Electrostatic actuating bendable flat electrode for micro electrochemical machining

Micro electrochemical machining (μECM) is a non-conventional machining process based on electrolysis. It has become an attractive research area due to the fact that this process does not create any defective layer after machining and obtains better surface. This machining technology is being developed to meet the increasing demand driven by aerospace, microelectronics, automotive and electronics. It enables the machining of micro-size features with high accuracy and high aspect ratio in materials with high hardness and stiffness.

Recent development in this area aims at achieving better accuracy and productivity. For example, the jet electrochemical machining (Jet-ECM), which is also called Electrolytic Jet Machining, is one kind of ECM technique that the electrolyte is pumped through a nozzle to form a free jet. The Jet-ECM can be used to quickly fabricate a microstructure of metal parts. It suffers no thermal impact, mechanical impact or tool wear, so the surface of workpiece is very smooth.¹ But the Jet-ECM is dependent on the unpredictable jet shape and the micro-nozzles are difficult to manufacture, restricting its use in high-resolution micro-part processing. Mask electrolytic machining (TMECM) has been feasible for machining metal parts with hole arrays. This technology takes the unprotected area of the workpiece as a node, which is an efficient and low-cost processing method. However, TMECM cannot avoid the lateral etching which reduces the processing accuracy.² Many scholars have tried other ways to achieve more acceptable machining results, such as developing servo system specially, adjusting the pulse frequency and pulse width, or reducing the voltage and electrolyte concentration.^3–7 Nevertheless, all these μECM processes require a very small gap (size of a few μm) between the anode (workpiece) and the cathode (tool electrode) to get proper energy in machining.⁸ Although the progress has been made in μECM over the past 20 years, the controllability and the monitoring of the electrode gap in the process are still big issues.

Traditional μECM always has a complex processing device and high precision X, Y and Z stages to ensure higher precision of micro-feeding.⁹ But only improving the precision of the device is not reliable because μECM environment is complex and uncontrollable in practice. It has been found that the thin metal sheet undergoes bending deformation due to the electrostatic force in the electrostatic field. Electrostatic actuation enjoys the advantages of short response time, low loss and high stability.^10–12 So electrostatic actuation is applied to the solution of longstanding small gap control issues in μECM. During the machining process, the flat electrode bends due to electrostatic force in pulse on-time, which self-adaptively narrows the gap between the electrode and the workpiece. The workpiece material will be removed once the gap meets the processing condition. The flat electrode quickly restores to its original condition when it is in pulse off-time, which makes the gap much larger than traditional electrochemical machining (ECM). It benefits the debris removing. This work explores the feasibility of the new method that uses electrostatic actuating flat electrode for micro self-adaptive feeding and debris removing. By using the proposed method, the characterization of the process with varying machining parameters will be discussed.

When a metal piece dipped in an electrolytic solution, the highly energetic surface atom leaves the surface as metal ions and dispersed into the solution. The simultaneous discharge of ions from the solution forms a layer over the metal surface. The equilibrium is reached when the total charge (electron) left in the metal contributes to the formation of layer of ions whose cumulative charge is equal and opposite to that of metal surface.⁹ In μECM, the electrode and workpiece both experience this phenomenon and these two sides behave as an electrical capacitor when the applied voltage is very low (up to 20 V) which leads to the charging of capacitor. The charging and discharging of capacitor at cathode and anode interface affect the electrochemical dissolution process.

The principle for the proposed process is illustrated in Fig. 1. A high frequency pulse generator, which is connected to the flat electrode and workpiece, is applied to perform the μECM process. The charging current flows through the electrolyte along the least resistance path which is proportional to the gap width (distance) between the electrodes. Therefore, the distance between electrode and workpiece should be as short as possible. At minimum gap distance, a portion of electrode is substantially charged where the time constant for the formed capacitor does not exceed the duration of pulse on-time of the power supply, for which localized material dissolution takes place.¹³ By controlling the inter electrode gap and the pulse on-time of the DC power supply, micro 3D letter or cylinder can be fabricated. Before applying pulse generator to the flat electrode and the workpiece, the flat electrode is stationary (Fig. 1(a)). When the pulse generator is applied, an electric field along with electrostatic force will be generated immediately between the flat electrode and the workpiece. This electric field bends the flat electrode and then decreases the gap. The workpiece will be processed once the gap meets the processing condition (Fig. 1(b)). After an electrical pulse, the electric field will disappear and the flat electrode will quickly recover (Fig. 1(c)), which increases the gap and eliminates the debris. When the process is repeated, the desired shape will appear as shown in Fig. 1(d).

Electrostatic actuation electrochemical machining setup is shown in Fig. 2. It mainly includes electrostatic actuation system, pulse generator and electrolyte circulation system. The electrostatic actuation mechanism is installed on the Z axis head, which is made for micro self-adaptive feed. The pulse generator provides the high-frequency pulse. It has the current detection function to detect the processing status in real time. The circulation system filters out the debris and provides fresh working fluid.

A commercially available thin copper sheet was employed as a flat electrode. The pattern on the electrode was etched by EDM (Fig. 3). The distance between the flat electrode and the workpiece was determined by electrical surface detection with a small voltage (10 V). Using the Z axis to make the flat electrode approach the workpiece, then retraction of the flat electrode defined the gap distance. After applying electrolyte and a machining voltage between the flat electrode and the workpiece, μECM patterning was performed by flat electrode electrostatic self-adaptive micro feeding with Z axis fixed. Z axis would servo feed according to gap current which was monitored by current sensor. The experimental conditions used for process testing are outlined in Table 1.

Experimental plans for machining tests were conducted to explore the efficiency and accuracy of the new method. The efficiency and accuracy of μECM are mainly related to the conclusive factor of current density which is related to the process parameters of voltage and electrolyte concentration. A series of experiments were performed. For experimentation, the voltage and electrolyte concentration were variables. Material removal rate and process gap respectively represented efficiency and accuracy. After conducting all the experiments, the differences in material removal rate and process gap were recorded and characteristics graphs were drawn.

Fig. 4 shows the variation of material removal rate in fabrication of cylinder arrays. It is found that the material removal rate increases with the increase in voltage (Fig. 4(a), electrolyte concentration 10 g/L, area of flat electrode 9 mm²) and increases with the increase in electrolyte concentration (Fig. 4(b), voltage 20 V, area of flat electrode 9 mm²). With the increase of amplitude of pulse voltage, high charging and discharging of electrical capacitor take place that accelerates the electrochemical reaction and increases the material removal. As the concentration of electrolyte increases, current density increases due to the frequent collision between the ions. For this reason, material removal rate increases. Therefore, the process efficiency can be increased by increasing the voltage and electrolyte concentration.

The accuracy of μECM manifests as process gap that is related to voltage and electrolyte concentration. Fig. 5 shows the variation of process gap in fabrication of cylinder arrays. The conditions of Fig. 5(a) and Fig. 5(b) are the same as those of Fig. 4(a) and Fig. 4(b) respectively. It is found that the process gap increases in each plot with the increase in process parameters. When the voltage increases, the discharge gap or the distance between the electrode and the workpiece increases. It results in the problem that the region of material removal becomes larger. For this reason, the process gap increases. Therefore, the accuracy will decrease when the voltage and electrolyte concentration increase.

Although the processing efficiency can be increased by increasing the voltage and electrolyte concentration, the processing accuracy decreases significantly simultaneously. Therefore, taking into account both efficiency and accuracy, the optimal voltage is 25 V and electrolyte concentration is around 5 g/L.

The optical image of the fabricated two-dimensional letters with the voltage of 20 V and the electrolyte concentration of 5 g/L is represented in Fig. 6(b) (depth 30 µm). This was performed for 30 minutes only using the new method of electrostatic actuation. It shows that electrostatic actuation can make the machining depth reach 30 µm with no servo feed. It is verified that μECM with electrostatic actuating flat electrode is effective to implement a micro-scale removal on the surface of workpiece.

The electrostatic actuation micro feed can combine with servo feed to get a deeper result. Servo feed with macro driver kept a favorable machining gap, which was on the basis of gap current. The gap current was monitored by the current sensor. The optical images of the fabricated cylindrical arrays with electrolyte concentrations of 5 g/L and voltage of 20 V are represented in Fig. 7. The result in Fig. 7(a) has the depth of 2 mm and the result in Fig. 7(b) has the depth of 300 µm. It is worth noting that the holes in flat electrode contribute to release bubbles produced in the process, which improves the stability of machining.

The experiments were performed successfully with the proposed setup. This work has experimentally explored the feasibility of electrostatic actuating flat electrode for μECM processing to overcome the issues of gap control. The proposed method using electrostatic actuating flat electrode is indeed effective to fabricate micro patterning on the workpiece. The machining depth is around 30 µm, the cylinder’s diameter is 0.5 mm and the surface roughness is small. The material removal rate increases with the increases of voltage and electrolyte concentration. But the accuracy will decrease when the voltage and electrolyte concentration increase. The optimal voltage is 25 V and electrolyte concentration is around 5 g/L. So this method has advantages of reducing dependence on high-precision machine tools and avoiding complex servo control. Meanwhile, the machining accuracy is improved because of the large gap for debris removing. The developed method is promising for rapid and low-cost production of micro patterns. The experimental results encourage further development to improve the depth.

This work was supported by the National Natural Science Foundation of China (Grant No. 51105110, 51475107), and Shenzhen Basic Research Program (Grant No. JCYJ20170811160440239).