Material Removal and Surface Modification of Copper under Ultrasonic-Assisted Electrochemical Polishing

Abstract

Electrochemical polishing exhibits high efficiency and simplicity of operation and presents broad prospects in metal field processing. However, the poor conductivity of the surface oxides generated during electrochemical polishing may lead to uneven electrolysis and surface protrusions if not promptly removed. This study combined ultrasonic treatment with electrochemical polishing and adjusted the angle of the ultrasonic jet to investigate the effect of ultrasonic-assisted electrochemical polishing on the removal of protruding microstructures. The study examined the surface morphology, hardness, residual stress, and workpiece contact angle before and after processing. The results demonstrated that ultrasonic assistance can effectively promote electrochemical reactions and improve the removal efficiency of the workpiece surface. With an increase in ultrasonic power and processing time, the corrosion potential of the workpiece decreased, which accelerated the material removal rate. The roughness of the workpiece surface increased within the threshold. Additionally, the surface hardness increased to 105.3 HV, the residual stress was enhanced by 517.89 MPa, and the contact angle increased to 104.7°. The erosion characteristics and hydrophobicity of the workpiece were also enhanced.

1. Introduction

Processing methods for metallic materials vary according to the material type, surface conditions, and required surface smoothness. Common processing methods include mechanical, chemical, and electrochemical polishing [1]. Mechanical machining methods can process complex surfaces; however, direct contact between the tool and workpiece can cause heating and deformation [2]. After chemical processing, the residual liquid corrodes the substrate, and the chemical agents can pollute the environment [3]. Owing to its non-contact and non-thermal processing characteristics, electrochemical technology has attracted widespread interest in eliminating the force or heat applied during traditional processing processes and maintaining the mechanical and metallurgical properties of polished components intact [4]. Electrochemical polishing is another effective method for processing conductive materials and complex shapes [5]. However, electrochemical products cannot be rapidly removed in electrolytic processing with narrow electrode gaps, leading to uneven material corrosion, which reduces the processing stability and accuracy. In addition, electrochemical polishing can remove the cold, work-hardened layer formed on the workpiece surface during mechanical machining, thereby improving the surface performance of the workpiece.

Ultrasound technology has been applied to electrochemical machining to promote the elimination of electrochemical products and improve processing efficiency. Ultrasound-assisted electrochemical polishing is studied widely to overcome the limitations of traditional processing methods. The shock wave released by the cavitation collapse causes enormous pressure and microplastic deformation on the surface of the material, which changes workpiece surface and material properties [6]. Ultrasonic cavitation, as a nonlinear acoustic phenomenon, involves the dynamic processes of expansion, oscillation, compression, and collapse in a liquid medium [6,7,8]. Studies have reported that ultrasonic vibration helps to eliminate bubbles in the electroplating solution and prevents interference from hydrogen bubbles during the coating formation process, thereby increasing the density and crystallinity of the coating, reducing the formation of pores, and promoting denser coating formation, resulting in submicron-level grain refinement on the eroded surface [9,10]. Liu et al. [11] found that ultrasonic cavitation is suitable for processing the surface microstructure of workpieces to improve processing for enhanced bonding strength. Research has shown that ultrasonic vibrations can accelerate the discharge of electrochemical products through cavitation and liquid-phase mass-transfer effects, thereby promoting electrochemical dissolution [12,13,14,15]. The micro/nano surface morphology of metals was modified through cavitation collapse, demonstrating the ability of cavitation to modify metal surfaces [16,17,18]. Li et al. [19] found that the ultrasonic-assisted electrodeposition of nickel/diamond coatings exhibited better corrosion resistance than coatings prepared under mechanical stirring conditions. Research has shown that ultrasonic-assisted electrochemical polishing can promote the discharge of electrochemical products, induce micro-deformations on material surfaces, further influence the material microstructure and surface morphology, improve corrosion resistance, and achieve surface modification of metal materials. However, there is a lack of detailed research on the specific changes in material surface properties resulting from ultrasonic-assisted electrochemical polishing.

Experiments were conducted to investigate the effects of ultrasonic-assisted electrochemical polishing on material surface properties. This study introduced ultrasound into electrolyte processing to induce cavitation effects, thereby accelerating mass transfer and reaction rates and achieving efficient processing and modification of material surfaces. The effects of different ultrasonic power levels and processing time on the workpiece surface were studied. In addition, the impact of the ultrasonic-assisted electrochemical polishing on the workpiece surface is described in terms of surface morphology, hardness, residual stress, erosion characteristics, and wetting properties.

2. Experimental System and Characterization Equipment

2.1. Experimental Installation

Figure 1a shows a schematic of the ultrasound-assisted electrochemical polishing experiment. Figure 2 shows a physical diagram. The device is divided into electrochemical and ultrasonic parts. The electrochemical part primarily consists of an electrochemical workstation, a working electrode, an auxiliary electrode, and a reference electrode. In this study, we applied different potentials and electrolysis time to the electrolytes. The ultrasonic component consists mainly of an ultrasonic vibration-driving device, ultrasonic transducer, amplitude rod, and tool head. The common transducer frequencies on the market are 28 kHz, 30 kHz, 100 kHz, and 120 kHz. With the increase in frequency, the amplitude and sound pressure of the transducer decrease, and the influence on the processing is not obvious. An ultrasonic transducer with a frequency of 28 kHz was used in this experiment.

Table 1. The main parameters of the transducer.

Parameter	Parameter Value
Resonance frequency (kHz)	28
Capacitance (pF)	3100 ± 10%
Resonant impedance (Ω)	≤25
Radius (mm)	8
Tool head material	Die steel

lists the specific transducer parameters. The distance between the transducer and workpiece is 10 mm. All the experiments were performed at room temperature. The ultrasonic vibration generator was capable of automatic frequency tracking. The intensity of the ultrasonic vibration could be adjusted by changing the ultrasonic power. The ultrasonic vibration generator converts the alternating current into high-frequency alternating current signals matched with the transducer, which then converts it into mechanical vibration. The tool head was fixed at the end of the amplitude rod and vibrated at the same frequency.

The main process involved the induction of electrochemical dissolution at the anode to form a passivation film on the surface using an electrochemical workstation. Ultrasonic vibrations disrupt the interactions between electrolytes, generating cavitation effects. When the cavitation bubbles collapse near the workpiece surface, they produce microjets (as shown in Figure 1b,c) to remove the passivation film from the surface, exposing the substrate to rapid dissolution in the electrolyte. Simultaneously, by varying the inclination angle of the workpiece (Figure 1d), the electrolyte can be quickly refreshed under the continuous impact of ultrasonic vibration, gradually achieving material removal, further promoting the removal rate of the electrochemical products, and enhancing processing efficiency.

2.2. Ultrasonic Transducer

The probe hydrophone and oscilloscope are used to detect the sound pressure of the machining center. The detected value of the oscilloscope is the effective value of the voltage value. The amplitude of the transducer is detected by a vibrometer. Changes in the amplitude and sound pressure in the ultrasonic transducer at different powers are shown in Figure 3.

2.3. Characterization Equipment

An electrochemical workstation (CHI760E, Chenhua, Shanghai, China) was employed to apply voltage to the electrolyte at different electrolysis times to study the polarization characteristics of the material. The surface morphology and roughness variations were investigated using a white-light interferometer (Super View W1) manufactured by CHOTEST, Shenzhen, China. Surface hardness was determined using a Vickers microhardness tester, and residual stress was measured using a D8 Advance X-ray diffractometer (XRD) analyzer (Bruker, Mannheim, Germany). Contact angle measurements were conducted using a DSA30S contact angle tension meter.

3. Experiment

3.1. Electrochemical Passivation Experiment

The formation of a passivation film on the surface of the copper workpiece was a prerequisite for the experiment. A 0.03 M benzotriazole (BTA) solution was prepared using an 18% mass fraction hydroxyethylidene diphosphonic acid (HEDP) solution and deionized water as the base electrolyte. Copper specimens were used as working electrodes, platinum plates were used as auxiliary electrodes, and a saturated calomel electrode (SCE) type 232 was used as the reference electrode for the experiments conducted using a CHI760E electrochemical analyzer.

Table 2. Experimental conditions of different electrolytic voltage.

Parameter	Value
Electrolytic voltage/V	0.2; 0.3; 0.4; 0.5; 0.6
Electrolyte	18% HEDP + 0.03 M BTA
Processing period/s	200
Workpiece size	Φ20 mm× 7 mm
Workpiece material	Copper

lists the experimental parameters for this process.

Figure 4a depicts the I-t current curves of the workpiece at different polarization potentials, where the polarization current increases with increasing polarization potential. Research has shown that the passivation of copper anodes initiated locally extends into a sheet and increases the coverage over time. Eventually, only small local holes or grooves continue to undergo electrode reactions, whereas other areas are protected by the passivation film [20]. Figure 4b shows the changes in the AC impedance spectrum of the workpiece under different polarization potentials, where the capacitive arc radius reaches its maximum at a polarization potential of 0.2 V, and the diffusion phenomena are more pronounced in the high-frequency region. Figure 5 shows the SEM images of the workpiece surface at polarization potentials of 0.3 V and 0.4 V, indicating that when the polarization potential reaches 0.4 V, corrosion pits (indicated by the arrows) appear on the workpiece surface, leading to a deterioration in surface quality. Based on these results, a polarization potential of 0.3 V is selected.

3.2. Feasibility Experiment

When ultrasonic vibration acts on an electrochemical system, its main effect is to promote the electrochemical reaction, and ultrasonic action can increase the corrosion current of copper. The specific parameters for the feasibility experiments are listed in

Table 3. Experimental conditions of different ultrasonic power.

Parameter	Value
Electrolytic voltage/V	0.3
Electrolyte	18% HEDP + 0.03 M BTA
Electrolyzing time/s	60
Ultrasonic jet angle/°	30, 45, 60, 90
Ultrasonic processing period/s	30
Workpiece size	Φ 20 mm × 7 mm

. Figure 6 shows the I-t current curves of the workpiece under the influence of an ultrasonic jet at different angles. After electrochemical polishing, the polarization current of the workpiece increased with the jet angle, indicating that the current density on the workpiece surface can be adjusted by changing the jet angle. With an increase in the jet angle, the lateral jet velocity on the front side of the jet exceeded that on the back side, increasing the volume pressure on the back side, thereby increasing the pressure on the workpiece surface [21]. In addition, when the workpiece was tilted, the electrochemical products were rapidly removed from the workpiece surface under the impact of ultrasonic vibration, thereby accelerating the electrochemical polishing rate on the workpiece surface. However, when the jet angle exceeded 60°, the current density rapidly increased and then decreased to the level at an angle of 60°. This indicates that the jet angle has a threshold effect on the electrochemical polishing rate of the workpiece surface, and exceeding this threshold results in a decrease in the rate. Therefore, a comprehensive selection of an ultrasonic jet angle of 60°is recommended.

After passivation, Cu formed a passivation structure with layered Cu₂O/CuO/Cu(OH)₂ on the electrode surface [22], as shown in Equation (1).

$$\left\{\begin{array}{l}2\mathrm{C}\mathrm{u}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}{\mathrm{u}}_2\mathrm{O}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\\ \mathrm{C}{\mathrm{u}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{C}\mathrm{u}\mathrm{O}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\\ \mathrm{C}{\mathrm{u}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{C}\mathrm{u}{(\mathrm{O}\mathrm{H})}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\end{array}\right.$$

(1)

In acidic electrolytes, the oxide layer on copper is unstable if there is no other insoluble covering layer to protect it, and the copper oxide reacts with hydrogen ions in the solution, leading to dissolution [22].

$$\left\{\begin{array}{l}\mathrm{C}{\mathrm{u}}_2\mathrm{O}+2{\mathrm{H}}^{+}\to 2\mathrm{C}{\mathrm{u}}^{+}+{\mathrm{H}}_2\mathrm{O}\\ \mathrm{C}\mathrm{u}\mathrm{O}+2{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{u}}^{2+}+{\mathrm{H}}_2\mathrm{O}\\ \mathrm{C}\mathrm{u}{(\mathrm{O}\mathrm{H})}_2+2{\mathrm{H}}^{+}\to \mathrm{C}{\mathrm{u}}^{2+}+2{\mathrm{H}}_2\mathrm{O}\end{array}\right.$$

(2)

The primary mechanism of ultrasound in liquids is cavitation, in which small gas-bubble nuclei undergo dynamic processes under the influence of both positive and negative pressures from ultrasonic vibrations, including expansion, compression, collapse, and oscillation. This rapid collapse instantly generates localized high temperatures and pressures that affect the surrounding tissues. Ultrasonic vibration disrupts the interaction between electrolytes, promoting electrolytic reactions through agitation and cavitation effects, thus enhancing the electrolysis efficiency. When cavitation bubbles collapse near the workpiece surface, microjets are produced, and high-speed impacts create protrusions and cavities on the workpiece surface, accompanied by workpiece surface plastic deformation. Under ultrasonic cavitation, water undergoes the following reactions:

$${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}^{+}+\mathrm{O}{\mathrm{H}}^{-}$$

(3)

Equations (2) and (3) show that the H⁺ generated under ultrasonic cavitation reacts with the copper oxide produced during electrolysis, thereby enhancing the decomposition efficiency of the oxide and promoting electrolytic processing.

3.3. Ultrasonic-Assisted Electrochemical Machining

At a polarization potential of 0.3 V and under the influence of a 60° ultrasonic jet, a passivation film was formed on the surface of the workpiece. Ultrasonic vibration causes erosion of the passivation film on the protrusions of the workpiece by cavitation bubbles, exposing the copper substrate and thereby deepening electrolysis. To investigate the removal effect of ultrasonic vibration on the protruding areas during electrochemical polishing, experiments were conducted using ultrasonic vibration-assisted electrochemical polishing with different ultrasonic power levels and processing times, as shown in

Table 4. Experimental conditions of different ultrasonic power.

Parameter	Value
Electrolytic voltage/V	0.3
Electrolyte	18% HEDP + 0.03 M BTA
Ultrasonic power/W	60, 80, 100, 120
Ultrasonic processing period/s	30
Workpiece size	Φ 20 mm × 7 mm

and

Table 5. Experimental conditions of different processing times.

Parameter	Value
Electrolytic voltage/V	0.3
Electrolyte	18% HEDP + 0.03 M BTA
Ultrasonic power/W	100
Ultrasonic processing period/s	60, 150, 300, 400
Workpiece size	Φ 20 mm × 7 mm

, for processing parameters.

4. Result and Discussion

4.1. Surface Morphology under Ultrasonic Electrochemical Action

4.1.1. The Effect of Ultrasonic Power on Surface Morphology

Figure 7 and Figure 8 show the three-dimensional and optical morphologies of the workpiece surface after ultrasonic electrochemical polishing. Observing the workpiece surface at an ultrasonic power of 60 W, it is evident that it exhibits uneven and prominent scratches (indicated by the arrows). This is attributed to the low current density on the workpiece surface, resulting in uneven metal dissolution where surface protrusions cannot dissolve completely. In addition, scratches on the workpiece surface are more pronounced because electrolysis occurs preferentially in damaged areas [23]. When the ultrasonic power was low, the removal effect on the passivation film was poor, leading to a situation in which the electrolysis rate exceeded the ultrasonic removal rate, resulting in a residual passivation film on the workpiece surface. As the ultrasonic power increased, the workpiece surface gradually flattened. It can also be seen from the marks in Figure 8. This is because ultrasonic vibration promotes electrolyte agitation and enhances mass transfer rates on the electrode surface [24,25]. Consequently, the ultrasonic removal rate exceeded the electrolysis rate, which reduces the residual passivation film and the fading of scratches on the workpiece surface. The addition of ultrasonic vibration reduced the variability in the surface contours and decreased surface protrusions. At an ultrasonic power of 100 W, the variability in the contours reached a minimum, indicating the best surface smoothness. However, when the ultrasonic power exceeds 100 W, large cavitation pits appear on the surface of the workpiece, resulting in significant material removal and substantial surface damage. Figure 6 shows the variations in the surface roughness (Sa) under different ultrasonic power levels of the workpiece. As the ultrasonic power increases, the Sa of the workpiece increases continuously. When the power was 100 W, there was a downward trend in Sa, indicating better surface quality. Overall, Figure 7, Figure 8 and Figure 9 demonstrate that the workpiece obtained a better surface quality at an ultrasonic power of 100 W. Therefore, an ultrasonic vibration of 100 W was recommended for further investigation of the effects of different processing times.

4.1.2. The Effect of Processing Time on Surface Morphology

Figure 10 and Figure 11 show the three-dimensional and optical surface morphologies of the workpiece surface under different processing times with an ultrasonic power of 100 W. It can be observed that with increasing processing time, the cavities formed by ultrasonic cavitation gradually removed the protruding portions of the workpiece surface, resulting in the fading of scratches on the workpiece surface and a gradual flattening of the workpiece. However, when the time exceeded 300 s, numerous cavitation pits appeared on the workpiece surface, indicating deterioration in the surface quality. Figure 12 shows the variation in the Sa of the workpiece at different processing times. As the processing time increased, the surface roughness of the workpiece increased continuously. The change in the Sa of the workpiece was insignificant when the processing time was within 300 s. However, when the processing time exceeded 300 s, a noticeable increase in Sa was observed. This is because, under continuous ultrasonic impact, cavitation bubbles collapse, causing an increase in liquid temperature and current density. The weakening of cavitation effects leads to electrolysis rates exceeding the ultrasonic removal rate, resulting in passivation of the workpiece after continuous electrochemical polishing in the electrolyte. Consequently, the workpiece surface gradually corrodes. Therefore, selecting an appropriate processing time is crucial for ensuring the quality of the workpiece surface.

4.2. Surface Hardness and Residual Stress

4.2.1. The Change in Surface Hardness and Residual Stress

The surface hardness is an important indicator of the surface performance and quality of materials after ultrasonic electrochemical polishing. The Vickers hardness of the copperplate surface after ultrasonic-assisted electrochemical polishing was measured using an HV-1000 microhardness tester. Figure 13a shows the variation in the surface hardness of the workpiece after ultrasonic-assisted processing at different ultrasonic power levels. The hardness increased with increasing ultrasonic power; however, the change in hardness was not significant when the power exceeded 100 W. Figure 13b shows the variation in the surface hardness of the workpiece after different processing times at an ultrasonic power of 100 W. With an increase in processing time, the surface hardness of the workpiece increased slightly, indicating that although electrochemical polishing could maintain the metallurgical properties of the workpiece surface intact, the impact of high-speed microjets and cavitation collapse shock waves on the surface hardness of the materials was affected by ultrasonic action [17]. High-energy cavitation collapse releases high-speed microjets and shock waves, causing gigapascal pressure and microplastic deformation of the material surface. The intense high-pressure impact caused by microjets and shock waves leads to an increase in surface hardness [26,27].

4.2.2. The Change of Residual Stress

Electrochemical polishing induces material surface transfer and structural changes during processing, which may affect the residual stress state of the workpiece. Therefore, the change in the residual stress on the workpiece surface after ultrasonic-assisted electrochemical polishing is also an important indicator for evaluating the material performance. Figure 14 and Figure 15 show the X-ray diffraction (XRD) patterns and changes in the residual stress of the workpiece under different ultrasonic power levels and processing times. With an increase in ultrasonic power and processing time, the overall change trend was not significant; however, there was a trend of peak shift to the right in the diffraction peaks, indicating the presence of residual compressive stress on the workpiece surface. Additionally, with an increase in the ultrasonic power level and processing time, the residual compressive stress increased. This is because the cavitation effects of ultrasonic vibration induce plastic deformation on the material surface, further affecting its microstructure and surface morphology [28,29,30]. During ultrasonic vibration processing, the microjet generated by cavitation continuously impacts the surface of the specimen, and the normal force perpendicular to the workpiece induces a more uniform deformation and introduces higher compressive stress values. The residual compressive stress on the workpiece surface improves fatigue strength, fatigue life, and workpiece wear resistance.

4.3. Erosion Characteristics and Wettability

4.3.1. The Change in Erosion Characteristics and Wettability

Electrochemical polishing is influenced by various factors, including the electrolyte composition, current density, and processing time, which affect the erosion characteristics of the materials [31,32]. Erosion characteristics are important indicators for measuring the impact of ultrasonic-assisted electrochemical polishing on the surface quality of workpieces. Figure 16 shows the polarization curves of the workpiece surface under different ultrasonic power levels and processing time. The results indicate that as the ultrasonic power and processing time increased, the corrosion potential decreased, and the corrosion current increased, which accelerated the dissolution reaction and corrosion rate of the materials. This is mainly because, during processing, several grain boundaries provide active dissolution sites, thereby accelerating the diffusion of elements. This suggests that ultrasonic vibration assistance during electrochemical polishing is beneficial for increasing the erosion rate of the material, thereby enhancing processing efficiency.

4.3.2. The Change in Wettability

The impact of ultrasonic vibration can lead to changes in the surface microstructure of the workpiece, such as the formation of micro-concave structures or nanosized features, which may affect the surface contact angle and wetting properties. Meanwhile, the hydrophobicity of materials is an important criterion for evaluating metal surface quality. Figure 17 shows the contact angle of the workpiece at different ultrasonic power levels and processing time. The results indicate that as the ultrasonic power and processing time increased, the contact angle gradually increased, demonstrating the enhanced hydrophobicity of the workpiece. The results were generally consistent with the changes in the sound power and amplitude after processing. Moreover, this improvement in hydrophobicity can effectively prevent corrosion from external sources, reduce frictional resistance, and achieve self-cleaning of the metal surface. Metal-based superhydrophobic surfaces have always been a research hotspot; however, owing to factors such as high production costs and limitations in large-scale production, metal-based superhydrophobic products are not commonly seen in daily life and industrial production. Ultrasonic-assisted electrochemical polishing provides a new approach to producing superhydrophobic metal surfaces.

5. Conclusions

This paper presents an experimental study of the removal and surface modification of copper using ultrasonic-assisted electrochemical polishing. The effects of the ultrasonic power and processing time on the surface morphology, roughness, residual stress, Vickers hardness, and erosion characteristics of the workpiece were analyzed in detail. The main conclusions are as follows:

(1)

The addition of ultrasonic vibration assistance to electrochemical polishing can increase the current density, which increases with increasing ultrasonic power. Furthermore, changing the jet angle can alter the current density, with the most significant effect of ultrasonic vibration assistance observed when the jet angle is 60°. Changes of material removal or surface topography are consistent with the variation of current density.

(2)

Surface protrusions on the workpiece gradually diminish with prolonged processing time. However, when the time is excessively long, cavitation bubbles collapse, increasing the electrolyte temperature and weakening the cavitation effects. The electrolysis rate exceeds the ultrasonic removal rate, causing the workpiece to be excessively passivated. Consequently, the current density on the workpiece surface increases sharply, leading to deteriorated surface quality.

(3)

As the ultrasonic power and processing time increase, the corrosion potential of the workpiece decreases, enhancing the material erosion rate during electrochemical polishing. Simultaneously, residual compressive stress appears on the workpiece surface, and there is a slight increase in the Vickers hardness. Additionally, under the impact of ultrasonic vibration, the hydrophobicity of the workpiece surface is improved compared to that before processing, which helps enhance the self-cleaning ability of the workpiece. It can be seen that all these changes (including surface morphology, hardness, residual stress, erosion characteristics, and wettability) are generally consistent with the changes in power and amplitude, indicating the effect of ultrasonic assistance on electrochemical polishing modification.