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Fabrication of an Electrode Insulated by Using Hot Dip Aluminizing and Micro-arc Oxidation Method for Electrochemical Microhole Machining
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 438 – 443
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Fabrication of an electrode insulated by using hot dip aluminizing and micro-arc oxidation method for electrochemical microhole machining Jung-Chou Hunga,*, Chih-Yang Kua,b, Zhi-Wen Fanc b
a Department of Mechanical and Computer Aided Engineering, Feng Chia University, Taichung 407, Taiwan Structure and Material Systems Department, Aeronautical Systems Research Division, NATIONAL CHUNG-SHAN INSTITUTE OF SCIENCE & TECHNOLOGY, Taichung 407, Taiwan c Regional R&D Service Department, Metal Industries Research & Development Centre, Taichung 407, Taiwan
* Corresponding author. Tel.: +886-4-24517250 Ext. 3532; fax: +886-4-24516545.E-mail address: [email protected].
Abstract In this research, electrode insulation of the electrochemical machining tool has been produced by hot dip aluminized micro arc oxidation techniques. In order to form an effective layer on the tungsten carbide tool and achieve higher precision, the tool will then be microarc oxidizing, convert the aluminum-rich into aluminum oxide insulating layer. Withstanding Voltage of the tool is being tested afterward by sodium chloride electrolysis method, and the surface and the cross section is being observed. Eventually, the tools were examined and discussed for their machining performance by ECM drilling through stainless steel plate. We were expecting to produce a precise and better insulation. By reducing ECM stray effect, the precision can be achieved. The experimental result shows the optimum parameters are aluminum dipping for 4 minutes, 450 V of micro-arc voltage for 20 minutes and 40 minutes of boiling water sealing, reaching 10.2V of withstanding voltage. A further electrochemical drilling has been done to explore the effect of insulation. Two electrodes, with and without insulation had been compared. The difference between the entrance and the exit diameter of the hole of insulated tool reaches 17um, compare to the one without insulation, we have successfully reduced 81um. The result shows that the insulation has effectively reduced the stray effect, and achieve high precision machining. ©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 the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Hot dip aluminizing, aluminum oxide, micro-arc oxidation, insulation, electrochemical machining.
1. Introduction During the precise electrochemical drilling process, the stray current will greatly affect the machining precision, the most effective way to overcome the problem is to cover the cathode surface with insulating material, that the current can be accurately discharged thus reduce the stray electrolysis. At present, the material for such insulation is still being further studied, due to a variety of physical and chemical impact effects, such as pulse voltage, resistance heat and reaction heat, High-pressure flow, etc., will damage and stripping the insulation. To overcome the extreme environment, the bonding force between the insulation and the electrode was strengthened, along with the density and corrosion resistance can effectively promote the machining accuracy.
Micro-arc oxidation is composed of electrochemical reaction, micro-arc oxidation reaction and thermal diffusion, the process including melting, molten flow, re-solidification, diffusion, sintering and densification of the continuous process [4]. Micro-arc oxide layer was consisting of two structural layers, the outer layer filled with micro-holes, cracks and other structural defects, the inner layer performs dense, good adhesion and excellent mechanical properties. The combination of Hot-dip aluminizing and micro-arc oxidation is conducive to the fabrication of excellent insulation. Several studies against hot dip aluminizing and micro-arc oxidation has been made in recent year, for incident, characterization of the intermetallic surface layers of hot-dip Al-coated steel has been studied by researchers leading by Antoine Van Alboom in 2017, and Characterization of micro-arc oxidation coatings
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.2017.12.092
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on aluminum drillpipes at different current density by researchers leading by PingWang in 2017. This study focuses on the fabrication of precision electrochemical processing electrode insulation by hot-dip aluminizing alloy micro-arc oxidation and discusses whether the insulation can be processed under high current density, pulse current, resistance heat environment. Thus, it is important to promote the density and corrosion resistance of the insulation in the harsh environment, resulting in better accuracy. 2. Experimental setup and Process 2.1. Hot Dip Aluminizing Hot Dip Aluminizing (HDA) is a technology that a layer of aluminum is coated on the substrate surface after proper pretreatment. The pretreatment is to make the aluminum easier to attach to the substrate surface. Afterward, dipped the substrate into the molten aluminum for a period of time. As the substrate leaves the molten aluminum, a series process of Nucleation, growth, diffusion and the formation of solid aluminum and aluminum alloy layer will occur on the dipped surface [5], as shown in Figure 1. Material for Hot-dip aluminizing is pure aluminum ingot, the ingot was put into the alumina crucible, and placed into the furnace as shown in Figure 2, the furnace heating up to 750, the molten aluminum will form an Oxide film on the surface from which contacting the air. The oxide film will decrease the adhesion between the electrode and the molten aluminum, so before the electrode was dipped into the molten aluminum the following procedure will be processed: (1) the electrode surface will be coated with a layer of solvent, (2) the molten aluminum was also sprinkled with dry flux to remove the oxide film on the surface. After the Flux treatment, dip plating can be carried out. Dipping parameters are dipping temperature of 750, dipping time of 1 to 6 minutes, and dipping, pulling speed of 80 cm/min.
Fig. 1. Hot-Dip Aluminum Process
Fig. 2. Hot Dip Aluminum Furnace.
2.2. Micro-Arc Oxidation Micro-Arc Oxidation (MAO), also known as Plasma Electrolytic Oxidation (PEO), Microarc Discharge Oxidation (MDO) and Anodic Spark Deposition (SPD). The Micro-arc oxidation is an anodic oxidation by high voltage technology, suitable for non-ferrous metals such as Ti, Al, Mg, Nb and other metals [7]. The partial temperature reaches 103 ~ 104K, and partial pressure reaches 102 ~ 103MPa [8], the entire process is a complex physical and chemical process including melting, vaporization, electrical collapse and electrophoresis. [9]. Micro-arc oxidation will produce four different oxidation stages, namely passive film, porous oxidation, spark discharge and micro-arc discharge [10]. It can also be divided into three areas (1) Faraday area, (2) spark discharge area, (3) micro-arc discharge area [11], as shown in Figure 3. When the voltage rises, similar to traditional anode treatment, the Faraday reaction occurs and form a passivation film. The voltage continues to rise, a large number of bubbles will continuously be generated and break through the passivation film, high voltage then breakthrough white arc point. As the electric field strength continues to increase causes the occurrence of tunneling effect, the spark began to appear and the oxide film also starts to grow rapidly, as shown in Figure 4. When the voltage stabilized, into the micro-arc discharge area, and reached the collapse voltage of the surface, the surface ceramic oxide film continues to grow, ionization turned into the thermal ionization. Micro-arc oxidation equipment includes a power supply, stainless steel tank, cooling system. The electrode should be acetone cleaned before the micro-arc process. The electrode then is placed at the anode, stainless steel tank connected with the cathode and fill the tank with electrolyte. Parameters for micro-arc oxidation are shown in Table.1. Afterward, the micro-arc process is able to begin, processing diagram was shown in Figure 5.
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2.3. Withstanding voltage
Fig. 3. Stages during micro-arc oxidation.
During Electrochemical process, current will pass through the electrode, so the cathode insulation layer should have withstood the machining voltage without disintegration. Therefore, the withstand voltage of the micro-arced electrode was tested by electrolysis method. The anode connected to a piece of stainless steel, the cathode connected to the treated electrode. The electrolyte is 15% of the sodium chloride aqueous solution, and the constant voltage was applied until the bubble was generated from the crack of the insulation, eventually causing the insulation to peel. In order to facilitate the observation of the test, in the electrolysis using inclined release method to observe the insulation when the bubble break, So the electrode is divided into upper, middle and lower part. Withstanding voltage test diagram was shown in Figure 6.
Fig. 6. Withstand Voltage test setup.
2.4. Electrochemical Drilling Test
Fig. 4. Micro-arc generation diagram.
In this experiment, the electrochemical equipment was erected on the ACRO PNC S-325 EDM machine. The equipment includes an electrolyte circulation system, an acrylic tank, clip Fixture, power supply and so on. The electrode was placed on the anode, and the stainless-steel work piece was placed on the anode, with NaNO3 as the electrolyte. The electrochemical drilling setups were shown in Fig7.
Fig. 5. Micro-Arc Processing Diagram. Table 1. Parameters for micro-arc oxidation Parameter
Value
Toll electrode martail
Hot dip aluminized electrode
Processing time(min)
10152025
Electrolyte concentration
10g/L Na2Al2O4+1g/L KOH
Electrolyte temperature()
5
Power supply voltage (V)
450
Fig. 7. Electrochemical drilling diagram.
3. Experimental Result and Discussion 3.1. Hot-dipping Aluminizing The thickness of the coating is one of the key factors in this experiment, and it is also a related to the hot dipping process,
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thus, the effect of the hot dipping time on the coating was being tested in this experiment. The experimental parameters were hot-dipping temperature 750 , dipping and pulling speed 80cm / min, dipping time was set to 1 minute, 2 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, other parameters remain unchanged. The experimental parameters are shown in Table 2.
(a)1min
(b)2min
(c)3min
(d)4min
(e)5min
(f)6min
Table 2. Hot-dipping aluminizing parameters. Parameter
Value
Toll electrode material
Tungsten electrode
Processing time(min)
123456
Hot dip temperature ()
750
Speed of rising and falling(cm/min)
80
3.2. Surface observation and structure analysis Fig. 8 shows the surface morphology after hot-dipping, among them, the surface condition performed relatively better under 4min of dipping time, Figure.8 (d). Shorter dipping time such as Figure.8 (a), (b) and (c) shows no aluminum coated or uneven coating result caused by the lack of dipping time. Figure.8 (e) and (f) shows the result under 5 and 6 minutes dipping time. The surface of the coating was full of micro hole and cracks after longer dipping time. From the different hot-dip plating time and cross-section diagram shown in Figure 9, it was found that dipping for 1min, the thickness of the tungsten-aluminum intercalation layer was 7.6 μm, the thickness of the impregnated layer was 10.3 μm, the tungsten carbide electrode diameter was 196.4mm. The thickness of the tungsten-aluminum intercalation layer increased to 10.7 μm, the thickness of the impregnated layer was 15.5 μm, and the diameter of the tungsten carbide electrode was reduced to 194.6 μm under 2min dipping time. When the dipping time increased to 3 minutes, 4 minutes, 5 minutes, 6 minutes, tungsten aluminum mulching layer increased to 11.7, 12.9, 13.3, 14.7μm, dip aluminum thickness increased to 19.4, 25.5, 32.5, 43.7μm, the tungsten carbide electrode diameter is 192.8, 191.1, 190.2, 188.9mm. According to Fig. 10, the thickness of the mulching layer is thickened by the increase of the thickness of the mope layer, but the thickening of the miscible layer represents the phenomenon of diffusion to both sides resulting in a reduction in the original diameter of the tungsten carbide electrode. Figure 11 shows the distribution of elements after hot dip aluminization, the outskirt is alumina, the oxide protective layer formed after dipping, and then the interior is the aluminum layer, followed by aluminum tungsten miscible layer, and tungsten carbide substrate. From the results above, it can be seen that the plating time affects the surface morphology of the electrode. When the dip plating time is short, the surface coating is uneven, and when the plating time is too long, the surface of the electrode coating is too rough. Therefore, 4 minutes is considered better.
Fig. 8. Hot dip aluminum at 750 for different dipping time. (a)1min, (b)2min, (c)3min, (d)4min, (e)5min, (f)6min.
(a)1min
(b)2min
(c)3min
(d)4min
(e)5min
(f)6min
Fig. 9. Hot dip aluminum at 750 for different dipping time.( Cross section ): (a)1min, (b)2min, (c)3min, (d)4min, (e)5min, (f)6min.
Jung-Chou Hung et al. / Procedia CIRP 68 (2018) 438 – 443
Thickness( μm)
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340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0
Tool electorde material: Tungsten carbide
Tungsten carbide substrate Aluminizing layer Solution layer
Hot dip temperature(OC): 750
Table 3. ithstand voltage for different micro-arc time against the electrode dipped for 4 minutes, micro-arc processing under 450 V and sealing in boiling water for 40 minutes. Micro-arc time
Speed of rising and falling(cm/min): 80
1min
5min
10min
Lower end
6V
6.8V
7.8V
Middle
6.2V
7.2V
8V
Upper end
6.2V
7.4V
8.2V
Observe point
Micro-arc time
15min
20min
25min
Lower end
8.8V
9.4V
8.4V
Middle
9V
9.8V
8.8V
Upper end
9.2V
10.2V
8.8V
Observe point
0
2
4
6
Processing time (min)
3.4. Electrochemical drilling analysis Fig. 10. Relationship between the mulching layer and processing time.
In this experiment, the parameters of the electrode were hot dipping for 4 minutes, 450V micro arc voltage, 20 minutes micro arc time, and 40 minutes of boiling water sealing, testing the drilling performance under different machining voltage, electrolyte concentration and feeding rate. Eventually comparing the entrance and the exit diameter of the noninsulated electrode. The machining parameters are shown in table 4. Figure 12 shows the SEM graphic of the electrochemical drilled hole. The result shows that the entrance and exit diameter machined by the insulated electrode are 295μm and 278μm, the difference is 17μm. For the hole machined by the uninsulated electrode, the entrance and exit are 406μm and 308μm, the difference is 98μm. The insulation has successfully decrease the entrance-exit differences by 81μm, which the insulation successfully reduced a large amount of stray current and improve the accuracy of the micro holes. Table 4. Electrochemical drilling parameters.
Fig. 11. Distribution of elements after hot dip aluminization
3.3. Withstand voltage test Table 3 shows the withstand voltage for different micro-arc time against the electrode dipped for 4 minutes, micro-arc processing under 450 V and sealing in boiling water for 40 minutes. The maximum withstands voltage has been successfully improved to 10.2V with the extra sealing process.
Parameter
Value
Diameter of uninsulated electrode (μm)
200
Diameter of insulated electrode (μm)
265
Voltage (V)
5
Pulse On-time (μs)
50
Pulse Off-time (μs)
150
Initial Gap (μm)
20
Electrode Feeding Rate(μm/s)
1
Jung-Chou Hung et al. / Procedia CIRP 68 (2018) 438 – 443
(a) Entrance by insulated electrode
(b) Exit by insulated electrode
443
(5) The experimental parameters for hot dip aluminizing and micro-arc oxidation method were as follows: the processing voltage was 5V, pulse on time was 50μs, pulse off time was 150μs, the initial gap was 20μm, and the electrode feeding rate was 1m/s. The electrochemical drilling experimental results were as follows: The difference between the entrance and exit of the electrochemical drilled hole is generally reduced from 98μm to 17μm. References
(c) Entrance by uninsulated
(d) Exit by uninsulated electrode
electrode
Fig. 12. Electrochemical drilling entrance-exit comparison
4. Conclusion In this experiment, the insulation for the electrochemical processing electrode was being made by hot-dip aluminum micro-arc oxidation method. The electrode was expected to reduce the stray current and promote the machining precision. The insulation of the electrode shall be subjected to a withstand voltage test to evaluate the corrosion resistance, withstand voltage and the bonding force of the insulation. And further examine the electrochemical drilling performance, to explore the effect under different parameters, the experimental results can be summarized as following conclusions: (1) In this study, a fine insulation for electrochemical drilling electrode has been made by hot-dip aluminizing and micro-arc oxidation. (2) The insulation produced by micro-arc oxidation technology can be applied to electrochemical processing to avoid the generation of stray current and improve the machining accuracy. (4) After boiling water sealed, the withstand voltage test showed the better micro-arc processing time is 20 minutes, the maximum withstands voltage is 10.2V.
[1] L.O. Snizhko, A.L. Yerokhin, A. Pilkington, N.L. Gurevina, D.O. Misnyankin, A. Leyland, A. Matthews “Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions,”Electrichim. Acta, 49, 2085-2095, 2004. [2] W. Xue, Z. Deng, R. Chen, T. Zhang “Growth regularity of ceramic coatings formed by microarc oxidation on Al–Cu–Mg alloy,” Thin Solid Films, 32, 114-117, 2000. [3] L. Chang “Growth regularity of ceramic coating on magnesium alloy by plasma electrolytic oxidation,” J. Alloys Compd” 468, 462-465, 2009. [4] R.O. Hussein, X. Nie, D.O. Northwood “The application of plasma electrolytic oxidation (PEO) to the production of corrosion resistant coatings on magnesium alloys: a review,” Corros. Mater ” 38, 55-65, 2013. [5] Jordan., C. E. and Marder., A. R., “Fe-Zn Phase Formation in InterstitialFree Steels Hot-Dip Galvanized at 450°C: Part I 0.00 wt% Al-Zn Baths,” Journal of materials science, Vol. 32, pp. 5593-5602, 1997. [6] Yerokhin., A. L., Leyland, A., and Matthews, A., “Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation,” Applied Surface Science, Vol. 200, pp. 172-184, 2002. [7] Wirtz, G. P., Brown, S. D., and Kriven, W. M., “Ceramic Coatings by Anodic Spark Deposition,” Material and Manufacturing Process, Vol. 6, pp. 87-115, 1991. [8] Yerokhin A.L., Nie X., Leyland A., Matthews A., Dowey S.J. “Plasma electrolysis for surface engineering,” Surface and Coatings Technology, 122, 73-93, 1999. [9] Lv, G., “Characteristic of Ceramic Coatings on Aluminum by Plasma Electrolytic Oxidation in Silicate and Phosphate Electrolyte,” Applied Surface Science, Vol. 253, pp. 2947-2952, 2006 [10] M.M. Al Bosta, K.-J. Ma, H.-H. Chien“The effect of MAO processing time on surface properties and low temperature infrared emissivity of ceramic coating on aluminium 6061 alloy,” Infrared Phys. Techmol ” 60, 323-334, 2013. [11] F. García, A. Salinas, E. Nava “ The role of Si and Ti additions on the formation of the alloy layer at the interface of hot-dip Al–Zn coatings on steel strips,” Mater. Lett ” 60, 775-778, 2006.
ScienceDirect Procedia CIRP 68 (2018) 438 – 443
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Fabrication of an electrode insulated by using hot dip aluminizing and micro-arc oxidation method for electrochemical microhole machining Jung-Chou Hunga,*, Chih-Yang Kua,b, Zhi-Wen Fanc b
a Department of Mechanical and Computer Aided Engineering, Feng Chia University, Taichung 407, Taiwan Structure and Material Systems Department, Aeronautical Systems Research Division, NATIONAL CHUNG-SHAN INSTITUTE OF SCIENCE & TECHNOLOGY, Taichung 407, Taiwan c Regional R&D Service Department, Metal Industries Research & Development Centre, Taichung 407, Taiwan
* Corresponding author. Tel.: +886-4-24517250 Ext. 3532; fax: +886-4-24516545.E-mail address: [email protected].
Abstract In this research, electrode insulation of the electrochemical machining tool has been produced by hot dip aluminized micro arc oxidation techniques. In order to form an effective layer on the tungsten carbide tool and achieve higher precision, the tool will then be microarc oxidizing, convert the aluminum-rich into aluminum oxide insulating layer. Withstanding Voltage of the tool is being tested afterward by sodium chloride electrolysis method, and the surface and the cross section is being observed. Eventually, the tools were examined and discussed for their machining performance by ECM drilling through stainless steel plate. We were expecting to produce a precise and better insulation. By reducing ECM stray effect, the precision can be achieved. The experimental result shows the optimum parameters are aluminum dipping for 4 minutes, 450 V of micro-arc voltage for 20 minutes and 40 minutes of boiling water sealing, reaching 10.2V of withstanding voltage. A further electrochemical drilling has been done to explore the effect of insulation. Two electrodes, with and without insulation had been compared. The difference between the entrance and the exit diameter of the hole of insulated tool reaches 17um, compare to the one without insulation, we have successfully reduced 81um. The result shows that the insulation has effectively reduced the stray effect, and achieve high precision machining. ©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 the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Hot dip aluminizing, aluminum oxide, micro-arc oxidation, insulation, electrochemical machining.
1. Introduction During the precise electrochemical drilling process, the stray current will greatly affect the machining precision, the most effective way to overcome the problem is to cover the cathode surface with insulating material, that the current can be accurately discharged thus reduce the stray electrolysis. At present, the material for such insulation is still being further studied, due to a variety of physical and chemical impact effects, such as pulse voltage, resistance heat and reaction heat, High-pressure flow, etc., will damage and stripping the insulation. To overcome the extreme environment, the bonding force between the insulation and the electrode was strengthened, along with the density and corrosion resistance can effectively promote the machining accuracy.
Micro-arc oxidation is composed of electrochemical reaction, micro-arc oxidation reaction and thermal diffusion, the process including melting, molten flow, re-solidification, diffusion, sintering and densification of the continuous process [4]. Micro-arc oxide layer was consisting of two structural layers, the outer layer filled with micro-holes, cracks and other structural defects, the inner layer performs dense, good adhesion and excellent mechanical properties. The combination of Hot-dip aluminizing and micro-arc oxidation is conducive to the fabrication of excellent insulation. Several studies against hot dip aluminizing and micro-arc oxidation has been made in recent year, for incident, characterization of the intermetallic surface layers of hot-dip Al-coated steel has been studied by researchers leading by Antoine Van Alboom in 2017, and Characterization of micro-arc oxidation coatings
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.2017.12.092
Jung-Chou Hung et al. / Procedia CIRP 68 (2018) 438 – 443
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on aluminum drillpipes at different current density by researchers leading by PingWang in 2017. This study focuses on the fabrication of precision electrochemical processing electrode insulation by hot-dip aluminizing alloy micro-arc oxidation and discusses whether the insulation can be processed under high current density, pulse current, resistance heat environment. Thus, it is important to promote the density and corrosion resistance of the insulation in the harsh environment, resulting in better accuracy. 2. Experimental setup and Process 2.1. Hot Dip Aluminizing Hot Dip Aluminizing (HDA) is a technology that a layer of aluminum is coated on the substrate surface after proper pretreatment. The pretreatment is to make the aluminum easier to attach to the substrate surface. Afterward, dipped the substrate into the molten aluminum for a period of time. As the substrate leaves the molten aluminum, a series process of Nucleation, growth, diffusion and the formation of solid aluminum and aluminum alloy layer will occur on the dipped surface [5], as shown in Figure 1. Material for Hot-dip aluminizing is pure aluminum ingot, the ingot was put into the alumina crucible, and placed into the furnace as shown in Figure 2, the furnace heating up to 750, the molten aluminum will form an Oxide film on the surface from which contacting the air. The oxide film will decrease the adhesion between the electrode and the molten aluminum, so before the electrode was dipped into the molten aluminum the following procedure will be processed: (1) the electrode surface will be coated with a layer of solvent, (2) the molten aluminum was also sprinkled with dry flux to remove the oxide film on the surface. After the Flux treatment, dip plating can be carried out. Dipping parameters are dipping temperature of 750, dipping time of 1 to 6 minutes, and dipping, pulling speed of 80 cm/min.
Fig. 1. Hot-Dip Aluminum Process
Fig. 2. Hot Dip Aluminum Furnace.
2.2. Micro-Arc Oxidation Micro-Arc Oxidation (MAO), also known as Plasma Electrolytic Oxidation (PEO), Microarc Discharge Oxidation (MDO) and Anodic Spark Deposition (SPD). The Micro-arc oxidation is an anodic oxidation by high voltage technology, suitable for non-ferrous metals such as Ti, Al, Mg, Nb and other metals [7]. The partial temperature reaches 103 ~ 104K, and partial pressure reaches 102 ~ 103MPa [8], the entire process is a complex physical and chemical process including melting, vaporization, electrical collapse and electrophoresis. [9]. Micro-arc oxidation will produce four different oxidation stages, namely passive film, porous oxidation, spark discharge and micro-arc discharge [10]. It can also be divided into three areas (1) Faraday area, (2) spark discharge area, (3) micro-arc discharge area [11], as shown in Figure 3. When the voltage rises, similar to traditional anode treatment, the Faraday reaction occurs and form a passivation film. The voltage continues to rise, a large number of bubbles will continuously be generated and break through the passivation film, high voltage then breakthrough white arc point. As the electric field strength continues to increase causes the occurrence of tunneling effect, the spark began to appear and the oxide film also starts to grow rapidly, as shown in Figure 4. When the voltage stabilized, into the micro-arc discharge area, and reached the collapse voltage of the surface, the surface ceramic oxide film continues to grow, ionization turned into the thermal ionization. Micro-arc oxidation equipment includes a power supply, stainless steel tank, cooling system. The electrode should be acetone cleaned before the micro-arc process. The electrode then is placed at the anode, stainless steel tank connected with the cathode and fill the tank with electrolyte. Parameters for micro-arc oxidation are shown in Table.1. Afterward, the micro-arc process is able to begin, processing diagram was shown in Figure 5.
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2.3. Withstanding voltage
Fig. 3. Stages during micro-arc oxidation.
During Electrochemical process, current will pass through the electrode, so the cathode insulation layer should have withstood the machining voltage without disintegration. Therefore, the withstand voltage of the micro-arced electrode was tested by electrolysis method. The anode connected to a piece of stainless steel, the cathode connected to the treated electrode. The electrolyte is 15% of the sodium chloride aqueous solution, and the constant voltage was applied until the bubble was generated from the crack of the insulation, eventually causing the insulation to peel. In order to facilitate the observation of the test, in the electrolysis using inclined release method to observe the insulation when the bubble break, So the electrode is divided into upper, middle and lower part. Withstanding voltage test diagram was shown in Figure 6.
Fig. 6. Withstand Voltage test setup.
2.4. Electrochemical Drilling Test
Fig. 4. Micro-arc generation diagram.
In this experiment, the electrochemical equipment was erected on the ACRO PNC S-325 EDM machine. The equipment includes an electrolyte circulation system, an acrylic tank, clip Fixture, power supply and so on. The electrode was placed on the anode, and the stainless-steel work piece was placed on the anode, with NaNO3 as the electrolyte. The electrochemical drilling setups were shown in Fig7.
Fig. 5. Micro-Arc Processing Diagram. Table 1. Parameters for micro-arc oxidation Parameter
Value
Toll electrode martail
Hot dip aluminized electrode
Processing time(min)
10152025
Electrolyte concentration
10g/L Na2Al2O4+1g/L KOH
Electrolyte temperature()
5
Power supply voltage (V)
450
Fig. 7. Electrochemical drilling diagram.
3. Experimental Result and Discussion 3.1. Hot-dipping Aluminizing The thickness of the coating is one of the key factors in this experiment, and it is also a related to the hot dipping process,
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thus, the effect of the hot dipping time on the coating was being tested in this experiment. The experimental parameters were hot-dipping temperature 750 , dipping and pulling speed 80cm / min, dipping time was set to 1 minute, 2 minutes, 3 minutes, 4 minutes, 5 minutes, 6 minutes, other parameters remain unchanged. The experimental parameters are shown in Table 2.
(a)1min
(b)2min
(c)3min
(d)4min
(e)5min
(f)6min
Table 2. Hot-dipping aluminizing parameters. Parameter
Value
Toll electrode material
Tungsten electrode
Processing time(min)
123456
Hot dip temperature ()
750
Speed of rising and falling(cm/min)
80
3.2. Surface observation and structure analysis Fig. 8 shows the surface morphology after hot-dipping, among them, the surface condition performed relatively better under 4min of dipping time, Figure.8 (d). Shorter dipping time such as Figure.8 (a), (b) and (c) shows no aluminum coated or uneven coating result caused by the lack of dipping time. Figure.8 (e) and (f) shows the result under 5 and 6 minutes dipping time. The surface of the coating was full of micro hole and cracks after longer dipping time. From the different hot-dip plating time and cross-section diagram shown in Figure 9, it was found that dipping for 1min, the thickness of the tungsten-aluminum intercalation layer was 7.6 μm, the thickness of the impregnated layer was 10.3 μm, the tungsten carbide electrode diameter was 196.4mm. The thickness of the tungsten-aluminum intercalation layer increased to 10.7 μm, the thickness of the impregnated layer was 15.5 μm, and the diameter of the tungsten carbide electrode was reduced to 194.6 μm under 2min dipping time. When the dipping time increased to 3 minutes, 4 minutes, 5 minutes, 6 minutes, tungsten aluminum mulching layer increased to 11.7, 12.9, 13.3, 14.7μm, dip aluminum thickness increased to 19.4, 25.5, 32.5, 43.7μm, the tungsten carbide electrode diameter is 192.8, 191.1, 190.2, 188.9mm. According to Fig. 10, the thickness of the mulching layer is thickened by the increase of the thickness of the mope layer, but the thickening of the miscible layer represents the phenomenon of diffusion to both sides resulting in a reduction in the original diameter of the tungsten carbide electrode. Figure 11 shows the distribution of elements after hot dip aluminization, the outskirt is alumina, the oxide protective layer formed after dipping, and then the interior is the aluminum layer, followed by aluminum tungsten miscible layer, and tungsten carbide substrate. From the results above, it can be seen that the plating time affects the surface morphology of the electrode. When the dip plating time is short, the surface coating is uneven, and when the plating time is too long, the surface of the electrode coating is too rough. Therefore, 4 minutes is considered better.
Fig. 8. Hot dip aluminum at 750 for different dipping time. (a)1min, (b)2min, (c)3min, (d)4min, (e)5min, (f)6min.
(a)1min
(b)2min
(c)3min
(d)4min
(e)5min
(f)6min
Fig. 9. Hot dip aluminum at 750 for different dipping time.( Cross section ): (a)1min, (b)2min, (c)3min, (d)4min, (e)5min, (f)6min.
Jung-Chou Hung et al. / Procedia CIRP 68 (2018) 438 – 443
Thickness( μm)
442
340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0
Tool electorde material: Tungsten carbide
Tungsten carbide substrate Aluminizing layer Solution layer
Hot dip temperature(OC): 750
Table 3. ithstand voltage for different micro-arc time against the electrode dipped for 4 minutes, micro-arc processing under 450 V and sealing in boiling water for 40 minutes. Micro-arc time
Speed of rising and falling(cm/min): 80
1min
5min
10min
Lower end
6V
6.8V
7.8V
Middle
6.2V
7.2V
8V
Upper end
6.2V
7.4V
8.2V
Observe point
Micro-arc time
15min
20min
25min
Lower end
8.8V
9.4V
8.4V
Middle
9V
9.8V
8.8V
Upper end
9.2V
10.2V
8.8V
Observe point
0
2
4
6
Processing time (min)
3.4. Electrochemical drilling analysis Fig. 10. Relationship between the mulching layer and processing time.
In this experiment, the parameters of the electrode were hot dipping for 4 minutes, 450V micro arc voltage, 20 minutes micro arc time, and 40 minutes of boiling water sealing, testing the drilling performance under different machining voltage, electrolyte concentration and feeding rate. Eventually comparing the entrance and the exit diameter of the noninsulated electrode. The machining parameters are shown in table 4. Figure 12 shows the SEM graphic of the electrochemical drilled hole. The result shows that the entrance and exit diameter machined by the insulated electrode are 295μm and 278μm, the difference is 17μm. For the hole machined by the uninsulated electrode, the entrance and exit are 406μm and 308μm, the difference is 98μm. The insulation has successfully decrease the entrance-exit differences by 81μm, which the insulation successfully reduced a large amount of stray current and improve the accuracy of the micro holes. Table 4. Electrochemical drilling parameters.
Fig. 11. Distribution of elements after hot dip aluminization
3.3. Withstand voltage test Table 3 shows the withstand voltage for different micro-arc time against the electrode dipped for 4 minutes, micro-arc processing under 450 V and sealing in boiling water for 40 minutes. The maximum withstands voltage has been successfully improved to 10.2V with the extra sealing process.
Parameter
Value
Diameter of uninsulated electrode (μm)
200
Diameter of insulated electrode (μm)
265
Voltage (V)
5
Pulse On-time (μs)
50
Pulse Off-time (μs)
150
Initial Gap (μm)
20
Electrode Feeding Rate(μm/s)
1
Jung-Chou Hung et al. / Procedia CIRP 68 (2018) 438 – 443
(a) Entrance by insulated electrode
(b) Exit by insulated electrode
443
(5) The experimental parameters for hot dip aluminizing and micro-arc oxidation method were as follows: the processing voltage was 5V, pulse on time was 50μs, pulse off time was 150μs, the initial gap was 20μm, and the electrode feeding rate was 1m/s. The electrochemical drilling experimental results were as follows: The difference between the entrance and exit of the electrochemical drilled hole is generally reduced from 98μm to 17μm. References
(c) Entrance by uninsulated
(d) Exit by uninsulated electrode
electrode
Fig. 12. Electrochemical drilling entrance-exit comparison
4. Conclusion In this experiment, the insulation for the electrochemical processing electrode was being made by hot-dip aluminum micro-arc oxidation method. The electrode was expected to reduce the stray current and promote the machining precision. The insulation of the electrode shall be subjected to a withstand voltage test to evaluate the corrosion resistance, withstand voltage and the bonding force of the insulation. And further examine the electrochemical drilling performance, to explore the effect under different parameters, the experimental results can be summarized as following conclusions: (1) In this study, a fine insulation for electrochemical drilling electrode has been made by hot-dip aluminizing and micro-arc oxidation. (2) The insulation produced by micro-arc oxidation technology can be applied to electrochemical processing to avoid the generation of stray current and improve the machining accuracy. (4) After boiling water sealed, the withstand voltage test showed the better micro-arc processing time is 20 minutes, the maximum withstands voltage is 10.2V.
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