Development of a reversible machining method for fabrication of microstructures by using micro-EDM

A new micromachining method for the fabrication of micro-metal structures by using micro-reversible electrical discharge machining (EDM) was investigated. The reversible machining combines the micro-EDM deposition process with the selective removal process, which provides the ability of depositing or removing metal material using the same micro-EDM machining system. From the discharge mechanism of micro-EDM, the process conditions of micro-EDM deposition were analyzed firstly. Using the brass and steel materials as a tool electrode, the micro-cylinders with 200 μm in diameter and height-to-diameter ratio of more than 5 were deposited on a high-speed steel surface. Then the machining procedure was transformed easily from deposition to selective removal process by switching the process conditions. Different removal strategies including micro-EDM drilling and micro-EDM milling were used in the machining. Micro-holes with 80 μm in diameter are drilled successfully in the radial direction of the deposited micro-steel cylinder. Also, a brass square column with 70 μm in side length and 750 μm in height, and a micro-cylinder with 135 μm in diameter and 1445 μm in height are obtained by using micro-EDM milling. Finally, the characteristics of the deposited material were analyzed. The results show that the material components of a deposited micro-cylinder are almost the same as those of the tool electrode, and the metallurgical bonding has been formed on the interface. In addition, the Vickers-hardness of 454Hv of the steel deposited material is higher when compared to the hardness of 200Hv of the raw steel electrode.     

A new tool wear compensation method based on real-time estimation of material removal volume in micro-EDM

Owing to the reduced tool area and poor flushing conditions in deep holes, tool wear in micro-electrical discharge machining (EDM) is more significant than in macro-EDM. In micro-EDM drilling, the z-axis of the tool position is monitored as machining progresses. However, due to significant electrode wear, the machined hole depth is not identical to the programmed depth of the hole, and thus this will result in geometrical inaccuracy. This paper presents a new micro-EDM drilling method, in which the material removal volume is estimated as machining progresses. Compensation length is calculated and adjustment is made repeatedly along the tool path until the targeted material removal volume is reached. A real-time material removal volume estimator is developed based on the theoretical electro-thermal model, number of discharge pulse and pulse discrimination system. Under various energy input and machining depth settings, the experimental and estimated results are found to be in satisfactory agreement with average error lower than 14.3% for stainless steel, titanium, and nickel alloy work materials. The proposed drilling method can compensate the tool wear and produce more accurate micro-holes as compared to other methods. Experimental work also shows that the proposed method is more reliable as compared to the uniform wear method. In drilling micro-holes of 900 μm depth, the depth error can be reduced to 4% using the proposed method.     

TEM study on the electrical discharge machined surface of single-crystal silicon

EDM is a useful process for machining high-aspect ratio features with good accuracy in electrically conductive materials irrespective of their mechanical properties. With the ability of micro-EDM to compete with the resolution of conventional semi-conductor processing techniques, the process has attracted interest for the potential machining of single-crystal silicon. In order for the process to be feasible, the damage mechanism occurring during machining must be characterised to assess the need for secondary processing. Despite this the microstructural transformations induced by the process on the surface of the workpiece have not yet been assessed. In this study transmission electron microscopy (TEM) and laser-Raman spectroscopy are employed to characterise the microstructural changes as well as the presence of any contaminants and defects at the nano-scale. A twinned-crystalline structure created by epitaxial growth is formed in the recast layer. Some amorphous phase is also present. Findings indicate sub-surface pores between 10 nm and 200 nm diameter formed by gas expansion are observed. If the formation of such pores can be generalised for EDM processing of other materials, this phenomenon may contribute to the reduced mechanical integrity of such machined surfaces. Significant tool electrode material deposition with crystals of down to 3 nm diameter also occurred in the workpiece surface. The nano-scale of embedded material may have implications for the progress of electrical discharge machining as a coating process and the properties of such coatings.     

Compound machining of titanium alloy by super high speed EDM milling and arc machining

A novel compound machining of titanium alloy (Ti6Al4V) by super high speed electrical discharge machining (EDM) milling and arc machining was proposed in this paper. The power supply consisted of a pulse generator and a DC power source which were isolated from each other. A rotating pipe graphite electrode was connected to the negative pole of the power supply. The plasma channel was able to deionize, and maximum material removal rate (MRR) reached 21,494 mm³/min with a relative electrode wear ratio (REWR) of 1.7% because of high current and efficient flushing. Compared with traditional EDM, the compound machining achieved a significantly higher MRR but a similar REWR. To investigate the characteristics of the compound machining, the effects of electrode polarity, peak voltage, peak current, and flushing pressure on the performance of the process, including its MRR, REWR, and radius of overcut (ROC), were determined. In addition, scanning electron microscopy, X-ray diffraction, and microhardness analysis were conducted. Result shows that the proposed method can machine difficult-to-machine materials efficiently.     

Ultrasonic and electric discharge machining to deep and small hole on titanium alloy

Being a difficult-to-cut material, titanium alloy suffers poor machinability for most cutting process, let alone the drilling of small and deep holes using traditional machining methods. Although electric discharge machining (EDM) is suitable to handle titanium alloys, it is not ideal for small and deep holes due to titanium alloys’ low heating conductivity and high tenacity. This paper introduces ultrasonic vibration into micro-EDM and analyzes the effect of ultrasonic vibration on the EDM process. A four-axis EDM machine tool which combines ultrasonic and micro-EDM has been developed. A wire electric discharge grinding (WEDG) unit which can fabricate a micro-electrode on-line, as well as a measuring unit, is set up on this equipment. With a cylindrical tool electrode, made of hard carbide, which has high stiffness, a single-side notch was made along the electrode. Ultrasonic vibration is then introduced into the micro-EDM. Experiments have been carried out and results have shown that holes with a diameter of less than Ø0.2 mm and a depth/diameter ratio of more than 15 can be drilled steadily using this equipment and technology.     

Effect of laminate configuration and feed rate on cutting performance when drilling holes in carbon fibre reinforced plastic composites

Composites use in the aerospace industry is expanding, in particular carbon fibre reinforced plastics (CFRP) for structural components. Machinability can however be problematic especially when drilling, due to CFRP's inherent anisotropy/in-homogeneity, limited plastic deformation and abrasive characteristics. Following a brief review on composites development and associated machining, the paper outlines experimental results when twist drilling 1.5 mm diameter holes in 3 mm thick CFRP laminate using tungsten carbide (WC) stepped drills. The control variables considered were prepreg type (3 types) and form (unidirectional (UD) and woven), together with drill feed rate (0.2 and 0.4 mm/rev). A full factorial experimental design was used involving 12 tests. Response variables included the number of drilled holes (wear criterion VB_Bmax ≤ 100 μm), thrust force and torque, together with entry and exit delamination (conventional and adjusted delamination factor values calculated) and hole diameter. Best results were obtained with woven MTM44-1/HTS oven cured material (3750 holes) while the effect of prepreg form on tool life was evident only when operating at the higher level of feed rate. Thrust forces were typically under 125 N with torque values generally below 65 Nmm over the range of operating parameters employed. Finally, the delamination factor (F_d) measured at hole entry and exit ranged between ～1.2–1.8 and 1.0–2.1 respectively.     

Abrasive slurry jet micro-machining of holes in brittle and ductile materials

This paper investigated the effects of elasticity and viscosity, induced by a dilute high-molecular-weight polymer solution, on the shape, depth, and diameter of micro-holes drilled in borosilicate glass and in plates of 6061-T6 aluminum alloy, 110 copper, and 316 stainless steel using low-pressure abrasive slurry jet micro-machining (ASJM). Holes were machined using aqueous jets with 1 wt% 10 μm Al₂O₃ particles. The 180 μm sapphire orifice produced a 140 μm diameter jet at pressures of 4 and 7 MPa. When the jet contained 50 wppm of dissolved 8 million molecular weight polyethylene oxide (PEO), the blind holes in glass were approximately 20% narrower and 30% shallower than holes drilled without the polymer, using the same abrasive concentration and pressure. The addition of PEO led to hole cross-sectional profiles that had a sharper edge at the glass surface and were more V-shaped compared with the U-shape of the holes produced without PEO. Hole symmetry in glass was maintained over depths ranging from about 80–900 μm by ensuring that the jets were aligned perpendicularly to within 0.2°. The changes in shape and size were brought about by normal stresses generated by the polymer. Jets containing this dissolved polymer were observed to oscillate laterally and non-periodically, with an amplitude reaching a value of 20 μm. For the first time, symmetric ASJM through-holes were drilled in a 3-mm-thick borosilicate glass plate without chipping around the exit edge.The depth of symmetric blind holes in metals was restricted to approximately 150 μm for jets with and without PEO. At greater depths, the holes became highly asymmetric, eroding in a specific direction to create a sub-surface slot. The asymmetry appeared to be caused by the extreme sensitivity of ductile materials to jet alignment. This sensitivity also caused the holes in metals to be less circular when PEO was included, apparently caused by the random jet oscillations induced by the polymer. Under identical conditions, hole depths increased in the order: borosilicate glass > 6061-T6 aluminum > 110 copper > 316 stainless steel. The edges of the holes in glass could be made sharper by machining through a sacrificial layer of glass or epoxy.     

Optimizing the EDM hole-drilling strain gage method for the measurement of residual stress

When using the electrical discharge machining (EDM) hole-drilling strain gage method to measure the residual stress within a component, the metallurgical transformation layer formed on the wall of the EDMed hole induces an extra stress, which can lead to significant measurement errors. Accordingly, the objective of the present work was to explore and determine the optimal EDM parameters which reduce the thickness of the metallurgical transformation layer and therefore minimize the magnitude of the hole-drilling induced stress. The experimental results demonstrated that by maintaining the relative stability coefficient of the discharge duty ratio at a value greater than 0.99, the induced stress emerged in EDM hole-drilling measurement can be reduced substantially and becomes insensitive to the parameters of the pulse current and pulse-on duration. Further investigations revealed that when the residual stress is to be measured accurately, using a hollow electrode instead of the usual solid electrode and the following parameters are recommended. The pulse current and pulse-on duration are in the ranges of 4–12 A and 9–23 μs, respectively, and the pulse-off duration needs to be longer than the value required to ensure that the relative stability coefficient of the discharge duty ratio exceeds 0.99.     

Fabrication of complex shape electrodes by localized electrochemical deposition

Localized electrochemical deposition (LECD) is a promising technology for fabrication of high-aspect ratio electrode of various materials. This technology is found to be one of the simple and inexpensive ways to fabricate electrodes for micro-EDM. This study presents a novel method to manufacture electrodes with complex cross-section using mask of non-conductive material. In this study, the mask is placed between the anode and cathode, which is immersed in mixed electrolyte of copper sulfate, 1.0 M sulfuric acid and as an additive agent 0.04 g/l of thiourea. The deposition of copper is localized on the cathode surface using a mask and applying ultra short voltage pulses between the anode and cathode. In this setup the cathode is placed above the anode and mask, so that the deposited electrode can be used directly for EDM or any application without changing tool orientation. The deposition characteristics such as size, shape, surface, and structural density according to gap between the anode and mask, applied voltage, pulse frequency and duty ratio have been investigated in this study. Finally, appropriate conditions have been found out for effective fabrication of smooth and fine-grained deposited electrodes based on the findings of the various experiments.     

Theoretical studies of electronic structure and hole drift mobility of host hole transporting material 4,4′-N,N′-dicarbazol-biphenyl

The most commonly used host hole transport material, 4,4′-dicarbazole-1,1′-biphenyl (CBP), its structure, electronic transition mechanism and hole mobility were studied by means of ab initio HF, DFT B3LYP methods and Marcus theory. The lowest singlet excited state (S1) has been studied by the singles configuration interaction (CIS) method and time-dependent density functional theory (TD-DFT). The lowest singlet electronic transition (S0 → S1) is π–π* electronic transitions between carbazole and biphenyl parts involving the charge transfer from N atom to biphenyl. TD-B3LYP calculations predict an emission wavelength of 403.3 nm. This is comparable to 400 nm observed experimentally for photoluminescence. Using an incoherent transport model we calculated its hole mobility (μ). Both its reorganization energy and electronic coupling, especially the electronic couplings are considered and calculated in detail. It has high hole transport efficiency (μ = 8.60 × 10^?2 cm²/(V s)) and the result was rationalized in terms of the spatial extent of the highest occupied molecular orbital (HOMO) and molecular structural character.     

Properties of Pb(Zr, Ti)O3 fibres with a radial gradient structure

One difficulty during the fabrication of lead zirconate titanate (PZT) materials is the high partial pressure and the accompanied evaporation of lead oxide (PbO) during sintering. To overcome this problem, atmospheric powders are used in the sintering step, whereby a composition and microstructure gradient across the fibre radius can be expected. In this study, therefore, the microstructure (porosity, grain size), the chemical and the phase composition as well as the ferroelectric properties were analysed across the fibre diameter. For these investigations, extruded PZT fibres with a green diameter of 300 μm and 70 μm were sintered at 1200 °C for 2 h in a PbO-enriched atmosphere. The measurements revealed that the chemical and thus the phase composition vary across the radius of the fibres for both fibre diameters, but the observed gradients are much more pronounced for 70 μm fibres. By removing the affected surface layer of a 300 μm diameter fibre, the maximum free strain could be increased by 20%.     

Variation of temperature at the bottom surface of a hole during drilling and its effect on tool wear

The temperature at the bottom surface of a hole being drilled is measured by using an infrared-radiation pyrometer equipped with two optical fibers. One of the optical fibers is inserted into the oil hole of an internal coolant carbide drill and passes through the machine-tool spindle. This optical fiber is connected to another optical fiber at the end of the spindle. Infrared rays radiating from the bottom surface of the hole being drilled are accepted and transmitted to the pyrometer by the two optical fibers. Temperature increases as drilling progresses, and it increases considerably near the bottom surface of the workpiece. In case of a 10-mm-thick carbon–steel workpiece, temperature reaches 190, 250, and 340 °C at drilling depths of 6, 8, and 10 mm, respectively. To investigate the effect of the increase in temperature on drill wear, a series of 10-mm-deep blind holes are drilled in workpieces with thicknesses of 10 and 25 mm. Tool wear is greater when the drill cuts a hole at the bottom of a 10-mm workpiece than that when the drill cuts a hole at the mid-depth of a 25-mm workpiece. This indicates that the rapid increase in temperature near the bottom of the workpiece effects the progress of drill wear.     

Analysis of the electrochemical behaviors of WC–Co alloy for micro ECM

Micro electrochemical machining (ECM) of tungsten carbide with cobalt binder (WC–Co) was studied using ultrashort pulses. In ECM, the machining characteristics were investigated according to machining conditions such as electrolyte, workpiece potential, and applied voltage pulse. Using a mixture of sulfuric acid and nitric acid, microstructures with a sharp edge and good surface quality were machined on tungsten carbide alloy. The potentials of workpiece electrode and tool electrode were determined by considering the machining rate, machining stability, and surface quality of products. With the negative potential of the workpiece electrode, oxide formation was successfully prevented and shape with good surface quality in the range from R_a 0.069 μm to 0.075 μm were obtained by electrochemical machining. Moreover, the performance of ECM, which includes machining gap, tapering, surface roughness, and machining time, without tool wear was compared with that of electrical discharge machining (EDM). Microstructures of WC–Co with a sharp edge and good surface quality were obtained by electrochemical milling and electrochemical drilling. Micro electrochemical turning was also introduced to fabricate micro shafts.     

Large spectral shifts of electronic transitions in MoSI molecular wire dispersions as a function of bundle diameter

A systematic study of optical absorption spectra of Mo₆S_9?xI_x (x = 6) molecular wire dispersions in ethanol, fractionated into different bundle diameter populations shows that electronic transitions shift significantly as a function of bundle diameter. Two electronic transitions show significant shifts: the Mo–S charge transfer peak shifts from 1.8 to 1.5 eV and the next inter-band transition shifts from 2.7 to 2.4 eV with increasing bundle diameter d, in the range 5–100 nm. This empirical observation hugely simplifies characterization of Mo₆S_9?xI_x wire dispersions according to diameter, opening the way to rapid advances in processing of these materials. We discuss the possible origin of the shift, dismissing quantum size effects, impurities and solvatochromism as well as stoichiometric variations between x = 6 and x = 4.5.     

电火花摇动加工微细阵列轴和孔的试验研究

针对微细阵列轴和孔的电火花加工,提出了利用数控电火花加工机床摇动功能的摇动加工微细阵列轴和孔的方法.此法是基于电火花反拷贝加工的原理,先用丝电极在薄平板(中间电极)上按要加工的阵列轴和孔间距或数倍间距加工阵列小孔(直径0.1 mm以上),然后用加工的薄平板(中间电极)作电极,电火花摇动加工微细阵列轴(电极),最后用此微细阵列电极加工阵列孔.进行了电火花摇动加工微细阵列电极试验,得到了单电极直径为50 μm、长径比为16的3×3阵列电极,并用此电极在70 μm厚的不锈钢板上加工出单孔直径为70 μm的3×3微细阵列孔.试验结果表明,电火花摇动加工方法可实现微细阵列轴和孔的加工.     

Ultra-precision finishing of micro-aspheric mold using a magnetostrictive vibrating polisher

Demands of micro-aspheric glass lenses are increasing in optical devices such as digital cameras and blu-ray players. In this paper, a novel vibration-assisted polishing machine using a magnetostrictive vibrating polisher is proposed and developed to improve the efficiency, surface roughness and stability of finishing. The magnetostrictive vibrating polisher can generate a radius of 30 μm circular vibrating motion at frequency 9.2 kHz. From the polishing experiments, a smooth removal function was obtained. The form accuracy was improved to less than 100 nm P–V and the surface roughness was reduced to 3.3 nm Rz (0.4 nm Ra).     

A microstructural investigation of the surface of a drilled hole in carbon steels

The microstructure of the surface of drilled holes generated under different drilling conditions in carbon steels has been investigated. It is found that the surface microstructure depends strongly on the drilling parameters and the hardness of the matrix. White etching layers, composed of an equiaxed nanocrystalline structure layer with an average grain size of the order of several 10 nm and a submicron grained layer containing fresh martensite along the depth, formed on the hole surfaces during drilling at moderate to high cutting speed in carbon steels with high matrix hardness. The existence of a high content of austenite at the hole surface suggests that dynamic phase transformation (DPT) from body-centered cubic to face-centered cubic occurred during high-speed drilling. It is proposed that the ultrafine structure layer on the surface of a drilled hole is produced by severe plastic deformation-induced DPT together with a large strain gradient and high strain rate.     

On-machine dry electric discharge truing of diamond wheels for micro-structured surfaces grinding

Precision grinding with diamond wheels gives a promising alternative to achieve high quality micro-structured surfaces on optical molds. However, it is difficult to true these diamond wheels efficiently, because of the remarkable resistance property and the geometrical limitation of small wheel profile. In this paper, an on-machine dry-EDT method to precision shape and prepare diamond wheels with various profiles was proposed for micro-structured surface grinding. Firstly, the fundamental truing errors were analyzed based on the dry-EDT kinematics. And then the capabilities of dry-EDT truing for high abrasive concentration metal bonded diamond wheels were presented. Next, the effects of kinematic parameters variables on trued wheel profile accuracy were investigated. Finally, the micro-structured surfaces on SiC ceramic and tungsten carbide WC were ground by these trued diamond wheels. The experiments results showed that the arc-shaped diamond wheel (diameter of 200 mm) with 4 μm profile error (PV) and 1.0023 mm profile radius, and the V-shaped diamond wheel with 22.5 μm V-tip radius and 120.03° profile angle could be obtained by on-machine dry EDT. The kinematic parameters of dry-EDT have an important influence on truing profile accuracy of diamond wheels, especially for the tip of V-shaped wheel. The subsequent grinding show that the edge radius of V groove array on SiC is less than 2 μm, while the radius of included corner is around 55 μm. The PV error of ground arc groove array on WC is less than 5 μm. The surface roughness of ground micro-structured surface R_a is 142 nm and 97 nm for SiC and WC, respectively.     

Numerical simulation and experimentation of a novel micro scale laser high speed punching

A novel process technology for micro punching of thin sheet metals is presented in this paper. The laser induced shock waves act as the micro punch. The forming speed can be controlled by adjusting the laser energy. Micro holes of 250 μm in diameter were successfully punched on sheet metal of 10 μm in thickness by single pulse, and good edge quality was obtained. The micro die-opening is produced by electrical discharge machining (EDM) using the copper electrode of φ=220 μm. In the experiment, a sacrificial coating is used to generate high-pressure plasma under a laser pulse, so no signs of melting, burning, or ablation were observed on the workpiece. The novel process of micro scale laser high speed punching is also numerically studied. In addition, this method can be used to fabricate noncircular micro holes on thin sheet metals with noncircular micro die-openings. With further development, laser high speed punching may become an important micro punching technology, which is characterized by non-contact, low cost, and high efficiency.     

Fuzzy nanofibrous network of polyaniline electrode for supercapacitor application

Fuzzy nanofibrous network of polyaniline electrode is successfully electrosynthesized for supercapacitor application. The nanofibre network of polyaniline electrode is characterized using Fourier transforms infrared spectroscopy (FTIR), scanning electron microscope (SEM) and optical absorption studies. Network of polyaniline is highly porous with interconnected fuzzy nanofibres having diameter typically between 120 and 125 nm. The supercapacitive performance of polyaniline electrode is tested using cyclic voltammetry (C-V) technique in H₂SO₄ electrolyte within potential range of ?100 to 800 mV. The effect of scan rate on the capacitance of polyaniline electrode is studied. The highest specific capacitance of 839 F g^?1 at the voltage scan rate of 10 mV s^?1 is achieved. Additionally stability and charging–discharging of polyaniline electrode are studied.