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Advances in Mechanical Engineering

Volume 2014 (2014), Article ID 382105, 7 pages

http://dx.doi.org/10.1155/2014/382105
Research Article

Fabrication of Metal Microtool Applying Wire Electrochemical Machining

1Jiangsu Key Laboratory of Precision and Micro-Manufacturing Technology, Nanjing 210016, China

2College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, China

Received 2 February 2014; Accepted 23 May 2014; Published 9 June 2014

Academic Editor: Chien Hung Liu

Copyright © 2014 Zhuang Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Metal microtools with various shapes can be used for micromachining technologies due to their specific characteristics. Wire electrochemical machining (wire ECM) shows high potential to produce complex microstructures with repetitive usage of wire electrode and absence of thermal effects. This study presented an investigation of feasibility on fabricating metal microtool with various shapes using microwire ECM process. The experiments were conducted under a condition of ø300 μm tungsten rod as anodic specimen, ø20 μm tungsten wire as cathode, KOH as electrolytic solution, and ultrashort pulsed current as power supply. Effects of pulse-on time, applied voltage, wire feeding rate, and solution concentration on overcut and machining stability were evaluated in order to obtain optimal process parameters. Microtools with various shapes were fabricated thereafter with the optimal condition. The results reveal that the presented approach is capable of producing microtools with complex shapes effectively.

1. Introduction

One of the challenging works of micromachining technologies is to fabricate proper metal tools, for example, micropins, microelectrodes, and micromilling cutters. These metal tools can be used to machine a variety of microstructures in different industries such as aerospace, biomedical, automobile, healthcare, and consumer electronics [1, 2].

There are several approaches of microtool fabrication that have been developed over the past decade. These approaches are mainly based on technologies such as wire electrodischarge grinding (WEDG), microelectrodischarge machining (micro-EDM), and electrochemical etching. WEDG and micro-EDM are well known thermal processes which remove material by repeated discharge caused by pulsed voltage applied between tool electrode and workpiece. Sheu developed a hybrid technique combining WEDG technology with one-pulse electrodischarge to fabricate multimicrospherical probes with diameters between 15 μm and 60 μm [3]. Egashira et al. presented cemented tungsten carbide microcutting tools of 3 μm diameter produced by EDM technology [4]. Cheng et al. introduced an approach of fabricating micromilling tool with helical surfaces applying EDM process [5]. However, WEDG and EDM processes suffer through tool wear and microcracks on machined surface due to thermal effects [6]. Electrochemical etching process has been studied by researchers for making metal microtools because it removes material by anodic dissolution that does not produce any residual stress and heat affected zone. For instance, Choi et al. utilized electrochemical etching technology to generate ø5 μm shaft on tungsten carbide with H2SO4 solution as an electrolyte [7]. Nevertheless, the electrochemical etching process can only fabricate microtools with rotational structure [8, 9].

Wire ECM process has been presented in 1980s and continuously been paid attention to by research community [1012]. A metal wire is employed as tool electrode rather than being preformed cathode during the wire ECM process. Comparing with preformed tool electrode, wire electrode is inexpensive and flexible to cut complicated shapes with no need for large power supply. Since 2000s, scientists have started applying wire ECM to micromachining research [1315]. Kim et al. presented microgrooves array with 20 μm width produced by wire ECM on material 304SS [13]. Zhu et al. established a theoretical model trying to predict cutting slit width of microwire ECM using nanosecond pulse voltage [14]. Zeng et al. discussed mass transport within machining zone of microwire ECM and demonstrated machined microgrooves and slots on stainless steel [15]. These results reveal that microwire ECM technology is a promising process for producing three-dimensional features with high flexibility regardless of material’s mechanical characteristics.

Tungsten material is considered as a favorable material for microtool due to its specific characters, for example, good electrical conductivity, thermal conductivity, and corrosion resistance. Literature very much lacks for research and experiments of using wire ECM to produce microtungsten tools with complex shape. The aim of this work is to study the feasibility of fabricating microtool on tungsten material applying wire ECM process. The effects of pulse-on time, applied voltage, wire feeding rate, and solution concentration on overcut and machining stability were experimentally investigated on a developed apparatus. Thereafter, fabrication of various shaped microtools by presented approach was followed.

2. Principle of Microtool Fabrication Using Wire ECM

The microwire ECM process, as described by Kim et al., uses a metal wire as cathode to electrochemically cut metals with working voltage applied between cathode and anode, which are immerged into electrolyte [13, 14]. Figure 1 shows principle of microtool fabrication using wire ECM. A metal rod is regarded as a targeted workpiece which is able to be moved vertically (linear motion on axis) and rotationally (rotation on axis) and linked to positive pole of power source. A microwire with diameter of tens of microns is employed as cathode and connected with negative pole of the power. The microwire can be linearly fed along axis which is perpendicular to workpiece rotating axis ( axis). As a result, the proposed process is able to produce microtool with varied shapes under coupling of linear motion of wire electrode on axis, linear motion, and rotation of workpiece on axis.

382105.fig.001
Figure 1: Principle of microtool fabrication by wire ECM.

In the case of processing tungsten by ECM, the anode surface tends to generate a thin oxidized layer when applying neutral electrolytic solutions, for example, NaCl and NaNO3. This oxidized layer is indissoluble within neutral electrolyte environment and prevents the anode participating electrochemical reaction. However, the oxidized layer is able to be removed by alkaline solution. As a result, this work selected KOH as electrolytic solution.

3. Optimization of Process Parameters: Experiments and Discussion

Influences of several key process factors, that is, working voltage, pulse-on time, wire feeding rate, and solution concentration, on overcut and machining stability were investigated so as to acquire optimal parameters for fabricating microtools. A ø20 μm wire electrode was electrochemically etched through a ø300 μm tungsten wire under 2 M KOH solution at room temperature of 25°C, which was detailed and described by Zhu et al. [14]. A dynamometer was used to help fix the wire electrode on a clamp with tension force of 1.5 N. The nonmachining area of wire electrode was insulated by silica gel in order to avoid the impact of additional electrical field on the cutting result.

Figure 2 shows a general geometrical cutting model of microwire ECM, where is the interelectrodes’ frontal gap, is the machining side gap, is cutting slit width at entrance, is wire diameter, and is wire feeding rate. The slit lateral overcut was defined as . The targeted workpiece was a tungsten rod of ø300 μm. A series of slits were cut by feeding wire electrode at 100 μm distance along axis under various processing parameters. The obtained cutting slit was photographed to assess overcut at entrance. The number of short circuits that occurred during cutting process was recorded to evaluate machining stability.

382105.fig.002
Figure 2: Geometrical model of microwire ECM.

The experimental conditions were electrolytic solution of KOH with concentration between 0.6 M and 1.1 M, applied voltage from 3 V to 5 V, pulse rate of 1 MHz, pulse-on time from 110 ns to 210 ns, and wire feeding rate from 0.1 μm/s to 0.3 μm/s. A threshold number of ten was set up to identify the success or failure of each cutting experiment in terms of short circuit. A cutting test was regarded as failure if over 10 times short circuit has been observed during process procedure. Slit width or lateral overcut would not be discussed for that case as well. Figure 3 demonstrates a typical cutting slit obtained by wire ECM.

382105.fig.003
Figure 3: Cutting slit obtained using 4.5 V working voltage, 1 MHz pulse rate, 150 ns pulse-on time, 0.8 M-KOH solution, and 0.2 μm/s wire feeding rate.
3.1. Influence of Working Voltage

Table 1 lists the experimental results at various working voltages. Constant machining parameters were pulse rate of 1 MHz, wire feeding rate of 0.2 μm/s, pulse-on time of 150 ns, and solution concentration of 0.8 M. Figure 4 plots the cutting slit width and overcut at working voltage varied from 4 V to 5 V. Figure 5 illustrates the machining stability in terms of short circuits observed during process. It shows that cutting slit and overcut increase with working voltage. Applied voltage at 4.5 V and 4.7 V shared best machining stability among those results. The cutting process with working voltage of 3 V and 3.5 V had short circuit too frequently to complete machining target.

tab1
Table 1: Experimental results at various working voltages.
382105.fig.004
Figure 4: Slit width and overcut at various working voltages.
382105.fig.005
Figure 5: Short circuits at various working voltages.

From ultrashort pulsed ECM point of view, reaction current ( ) and anode dissolution rate ( ) in one pulse period are able to be described as follows [13]: where is the exchange current density, is the transfer coefficient, ( is the Faraday constant, is the gas constant, and is the temperature), is on-time voltage, is time variable, is pulse period, is pulse-on time, is time constant, and is the electrochemical equivalent number. Therefore the anode dissolution rate is proportional to reaction current ( ) which is dependent on pulsed voltage ( ) applied between electrodes. This explains why cutting slit and overcut increase continuously with on-time voltage from 4 V to 5 V.

There are several factors influencing machining stability of microwire ECM process, that is, anode dissolution rate, wire feeding rate, and reaction products transport rate. The problems of driving out reaction products and metal removal rate must be considered simultaneously as they may cause the microtool to touch the workpiece when any one of the two factors is under poor situation [16]. In addition, the wire feeding rate should not exceed anode dissolution rate in order to prevent any touch between microwire and workpiece. At the working voltage of 3 V and 3.5 V, the cutting procedure exhibited severely poor machining stability because anode dissolution rate was far behind wire feeding rate. At the working voltage of 4.5 V and 4.7 V, the anode dissolution rate matched wire feeding speed and reaction products were transported promptly, thus achieving best machining stability. However, the machining stability became worse and short circuits were observed at applied voltage of 5 V due to reaction products (sludge and bubbles) transport rate which was inferior to anode dissolution rate.

3.2. Influence of Pulse-On Time

Table 2 shows the experimental results at various pulse-on times under condition of working voltage of 4.5 V, pulse rate of 1 MHz, wire feeding rate of 0.2 μm/s, and solution concentration of 0.8 M. Figure 6 plots the cutting slit width and overcut, and Figure 7 illustrates the machining stability in terms of short circuits. It indicates that overcut rises with pulse-on time and the machining process has a high stability at pulse-on time between 150 ns and 190 ns among results.

tab2
Table 2: Experimental results at various pulse-on times.
382105.fig.006
Figure 6: Slit width and overcut at various pulse-on times.
382105.fig.007
Figure 7: Short circuit at various pulse-on times.

The anode dissolution rate is proportional to pulse-on time according to the double layer theory of ultrashort pulsed ECM (2). Increased pulse-on time ( ) will cause anode dissolution rate ( ) to rise and therefore enlarge lateral overcut in the meantime. It is convincing that if pulse-on time ( ) is greater than time constant ( ), the electrode’s double layer is able to be fully charged in that period and electrochemical reaction is regarded as a stable procedure. On the contrary, the electrode’s double layer is undercharged if is less than and therefore the electrode potential cannot reach the necessary value to maintain electrochemical reaction in a stable level. This explains why the cutting procedure could not be complete at pulse-on time of 110 ns and was unstable at pulse-on time of 130 ns. At pulse-on time of 210 ns, short circuit phenomenon was observed again because sludge and gas bubbles generation rate exceeds its transportation rate in this case.

3.3. Influence of Wire Feeding Rate

Table 3 displays the experimental results at various wire feeding rates under condition of working voltage of 4.5 V, pulse rate of 1 MHz, pulse-on time of 150 ns, and solution concentration of 0.8 M. Figure 8 illustrates cutting slit width and overcut at wire feeding rate of 0.1 μm/s and 0.2 μm/s. Applying feeding rate of 0.3 μm/s was unable to perform the process because of frequent short circuit phenomenon. It reveals that high wire feeding rate is capable of achieving small lateral overcut and less risk of short circuit at the same time.

tab3
Table 3: Experimental results at various wire feeding rates.
382105.fig.008
Figure 8: Slit width and overcut with various wire feeding rates.

The wire feeding rate ( ) can be defined as on the classical electrochemical machining theory basis, where is the current efficiency, is electrochemical equivalency of anode metal, is electrolyte conductivity, is applied working voltage between electrodes, and is the interelectrodes frontal gap. Therefore, the interelectrodes frontal gap ( ) is inversely proportional to wire feeding rate when other parameters are treated as constant. On the other hand, is a function of cutting side gap ( ) which can be described as in microwire ECM process [14]. Consequently, the wire feeding rate is inversely proportional to slit width which is affected by frontal gap ( ). This explains why cutting result of wire feeding rate 0.2 μm/s possesses smaller overcut than that of 0.1 μm/s. The wire feeding rates of 0.1 μm/s and 0.2 μm/s show excellent machining stability according to the results. The failed test of 0.3 μm/s feeding rate attributes to unmatchable condition between anode dissolution rate and wire feeding speed.

3.4. Influence of Solution Concentration

Table 4 presents the experimental results at various solution concentrations under machining condition of working voltage of 4.5 V, pulse rate of 1 MHz, wire feeding rate of 0.2 μm/s, and pulse-on time of 150 ns. Figure 9 illustrates the cutting slit width and lateral overcut of this test. Figure 10 demonstrates machining stability in terms of short circuits recorded in the process. It indicates that overcut increases with solution concentration from 0.7 M to 1.1 M. Concentrations of 0.9 M and 1.0 M have the highest stable machining among these results.

tab4
Table 4: Experimental results obtained at various solution concentrations.
382105.fig.009
Figure 9: Slit width and overcut with various solution concentrations.
382105.fig.0010
Figure 10: Short circuit with various solution concentrations.

Solution concentration certainly affects electrolyte conductivity in ECM process. The waviness of electrolyte conductivity ( ) influences interelectrodes front gap ( ) according to equilibrium gap theory of . Therefore, the interelectrodes front gap ( ) increases with electrolyte conductivity assuming other parameters are constant. As discussed in Section 3.3, cutting side gap ( ) is proportional to interelectrodes front gap ( ). As a result, lateral overcut rises with solution concentration.

However, the anode dissolution rate is supposed to be in a low level when electrolyte conductivity becomes low. This will cause wire feeding rate to exceed anode dissolution rate and consequently lead to touch between microwire and workpiece. This situation occurred when solution concentration is lower than 0.8 M according to the experimental results. At a concentration of 1.1 M machining, short circuit was observed again because sludge and bubbles transport rate fell behind anode dissolution rate.

4. Fabrication of Microtools

Several shaped microtools were made by the presented process with optimal process parameters from the previous experiments. The machining conditions were pulsed voltage of 4.5 V, pulse-on time of 150 ns, pulse rate of 1 MHz, wire feeding rate of 0.2 μm/s, and solution concentration of 0.8 M. The consideration of using solution concentration of 0.8 M is that it has smaller overcut than 0.9 M and only one time short circuit during the experiments in Section 3.4; consequently better cutting precision can be obtained with a relatively high machining stability. Figure 11(a) displays a produced semicylinder tool with high aspect ratio. Figure 11(b) presents a produced thin slice microtool with 224 μm in length and 21 μm in width. Figure 11(c) exhibits a produced triangular prism with 100 μm in edge length.

fig11
Figure 11: Fabrication of various shaped microtools.

5. Conclusions

This work presented a new approach of fabricating micrometal tool with complex shapes. Effects of pulse-on time, applied voltage, wire feeding rate, and solution concentration on overcut and machining stability were experimentally evaluated and optimal processing parameters were obtained accordingly. Fabrication of metal microtool with various shapes was carried out under the optimal machining condition. The implications conducted from this study are as follows.(i)As a novel approach, the wire ECM process possesses capability of producing complex shaped micrometal tool effectively, with coupling motion control between rotation of workpiece and horizontal and vertical feeding of microwire.(ii)Pulse-on time, working voltage, and solution concentration influence the overcut of cutting slit markedly. The machining stability correlates with anode dissolution rate, wire feeding speed, and reaction products transport rate, which must be considered simultaneously to achieve a steady machining condition.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was conducted under the sponsorship of the National Natural Science Foundation of China (51375238) and the Jiang Su Natural Science Foundation (BK20131361). Zhuang Liu gratefully acknowledges the China Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents as well as Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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