with an expectation to enhance the machining localization by localizing the electrolytic current of the impinging jet. In this newly proposed Jet-ECM, the nozzle, the workpiece, and the electrolyte jet are all submerged in the kerosene. Kerosene is an excellent electrical insulating medium which is commonly used in electro discharge machining processes due to its some favorable properties. These properties include good flowability even at a low temperature, good thermal stability and oxidation resistance, low attack to metal parts and few impurities, etc. Additionally, we found from our experiments and simulations that, reflection of the electrolyte caused by hitting of the electrolyte jet against the solid surface was dramatically inhibited. The reflection of the electrolyte usually results in a re-erosion occurring at the regions the wall electrolyte-jet flow film contacted. Consequently, by using the kerosene submerged Jet-ECM, it is expected that the removal rate and the localization (machining accuracy) could be greatly improved. In the following sections, experimental and simulation investigations will be described.
433
the electrolyte and the kerosene surrounds the electrolyte jet. a
b
2. Principle, simulations and experimental 2.1. Principle of kerosene submerged Jet-ECM The schematic diagram of the kerosene submerged Jet-ECM is illustrated in Fig.1. In the kerosene submerged Jet-ECM, the common aqueous electrolyte is used. When the pressurized electrolyte is ejected from the cathode nozzle, it forms an impinging electrolyte jet, then the electrolyte jet hits the anode surface of the workpiece after passing through the kerosene layer. If an appropriate voltage is applied to the gap between the cathode nozzle and the workpiece, the materials which are impinged by the jet will be selectively removed electrolytically.
Fig.2 Simulated flow field distribution during electrolytic jet drilling with and without kerosene. (a) with kerosene; (b) without kerosene.
This mixture flow helps to inhibit the stray current because of its higher electric resistance compared to the pure electrolyte flow. It was also found from the simulations that, with the kerosene, a very small fraction of the electrolyte can be reflected to the outside walls of the nozzle, and further the reflected electrolyte discontinuously covers the nozzles after it mixes the kerosene. In such a situation, the possibility of supplying electrolytic current for the outside walls of the nozzle with the workpiece surface through the reflected electrolyte is greatly reduced, thus significantly decreasing stray current. In contrast, as shown in Fig. 2(b), in the traditional jet electrochemical drilling process, the reflection of the hitting electrolyte is very significant because of very small flow resistance from an air medium, and thus the interelectrode gap is fully filled by the electrolyte and most outside walls of the nozzle are covered by the reflected electrolyte. Therefore, in the unsubmerged Jet-ECM, stray-current erosion is generally serious.
Fig.1 Schematic view of the kerosene submerged Jet-ECM
Consequently, in theory, compared to the unsubmerged JetECM process, the kerosene submerged Jet-ECM has more localized and centralized electrolytic current to dissolve the materials, producing an improved machining rate and accuracy.
2.2. Simulation of kerosene submerged Jet-ECM 2.3. Experimental setup Fig.2 shows flow field distribution during Jet-ECM-drilling micro-dimple without considering electric field using a commercial COMOSL Multiphysics software. It can be seen from the Fig.2 that, during machining, some kerosene is entrained into the electrolyte jet and a mixture composing of
Fig. 3 shows the experimental setup. The workpiece is installed in an electrolytic cell which is placed on an XY horizontal table. The nozzle is fixed vertically on a Z table
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which can be moved up and down. In order to eliminate the possible fluctuation of jet flow during ejecting, the pressure
a
b
Fig.3 Schematic diagram of the setup used for the kerosene submerged Jet-ECM
applied to the electrolyte is produced by a constant highpressure nitrogen gas tank. To maintain a constant height of the kerosene in the electrolytic cell, a throttle valve was set on the electrolytic cell for regulating the flow-out velocity of the used electrolyte.
Fig.4 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.1mm. (a) diameter, depth; (b) aspect ratio.
a
Since pulsed jet was used, to avoid interference of the meniscus-shaped electrolyte droplet with filling of the kerosene and fresh electrolyte and discharging of electrolytic products in a small working gap during the pulse-off period, a controlling sub-system was specially added to the setup. The meniscusshaped electrolyte droplet is usually observed at the exit of the small nozzle after the jet is stopped. 2.4. Materials and methods In this study, a sodium nitrate (NaNO3) aqueous solution with a concentration of 20 wt.% was used as the electrolyte. Different inner diameter of the stainless steel (SUS 304) nozzle was selected, which ranges from about 170μm to 340μm. The working gap between the nozzle and the workpiece surface was adjusted in the range of 100μm-300μm. A stainless steel SUS304 sheet was used as the workpiece. Pulsed jet with a pulse-on time, 3s, and a pulse-off time, 3s, respectively, was used. Output pressure of the electrolyte was 1.0 MPa. A digital microscope (VHX-2000, Keyence, Japan) and a scanning electron microscope (Merlin Compact, Carl Zeiss NTS GmbH, German) were used to characterize qualitatively and quantitatively the machined micro-structures. An interferometric surface profilometer (Talysurf CCI6000, Taylor, UK) was employed to measure surface roughness of the machined surfaces.
b
Fig.5 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.2mm. (a) diameter, depth; (b) aspect ratio.
Pingmei Ming et al. / Procedia CIRP 68 (2018) 432 – 437
3. Results and discussion 3.1. Geometric dimensions To evaluate the effectiveness of the proposed Jet-ECM, a
b
435
the effects of working gap on aspect ratio of the machined micro-dimples produced with and without kerosene. However, in comparison with the unsubmerged Jet-ECM, the kerosene submerged Jet-ECM shows a better machining localization and a bigger removal rate if appropriate processing conditions are provided. According to present experiments, the appropriate processing conditions are working gap of 0.2mm, applied voltages of less than 14V. Of course, if other working gaps such as 0.1mm and 0.3mm were chosen, the kerosene submerged Jet-ECM can only produce either smaller or deeper micro-dimples. In addition, the voltage at which the biggest diameter-difference and depth-difference between the microdimples formed with and without kerosene depends on the applied working gap. For example, as 0.2mm working gap was set, biggest diameter-difference is obtained at 8V, with the micro-dimple produced with kerosene being 11.7μm smaller than the one produced without kerosene, but when 0.3mm working gap was used, the voltage at which biggest diameterdifference of about 14μm is produced is 6V. It was found from Fig.7 that, if a very small working gap was chosen, i.e., 0.1mm, the achieved aspect ratio of microdimples generated at very low voltages under the kerosene was higher than that from the unsubmerged process; if the working gap was increased to 0.3 mm, the voltage range in which bigger aspect-ratio micro-dimples can be created was further widen; whereas if the working gap was appropriately selected, i.e., 0.2mm, at most of the examined voltages, the micro-dimples created under the kerosene showed a higher aspect ratio than those fabricated without kerosene.
Fig.6 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.3mm. (a) diameter, depth; (b) aspect ratio.
machining of micro-dimples was implemented in a stationary manner by using two kinds of inner-diameter nozzle, 0.17 mm and 0.34 mm, respectively. Fig.4 -Fig. 6 show the change trend of depth and diameter of the machined micro-dimples with the applied voltage when the 0.17mm-innner-diameter nozzle was used. Like the traditional Jet-ECM processes carried out in an ambient air environment, the depth and diameter of the micro-dimples fabricated in the kerosene submerged Jet-ECM both increase with increasing the applied voltage, with a gradually decreasing increment, when a given working gap is set. Fig. 7 summarizes
Fig.7 Change of aspect ratio of the machined micro-dimple with the applied voltage and the working gap.
For a large nozzle (inner diameter, 0.34mm), it was showed that, compared to the small nozzle, more satisfied microdimples can be produced by the kerosene submerged Jet-ECM, and similar change trends involving the above-mentioned variable relationships can also be observed. By using the large nozzle, better localization and/or bigger removal rate can be achieved. Moreover, in the case of using the large nozzle, the micro-dimples with smoother surfaces can be obtained. This is due mainly to bigger jet rate and better mass transfer conditions that the large nozzle can give. In summary, in comparison with the unsubmerged Jet-ECM, if appropriately low voltages were applied, the kerosene submerged Jet-ECM can produce the micro-dimples with a higher aspect-ratio regardless of the chosen working gap; on the other hand, if an appropriate working gap was selected, it can create the micro-dimples with a higher aspect ratio in a large range of voltages. This implies that, compared with the unsubmerged Jet-ECM, both a bigger anodic dissolution rate and a better localization can be achieved in the kerosene submerged Jet-ECM if appropriate process conditions are satisfied. The reasons for the above findings are described as follows. On the one hand, as described in section 2.2, the inhibition effects of the kerosene on stray current play a dominant role on reducing stray-current erosion and centralizing the current. On the other hand, at some less suitable processing conditions, blocking effects of the kerosene film on discharging of the
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electrolytic products away from the machining space start to dominate, which generally increases overcut and coarsens the machined surface. These less suitable processing conditions include a high applied voltage which means more electrolytic products are produced, a narrow working gap which means that the electrolytic products produced cannot be expelled away smoothly, and a slow jet velocity which means a relatively low mass transfer rate.
and 66.62nm for Rz, being in the mirror-finish range. Fig.9 shows the change of the smallest surface roughness, Ra, measured at the central zone of micro-dimple, with the applied voltage in the kerosene submerged and unsubmerged Jet-ECM experiments with a 0.3 mm working gap. 4. Machining of micro-dimple array and microperforations
3.2. Morphologies and surface roughness Fig.8 illustrates morphologies and profiles of the machined micro-dimples under the conditions of 26V voltage, 0.2mm a b
Fig. 10 Micro-dimple arrays fabricated by the kerosene submerged JetECM using the same conditions.
c
d
Fig.8 Profiles of the machined micro-dimples using the nozzle with two different diameters by kerosene submerged Jet-ECM. (a) 0.17mm, topography; (b) 0.34mm, topography; (c) 0.17mm, SEM; (d) 0.34mm, SEM.
working gap and with the kerosene. It can be seen that smooth surface appears at exit of the micro-dimples and few straycurrent erosions are observed on the surface surrounding the micro-dimples. It was also found from our experimental results that glossy surfaces can be observed in all the micro-dimples produced at all examined voltages and working gaps. The range of surface roughness at the central zone of micro-dimple obtained at the voltage of 16V and the working gap of 0.3mm is 24.05nm-
Fig.11 Micro-perforations drilled by the kerosene submerged Jet-ECM
Fig.10 and Fig.11 respectively show SEM image of the micro-dimples and through holes fabricated by the kerosene submerged Jet-ECM under the same conditions in a stationary manner. For the micro-dimples, the processing parameters were working gap, 200μm; nozzle’s inner diameter, 172.6μm; applied voltage, 8V; machining time, 15 s; and for the through holes, they were working gap, 200μm; nozzle’s inner diameter, 172.6μm; applied voltage, 14V; machining time, 15s. These machined microstructure arrays showed a fairly identical profile features, meaning that the kerosene submerged JetECM has a good reproducibility in micro-machining. 5. Conclusions Aiming at further improving the machining accuracy and rate of Jet-ECM, this paper proposed a novel process, in which, the nozzle and the workpiece are both submerged in the kerosene. Some evaluations were carried out experimentally on this proposed Jet-ECM, and the conclusions are described as follows.
Fig.9 Influence of the applied voltage on surface roughness, Ra, with the working gap of 0.3 mm.
33.99nm. The smallest surface roughness is 24.05nm for Ra
(1) In the kerosene submerged Jet-ECM, machining localization and removal rate can be improved under the suitable processing conditions.
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(2) To maximize the capacities of the kerosene submerged Jet-ECM, relatively low voltages and an appropriate working gap are preferentially used. (3) Uniform arrayed micro structures including microdimples and micro-perforations can be successfully fabricated using the same electrolyte jet under the kerosene. Acknowledgements This work was financially supported by the National Natural Science Foundation of China [No.51475149], Plan For Scientific Innovation Talent of Henan Province [No.154100510008]; and Key Program of Science & Technology of Henan Province [No.172102210025].
References [1] Kozak J, Rajurkar KP, Balkrishna R. Study of electrochemical jet machining process. J Manuf Sci Eng 1996;118: 490-498. [2] Kunieda M. Influence of micro indents formed by electro-chemical jet machining on rolling bearing fatigue life. ASME PED 1993; 64: 693-699.
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[3] Kawanaka T, Kato S, Kunieda M, et al. Selective surface texturing using electrolyte jet machining. Procedia CIRP 2014; 13: 345-349. [4] Hackert-Oschätzchen M, Meichsner G, Zinecker M, et al. Micro machining with continuous electrolytic free jet. Precision Eng 2012; 36: 612-619. [5] Kai S, Sai H, Kunieda M, et al. Study on electrolyte jet cutting. Procedia CIRP 2012; 1: 627-632. [6] Kunieda M, Mizugai K, Watanabe S, et al. Electrochemical micromachining using flat electrolyte jet. CIRP Annals Manuf Tech 2011; (1): 251-254. [7] Kawanaka T, Kunieda M. Mirror-like finishing by electrolyte jet machining. CIRP Annal Manuf Tech 2015; (1): 237-240. [8] Natsu W, Ikeda T, Kunieda M. Generating complicated surface with electrolyte jet machining. Precision Eng 2007; (1): 33-39. [9] Natsu W, Ooshiro S, Kunieda M. Research on generation of threedimensional surface with micro-electrolyte jet machining. CIRP J Manuf Sci Tech 2008; (1): 27-34. [10] DeSilva AKM, Pajak PT, Harrison DK, et al. Modelling and experimental investigation of laser assisted jet electrochemical machining. CIRP Annal Manuf Tech 2004; (1): 179-182. [11] Pajak PT, Desilva AKM, Harrison DK, et al. Precision and efficiency of laser assisted jet electrochemical machining. Precision Eng 2006; (3): 288298.
ScienceDirect Procedia CIRP 68 (2018) 432 – 437
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Study on Kerosene Submerged Jet Electrolytic Micromachining Pingmei Ming*, Xinchao Li, Xinmin Zhang, Xudong Song, Jintao Cai, Ge Qin, Liang Yan, Xinshuai Zheng Institute of Non-traditionary Machining and Equipment, Henan Polytechnic University, Jiaozuo 454000, China
* Corresponding author. Tel.: +86-391-398-7530; fax: +86-391-398-7511. E-mail address: [email protected]
Abstract
This paper proposes a non-conventional jet electrolytic machining process to further improve localization of anodic dissolution by submerging both the nozzle and the workpiece in the kerosene, and verification experiments are conducted by fabricating micro-dimples. The experimental results show that, compared to the traditional unsubmerged Jet-ECM processes, the kerosene submerged Jet-ECM process shows a better localization if appropriately low voltages are chosen, and further, it exhibits both a better localization and a bigger removal rate when a suitable working gap is also satisfied. In addition, smoother surfaces can be observed, showing a smaller surface roughness, in the all micro-dimples machined under the kerosene. ©2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of thecscientific committee of the 19th CIRP Conference and Chemical Machining. Peer-review under responsibility of the scientifi committee of the 19th CIRP Conference on Electro Physical on andElectro ChemicalPhysical Machining Keywords: Jet Electrolytic Micromachining; Submerged Impinging Jet; Jet Electrochemical Machining; Kerosene; Jet-ECM
1. Introduction Jet electrolytic machining (Jet-ECM), which is based on the anodic dissolution process using an impinging free electrolytic jet ejected from a cathodic nozzle, has been closely studied in recent years for its significant potential applications as a high precision and fast micro-machining technique. In Jet-ECM, metallic workpiece can be machined selectively under a relatively low temperature, showing no residual stresses and cracks, burrs, and heat affected layers [1]. Up to now, Jet-ECM has been used as a variety of machining tools for pitting [2], surface texturing [3], perforating [4], cutting [4-6], milling [46], and polishing [7], grooving [4], turning [6], and making complicated structures [8,9], etc. In most cases, Jet-ECM processes are carried out in an unsubmerged condition where the impinging free jet is surrounded only by the ambient air, and interaction effects between the air and the liquid jet such as entraining effect are
usually neglected. In such a situation, the electrolyte flow film is generally formed on the workpiece surface surrounding the stagnation point of the impinging jet, extending in radial directions, and a hydraulic jump phenomenon can be frequently observed in the region close to the stagnation point during the flow film extending, which further thickens the electrolyte film. The electrolyte flow film inevitably delocalizes the current distribution of the electrolyte jet because it carries active ions and conducts stray current. This would lead to overcut, decreasing machining accuracy, and also a decrease in machining rate during Jet-ECM. Therefore, further confining the electrolytic jet current into the designated zone has become one of common attempts to improve capacities of the Jet-ECM processes. It has been verified that, the addition of laser beam to the electrolyte jet co-axially can achieve a higher dissolution rate and a better localization and surface finish [10,11]. Recently, the air jet has been additionally superimposed to the electrolyte jet to enhance the dissolution rate and machining
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.091
Pingmei Ming et al. / Procedia CIRP 68 (2018) 432 – 437
accuracies of Jet-ECM process by reducing the electrolyte film and localizing the electrolytic current [5]. This paper presents a non-traditional Jet-ECM process, which is carried out in an insulating liquid medium (such as kerosene) indissoluble with the electrolyte, with an expectation to enhance the machining localization by localizing the electrolytic current of the impinging jet. In this newly proposed Jet-ECM, the nozzle, the workpiece, and the electrolyte jet are all submerged in the kerosene. Kerosene is an excellent electrical insulating medium which is commonly used in electro discharge machining processes due to its some favorable properties. These properties include good flowability even at a low temperature, good thermal stability and oxidation resistance, low attack to metal parts and few impurities, etc. Additionally, we found from our experiments and simulations that, reflection of the electrolyte caused by hitting of the electrolyte jet against the solid surface was dramatically inhibited. The reflection of the electrolyte usually results in a re-erosion occurring at the regions the wall electrolyte-jet flow film contacted. Consequently, by using the kerosene submerged Jet-ECM, it is expected that the removal rate and the localization (machining accuracy) could be greatly improved. In the following sections, experimental and simulation investigations will be described.
433
the electrolyte and the kerosene surrounds the electrolyte jet. a
b
2. Principle, simulations and experimental 2.1. Principle of kerosene submerged Jet-ECM The schematic diagram of the kerosene submerged Jet-ECM is illustrated in Fig.1. In the kerosene submerged Jet-ECM, the common aqueous electrolyte is used. When the pressurized electrolyte is ejected from the cathode nozzle, it forms an impinging electrolyte jet, then the electrolyte jet hits the anode surface of the workpiece after passing through the kerosene layer. If an appropriate voltage is applied to the gap between the cathode nozzle and the workpiece, the materials which are impinged by the jet will be selectively removed electrolytically.
Fig.2 Simulated flow field distribution during electrolytic jet drilling with and without kerosene. (a) with kerosene; (b) without kerosene.
This mixture flow helps to inhibit the stray current because of its higher electric resistance compared to the pure electrolyte flow. It was also found from the simulations that, with the kerosene, a very small fraction of the electrolyte can be reflected to the outside walls of the nozzle, and further the reflected electrolyte discontinuously covers the nozzles after it mixes the kerosene. In such a situation, the possibility of supplying electrolytic current for the outside walls of the nozzle with the workpiece surface through the reflected electrolyte is greatly reduced, thus significantly decreasing stray current. In contrast, as shown in Fig. 2(b), in the traditional jet electrochemical drilling process, the reflection of the hitting electrolyte is very significant because of very small flow resistance from an air medium, and thus the interelectrode gap is fully filled by the electrolyte and most outside walls of the nozzle are covered by the reflected electrolyte. Therefore, in the unsubmerged Jet-ECM, stray-current erosion is generally serious.
Fig.1 Schematic view of the kerosene submerged Jet-ECM
Consequently, in theory, compared to the unsubmerged JetECM process, the kerosene submerged Jet-ECM has more localized and centralized electrolytic current to dissolve the materials, producing an improved machining rate and accuracy.
2.2. Simulation of kerosene submerged Jet-ECM 2.3. Experimental setup Fig.2 shows flow field distribution during Jet-ECM-drilling micro-dimple without considering electric field using a commercial COMOSL Multiphysics software. It can be seen from the Fig.2 that, during machining, some kerosene is entrained into the electrolyte jet and a mixture composing of
Fig. 3 shows the experimental setup. The workpiece is installed in an electrolytic cell which is placed on an XY horizontal table. The nozzle is fixed vertically on a Z table
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Pingmei Ming et al. / Procedia CIRP 68 (2018) 432 – 437
which can be moved up and down. In order to eliminate the possible fluctuation of jet flow during ejecting, the pressure
a
b
Fig.3 Schematic diagram of the setup used for the kerosene submerged Jet-ECM
applied to the electrolyte is produced by a constant highpressure nitrogen gas tank. To maintain a constant height of the kerosene in the electrolytic cell, a throttle valve was set on the electrolytic cell for regulating the flow-out velocity of the used electrolyte.
Fig.4 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.1mm. (a) diameter, depth; (b) aspect ratio.
a
Since pulsed jet was used, to avoid interference of the meniscus-shaped electrolyte droplet with filling of the kerosene and fresh electrolyte and discharging of electrolytic products in a small working gap during the pulse-off period, a controlling sub-system was specially added to the setup. The meniscusshaped electrolyte droplet is usually observed at the exit of the small nozzle after the jet is stopped. 2.4. Materials and methods In this study, a sodium nitrate (NaNO3) aqueous solution with a concentration of 20 wt.% was used as the electrolyte. Different inner diameter of the stainless steel (SUS 304) nozzle was selected, which ranges from about 170μm to 340μm. The working gap between the nozzle and the workpiece surface was adjusted in the range of 100μm-300μm. A stainless steel SUS304 sheet was used as the workpiece. Pulsed jet with a pulse-on time, 3s, and a pulse-off time, 3s, respectively, was used. Output pressure of the electrolyte was 1.0 MPa. A digital microscope (VHX-2000, Keyence, Japan) and a scanning electron microscope (Merlin Compact, Carl Zeiss NTS GmbH, German) were used to characterize qualitatively and quantitatively the machined micro-structures. An interferometric surface profilometer (Talysurf CCI6000, Taylor, UK) was employed to measure surface roughness of the machined surfaces.
b
Fig.5 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.2mm. (a) diameter, depth; (b) aspect ratio.
Pingmei Ming et al. / Procedia CIRP 68 (2018) 432 – 437
3. Results and discussion 3.1. Geometric dimensions To evaluate the effectiveness of the proposed Jet-ECM, a
b
435
the effects of working gap on aspect ratio of the machined micro-dimples produced with and without kerosene. However, in comparison with the unsubmerged Jet-ECM, the kerosene submerged Jet-ECM shows a better machining localization and a bigger removal rate if appropriate processing conditions are provided. According to present experiments, the appropriate processing conditions are working gap of 0.2mm, applied voltages of less than 14V. Of course, if other working gaps such as 0.1mm and 0.3mm were chosen, the kerosene submerged Jet-ECM can only produce either smaller or deeper micro-dimples. In addition, the voltage at which the biggest diameter-difference and depth-difference between the microdimples formed with and without kerosene depends on the applied working gap. For example, as 0.2mm working gap was set, biggest diameter-difference is obtained at 8V, with the micro-dimple produced with kerosene being 11.7μm smaller than the one produced without kerosene, but when 0.3mm working gap was used, the voltage at which biggest diameterdifference of about 14μm is produced is 6V. It was found from Fig.7 that, if a very small working gap was chosen, i.e., 0.1mm, the achieved aspect ratio of microdimples generated at very low voltages under the kerosene was higher than that from the unsubmerged process; if the working gap was increased to 0.3 mm, the voltage range in which bigger aspect-ratio micro-dimples can be created was further widen; whereas if the working gap was appropriately selected, i.e., 0.2mm, at most of the examined voltages, the micro-dimples created under the kerosene showed a higher aspect ratio than those fabricated without kerosene.
Fig.6 Change of geometric dimensional features (diameter, depth, and aspect ratio) of the machined micro-dimple with the applied voltage when working gap is 0.3mm. (a) diameter, depth; (b) aspect ratio.
machining of micro-dimples was implemented in a stationary manner by using two kinds of inner-diameter nozzle, 0.17 mm and 0.34 mm, respectively. Fig.4 -Fig. 6 show the change trend of depth and diameter of the machined micro-dimples with the applied voltage when the 0.17mm-innner-diameter nozzle was used. Like the traditional Jet-ECM processes carried out in an ambient air environment, the depth and diameter of the micro-dimples fabricated in the kerosene submerged Jet-ECM both increase with increasing the applied voltage, with a gradually decreasing increment, when a given working gap is set. Fig. 7 summarizes
Fig.7 Change of aspect ratio of the machined micro-dimple with the applied voltage and the working gap.
For a large nozzle (inner diameter, 0.34mm), it was showed that, compared to the small nozzle, more satisfied microdimples can be produced by the kerosene submerged Jet-ECM, and similar change trends involving the above-mentioned variable relationships can also be observed. By using the large nozzle, better localization and/or bigger removal rate can be achieved. Moreover, in the case of using the large nozzle, the micro-dimples with smoother surfaces can be obtained. This is due mainly to bigger jet rate and better mass transfer conditions that the large nozzle can give. In summary, in comparison with the unsubmerged Jet-ECM, if appropriately low voltages were applied, the kerosene submerged Jet-ECM can produce the micro-dimples with a higher aspect-ratio regardless of the chosen working gap; on the other hand, if an appropriate working gap was selected, it can create the micro-dimples with a higher aspect ratio in a large range of voltages. This implies that, compared with the unsubmerged Jet-ECM, both a bigger anodic dissolution rate and a better localization can be achieved in the kerosene submerged Jet-ECM if appropriate process conditions are satisfied. The reasons for the above findings are described as follows. On the one hand, as described in section 2.2, the inhibition effects of the kerosene on stray current play a dominant role on reducing stray-current erosion and centralizing the current. On the other hand, at some less suitable processing conditions, blocking effects of the kerosene film on discharging of the
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electrolytic products away from the machining space start to dominate, which generally increases overcut and coarsens the machined surface. These less suitable processing conditions include a high applied voltage which means more electrolytic products are produced, a narrow working gap which means that the electrolytic products produced cannot be expelled away smoothly, and a slow jet velocity which means a relatively low mass transfer rate.
and 66.62nm for Rz, being in the mirror-finish range. Fig.9 shows the change of the smallest surface roughness, Ra, measured at the central zone of micro-dimple, with the applied voltage in the kerosene submerged and unsubmerged Jet-ECM experiments with a 0.3 mm working gap. 4. Machining of micro-dimple array and microperforations
3.2. Morphologies and surface roughness Fig.8 illustrates morphologies and profiles of the machined micro-dimples under the conditions of 26V voltage, 0.2mm a b
Fig. 10 Micro-dimple arrays fabricated by the kerosene submerged JetECM using the same conditions.
c
d
Fig.8 Profiles of the machined micro-dimples using the nozzle with two different diameters by kerosene submerged Jet-ECM. (a) 0.17mm, topography; (b) 0.34mm, topography; (c) 0.17mm, SEM; (d) 0.34mm, SEM.
working gap and with the kerosene. It can be seen that smooth surface appears at exit of the micro-dimples and few straycurrent erosions are observed on the surface surrounding the micro-dimples. It was also found from our experimental results that glossy surfaces can be observed in all the micro-dimples produced at all examined voltages and working gaps. The range of surface roughness at the central zone of micro-dimple obtained at the voltage of 16V and the working gap of 0.3mm is 24.05nm-
Fig.11 Micro-perforations drilled by the kerosene submerged Jet-ECM
Fig.10 and Fig.11 respectively show SEM image of the micro-dimples and through holes fabricated by the kerosene submerged Jet-ECM under the same conditions in a stationary manner. For the micro-dimples, the processing parameters were working gap, 200μm; nozzle’s inner diameter, 172.6μm; applied voltage, 8V; machining time, 15 s; and for the through holes, they were working gap, 200μm; nozzle’s inner diameter, 172.6μm; applied voltage, 14V; machining time, 15s. These machined microstructure arrays showed a fairly identical profile features, meaning that the kerosene submerged JetECM has a good reproducibility in micro-machining. 5. Conclusions Aiming at further improving the machining accuracy and rate of Jet-ECM, this paper proposed a novel process, in which, the nozzle and the workpiece are both submerged in the kerosene. Some evaluations were carried out experimentally on this proposed Jet-ECM, and the conclusions are described as follows.
Fig.9 Influence of the applied voltage on surface roughness, Ra, with the working gap of 0.3 mm.
33.99nm. The smallest surface roughness is 24.05nm for Ra
(1) In the kerosene submerged Jet-ECM, machining localization and removal rate can be improved under the suitable processing conditions.
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(2) To maximize the capacities of the kerosene submerged Jet-ECM, relatively low voltages and an appropriate working gap are preferentially used. (3) Uniform arrayed micro structures including microdimples and micro-perforations can be successfully fabricated using the same electrolyte jet under the kerosene. Acknowledgements This work was financially supported by the National Natural Science Foundation of China [No.51475149], Plan For Scientific Innovation Talent of Henan Province [No.154100510008]; and Key Program of Science & Technology of Henan Province [No.172102210025].
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