Do Kwan Chung

Micro electrical discharge milling using deionized water as a dielectric fluid

In electrical discharge machining, dielectric fluid is an important factor affecting machining characteristics. Generally, kerosene and deionized water have been used as dielectric fluids. In micro electrical discharge milling, which uses a micro electrode as a tool, the wear of the tool electrode decreases the machining accuracy. However, the use of deionized water instead of kerosene can reduce the tool wear and increase the machining speed. This paper investigates micro electrical discharge milling using deionized water. Deionized water with high resistivity was used to minimize the machining gap. Machining characteristics such as the tool wear, machining gap and machining rate were investigated according to resistivity of deionized water. As the resistivity of deionized water decreased, the tool wear was reduced, but the machining gap increased due to electrochemical dissolution. Micro hemispheres were machined for the purpose of investigating machining efficiency between dielectric fluids, kerosene and deionized water.

One-step process for superhydrophobic metallic surfaces by wire electrical discharge machining.

We present a direct one-step method to fabricate dual-scale superhydrophobic metallic surfaces using wire electrical discharge machining (WEDM). A dual-scale structure was spontaneously formed by the nature of exfoliation characteristic of Al 7075 alloy surface during WEDM process. A primary microscale sinusoidal pattern was formed via a programmed WEDM process, with the wavelength in the range of 200 to 500 μm. Notably, a secondary roughness in the form of microcraters (average roughness, Ra: 4.16 to 0.41 μm) was generated during the exfoliation process without additional chemical treatment. The low surface energy of Al 7075 alloy (γ = 30.65 mJ/m(2)) together with the presence of dual-scale structures appears to contribute to the observed superhydrophobicity with a static contact angle of 156° and a hysteresis less than 3°. To explain the wetting characteristics on dual-scale structures, we used a simple theoretical model. It was found that Cassie state is likely to present on the secondary roughness in all fabricated surfaces. On the other hand, either Wenzel or Cassie state can present on the primary roughness depending on the characteristic length of sinusoidal pattern. In an optimal condition of the serial cutting steps with applied powers of ∼30 and ∼8 kW, respectively, a stable, superhydrophobic metallic surface was created with a sinusoidal pattern of 500 μm wavelength.

Micro electrical discharge drilling of tungsten carbide using deionized water

Micro electrical discharge machining (micro EDM) is an effective machining method for cobalt-bonded tungsten carbide (WC-Co); however, this material is susceptible to electrolytic corrosion when deionized water is used as the working fluid with a dc power source for the RC circuit. In this study, a bipolar pulse power source and a triangular electrode were used in order to reduce the electrolytic corrosion phenomenon during micro EDM using an RC discharge circuit. A bipolar pulse power source reduces the positive polarity period of the workpiece by periodically alternating the polarity of the workpiece and electrode and decreases the average gap voltage at the machining gap. Therefore, electrolytic corrosion, which is a type of electrochemical reaction on the positively charged workpiece, is reduced by these electrical conditions. The triangular electrode has a smaller side area as compared with the cylindrical electrode. Since the electrolytic corrosion is an electrochemical reaction between the side of the electrode and the surface of the workpiece, the small side area of the triangular electrode could reduce these reactions. With the aid of the bipolar pulse power source and the triangular electrode, an electrolytic-corrosion-free hole could be machined on the WC-Co workpiece using deionized water.

Surface finishing of micro-EDM holes using deionized water

In this paper, with the use of deionized water, a finishing process of micro hole surfaces processed by micro electrical discharge machining (micro-EDM) is investigated. A micro hole is machined by micro-EDM using deionized water as a dielectric fluid. The inner surface of the hole is finished successfully via electrochemical dissolution in deionized water. The effects of finishing conditions such as the resistivity of deionized water, the voltage, the tool rotation and the finishing time on the surface quality and accuracy of the shape were investigated. After a finishing process using deionized water with a resistivity of 2 MΩ cm, a voltage of 80 V, a tool rotation of 1200 rpm and a finishing time of 6 min, the surface roughness was reduced considerably from 0.225 µm Ra after micro-EDM to 0.066 µm Ra.

Hybrid micromachining using a nanosecond pulsed laser and micro EDM

Micro electrical discharge machining (micro EDM) is a well-known precise machining process that achieves micro structures of excellent quality for any conductive material. However, the slow machining speed and high tool wear are main drawbacks of this process. Though the use of deionized water instead of kerosene as a dielectric fluid can reduce the tool wear and increase the machine speed, the material removal rate (MRR) is still low. In contrast, laser ablation using a nanosecond pulsed laser is a fast and non-wear machining process but achieves micro figures of rather low quality. Therefore, the integration of these two processes can overcome the respective disadvantages. This paper reports a hybrid process of a nanosecond pulsed laser and micro EDM for micromachining. A novel hybrid micromachining system that combines the two discrete machining processes is introduced. Then, the feasibility and characteristics of the hybrid machining process are investigated compared to conventional EDM and laser ablation. It is verified experimentally that the machining time can be effectively reduced in both EDM drilling and milling by rapid laser pre-machining prior to micro EDM. Finally, some examples of complicated 3D micro structures fabricated by the hybrid process are shown.

Micro electrical discharge milling of WC-Co using a deionized water spray and a bipolar pulse

Micro electrical discharge milling (ED-milling) is an effective machining process for manufacturing micro structures on hard metals. This method of machining generally uses kerosene or deionized water as the working fluid, both of which are associated with some problems. Kerosene results in considerable electrode wear and deionized water causes electrolytic corrosion in workpieces. In particular, when cemented tungsten carbide (WC-Co), which has superior strength, hardness and wear resistance, is machined by electrical discharge machining (EDM), the problem of electrolytic corrosion arises as a matter of course since the material is very susceptible to electrolyzation. In this study, spray ED-milling with a bipolar pulsed power source and deionized water was conducted to solve the above problems. This method uses a water spray, which is a mixture of compressed air and deionized water. The spray is injected into the machining gap between the electrode and the workpiece. WC-Co was used for the workpiece and micro grooves were machined on the workpiece. As a result, using the spray ED-milling method, high-quality micro grooves were manufactured on the WC-Co workpiece with no electrolytic corrosion and almost-zero electrode wear.

Electrochemical etching of stainless steel through laser masking

A new micro patterning process without the need for a mask is proposed in this paper. It is a combination of laser masking and electrochemical etching. In electrochemical etching through laser masking (EELM), selective electrochemical dissolution of stainless steel is achieved in a 2 M sodium nitrite electrolyte. The micro-patterned surface layer of stainless steel is formed by laser marking using an ytterbium pulsed fiber laser. After the laser masking process, the patterned surface of stainless steel is selectively dissolved under an appropriate electrochemical etching condition because the laser-marked area can temporarily act as a barrier during electrochemical etching. The characteristics of the laser-marked surfaces have been investigated by scanning electron microscopy (SEM), x-ray diffractometry (XRD) and x-ray photoelectron spectroscopy (XPS). The polarization curve has been used to evaluate electrochemical dissolution behaviors. The EELM process enables the electrochemical micro-machining of stainless steel without photolithography technology; thus, it can be applied in micro fabrication, micro patterning and surface texturing.

Micro Electrochemical Machining for Complex Internal Micro Features

Micromachining of internal features by electrochemical machining (ECM) is investigated. By controlling pulse conditions and using a customized tool electrode, micro features were machined on the side wall of a micro hole. In micro ECM, longer pulse on-time enlarges the side gap. A reverse tapered hole and a barrel-shape hole were fabricated through pulse duration control. Micro cavity was machined by controlling the pulse duration and machining time in similar manner. A micro disk-shape electrode was used to machine microgroove inside the hole, which is similar to turning lathe process.

Electrical discharge machining of carbon nanomaterials: Mechanisms and the advanced field emission applications

Carbon nanomaterials such as carbon nanotubes and carbon nanofibers (CNFs) have been intensively investigated for various nanoelectronic applications due to their superb properties, good stability, and high aspect ratio. There have been many efforts to develop flat panel displays, light sources, and backlight units by using carbon nanomaterials. For wider applications, carbon nanomaterials often need to be cut, leveled, patterned, and figured. A reliable and precise micro-machining process, electrical discharge machining (EDM), was investigated in depth as a very neat method for the effective and multifunctional engineering of carbon nanomaterials. In the present study, the mechanisms of the EDM engineering of carbon nanomaterials are systematically examined, and the advanced field emission applications derived from the EDM method are presented.