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Ultrason Sonochem
Ultrason Sonochem
Ultrasonics Sonochemistry
1350-4177
1873-2828
Elsevier

S1350-4177(24)00302-X
10.1016/j.ultsonch.2024.107054
107054
Original Research Article
Research on jet electrochemical machining with coaxial megasonic assistance
Zhai Ke kezhai@hbu.edu.cn
a⁎
Liang Yongkang a
Li Tengnan a
Ma Shihao a
Du Liqun b
a National & Local Joint Engineering Research Center of Metrology Instrument and System, College of Quality and Technical Supervision, Hebei University, Baoding 071002, China
b State Key Laboratory of High-Performance Precision Manufacturing, School of Mechanical Engineering, Dalian University of Technology, Dalian 116024, China
⁎ Corresponding author. kezhai@hbu.edu.cn
30 8 2024
11 2024
30 8 2024
110 10705417 2 2024
25 8 2024
29 8 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
In order to address the problem of poor localization in electrochemical machining (ECM), a coaxial megasonic assisted jet ECM method was proposed. Based on theoretical analysis, experiments were conducted to compare the effects of various electrolyte flow rates, electrolytic voltage and megasonic power levels on pit ECM. The results indicate that, in the range of experimental parameters, the increase of electrolyte flow rate and megasonic power can increase the machining depth, so as to improve the depth-diameter ratio of ECM pits. The use of coaxial megasonic-assisted jet ECM can enhance the depth-diameter ratio of etched pits compared to the without megasonic one. When applying a megasonic power of 22 W, the dimensions of the ECM pit were measured as 0.81 mm in depth and 5.73 mm in diameter, resulting in an depth-diameter ratio of 0.140. Under the same conditions, without megasonic assistance, the pit diameter is reduced to 0.65 mm while the pit depth increases to 6.36 mm, resulting in a depth-diameter ratio of 0.102. Additionally, The results also demonstrate that, the increase of electrolytic voltage makes the depth to diameter ratio of pit further increase on the original basis. With an electrolyte flow rate of 0.9 L/min and a megasonic power of 22 W, the use of electrolysis voltage of 50 V increased the depth-diameter ratio of etched pits to 0.173. Using the above preferred parameters, electrolytic milling of the wide groove is carried out. The depth-diameter ratio of the wide groove is increased from 0.039 to 0.059 by appending coaxial megasonic. This further verified the effectiveness of the coaxial megasonic-assisted jet ECM method.

Keywords

Jet ECM
Megasonic
Depth-diameter ratio
Pit etching
Groove etching
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pmc1 Introduction

ECM is a fast and effective technology for machining metal materials [1]. Based on the controlled anodic dissolution, ECM is ideally suited for machining difficult-to-cut metals, offering advantages such as no tool wear, high efficiency, and no surface hardening layer [2]. Jet-ECM is a unique ECM process that utilizes the electrolyte-jet as the cathode tool to electrochemically fabricate desired metallic microstructures cost-effectively and flexibly. In addition to the inherent advantages of ECM, Jet-ECM also features higher machining selectivity, larger material removal rate (MRR), and greater operating flexibility [3]. Based on these advantages, Jet-ECM can be widely used in aviation, automotive, biomedicine, electronics, and many other industrial fields [4], [5], [6]. Nonetheless, certain issues persist, including suboptimal surface machining quality, stray current corrosion, and processing efficiency that is constrained by the transfer of reactants.

To address the challenges associated with Jet-ECM, optimizing the parameters of the jet electrolysis process can serve as a viable solution. A study by Zhang et al. [7] explored the influence of electrolytic jet orientation on electromachining characteristics. Comparing various jet directions revealed that a horizontal jet configuration achieved superior machining accuracy. Furthermore, the study found that a horizontal jet enhances mass transfer and current density concentration, resulting in higher material removal rates. Utilizing ultra-high density current in the jet electrochemical milling of Ti-6Al-4 V, Huang et al. [8] discovered that intense current mitigates stray corrosion within the electrolytic process. However, it is noted that high current application can enhance material removal rates but potentially compromising current efficiency. Lin et al. [9] conducted a study on the high-precision ECM of a kerosene-immersed horizontal jet and observed that optimizing the orientation of the horizontal jet not only enhances mass transfer conditions but also stabilizes the machining process, mitigates stray corrosion, and improves the accuracy of kerosene-immersed jet ECM.

In addition to optimizing ECM parameters, auxiliary techniques such as laser, ultrasound, and electric spark can further enhance precision and quality of ECM. Malik et al. [10] utilized pulsed laser to assist the jet ECM process in precision microhole drilling and compared the results with those obtained solely by jet ECM. They found that the average reduction rates for taper, overcut, and an unspecified third parameter in the spark-affected area were 32.3 %, 27.8 %, and 50.2 %, respectively. This indicates that the addition of laser auxiliary processing can significantly enhance both processing accuracy and quality. Ultrasonic vibration and ultrasonic cavitation have proven to be effective in enhancing ion mass transfer on the workpiece surface, thereby accelerating ion exchange and the rate of movement within the solution. This unique combination of effects has led to ultrasonic-electrochemical compound machining (UECM) achieving remarkable results and showcasing significant potential for further development. Specifically, UECM has the potential to dramatically boost processing efficiency, refine surface quality, and elevate processing accuracy, as evidenced by studies in the literature [11], [12], [13]. Sonia et al. [14] analysed the role of ultrasonic assistance in electrolytic machining, where ultrasonic vibrations can generate microcavitation bubbles in the machining zone. The collapsing of micro bubbles improves the possibility for increasing the electric charge transportation and intensification of mass. Liu et al. [15] combined ultrasonic vibration with a high-speed rotating spiral electrode for micro-electrochemical milling and discovered that ultrasonic composite ECM can reduce surface roughness, improve machining quality and efficiency, thereby verifying the effectiveness of ultrasonic-assisted micro-electrochemical machining. Dalabehera et al. [16] conducted experimental analysis on ultrasonic composite continuous pulse electro-jet machining and verified that ultrasonic vibration can control the formation of more precise and fine features at high frequencies. Zhang et al. [17] investigated ultrasonic pulsed electrolytic machining based on particles and found that the addition of particles can further enhance ultrasonic-assisted electrolytic machining, thereby improving the machining accuracy and surface quality. Goel et al. [18] utilized multi-physics modeling and simulation methods to analyze the processing laws of jet electrolytic drilling and ultrasonic-assisted jet electrolytic drilling processes. The results indicated that when ultrasonic waves are superimposed on the electrolyte jet, the material removal rate (MRR) significantly improves. Wang et al. [19] examined the impact of ultrasonic stirring parameters on mass transfer in mask electrolytic processing. Through numerical methods, they observed periodic changes in sound pressure in the electrolyte over time and achieved optimized mass transfer with ultrasonic, resulting in a 30 μm micro-pit array with smooth etched surfaces on large-scale tin bronze substrates. Additionally, Wang et al. [20] introduced a new hybrid electrolytic processing technology that combines air assistance and ultrasonic assistance to achieve efficient material removal and surface quality improvement.

Megasonic waves exhibit shorter wavelengths, higher vibration frequencies, superior directivity, and a higher cavitation threshold compared to ultrasonic waves. Additionally, their acoustic-fluid effect is more pronounced, significantly enhancing fluid flow and facilitating mass transfer of electrolyte, as noted in [21], [22]. This paper delves into the theoretical and experimental results of coaxial megasonic-assisted jet ECM, and carried out single-point machining and wide groove milling processes. The purpose of using coaxial megasonic is to accelerate the updating of the electrolyte in the machining zone, thereby enhancing the precision and efficiency of ECM.

2 Method and principle

2.1 Mathematical model

The megasonic used in this method propagates in liquid, and the waveform is mainly longitudinal wave. Satisfy the wave equation:(1) ∇·-1ρ0·∇p+1ρ0c2∂2p∂t2=0

The non-homogeneous Helmholtz equation is obtained by separating the time terms:(2) ∇·-1ρc·∇P-1ρcωcc2P=0

The formula mentioned above includes the following terms:

P represents sound pressure,

ω represents acoustic angular frequency,

ρc represents composite density, and.

cc represents composite sound velocity.

Compared to other sound waves, the megasonic wave has a shorter wavelength, a higher frequency, and good linearity. Under the action of megasonic sound, the electrolyte is perturbed by the volumetric force of megasonic sound action, which promotes the renewal of electrolyte in the anode processing area, thus achieving the purpose of enhanced convective mass transfer. Therefore, under the action of megasonic field, the volume force F in the fluid can be expressed as:(3) F=2αρ0c02P2

In the electrolysis reaction, a very small number of dissolved cations at the anode combine with hydroxide ions to produce hydroxide precipitates, but this amount is negligible compared to the amount of hydrogen gas produced at the cathode. Therefore, it can be considered that the flow field is mainly a two-phase flow of gas and liquid composed of hydrogen gas and electrolyte. Assuming that the electrolyte is an incompressible fluid, the flow field satisfies the Navier-Stokes equation [23]:(4) ρ∂u∂t+ρu·∇u=∇-p+μ∇u+F

Among them, F represents the volume force between electrolyte particles; μ represents the dynamic viscosity of the electrolyte during electrolytic processing; u represents the flow velocity of the sodium chloride solution.

In addition, the mass conservation equation of the flow field can be described as follows:(5) ∂∂tφlρl+φgρg+∇·φlρlul+φgρgug=0

And(6) ul=u,ρ=φlρl+φgρg

In the formula, ul represents the liquid phase velocity; ug represents the gas phase velocity; φg represents the percentage of hydrogen gas in the cross-section of the electrolyte; φl represents the percentage of sodium chloride solution in the cross-section of the electrolyte; ρl represents the density of the electrolyte; ρg represents the density of hydrogen gas.

Furthermore, during electrolysis, a small quantity of anode gas is produced in the vicinity of the anode, whereas a substantial amount of hydrogen is evolved near the cathode. Concurrently, the electrolytic reaction generates heat, causing the temperature of the electrolyte to rise, which in turn leads to a decrease in the electrolyte's conductivity. The conductivity of the electrolyte can be expressed as:(7) κ=κ0*1-βn1+γT-T0

Among them, κ0 represents the original conductivity of the sodium chloride solution; β represents the volume fraction of hydrogen gas in the sodium chloride solution; n represents the Brugemann coefficient;T0 represents the initial temperature of the sodium chloride solution; T represents the solution temperature at a certain time during electrolytic processing; γ represents the temperature coefficient of sodium chloride solution during electrolytic processing.

In the actual electrolysis process, the electrolyte is placed in a constant temperature bath, where the default temperature is constant. According to this Eq. (7), the conductivity of the electrolyte can be simplified as:(8) κ=κ0*1-βn

At this point, electrolyte conductivity has a significant impact on the ECM process, mainly reflected in the impact on current density. Considering the distribution of primary current, the relationship between current density and electrolyte conductivity is shown in the formula below:(9) ie=κ*∇Φ

where ie represents the current density of the electrolyte during ECM, and Φ represents the electrode potential during ECM.

Based on the aforementioned analysis, during ECM, an increase in the volume percentage of hydrogen in the electrolyte directly impacts the electrolyte conductivity, leading to a decrease in current density and subsequently a reduction in the anode dissolution rate. The integration of megasonic into jet ECM processes can, on the one hand, elevate the electrolyte flow rate, ensuring continuous renewal of the electrolyte within the anode machined area; on the other hand, the supplementary megasonic waves can facilitate the dissipation of reaction gases. It is anticipated that the combined effects of these two aspects will enhance both the accuracy and efficiency of jet ECM.

2.2 Experimental set up

The design of the nozzle for coaxial megasonic-assisted electrochemical jet machining is outlined in Fig. 1(a). The piezoceramics plate, used to generate the megasonic wave, is coaxial with both the electrolytic cathode and the electrolyte jet. Beneath the nozzle lies the workpiece, which acts as the electrolytic anode. The electrolytic cathode is in the shape of a flange, as depicted in Fig. 1(b). The electrolyte flows into the cathode through eight small holes, each with a diameter of 4.5 mm, and then out through a large hole in the middle, with a diameter of 12 mm. Finally, it is sprayed onto the surface of the workpiece via a nozzle that has a diameter of 5 mm. This design ensures that the electrolyte is in full contact with the piezoelectric oscillator, allowing the megasonic wave to be fully propagated through the liquid stream. During experimental process, the electrolyte flows smoothly from the side of the nozzle to the workpiece surface. Simultaneously, the piezoceramics plate emits megasonic waves which propagate parallel to the electrolyte jet, ultimately reaching the workpiece surface. To facilitate the experimental process, the nozzle is securely mounted on a three-axis motion platform, enabling precise movement for either single-point processing or electrochemical milling. The nozzle maintains a consistent distance from the anode plate, and the flow rate of the electrolyte is precisely controlled by adjusting the voltage of the peristaltic pump through a power supply. For electrochemical processing, a DC power supply is used to establish electrical contact between the anode and cathode. The experimental setup is illustrated in Fig. 1(c).Fig. 1 Coaxial megasonic assisted jet ECM device. (a) Structural schematic diagram, (b)Shape of the electrolytic cathode, (c) Device photograph.

In the experiment, a 15 % NaCl solution was used as the electrolyte; the 304 stainless steel plate was used as the anode workpiece. Due to the loss of hydrogen during the electrolysis process, the hydroxide ion and the dissolved cation at the anode can easily form a precipitate product, which affects the efficiency of the anodic reaction. Therefore, it is necessary to add an amount of acidic solution to adjust the pH value of the solution. Hydrochloric acid of 1 mol/L concentration was dripped into the electrolytic solution to control the pH to be within the range of 1.5–2. Additionally, the distance between the anode plate and the nozzle was set to 0.5 mm, and the frequency of the megasonic transducer was adjusted to 1 MHz. In the experiment, pit etching was first studied under different process conditions, and groove electrolytic milling was subsequently carried out based on the results of the pit etching, as shown in Table 1.Table 1 Experimental parameters.

Pit etching	Flow (L/min)	Electrolytic voltage(V)	Megasonic power(W)	
0.45	40	0	
0.90	40	0	
1.22	40	0	
Flow (L/min)	Electrolytic voltage(V)	Megasonic power(W)	
0.90	40	0	
0.90	40	8	
0.90	40	16	
0.90	40	22	
Flow (L/min)	Electrolytic voltage(V)	Megasonic power(W)	
0.90	30	0	
0.90	30	22	
0.90	40	0	
0.90	40	22	
0.90	50	0	
0.90	50	22	


	
groove electrolytic milling	Flow (L/min)	Voltage(V)	Megasonic power(W)	
0.90	50	22	
0.90	50	0	

3 Experimental results and discussion

3.1 Electrolyte flow

In the initial phase of the study, the impact of electrolyte flow rate on the etching effect in jet ECM was thoroughly examined. To conduct the experiments, the electrolytic voltage was set to 40 V, and the electrolyte flow was adjusted using a peristaltic pump at rates of 0.45 L/min, 0.90 L/min, and 1.22 L/min. Subsequently, a measuring tool microscope was used to meticulously measure the pit diameter, pit depth of the etched pits. As shown in Fig. 2, when the electrolyte flow rate is 0.45 L/min, the etched pit has a diameter of 6.00 mm, a depth of 0.59 mm, and an depth-diameter ratio of 0.098. At an electrolyte flow rate of 0.90 L/min, the etched pit has a diameter of 6.36 mm, a depth of 0.65 mm, and an depth-diameter ratio of 0.102. When the electrolyte flow rate increases to 1.22 L/min, the etched pit exhibits a diameter of 6.91 mm, a depth of 0.73 mm, and an depth-diameter ratio of 0.105. The results indicate that as the electrolyte flow rate increases, both the pit diameter and pit depth of the etched pits gradually enlarge. Furthermore, the depth-diameter ratio of the etched pits exhibits a slight increase. However, when the peristaltic pump flow rate reached 1.22 L/min, pipeline vibrations became severe, leading to inconsistent processing effects and impeding further processing. Therefore, for the next phase of etching, an electrolyte flow rate of 0.90 L/min was deliberately selected. This rate provided an optimal balance between processing effectiveness and consistency, ensuring optimal results during the subsequent phase of the study.Fig. 2 Pit etching results of different electrolyte flow rates. (a) Photo of the pit etched with 0.45 L/min electrolyte flow rate. (b) Photo of the pit etched with 0.90 L/min electrolyte flow rate. (c)Photo of the pit etched with 1.22 L/min electrolyte flow rate. (d) Results of diameter. (e) Results of depth. (f) Results of depth to diameter ratio.

3.2 Megasonic power

In the experiments of electrochemical jet machining, the electrolyte flow rate was fixed at 0.9 L/min, and a electrolytic voltage of 40 V was maintained between the anode and cathode. The megasonic power was then varied among 8 W, 16 W, and 22 W. Following the experiments, the pit depth and diameter of the etched pits were measured, and the depth-diameter ratio was calculated. As shown in Fig. 3, at a megasonic power of 8 W, the etched pit has a diameter of 5.76 mm, a depth of 0.58 mm, and an depth-diameter ratio of 0.101. When the megasonic power increases to 16 W, the etched pit has a diameter of 6.09 mm, a depth of 0.72 mm, and an depth-diameter ratio of 0.118. At a megasonic power of 22 W, the etched pit exhibits a diameter of 5.73 mm, a depth of 0.81 mm, and an depth-diameter ratio of 0.140. The results reveal that as the megasonic power increases, the pit depth of the etched pits deepens, while there is no significant increase in the etching pit diameter. This suggests that, under the influence of megasonic waves, the coaxial propagation of these waves effectively vibrates and removes electrolytic products from the bottom of the electrolytic pits. This promotes electrolytic reactions to proceed in the depth direction, enhancing the deep etching ability of electrolytic processing. Consequently, the depth-diameter ratio of etched pits also increases with the augmentation of megasonic power. Interestingly, when the megasonic power is relatively low (8 W), the depth-diameter ratio of micro-pits etching is close to that without the megasonic effect, indicating that the megasonic effect is not prominent at low powers. However, as the megasonic power escalates, its impact on enhancing etching becomes more evident, demonstrating its effectiveness in promoting electrolytic reactions and enhancing etching capabilities.Fig. 3 Etching results of aperture, pit depth, and depth-diameter ratio at different megasonic power. (a) Results of diameter. (b) Results of pit depth. (c) Results of depth-diameter ratio.

3.3 Electrolytic voltage

To further verify the effectiveness of megasonic-assisted jet ECM, based on an electrolyte flow rate of 0.9 L/min and a megasonic power of 22 W, experiments were carried out at electrolytic voltages of 30 V, 40 V, and 50 V to study jet ECM pits. The pit diameters and depths were measured and compared with and without megasonic assistance. As shown in Fig. 4(a–f), without megasonic waves, when the electrolytic voltage is 30 V, the pit diameter is 6.23 mm, the pit depth is 0.38 mm, and the depth-diameter ratio is 0.061. As the electrolytic voltage increases to 40 V, the pit diameter shrinks to 6.18 mm, while the depth deepens to 0.63 mm and the depth-diameter ratio increases to 0.102. At 50 V, the pit diameter further diminishes to 5.91 mm, the pit depth reaches 0.91 mm, and the depth-diameter ratio is 0.154. However, when megasonic waves are applied, distinct differences are observed. With a voltage of 30 V, the pit diameter is reduced to 5.92 mm, the pit depth remains at 0.37 mm, and the depth-diameter ratio is 0.062. At 40 V, the pit diameter further shrinks to 5.73 mm, but the pit depth increases to 0.81 mm and the depth-diameter ratio jumps to 0.140. Finally, at 50 V, the pit diameter slightly increases to 6.00 mm, but the depth reaches its maximum of 1.04 mm and the depth-diameter ratio is 0.173. A comparison of the etching results for pit diameter, depth, and depth-diameter ratio under different conditions is provided in Fig. 4(g–i). Additionally, current values under various working conditions were measured. Without megasonic waves, the electrolytic currents at different electrolytic voltages are 0.36A, 0.51A, and 0.89A, respectively. With megasonic waves, the electrolytic currents at different voltages increase to 0.42A, 0.87A, and 1.00A, respectively (Fig. 4j). At a constant voltage, the electrolytic currents with megasonic action are all larger than those without megasonic action, indicating that megasonic waves play a significant role in improving the etching rate.Fig. 4 Etching results of diameter, pit depth and depth-diameter ratio under different electrolytic voltages. (a) U=30 V, megasonic 22 W. (b) U=40 V, magesonic 22 W. (c) U=50 V, megasonic 22 W. (d) U=30 V, Silent sound. (e) U=40 V, Silent sound. (f) U=50 V, Silent sound. (g) Results of aperture. (h) Results of pit depth. (i)Results of depth-diameter ratio. (j) Results of electrolytic current.

3.4 Groove electrolytic milling

To explore the feasibility of using the coaxial megasonic assisted jet ECM method for electrolytic milling processing, the electrolyte flow rate was set to 0.9 L/min, the electrolytic voltage was fixed at 50 V, the nozzle movement speed was adjusted to 0.8 mm/s, and the reciprocating movement was repeated 10 times while other parameters remained unchanged. The electrolytic milling effects with and without megasonic waves were then compared, as shown in Fig. 5(a) and (b). After the milling process, the line width and depth of the micro-grooves were precisely measured as shown in Fig. 5(c–f). In the absence of megasonic waves, the groove line width was measured to be 6.61 mm, the groove etching depth reached 0.26 mm, and the depth-diameter ratio was calculated as 0.039. At this point, the current between the anode and cathode was recorded at 0.65A. However, when megasonic waves were applied at a power of 22 W, notable differences were observed. The groove line width decreased to 5.98 mm, the groove etching depth increased to 0.35 mm, and the depth-diameter ratio improved to 0.059. At this time, the current between the anode and cathode was recorded at 1.13A. These findings suggest that the coaxial megasonic assisted jet ECM method can indeed serve as an effective approach for electrolytic milling of structures.Fig. 5 Groove electrolytic milling results. (a) No acoustic electrolytic milling. (b) Electrolytic milling with megasonic power 22 W. (c) Groove width. (d) Groove depth. (e) depth to width ratio. (f) Electrolytic current at different working conditions.

4 Conclusion

(1) The theoretical analysis delves into the significance of megasonic in ECM. The results of analysis indicate that megasonic enhances the electrolytic reaction by enhancing electrolyte renewal and facilitating the discharge of electrolytic products such as electrolytic gas. This leads to improved machining accuracy and efficiency.

(2) Furthermore, the effectiveness of megasonic assisted jet ECM is confirmed through rigorous testing. When compared to traditional methods, the application of megasonic jet ECM results in a noticeable increase in etching depth, while maintaining a consistent line width. This improvement in etching depth-diameter ratio highlights the superior localized processing capabilities and etching efficiency of this technique.

(3) The coaxial megasonic assited jet ECM is applied to electrolytic milling, and the depth-diameter ratio of the wide groove is increased from 0.039 to 0.059 by the coaxial megasonic assisted electrolytic milling method. This shows that it is an effective method to use megasonic in electrolytic milling.

CRediT authorship contribution statement

Ke Zhai: Writing – review & editing, Supervision, Funding acquisition, Conceptualization. Yongkang Liang: Writing – original draft, Methodology. Tengnan Li: Validation, Investigation. Shihao Ma: Supervision, Project administration, Conceptualization. Liqun Du: Writing – review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the 10.13039/501100012166 National Key Research and Development Program of China (No. 2022YFB4601602 ), the 10.13039/501100003787 Natural Science Foundation of Hebei Province (No. E2022201028 ).
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