Abstract
In this study, a new electrostatic field–induced electrolyte jet electrical discharge machining method has been proposed, which can automatically generate the tool electrode. Then, a series of experiments have been carried out to reveal the machining mechanism and test the machining ability of this method. The continuous observation experiments and the online current detection experiments have demonstrated that the electrolyte jet discharge machining is a pulsing, dynamic and cyclic process. Moreover, the 20-min time long reverse polarity experiments on the silicon surface have revealed that the machining is an electrical discharge machining process during the negative polarity machining; however, in the positive polarity machining, it is a hybrid electrical discharge machining and electrochemical machining process. Furthermore, the craters as small as 2 µm in diameter on stainless steel and silicon are produced by this electrolyte jet electrical discharge machining, which has proved the micro-machining ability of this method.
Keywords
Introduction
Drilling tiny holes and machining shaped slots in some materials, especially on rigid and brittle material, are difficult problems which are nonetheless importantly and frequently encountered in the aerospace and electronics industries, such as printed circuit boards, fuel injection nozzles, optical apertures, micro-pipettes, micro-mold and cavities and micro-air turbines. 1 Since the conventional machining method is based on a hard tool drilling a soft workpiece, tool rigidity limits drilling small and deep holes. Therefore, various nontraditional processes have been attempted to overcome these limitations.2,3
Electrical discharge machining (EDM) is a promising candidate among nonconventional process methods to replace the conventional machining techniques at micro-scales. The machining mechanism of EDM is based on thermal energy produced by the discharge between two electrically conductive parts, so it can machine hard material. Moreover, there is always a gap between the tool electrode and the workpiece, which can eliminate mechanical stresses during machining. 4 Thus, EDM is suited to machining micro-hole, micro-shaft and complex micro-cavities especially on hard-to-machine material. 5 Masuzawa 6 has emphasized applying the EDM method to process micro-parts such as micro-pins, holes, slots and cavities with three dimensions (3D). In addition, EDM can also deal with photo-masks in an integrated circuit. 7
As originally conceived, the tool electrode process is inevitable during conventional EDM because it is based on an electro-erosion phenomenon in which both tool electrode material and the workpiece material are simultaneously ablated. With the appearance of the wire electro-discharge grinding (WEDG) method, 8 the puzzle of the online tool electrode machining of micro-EDM has been settled and the micro-EDM comes into practice. However, due to the uneven diameter of the wire, the unsmooth surface of the wire and the instability of the wire running, micro-EDM is significantly influenced by this tool electrode. From then on, many new tool electrode process methods have been investigated, such as the block electrode discharge grinding, the edge electrode discharge grinding, 9 the micro-electrochemical machining(u-ECM) method, 10 the hybrid WEDG and ECM machining method, 11 and the carbon fiber tool electrode process method.12,13 Although much progress has been made in tool electrode, the problem of high product cost and complicated process technology still limit the wide utility of EDM, especially micro- or nano-EDM.
Considering this problem, this article breaks away from convention that the tool electrode must be produced well before EDM process and proposes a new electrostatic field–induced electrolyte jet (E-jet) EDM method in which the electrostatic field–induced E-jet is used as the tool electrode. In this method, the pulsing and cyclic discharge between the jet and the workpiece has been utilized to remove the debris from the workpiece. In section “The process of the E-jet EDM,” the machining process of this method has been described; in section “Experiment facilities,” the observation experiment and the current flow detection experiment have been carried out to analyze the feature of the E-jet discharge process; the micro-holes machined on the surface of a silicon workpiece by this method under positive and negative polarities have been compared and the machining process mechanism has been revealed in section “The E-jet electrical discharge process observation experiment,” and moreover, the craters produced by a single-discharge crater machining experiment (using stainless steel) and a blind-hole machining experiment (using a silicon wafer) have been conducted to further testify the micro-machining ability of this method. Finally, the conclusions have been drawn.
The process of the E-jet EDM
Figure 1 shows the process of this new electrostatic field–induced E-jet EDM. In Figure 1, (i) an intense electric field is applied between a nozzle and a workpiece. Electrolytes can be drawn out from the nozzle and form a liquid Taylor Cone at the nozzle outlet. (ii) When more and more ions or induced charge are accumulating on the surface of the Taylor Cone, which makes the electric force overwhelm the surface tension, and the tip of the cone ejects a narrow jet. 14 When the jet is about to touch the surface of the workpiece, a dielectric breakdown occurs between the tip of the fine jet and the workpiece because of the intense electric field. This creates a short-lived plasma channel followed by an electric discharge that rapidly heats the discharge point on the workpiece, leading to the evaporation or scattering of some of the material. (iii) After each discharge, an extremely tiny crater appeared on the workpiece. The cone withdraws from the workpiece to some distance larger than the minimum discharge distance because of the loss of the induced charge on the jet surface that made the electrical force of the jet smaller than the surface tension. Then, under the intense electric field, the ions or induced charge will gather again to form the Taylor Cone (i). As the result, the automatic, cyclic and pulsing discharge process has been formed. During each discharge, the fine jet is used as the tool electrode. Compared to the conventional tool electrode machining, this is a new kind of tool electrode process method, which has lower product cost and is easy to generate.
The process of the electrostatic field–induced electrolyte jet micro-electrical machining method.
Experiment facilities
Experimental equipment
The schematic and the basic components of our experimental setup are shown in Figure 2. The electrostatic field–induced E-jet EDM system is composed of a syringe with needle nozzle, a workpiece, a high-voltage direct current (HVDC) power supply, a three-axis motion platform and auxiliary insulating supporting table. The syringe is used to store the electrolyte. The HVDC power supply is adopted to generate an intense electric field between the workpiece and the nozzle. The syringe needle is mounted on a three-axis motion platform, by which the relative position between the nozzle and workpiece can be controlled either manually or by a computer.
Schematic diagram of electrostatic field–induced electrolyte jet (E-jet) electrical discharge machining setup.
The electric field intensity can be controlled by regulating the output of the HVDC power supply or changing the gap width between the needle nozzle and the workpiece. Generally, gap control is preferred to voltage control because conventional EDM controllers work with gap control.
Measurement equipment and method
A high-speed digital video camera is installed on the platform to observe the Taylor Cone formation and the field-induced E-jet micro-electrical discharge process. The maximum speed of this camera can be set at 60,000 frames per second (fps) and the pixel array is 1280 × 1024. The high-speed digital picoammeter is connected in series to record the transient current change in the electrical discharge. With these equipment, the features of the dynamic electrical discharge process can be analyzed.
The morphology and composition of the craters produced on the workpiece can be characterized with a scanning electron microscope (SEM) integrated with the energy-dispersive X-ray spectroscopy (EDS). 15 With the help of these equipment, the machining mechanism can be revealed.
The E-jet electrical discharge process observation experiment
E-jet micro-EDM process verification experiment
In this experiment, the electrolyte is a sodium nitrate aqueous solution of 20 wt%, while stainless steel is selected as the workpiece. The outer and inner diameters of the nozzle are 0.55 and 0.30 mm, respectively. The voltage applied between the nozzle and stainless steel is set at 3.5 kV all through the machining process. Figures 3 and 4 show the sequences of high-speed photographic images of the field-induced E-jet from the nozzle interface.
The formation of the Taylor Cone, photographed at (a) 0 ms, (b) 100 ms, (c) 125 ms, (d) 140 ms, (e) 141 ms and (f) 141.175 ms.
The evolution of the electrostatic field–induced jet micro-discharge seen at (a) −50 µs before the discharge, (b) −25 µs before the discharge, (c) 0 µs, (d) 25 µs after the discharge and (e) 50 µs after the discharge.
The formation process of the Taylor Cone is clearly demonstrated in Figure 3. The high-speed digital video camera sample rate is set to be 40,000 fps (25 µs between frames). Figure 3(a) (frame 0 ms) shows the initial state when no voltage is applied between the nozzle and the workpiece. Since the nozzle is oriented perpendicular to the direction of gravity, the solution does not flow. However, due to the surface tension of the solution, the surface of the liquid bulges slightly beyond the nozzle outlet, appearing like a droplet.
When high voltage is applied, the liquid surface reacts to the electric field force because the charge is induced on the surface of the droplet. The polarity of this induced charge is opposite to the polarity of the workpiece. Due to the combined effect of electric field force and the surface tension, the surface of the solution at the outlet slowly changes into a semi-spherical crown, shown in Figure 3(b) (frame 100 ms). Caused by the electric field intensity, more ions or induced charge aggregate on the surface of the droplet. As a result, the surface gradually bulges outward as shown in Figure 3(c)–(e). In Figure 3(c) (frame 125 ms), the semi-spherical crown grows higher. While in Figure 3(d) (frame 140 ms), the contour surface of the droplet has gradually changed from the semi-spherical crown into a cone with round tip. With the charge on the surface of droplet further accumulating, the tip of the cone grows sharper, shown in Figure 3(e) (frame 141 ms), until at last the droplet’s surface formed a stable Taylor cone with cone angle approximately 98.6°, shown in Figure 3(f) (frame 141.175 ms).
When the distance between the nozzle and the workpiece becomes smaller than that in Figure 3, the intensity of the electric field gradually increases, leading to the formation process of the Taylor cone speeding up and the E-jet appearance. Then, with more and more charge further gathering on the Taylor cone, the fine jet from the tip of the Taylor Cone occurs and this jet tool electrode is generated. Figure 4 shows the jet and discharge process. During these frames, the start discharge denotes as the reference time (0 µs). In Figure 4(a) (frame −50 µs), the droplet at the nozzle has already transformed into the cone by the applied electric field, as shown in Figure 3. The cone angle becomes sharper in Figure 4(b) (frame −25 µs), and a shining jet emanates from the tip of the cone in Figure 4(c) (frames 0 µs). Finally, in Figure 4(d) (frame 25 µs), a fine jet is ejected from the tip of the cone, and it is clear that the discharge is occurring between the tip of the jet and the workpiece. After 25 µs, the jet disappears due to the discharge and then the droplet returns to round tip again, shown in Figure 4(e) (frame 50 µs). The jet lasts less than 50 µs, and the diameter of the jet might be smaller than 5 µm by calculating the pixels in Figure 4(d); therefore, the volume of flow in each jet is extremely tiny. Moreover, due to the capillary effect, this small amount of fluid lost can be automatically compensated from the syringe. Therefore, the jet at the nozzle outlet forms a cyclic dynamic process.
Discharge current observations and analysis
A silicon wafer as the workpiece and deionized water as the induced jet solution have been selected to investigate the current during the E-jet EDM process. Contrasted to the E-jet process observation experiment, aside from the material, the other experimental parameters are the same. The discharge is controlled by adjusting the distance between the nozzle tip and the workpiece electrode, which is about 1.65 mm. The same E-jet discharge phenomena have been observed as those described in Figure 4.
The representative current waveform obtained during this discharge machining process is shown in Figure 5. It can be found that a train of the current pulse with stationary pulse width and pulse interval can be observed during the discharge process, where the pulse width means the interval between the beginning of the jet discharge and the disruption of the jet after discharge. The pulse interval means the period between two successive jets. Moreover, the jet discharge is a short but continuous process, and the discharge period is stable, approximately 2 ms. The maximum value of the pulse current can reach 60 mA during the discharge and the pulse width lasts about 0.2 ms. It is also found in the experiment that these parameters are related to the machining conditions, such as the positive or negative polarity, the concentration of the solution and the material of the workpiece.
The typical current waveform during electrostatic field–induced electrolyte jet (E-jet) electrical discharge machining.
From Figures 4 and 5, it is shown that the electrostatic field–induced E-jet electrical discharge process is a pulsing, spontaneous and cyclic process. As a result, with the current feedback, by regulating the key machining parameters such as the applied voltage and the distance between the nozzle and the workpiece, the continuous and steady field-induced E-jet micro-electrical discharge process can be maintained.
Machining results and discussions
The phenomena of the field-induced E-jet discharge and the current detection experiment have confirmed that the cyclic discharge has happened between the jet and the workpiece. In this section, the mechanism of this machining method is revealed through the analyses of the morphology and composition in the machined area.
The reverse polarity E-jet experiment and analysis
This experiment has been conducted to demonstrate the machining mechanism. This involved machining holes with deionized water and reversing the field polarity to see whether there is any effect on the morphology and composition of the craters produced. Commonly, the workpiece connected with the positive of the power supply and the tool electrode with the negative of the power supply, is named “positive polarity,” contrariwise, the workpiece connected with the negative and the tool electrode with the positive is named “negative polarity.”
The deionized water is selected as the induced jet solution because it is neutral and cannot have a direct chemical reaction with the workpiece. In addition, the mineral ions have been removed from the water so that the jet surface can only be induced by surface electrostatic charges (H+ or OH−) when HVDC is applied.
The manual control is chosen here. As for the workpiece, the silicon wafer is polished to remove any intrinsic flaws so that the machined area can be distinguished from the un-machined. However, it is still difficult to identify the small machined crater due to the semi-conductive nature of the silicon and the way that the SEM works. Therefore, the strategy of the increase in machining duration (up to 20 min) and area is adopted so that many craters or deeper craters can be produced.
During the negative polarity machining experiment, the HVDC power supply is set with 3.5 kV in the output voltage. The motion platform is manually adjusted to lead the nozzle close to the workpiece until the discharge between the tip of the fine jet and the workpiece appears. Subsequently, maintain this distance and continue machining for 20 min, while moving the nozzle around the designated point in the XY plane (on the workpiece). Finally, the workpiece is washed with high purity ethanol and imaged with the SEM.
As shown in Figure 6(a), it is illustrated that there are some micro-holes in the machined area with approximately uniform size of less than 5 µm in diameter. The image captured with the SEM for the positive polarity setup is shown in Figure 6(b). Aside from the polarity switch, the other experimental parameters are the same. As shown in Figure 6(b), with positive polarity machining, the deionized water can also machine the silicon surface with holes, which is less than 4 µm in diameter.
SEM view of machined area in silicon wafer with 3.5 kV in power supply and the deionized water E-jet with (a) negative polarity and (b) positive polarity.
Figure 6 illustrates that, regardless of the polarity, the fine jet can generate the violent discharge on the surface of workpiece. The craters are not manufactured by chemical reaction due to the chemical property of the deionized water, and moreover, they are not caused by the pressure machining for there is no mechanical pressure applied. Furthermore, during the negative polarity machining experiment where the workpiece is connected with the negative terminal of the power supply in Figure 6(a), the possibility that the craters are produced by ECM processes can also be excluded because the ECM process is based on the theory of anodic dissolution to remove material. Together with the phenomena described in section “The E-jet electrical discharge process observation experiment,” it can be concluded that it is the E-jet discharge that machines the holes on the workpiece during this negative polarity machining. In Figure 6(b), contrasted to the negative polarity machining, besides the holes, some small cracks and bump area appear.
EDS spectrum analyses of chemical composition of the workpiece surface for the negative polarity and the positive polarity experiments have further confirmed the conclusion in last paragraph. Before the experiment, the workpiece surface is made from 100 wt % silicon. However, after the experiment, in these two samples, only two elements—oxygen and silicon—are found on the machined surface. The chemical composition at different points or zone is listed in Table 1. It is 100 wt% silicon at the zone A on the bottom of the micro-blind hole both in negative and positive polarity experiment. While on the circular (zone C) area around these blind holes during these two experiments, the oxygen element and silicon element are found. This is a typical EDM phenomena, 16 for when the heat generated by the discharge makes the discharge point melted and throws the debris out, the melted debris oxidizes when meeting the oxygen in the air, lending to the silicon in combine with the oxygen, and then a kind of recast layer can be formed. At zone B, more oxygen atoms on the embossment parts during positive polarity machining confirm that the oxidization is more violent than that in negative polarity machining, which means that there should be more pure oxygen. This is caused by the reason that when the distance between the workpiece and the nozzle increases after some craters have been created at the beginning of E-jet discharge, the E-jet condition has been changed, leading to some un-thorough discharge, which leaves some unevaporated liquid on the workpiece. For the E-jet discharge happens at the point where the electric field is most intense, the discharge current must happen between the nozzle (made from copper) and the unevaporated liquid on the workpiece surface. And then the ECM environment comes into being. Therefore, the ECM process appears on the interface between the workpiece and the electrolyte, and then the oxygen is generated on the workpiece surface. Together with the heat generated by the discharge, the violent oxidization happens on the surface workpiece, leading to the silicon on the discharge point into silica. On the other hand, for the bad machining environment, the machined material debris cannot be flushed out by the electrolyte flow as conventional ECM process and EDM process. Therefore, the products pile up and lead to some bump areas on the workpiece. However, in the negative machining, for there is no extra oxygen from the ECM process, the oxidation only relies on the oxygen in the air; as a result, the oxidation process is a little weaker, resulting in the smaller concentration of oxygen atom on the debris during negative polarity machining. The same reasons can also explain the reason that the concentration of oxygen atom in zone D in Figure 6(b) during positive polarity machining is larger than the corresponding zone D in negative machining in Figure 6(a).
The chemical composition analysis of different features under the negative polarity machining and positive machining with the deionized water as the jet liquid.
As for the zone E in positive polarity machining, the chemical composition is almost 100% silicon. This is caused by the unstable discharge along the border of the liquid on the workpiece surface, leading to EDM process along the contact border of the liquid and the workpiece surface. Furthermore, it should be noted that there is no other metal element in the composition on the machined workpiece surface; therefore, it can be concluded that it is not the nozzle itself, but the fine jet that acted as the tool electrode in this EDM process. As a result, unlike the negative polarity machining, it is the hybrid ECM and EDM that has machined the craters during the positive polarity machining.
The machining experiment
With the discharge phenomena observation experiment, SEM analysis of machined workpiece with the long time and large area machining and the EDS spectrum analyses have revealed the mechanism of electrostatic field–induced E-jet EDM. The utility of this process also depends on how small a hole it can make. Figure 7 shows two examples of holes machined with this process. Figure 7(a) shows a crater produced using a stainless steel workpiece and sodium nitrate solution 20 wt% as the jet solution. The applied voltage is stabilized at 3.5 kV. The single crater is 2 µm in the inlet diameter, the deepest point is about 0.3 µm and this crater is machined by a single discharge between the tip of the jet and the workpiece. The crater is surrounded by a semilunar recast area, 17 which also confirms that the machining is a kind of thermal fabrication. Figure 7(b) shows the result of the 2-min discharge machining of a silicon surface with the sodium nitrate solution 20 wt% as the jet solution. It shows that a blind hole with 3 µm in diameter and with 1.2 µm in depth can be produced.
(a) Morphology of a single-discharge crater on stainless steel with 6000 times magnification and (b) morphology of a discharge-machined crater on silicon after 2 min with 5000 times magnification.
Moreover, previous studies of electrodynamic jets had emphasized that the fine jet could reach almost several nanometers in diameter during the electro-spinning process. 18 Therefore, if the process were controlled precisely, this E-jet micro-EDM could have a promising application on the conductive or semi-conductive component micro- or nano-machining.
Conclusion
Considering the tool electrode process problem in the EDM, this article opens a new path and proposes a new electrostatic field–induced E-jet micro-EDM method in which the electrostatic field–induced E-jet is used as the tool electrode. The machining process of this method has been revealed by the E-jet observation and current detection experiments. In the observation experiments, it has found that the electrostatic field–induced E-jet electrical discharge is a pulsing, dynamic and cyclic process. Machining experiments have confirmed that the method proposed in this article is mainly the EDM process. However, in the positive polarity machining, besides the EDM process, when some electrolyte does not evaporate but resides on the workpiece, sometime ECM can appear. Furthermore, the single-discharge machining experiment and 2-min fixed point experiment have confirmed the micro-machining ability of this E-jet machining method.
Footnotes
Acknowledgements
The authors would thank the S. M. Wu Manufacturing Research Center (WuMRC) of the University of Michigan and Assistant Research Scientist Edmund Hodges-Kluck at the Department of Astronomy in the University of Michigan.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by grants from the National Natural Science Foundation of China (NSFC) (grant no. 51175336), China Scholarship Council (CSC) and Young Researcher Foundation in Shanghai Jiao Tong University (SJTU).