A Hybrid Process of Electrochemical Discharge Machining (ECDM) and High-Speed Milling (HSM) on Quartz Glass

doi:10.21203/rs.3.rs-5304835/v1

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A Hybrid Process of Electrochemical Discharge Machining (ECDM) and High-Speed Milling (HSM) on Quartz Glass

https://doi.org/10.21203/rs.3.rs-5304835/v1

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In traditional machining processes, surface microcracks are prone to generate due to the hardness and brittleness of quartz glass. A hybrid process of electrochemical discharge machining (ECDM) and high-speed milling (HSM) is proposed for efficiently machining quartz glass without surface microcracks. During ECDM-HSM process, a "mushroom cloud" gas film is found by a high-speed camera, which facilitates to constraint discharges to a tool-electrode end. Machining experiments of ECDM-HSM obtains the processing effect of key factors of tool rotation speed, applied voltage, and feed rate. The higher rotation speed requires higher critical voltage to start discharges. The high feed rate can improve machining efficiency and quality significantly owing to the combined action of plastic cutting, discharges, and chemical reaction. The homogenization mechanism of discharging energy and the transition mechanism of plastic cutting are proposed for illuminating the processing effect. The matching mapping of key parameters is established for optimizing the processing parameters. As an example, a regular groove without surface microcracks is machined with the processing efficiency improved up to 20 times as compared with that of HSM.

electrochemical discharge machining

high speed milling

quartz glass

narrow groove

machining efficiency

high speed camera

Quartz glass has broad application prospects in the fields of semiconductors, optical instruments, sensors, etc. due to the excellent properties of non-magnetic, transparent, high hardness, good biocompatibility, and corrosion resistance. Quartz glass brings the growing tendency to apply in small/micro systems such as oxide fuel cells, fluid pumps, fluidic chips, quartz crystal resonators, and so on. These small/micro structures require an advanced machining process to achieve high precision and no-damage surfaces. Taking the microfluidic chip as an example, the micro channels required the processing accuracy of micron level [1].

In recent years, the hybrid processes of ECDM and mechanical machining have been explored on quartz glass. In 2002, Jain et al. [8] applied grinding tools as tool electrodes in drilling deeper holes. In 2017, Elhami and Razfar [9] improved the drilling efficiency by 22–67% by the hybrid process of ECDM and mechanical drilling with a tool rotation speed of about 1,500 r/min. In 2019, Liu et al. [10] adopted a Φ105 µm spiral tool with a rotation speed of 3,000 r/min in ECDM milling experiments, and they machined microchannels at a low feed rate < 2 µm/s. These experiments show that the hybrid processes are beneficial to improve the machining accuracy and efficiency. However, the hybrid process research is still lacking, especially for that under the condition of tool-rotation speed up to > 10,000 rpm.

In this research, the hybrid process of ECDM and high-speed milling (HSM) using a pneumatic spindle with an end-milling tool is proposed, aiming at high-efficiency machining grooves without surface microcracks on quartz glass. The influence of ultra-high rotation speed of end-milling tool on the formation and movement of bubbles/gas films is observed firstly. The effect of key parameters of tool-rotation speed, applied voltage, and electrolyte concentration on spark discharges are comprehensively understood and analyzed. Furthermore, machining experiments are carried out on quartz glass to obtain the processing law. The matching mappings of key parameters are established for understanding and optimizing the hybrid process, as a result that a typical groove is successfully machined.

Figure 1 shows the basic composition and principle of the ECDM-HSM hybrid process on quartz glass. Applying a voltage between the end-milling tool linked to the cathode and the auxiliary electrode linked to the anode, the electrochemical reaction as Eq. (1) generates a large number of bubbles around the tool surface immersed in the electrolyte, and then a gas film can form with the increase and emergence of the bubbles. When the local electric field strength is more than a certain threshold called as a critical voltage, spark discharges generate to break down the gas film. The high temperature and pressure from the discharges can melt or even vaporize the workpiece material of quartz glass. The instantaneous high temperature promotes the dissolution of quartz glass through the chemical reaction of Eq. (2). Simultaneously, the cutting edge of milling tool also removes the workpiece materials through the mechanical force. The processed debris is easy to be removed by the electrolyte flowing and renewal through the tool grooves with a rotation speed. The machining mechanism is worth studying, including the key processes of gas film formation, discharging behaviour, and machining characteristics.

$${\text{2}}{{\text{H}}_{\text{2}}}{\text{O+2}}{{\text{e}}^{\text{-}}} \to {\text{2O}}{{\text{H}}^{\text{-}}}{\text{+}}{{\text{H}}_{\text{2}}} \uparrow$$

$${\text{2O}}{{\text{H}}^{\text{-}}}{\text{+Si}}{{\text{O}}_{\text{2}}} \to {\text{Si}}{{\text{O}}_{\text{3}}}^{{{\text{2-}}}}{\text{+}}{{\text{H}}_{\text{2}}}{\text{O}}$$

The mechanism of spark discharges in ECDM itself has not been fully discovered so far. It is known that there is a threshold of critical voltage for breaking down a gas film, that is, spark discharges occur only when an interelectrode voltage exceeds the threshold. The air-film formation and morphology is the key affecting spark discharges. The stability and the thickness of the gas film are mainly affected by multi factors of tool shape, surface morphology, rotation speed, current density, and electrolyte concentration. The centrifugal force coming from high-speed rotation of the spiral end-milling tool is taken into account as an important factor, which also brings the hydrodynamic effect on the thickness of the gas film. The tip effect was found as the positions at the tips and edges of tool electrodes are easier to discharge [11]. The high-speed rotation of the end-milling tool is expected to avoid the discharging concentration at the same point.

The traditional mechanical milling on quartz glass depends on the selected key parameters of tool rotation speed, cutting depth, and feed rate. The published result [2] has shown that the processing accuracy and surface quality can be improved obviously, but it must use a small cutting depth and a low feed rate. The discharges in the proposed hybrid process of ECDM-HSM not only remove a part of the workpiece materials, but also heat the workpiece of quartz glass into local softening. In this way, the favorable effect is expected to be obtained as the transition from brittle cutting into plastic cutting, thereby reducing even to avoiding the overcutting and microcracks. The transition provides the possibility of high feed rate and large cutting depth.

This research aims to improve the processing efficiency, dimensional accuracy, and surface quality simultaneously. Furthermore, machining characteristics and removing mechanism of the proposed process of ECDM-HSM are expected to illuminate, which are affected by multi-parameters of tool rotation speed, applied voltage, electrolyte concentration, and feed rate.

3.1. Experimental system and conditions

As shown in Fig. 2, the experimental system mainly includes a pulsed power supply, an XY-axis motion platform, a high-speed rotation pneumatic spindle fixed on a Z-axis platform, an electrolyte circulation system, a high-speed camera, and a CNC system. The pulsed power supply can output the minimum pulse width of 1 µs. The tool electrode uses an end-milling tool, and the auxiliary electrode is made of graphite. The XY-axis motion platform has the displacement resolution of 1 µm and the positioning accuracy of ± 2 µm. The Z axis has the positioning accuracy of ± 0.2 µm, which is equipped with a high-speed pneumatic spindle with the rotation speed up to 50,000 r/min. The pneumatic spindle has the special advantage as conducting the pulsed electricity by its internal conductive parts instead of a traditional electric brush, avoiding the brush force interference on dynamic balance of high speed rotation. In this way, the high-speed rotation and electricity conduction can be acted on the tool electrode when the rotation is driven by the air pressure instead of a traditional motor. The tool electrode is a micro end-milling tool made of tungsten steel with a diameter of 500 µm. The electrolyte circulation system can circulate electrolyte solution to keep its consistent depth. The high-speed camera can shoot a video with the frame up to 15,000 fps and the maximum resolution of 1920×1080. The CNC system is composed of an industrial computer and a programmable multi-axis controller. During the hybrid process of ECDM-HSM, the voltage and current waveforms are recorded by an oscilloscope with a current sensor.

Table 1 presents the physical properties of the tool electrode. Table 2 presents the workpiece material properties of quartz glass. During the experimental process, the profile and accuracy of the machined grooves is observed and measured by an optical microscope, a white light interference profiler (WLIP), and a scanning electron microscope (SEM). The experimental parameters are presented in Table 3.

Table 1

Physical properties of tool electrode.
Parameter	Description
Melting point (℃)	2850
Rockwell hardness (HRA)	90
Resistivity (Ω∙m)	2×10^− 7
Diameter (µm)	500
Thermal conductivity (Wm^− 1K^− 1)	50.2

Table 2

Workpiece material properties of quartz glass.
Parameter	Description
Elastic modulus (GPa)	72
Shear modulus (GPa)	31
Density (g/cm³)	2.2
Tensile strength (MPa)	48.5
Compressive strength (MPa)	1060
Poisson ratio	0.17

Basic experiments are carried out for studying the key processes of gas film formation, discharging behavior, and machining characteristics. Firstly, a digital camera (Fig. 3) and a high-speed camera are used to observe the formation and morphology of bubbles/gas films under high-speed rotation of tool electrode. Secondly, a CCD (charge coupled device) camera is adopted to observe the discharging phenomenon on the tool surface. The influence of applied voltage, tool rotation speed, and electrolyte concentration on the discharging behavior is analyzed for selecting the optimal parameters. At last, the machining experiments of grooves on quartz glass are carried out to compare ECDM-HSM with HSM with a relative large cutting depth of 200 µm. By obtaining the effect of the tool feed rate, tool rotation, and applied voltage on the size, shape, overcut rate, and surface roughness, the processing mechanism of ECDM-HSM is analyzed. A typical groove as an example is machined to demonstrate the optimal effect.

Table 3

Experimental parameters
Parameter	Hybrid process of ECDM and HSM	HSM
Material of end-milling tool	Tungsten steel
Rotation speed	5,000 r/min, 8,000 r/min, 10,000 r/min, 20,000 r/min, and 30,000 r/min
Feed method	Constant feed rate
Electrolyte	NaNO3 (1wt%, 5wt%, 10wt%, and 20wt%)
Interelectrode voltage	100–200 V	-
Pulse width/pulse interval	16 µs/8 µs	-
Tool diameter	0.5 mm
Immersion depth	2 mm

3.2. Effect of key parameters on bubbles and discharging behavior

Figure 4 shows the morphology of bubbles and gas films at different tool rotation speed. When the rotation speed v was 0 rpm, most of larger bubbles gathered on the tool surface to form a thicker gas film, and a few smaller bubbles merged into the large bubbles away from the tool (Fig. 4a). When v = 3,000 r/min, the bubbles separated from the tool surface and spread out quickly (Fig. 4b). When v = 20,000 r/min, the bubbles on the tool electrode surface were thrown out at high speed along the tangential direction under the centrifugal force of the high-speed rotation (Fig. 4c). With the increase of rotation speed, the bubbles were difficult to concentrate near the tool surface. Figure 5 shows the microscopic and instantaneous process of bubbles under the liquid surface, observing by the high-speed camera. When v = 0 rpm, the bubbles gradually grew up with electrochemical reaction to form a thin gas film. Continuing the electrochemical reaction, a number of smaller bubbles grew and merged into the larger bubbles in the liquid. These larger bubbles caused the uneven thickness of the gas film, which could affect the discharging stability of ECDM. When v = 20,000 r/min, the bubbles gradually formed a "mushroom cloud" shaped air film under the combined effect of the centrifugal force and bubble buoyancy force come from the spiral groove sidewall of the end-milling tool. Although the "mushroom cloud" had a larger range, the bubbles that make up the gas film were smaller and denser compared with that of v = 0 rpm. Moreover, a favorable phenomenon was found that the thickness of the gas film at the end of the tool was thinner and denser, which would facilitate the discharging breakdown there during ECDM process. That is, the advantageous effect of concentrating the discharges to the tool end can be expected.

Figure 6 shows the discharging effect under various conditions of tool rotation speed (0–20,000 rpm), applied voltage (110–160 V), and electrolyte concentration (1wt%, 5wt% and 20wt%). Comparing the horizontal rows in Fig. 6, the discharging phenomenon occurred with the voltage increase until exceeding a critical voltage. Due to the sharp-edge discharging characteristic [11], the discharges firstly occurred at the tip and cutting edge of the tool. As the voltage gradually increased, the discharging area gradually spread upward and covered the lateral cutting edge almost. Comparing the longitudinal columns in Fig. 6, in the case of high-speed rotation, a higher voltage was required to achieve the discharges. We believe that the result is related to the “mushroom cloud” shaped gas film formed by the tool high-speed rotation (Fig. 5). The large centrifugal force caused the bubbles to leave the tool surface quickly, thereby accelerating the H₂-gas dissipation so as to be difficult to form a stable insulating film [12]. The higher voltage can speed up the formation rate of electrolytic bubbles for compensating the gas dissipation. When the bubbles formed fast enough to form a gas film, the discharges occurred. Comparing Fig. 6(a) with Fig. 6(b), when the concentration increased from 1wt% to 5wt%, there was no change of the critical voltage. The discharges at the 5wt% concentration looks brighter than that at the concentration of 1wt%, indicating that the larger concentration can bring the denser discharges. Comparing Fig. 6(b) and Fig. 6(c), when the electrolyte concentration was further increased to 20wt%, there was a discharging suppression phenomenon (more difficult to generate discharges), especially for the rotating speed of 20,000 r/min. When the speed was 10,000 r/min and the voltage was 150 V, the discharges only occurred at the electrode tip (as shown in Fig. 6(c)b-5). The experiments found there was not obvious discharges on the tool surface under the condition of the rotation speed ≥ 20,000 r/min (for example Fig. 6(c)a-6).

Figure 7 shows the voltage-current curves and discharging waveforms. The peak voltage and the peak current increase with the increase of applied voltage, and the critical voltage (point I, II, III and IV) increased with the increase of the tool rotation speed. The critical voltage was 120 V at the speed of 0 rpm, while the critical voltage increased up to 135 V, 140 V, and 145 V at the speed of 3,000 r/min, 10,000 r/min, and 20,000 r/min respectively. If the rotation speed was higher, the applied voltage required to be higher in order to achieve the same peak voltage and peak current during ECDM. For example, aiming at the peak current of 1.4 A, the applied voltage needed to reach 145 V at the rotation speed of 20,000 r/min, compared with 120 V at the rotation speed of 0 r/min.

The critical voltages at different electrolyte concentration and different rotation speed are summarized in Fig. 8. The voltages increase with the increase of rotation speed regardless of the electrolyte concentration. In other words, the discharges began more difficult if the rotation speed was higher. However, the excessive peek voltage could make the peak current increase sharply, resulting in excessive overcutting and surface damage caused by the instantaneous huge energy. Therefore, the preferred applied voltage should be slightly higher than the critical voltages to avoid the excessive peak current. According to Fig. 8, the 5wt% concentration of NaNO₃ electrolyte is selected for the following machining experiments because its critical voltage is relatively lower at the condition of high rotation speed.

3.3. Machining experiments for comparing HSM with ECDM-HSM

Figure 9 shows the machined results measured by WLIP with the lower feed rate of 0.01 mm/s and 0.05 mm/s at the cutting depth of 200 µm. As known from the HSM results (Fig. 8a1-5 and b1-5), with the increase of rotation speed, the machined surface quality was improved by observing the milling mark and surface roughness R_a. For example, the best result of surface roughness R_a <1 µm (Fig. 9a5) without obvious microcracks was obtained at the speed of 30,000 r/min and the feed rate of 0.01 mm/s. However, the feed rate of 0.05 mm/s resulted in the rougher surfaces and the larger overcutting edges. As for the ECDM-HSM results (Fig. 9A1-5 and B1-5), the width of grooves machined by ECDM-HSM under the combined energy fields was larger than that machined by HSM. The lateral overcutting of 5,000 r/min & 120V and 8,000 r/min & 130 V was 14 µm and 24 µm, respectively. When the rotation speed was higher than 10,000 r/min, the overcutting was sharply increased, and there was a serious surface damage. We believe two possible reasons causing this result. First, the discharging critical voltage was higher corresponding to the higher rotating speed, therefore a larger discharging energy caused the excessive removal of workpiece material (e.g. Figure 9A5). Second, the higher rotating speed led to the unstable gas film, resulting in the unstable energy distribution of the discharges. This is easy to cause the centralized discharges in some positions, which produced the large pits and the unregulated shape of the grooves. In the case of same rotation speed, the lateral overcutting and the machined depth (Fig. 9B1-5) at the feed rate of 0.05 mm/s were smaller than that (Fig. 9A1-5) of 0.01 mm/s, which can be interpreted as the higher feed rate can disperse the discharge energy over a larger area in unit time.

Furthermore, the comparison experiments were carried out by increasing the feed rate. Figure 10 shows the machined results measured by WLIP with the higher feed rate of 0.1 mm/s and 0.5 mm/s at the cutting depth of 200 µm. Compared with the results at lower feed rates of 0.01 mm/s and 0.05 mm/s (Fig. 9), the machined accuracies of 0.1 mm/s and 0.5 mm/s were significantly improved by the hybrid process of ECDM-HSM. Even with the larger discharging energy (160 V and 180 V), the heat damage areas on groove edges were reduced obviously. When the feed rate was 0.1 mm/s, the surface roughness at different rotation speed was basically same as R_a 2.1 µm. When the feed rate was 0.5 mm/s, the surface roughness (Fig. 10B1-5) was almost better than that (Fig. 10A1-5) of 0.1 mm/s. In contrast, the edge break and the tool milling marks in HSM become more serious than that in ECDM-HSM. The HSM process was probably in brittle cutting stage of quartz glass, resulting in the deteriorative surface roughness (for example, R_a 2.6 µm in Fig. 10b-1). According to the comparison, ECDM-HSM can change the brittle cutting into the plastic cutting under the combined action of discharging temperature and chemical reaction, so as to improve the processing effect of the machined accuracies.

The width and the depth of the machined grooves by ECDM-HSM are summarized in Fig. 11. The overcutting under a low feed rate has unfavorable effect on the groove width and depth. With the increase of feed rate, the machined accuracy was significantly improved. As for the cases of rotation speed 10,000–20,000 rpm under feed rate 0.5 mm/s, the errors of groove width and groove depth can be controlled < 10 µm and < 5 µm respectively. Therefore, the optimized strategy of ECDM-HSM was proposed as the idea of a high feed rate with a favorable high-speed tool rotation for equilibrating the combined energy of electrical discharge erosion, electrochemical dissolution, and mechanical cutting. This goal is to avoid the excessive concentration of electrical discharge energy and achieve the state of plastic cutting.

The machining experiments were further carried out by increasing the feed rate. Figure 12 summaries the relationship between feed rate and surface roughness. As for the lower feed rate (≤ 0.05 mm/s), the surface quality of ECDM-HSM was obviously worse than that of HSM due to the high temperature damage from the discharging overconcentration. Under the condition of higher feed rate (≥ 0.1 mm/s), the surface quality of ECDM-HSM was improved significantly and better than that of HSM owing to the effect of the combined energy fields. Using the process of ECDM-HSM at the feed rate of 0.5 mm/s, the value of surface roughness R_a can reach the best one as 1.7 µm (tool rotation 10,000 r/min). Moreover, the improved effect of ECDM-HSM was more obvious in the case of the higher feed rate. Taking the feed rate of 1.0 mm/s (tool rotation 10,000 r/min) as an example, the surface roughness R_a can be improved from 6.3 µm of HSM to 3.0 µm of ECDM-HSM, although the surfaces become worse in both processes of ECDM-HSM and HSM compared with that of 0.5 mm/s. In other words, given the similar surface roughness and accuracy, the hybrid process of ECDM-HSM can apply higher feed rate for improving machining efficiency. Considering the similar surface quality by comparing the case of 0.01 mm/s and 10,000 r/min (Fig. 9a-3) with the case of 0.5 mm/s and 10,000 r/min (Fig. 10B-3), ECDM-HSM can improve the processing efficiency by 50 times more than that of HSM. In contrast to the ECDM machining efficiency of existing experimental result as R_a ~2µm [8], that of ECDM-HSM can improve by 10 times. The hybrid process of ECDM-HSM can better balance the processing accuracy and efficiency.

4.1. Mechanism of ECDM-HSM on quartz glass

Figure 13 compares the machined grooves between HSM and ECDM-HSM. The mechanical force of HSM broke the surface into the long pits (Fig. 13a). In addition, the edge collapse reached the sidewall range of about 50 µm. As for ECDM-HSM, the machined groove was relatively smooth as its edges without the chipped morphology (Fig. 13b). According to the experimental results of different feed rates in the hybrid process of ECDM-HSM, the large pits and the excessive overcutting at the low feed rate (≤ 0.05 mm/s) were caused by the discharging effect of high temperature and high pressure. Figure 14 shows the machined grooves of the lower feed rate (0.01 mm/s) and the higher feed rate (0.1 mm/s). As for the case of lower feed rate, the high temperature and high pressure of continuous discharges caused the more serious overcutting edges, irregular wave patterns, and deep cracks (Fig. 14a). As for the case of higher feed rate, the regular groove was machined into the surface without microcracks (Fig. 14b). The higher feed rate can avoid the concentrated discharges at a certain point.

Considering material properties of quartz glass as a kind of brittle material, the accuracy and surface improvement indicates the transition mechanism from the brittle cutting of HSM into the plastic cutting of ECDM-HSM. The temperature was the main contribution except for the chemical reaction of ECDM. During the ECDM-HSM process, the temperature around tool tip can rise sharply under the action of discharges. This temperature can soften the quartz glass to decrease the hardness and stiffness even into melt state at some local positions. The HSM tool removed the materials by brittle cutting, while the ECDM-HSM tool removed the materials under the combined action of plastic cutting, discharges, and chemical reaction.

4.2. Matching parameters and application of ECDM-HSM on quartz glass

The hybrid process of ECDM-HSM is influenced by the main factors of feed rate, tool rotation speed, cutting depth, applied voltage, and electrolyte concentration. These factors are coupling effect rather than simple superposition relationship. The matching and selection of these parameters is important for achieving the optimized processing effect. There is a reasonable matching relationship between the tool rotation speed and the applied voltage. The applied voltage lower than the critical voltage cannot trigger discharges, while the too high voltage is easy to damage the machined surfaces. According to the homogenization mechanism of discharging energy and the transition mechanism of plastic cutting, the matching effect between tool rotation speed (TRS), applied voltage (AV), and tool feed rate (TFR) is key for improving the process. The excessive combination of high TRS, high AV, and low TFR trends to surface heat damage, while the excessive combination of low TRS, low AV, and high TFR trends to mechanical damage of brittle cutting.

Figure 15 (a) summarizes the matching mapping of three key parameters for ECDM-HSM on quartz glass according to the experimental results. Wherein the boundaries are defined as follows: surface roughness R_a≥6 µm and machined depth MD ≥ 360 µm as High Temperature Burn; 6 > R_a≥3 µm and 360 > MD ≥ 230 µm as Slight Burn; 3 > R_a>2.4 µm as Slight Brittle Cutting with light broken edges; obvious broken edges as Brittle Cutting; R_a≤2.4 µm and no-damage surface as Plastic Cutting with regular edges. Compared with the matching mapping of HSM (Fig. 15b), Fig. 15 (a) indicates that ECDM-HSM can achieve the conditions of plastic cutting in a large range for machining quartz glass. ECDM-HSM is much easier than HSM to obtain the transition conditions of plastic cutting for improving the processing accuracy and efficiency.

According to the Fig. 15 (a), selecting the processing parameters that locate in Plastic Cutting section, a typical groove was machined on quartz glass by ECDM-HSM as shown in Fig. 16. The material removal rate reaches 2.036×10⁷ µm³/s (total machining time of 140 s). As for this example with the feed rate of 0.2 mm/s, the machining efficiency of ECDM-HSM can improve to 20 times that of HSM with the feed rate of 0.01 mm/s, considering the similar machining accuracy.

The hybrid process of electrochemical discharge machining (ECDM) and mechanical high speed milling (HSM) using a spiral end-milling tool was proposed for improving the processing effect on quartz glass. The high-speed rotation up to 30,000 rpm was realized by using a pneumatic spindle instead of a traditional motor. This way has the special advantage as conducting the pulsed electricity on the tool by the internal conductive parts instead of a traditional electric brush, avoiding the brush force interference on dynamic balance of high speed rotation.

The unique morphology of bubbles and gas films under high speed rotation was observed by a high-speed camera with the frame up to 15,000 fps. The bubbles can spread outward into the "mushroom cloud" shaped gas film with the thinner thickness at the tip of too electrode, under the action of the centrifugal force of high-speed tool rotation. The sharp points and edges of the tool were easier to discharge than other positions due to the sharp-edge discharging characteristic. The tool rotation makes the discharging breakdown more difficult, especially for the higher rotation speed that requires the higher critical voltage for starting to discharge. The preferred applied voltage should be slightly higher than the critical voltages to avoid the excessive peak current.

As for HSM, the extremely low feed rate (0.01 mm/s) is a precondition for achieve high machining accuracy and good surface quality, and the high speed of tool rotation can improve significantly the HSM effect. The machining experiment (0.01 mm/s) with the rotation speed of 30,000 rpm achieved the best results as the groove width error of 2 µm, depth error of 6 µm, and surface roughness of Ra 0.9 µm. However, with the increase of feed rate, the processing effect of HSM became worse into the typical brittle cutting as the damage features of overcutting, long pits, and deep cracks.

As for ECDM-HSM, the low feed rate (≤ 0.05 mm/s) resulted in the serous overcutting and surface damages because of the discharging high temperature and pressure concentrated on some local positions, especially for the cases of higher tool-rotation speed needing the higher critical voltage. With the increase of feed rate, the processing effect can be improved significantly by homogenizing the discharging energy. Under the condition of higher feed rate (≥ 0.1 mm/s), the machined quality of ECDM-HSM was better than that of HSM owing to the combined action of plastic cutting, discharges, and chemical reaction. Under 0.1–0.5 mm/s and 8,000–20,000 rpm, the smooth surfaces without microcracks can be obtained by ECDM-HSM. When the feed rate was further increased to 1 mm/s, although the trend of brittle cutting appeared gradually to deteriorate the process, the processing results in ECDM-HSM were obviously better than that of HSM.

The homogenization mechanism of discharging energy and the transition mechanism of plastic cutting are proposed for illuminating the processing effect of ECDM-HSM. The matching mapping of three key parameters were summarized for ECDM-HSM and HSM on quartz glass. ECDM-HSM can achieve the conditions of plastic cutting in a large range. The transition from brittle cutting into plastic cutting provides the possibility of high feed rate and large cutting depth for improving the processing efficiency significantly. A typical groove was machined by ECDM-HSM with the processing efficiency up to 2.036×107 µm3/s, which improved up to 20 times as compared with that of HSM.

Competing interests:

The authors declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. This manuscript has not been published or presented elsewhere in part or in entirety and is not under consideration by another journal.

Ethics approval:

All the authors declare that no animals or human participants are involved in this research. There are no ethical issues to declare.

Consent to participate:

Only authors participate in the research work of this paper.

Consent to publish:

All authors agree to publish the research results in this paper.

Funding:

This work was supported by the Science Center for Gas Turbine Project [grant No. P2022-A-IV-002-003], the Freedom Exploration Fund Project of Department of Mechanical Engineering at Tsinghua University [grant No.2024JXA01], and the Independent Research Project of the State Key Laboratory of Tribology of China [grant No. SKLT2022B08].

Acknowledgement

Availability of data and material:

The datasets used or analyzed during the current study are available from the corresponding author or the first author on reasonable request.

Code availability:

The code used during the current study are available from the corresponding author or the first author on reasonable request.

Authors’ contributions: Hao Tong: Conceptualization, Methodology, Project administration, Funding acquisition, Resources, Software, Supervision, Writing - reviewing & editing. Yuge Luo: Investigation, Methodology, Data collection and analysis, Validation, Writing - original draft. Guodong Liu: Investigation, Visualization, Writing - editing. Yong Li: Supervision, Resources. Shan Ali Nawaz: Investigation, Validation.

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A Hybrid Process of Electrochemical Discharge Machining (ECDM) and High-Speed Milling (HSM) on Quartz Glass

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Figures

1. Introduction

2. Principle analysis of ECDM-HSM

3. Basic experiments and discussion of key influencing factors

3.1. Experimental system and conditions

3.2. Effect of key parameters on bubbles and discharging behavior

3.3. Machining experiments for comparing HSM with ECDM-HSM

4. Mechanism and application of ECDM-HSM on quartz glass

4.1. Mechanism of ECDM-HSM on quartz glass

4.2. Matching parameters and application of ECDM-HSM on quartz glass

5. Conclusions

Declarations

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Acknowledgement

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References

Status:

Version 1