Research on Unidirectional Traveling Wire Electrochemical Discharge Micromachining of Thick Metal Materials

Abstract

Wire electrochemical discharge machining (WECDM) integrates the effectiveness of electrical discharge machining (EDM) with the superior quality of electrochemical machining (ECM), leading to enhanced machining efficiency, excellent surface finish, and significant potential for advancement. However, previous research has mainly focused on the processing of non-metallic materials, with little research in the field of the microfabrication of thick metal materials. The wire electrochemical discharge machining process with large aspect ratios is more complex. Accordingly, a unidirectional traveling wire electrochemical discharge micromachining (UWECDMM) method using a glycol-based electrolyte was proposed. The method employs a glycol solution with low conductivity and a neutral salt, facilitating enhanced mass transfer efficiency through a unidirectional traveling wire, and enabling the realization of high-efficiency, high-precision, and recast-free processing. The phenomenon of discharge in UWECDMM was observed in real-time with a high-speed camera, while the voltage and current waveforms throughout the machining process were carefully analyzed. It was found that electrolysis and discharge alternate. Experiments were conducted to investigate the wire traveling pattern, the recast layer, and the wear of the wire electrode. It was found that due to the small energy of a single discharge, the wear of wire electrodes is minimal after multiple uses and can be reused. Under optimal parameters, a machined surface without a recast layer can be obtained. In the final stages, a standard structure was machined on plates of 10 mm thickness made of pure nickel and 304 stainless steel, using a tungsten wire measuring 30 μm in diameter. The feed rate achieved was 1 μm/s, the surface roughness (Ra) measured 0.06 μm, and the absence of a recast layer confirmed the method’s sustainability and quality traits, indicating significant potential in microfabrication.

1. Introduction

In recent times, the swift advancement of manufacturing technology has led to a significant move toward miniaturization and precision in product requirements, creating considerable challenges for micro-manufacturing technology. The most commonly utilized micromachining techniques encompass mechanical micromachining, laser processing [1,2], electrical discharge micromachining (EDM), electrochemical micromachining (ECM), and other related processes [3,4]. The technology of combining multiple energy fields for processing has been widely studied, such as laser electrochemical composite processing [5,6], electrolytic discharge composite processing, etc. This could combine the advantages of single energy field processing technology to achieve higher processing efficiency and surface quality. Wire electrical discharge micromachining (WEDMM) demonstrates exceptional processing efficiency due to its non-contact mechanism, eliminating mechanical interactions between the tool and workpiece. The process relies on transient electrical discharges that generate localized ultra-high temperatures exceeding 8000–12,000 °C. This intense thermal energy induces rapid material phase transitions within the machining zone, facilitating precise material removal through controlled fusion and vaporization mechanisms. The product transportation efficiency is high, which can realize the processing of large thickness workpieces, but the machined surface will produce heat-affected zones, forming recast layers and discharge pits, which will affect the material performance [7]. Wire electrochemical micromachining (WECMM) utilizes the electrochemical dissolution mechanism at the anode, employing a micrometer-scale metallic wire cathode to achieve submicron-level precision with negligible tool wear. This method holds significant promise for fabricating microscale components [8]. Nevertheless, the confined interelectrode gap impedes the efficient evacuation of electrolytic byproducts, despite the reciprocating motion of the wire electrode designed to enhance debris removal. Notably, ECM processes prioritize precision through optimized parameters, such as short-duration pulses, low-voltage operation, and minimized current inputs [9], inherently compromising material removal rates. In a hybrid manufacturing approach, Kurita et al. [10] implemented sequential EDM shaping, followed by ECM finishing on a unified platform, achieving a surface roughness of Ra 0.06 μm. This synergistic methodology underscores ECM’s efficacy in mitigating surface irregularities induced by prior EDM operations.

WECDM integrates the principles of ECM and EDM, combining their respective advantages to achieve high-efficiency, high-precision material processing. Widely applied in non-metallic material fabrication, this hybrid process employs diverse cathode configurations, such as metallic wires, textured tools [11,12], and helical electrodes [13]. The processed materials span composites [14], quartz [15], and others. Figure 1 shows a schematic diagram of the ECDM processing mechanisms for conductive and non-conductive materials. As depicted in Figure 1a, the tool electrode acts as the cathode, paired with an auxiliary anode in alkaline electrolytes (e.g., NaOH/KOH). A critical voltage induces gas film breakdown on the tool surface, triggering localized discharges near the workpiece that remove material via melting or vaporization, as extensively documented [16,17]. Recent advancements include magnetic field-assisted WECDM by Rattan et al. [18], where magnetohydrodynamic (MHD) convection improved electrolyte circulation, boosting material removal rates by 9.09–200%. Peng et al. [19] demonstrated pulse DC power’s superior spark stability over conventional DC in optical glass and quartz machining. Horizontal wire electrode alignment, typical in non-metallic ECDM, limits processing to simple geometries like microgrooves. To address this, Wang et al. [20] developed an oil-film-assisted method, forming a stable insulating layer on the wire for vertical quartz glass machining via spray cooling.

Metallic ECDM mechanisms differ from non-metallic counterparts by incorporating electrochemical dissolution alongside gas film formation [21,22]. As shown in Figure 1b, low-concentration saline solution dominates conductive material ECDM [23,24], while glycol-based electrolytes show promise. Wu et al. [25] reduced EDM-induced surface defects on stainless steel using deionized water, whereas Geng et al. [26] pioneered high-speed ECDM drilling with NaNO3-glycol electrolyte and visual modeling. Kong et al. [27] achieved an Ra 0.12 μm surface roughness on Inconel 718 via spiral wire ECDM, while Han et al. [28] developed a glycol-based energy regulation method, producing Ra < 0.2 μm features on metals (304 SS, TC4, etc.). These studies have demonstrated the feasibility of using ethylene glycol-based solutions as ECDM processing media. However, existing WECDM systems face limitations in micro/nano-structure fabrication due to large cathode diameters and complex bubble dynamics in submicron gaps.

This study investigates NaNO₃-glycol electrolyte’s discharge mechanisms using a high-speed camera and electrical signal analysis. The process parameters (voltage, frequency, and duty cycle) were optimized to fabricate microstructures, validating the feasibility of UWECDMM.

2. Principle of UWECDMM

Figure 2 illustrates the material removal mechanism. The gray area represents the wire electrode, the yellow area represents the workpiece, and the blue area represents the electrolyte. Sodium nitrate and glycol come from a professional supplier in Jiangsu Province (China National Pharmaceutical Group Chemical Reagent Co., Ltd, Beijing, China). Nickel and GH4202 are professional material suppliers from Hebei Province (Qinghe Hengyu Metal Materials Co., Ltd., Qinghe, China). A low-conductivity NaNO₃-glycol electrolyte is employed in a pulse power configuration with the workpiece as the anode and the wire electrode as the cathode. Low-conductivity electrolytes can undergo electrolysis and have a certain impedance that is conducive to generating electricity. The bubbles generated by electrolysis are more conducive to discharge on the surface of the electrode. Primary material removal occurs through controlled discharges that generate a recast layer, while electrochemical dissolution at the slit sidewalls addresses residual defects. As the interelectrode gap narrows, the intensified potential gradient exceeds the critical breakdown voltage, initiating localized discharges (EDM zone). The workpiece material is removed under the action of discharge, and then the material changes from a molten state to a solid state, forming a recast layer on the surface of the workpiece. Concurrently, electrochemical reactions dominate in wider gaps (ECM zone), selectively removing the recast layer. The wire’s unidirectional motion enhances debris evacuation, optimizing mass transfer and process stability.

The NaNO₃-glycol electrolyte is chosen for its oxygen suppression capability during ECM, preventing passive film regeneration on workpiece surfaces. Key electrolytic reactions include the following:

Anode:

$$M-{ne}^{-}\to M^{n+}$$

(1)

Cathode:

$$2{HOCH}_2{CH}_{2}OH+{2e}^{-}\to {2{OHCH}_2{CH}_{2}O}^{-}+H_2\uparrow$$

(2)

When metal materials undergo anodic dissolution during electrolytic machining, the relationship between the dissolution amount and the electricity follows Faraday’s law. According to Faraday’s first law, the mass of dissolved metal at the anode is given by the following:

M = kQ = kIt

(3)

where M represents the mass of dissolved metal at the anode (g), k denotes the mass of dissolved elements per unit electricity [g/(A·s)], Q is the total electricity passing through the two-phase interface (A·s), I is the current intensity (A), and t is the duration of the current flow (s).

In the context of machining processes, the proportion of electrolytic pulses (${\mathrm{\varnothing }}_0$) combined with the pulse power characteristics (WECM) allows the calculation of the dissolved material volume ($V_1$) through the following equation [29]:

$$V_1={\displaystyle \frac{{\varnothing }_0}{2T_p}}\pi dl\omega ∆t{\int }_0^{t_{on}}{i}_{r}dt$$

(4)

where $T_p$is the pulse period, d is the diameter of the electrode wire, $t_{on}$ is the pulse width, l is the workpiece thickness, ω is the volume electrochemical equivalent of the element, and $i_r$ is the electrochemical reaction current density.

The material removal model of WECDM is shown in Figure 3, where the yellow area represents the workpiece, the blue area represents the electrolyte, and the white area represents the recast layer. Within the time interval $∆t$, the wire electrode is fed from point A to point B at a feed rate of v. In the figure, $d_s$ represents the discharge gap, $d_h$ is the depth of the discharge pit, $d_r$ is the thickness of the recast layer, and $d_k$ is the width of the slit.

So, at time $t+∆t$, the volume $V_{t+∆t}$ of the recast layer formed by discharge is

$$V_{t+∆t}={\displaystyle \frac{1}{2}}\pi l{d}_r(d+2d_s+2d_h-d_r)$$

(5)

Due to the unchanged processing parameters, the recast layer thickness $d_r$ formed by each discharge remains consistent. Therefore, the total number of recast layers N generated during the $∆t$ interval is

$$N={\displaystyle \frac{2v∆t}{d+2d_s+2d_h-d_k}}$$

(6)

Therefore, the overall volume $V_2$ of the recast layer generated by discharge within $∆t$ is approximately

$$V_2=N·V_{t+∆t}$$

(7)

Let δ represent the coefficient of the electrolytic dissolution. When the proportion of electrolytic pulses is small, material removal is primarily attributed to electric spark action, with δ approaching 0. Conversely, when the feed rate is very low, the pulse ratio of electrolysis will approach 100%, and the material will be removed by electrolysis. At this time, δ is approximately 1, so the range of δ values is (0, 1). Combining Equation (4), the following can be obtained:

$$V_1=\delta V_2$$

(8)

In utilizing Equations (4)–(7) into Equation (8), the following can be obtained:

$$\mathsf{\delta }={\displaystyle \frac{{\varnothing }_0}{v}}·{\displaystyle \frac{d+{2d}_s+2d_h-d_k}{d_r\left(d+2d_s+2d_h-d_r\right)}}·{\displaystyle \frac{1}{2T_p}}d\omega {\int }_0^{t_{on}}{i}_{r}dt$$

(9)

From Equation (9), it is evident that the electrolytic dissolution coefficient δ is influenced by three primary factors: the proportion of electrolytic pulses (${\varnothing }_0$), feed rate (v), and electrochemical reaction current density ($i_r$). The electrochemical reaction current affects the electrolysis process, and the feed rate affects the proportion of electrolysis pulses, all of which interact with each other. Additionally, the conductivity of the electrolyte also affects the current density. Therefore, in order to achieve non-recast-layer electrochemical discharge machining, it is possible to consider increasing the proportion of electrolytic action pulses, appropriately reducing the feed rate, and improving the conductivity of the electrolyte.

3. Experimental System and Methods

Figure 4a illustrates a schematic representation of the experimental setup, while Figure 4b provides its physical counterpart. The experimental apparatus comprises an X–Y–Z motion stage, an oscilloscope, a pulse generator, a wire feed system, a fixture, a working liquid tank, a flow pump, etc. In this configuration, the workpiece is connected to the positive terminal of the power supply, whereas the wire electrode is linked to the negative terminal via a conductive roller. The workpiece, mounted on the Z-axis, traverses radially toward the wire electrode along the predefined toolpath on the X–Y plane. Horizontal flushing is employed to remove byproducts from the machining zone, ensuring continuous electrolyte renewal to minimize stray corrosion on the workpiece surface. An oscilloscope monitors and records the applied voltage and machining current, while a computer-controlled CCD camera facilitates the real-time observation of the process. The electrolyte conductivity is measured using a conductivity meter (Seven Compact S230, Mettler Toledo, Zurich, Switzerland), and a high-speed camera (Dimax HS1, PCO, North Rhine-Westphalia, Germany) captures the discharge dynamics.

Tungsten wire electrodes, owing to their straightforward fabrication and ability to achieve diameters as small as a few microns, are well-suited for producing narrow slots and high-precision microstructures. Consequently, a 30 µm diameter tungsten wire is employed as the cathode in this study. Prior to experimentation, a 10 mm thick nickel plate (10 × 25 mm) undergoes surface preparation through sandpaper polishing, followed by ultrasonic cleaning in alcohol. The electrolyte consists of a NaNO₃-glycol electrolyte. Post-processing analysis includes surface morphology and elemental distribution examination using a scanning electron microscope (SEM, S-4800N, Hitachi, Tokyo, Japan) equipped with an energy-dispersive spectroscopy system (EDS, X-flash 5030, Bruker, Billerica, MA, USA). The surface roughness (Ra) is quantified using an atomic force microscope (AFM, Dimension Edge SPM, Bruker, Billerica, MA, USA), while slot width measurements are performed with an optical microscope (VHX 6000, Keyence, Minato, Japan). To ensure measurement reliability, each data point is recorded at least three times. Detailed experimental parameters are summarized in

Table 1. Experimental conditions for UWECDMM.

Parameters	Values
Workpiece	10 mm thick Ni plate
Tool electrode	$\mathsf{\varphi }$30 µm, tungsten wire
Electrolyte conductivity	200 µS/cm
Pulse voltage	55–70 V
Pulse frequency	30–90 KHz
Duty cycle	30–45%
Wire speed	20–65 mm/s
Feed rate	0.6–1.8 µm/s

.

4. Results

4.1. Observation of Discharge

To explore the machining mechanism and principles of UWECDMM, the discharge phenomenon in a NaNO₃–glycol electrolyte was analyzed using a high-speed camera (capture rate: 18,000 fps (50.5 µs), resolution: 640 × 480 pixels). A 1 mm thick acrylic plate was securely bonded to the workpiece using epoxy resin adhesive. The wire electrode was positioned to cut into the workpiece edge, and the machining gap was recorded from the front using the high-speed camera (Figure 5). The experimental parameters are detailed in

Table 2. Parameters used for high-speed imaging experiments.

Parameters	Values
Electrolyte conductivity	200 µS/cm
Pulse voltage	65 V
Pulse frequency	50 KHz
Duty cycle	35%
Wire speed	50 mm/s
Feed rate	1.0 µm/s

, and the observed discharge phenomena are illustrated in Figure 6. Upon applying a pulse voltage between the anode and cathode, tiny bubbles emerge near the electrode as a result of electrolysis, which progressively combine and enlarge (Figure 6a). As the wire electrode advances, the equivalent resistance of the electrolyte decreases, and the potential gradient between the electrodes exceeds the critical discharge limit. This will cause the electrolyte to be broken down, resulting in a discharge phenomenon, instantly melting and removing the material at high temperature, forming a melt pit (Figure 6b). The high temperature generated during the discharge process will cause the electrolyte to vaporize and produce more bubbles (Figure 6c). These newly created bubbles merge with adjacent ones, and the discharge cycle ends as the spark fades (Figure 6d), marking the beginning of the subsequent process phase.

4.2. Analysis of Processed Waveforms

The processing waveform serves as an indicator of the machining state, aiding in the analysis of the process and the understanding of its underlying mechanism. Using the parameters listed in Table 2, the experiment collected waveforms within 0.5 s using an oscilloscope. As illustrated in Figure 7a, the enlarged section reveals three distinct waveform types: electrolysis, discharge, and a combination of electrolysis and discharge, with these processes alternating. During the electrochemical dissolution phase, the voltage between the anode and cathode is 50 V, with a current of 0.26 A. As the electrode gap narrows and the voltage surpasses the critical threshold, dielectric breakdown occurs, leading to discharge. During discharge, the voltage drops to 16.8 V, and the current peaks at 0.88 A. In addition to pure electrolysis and discharge waveforms, a combined ECM + EDM waveform is observed. Waveform I (Figure 7a) exhibits electrolysis, followed by discharge due to a delayed discharge, where the delay is less than half the pulse width, allowing the current to reach the normal discharge state (0.88 A). Waveform II is similar but with a discharge delay exceeding half the pulse width, resulting in a shorter current rise time and a lower peak current (0.78 A) compared to the normal discharge state. Waveform III lacks a discharge delay, discharging first and then undergoing electrolysis. This may occur because the system remains in a discharge state from the previous pulse, maintaining discharge conditions at the start of the new pulse. Subsequent electrolysis may arise due to electrode vibrations and an increased electrode gap caused by the discharge.

Figure 7b presents the statistical distribution of the machining waveforms at varying conductivities. Electrolysis dominates, accounting for over 50% of the waveforms, with its proportion increasing as the electrolyte conductivity rises. The ECM proportion grows from 51.12% to 85.09%, while the EDM proportion declines from 48.08% to 14.86%. This reduction in EDM is attributed to decreased electrolyte resistance, leading to lower peak voltages and discharge frequencies. At low conductivity (0.2 µS/cm), short circuits are more frequent, but their occurrence diminishes significantly as their conductivity increases.

4.3. The Influence of Process Parameters on Machining

4.3.1. Characteristics of Processed Surfaces

When discharge occurs, the material is removed to form discharge pits, and the material area larger than the discharge gap undergoes electrochemical dissolution reactions. The experimental parameters are detailed in Table 2. Figure 8 shows the machined surface of UWECDMM, which can be divided into the EDM area, transition area, and ECM area. At the end of processing, the surface near the wire electrode is the EDM area, where discharge mainly occurs, and the surface presents a large number of discharge pits and recast layers. In the transition area, the ECM effect is enhanced, and the discharge pits and recast layer begin to dissolve, making the surface smoother. In the ECM area, electrochemical reactions further dissolve the discharge pits and recast layers, forming the final machined surface. Therefore, it is possible to achieve non-recast-layer processing by using reasonable processing parameters.

4.3.2. The Direction and Speed of the Traveling Wire

The traveling wire direction in UWECDMM includes upward (Figure 9a) and downward (Figure 9c). The experimental parameters are detailed in Table 2. As shown in Figure 9a, the red arrow indicates the direction of workpiece feed. When the wire electrode moves upward, electrolysis-generated bubbles move upward due to buoyancy, while insoluble machining products deposit downward due to gravity. The upward traveling wire aligns with the bubble movement, facilitating bubble discharge but hindering the removal of insoluble products, potentially leading to their accumulation on the workpiece surface. The surface morphology of the workpiece processed when the wire electrode moves upward is shown in Figure 9b. The right side represents the discharge area at the front end of the wire electrode, primarily influenced by the electrical discharge, while the left side shows the surface after electrochemical machining. However, in the ECM area, there are many bulges on the processing surface of the upward wire, which are relatively rough.

Conversely, as shown in Figure 9c, the downward traveling wire hinders bubble discharge but promotes the removal of insoluble products. Under identical experimental conditions, Figure 9d depict the sidewall morphology of the narrow slits processed by the upward and downward traveling wire methods. No significant morphological differences were observed between the upward and downward wire methods in the EDM area. However, in the ECM area, the downward wire showed markedly reduced surface bulges, resulting in higher surface quality. This indicates that the timely discharge of insoluble products significantly impacts the electrochemical effect, making it more challenging to discharge products with the upward traveling wire, which severely affects the leveling effect of the electrochemical process on the EDM surface. Consequently, subsequent experiments will utilize the downward traveling wire method.

Figure 10 illustrates the morphology of the slit sidewall processed at different wire speeds. At a wire speed of 20 mm/s, the processed surface exhibited numerous bulges and pits (Figure 10a), indicating that the molten pits generated by discharge were not leveled by electrolytic action. When the wire speed was increased to 50 mm/s, the protrusions and pits on the processed surface decreased, and the surface quality improved (Figure 10b), suggesting that increasing the wire speed is beneficial for enhancing surface quality. This is due to the improved mass transfer efficiency of products processed at higher wire speed, which can eliminate the products in the processing gap in time. This is conducive to making the machined surface smooth and improving the surface quality through electrolysis.

4.3.3. Slit Width

According to the parameters specified in Table 1, a series of 0.5 mm-long slits was fabricated on a 10 mm-thick nickel substrate. Systematic investigations were conducted to evaluate the influence of key processing parameters (voltage, frequency, duty cycle, and feed rate) on the slit width characteristics. As illustrated in Figure 11, the feed rate exhibited predominant control over the slit dimensions. When increasing the feed rate from 0.6 to 1.8 μm/s, the slit width S demonstrated a linear reduction from 57.4 μm to 44.1 μm. However, accelerated feed rates compromised the processing integrity due to the insufficient electrolytic removal of the recast layer under high-speed conditions, resulting in serrated morphologies along the slit edges. The experimental data revealed positive correlations between slit width and both voltage and duty cycle parameters, while power frequency showed negligible effects on dimensional characteristics.

5. Discussion

5.1. Removal of Recast Layer and Evolution of Surface Morphology

The formation of the recast layers during EDM critically compromises material performance, necessitating effective removal strategies. In the UWECDMM process, the electrolytic proportion, predominantly governed by processing parameters, determines the recast layer dissolution efficiency. This investigation systematically elucidates the conductivity-dependent recast layer evolution through machining experiments in varied dielectric media, supported by surface morphology characterization and elemental analysis. As demonstrated in Figure 12a, machining in pure glycol (conductivity: 0.2 μS/cm) solely involves discharge effects. When discharge occurs, a molten pool is formed on the surface of the workpiece, and a recast layer is formed after solidification. The thickness of the recast layer is about 14.25 μm. EDS reveals elevated oxygen (9.64%) and tungsten (41.87%) concentrations (Figure 13a), indicating intensive tungsten wire sputtering during discharge. At a 100 μS/cm conductivity (Figure 12b), incipient electrolysis partially dissolves the recast layer, reducing oxygen and tungsten contents to 2.06% and 6.67%, respectively (Figure 13b), though incomplete removal persists due to insufficient electrochemical action. Remarkably, enhanced electrolysis at 200 μS/cm (Figure 12c) achieves complete recast layer elimination, yielding smooth sidewalls with residual oxygen (0.14%) and undetectable tungsten element (Figure 13c).

5.2. Analysis of Wear of Wire Electrode

This study evaluated the sustainable processing potential of tungsten electrodes through cyclic utilization experiments. Using virgin and recycled tungsten wires (one, two, and eight use cycles) under identical parameters (Table 2), systematic investigations were conducted on electrode surface evolution and elemental migration. As shown in Figure 14, the surface of the original tungsten wire was smooth (Figure 14a). After the first use, sparse discharge pits were generated on the surface of the tungsten wire due to discharge (Figure 14b), with a tungsten element content of 96.32% (Figure 15a). At the same time, ethylene glycol decomposed at high temperatures during discharge, and carbon and oxygen adhered to the surface of the tungsten wire. Secondary use increases the pit density and size (Figure 14c), reducing the tungsten content to 84.87% with carbon accumulation (13.56%, Figure 15b). After nine usage cycles, the number of discharge pits on the surface of the tungsten wire further increased and merged into a sheet-like distribution (Figure 14d), with the W element content decreasing to 65.30% and the O element increasing to 17.69% (Figure 15c). Therefore, as the number of times the tungsten wire is used increases, the number of discharge pits on the surface also increases.

In order to quantitatively analyze the wear of wire electrodes, the diameter relative wear ratio was used to evaluate the wear of wire as follows:

$${\theta }_d={\displaystyle \frac{{D_1-D}_0}{D_0}}·100\mathrm{\%}$$

(10)

where ${\theta }_d$ represents the diameter relative wear ratio (%), $D_1$ represents the diameter of the wire electrode (μm), and $D_0$ represents the initial diameter of the wire electrode (μm).

From Figure 16, it can be seen that after eight uses, the diameter of the wire electrode decreased from the original size of 30.08 μm to 29.73 μm, and the relative wear rate of the diameter was only 1.163%. This indicates that the wear of the wire electrode is very small after multiple uses. As shown in Figure 17, this is because the discharge energy is small, and each discharge forms a small discharge pit on the surface of the wire electrode. The gap between the electrode surface without a discharge pit and the workpiece surface is smaller, making it easier for discharge to occur at this position (Figure 17b). Therefore, the discharge pit gradually increases with the number of uses and forms a sheet shape. Finally, the wire electrode only removes a thin layer of material after multiple uses (Figure 17c).

Figure 18 illustrates the evolution of the slit sidewall morphology in WECDM processes, utilizing virgin and recycled tungsten wires (one, two, and eight use cycles). Microscopic analysis reveals consistent surface integrity with negligible roughness variation. This is because although the number of discharge pits on the surface of the tungsten wire increases with the use of the tungsten wire, the diameter change is very small and has no effect on the machining effect. This indicates that the tungsten wire can still achieve high-quality processing surfaces without recast layers after multiple uses. Therefore, in the processing method proposed in this paper, the tungsten wire can be reused multiple times to reduce experimental costs, which is in line with the concept of sustainable development.

5.3. Typical Microstructure Processing

To validate the process stability and material versatility of UWECDMM, optimized parameters (Table 2) were applied to fabricate typical microstructures. Figure 19a shows the profile of a “ratchet”, with an outer circle diameter of less than 500 μm on nickel, and Figure 19b shows a T-shaped microgroove machined on nickel-based high-temperature alloy GH 4202. At a feed rate of 1 μm/s, the processed surface is smooth and free of recast layers, with a surface roughness Ra of approximately 0.06 μm. The 10 mm-thick workpieces exhibited slit widths of 50 μm, yielding depth diameter ratios exceeding 200. AFM characterization (Orange dashed box image) confirmed nanoscale surface topography. These results substantiate the efficacy of the glycol-based UWECDMM in achieving recast-free, high-aspect-ratio features, establishing a viable methodology for the precision micromachining of conductive substrates.

6. Conclusions

In this study, an electrolyte based on glycol was used to study WECDMM, and the unidirectional wire walking method was used to realize the WECDMM of conductive metal materials with high efficiency and high precision. On this basis, the discharge phenomenon was observed, and the effects of this method on the surface quality, recast layer, and wire electrode wear were discussed. The typical microstructure was processed.

• The discharge phenomenon of UWECDMM was directly observed through a high-speed camera. The results show that the discharge is mainly breakdown electrolyte discharge at low conductivity. Waveform analysis shows that ECM and EDM alternate, and as the electrolyte conductivity increases, the discharge proportion decreases from 48.08% to 14.86%. The discharge current is less than 1 A (0.88 A), and the discharge energy is small, which is conducive to high-precision machining of microstructures.
• There is no significant difference in the morphology between the upward and downward traveling wires in the EDM area. However, in the ECM area, the surface of the upward traveling wire exhibits numerous bulges, while the bulges on the surface of the downward traveling wire are significantly reduced, resulting in higher surface quality. Additionally, the surface quality of the high-speed traveling wire is superior to that of the low-speed traveling wire.
• After discharge, a recast layer was formed on the machined surface. When the conductivity of the electrolyte is 200 μS/cm, the recast layer on the side wall of the slit is completely dissolved, achieving processing without a recast layer, and the surface roughness is only Ra 0.06 μm.
• After multiple uses, the surface discharge pits of the tungsten wire increased, but the diameter did not significantly decrease. After eight uses, the relative loss rate of the diameter was only 1.163%, and it was still able to obtain high-quality non-recast-layer machined surfaces. Reducing experimental costs is in line with the concept of sustainable development.
• By using optimized process parameters to process typical microstructures, the surface roughness was only Ra 0.06 μm, and the depth diameter ratio could reach 200. By processing typical microstructures, the feasibility of ethylene glycol-based UWECDMM was verified, providing a method for efficient and high-precision microfabrication of conductive materials.

UWECDMM has shown promising applications and will play an important role in the field of microfabrication. In the future, this method can be used for processing microstructures of metal materials. This method significantly improves the machining efficiency of WECMM and can obtain high-precision machined surfaces. In addition, the wire electrode can be reused, saving production costs and achieving sustainable green processing. Therefore, it has enormous potential for practical value.