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Electrode insulation layer for electrochemical machining fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate
Surface & Coatings Technology xxx (xxxx) xxxx
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Electrode insulation layer for electrochemical machining fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate Jung-Chou Hunga,∗, Yi-Ren Liua, Hai-Ping Tsuib, Zhi-Wen Fanc a
Department of Mechanical and Computer-Aided Engineering, Feng Chia University, Taichung 407, Taiwan Department of Mechanical Engineering, National Central University, Chung-Li 320, Taiwan c Regional R&D Service Department, Metal Industries Research & Development Centre, Taichung 407, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical machining Hot-dip aluminizing Microarc oxidation Stainless-steel substrate Insulation layer
Insulation of an electrode for use with an electrochemical machining (ECM) was achieved through hot-dip aluminizing and microarc oxidation techniques. To form an effective insulation layer on a stainless-steel substrate and achieve high precision in ECM, an electrode was processed through microarc oxidation, and the aluminum-rich layer was converted into an aluminum oxide insulating layer. The withstand voltage of the tool was evaluated through sodium chloride electrolysis, and the surface and cross-section were observed. The ECM performance of various tools with and without aluminum oxide insulating layer was examined by drilling on a stainless-steel workpiece. A precise and robust insulation layer must be produced. Precision can be achieved by reducing the stray effects of ECM. Experimental results indicated that the optimal parameters were aluminum dipping were 4 min, microarc voltage of 475 V for 10 min with a withstand voltage of 9.3 V. Electrochemical drilling was used to examine electrodes with and without an insulation layer. Improvement of variation between entrance and exit could be 64.58 %. The differences between the entrance and exit values indicated that the electrochemical insulating layer considerably reduces stray current and improves drilling accuracy.
1. Introduction Electrochemical machining (ECM) was an electrochemical corrosion reaction method that has been developed and widely used for numerous years, particularly in defense aerospace, vehicles, and medical equipment applications. In conventional electrochemical processing, the large size of a processing electrode means that a relatively thick insulating layer can be used to isolate an electric field effect. Conventionally used insulating materials, such as ceramics, engineering plastics, glass fibers, and other composite materials, were used for nesting and inlaying. Techniques, such as locking, coating, and gluing were combined. An insulating layer was formed around the electrode for improved shape accuracy, and the electrolyte required for processing can be injected from inside the electrode into the drilling area. The electrolyte in the processing area was renewed using the pressure difference of the fluid. The renewal rate and flow direction of the electrolyte were highly correlated with the smoothness and accuracy of the electrode and of the machined surface. However, for precise ECM, stray corrosion and secondary processing considerably affect processing precision, and fine processing electrodes and high dimensional
∗
accuracy were required. Machining accuracy was affected by stray current in precision ECM as shown in Fig. 1 [1]. Coating an insulation layer on the electrode surface was the most effective approach to improve accuracy. In this approach, current can only be accurately released from the desired area as shown in Fig. 1(b). Precise and durable insulation of the electrode for microstructure machining must be further studied because the gap between the two poles used in ECM was only tens of microns. Various physical and chemical effects, such as pulse voltage, thermal resistance, thermal action, and high-pressure intense flush, can destroy or remove the insulation layer. The bonding force, insulation density, and corrosion resistance must be improved under such an environment, and the life of electrodes must be extended. Precision machining of accurate shapes was crucial. During electrochemical microdrilling of a microelectrode with an insulating layer, the insulating layer concentrates the processing current on the processing area, thus eliminating the effects of secondary spurious current on both sides of the electrode, as discovered by Li [2], who bored a micropore with a diameter of 220 μm and depth of 300 μm into the surface of an insulated copper electrode. Park et al. [3] used an
Corresponding author. E-mail address: [email protected] (J.-C. Hung).
https://doi.org/10.1016/j.surfcoat.2019.124995 Received 15 July 2019; Received in revised form 9 September 2019; Accepted 14 September 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jung-Chou Hung, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.124995
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Table 3 Microarc oxidation parameters. Parameter
Value
HDA Time (min) MAO time (min) Electrolyte temperature (°C) Voltage (V) Pulse on time (μs) Pulse off time (μs)
4 8, 10 20 425, 450, 475, 500 150 50
Table 4 The MAO electrolyte formula.
Fig. 1. Schematic diagram of tool side effect in ECM [1].
Parameter
Value
Na2SiO3 (g/L) NaOH (g/L)
14 10
Fig. 2. Hot-dip aluminizing process schematic diagram. Table 1 Hot-dip aluminizing parameters. Parameter
Value
Workpiece tube material Dipping time (min) Hot-dip temperature (°C) Speed of rising and falling (cm/min)
AWASI 304 stainless steel 2, 3, 4, 5, 6 750 80
Fig. 4. Withstand voltage test schematic diagram. Table 5 Electrochemical drilling parameters.
Table 2 The main composition of the flux. Parameter
Value
LiCl ZnCl2 KC NaCl
40% 10% 10% 40%
Parameter
Value
AWASI 304 Workpiece thickness (mm) AWASI 304 Tool electrode tube size (mm)
25 Outer Diameter 3.0 Inner Diameter 2.0 Length 100 100 8 10 25 10 240 4
Initial electrode gap (μm) Electrolytic voltage (V) Electrolyte NaNO3 concentration (wt.%) Electrolyte temperature (°C) Electrolyte input pressure (kgf/cm2) Electrode rotational speed (rpm) Electrode feed rate (μm/s)
Moreover, van den Brand et al. [4] proposed two methods for improving bonding, adsorption, and durability between an epoxy resin coating and aluminum substrate; before coating the epoxy resin, a polymer layer of approximately 10-nm thickness was coated on the aluminum substrate, and the polymer participates in the formation of the epoxy resin. The second method entails hydrating the aluminum substrate for fabricating the surface of the porous layer of substrate. Hot-dip aluminizing (HDA) was adopted in this study to examine the effect of an insulation layer coated on an electrode substrate and to integrate aluminum into the alumina insulation layer for microarc oxidation (MAO). A dense oxide insulation layer was coated on an electrode through aluminum MAO for ECM. During HDA, a layer of aluminum was coated on the substrate surface after a proper pretreatment. Pretreatment enables easy adhesion of aluminum to the substrate surface. The substrate was then dipped in molten aluminum for a certain duration. When the substrate was removed from the molten
Fig. 3. Microarc oxidation process schematic diagram.
insulated electrode to machine a deep hole with a high aspect ratio in stainless steel. Liu et al. [1] used an insulated electrode with a diameter of 80–100 μm to machine a micropore in a 0.5-mm stainless-steel plate. 2
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Fig. 7. The state of the HDA layer was observed in cross-section.
A study of hot-dip-aluminized aluminum alloys [5] used carbon steel as a substrate and employed normalization, furnace cooling, water quenching, and spheroidization to obtain different microstructures and then 700 °C. The hot-dip-galvanized pure aluminum was used to study the effect of a carbon steel microstructure on the growth of an aluminized layer and outer aluminum layer. The coating of the hot-dip aluminized test piece comprised an outer aluminum layer, iron–aluminum metal layer, and substrates of different microstructures, and the iron–aluminum metal layer was primarily composed of Fe2Al5. The
Fig. 5. Electrochemical drilling setups schematic diagram.
aluminum, a series of processes including the nucleation, growth, diffusion, and formation of solid aluminum occur, and an aluminum alloy layer forms on the dipped surface.
Fig. 6. Surface topography after different immersion times for hot-dip aluminizing temperature of 750 °C: (a) 2 min, (b) 3 min, (c) 4 min, (d) 5 min, (e) 6 min. 3
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Fig. 8. Cross-sectional diagram showing layers formed after different immersion times for hot-dip aluminizing at 750 °C: (a) 2 min, (b) 3 min, (c) 4 min, (d) 5 min, (e) 6 min.
higher diffusivity in this phase coupled with its marked ferrite-stabilizer behavior. Ni and Cr contents in the ferritic matrix have an influence but not highly relevant. MAO, which was also called plasma electrolytic oxidation, was a novel surface engineering technique used to form metallurgically bonded ceramics on some nonferrous metals. This method was widely employed for the surface modification of elements, such as Ti, Al, and Mg, and its alloys to improve their wear, corrosion, and thermal properties [8–12]. MAO was based on an anodizing process with a high applied voltage and plasma discharge channels. This process provides excellent adhesion of a coating to a substrate, environmental friendliness, and ease of control. MAO comprises electrochemical reactions, microarc oxidation, and thermodiffusion [13–15]. The processes include repeated melting, melt-flow, resolidification, diffusion, sintering, and continuous densification [16]. The composition formed through MAO has two bilayers [17]. The surfaces of the bilayers were
distribution of the carbon substrate affected the uniformity of the Fe2Al5 layer; the uniformity of the carbon distribution negatively corresponded to the variation in the Fe2Al5 layer thickness. The thickness of the outer aluminum layer was strongly affected by the content of carbides in the iron–aluminum layer; that was, the thickness increased with the carbides in the steel. However, the changes in thickness were not affected by the carbon content of the steel. Both shrunk with the increasing duration of hot dip. K. Bouche et al. [6] performed intermetallic compound layer growth between solid iron and molten aluminium. The intermetallic layers formed at the solid–liquid interface and their growth mechanisms are characterized. The Fe2Al5 and FeAl3 phases are identified in the temperature range from 700 to 900 °C, and their growth was found to be mainly controlled by a diffusion regime. A.-F. Ciuffini et al. [7] conducted a hot-dip aluminizing reaction using on AISI F55–UNS S32760 super duplex stainless steel. Molten aluminum interacts exclusively with the ferritic phase due to its much
4
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Wang et al. [22] controlled frequencies and voltages in a pulse power supply to produce different surface morphologies of ceramic oxide film. When the control voltage was 500 V and the negative frequency was of greater magnitude than the positive frequency, the surface roughness of the ceramic oxide film was reduced. With fewer holes, a low coefficient of friction (0.2–0.3) was obtained in the friction test. Yerokhin et al. [23] performed a microarc reaction with an alternating current (AC) power source and a reaction with a pulse power source. The film thickness was high, whereas the surface roughness was low for the pulse power source; however, the surface adhesion was high when the AC source was used. Moreover, an energy dispersive spectrometer (EDS) map function was used in the study, which indicated that the surface was comprised primarily of aluminum and oxygen, with extremely low quantities of other elements. Butyagin [24] and others reported that during MAO of an aluminum alloy, current decreased with time under a constant voltage, and the current reduction for various materials was, in descending order, KOH > ultradispersable powder of α-Al2O3 > Na2SiO3 > Co(NO3)2. In this study, the stainless-steel material was used as tool electrode substrate. For the production of the insulating layer, HDA was first performed. After the aluminum layer was coated on the surface of the workpiece using different durations of HAD and the MAO process was conducted with optimal parameters to convert the aluminum layer into a ceramic insulation layer, the surface and aluminum layer of the crosssection were observed through SEM and EDS. Owing to the ceramic oxide film provided with good properties such as insulation, abrasion resistance, mechanical strength and high temperature resistance, hence after preparing the MAO insulation layer, the tool electrode was used with different MAO parameters for an electrolytic withstand voltage test using a sodium chloride solution. Furthermore, the ECM performance of various tools with and without aluminum oxide insulating layer was examined by drilling using direct current (DC) power supply conditions on a stainless-steel workpiece, the processing performance could be understood by the method. The differences between the entrance and exit values indicated that the electrochemical insulating layer considerably reduces stray current and improves drilling accuracy. Finally, the development of a tool electrode with precise and robust insulation layer be expected to achieve precise ECM effects.
Fig. 9. Hot-dip aluminizing processing time versus coating thickness.
2. Materials and methods Fig. 10. Element distribution after hot-dip aluminizing of 4 min.
2.1. Pretreatment of substrate
permeated with pores and cracks. The inner layer was specifically intended to possess high binding affinity and excellent mechanical properties. The wavy, jagged appearance generated by an oxide layer and a substrate was attributed to the melting effect in the early stage of MAO. Sundararajan et al. [18] performed a microarc test on a 7075 aluminum alloy. The surface roughness increased with the time and thickness of a ceramic oxide film, and surface holes were obtained; software was used to process scanning electron microscopy (SEM) images. With time, the number exponentially decreased, and the diameter of the hole linearly increased. Tian et al. [19] performed MAO of an aluminum alloy with alkaline solution of KOH and collected the generated gas using a gas collector. The generated gas was primarily hydrogen and oxygen. The rate of gas generation increased with the current density and time but decreased as the amount of added KOH increased. Finally, the ceramic oxide film thickness was optimal with 1 g/L of KOH. Xue et al. [20] conducted a reaction using 2024 aluminum alloy. The trend of the elastic modulus was similar to the hardness curve, and the hardness reached 32 GPa. Sun et al. [21] used fabricated a ceramic oxide film through MAO on a light alloy to improve the roughness and compactness of the film at various cathode/ anode current densities, with the cathode current density higher than anode current density. The roughness of 2 times or more will become significantly smaller, and the thickness of the dense layer increased.
This experiment used an American Iron and Steel Institute 304 stainless-steel substrate. The size of the tool electrode tube was outer diameter of 3.0 mm, inner diameter of 2.0 mm and length of 100 mm. Before HDA, an aluminum workpiece was first washed with acetone, alkali-washed with 10 wt% NaOH, acid-washed with 15 wt% H3PO4, neutralized with 5 wt% NaOH, and then dried and subjected to hot-dip aluminizing. 2.2. Hot-dip aluminizing The material used for HDA was pure aluminum ingot, and the aluminum ingot was placed in an alumina crucible. Fig. 2 shows the hotdipping furnace. The furnace was heated to 750 °C. The immersion aluminizing parameters were as Table 1. Aluminum acted as a solution in the HDA process and was poured into the alumina crucible; however, the air in contact with the aluminum surface of the molten liquid formed an oxide layer. Such an oxide layer will cause poor adhesion between the electrode and molten aluminum during HDA. Before the aluminum piece was molten, the oxide layer must be removed. Therefore, the following two steps were used to eliminate the oxide layer: (1) a layer of a wet flux coating was applied to the specimen surface and left to dry (a ratio of 10:1 for flux to water), and (2) dry flux was added to remove the oxide layer from the molten aluminum surface. The main 5
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Fig. 11. Surface state diagram of MAO layers formed with different voltages and fixed MAO time of 8 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
Fig. 12. Surface state diagram of MAO layers formed with different voltages and fixed MAO time of 10 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
composition of the flux was shown in Table 2. After the flux treatment, HDA was performed. The parameters were a temperature of 750 °C, dipping time of 2–6 min, and rising and falling speed of 80 cm/min.
stainless-steel electrolytic bath and was steeped in a receptacle with dimensions of 150 × 150 × 150 mm3. The electrode was set as the anode, and the stainless-steel tank was set as the cathode. The electrolyte was then poured. After the electrolyte had set completely, microarc oxidization was performed, and Fig. 3 shows the processing diagram. The experimental parameters for MAO were different constant voltages and different oxidation times. The MAO parameters were as follows: voltages of 425, 450, 475, and 500 V and times of 8 and 10 min (Table 3). Table 4 shows the electrolyte formula of MAO process.
2.3. Microarc oxidation MAO equipment includes a power supply, a stainless-steel electrolytic tank, and cooling systems. Before MAO, the equipment must undergo a surface treatment. After ultrasonic cleaning in deionized water for 10 min, the components were rinsed with acetone and immediately dried in air. The electrode was placed perpendicular to the center of a 6
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electrode prepared through hot-dip aluminizing and MAO was placed on a cathode Z-axis chuck. The stainless-steel workpiece was placed on the anode, and NaNO3 electrolyte was used for electrochemical drilling as shown in Table 5. Fig. 5 shows the electrochemical drilling setups. 3. Results and discussion 3.1. Effect of HDA parameters and observation of the coating state 3.1.1. Effect of aluminum morphology and structure obtained through HDA time The thickness of the aluminized layer and the surface morphology were the most crucial factors in this experiment. Therefore, this experiment examined the effect of hot-dip aluminizing time on the coating. First, the experimental results of HDA were analyzed. Fig. 6 shows surface morphology after hot-dip aluminizing for 2 min (a) and 3 min (b); it can be seen from the surface that there are distinct intermetallic compounds and the aluminum coating is relatively thin and rough due to thermal unevenness and low immersion time. According to literature [6], intermetallic compounds growth was found to be mainly controlled by a diffusion regime. As can be seen that the coverage area of aluminum coating increases with the increase of HDA time. Fig. 6(c) illustrates hot-dip aluminizing for 4 min, and it can be found that the aluminum coating is completely covered and the surface topography is relatively flat. Fig. 6(d) and (e) show hot-dip aluminizing for immersion times of 5 and 6 min. After these considerably longer immersion times, the surface was rough and uneven because of holes and defects caused by thicker aluminum coating.
Fig. 13. Microarc oxidation processing voltage versus coating roughness.
2.4. Voltage tolerance of insulation layer An electrolysis reaction was used to achieve the effect in this study. The resistance voltage of the insulated electrode could be successfully determined through an electrolytic method (Fig. 4). A stainless-steel substrate and microarc-oxidized layer electrode were used as the anode and cathode, respectively. The electrolyte was a 15 wt% aqueous sodium chloride solution and continuously energized at a constant voltage until the cathode boiled and fell off. For evaluating the voltage tolerance, the breakthrough voltage of the insulation layer must be determined. A continuous energizing current was supplied at a low voltage of 0.2 V until the cathode boiled, at which point, the time was recorded. The voltage could be easily examined by tilting the oxidized layer during electrolysis, which was advantageous for observing the bubbles during the breakthrough of the insulation layer.
3.1.2. Relationship between HDA parameters and coating thickness The state of the HDA layer at immersion time of 2 min was observed in cross-section as shown in Fig. 7. The protective outer oxide layer was composed of alumina and was formed after immersion aluminum aluminizing. The inner layer was the aluminum layer, which was atop an iron–aluminum miscible layer. The innermost part of the fixed aluminized layer was a stainless-steel substrate. According to research, the HDA process comprises an electrochemical reaction, the HDA itself, and thermal diffusion. The process includes melting, melt-flow, resolidification and diffusion. The hot-dip aluminizing layer in this study comprised two structural layers divided as aluminum layer and ironaluminum miscible layer. During hot dip coating, the substrate will be mutually soluble and diffused with aluminum soup aluminum because aluminum atoms were larger than iron atoms. When diffused, the
2.5. Electrochemical drilling The experimental processing equipment included a self-set electrochemical drilling machine. This machine included an electrolyte circulation system, acrylic tank, clip fixture, and DC power supply. The
Fig. 14. Cross-section of microarc oxidation layer structure at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min. 7
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Fig. 15. MAO layer profile of MAO layers formed with different voltages and fixed MAO time of 8 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
Fig. 16. MAO layer profile of MAO layers formed with different voltages and fixed MAO time of 10 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
layer was 46.5 μm. When the immersion time was increased to 4, 5, and 6 min, the average thickness of the immersed aluminum was increased by 106.3, 140, and 149 μm. According to this trend, an increase in the thickness of the miscible layer prompts an increase in the thickness of the immersed aluminum layer as shown in Fig. 9. In the HDA experiment, the workpiece temperature was same as room temperature. Therefore, it was inferred that the heat conduction on the surface of the workpiece was uneven at the initial stage of immersion plating. After the surface was thermally balanced, the thickness of the coating would grow at another ratio, resulting in a different ratio of initial (2–3 min) to late (4–6 min) as can be seen from Fig. 9. The above coating thickness data was an average thickness, which was an average thickness of 6 points measured for the same workpiece. The measurement was
atomic size will be dominated by iron diffusion. The iron atoms diffuse into the aluminum layer in a large amount, relatively few. The aluminum atoms diffuse inward to achieve a miscible alloying to form the intermetallic compounds. From the Fe/Al phase diagram in which it can be seen that five types of intermetallic compounds (Fe3Al, FeAl, FeAl2, Fe2Al5 and FeAl3 phases) exist. Moreover, it was confirmed from the results of XRD that the surface and lower layers were aluminum and Fe2Al5, respectively. These results were consistent with the literature [5,6]. Fig. 8 shows cross-sections after various hot-dip aluminizing times. The average thickness of the immersed aluminum layer of the electrode was 12.5 μm for an immersion time of 2 min. The playing immersion time was 3 min, and the average thickness of the immersed aluminum 8
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an influence but not highly relevant. From the experimental results showed that the Ni element has no diffusion at all. The aforementioned results indicate that the duration of immersion affects the surface morphology of the electrode. After a short immersion time, the aluminum of the surface coating was uneven. After a long immersion time, the surface of the electrode aluminizing was excessively undulated and flat. Therefore, when the immersion aluminizing time was 4 min, the electrode aluminizing flatness was improved; therefore, subsequent MAO would employ an immersion aluminizing time of 4 min. 3.2. Effect of MAO parameters and observation of the coating state 3.2.1. Effect of MAO on morphology and structure Because of the aforementioned HDA, an experimental product with a relatively flat surface was used for MAO; that was, the workpiece prepared through HDA for 4 min underwent MAO. First, for the MAO parameters, microarc voltage controlled at 425, 450, 475, or 500 V and MAO times of 8 or 10 min were used, and the surface condition of the MAO layer was observed. Fig. 11 shows the surface state of the MAO layer at different voltages for 8 min of fixed MAO time. Fig. 11 (a), (b), (c), and (d) show the surface morphologies obtained with microarc voltages of 425, 450, 475, and 500 V, respectively. When the microarc voltage was 425 V, microcavities were formed on the surface of the MAO layer. The characteristics of spark holes were not evident, and spark holes were only generated by high-voltage discharge; however, the microarc ceramic surface was incomplete. Fig. 11 (b) shows the results obtained with a microarc voltage of 450 V; the surface exhibited microarc features, high density, and a discharge tunnel. For the same MAO time of 8 min, the surface morphology was observed. Although some microarc features were more evident, no flat or dense surface was observed, and the surface microholes comprised numerous ridges. When the MAO voltage was increased, the number of strong-discharge holes on the surface increased, and they grew in size, exhibiting local cracks and pores on the surface tend to cover a large area as shown in Fig. 11 (d). Fig. 12 shows a surface state diagram of the MAO layer for a MAO time of 10 min. Fig. 12 (a), (b), (c), and (d) microarc voltages of 425, 450, 475, and 500 V, respectively. When the MAO time was increased as 10 min, the microarc features of the surface layer were significantly improved. Fig. 12 (c) shows a microarc with a voltage of 475 V and a time of 10 min, the microarc ceramic surface hole was covered using the remaining dense microarc ceramic surface; thus, the discharge holes were respectively flatter and smaller than those for other microarc voltages. Fig. 12 (d) corresponds to a microarc voltage of 500 V. The figure shows that the number of discharge cells and holes increased considerably because of the large discharge voltage, and the MAO ceramic layer grew extremely rapidly; thus, these structures were crowded. The resulting pressure formed surface cracks, and the MAO layer and stainless-steel tube substrate were also cracked. Since the tool electrode material was a stainless steel tube with a wall thickness of 0.5 mm, under the experimental conditions of high micro-arc oxidation voltage, the thickness of the stainless steel decreases as the micro-arc time increases, so when the voltage was too high lead to strong discharge energy, it will easily occur the plating peels off or penetrates the stainless steel tube. Fig. 13 shows the average surface roughness of MAO-treated hotdip-aluminized test strips obtained using different operating voltages. The surface roughness measurement was performed using a hand-held roughness meter, and each tool electrode tube was rotated and measured for 10 samples and then averaged. When the operating voltage was 425 V, some arcs were observed on the surface of the workpiece during the process, but they were sparse, thus resulting in minimally evident surface microarc characteristics and generating slight changes in surface roughness. When the operating voltage was increased, the MAO layer was relatively dense on the workpiece surface. Surface
Fig. 17. Microarc oxidation processing voltage versus alumina thickness.
Fig. 18. Element distribution after microarc oxidation.
performed using an SEM-attached measuring tool ranging from substrate to coating tip thickness. Fig. 10 shows the element distribution after HDA of 4 min. In order to observe the diffusion of aluminum, the elements of C, O, Al, Fe and Ni were adopted. As can be seen from Fig. 10, the Al element diffuses from the outer layer to the inner layer and the Fe element was the opposite. In addition, from the spectrum 1 of Fig. 10, it can be found that in the aluminum-rich layer, there was a phenomenon of crystal growth of the intermetallic compound Fe2Al5. According to the literature [7] that conducted a HDA reaction using on super duplex stainless steel. Molten aluminum interacts exclusively with the ferritic phase due to its much higher diffusivity in this phase coupled with its marked ferrite-stabilizer behavior. Ni and Cr contents in the ferritic matrix have 9
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Fig. 19. Electrolytic withstand voltage with different microarc voltages and electrode areas.
Fig. 20. Electrochemical drilling results comparison (a) entrance by using uninsulated electrode (b) exit by using uninsulated electrode (c) entrance by using insulated electrode (d) exit by using insulated electrode.
diffusion, sintering, and densification. The microarc oxide layer in this study comprised two structural layers; the outer layer comprised structural defects, such as micropores and cracks, whereas the inner layer was dense and highly bonded, with excellent mechanical properties. The third and fourth layer were aluminum-rich and iron-aluminum miscible layer (intermetallic compounds) [5,6] as shown in Fig. 13. Owing to the tool electrode was immersed into electrolyte during ECM, electrical transmission via liquid phase to anode. The micropores and cracks will be a tunnel to help the electron pass through. Therefore, the denser oxide layer was needed. Fig. 15 shows the MAO layer profile state diagram with different microarc voltages and 8 min of fixed MAO time. Fig. 15 (a), (b), (c), and (d) respectively illustrate the results of microarc voltages of 425, 450, 475, and 500 V. The cross-section of the workpiece after a MAO time of
microarc characteristics were relatively evident. Holes and grooves were generated in the MAO layer, thus increasing surface roughness. However, when the microarc voltage was 500 V, the discharge energy was sufficiently large such that the aluminum layer was exhausted, the ceramic layer cracked, and the substrate was separated from the ceramic layer, thus generating a broken surface with the highest observed surface roughness. 3.2.2. Effect of MAO parameters and cross-sectional state observations The state of the oxide layer at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min was observed in crosssection as shown in Fig. 14. According to research, the MAO process comprises an electrochemical reaction, the MAO itself, and thermal diffusion. The process includes melting, melt-flow, resolidification, 10
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content was the strongest, and the immersed aluminum layer and the aluminum–iron interdiffusion layer (Fe2Al5) were underneath, where the aluminum aluminizing layer was fixed. The innermost part was the stainless-steel substrate, and thus, the iron content was relatively high. 3.3. Withstand voltage test for different MAO parameters Fig. 19 shows the electrolytic withstand voltages of the insulating layer after microarc at various microarc voltages, MAO time of 10 min, and immersion HDA time of 4 min. As can be seen that the maximum withstand voltage was 9.3 V with the microarc voltage was 475 V. Thus according to the tendency of Fig. 18, the electrolytic withstand voltage of the tool surface was proportional to alumina thickness. The obtained MAO layer of the insulating layer had a large hole in its surface, and the ceramic layer and workpiece substrate collapsed. Therefore, during the withstand voltage test, these conditions may result in easy penetration of the insulating layer by the electrolyte. Electrolytic corrosion disintegrates layers, thus resulting in unsatisfactory insulation and reduced voltage resistance. Therefore, the insulating layer obtained at 475 V exhibited a relatively high density and small surface holes, and the withstand voltage was relatively high. The voltage test indicates that although the insulating layer could be fabricated through the MAO process, the withstand voltage reached 9.3 V; however, the actual ECM withstand voltage of the experimental target was slightly different because it was in the production circle. When a rod-shaped material was subjected to MAO after the HDA process, the current distribution was likely to be uneven; this may have caused the surface portion to be easily over-discharged in this study, thus resulting in the collapse of the MAO layer. This phenomenon may affect the withstand voltage.
Fig. 21. Surface characteristics of the finished electrode after microarc oxidation: surface morphology of the electrode (a) before electrochemical drilling and (b) after electrochemical drilling.
3.4. Electrochemical drilling analysis 3.4.1. Improvement of variation between entrance and exit The electrochemical drilling electrode used in this experiment was formed with an HDA time of 4 min, microarc voltage of 475 V, and MAO time of 10 min and comparison with electrodes without an insulation layer was performed. Fig. 20 shows the state of the entrance and exit of the electrochemically drilled product. When an uninsulated electrode was used, the finished workpiece could be observed at the processing inlet because of the uninsulated status of the electrode during processing. Because of generated stray current, the phenomenon of a concave arc shape produced using the electrolytic reaming was evident; and the average processing inlet aperture was 4.23 mm, and the average processing exit hole was 3.75 mm in diameter. Drilling with an electrode with an insulating layer was shown in the figure; the finished product processed using an electrode with an insulating layer could be improved by the effect of the stray current generated by the processing inlet. The average inlet aperture was 3.52 mm, and the average exit aperture was 3.35 mm. Improvement of variation between entrance and exit could be 64.58 % [25]. Multiple sets of processing experiments verified that this electrochemically processed electrode insulation method could effectively suppress stray current and improve processing accuracy.
8 min revealed that only the porous outer layer was present on the MAO layer's surface. The dense inner layer was observed, and it was discharged because of the high voltage. A relationship between the voltage and consumption of the aluminum-rich layer was slightly evident. Fig. 16 shows MAO layer profile state diagrams after MAO with different microarc voltages for a fixed MAO time of 10 min. Fig. 16 (a), (b), (c), and (d) show the results of microarc voltages of 425, 450, 475, and 500 V, respectively. The cross-section reveals that the oxide layer of the workpiece was dense after MAO with 10 min of MAO time, and the structure was solid. The cracks and holes were lower in comparison with those after 8 min, and the cross-section exhibited the highest compactness when the microarc voltage was 475 V in Fig. 16 (c). The least oxide layer in the hole, but after the voltage was raised, as shown in Fig. 16 (d) for a microarc voltage of 500 V, the MAO layer was broken because of excessively strong discharge energy, and cracks were observed at the interface between the substrate and oxide layer. Diffusion, which resulted in the formation of an oxide layer, as seen in the cross-section, exhibited unsatisfactory compactness and excessive cracking. During MAO, the ceramic oxide layer grows outwardly; however, the original aluminum layer was simultaneously consumed. The cross-sectional SEM images and Fig. 17 revealed that the operating voltage increased or the MAO time increased. As expected, the aluminum layer was consumed as the ceramic oxide layer grew in thickness. When the working voltage was increased to 500 V, the discharge energy was considerably large. The aluminum layer was exhausted, and only the ceramic layer was observed. In this condition, the cracking separated the substrate. Fig. 18 shows the elemental distribution of the outermost surface of the electrode after microarc oxidation at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min. The uppermost layer was an embedded resin layer, and thus, the carbon content was the highest. The middle layer was the MAO layer. Therefore, the oxygen
3.4.2. Surface observations of electrode usage status Fig. 21 shows the surface state of the stainless-steel electrode after MAO. After several rounds of electrochemical drilling, the surface was slightly cracked. Because of the electrochemical drilling, the electrolyte circulation system did not filter impurities. Electrolyte impurities cause small sparks between the electrode and processed workpiece. Sparks cause spark holes and cracks on the surface of the electrode surface; however, the integrity of the surface of the insulating layer was mostly maintained, and the diameter was used. Such an electrode insulation layer generated through MAO after HDA can be used for practical applications in electrochemical processing. 11
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4. Conclusion
micro electrochemical machining, CIRP Ann. - Manuf. Technol. 55 (2006) 197–200. [4] J. Brand, S. Van Gils, P.C.J. Beentjes, H. Terryn, V. Sivel, J.H. Wde Wit, Improving the adhesion between epoxy coatings and aluminium substrates, Prog. Org. Coat. 51 (2004) 339–350. [5] S. Kobayashi, T. Yakou, Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment, Mater. Sci. Eng. A 338 (2002) 44–53. [6] K. Bouche, F. Barbier, A. Coulet, Intermetallic compound layer growth between solid iron and molten aluminium, Mater. Sci. Eng. A 249 (1998) 167–175. [7] A.F. Ciuffini, S. Barella, C.D. Cecca, A. Gruttadauria, D. Mombelli, C. Mapelli, “Hotdip aluminizing on AISI F55–UNS S32760 super duplex stainless steel properties: effect of thermal treatments, Metals 7 (2017) 525. [8] R. Arrabal, E. Matykina, T. Hashimoto, P. Skeldon, G.E. Thompson, Characterization of AC PEO coatings on magnesium alloys, Surf. Coat. Technol. 203 (2009) 2207–2220. [9] R.O. Hussein, P. Zhang, Y. Xia, X. Nie, D.O. Northwood, The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62, Surf. Coat. Technol. 260 (2011) 1990–1997. [10] A. Ghasemi, V.S. Raja, C. Blawert, W. Dietzel, K.U. Kainer, Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy, Surf. Coat. Technol. 202 (2008) 3513–3518. [11] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Advanced Engineering Materials 8 (2006) 511–533. [12] C.E. Jordan, A.R. Marder, Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450°C: Part I 0.00 wt% Al-Zn baths, J. Mater. Sci. 32 (1997) 5593–5602. [13] A.L. Yerokhin, A. Leyland, A. Matthews, Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation, Appl. Surf. Sci. 200 (2002) 172–184. [14] G.P. Wirtz, S.D. Brown, W.M. Kriven, Ceramic coatings by anodic spark deposition, Mater. Manuf. Process. 6 (1991) 87–115. [15] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [16] G. Lv, Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte, Appl. Surf. Sci. 253 (2006) 2947–2952. [17] C.N. Hsu, C.J. Wang, Effect of microstructure on hot-dip aluminizing, J. Chin. Corros. Eng. 27 (2013) 117–130. [18] G. Sundararajan, L. Rama Krishna, Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology, Surf. Coat. Technol. 167 (2003) 269–277. [19] J. Tian, Z.G. Luo, S.K. Qi, X.J. Sun, Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy, Surf. Coat. Technol. 154 (2002) 1–7. [20] W. Xue, C. Wang, Z. Deng, R. Chen, Y. Li, T. Zhang, Evaluation of the mechanical properties of microarc oxidation coatings and 2024 aluminium alloy substrate, Physics Condensed 14 (109) (2002) 47–52. [21] X.T. Sun, Z.H. Jiang, S.G. Xin, Z.G. Yao, “Composition and mechanical properties of hard ceramic coating containing α-Al2O3 produced by microarc oxidation on Ti–6Al–4V alloy, Thin Solid Films 471 (2005) 194–199. [22] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation coatings formed on Ti6Al4V in Na2SiO3 system solution: microstructure, mechanical and tribological properties, Surf. Coat. Technol. 201 (2006) 82–89. [23] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Pilkington, A. Leyland, A. Matthews, Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process, Surf. Coat. Technol. 199 (2005) 150–157. [24] P.I. Butyagin, Y.V. Khokhryakov, A.I. Mamaev, Microplasma systems for creating coatings on aluminium alloys, Mater. Lett. 57 (2003) 1748–1751. [25] J.C. Hung, J.K. Lin, B.H. Yan, H.S. Liu, P.H. Ho, Using a helical micro-tool in microEDM combined with ultrasonic vibration for micro-hole machining, J. Micromech. Microeng. 16 (2006) 2705–2713.
After the experimental results were analyzed, the following conclusions were drawn: 1. The electrode insulation layer for electrochemical machining could be fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate. 2. The experimental results indicate that the duration of immersion affects the surface morphology of the electrode and coating thickness. When HDA time was 4 min, a denser and flatter aluminum layer resulted. 3. During MAO, the aluminum layer was consumed as the ceramic oxide layer grew in thickness. The MAO layer was broken because of excessively strong discharge energy, and cracks were observed at the interface between the substrate and oxide layer. The micropores and cracks will be a tunnel to help the electron pass through. Therefore, the denser oxide layer was needed. 4. The electrolytic withstand voltage of the tool surface was proportional to alumina thickness. The oxide layer with smallest and most dense spark holes could be obtained using a microarc voltage of 475 V and MAO time of 10 min which demonstrated a voltage tolerance of up to 9.3 V. 5. The MAOed precision electrode used for electrochemical drilling was prepared. The ECM processing voltage, initial processing gap, electrode feed rate, and electrolyte concentration were 5 V, 100 μm, 4 μm/s, and 10 wt%, respectively. The hole expansion phenomenon affected by stray current at the inlet used for drilling was significantly improved. After electrochemical drilling, improvement of variation between entrance and exit could be 64.58 %. The difference between the entrance and exit values indicated that the electrochemically processed insulating layer successfully reduced stray current and improved the precision of drilling. Acknowledgements The authors would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract No. MOST107-2221-E-035-045-MY2. References [1] Z. Liu, Y. Liu, Z. Qiu, N. Qu, Effect of tool electrode insulation on electrochemical micro drilling accuracy, Nanotechnology and Precision Engineering 7 (2009) 355–360. [2] Y. Li, Y. Zheng, G. Yang, L. Peng, Localized electrochemical micromachining with gap control, Sens. Actuators A Phys. 108 (2003) 144–148. [3] B.J. Park, B.H. Kim, C.N. Chu, The effect of tool electrode size on characteristics of
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Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Electrode insulation layer for electrochemical machining fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate Jung-Chou Hunga,∗, Yi-Ren Liua, Hai-Ping Tsuib, Zhi-Wen Fanc a
Department of Mechanical and Computer-Aided Engineering, Feng Chia University, Taichung 407, Taiwan Department of Mechanical Engineering, National Central University, Chung-Li 320, Taiwan c Regional R&D Service Department, Metal Industries Research & Development Centre, Taichung 407, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Electrochemical machining Hot-dip aluminizing Microarc oxidation Stainless-steel substrate Insulation layer
Insulation of an electrode for use with an electrochemical machining (ECM) was achieved through hot-dip aluminizing and microarc oxidation techniques. To form an effective insulation layer on a stainless-steel substrate and achieve high precision in ECM, an electrode was processed through microarc oxidation, and the aluminum-rich layer was converted into an aluminum oxide insulating layer. The withstand voltage of the tool was evaluated through sodium chloride electrolysis, and the surface and cross-section were observed. The ECM performance of various tools with and without aluminum oxide insulating layer was examined by drilling on a stainless-steel workpiece. A precise and robust insulation layer must be produced. Precision can be achieved by reducing the stray effects of ECM. Experimental results indicated that the optimal parameters were aluminum dipping were 4 min, microarc voltage of 475 V for 10 min with a withstand voltage of 9.3 V. Electrochemical drilling was used to examine electrodes with and without an insulation layer. Improvement of variation between entrance and exit could be 64.58 %. The differences between the entrance and exit values indicated that the electrochemical insulating layer considerably reduces stray current and improves drilling accuracy.
1. Introduction Electrochemical machining (ECM) was an electrochemical corrosion reaction method that has been developed and widely used for numerous years, particularly in defense aerospace, vehicles, and medical equipment applications. In conventional electrochemical processing, the large size of a processing electrode means that a relatively thick insulating layer can be used to isolate an electric field effect. Conventionally used insulating materials, such as ceramics, engineering plastics, glass fibers, and other composite materials, were used for nesting and inlaying. Techniques, such as locking, coating, and gluing were combined. An insulating layer was formed around the electrode for improved shape accuracy, and the electrolyte required for processing can be injected from inside the electrode into the drilling area. The electrolyte in the processing area was renewed using the pressure difference of the fluid. The renewal rate and flow direction of the electrolyte were highly correlated with the smoothness and accuracy of the electrode and of the machined surface. However, for precise ECM, stray corrosion and secondary processing considerably affect processing precision, and fine processing electrodes and high dimensional
∗
accuracy were required. Machining accuracy was affected by stray current in precision ECM as shown in Fig. 1 [1]. Coating an insulation layer on the electrode surface was the most effective approach to improve accuracy. In this approach, current can only be accurately released from the desired area as shown in Fig. 1(b). Precise and durable insulation of the electrode for microstructure machining must be further studied because the gap between the two poles used in ECM was only tens of microns. Various physical and chemical effects, such as pulse voltage, thermal resistance, thermal action, and high-pressure intense flush, can destroy or remove the insulation layer. The bonding force, insulation density, and corrosion resistance must be improved under such an environment, and the life of electrodes must be extended. Precision machining of accurate shapes was crucial. During electrochemical microdrilling of a microelectrode with an insulating layer, the insulating layer concentrates the processing current on the processing area, thus eliminating the effects of secondary spurious current on both sides of the electrode, as discovered by Li [2], who bored a micropore with a diameter of 220 μm and depth of 300 μm into the surface of an insulated copper electrode. Park et al. [3] used an
Corresponding author. E-mail address: [email protected] (J.-C. Hung).
https://doi.org/10.1016/j.surfcoat.2019.124995 Received 15 July 2019; Received in revised form 9 September 2019; Accepted 14 September 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Jung-Chou Hung, et al., Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.124995
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Table 3 Microarc oxidation parameters. Parameter
Value
HDA Time (min) MAO time (min) Electrolyte temperature (°C) Voltage (V) Pulse on time (μs) Pulse off time (μs)
4 8, 10 20 425, 450, 475, 500 150 50
Table 4 The MAO electrolyte formula.
Fig. 1. Schematic diagram of tool side effect in ECM [1].
Parameter
Value
Na2SiO3 (g/L) NaOH (g/L)
14 10
Fig. 2. Hot-dip aluminizing process schematic diagram. Table 1 Hot-dip aluminizing parameters. Parameter
Value
Workpiece tube material Dipping time (min) Hot-dip temperature (°C) Speed of rising and falling (cm/min)
AWASI 304 stainless steel 2, 3, 4, 5, 6 750 80
Fig. 4. Withstand voltage test schematic diagram. Table 5 Electrochemical drilling parameters.
Table 2 The main composition of the flux. Parameter
Value
LiCl ZnCl2 KC NaCl
40% 10% 10% 40%
Parameter
Value
AWASI 304 Workpiece thickness (mm) AWASI 304 Tool electrode tube size (mm)
25 Outer Diameter 3.0 Inner Diameter 2.0 Length 100 100 8 10 25 10 240 4
Initial electrode gap (μm) Electrolytic voltage (V) Electrolyte NaNO3 concentration (wt.%) Electrolyte temperature (°C) Electrolyte input pressure (kgf/cm2) Electrode rotational speed (rpm) Electrode feed rate (μm/s)
Moreover, van den Brand et al. [4] proposed two methods for improving bonding, adsorption, and durability between an epoxy resin coating and aluminum substrate; before coating the epoxy resin, a polymer layer of approximately 10-nm thickness was coated on the aluminum substrate, and the polymer participates in the formation of the epoxy resin. The second method entails hydrating the aluminum substrate for fabricating the surface of the porous layer of substrate. Hot-dip aluminizing (HDA) was adopted in this study to examine the effect of an insulation layer coated on an electrode substrate and to integrate aluminum into the alumina insulation layer for microarc oxidation (MAO). A dense oxide insulation layer was coated on an electrode through aluminum MAO for ECM. During HDA, a layer of aluminum was coated on the substrate surface after a proper pretreatment. Pretreatment enables easy adhesion of aluminum to the substrate surface. The substrate was then dipped in molten aluminum for a certain duration. When the substrate was removed from the molten
Fig. 3. Microarc oxidation process schematic diagram.
insulated electrode to machine a deep hole with a high aspect ratio in stainless steel. Liu et al. [1] used an insulated electrode with a diameter of 80–100 μm to machine a micropore in a 0.5-mm stainless-steel plate. 2
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Fig. 7. The state of the HDA layer was observed in cross-section.
A study of hot-dip-aluminized aluminum alloys [5] used carbon steel as a substrate and employed normalization, furnace cooling, water quenching, and spheroidization to obtain different microstructures and then 700 °C. The hot-dip-galvanized pure aluminum was used to study the effect of a carbon steel microstructure on the growth of an aluminized layer and outer aluminum layer. The coating of the hot-dip aluminized test piece comprised an outer aluminum layer, iron–aluminum metal layer, and substrates of different microstructures, and the iron–aluminum metal layer was primarily composed of Fe2Al5. The
Fig. 5. Electrochemical drilling setups schematic diagram.
aluminum, a series of processes including the nucleation, growth, diffusion, and formation of solid aluminum occur, and an aluminum alloy layer forms on the dipped surface.
Fig. 6. Surface topography after different immersion times for hot-dip aluminizing temperature of 750 °C: (a) 2 min, (b) 3 min, (c) 4 min, (d) 5 min, (e) 6 min. 3
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Fig. 8. Cross-sectional diagram showing layers formed after different immersion times for hot-dip aluminizing at 750 °C: (a) 2 min, (b) 3 min, (c) 4 min, (d) 5 min, (e) 6 min.
higher diffusivity in this phase coupled with its marked ferrite-stabilizer behavior. Ni and Cr contents in the ferritic matrix have an influence but not highly relevant. MAO, which was also called plasma electrolytic oxidation, was a novel surface engineering technique used to form metallurgically bonded ceramics on some nonferrous metals. This method was widely employed for the surface modification of elements, such as Ti, Al, and Mg, and its alloys to improve their wear, corrosion, and thermal properties [8–12]. MAO was based on an anodizing process with a high applied voltage and plasma discharge channels. This process provides excellent adhesion of a coating to a substrate, environmental friendliness, and ease of control. MAO comprises electrochemical reactions, microarc oxidation, and thermodiffusion [13–15]. The processes include repeated melting, melt-flow, resolidification, diffusion, sintering, and continuous densification [16]. The composition formed through MAO has two bilayers [17]. The surfaces of the bilayers were
distribution of the carbon substrate affected the uniformity of the Fe2Al5 layer; the uniformity of the carbon distribution negatively corresponded to the variation in the Fe2Al5 layer thickness. The thickness of the outer aluminum layer was strongly affected by the content of carbides in the iron–aluminum layer; that was, the thickness increased with the carbides in the steel. However, the changes in thickness were not affected by the carbon content of the steel. Both shrunk with the increasing duration of hot dip. K. Bouche et al. [6] performed intermetallic compound layer growth between solid iron and molten aluminium. The intermetallic layers formed at the solid–liquid interface and their growth mechanisms are characterized. The Fe2Al5 and FeAl3 phases are identified in the temperature range from 700 to 900 °C, and their growth was found to be mainly controlled by a diffusion regime. A.-F. Ciuffini et al. [7] conducted a hot-dip aluminizing reaction using on AISI F55–UNS S32760 super duplex stainless steel. Molten aluminum interacts exclusively with the ferritic phase due to its much
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Wang et al. [22] controlled frequencies and voltages in a pulse power supply to produce different surface morphologies of ceramic oxide film. When the control voltage was 500 V and the negative frequency was of greater magnitude than the positive frequency, the surface roughness of the ceramic oxide film was reduced. With fewer holes, a low coefficient of friction (0.2–0.3) was obtained in the friction test. Yerokhin et al. [23] performed a microarc reaction with an alternating current (AC) power source and a reaction with a pulse power source. The film thickness was high, whereas the surface roughness was low for the pulse power source; however, the surface adhesion was high when the AC source was used. Moreover, an energy dispersive spectrometer (EDS) map function was used in the study, which indicated that the surface was comprised primarily of aluminum and oxygen, with extremely low quantities of other elements. Butyagin [24] and others reported that during MAO of an aluminum alloy, current decreased with time under a constant voltage, and the current reduction for various materials was, in descending order, KOH > ultradispersable powder of α-Al2O3 > Na2SiO3 > Co(NO3)2. In this study, the stainless-steel material was used as tool electrode substrate. For the production of the insulating layer, HDA was first performed. After the aluminum layer was coated on the surface of the workpiece using different durations of HAD and the MAO process was conducted with optimal parameters to convert the aluminum layer into a ceramic insulation layer, the surface and aluminum layer of the crosssection were observed through SEM and EDS. Owing to the ceramic oxide film provided with good properties such as insulation, abrasion resistance, mechanical strength and high temperature resistance, hence after preparing the MAO insulation layer, the tool electrode was used with different MAO parameters for an electrolytic withstand voltage test using a sodium chloride solution. Furthermore, the ECM performance of various tools with and without aluminum oxide insulating layer was examined by drilling using direct current (DC) power supply conditions on a stainless-steel workpiece, the processing performance could be understood by the method. The differences between the entrance and exit values indicated that the electrochemical insulating layer considerably reduces stray current and improves drilling accuracy. Finally, the development of a tool electrode with precise and robust insulation layer be expected to achieve precise ECM effects.
Fig. 9. Hot-dip aluminizing processing time versus coating thickness.
2. Materials and methods Fig. 10. Element distribution after hot-dip aluminizing of 4 min.
2.1. Pretreatment of substrate
permeated with pores and cracks. The inner layer was specifically intended to possess high binding affinity and excellent mechanical properties. The wavy, jagged appearance generated by an oxide layer and a substrate was attributed to the melting effect in the early stage of MAO. Sundararajan et al. [18] performed a microarc test on a 7075 aluminum alloy. The surface roughness increased with the time and thickness of a ceramic oxide film, and surface holes were obtained; software was used to process scanning electron microscopy (SEM) images. With time, the number exponentially decreased, and the diameter of the hole linearly increased. Tian et al. [19] performed MAO of an aluminum alloy with alkaline solution of KOH and collected the generated gas using a gas collector. The generated gas was primarily hydrogen and oxygen. The rate of gas generation increased with the current density and time but decreased as the amount of added KOH increased. Finally, the ceramic oxide film thickness was optimal with 1 g/L of KOH. Xue et al. [20] conducted a reaction using 2024 aluminum alloy. The trend of the elastic modulus was similar to the hardness curve, and the hardness reached 32 GPa. Sun et al. [21] used fabricated a ceramic oxide film through MAO on a light alloy to improve the roughness and compactness of the film at various cathode/ anode current densities, with the cathode current density higher than anode current density. The roughness of 2 times or more will become significantly smaller, and the thickness of the dense layer increased.
This experiment used an American Iron and Steel Institute 304 stainless-steel substrate. The size of the tool electrode tube was outer diameter of 3.0 mm, inner diameter of 2.0 mm and length of 100 mm. Before HDA, an aluminum workpiece was first washed with acetone, alkali-washed with 10 wt% NaOH, acid-washed with 15 wt% H3PO4, neutralized with 5 wt% NaOH, and then dried and subjected to hot-dip aluminizing. 2.2. Hot-dip aluminizing The material used for HDA was pure aluminum ingot, and the aluminum ingot was placed in an alumina crucible. Fig. 2 shows the hotdipping furnace. The furnace was heated to 750 °C. The immersion aluminizing parameters were as Table 1. Aluminum acted as a solution in the HDA process and was poured into the alumina crucible; however, the air in contact with the aluminum surface of the molten liquid formed an oxide layer. Such an oxide layer will cause poor adhesion between the electrode and molten aluminum during HDA. Before the aluminum piece was molten, the oxide layer must be removed. Therefore, the following two steps were used to eliminate the oxide layer: (1) a layer of a wet flux coating was applied to the specimen surface and left to dry (a ratio of 10:1 for flux to water), and (2) dry flux was added to remove the oxide layer from the molten aluminum surface. The main 5
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Fig. 11. Surface state diagram of MAO layers formed with different voltages and fixed MAO time of 8 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
Fig. 12. Surface state diagram of MAO layers formed with different voltages and fixed MAO time of 10 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
composition of the flux was shown in Table 2. After the flux treatment, HDA was performed. The parameters were a temperature of 750 °C, dipping time of 2–6 min, and rising and falling speed of 80 cm/min.
stainless-steel electrolytic bath and was steeped in a receptacle with dimensions of 150 × 150 × 150 mm3. The electrode was set as the anode, and the stainless-steel tank was set as the cathode. The electrolyte was then poured. After the electrolyte had set completely, microarc oxidization was performed, and Fig. 3 shows the processing diagram. The experimental parameters for MAO were different constant voltages and different oxidation times. The MAO parameters were as follows: voltages of 425, 450, 475, and 500 V and times of 8 and 10 min (Table 3). Table 4 shows the electrolyte formula of MAO process.
2.3. Microarc oxidation MAO equipment includes a power supply, a stainless-steel electrolytic tank, and cooling systems. Before MAO, the equipment must undergo a surface treatment. After ultrasonic cleaning in deionized water for 10 min, the components were rinsed with acetone and immediately dried in air. The electrode was placed perpendicular to the center of a 6
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electrode prepared through hot-dip aluminizing and MAO was placed on a cathode Z-axis chuck. The stainless-steel workpiece was placed on the anode, and NaNO3 electrolyte was used for electrochemical drilling as shown in Table 5. Fig. 5 shows the electrochemical drilling setups. 3. Results and discussion 3.1. Effect of HDA parameters and observation of the coating state 3.1.1. Effect of aluminum morphology and structure obtained through HDA time The thickness of the aluminized layer and the surface morphology were the most crucial factors in this experiment. Therefore, this experiment examined the effect of hot-dip aluminizing time on the coating. First, the experimental results of HDA were analyzed. Fig. 6 shows surface morphology after hot-dip aluminizing for 2 min (a) and 3 min (b); it can be seen from the surface that there are distinct intermetallic compounds and the aluminum coating is relatively thin and rough due to thermal unevenness and low immersion time. According to literature [6], intermetallic compounds growth was found to be mainly controlled by a diffusion regime. As can be seen that the coverage area of aluminum coating increases with the increase of HDA time. Fig. 6(c) illustrates hot-dip aluminizing for 4 min, and it can be found that the aluminum coating is completely covered and the surface topography is relatively flat. Fig. 6(d) and (e) show hot-dip aluminizing for immersion times of 5 and 6 min. After these considerably longer immersion times, the surface was rough and uneven because of holes and defects caused by thicker aluminum coating.
Fig. 13. Microarc oxidation processing voltage versus coating roughness.
2.4. Voltage tolerance of insulation layer An electrolysis reaction was used to achieve the effect in this study. The resistance voltage of the insulated electrode could be successfully determined through an electrolytic method (Fig. 4). A stainless-steel substrate and microarc-oxidized layer electrode were used as the anode and cathode, respectively. The electrolyte was a 15 wt% aqueous sodium chloride solution and continuously energized at a constant voltage until the cathode boiled and fell off. For evaluating the voltage tolerance, the breakthrough voltage of the insulation layer must be determined. A continuous energizing current was supplied at a low voltage of 0.2 V until the cathode boiled, at which point, the time was recorded. The voltage could be easily examined by tilting the oxidized layer during electrolysis, which was advantageous for observing the bubbles during the breakthrough of the insulation layer.
3.1.2. Relationship between HDA parameters and coating thickness The state of the HDA layer at immersion time of 2 min was observed in cross-section as shown in Fig. 7. The protective outer oxide layer was composed of alumina and was formed after immersion aluminum aluminizing. The inner layer was the aluminum layer, which was atop an iron–aluminum miscible layer. The innermost part of the fixed aluminized layer was a stainless-steel substrate. According to research, the HDA process comprises an electrochemical reaction, the HDA itself, and thermal diffusion. The process includes melting, melt-flow, resolidification and diffusion. The hot-dip aluminizing layer in this study comprised two structural layers divided as aluminum layer and ironaluminum miscible layer. During hot dip coating, the substrate will be mutually soluble and diffused with aluminum soup aluminum because aluminum atoms were larger than iron atoms. When diffused, the
2.5. Electrochemical drilling The experimental processing equipment included a self-set electrochemical drilling machine. This machine included an electrolyte circulation system, acrylic tank, clip fixture, and DC power supply. The
Fig. 14. Cross-section of microarc oxidation layer structure at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min. 7
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Fig. 15. MAO layer profile of MAO layers formed with different voltages and fixed MAO time of 8 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
Fig. 16. MAO layer profile of MAO layers formed with different voltages and fixed MAO time of 10 min: (a) 425 V, (b) 450 V, (c) 475 V, (d) 500 V.
layer was 46.5 μm. When the immersion time was increased to 4, 5, and 6 min, the average thickness of the immersed aluminum was increased by 106.3, 140, and 149 μm. According to this trend, an increase in the thickness of the miscible layer prompts an increase in the thickness of the immersed aluminum layer as shown in Fig. 9. In the HDA experiment, the workpiece temperature was same as room temperature. Therefore, it was inferred that the heat conduction on the surface of the workpiece was uneven at the initial stage of immersion plating. After the surface was thermally balanced, the thickness of the coating would grow at another ratio, resulting in a different ratio of initial (2–3 min) to late (4–6 min) as can be seen from Fig. 9. The above coating thickness data was an average thickness, which was an average thickness of 6 points measured for the same workpiece. The measurement was
atomic size will be dominated by iron diffusion. The iron atoms diffuse into the aluminum layer in a large amount, relatively few. The aluminum atoms diffuse inward to achieve a miscible alloying to form the intermetallic compounds. From the Fe/Al phase diagram in which it can be seen that five types of intermetallic compounds (Fe3Al, FeAl, FeAl2, Fe2Al5 and FeAl3 phases) exist. Moreover, it was confirmed from the results of XRD that the surface and lower layers were aluminum and Fe2Al5, respectively. These results were consistent with the literature [5,6]. Fig. 8 shows cross-sections after various hot-dip aluminizing times. The average thickness of the immersed aluminum layer of the electrode was 12.5 μm for an immersion time of 2 min. The playing immersion time was 3 min, and the average thickness of the immersed aluminum 8
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an influence but not highly relevant. From the experimental results showed that the Ni element has no diffusion at all. The aforementioned results indicate that the duration of immersion affects the surface morphology of the electrode. After a short immersion time, the aluminum of the surface coating was uneven. After a long immersion time, the surface of the electrode aluminizing was excessively undulated and flat. Therefore, when the immersion aluminizing time was 4 min, the electrode aluminizing flatness was improved; therefore, subsequent MAO would employ an immersion aluminizing time of 4 min. 3.2. Effect of MAO parameters and observation of the coating state 3.2.1. Effect of MAO on morphology and structure Because of the aforementioned HDA, an experimental product with a relatively flat surface was used for MAO; that was, the workpiece prepared through HDA for 4 min underwent MAO. First, for the MAO parameters, microarc voltage controlled at 425, 450, 475, or 500 V and MAO times of 8 or 10 min were used, and the surface condition of the MAO layer was observed. Fig. 11 shows the surface state of the MAO layer at different voltages for 8 min of fixed MAO time. Fig. 11 (a), (b), (c), and (d) show the surface morphologies obtained with microarc voltages of 425, 450, 475, and 500 V, respectively. When the microarc voltage was 425 V, microcavities were formed on the surface of the MAO layer. The characteristics of spark holes were not evident, and spark holes were only generated by high-voltage discharge; however, the microarc ceramic surface was incomplete. Fig. 11 (b) shows the results obtained with a microarc voltage of 450 V; the surface exhibited microarc features, high density, and a discharge tunnel. For the same MAO time of 8 min, the surface morphology was observed. Although some microarc features were more evident, no flat or dense surface was observed, and the surface microholes comprised numerous ridges. When the MAO voltage was increased, the number of strong-discharge holes on the surface increased, and they grew in size, exhibiting local cracks and pores on the surface tend to cover a large area as shown in Fig. 11 (d). Fig. 12 shows a surface state diagram of the MAO layer for a MAO time of 10 min. Fig. 12 (a), (b), (c), and (d) microarc voltages of 425, 450, 475, and 500 V, respectively. When the MAO time was increased as 10 min, the microarc features of the surface layer were significantly improved. Fig. 12 (c) shows a microarc with a voltage of 475 V and a time of 10 min, the microarc ceramic surface hole was covered using the remaining dense microarc ceramic surface; thus, the discharge holes were respectively flatter and smaller than those for other microarc voltages. Fig. 12 (d) corresponds to a microarc voltage of 500 V. The figure shows that the number of discharge cells and holes increased considerably because of the large discharge voltage, and the MAO ceramic layer grew extremely rapidly; thus, these structures were crowded. The resulting pressure formed surface cracks, and the MAO layer and stainless-steel tube substrate were also cracked. Since the tool electrode material was a stainless steel tube with a wall thickness of 0.5 mm, under the experimental conditions of high micro-arc oxidation voltage, the thickness of the stainless steel decreases as the micro-arc time increases, so when the voltage was too high lead to strong discharge energy, it will easily occur the plating peels off or penetrates the stainless steel tube. Fig. 13 shows the average surface roughness of MAO-treated hotdip-aluminized test strips obtained using different operating voltages. The surface roughness measurement was performed using a hand-held roughness meter, and each tool electrode tube was rotated and measured for 10 samples and then averaged. When the operating voltage was 425 V, some arcs were observed on the surface of the workpiece during the process, but they were sparse, thus resulting in minimally evident surface microarc characteristics and generating slight changes in surface roughness. When the operating voltage was increased, the MAO layer was relatively dense on the workpiece surface. Surface
Fig. 17. Microarc oxidation processing voltage versus alumina thickness.
Fig. 18. Element distribution after microarc oxidation.
performed using an SEM-attached measuring tool ranging from substrate to coating tip thickness. Fig. 10 shows the element distribution after HDA of 4 min. In order to observe the diffusion of aluminum, the elements of C, O, Al, Fe and Ni were adopted. As can be seen from Fig. 10, the Al element diffuses from the outer layer to the inner layer and the Fe element was the opposite. In addition, from the spectrum 1 of Fig. 10, it can be found that in the aluminum-rich layer, there was a phenomenon of crystal growth of the intermetallic compound Fe2Al5. According to the literature [7] that conducted a HDA reaction using on super duplex stainless steel. Molten aluminum interacts exclusively with the ferritic phase due to its much higher diffusivity in this phase coupled with its marked ferrite-stabilizer behavior. Ni and Cr contents in the ferritic matrix have 9
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Fig. 19. Electrolytic withstand voltage with different microarc voltages and electrode areas.
Fig. 20. Electrochemical drilling results comparison (a) entrance by using uninsulated electrode (b) exit by using uninsulated electrode (c) entrance by using insulated electrode (d) exit by using insulated electrode.
diffusion, sintering, and densification. The microarc oxide layer in this study comprised two structural layers; the outer layer comprised structural defects, such as micropores and cracks, whereas the inner layer was dense and highly bonded, with excellent mechanical properties. The third and fourth layer were aluminum-rich and iron-aluminum miscible layer (intermetallic compounds) [5,6] as shown in Fig. 13. Owing to the tool electrode was immersed into electrolyte during ECM, electrical transmission via liquid phase to anode. The micropores and cracks will be a tunnel to help the electron pass through. Therefore, the denser oxide layer was needed. Fig. 15 shows the MAO layer profile state diagram with different microarc voltages and 8 min of fixed MAO time. Fig. 15 (a), (b), (c), and (d) respectively illustrate the results of microarc voltages of 425, 450, 475, and 500 V. The cross-section of the workpiece after a MAO time of
microarc characteristics were relatively evident. Holes and grooves were generated in the MAO layer, thus increasing surface roughness. However, when the microarc voltage was 500 V, the discharge energy was sufficiently large such that the aluminum layer was exhausted, the ceramic layer cracked, and the substrate was separated from the ceramic layer, thus generating a broken surface with the highest observed surface roughness. 3.2.2. Effect of MAO parameters and cross-sectional state observations The state of the oxide layer at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min was observed in crosssection as shown in Fig. 14. According to research, the MAO process comprises an electrochemical reaction, the MAO itself, and thermal diffusion. The process includes melting, melt-flow, resolidification, 10
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content was the strongest, and the immersed aluminum layer and the aluminum–iron interdiffusion layer (Fe2Al5) were underneath, where the aluminum aluminizing layer was fixed. The innermost part was the stainless-steel substrate, and thus, the iron content was relatively high. 3.3. Withstand voltage test for different MAO parameters Fig. 19 shows the electrolytic withstand voltages of the insulating layer after microarc at various microarc voltages, MAO time of 10 min, and immersion HDA time of 4 min. As can be seen that the maximum withstand voltage was 9.3 V with the microarc voltage was 475 V. Thus according to the tendency of Fig. 18, the electrolytic withstand voltage of the tool surface was proportional to alumina thickness. The obtained MAO layer of the insulating layer had a large hole in its surface, and the ceramic layer and workpiece substrate collapsed. Therefore, during the withstand voltage test, these conditions may result in easy penetration of the insulating layer by the electrolyte. Electrolytic corrosion disintegrates layers, thus resulting in unsatisfactory insulation and reduced voltage resistance. Therefore, the insulating layer obtained at 475 V exhibited a relatively high density and small surface holes, and the withstand voltage was relatively high. The voltage test indicates that although the insulating layer could be fabricated through the MAO process, the withstand voltage reached 9.3 V; however, the actual ECM withstand voltage of the experimental target was slightly different because it was in the production circle. When a rod-shaped material was subjected to MAO after the HDA process, the current distribution was likely to be uneven; this may have caused the surface portion to be easily over-discharged in this study, thus resulting in the collapse of the MAO layer. This phenomenon may affect the withstand voltage.
Fig. 21. Surface characteristics of the finished electrode after microarc oxidation: surface morphology of the electrode (a) before electrochemical drilling and (b) after electrochemical drilling.
3.4. Electrochemical drilling analysis 3.4.1. Improvement of variation between entrance and exit The electrochemical drilling electrode used in this experiment was formed with an HDA time of 4 min, microarc voltage of 475 V, and MAO time of 10 min and comparison with electrodes without an insulation layer was performed. Fig. 20 shows the state of the entrance and exit of the electrochemically drilled product. When an uninsulated electrode was used, the finished workpiece could be observed at the processing inlet because of the uninsulated status of the electrode during processing. Because of generated stray current, the phenomenon of a concave arc shape produced using the electrolytic reaming was evident; and the average processing inlet aperture was 4.23 mm, and the average processing exit hole was 3.75 mm in diameter. Drilling with an electrode with an insulating layer was shown in the figure; the finished product processed using an electrode with an insulating layer could be improved by the effect of the stray current generated by the processing inlet. The average inlet aperture was 3.52 mm, and the average exit aperture was 3.35 mm. Improvement of variation between entrance and exit could be 64.58 % [25]. Multiple sets of processing experiments verified that this electrochemically processed electrode insulation method could effectively suppress stray current and improve processing accuracy.
8 min revealed that only the porous outer layer was present on the MAO layer's surface. The dense inner layer was observed, and it was discharged because of the high voltage. A relationship between the voltage and consumption of the aluminum-rich layer was slightly evident. Fig. 16 shows MAO layer profile state diagrams after MAO with different microarc voltages for a fixed MAO time of 10 min. Fig. 16 (a), (b), (c), and (d) show the results of microarc voltages of 425, 450, 475, and 500 V, respectively. The cross-section reveals that the oxide layer of the workpiece was dense after MAO with 10 min of MAO time, and the structure was solid. The cracks and holes were lower in comparison with those after 8 min, and the cross-section exhibited the highest compactness when the microarc voltage was 475 V in Fig. 16 (c). The least oxide layer in the hole, but after the voltage was raised, as shown in Fig. 16 (d) for a microarc voltage of 500 V, the MAO layer was broken because of excessively strong discharge energy, and cracks were observed at the interface between the substrate and oxide layer. Diffusion, which resulted in the formation of an oxide layer, as seen in the cross-section, exhibited unsatisfactory compactness and excessive cracking. During MAO, the ceramic oxide layer grows outwardly; however, the original aluminum layer was simultaneously consumed. The cross-sectional SEM images and Fig. 17 revealed that the operating voltage increased or the MAO time increased. As expected, the aluminum layer was consumed as the ceramic oxide layer grew in thickness. When the working voltage was increased to 500 V, the discharge energy was considerably large. The aluminum layer was exhausted, and only the ceramic layer was observed. In this condition, the cracking separated the substrate. Fig. 18 shows the elemental distribution of the outermost surface of the electrode after microarc oxidation at microarc voltage of 475 V, MAO time of 10 min, and immersion HDA time of 4 min. The uppermost layer was an embedded resin layer, and thus, the carbon content was the highest. The middle layer was the MAO layer. Therefore, the oxygen
3.4.2. Surface observations of electrode usage status Fig. 21 shows the surface state of the stainless-steel electrode after MAO. After several rounds of electrochemical drilling, the surface was slightly cracked. Because of the electrochemical drilling, the electrolyte circulation system did not filter impurities. Electrolyte impurities cause small sparks between the electrode and processed workpiece. Sparks cause spark holes and cracks on the surface of the electrode surface; however, the integrity of the surface of the insulating layer was mostly maintained, and the diameter was used. Such an electrode insulation layer generated through MAO after HDA can be used for practical applications in electrochemical processing. 11
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4. Conclusion
micro electrochemical machining, CIRP Ann. - Manuf. Technol. 55 (2006) 197–200. [4] J. Brand, S. Van Gils, P.C.J. Beentjes, H. Terryn, V. Sivel, J.H. Wde Wit, Improving the adhesion between epoxy coatings and aluminium substrates, Prog. Org. Coat. 51 (2004) 339–350. [5] S. Kobayashi, T. Yakou, Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment, Mater. Sci. Eng. A 338 (2002) 44–53. [6] K. Bouche, F. Barbier, A. Coulet, Intermetallic compound layer growth between solid iron and molten aluminium, Mater. Sci. Eng. A 249 (1998) 167–175. [7] A.F. Ciuffini, S. Barella, C.D. Cecca, A. Gruttadauria, D. Mombelli, C. Mapelli, “Hotdip aluminizing on AISI F55–UNS S32760 super duplex stainless steel properties: effect of thermal treatments, Metals 7 (2017) 525. [8] R. Arrabal, E. Matykina, T. Hashimoto, P. Skeldon, G.E. Thompson, Characterization of AC PEO coatings on magnesium alloys, Surf. Coat. Technol. 203 (2009) 2207–2220. [9] R.O. Hussein, P. Zhang, Y. Xia, X. Nie, D.O. Northwood, The effect of current mode and discharge type on the corrosion resistance of plasma electrolytic oxidation (PEO) coated magnesium alloy AJ62, Surf. Coat. Technol. 260 (2011) 1990–1997. [10] A. Ghasemi, V.S. Raja, C. Blawert, W. Dietzel, K.U. Kainer, Study of the structure and corrosion behavior of PEO coatings on AM50 magnesium alloy by electrochemical impedance spectroscopy, Surf. Coat. Technol. 202 (2008) 3513–3518. [11] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Advanced Engineering Materials 8 (2006) 511–533. [12] C.E. Jordan, A.R. Marder, Fe-Zn phase formation in interstitial-free steels hot-dip galvanized at 450°C: Part I 0.00 wt% Al-Zn baths, J. Mater. Sci. 32 (1997) 5593–5602. [13] A.L. Yerokhin, A. Leyland, A. Matthews, Kinetic aspects of aluminium titanate layer formation on titanium alloys by plasma electrolytic oxidation, Appl. Surf. Sci. 200 (2002) 172–184. [14] G.P. Wirtz, S.D. Brown, W.M. Kriven, Ceramic coatings by anodic spark deposition, Mater. Manuf. Process. 6 (1991) 87–115. [15] A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [16] G. Lv, Characteristic of ceramic coatings on aluminum by plasma electrolytic oxidation in silicate and phosphate electrolyte, Appl. Surf. Sci. 253 (2006) 2947–2952. [17] C.N. Hsu, C.J. Wang, Effect of microstructure on hot-dip aluminizing, J. Chin. Corros. Eng. 27 (2013) 117–130. [18] G. Sundararajan, L. Rama Krishna, Mechanisms underlying the formation of thick alumina coatings through the MAO coating technology, Surf. Coat. Technol. 167 (2003) 269–277. [19] J. Tian, Z.G. Luo, S.K. Qi, X.J. Sun, Structure and antiwear behavior of micro-arc oxidized coatings on aluminum alloy, Surf. Coat. Technol. 154 (2002) 1–7. [20] W. Xue, C. Wang, Z. Deng, R. Chen, Y. Li, T. Zhang, Evaluation of the mechanical properties of microarc oxidation coatings and 2024 aluminium alloy substrate, Physics Condensed 14 (109) (2002) 47–52. [21] X.T. Sun, Z.H. Jiang, S.G. Xin, Z.G. Yao, “Composition and mechanical properties of hard ceramic coating containing α-Al2O3 produced by microarc oxidation on Ti–6Al–4V alloy, Thin Solid Films 471 (2005) 194–199. [22] Y.M. Wang, B.L. Jiang, T.Q. Lei, L.X. Guo, Microarc oxidation coatings formed on Ti6Al4V in Na2SiO3 system solution: microstructure, mechanical and tribological properties, Surf. Coat. Technol. 201 (2006) 82–89. [23] A.L. Yerokhin, A. Shatrov, V. Samsonov, P. Shashkov, A. Pilkington, A. Leyland, A. Matthews, Oxide ceramic coatings on aluminium alloys produced by a pulsed bipolar plasma electrolytic oxidation process, Surf. Coat. Technol. 199 (2005) 150–157. [24] P.I. Butyagin, Y.V. Khokhryakov, A.I. Mamaev, Microplasma systems for creating coatings on aluminium alloys, Mater. Lett. 57 (2003) 1748–1751. [25] J.C. Hung, J.K. Lin, B.H. Yan, H.S. Liu, P.H. Ho, Using a helical micro-tool in microEDM combined with ultrasonic vibration for micro-hole machining, J. Micromech. Microeng. 16 (2006) 2705–2713.
After the experimental results were analyzed, the following conclusions were drawn: 1. The electrode insulation layer for electrochemical machining could be fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate. 2. The experimental results indicate that the duration of immersion affects the surface morphology of the electrode and coating thickness. When HDA time was 4 min, a denser and flatter aluminum layer resulted. 3. During MAO, the aluminum layer was consumed as the ceramic oxide layer grew in thickness. The MAO layer was broken because of excessively strong discharge energy, and cracks were observed at the interface between the substrate and oxide layer. The micropores and cracks will be a tunnel to help the electron pass through. Therefore, the denser oxide layer was needed. 4. The electrolytic withstand voltage of the tool surface was proportional to alumina thickness. The oxide layer with smallest and most dense spark holes could be obtained using a microarc voltage of 475 V and MAO time of 10 min which demonstrated a voltage tolerance of up to 9.3 V. 5. The MAOed precision electrode used for electrochemical drilling was prepared. The ECM processing voltage, initial processing gap, electrode feed rate, and electrolyte concentration were 5 V, 100 μm, 4 μm/s, and 10 wt%, respectively. The hole expansion phenomenon affected by stray current at the inlet used for drilling was significantly improved. After electrochemical drilling, improvement of variation between entrance and exit could be 64.58 %. The difference between the entrance and exit values indicated that the electrochemically processed insulating layer successfully reduced stray current and improved the precision of drilling. Acknowledgements The authors would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract No. MOST107-2221-E-035-045-MY2. References [1] Z. Liu, Y. Liu, Z. Qiu, N. Qu, Effect of tool electrode insulation on electrochemical micro drilling accuracy, Nanotechnology and Precision Engineering 7 (2009) 355–360. [2] Y. Li, Y. Zheng, G. Yang, L. Peng, Localized electrochemical micromachining with gap control, Sens. Actuators A Phys. 108 (2003) 144–148. [3] B.J. Park, B.H. Kim, C.N. Chu, The effect of tool electrode size on characteristics of
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