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Influence of vibration on micro-tool fabrication by electrochemical machining
International Journal of Machine Tools & Manufacture 64 (2013) 49–59
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International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Influence of vibration on micro-tool fabrication by electrochemical machining B. Ghoshal, B. Bhattacharyya n Production Engineering Department, Jadavpur University, Kolkata-700032, India
a r t i c l e i n f o
abstract
Article history: Received 14 March 2012 Received in revised form 25 July 2012 Accepted 31 July 2012 Available online 23 August 2012
Recent trend in societies is to have micro products in limited space. Efficient micromachining technologies are essential to fabricate micro products which in turn will be helpful in saving material, energy and enhancing functionality. For micromachining, micro tool is very much essential. This paper is aimed at finding the most suitable and quickest method of micro tool fabrication by electrochemical machining. Tungsten micro tools were fabricated at different machining conditions to know the influences of voltage, frequency of tool vibration, amplitude of vibration of tungsten tool, concentrations of electrolyte and dipping length of tool inside the electrolyte. Fabrication of uniform diameter of micro tool is possible at each applied voltage starting at 2 V to higher volt utilizing vibration with appropriate amplitude. Good quality micro tools with different shapes can be fabricated by controlling a proper diffusion layer thickness within a very short time introducing the vibrations of micro tool. Finally, the fabricated micro tools were applied for machining precise micro holes and micro channel using electrochemical micromachining (EMM). & 2012 Elsevier Ltd. All rights reserved.
Keywords: Micro tool Vibration Electrochemical machining Tungsten tool fabrication
1. Introduction Electrochemical machining (ECM) was introduced in the late 1950s and early 1960s in aerospace and other heavy industries for shaping and finishing operations. ECM can be thought of a controlled anodic dissolution at atomic level due to flow of current through an electrolyte which can be water based neutral salt, dilute acid or alkali. Micro-machining refers to material removal of small dimensions ranging from 1 mm to 999 mm. Advanced micro-machining may consist of various ultra-precision machining activities such as micro-holes, slots, complex surfaces in large numbers, sometimes in a single work piece, especially in the electronic and computer industries. Micromachining technology plays a vital role in miniaturization of components in the fields of aviation (cooling holes in jet turbine blades), automobile, biomedical, electronics, sensors, computer, chemical micro-reactors and micro-electromechanical systems (MEMS) etc. Electrochemical machining (ECM) processes are thermal free and material removal takes place due to atomic level dissociation, thereby giving stress free and excellent surface finish. Chemical machining cannot be controlled precisely in this micromachining domain. When anodic dissolution is applied to
n
Corresponding author. Tel.: þ91 033 24146153; fax: þ 91 033 24137121. E-mail addresses: [email protected] (B. Ghoshal), [email protected] (B. Bhattacharyya). 0890-6955/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmachtools.2012.07.014
the micro-machining range of applications for manufacturing of ultra precision shapes, it is called Electrochemical Micromachining (EMM). EMM appears to be a very promising micromachining technology due its advantages that include no tool wear, absence of stress/burr, high material removal rate (MRR), bright surface finish, ability to machine complex shapes regardless of hardness, better precision and control, rapid machining, reliable, flexible, environmentally acceptable (electrolyte is less pollutant) and it also permits machining of chemically resistant materials like copper alloys, stainless steel, titanium and super alloys, which are widely used in bio-medical, electronic and MEMS applications. Recent trend in societies is also to have micro products in limited space. Micromachining technologies are thus helpful in saving material, energy and enhancing functionality. For micromachining, micro tool is very much essential. In recent years there is significant breakthrough in fabrication of microprobes, slender micro pin and micro tool etc. by etching process. Problems generally encountered during fabrication process are the nonuniform cross sectional shape of slender pin and poor surface quality. Previously micro tools up to |100 mm were used to be fabricated by reverse electro discharge machining, wire electro discharge grinding. But below |100 mm micro tool fabrication, cost becomes very high because of low productivity. ECM method has drawn attention nowadays because of low cost method of production of micro tool. Fan et al. [1] used pulsed power supply to fabricate the microelectrode and discussed the effect of pulsed discharge, duty ratio and linear variation of applied voltage on the shape of
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microelectrode. The need of ‘off time’ in a pulse was emphasized for removal of dissolved product away from the anode surface thereby reducing the thickness of diffusion layer. Fan and Hourng [2] discussed the effect of rotation of anodic tool on the diffusion layer and rate of dissolution with the increasing rotational speed. The tungsten rod was dipped as anode and nickel rod as cathode to a particular depth side by side, maintaining a desired inter electrode gap and etched by a D.C. current. It was observed by the authors that minimum time of 10 min of etching was required before the drop down of micro tool. Lim and Kim [3] used 5 M KOH solution as an electrolyte and etched tungsten rod with D.C current. Authors reported the presence of diffusion layer, which along with geometry and voltage forms the shape of micro tool. Wang et al. [4] used D.C. voltage, 2 M KOH electrolyte and cathode ring of inner diameter 8 mm to fabricate micro tool, controlling voltage and current density. But etching time was 20 min for shaping very small diameter from |300 mm. Choi et al. [5] fabricated WC micro-shaft at optimal 1.5 M H2SO4 solution because it can dissolve tungsten and cobalt simultaneously. Tungsten rod was dipped as anode and Platinum as cathode side by side maintaining a desired inter electrode gap and etched by a D.C. current. Skoczypiec [6] applied ultrasonic vibration to the cathode during electrochemical machining and analyzed the effect of ultrasonic vibration on the flowing electrolyte through the interelectrode gap and consequently, the change of conditions of electrochemical dissolution process. Yang et al. [7] used semi-cylindrical tool along with the ultrasonic vibrations for the drilling of deep holes with reduced machining time and machining gap. Forster et al. [8] introduced oscillating tool with 50 Hz frequency and large amplitude of 200 mm for die sinking in stainless steel with the objective of improving shaping accuracy and smooth surface. With all the above research reports it is clear that some efforts have been given to control the thickness of the diffusion layer by the rotation of anode, linear variation of voltage and variation of pulse parameters etc. during fabrication of micro tools. Ultrasonic vibrations and vibrations with low frequency have also been used during electrochemical machining with flowing electrolytes but not with tool fabrications at stagnant electrolyte. For all those investigations of micro tool fabrications, the times of machining of micro tools were considerably high. In this study, a stainless steel ring of 1600 mm diameter was used as cathode and |300 mm straight tungsten rod was immersed centrally to a certain depth for the electrochemical machining, applying vibration to the anodic tungsten rod. Variation of amplitude of vibration, frequency of vibration, voltage, electrolyte concentration and dipping length of tool inside the electrolyte have been investigated during micro tool fabrication. After analysis of the effects of the above mentioned parameters, proper condition of electrochemical machining has been suggested for the cylindrical micro tool fabrication within a very short time. Except the tip portion a series of straight micro tools have been developed up to very small diameters within 1–5 min depending on applied voltage, proper amplitude of vibration and frequency of vibration. Thus, vibration of the tungsten rod has been considered as an important parameter during the fabrication of micro tool for the disruption of the diffusion layer and enhanced diffusion and convection of ions due to hydrodynamic effects on the bubble behavior. For establishing this fact, in depth investigation has been performed and test results supported by various micrographs of fabricated micro tools, have been analyzed.
2. Electrochemical machining of tungsten micro tool 2.1. Principle of micro tool fabrication The micro tool is fabricated based on the Faraday’s two laws of electrolysis. Tungsten tool is connected to positive terminal and a
stainless steel ring is connected to negative terminal of DC supply. In between cathode and anode, NaOH electrolyte of proper concentration is applied. The anodic tungsten tool dissolves based on current density distribution. Coulomb’s law states that the electric field intensity, E applied to the micro tool is proportional to the charge density, q and inversely proportional to the square of the distance between the micro tool and the stainless steel ring, R, as given by the Eq. (1). q Ea 2 R
ð1Þ
Mass of tool that dissolves is transferred away from it, resulting in flow of mass transfer current. Mass transfer occurs due to migration, convection and diffusion of ions. Migration of ions depends on the electric field intensity. Again, electric intensity at any point in an electric field is equal to the potential gradient at that point. In other words, E is equal to rate of fall of potential in the direction of current flux and given by dV E¼ dX
ð2Þ
Convection of ions occurs due to the bodily movement of electrolyte and can be effected by flow of electrolytes or vibration of tool. Diffusion is the movement of ions from a region of higher concentration of ions, to their region of lower concentration through the concentration gradient. Mass transfer current or current density is enhanced due to diffusion process and thereby, MRR increases. But, Nernst diffusion layer is a layer of similar charges within which, gradient of the ion concentration is constant. This layer hinders the flow of current. The current density, J across the diffusion layer is well known and given by [9,10]. J¼
eDðC 0 -C d Þ
d
ð3Þ
where, D is effective diffusion coefficient, e is the electronic charge, C0 is tungsten ion concentration i.e. no. of ions per unit volume at the surface of tungsten, Cd is the tungsten ion concentration at the end of the diffusion layer and can be taken as the concentration of bulk solution and d is the diffusion layer thickness. Neglecting diffusion layer, it can be assumed that the removed amount of material from the micro tool is proportional to E. A simulation of the zone between the micro tool and ss-304 ring is analyzed as shown in Fig. 1. A stainless steel plate (SS 304) ring of thickness 300 mm is used as cathode and |300 mm tungsten rod is used as anode for the micro tool fabrication. The analysis has been done with the following assumptions: (i) The electric parameters are stable, (ii) The conductivity of the electrolyte remains constant, and (iii) The concentration gradient in the bulk electrolyte is negligible. The electrical potential, V in the interelectrode gap obeys the Laplace’s equation.
r2 V ¼ 0
ð4Þ
Boundary conditions are as follows: V9G1n2n3 ¼ 0 V ðat the cathodic ringÞ
ð5Þ
V9G4n5n6 ¼ 5 V ðat the anodic tool surfaceÞ
ð6Þ
@V @N9G7n8
ð7Þ
¼0
ðthe boundary conditionÞ
where, V ¼5 V applied between tungsten micro tool and cathodic ring and N is the surface normal. The Laplace’s equation is solved subjected to the given boundary conditions in MATLAB.
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Fig. 2. Micro tool fabrication with tool vibration.
Fig. 1. Distribution of equipotential curves in the gap of micro tool and stainless steel ring.
Equipotential distributions of curves are shown. The gradient of potential ( dV/dX) is more in the front end and lateral face of front end side i.e. distance between equipotential curves are small. From the Eq. (2) it can be predicted that in absence of diffusion layer, the rate of dissolution of micro tool will be highest at the front end and lateral face of front end side. At the low voltage, the chance of diffusion layer formation is less, as the rate of dissolution of metal will be less and hence, can diffuse away to the bulk solution. The conical tool formation is predicted. At the interface of electrolyte and air, the gradient of potential is high and hence higher dissolution can also be predicted for this zone. Later on, practical experiments continued to verify the simulation results. 2.2. Analysis of ECM with tool vibration Vibrations within the machining zone, at the stagnant electrolyte, have considerable influence on the diffusion and convection of dissolved metal ions due to hydrodynamic effects on the bubble behavior. An oscillatory vibrating tool transverse to the ring is applied as shown in the Fig. 2. The tool moves back and forth over the same path periodically. When the tool starts its motion from rest i.e. t ¼0, displacement, X¼0, the equations of motion are as follows: X ¼ A sinot
ð8Þ
V ¼ A o cosot
ð9Þ
where, A is termed as amplitude of vibration, the maximum displacement from mean position, o is angular velocity, t is instantaneous time, V is the velocity at time t. Eq. (9) explains that the velocity of the micro tool is variable during each cycle of vibration. The work is done on the viscous electrolyte by the vibrating tool having variable kinetic energy due to variable velocity. The amount of energy transfer due to tool vibration is given by: 1 PQ ¼ mV 2 2 1 or P ¼ re V 2 2
ð10Þ
where, P is the pressure developed inside IEG, Q is the volume of electrolyte displaced and m is the mass of electrolyte which is given a thrust, re is the density of the electrolyte. During the upward motion of the micro tool, there will be pressure drop in the inter-electrode gap (IEG) which can be explained from the
Eq. (10). This pressure drop breaks the forces holding the liquid molecules together and generates micro bubbles of electrolyte vapor or transient cavitation [11]. When the anodic micro tool moves downward, the pressure increases and rapid collapse of micro bubbles occur with the generation of temperature and pressure [12]. As the anode itself is vibrated, the bubble collapse will be maximum very near to the anodic tungsten tool. The potential energy of expanded cavity is converted to kinetic energy of liquid jet motion during the energetic collapse. The impact of the micro-jets on the anodic surface, results in the enhanced convective mass transport of dissolved ions, disruption of diffusion layer and supply of fresh electrolyte. Moreover, increased pressure drives out hydrogen bubbles from the IEG resulting in the improved conductivity. Depending on amplitude and frequency of vibration, there are possibilities of periodic size oscillation of bubbles, in the case of stable cavitation or escape from the solution due to mass convection. The vibration has important effect on the diffusion layer, which is formed when the rate of metal dissolution is greater than the rate at which the metal ions can diffuse away from an electrode. Concentration overpotential comes into play under the situation and current density at the zone of diffusion layer decreases. The extremely fast bubble collapse usually occur in less than one microsecond time [13]. These conditions are conducive for the quick charge transport and depolarization. Thus, vibration increases current density due to increase of convective mass transport, increase in the diffusion rate and improved conductivity. 2.3. Developed experimental set up Considering the influence of various process parameters, a well planned research program has been taken up for the experimentation in the developed EMM set up [14–16]. The set up consists of various sub-components such as electrical power and controlling system, stepper motor along with microprocessor for motion control of tool and job, Mechanical unit for holding the tool and job and Piezoelectric transducer (PZT) for vibrating the tool longitudinally. PZT is also very helpful for easy supply of electrolyte to the interelectrode gap (IEG) during electrochemical micromachining (EMM), thereby material removal becomes possible at very small diameter (10 mm) drilling or micro channel formation. PZT is connected to 230 V main power supply through amplifier module ( 20 V–130 V) for feed control in the nanometer range (resolution 0.12 nm) having maximum stroke 66 mm and a modulation input from a function generator for controlling the amplitude of vibration which varies with the variation of voltage. Frequency of vibration of the PZT is controlled by the frequency of input voltage. Electrical power and control is comprised of a function generator for the supply of pulsed DC voltage to the tool and work piece. Digital storage oscilloscope acts as data collection system which is inevitable for the control of inter electrode gap (IEG) during EMM. Stepper motors are used to give motion of the
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Fig. 3. Experimental set up for micromachining and micro tool fabrication.
tool and work piece along X, Y and Z direction. However, for very low feed rate and accurate control of IEG ( o10 mm), feed by PZT is preferred. Fig. 3 shows the various units of the developed experimental set up of EMM, used for the micro tool fabrication.
3. Planning for experimental analysis The designed and developed experimental set up is used for the micro tool fabrication and electrochemical micromachining purpose. During the fabrication of micro tool, a lot of experiments were performed to study the influence of various predominant parameters such as machining voltage, electrolyte concentration, immersion length of tool, amplitude of vibration of tool and frequency of vibration of tool etc. Applied voltage is of DC (frequency 1 mili Hz, period 1000 sec and pulse width 999 sec) from a function generator and is noted during the micro tool fabrication by a digital storage oscilloscope, DL 750 (YOKOGAWA, JAPAN). Micro tool measurements are performed by microscope (OLYMPUS, JAPAN) with a least count of 0.0005 mm through the lenses of 3 –50 . Images are taken with the help of software from the microscope. Cylindrical tools are mostly preferred for micromachining. To find the machining accuracy, 150 mm is excluded from the tip of fabricated tool as this portion is cut by electrochemical micromachining for getting final micro tool. At a gap of 400 mm, the readings of diameter are taken as D1, D2, D3, D4 and D5 as shown in Fig. 4, to measure the average diameter (Davg). To evaluate the deviation from the cylindrical shape, standard deviation (s) is calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðD1 Davg Þ2 þðD2 Davg Þ2 þ ðD3 Davg Þ2 þ ðD4 Davg Þ2 ð11Þ s¼ 4 Effect of voltage, frequency of vibration, amplitude of vibration and concentration of electrolyte are investigated on the final shape of the fabricated micro tools and finally, attempts have been made to achieve the optimum parametric conditions for the fabrication of cylindrical micro tool.
Micro tools developed were also used as cathode tool during the EMM operation for the machining of micro holes and micro channels. EMM was performed using 0.2 M H2SO4 electrolyte based on the ultra-short voltage pulse of 5 MHz frequency. H2SO4 was chosen as electrolyte because sludge is dissolved fully in the electrolyte, thereby reducing the side gap of micromachining and improving surface quality [17,18]. The reasons for choosing 0.2 M concentration of electrolyte are less machining time and generation of same side gap during machining of microchannel with 0.1 M and 0.2 M electrolyte. During drilling operation, it was found that the side gap at 0.1 M is less than that at 0.2 M. At 0.5 M concentration, machining became difficult and frequent short circuit occurred. Ultra-short pulse voltage of 5 MHz frequency was chosen for localization of current at the intended area of machining [18].
4. Fabrication of tungsten micro-tool Small pieces of tungsten rods are sheared from long tungsten rod of |300 mm diameter and ends are flattened by fine grinding and straightened. Then, the tool specimen is connected to the positive terminal of function generator and stainless steel ring of diameter 1600 mm is connected to the negative terminal. The tool specimen is centered to the hole and immersed vertically to a certain dipping length into NaOH electrolyte. Diameter variation of the fabricated tool can be kept within 70.5 mm tolerance at an appropriate combination of machining voltage and amplitude of vibration. Overall objective is to fabricate low cost and high production volume of micro tools with repeatability of production. When voltage is applied between anode and cathode the tungsten dissolves at the anode by the following reactions: Wþ 6OH -WO3 þ 3H2 O þ6e
ð12Þ
WO3 þ2OH -WO2 4 þ H2 O
ð13Þ
The electrochemical reaction on the cathode is: 6H2 Oþ 6e -3H2 m þ 6OH
ð14Þ
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Fig. 4. Micro tool measurement positions to get average diameter and std. deviation.
Fig. 5. Sharp conical shape formation at voltage 1.5 V.
In the reaction (12), the metal tungsten is oxidized to tungstate (WO3). In the reaction (13), the tungstate becomes ion (WO2 4 ) by reacting with excess hydroxyl ions and remains dissolved in the electrolyte. In the reaction (14), water is converted to hydroxyl ions and hydrogen evolves speedily at the cathode.
5. Results and discussions Experiments have been conducted in the present set up to analyze the effect of vibration and other parameters during the micro tool fabrication. In the present system |300 mm tungsten rod was dipped up to 900 mm inside the electrolyte (2 M NaOH), to start experimentation. However, machining reaction started at lower dipping length at high concentration (4 M NaOH). This happens due to the double layer phenomena. The cathode (stainless steel ring) is immersed fully in electrolyte; thereby voltage drop due to double layer potential in the interface between cathode and electrolyte is small. Whereas, large voltage drop occur between small surface area of micro tool (dipped area of anode) and electrolyte.
5.1. Influence of voltage on micro tool fabrication The applied voltage is one of the important parameter for the removal of material. Therefore, the investigation was started with variation of voltage. When applied voltage was less than 1.5 V, sharp conical formations of micro tool was found as shown in the Fig. 5. Here conical angle formed is 5.541 having diameter 64 mm at a distance of 712 mm from the tip of the micro tool. At this low voltage the rate of dissolution of tungsten metal is low and machining time was 18 min in 2 M NaOH electrolyte. Thus formation of diffusion layer is either minimum or absent as the rate of migration of ions will be equal to rate of formation of hydrated tungstate ions. As predicted by the simulation result at Fig. 1, the current density is high at front end and lateral face of front end of micro tungsten rod, resulting in higher rate of removal at the front end of tool. The tool becomes conical as predicted by the simulation result.
Fig. 6. (a) 70 s of machining (b) 105 s of machining (c) 130 s of machining (d) 135 s of machining.
Fig. 6 exhibits the formation of reversed conical shape of micro tool for the applied voltage greater than 2 V, without application of vibration. The machining was done at high voltage of 8 V without vibration. Evolution of shape is shown in four steps, initial dia. |300 and 2M/L NaOH electrolyte was used. Fig. 6(a) shows that after 70 s of machining, slight necking at the shank portion is observed as predicted by the simulation result. Fig. 6(b) describes the shape after 105 s of machining, where prominent reversed taper is observed. Fig. 6(c) shows that the front portion is about to drop off due to critical necking, after 130 s of machining. Fig. 6(d) shows the drop off after 135 s of machining. At higher voltage, the micro tool cuts off from the necking portion without much reduction in diameter. Necking formation
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5.2. Influence of vibration on micro tool fabrication Vibration has hydrodynamic effects on the bubble behavior, which has been utilized for the effective removal of sludge, hydrogen bubbles and replenishment of fresh electrolyte. The tool was vibrated longitudinally with definite combination of amplitude of vibration and frequency of vibration. Influence of these two important vibration parameters on the fabrication of micro tool is discussed here. 5.2.1. Effect of amplitude of vibration Amplitude of vibration has the most significant effect on the growth of cavitations bubbles and energetic collapse. Higher amplitude with the fixed frequency means higher velocity of the tool as can be explained from the Eq. (10). Again, higher velocity means higher pressure rise or fall, which in turn creates larger number of cavitations bubbles and more energetic collapse. As a result, the micro jets impinge on the surface of the tool breaking the diffusion layer and increasing the convective mass transport. Fig. 9(a)–(c) shows the shape evolution at 7 V applied
1200
1000
800
600
400
200
0 0
1
2
3
4
5
6
7
8
Applied voltage (V) Fig. 7. Maximum possible machining time without vibration at different voltages.
3500 3000
Length of tool (μm)
can be explained from the simulation result of Fig. 1. The current density is high at the interface between air and NaOH electrolyte in the absence of diffusion layer. Thus, the tools having high aspect ratio cannot be fabricated at high voltage without the help of vibration. Generation of reversed conical shape can be explained by the formation of diffusion layer [1,2]. With the application of high voltage, rate of dissolution of tungsten tool being fabricated is very high but because of limitation of migration of these cations to the bulk solution, accumulation of ions near the tungsten tool take place. At the very beginning of the applied voltage, current density is highest at front end and lateral face around it, as is evident from the simulation shown at Fig. 1. Hence, diffusion layer thickness growth is also highest at the front end and gradually decreases upwards. At the interface between electrolyte and air, the diffusion layer thickness around micro tool is low. Rising H2 gas bubbles bursts while reaching at the interface of air and electrolyte. Thereby, energy of burst breaks the diffusion layer at the interface between electrolyte and air. From the Eq. (3) it is evident that the current density is inversely proportional to the thickness of the diffusion layer. Thus, after the growth of diffusion layer, highest dissolution takes place at the shank portion, where diffusion layer is thin. Fig. 7 shows the machining time required to attain the lowest diameter or complete dissolution or drop off whichever is earlier with the variation of applied voltage. For example at 1.4 V, the maximum machining time is 1140 s (19 min) and within this time, machining is to be stopped to get a certain diameter of micro tool. Similarly at 7 V, machining time is 140 s. Within this time, machining is to be stopped to get a visible tool otherwise, drop off may take place. At complete dissolution, there will be no charging of double layer and hence, the applied voltage remains constant and can be monitored by the oscilloscope as zero voltage drops. Fig. 8 shows an important observation that fabricated micro tool length increases with the increase of applied voltage though the immersed tool length is 900 mm. Initially, the length of micro tool increases linearly up to 7 V and then increase in length is steep. Machined length is always greater than the dipping length of the tool. The electrolyte level over the tool is higher than the upper level of electrolyte due to surface tension. Again, as the voltage increases, the current density increases and rate of dissolution increases with the increased rate of evolution of hydrogen from the cathode ring. The temperature of the tungsten tool surface increases due to the Ohmic heating at high voltage, which further increase the level of electrolyte over the tungsten rod.
Machining time (sec)
54
2500 2000 1500 1000 500 0 0
2
4
6
8
10
12
Applied violtage (V) Fig. 8. Effect of voltage on the length of tool.
voltage, 5 mm amplitude of vibration, 2 M NaOH, 900 mm dipping length and 228 Hz frequency of vibration. Fig. 9(a) shows the conical shape evolution after 70 s of machining having average dia. |175 mm. Fig. 9(b) shows the conical shape of uniform taper with tip diameter 42 mm after 100 s of machining. Fig. 9(c) shows the shape having |13 mm at the interface of electrolyte and air, |39 mm at the middle and |16 mm at the tip after 131 s of machining. Fig. 9(d) shows the sharp taper having |9 mm tip diameter i.e. the current density becomes linearly variable, being highest at the front tip as explained in the simulation result of Fig. 1. Fig. 10 shows the comparison of standard deviation of machining between low amplitude of vibration and high amplitude of vibration at the applied voltage of 7 V. Machining is done at 2 M concentration of electrolyte, dipping length 900 mm and 240 Hz frequency of vibration. Variation of standard deviation at 7 V, 2 mm amplitude is less than steep increase of standard deviation at 7 V, 5 mm amplitude of vibration i.e. at 2 mm amplitude of vibration, the machined tool is more cylindrical than at 5 mm amplitude of vibration. For each voltage, there exists appropriate amplitude of vibration for minimum standard deviation and main idea is to make the current density uniform throughout the length. Higher voltage means higher dissolution of metal and hence more mass transport is required by convection and diffusion to avoid diffusion layer formation. Therefore, higher amplitude of vibration is needed for the higher applied voltage. Fabrication of uniform diameter of micro tool is possible at each applied voltage, starting with 2–10 V with appropriate amplitude
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Fig. 9. Shape evolution at high voltage and high amplitude of vibration (a) 70 s (b) 100 s (c) 131 s (d) 147 s.
std. deviation at 7 volt with different amplitude of vibration 35
Std. deviation (μm)
30 25 20
2 μm amplitude
15
5 μm amplitude
10 5 0 0
25
50
75
100
125
150
175
Time of machining (sec) Fig. 10. Standard deviation at low and high amplitude of vibration.
of vibration maintaining a uniform current density along the length of tool. During the machining time (147 s) at 7 V, 2 M NaOH electrolyte, 1 mm amplitude and 236 Hz frequency, it was observed from the oscilloscope that initially, the voltage drop was around half the applied voltage but after approximately 25 s, the voltage drop was very high, almost around two third of the applied voltage and towards the end of machining, voltage drop was very small. The above observation is reflected in the graph in Fig. 11 by actual volume removal rate. When the voltage is applied, no bubbles are found and clearly a wave of ion concentration gradient (migration) is visible from the micro tool towards the cathode ring. Applied electric field takes some time to start migration of ions. After 10–15 s, hydrodynamic effects on the bubble behavior starts and large no. of microbubbles are found, creating a highly unstable region between anodic tool and cathode ring. The reaction rate starts rising and reaches peak rate after 45 s and before 100 s, the rate of reaction decreases due to higher resistance to current flow, as the tool area undergoing dissolution decreases. As the amplitude of vibration increases, growth of cavitations bubbles and energetic collapse increases. The
Fig. 11. MRR increases with increase of amplitude of vibration.
current density increases and hence, MRR increases as shown in Fig. 11 at 3 mm and 5 mm amplitude of vibration. Thus cylindrical micro tool fabrication is possible at shortest time, utilizing amplitude of vibration.
5.2.2. Effect of frequency of vibration The shape of the fabricated micro tool becomes wavy and reversed taper, with the increase of frequency of vibration of the tungsten rod as shown in Fig. 12(a) where, the parameters are 300 Hz frequency of vibration, 2 mm amplitude of vibration and 3 V applied voltage, during the fabrication of micro tool. The reversed taper indicates that the cavitations phenomena diminish and disappear as the frequency is raised creating stable bubbles of electrolyte. The tungsten rod transmits vibration to the electrolyte, resulting in stretching and compression of the electrolyte. The bubbles at the rarefaction zone are expanded, decreasing the conductivity of the electrolyte and at the compression zone, the volume of bubbles decrease, increasing the conductivity of electrolyte. At the compression zone of the electrolyte, current density is high due to higher conductivity and as a result, node is
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std. deviation ( σ) at 2 Volt
25
std. deviation (μm)
Without vibration 20 With vibration frequency 228 Hz and 1 µm amplitude With vibration frequency 300 Hz and 1 µm amplitude
15 10 5 0 0
100
200
300
400
500
600
700
Machining time (sec) Fig. 13. Accuracy of machining with the variation of frequency of vibration.
300
formed. Again, at the rarefaction zone, the current density is low, due to low conductivity and as a result, antinode is formed. Nodes and antinodes formation can also be explained by the acoustic streaming phenomena [19,20]. Acoustic streaming gives rise to the circular flow along the surface of the tool at an interval of l/2 where, l is the wavelength of the transmitted wave through electrolyte. The circular flow increases convective mass transport resulting in higher dissolution at nodes. The nodes become prominent, with the increase of amplitude of vibration along with the increase of frequency, as shown in Fig. 12(b) where, 300 Hz frequency of vibration of the tungsten rod and 4 mm amplitude of vibration is applied, during the fabrication of micro tool. The end portion of the tool dropped from the remaining portion because, dissolution rate is high at high amplitude of vibration. Fig. 12(c) shows that at low frequency, there is no formation of nodes in the micro tools where, machining was done at 228 Hz frequency of vibration of tungsten rod and 2 mm amplitude of vibration. It is obvious from Fig. 12(c) and Fig. 13 that with low frequency of vibration of micro tool, the accuracy of machining increases i.e. std. deviation decreases. At high frequency of 300 Hz, 1 mm amplitude of vibration and 2 V machining voltage, std. deviation steeply increases with time. Other variables remaining same, the rate of increase of std. deviation is very low with 228 Hz frequency of vibration and it is lower than that of without vibration. Initially around 90 s of machining, std. deviation is approx. 1 mm for all the cases but at 600 s of machining std. deviation is 4 mm, 9 mm and 23 mm for 228 Hz frequency of vibration of tool, without vibration of tool and 300 Hz frequency of vibration of tool respectively. 5.3. Influence of concentration of electrolyte Fig. 14 shows the average diameter of fabricated tool at different concentrations of electrolyte after 5 min of machining at 2 V applied voltage, 1 mm amplitude of vibration and 232 Hz frequency of vibration of tungsten rod. It is obvious that material removal rate (MRR) increases with the increasing of concentration up to 4 M and then starts decreasing due to the decrease of activity coefficient at high concentration. Hydration of ions also decreases with the increase of concentration beyond 4 M. Minimum tool length (700 mm) can be machined at 4 M concentration, as the minimum required immersion length decreases with the increase of concentration. Machined micro tool length also decreases for the same dipping length with the increase of
Average diameter (μm)
Fig. 12. (a) 300 Hz and 2 mm (b) 300 Hz and 4 mm, (c) 228 Hz and 2 mm.
250 200 150 100 50 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Concentration (M/Litre) Fig. 14. Average diameter at 5 min of machining.
concentration due to high viscosity and less surface tension but standard deviation (departure from cylindrical shape) increases. In the present research, 2 M is most suitable for the machining of micro tool. However, for the fabrication of special tool tip, high concentration electrolyte is preferable as machining is possible at low dipping length and diffusion layer can be confined within the front tip only, by utilizing vibration. Fig. 15 shows the tool fabrication at concentration of 3 M NaOH, 5 V applied voltage and 228 Hz frequency of vibration. Fig. 15(a) shows that, at high concentration and low amplitude of vibration i.e. 1 mm, transient cavitations effect is small at the front tip of the tool and this type of shape of micro tools are suitable for taper less drilling in micromachining. Comparing Fig. 15(b) and (c) it is obvious that transient cavitations effect increases with the increase of amplitude of vibration. Fig. 15(b) shows the fabricated micro tool which was machined at 3 mm amplitude of vibration and that in Fig. 15(c) was machined at 4 mm amplitude of vibration. Fig. 15(d) shows an important fact that the standard deviation decreases with the increase of immersion length of the tool, along with the appropriate amplitude of vibration. Machining was done at 5 mm amplitude of vibration of tungsten rod and dipping length was increased to 1500 mm and fabricated tool length was 1970 mm. Each applied voltage has corresponding amplitude of vibration for the uniform current density on the surface of the tool.
6. Fabrication of suitable micro-tool From the earlier analysis on test results for the fabrication of cylindrical micro tool with small deviation, the values of parameters chosen as 228 Hz frequency of vibration of tungsten rod,
B. Ghoshal, B. Bhattacharyya / International Journal of Machine Tools & Manufacture 64 (2013) 49–59
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Fig. 15. (a) 1 mm amplitude (b) 3 mm amplitude (c) 4 mm amplitude (d) 5 mm amplitude. Table I Recommended applied voltage and appropriate amplitude of vibration for minimum diameter cylindrical tool fabrication. Volt (V) Amplitude of vibration(mm) Machining time (s)
10 6 60
9 5.5 70
8 4.5 100
7 4 118
6 3.5 137
5 3 158
4 2.5 208
3 2 283
2 1.5 305
350
Diameter (μm)
300 250 200 150 100 50
Fig. 17. Fabrication of micro tool in two steps.
0 0
20
40
60
80
100
120
140
160
180
Time (sec) Fig. 16. Diameter of cylindrical fabricated tool according to machining time.
2 M electrolyte concentration and dipping length 900. Fabrication was done at each and every voltage starting from 2 V to 10 V and appropriate amplitude of vibration for finding out minimum diameter cylindrical tool. Table I shows the recommended voltage and amplitude combination along with machining time for the present set of research. However, tools other than cylindrical shape are also very effective for particular type of machining some of which are also discussed in brief. Taking one combination from Table I, the machining time and corresponding diameters are shown in Fig. 16. Fabrication was done at 5 V and amplitude of vibration 3 mm. Minimum micro tool diameter of 10 mm was fabricated at 158 s of machining. The cylindrical tools of very small diameter are vulnerable to breakage in a single use if the length of the micro tool is high. To make the tool of small diameter and to maintain rigidity as well, the fabrication may be done in two or more than two steps. Fig. 17 depicts the fabrication of micro tool in two steps. Firstly, cylindrical
Fig. 18. Fabrication of micro tool in three steps.
tungsten tool of |80 mm was fabricated at 5 V and 3 mm amplitude of vibration within 2 min. Then fabrication continued at 1.5 V and without vibration for 5 min. Thus micro tool of length 150 mm, tip diameter of 1 mm and tip angle of 11.591 is fabricated. Fig. 18 exhibits the tool fabricated at three steps. Diameter reduced to 150 mm cylindrical in first step from 300 mm. Then, again reduced to
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Fig. 19. (a) Tool of 12 mm diameter (b) Entry side (c) Exit side.
Fig. 20. Blind micro channel of 50 mm depth.
|37 mm cylindrical followed by third step reduction in low voltage for conical shape generation having tip angle 261. This type of tools is suitable from rigidity point of view as bending stress will be less.
7. EMM application of micro tools A micro hole of entry diameter 24 mm and exit diameter 22.5 mm was drilled on a SS-304 plate of 35 mm thickness as shown in Fig. 19(b) and (c) with the tool of 150 mm long and 12 mm uniform diameter at the tip as shown in Fig. 19(a). EMM operation was performed at 5 MHz pulse power, pulse width 60 ns, average voltage 2.8 V, amplitude of vibration of micro tool 0.2 mm and frequency of vibration of micro tool 236 Hz. The total machining time for the generation of |22.5 mm hole was about 25 min. A blind micro channel of 1000 mm length, 135 mm width and 50 mm depth was machined on a SS-304 plate of 60 mm thickness as shown in Fig. 20. EMM was carried out at 5 MHz pulse power, pulse width 60 ns, average voltage 2.7 V, amplitude of vibration of micro tool 0.1 mm, frequency of vibration of micro tool 228 Hz and horizontal velocity of micro tool was 78 mm/s. The total machining time for the generation of micro channel was 55 min. The micro tool used for the machining is of reversed taper with tip diameter 60 mm. Such type of micro tool generates vertical side wall of micro channel.
8. Conclusions Fabrication of micro tools is essential for the manufacturing of micro components and features in micromachining area and preferably in EMM. In experimentation during fabrication of micro tools, some important observations can be drawn from the test results and analyses are as follows: (i) It is observed from the simulation of the machining zone that current density will be maximum at the front face and front side lateral face and also, at the interface between the electrolyte and air, when the diffusion layer is neglected.
(ii) Transient cavitations phenomena results in the increases the convective mass transport, disruption of the diffusion layer and replenishment of fresh electrolyte. Amplitude of vibration promotes the transient cavitations phenomena. Thus, volume removal rate increases with the increase of amplitude of vibration at a particular voltage and micro-tool fabrication is possible within the shortest time. (iii) Standard deviation increases with time both at low voltage and high voltage as the current density distribution varies with machining time. During the fabrication at 2 V, 228 Hz frequency and 1 mm amplitude of vibration, the std. deviation was 0.5 mm, after 90 s of machining and 4 mm, after 600 s of machining. During fabrication at 7 V, 240 Hz frequency and 2 mm amplitude of vibration, the std. deviation was 0.3 mm after 45 s of machining and 3 mm after 147 s of machining. (iv) Fabricated tool length increases with the increase of voltage, because of high rate of evolution of hydrogen, increase in reaction rate. Fabricated tool length was 1400 mm at 1.6 V and 3000 mm at 10 V, though dipping length was 900 mm in both the cases. (v) At high frequency nodes and antinodes are observed along with the reversed taper formation of the micro tool above 2 V applied voltage. Transient cavitations phenomena diminish and disappear as the frequency is raised, making the tool reversed taper. Nodes are formed due to acoustic streaming at the interval of half the wavelength. So both EMM and fabrication of micro tool should be done at low frequency i.e. less than 250 Hz.
Tools of various shape at the tip is very helpful for various purpose of micro machining. For micro hole generation reversed taper is essential for minimization of side gap. Similarly, micro tool tip with various end shape are also effective for channel machining or internal groove machining. Conical tools are useful for nozzle drilling as well as STM probe and stylus of various sensors. In this research paper, attempt has been made to establish the fact that micro tools of various shapes can be fabricated within shortest time utilizing amplitude of vibration. Thus, fabrication of complicated micro tools with the help of amplitude and frequency of vibration, along with the high electrolyte concentration may be possible in future and can be utilized for layer by layer machining and also, for free form micro machining.
References [1] Z.W. Fan, L.W. Hourng, C.Y. Wang, Fabrication of tungsten micro-electrodes using pulsed electrochemical machining, Precision Engineering 34 (2010) 489–496. [2] Z.W. Fan, L.W. Hourng, The analysis and investigation on the microelectrode fabrication by electrochemical machining, International Journal of Machine Tools & Manufacture 49 (2009) 659–666. [3] Y.M. Lim, S.H. Kim, An electrochemical fabrication method for extremely thin cylindrical micro pin, International Journal of Machine Tools & Manufacture 41 (2001) 2287–2296.
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[4] M.H. Wang, D. Zhu, W. Peng, Experimental research on electrochemical micromachining, Advanced Design and Manufacture to Gain a Competitive Edge, http: //dx.doi.org/10.1007/978-1-8-4800-2418-78, pp. 775–783 (Chapter 6). [5] S.H. Choi, S.H. Ryu, D.K. Choi, C.N. Chu, Fabrication of WC micro-shaft by using electrochemical etching, International Journal of Advanced Manufacturing Technology 31 (2007) 682–687, http://dx.doi.org/10.1007/s00170005-0241-4. [6] S. Skoczypiec, Research on ultrasonically assisted electrochemical machining process, International Journal of Advanced Manufacturing Technology 52 (2011) 565–574, http://dx.doi.org/10.1007/s00170-010-2774-4. [7] I. Yang, M.S. Park, C.N. Chu, Micro E.C.M. with ultrasonic vibrations using a semicylindrical tool, International Journal of Precision Engineering and Manufacturing 10 (2) (2009) 5–10, http://dx.doi.org/10.1007/s12541-009-0020-5. [8] R. Forster, A. Schoth, W. Menz, Micro-ECM for production of microsystems with a high aspect ratio, Microsystem Technology 11 (2005) 246–249. [9] J.A. McGeough, Principles of Electrochemical Machining, Chapman and Hall, London, 1974. [10] A.J. Bard, L.R. Faulkner, Electrochemical methods, 2nd ed., JohnWiley & Sons, 2001. [11] C. Banks, R.G. Compton, Voltammetric exploration and applications of ultrasonic cavitation, ChemPhysChem 4 (2003) 169–178. [12] S. Skoczypiec, A. Ruszaj, Discussion of cavitation phenomena influence on electrochemical machining process,, International Journal of Manufacturing Science and Technology 7 (2) (2005) 27–32.
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[13] K.S. Suslick, Y. Didenko, M.M. Fang, T. Hyeon, K.J. Kolbeck, W.B. McNamara, M.M. Mdlelen, M.W. Wong, Acoustic cavitation and its chemical consequences, Philosophical Transactions of the Royal Society of London Series A 357 (1999) 335–353. [14] B. Bhattacharyya, B. Doloi, P.S. Sridhar, Electrochemical micromachining: new possibilities for micro manufacturing, Journal of Materials Processing Technology 113 (2001) 301–305. [15] B. Bhattacharyya, M. Malapati, J. Munda, A. Sarkar, Influence of tool vibration on machining performance in electrochemical micro-machining of copper, International Journal of Machine Tools and Manufacture 47 (2007) 335–342. [16] A.K.M. De Silva, J.A. McGough, Process monitoring of electrochemical micromachining, Journal of Material Processing Technology 76 (1998) 165–169. [17] B.H. Kim, C.W. Na, Y.S. Lee, D.K. Choi, C.N. Chu, Micro electrochemical machining of 3D microstructure using dilute sulfuric acid, CIRP Annals 54 (1) (2005) 191–194. [18] S.H. Ahn, S.H. Ryu, D.K. Choi, C.N. Chu, Electro-chemical micro drilling using ultra short pulses, Precision Engineering 28 (2004) 129–134. [19] T.G. Leighton, The Acoustic Bubble, Academic Press, London, 1994. [20] V. Frenkel, R. Gurka, A. Liberzon, U. Shavit, E. Kimmel, Preliminary investigations of ultrasound induced acoustic streaming using a particle image velocimetry, Ultrasonics 39 (2001) 153–156.
Contents lists available at SciVerse ScienceDirect
International Journal of Machine Tools & Manufacture journal homepage: www.elsevier.com/locate/ijmactool
Influence of vibration on micro-tool fabrication by electrochemical machining B. Ghoshal, B. Bhattacharyya n Production Engineering Department, Jadavpur University, Kolkata-700032, India
a r t i c l e i n f o
abstract
Article history: Received 14 March 2012 Received in revised form 25 July 2012 Accepted 31 July 2012 Available online 23 August 2012
Recent trend in societies is to have micro products in limited space. Efficient micromachining technologies are essential to fabricate micro products which in turn will be helpful in saving material, energy and enhancing functionality. For micromachining, micro tool is very much essential. This paper is aimed at finding the most suitable and quickest method of micro tool fabrication by electrochemical machining. Tungsten micro tools were fabricated at different machining conditions to know the influences of voltage, frequency of tool vibration, amplitude of vibration of tungsten tool, concentrations of electrolyte and dipping length of tool inside the electrolyte. Fabrication of uniform diameter of micro tool is possible at each applied voltage starting at 2 V to higher volt utilizing vibration with appropriate amplitude. Good quality micro tools with different shapes can be fabricated by controlling a proper diffusion layer thickness within a very short time introducing the vibrations of micro tool. Finally, the fabricated micro tools were applied for machining precise micro holes and micro channel using electrochemical micromachining (EMM). & 2012 Elsevier Ltd. All rights reserved.
Keywords: Micro tool Vibration Electrochemical machining Tungsten tool fabrication
1. Introduction Electrochemical machining (ECM) was introduced in the late 1950s and early 1960s in aerospace and other heavy industries for shaping and finishing operations. ECM can be thought of a controlled anodic dissolution at atomic level due to flow of current through an electrolyte which can be water based neutral salt, dilute acid or alkali. Micro-machining refers to material removal of small dimensions ranging from 1 mm to 999 mm. Advanced micro-machining may consist of various ultra-precision machining activities such as micro-holes, slots, complex surfaces in large numbers, sometimes in a single work piece, especially in the electronic and computer industries. Micromachining technology plays a vital role in miniaturization of components in the fields of aviation (cooling holes in jet turbine blades), automobile, biomedical, electronics, sensors, computer, chemical micro-reactors and micro-electromechanical systems (MEMS) etc. Electrochemical machining (ECM) processes are thermal free and material removal takes place due to atomic level dissociation, thereby giving stress free and excellent surface finish. Chemical machining cannot be controlled precisely in this micromachining domain. When anodic dissolution is applied to
n
Corresponding author. Tel.: þ91 033 24146153; fax: þ 91 033 24137121. E-mail addresses: [email protected] (B. Ghoshal), [email protected] (B. Bhattacharyya). 0890-6955/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijmachtools.2012.07.014
the micro-machining range of applications for manufacturing of ultra precision shapes, it is called Electrochemical Micromachining (EMM). EMM appears to be a very promising micromachining technology due its advantages that include no tool wear, absence of stress/burr, high material removal rate (MRR), bright surface finish, ability to machine complex shapes regardless of hardness, better precision and control, rapid machining, reliable, flexible, environmentally acceptable (electrolyte is less pollutant) and it also permits machining of chemically resistant materials like copper alloys, stainless steel, titanium and super alloys, which are widely used in bio-medical, electronic and MEMS applications. Recent trend in societies is also to have micro products in limited space. Micromachining technologies are thus helpful in saving material, energy and enhancing functionality. For micromachining, micro tool is very much essential. In recent years there is significant breakthrough in fabrication of microprobes, slender micro pin and micro tool etc. by etching process. Problems generally encountered during fabrication process are the nonuniform cross sectional shape of slender pin and poor surface quality. Previously micro tools up to |100 mm were used to be fabricated by reverse electro discharge machining, wire electro discharge grinding. But below |100 mm micro tool fabrication, cost becomes very high because of low productivity. ECM method has drawn attention nowadays because of low cost method of production of micro tool. Fan et al. [1] used pulsed power supply to fabricate the microelectrode and discussed the effect of pulsed discharge, duty ratio and linear variation of applied voltage on the shape of
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microelectrode. The need of ‘off time’ in a pulse was emphasized for removal of dissolved product away from the anode surface thereby reducing the thickness of diffusion layer. Fan and Hourng [2] discussed the effect of rotation of anodic tool on the diffusion layer and rate of dissolution with the increasing rotational speed. The tungsten rod was dipped as anode and nickel rod as cathode to a particular depth side by side, maintaining a desired inter electrode gap and etched by a D.C. current. It was observed by the authors that minimum time of 10 min of etching was required before the drop down of micro tool. Lim and Kim [3] used 5 M KOH solution as an electrolyte and etched tungsten rod with D.C current. Authors reported the presence of diffusion layer, which along with geometry and voltage forms the shape of micro tool. Wang et al. [4] used D.C. voltage, 2 M KOH electrolyte and cathode ring of inner diameter 8 mm to fabricate micro tool, controlling voltage and current density. But etching time was 20 min for shaping very small diameter from |300 mm. Choi et al. [5] fabricated WC micro-shaft at optimal 1.5 M H2SO4 solution because it can dissolve tungsten and cobalt simultaneously. Tungsten rod was dipped as anode and Platinum as cathode side by side maintaining a desired inter electrode gap and etched by a D.C. current. Skoczypiec [6] applied ultrasonic vibration to the cathode during electrochemical machining and analyzed the effect of ultrasonic vibration on the flowing electrolyte through the interelectrode gap and consequently, the change of conditions of electrochemical dissolution process. Yang et al. [7] used semi-cylindrical tool along with the ultrasonic vibrations for the drilling of deep holes with reduced machining time and machining gap. Forster et al. [8] introduced oscillating tool with 50 Hz frequency and large amplitude of 200 mm for die sinking in stainless steel with the objective of improving shaping accuracy and smooth surface. With all the above research reports it is clear that some efforts have been given to control the thickness of the diffusion layer by the rotation of anode, linear variation of voltage and variation of pulse parameters etc. during fabrication of micro tools. Ultrasonic vibrations and vibrations with low frequency have also been used during electrochemical machining with flowing electrolytes but not with tool fabrications at stagnant electrolyte. For all those investigations of micro tool fabrications, the times of machining of micro tools were considerably high. In this study, a stainless steel ring of 1600 mm diameter was used as cathode and |300 mm straight tungsten rod was immersed centrally to a certain depth for the electrochemical machining, applying vibration to the anodic tungsten rod. Variation of amplitude of vibration, frequency of vibration, voltage, electrolyte concentration and dipping length of tool inside the electrolyte have been investigated during micro tool fabrication. After analysis of the effects of the above mentioned parameters, proper condition of electrochemical machining has been suggested for the cylindrical micro tool fabrication within a very short time. Except the tip portion a series of straight micro tools have been developed up to very small diameters within 1–5 min depending on applied voltage, proper amplitude of vibration and frequency of vibration. Thus, vibration of the tungsten rod has been considered as an important parameter during the fabrication of micro tool for the disruption of the diffusion layer and enhanced diffusion and convection of ions due to hydrodynamic effects on the bubble behavior. For establishing this fact, in depth investigation has been performed and test results supported by various micrographs of fabricated micro tools, have been analyzed.
2. Electrochemical machining of tungsten micro tool 2.1. Principle of micro tool fabrication The micro tool is fabricated based on the Faraday’s two laws of electrolysis. Tungsten tool is connected to positive terminal and a
stainless steel ring is connected to negative terminal of DC supply. In between cathode and anode, NaOH electrolyte of proper concentration is applied. The anodic tungsten tool dissolves based on current density distribution. Coulomb’s law states that the electric field intensity, E applied to the micro tool is proportional to the charge density, q and inversely proportional to the square of the distance between the micro tool and the stainless steel ring, R, as given by the Eq. (1). q Ea 2 R
ð1Þ
Mass of tool that dissolves is transferred away from it, resulting in flow of mass transfer current. Mass transfer occurs due to migration, convection and diffusion of ions. Migration of ions depends on the electric field intensity. Again, electric intensity at any point in an electric field is equal to the potential gradient at that point. In other words, E is equal to rate of fall of potential in the direction of current flux and given by dV E¼ dX
ð2Þ
Convection of ions occurs due to the bodily movement of electrolyte and can be effected by flow of electrolytes or vibration of tool. Diffusion is the movement of ions from a region of higher concentration of ions, to their region of lower concentration through the concentration gradient. Mass transfer current or current density is enhanced due to diffusion process and thereby, MRR increases. But, Nernst diffusion layer is a layer of similar charges within which, gradient of the ion concentration is constant. This layer hinders the flow of current. The current density, J across the diffusion layer is well known and given by [9,10]. J¼
eDðC 0 -C d Þ
d
ð3Þ
where, D is effective diffusion coefficient, e is the electronic charge, C0 is tungsten ion concentration i.e. no. of ions per unit volume at the surface of tungsten, Cd is the tungsten ion concentration at the end of the diffusion layer and can be taken as the concentration of bulk solution and d is the diffusion layer thickness. Neglecting diffusion layer, it can be assumed that the removed amount of material from the micro tool is proportional to E. A simulation of the zone between the micro tool and ss-304 ring is analyzed as shown in Fig. 1. A stainless steel plate (SS 304) ring of thickness 300 mm is used as cathode and |300 mm tungsten rod is used as anode for the micro tool fabrication. The analysis has been done with the following assumptions: (i) The electric parameters are stable, (ii) The conductivity of the electrolyte remains constant, and (iii) The concentration gradient in the bulk electrolyte is negligible. The electrical potential, V in the interelectrode gap obeys the Laplace’s equation.
r2 V ¼ 0
ð4Þ
Boundary conditions are as follows: V9G1n2n3 ¼ 0 V ðat the cathodic ringÞ
ð5Þ
V9G4n5n6 ¼ 5 V ðat the anodic tool surfaceÞ
ð6Þ
@V @N9G7n8
ð7Þ
¼0
ðthe boundary conditionÞ
where, V ¼5 V applied between tungsten micro tool and cathodic ring and N is the surface normal. The Laplace’s equation is solved subjected to the given boundary conditions in MATLAB.
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Fig. 2. Micro tool fabrication with tool vibration.
Fig. 1. Distribution of equipotential curves in the gap of micro tool and stainless steel ring.
Equipotential distributions of curves are shown. The gradient of potential ( dV/dX) is more in the front end and lateral face of front end side i.e. distance between equipotential curves are small. From the Eq. (2) it can be predicted that in absence of diffusion layer, the rate of dissolution of micro tool will be highest at the front end and lateral face of front end side. At the low voltage, the chance of diffusion layer formation is less, as the rate of dissolution of metal will be less and hence, can diffuse away to the bulk solution. The conical tool formation is predicted. At the interface of electrolyte and air, the gradient of potential is high and hence higher dissolution can also be predicted for this zone. Later on, practical experiments continued to verify the simulation results. 2.2. Analysis of ECM with tool vibration Vibrations within the machining zone, at the stagnant electrolyte, have considerable influence on the diffusion and convection of dissolved metal ions due to hydrodynamic effects on the bubble behavior. An oscillatory vibrating tool transverse to the ring is applied as shown in the Fig. 2. The tool moves back and forth over the same path periodically. When the tool starts its motion from rest i.e. t ¼0, displacement, X¼0, the equations of motion are as follows: X ¼ A sinot
ð8Þ
V ¼ A o cosot
ð9Þ
where, A is termed as amplitude of vibration, the maximum displacement from mean position, o is angular velocity, t is instantaneous time, V is the velocity at time t. Eq. (9) explains that the velocity of the micro tool is variable during each cycle of vibration. The work is done on the viscous electrolyte by the vibrating tool having variable kinetic energy due to variable velocity. The amount of energy transfer due to tool vibration is given by: 1 PQ ¼ mV 2 2 1 or P ¼ re V 2 2
ð10Þ
where, P is the pressure developed inside IEG, Q is the volume of electrolyte displaced and m is the mass of electrolyte which is given a thrust, re is the density of the electrolyte. During the upward motion of the micro tool, there will be pressure drop in the inter-electrode gap (IEG) which can be explained from the
Eq. (10). This pressure drop breaks the forces holding the liquid molecules together and generates micro bubbles of electrolyte vapor or transient cavitation [11]. When the anodic micro tool moves downward, the pressure increases and rapid collapse of micro bubbles occur with the generation of temperature and pressure [12]. As the anode itself is vibrated, the bubble collapse will be maximum very near to the anodic tungsten tool. The potential energy of expanded cavity is converted to kinetic energy of liquid jet motion during the energetic collapse. The impact of the micro-jets on the anodic surface, results in the enhanced convective mass transport of dissolved ions, disruption of diffusion layer and supply of fresh electrolyte. Moreover, increased pressure drives out hydrogen bubbles from the IEG resulting in the improved conductivity. Depending on amplitude and frequency of vibration, there are possibilities of periodic size oscillation of bubbles, in the case of stable cavitation or escape from the solution due to mass convection. The vibration has important effect on the diffusion layer, which is formed when the rate of metal dissolution is greater than the rate at which the metal ions can diffuse away from an electrode. Concentration overpotential comes into play under the situation and current density at the zone of diffusion layer decreases. The extremely fast bubble collapse usually occur in less than one microsecond time [13]. These conditions are conducive for the quick charge transport and depolarization. Thus, vibration increases current density due to increase of convective mass transport, increase in the diffusion rate and improved conductivity. 2.3. Developed experimental set up Considering the influence of various process parameters, a well planned research program has been taken up for the experimentation in the developed EMM set up [14–16]. The set up consists of various sub-components such as electrical power and controlling system, stepper motor along with microprocessor for motion control of tool and job, Mechanical unit for holding the tool and job and Piezoelectric transducer (PZT) for vibrating the tool longitudinally. PZT is also very helpful for easy supply of electrolyte to the interelectrode gap (IEG) during electrochemical micromachining (EMM), thereby material removal becomes possible at very small diameter (10 mm) drilling or micro channel formation. PZT is connected to 230 V main power supply through amplifier module ( 20 V–130 V) for feed control in the nanometer range (resolution 0.12 nm) having maximum stroke 66 mm and a modulation input from a function generator for controlling the amplitude of vibration which varies with the variation of voltage. Frequency of vibration of the PZT is controlled by the frequency of input voltage. Electrical power and control is comprised of a function generator for the supply of pulsed DC voltage to the tool and work piece. Digital storage oscilloscope acts as data collection system which is inevitable for the control of inter electrode gap (IEG) during EMM. Stepper motors are used to give motion of the
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Fig. 3. Experimental set up for micromachining and micro tool fabrication.
tool and work piece along X, Y and Z direction. However, for very low feed rate and accurate control of IEG ( o10 mm), feed by PZT is preferred. Fig. 3 shows the various units of the developed experimental set up of EMM, used for the micro tool fabrication.
3. Planning for experimental analysis The designed and developed experimental set up is used for the micro tool fabrication and electrochemical micromachining purpose. During the fabrication of micro tool, a lot of experiments were performed to study the influence of various predominant parameters such as machining voltage, electrolyte concentration, immersion length of tool, amplitude of vibration of tool and frequency of vibration of tool etc. Applied voltage is of DC (frequency 1 mili Hz, period 1000 sec and pulse width 999 sec) from a function generator and is noted during the micro tool fabrication by a digital storage oscilloscope, DL 750 (YOKOGAWA, JAPAN). Micro tool measurements are performed by microscope (OLYMPUS, JAPAN) with a least count of 0.0005 mm through the lenses of 3 –50 . Images are taken with the help of software from the microscope. Cylindrical tools are mostly preferred for micromachining. To find the machining accuracy, 150 mm is excluded from the tip of fabricated tool as this portion is cut by electrochemical micromachining for getting final micro tool. At a gap of 400 mm, the readings of diameter are taken as D1, D2, D3, D4 and D5 as shown in Fig. 4, to measure the average diameter (Davg). To evaluate the deviation from the cylindrical shape, standard deviation (s) is calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðD1 Davg Þ2 þðD2 Davg Þ2 þ ðD3 Davg Þ2 þ ðD4 Davg Þ2 ð11Þ s¼ 4 Effect of voltage, frequency of vibration, amplitude of vibration and concentration of electrolyte are investigated on the final shape of the fabricated micro tools and finally, attempts have been made to achieve the optimum parametric conditions for the fabrication of cylindrical micro tool.
Micro tools developed were also used as cathode tool during the EMM operation for the machining of micro holes and micro channels. EMM was performed using 0.2 M H2SO4 electrolyte based on the ultra-short voltage pulse of 5 MHz frequency. H2SO4 was chosen as electrolyte because sludge is dissolved fully in the electrolyte, thereby reducing the side gap of micromachining and improving surface quality [17,18]. The reasons for choosing 0.2 M concentration of electrolyte are less machining time and generation of same side gap during machining of microchannel with 0.1 M and 0.2 M electrolyte. During drilling operation, it was found that the side gap at 0.1 M is less than that at 0.2 M. At 0.5 M concentration, machining became difficult and frequent short circuit occurred. Ultra-short pulse voltage of 5 MHz frequency was chosen for localization of current at the intended area of machining [18].
4. Fabrication of tungsten micro-tool Small pieces of tungsten rods are sheared from long tungsten rod of |300 mm diameter and ends are flattened by fine grinding and straightened. Then, the tool specimen is connected to the positive terminal of function generator and stainless steel ring of diameter 1600 mm is connected to the negative terminal. The tool specimen is centered to the hole and immersed vertically to a certain dipping length into NaOH electrolyte. Diameter variation of the fabricated tool can be kept within 70.5 mm tolerance at an appropriate combination of machining voltage and amplitude of vibration. Overall objective is to fabricate low cost and high production volume of micro tools with repeatability of production. When voltage is applied between anode and cathode the tungsten dissolves at the anode by the following reactions: Wþ 6OH -WO3 þ 3H2 O þ6e
ð12Þ
WO3 þ2OH -WO2 4 þ H2 O
ð13Þ
The electrochemical reaction on the cathode is: 6H2 Oþ 6e -3H2 m þ 6OH
ð14Þ
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Fig. 4. Micro tool measurement positions to get average diameter and std. deviation.
Fig. 5. Sharp conical shape formation at voltage 1.5 V.
In the reaction (12), the metal tungsten is oxidized to tungstate (WO3). In the reaction (13), the tungstate becomes ion (WO2 4 ) by reacting with excess hydroxyl ions and remains dissolved in the electrolyte. In the reaction (14), water is converted to hydroxyl ions and hydrogen evolves speedily at the cathode.
5. Results and discussions Experiments have been conducted in the present set up to analyze the effect of vibration and other parameters during the micro tool fabrication. In the present system |300 mm tungsten rod was dipped up to 900 mm inside the electrolyte (2 M NaOH), to start experimentation. However, machining reaction started at lower dipping length at high concentration (4 M NaOH). This happens due to the double layer phenomena. The cathode (stainless steel ring) is immersed fully in electrolyte; thereby voltage drop due to double layer potential in the interface between cathode and electrolyte is small. Whereas, large voltage drop occur between small surface area of micro tool (dipped area of anode) and electrolyte.
5.1. Influence of voltage on micro tool fabrication The applied voltage is one of the important parameter for the removal of material. Therefore, the investigation was started with variation of voltage. When applied voltage was less than 1.5 V, sharp conical formations of micro tool was found as shown in the Fig. 5. Here conical angle formed is 5.541 having diameter 64 mm at a distance of 712 mm from the tip of the micro tool. At this low voltage the rate of dissolution of tungsten metal is low and machining time was 18 min in 2 M NaOH electrolyte. Thus formation of diffusion layer is either minimum or absent as the rate of migration of ions will be equal to rate of formation of hydrated tungstate ions. As predicted by the simulation result at Fig. 1, the current density is high at front end and lateral face of front end of micro tungsten rod, resulting in higher rate of removal at the front end of tool. The tool becomes conical as predicted by the simulation result.
Fig. 6. (a) 70 s of machining (b) 105 s of machining (c) 130 s of machining (d) 135 s of machining.
Fig. 6 exhibits the formation of reversed conical shape of micro tool for the applied voltage greater than 2 V, without application of vibration. The machining was done at high voltage of 8 V without vibration. Evolution of shape is shown in four steps, initial dia. |300 and 2M/L NaOH electrolyte was used. Fig. 6(a) shows that after 70 s of machining, slight necking at the shank portion is observed as predicted by the simulation result. Fig. 6(b) describes the shape after 105 s of machining, where prominent reversed taper is observed. Fig. 6(c) shows that the front portion is about to drop off due to critical necking, after 130 s of machining. Fig. 6(d) shows the drop off after 135 s of machining. At higher voltage, the micro tool cuts off from the necking portion without much reduction in diameter. Necking formation
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5.2. Influence of vibration on micro tool fabrication Vibration has hydrodynamic effects on the bubble behavior, which has been utilized for the effective removal of sludge, hydrogen bubbles and replenishment of fresh electrolyte. The tool was vibrated longitudinally with definite combination of amplitude of vibration and frequency of vibration. Influence of these two important vibration parameters on the fabrication of micro tool is discussed here. 5.2.1. Effect of amplitude of vibration Amplitude of vibration has the most significant effect on the growth of cavitations bubbles and energetic collapse. Higher amplitude with the fixed frequency means higher velocity of the tool as can be explained from the Eq. (10). Again, higher velocity means higher pressure rise or fall, which in turn creates larger number of cavitations bubbles and more energetic collapse. As a result, the micro jets impinge on the surface of the tool breaking the diffusion layer and increasing the convective mass transport. Fig. 9(a)–(c) shows the shape evolution at 7 V applied
1200
1000
800
600
400
200
0 0
1
2
3
4
5
6
7
8
Applied voltage (V) Fig. 7. Maximum possible machining time without vibration at different voltages.
3500 3000
Length of tool (μm)
can be explained from the simulation result of Fig. 1. The current density is high at the interface between air and NaOH electrolyte in the absence of diffusion layer. Thus, the tools having high aspect ratio cannot be fabricated at high voltage without the help of vibration. Generation of reversed conical shape can be explained by the formation of diffusion layer [1,2]. With the application of high voltage, rate of dissolution of tungsten tool being fabricated is very high but because of limitation of migration of these cations to the bulk solution, accumulation of ions near the tungsten tool take place. At the very beginning of the applied voltage, current density is highest at front end and lateral face around it, as is evident from the simulation shown at Fig. 1. Hence, diffusion layer thickness growth is also highest at the front end and gradually decreases upwards. At the interface between electrolyte and air, the diffusion layer thickness around micro tool is low. Rising H2 gas bubbles bursts while reaching at the interface of air and electrolyte. Thereby, energy of burst breaks the diffusion layer at the interface between electrolyte and air. From the Eq. (3) it is evident that the current density is inversely proportional to the thickness of the diffusion layer. Thus, after the growth of diffusion layer, highest dissolution takes place at the shank portion, where diffusion layer is thin. Fig. 7 shows the machining time required to attain the lowest diameter or complete dissolution or drop off whichever is earlier with the variation of applied voltage. For example at 1.4 V, the maximum machining time is 1140 s (19 min) and within this time, machining is to be stopped to get a certain diameter of micro tool. Similarly at 7 V, machining time is 140 s. Within this time, machining is to be stopped to get a visible tool otherwise, drop off may take place. At complete dissolution, there will be no charging of double layer and hence, the applied voltage remains constant and can be monitored by the oscilloscope as zero voltage drops. Fig. 8 shows an important observation that fabricated micro tool length increases with the increase of applied voltage though the immersed tool length is 900 mm. Initially, the length of micro tool increases linearly up to 7 V and then increase in length is steep. Machined length is always greater than the dipping length of the tool. The electrolyte level over the tool is higher than the upper level of electrolyte due to surface tension. Again, as the voltage increases, the current density increases and rate of dissolution increases with the increased rate of evolution of hydrogen from the cathode ring. The temperature of the tungsten tool surface increases due to the Ohmic heating at high voltage, which further increase the level of electrolyte over the tungsten rod.
Machining time (sec)
54
2500 2000 1500 1000 500 0 0
2
4
6
8
10
12
Applied violtage (V) Fig. 8. Effect of voltage on the length of tool.
voltage, 5 mm amplitude of vibration, 2 M NaOH, 900 mm dipping length and 228 Hz frequency of vibration. Fig. 9(a) shows the conical shape evolution after 70 s of machining having average dia. |175 mm. Fig. 9(b) shows the conical shape of uniform taper with tip diameter 42 mm after 100 s of machining. Fig. 9(c) shows the shape having |13 mm at the interface of electrolyte and air, |39 mm at the middle and |16 mm at the tip after 131 s of machining. Fig. 9(d) shows the sharp taper having |9 mm tip diameter i.e. the current density becomes linearly variable, being highest at the front tip as explained in the simulation result of Fig. 1. Fig. 10 shows the comparison of standard deviation of machining between low amplitude of vibration and high amplitude of vibration at the applied voltage of 7 V. Machining is done at 2 M concentration of electrolyte, dipping length 900 mm and 240 Hz frequency of vibration. Variation of standard deviation at 7 V, 2 mm amplitude is less than steep increase of standard deviation at 7 V, 5 mm amplitude of vibration i.e. at 2 mm amplitude of vibration, the machined tool is more cylindrical than at 5 mm amplitude of vibration. For each voltage, there exists appropriate amplitude of vibration for minimum standard deviation and main idea is to make the current density uniform throughout the length. Higher voltage means higher dissolution of metal and hence more mass transport is required by convection and diffusion to avoid diffusion layer formation. Therefore, higher amplitude of vibration is needed for the higher applied voltage. Fabrication of uniform diameter of micro tool is possible at each applied voltage, starting with 2–10 V with appropriate amplitude
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Fig. 9. Shape evolution at high voltage and high amplitude of vibration (a) 70 s (b) 100 s (c) 131 s (d) 147 s.
std. deviation at 7 volt with different amplitude of vibration 35
Std. deviation (μm)
30 25 20
2 μm amplitude
15
5 μm amplitude
10 5 0 0
25
50
75
100
125
150
175
Time of machining (sec) Fig. 10. Standard deviation at low and high amplitude of vibration.
of vibration maintaining a uniform current density along the length of tool. During the machining time (147 s) at 7 V, 2 M NaOH electrolyte, 1 mm amplitude and 236 Hz frequency, it was observed from the oscilloscope that initially, the voltage drop was around half the applied voltage but after approximately 25 s, the voltage drop was very high, almost around two third of the applied voltage and towards the end of machining, voltage drop was very small. The above observation is reflected in the graph in Fig. 11 by actual volume removal rate. When the voltage is applied, no bubbles are found and clearly a wave of ion concentration gradient (migration) is visible from the micro tool towards the cathode ring. Applied electric field takes some time to start migration of ions. After 10–15 s, hydrodynamic effects on the bubble behavior starts and large no. of microbubbles are found, creating a highly unstable region between anodic tool and cathode ring. The reaction rate starts rising and reaches peak rate after 45 s and before 100 s, the rate of reaction decreases due to higher resistance to current flow, as the tool area undergoing dissolution decreases. As the amplitude of vibration increases, growth of cavitations bubbles and energetic collapse increases. The
Fig. 11. MRR increases with increase of amplitude of vibration.
current density increases and hence, MRR increases as shown in Fig. 11 at 3 mm and 5 mm amplitude of vibration. Thus cylindrical micro tool fabrication is possible at shortest time, utilizing amplitude of vibration.
5.2.2. Effect of frequency of vibration The shape of the fabricated micro tool becomes wavy and reversed taper, with the increase of frequency of vibration of the tungsten rod as shown in Fig. 12(a) where, the parameters are 300 Hz frequency of vibration, 2 mm amplitude of vibration and 3 V applied voltage, during the fabrication of micro tool. The reversed taper indicates that the cavitations phenomena diminish and disappear as the frequency is raised creating stable bubbles of electrolyte. The tungsten rod transmits vibration to the electrolyte, resulting in stretching and compression of the electrolyte. The bubbles at the rarefaction zone are expanded, decreasing the conductivity of the electrolyte and at the compression zone, the volume of bubbles decrease, increasing the conductivity of electrolyte. At the compression zone of the electrolyte, current density is high due to higher conductivity and as a result, node is
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std. deviation ( σ) at 2 Volt
25
std. deviation (μm)
Without vibration 20 With vibration frequency 228 Hz and 1 µm amplitude With vibration frequency 300 Hz and 1 µm amplitude
15 10 5 0 0
100
200
300
400
500
600
700
Machining time (sec) Fig. 13. Accuracy of machining with the variation of frequency of vibration.
300
formed. Again, at the rarefaction zone, the current density is low, due to low conductivity and as a result, antinode is formed. Nodes and antinodes formation can also be explained by the acoustic streaming phenomena [19,20]. Acoustic streaming gives rise to the circular flow along the surface of the tool at an interval of l/2 where, l is the wavelength of the transmitted wave through electrolyte. The circular flow increases convective mass transport resulting in higher dissolution at nodes. The nodes become prominent, with the increase of amplitude of vibration along with the increase of frequency, as shown in Fig. 12(b) where, 300 Hz frequency of vibration of the tungsten rod and 4 mm amplitude of vibration is applied, during the fabrication of micro tool. The end portion of the tool dropped from the remaining portion because, dissolution rate is high at high amplitude of vibration. Fig. 12(c) shows that at low frequency, there is no formation of nodes in the micro tools where, machining was done at 228 Hz frequency of vibration of tungsten rod and 2 mm amplitude of vibration. It is obvious from Fig. 12(c) and Fig. 13 that with low frequency of vibration of micro tool, the accuracy of machining increases i.e. std. deviation decreases. At high frequency of 300 Hz, 1 mm amplitude of vibration and 2 V machining voltage, std. deviation steeply increases with time. Other variables remaining same, the rate of increase of std. deviation is very low with 228 Hz frequency of vibration and it is lower than that of without vibration. Initially around 90 s of machining, std. deviation is approx. 1 mm for all the cases but at 600 s of machining std. deviation is 4 mm, 9 mm and 23 mm for 228 Hz frequency of vibration of tool, without vibration of tool and 300 Hz frequency of vibration of tool respectively. 5.3. Influence of concentration of electrolyte Fig. 14 shows the average diameter of fabricated tool at different concentrations of electrolyte after 5 min of machining at 2 V applied voltage, 1 mm amplitude of vibration and 232 Hz frequency of vibration of tungsten rod. It is obvious that material removal rate (MRR) increases with the increasing of concentration up to 4 M and then starts decreasing due to the decrease of activity coefficient at high concentration. Hydration of ions also decreases with the increase of concentration beyond 4 M. Minimum tool length (700 mm) can be machined at 4 M concentration, as the minimum required immersion length decreases with the increase of concentration. Machined micro tool length also decreases for the same dipping length with the increase of
Average diameter (μm)
Fig. 12. (a) 300 Hz and 2 mm (b) 300 Hz and 4 mm, (c) 228 Hz and 2 mm.
250 200 150 100 50 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
Concentration (M/Litre) Fig. 14. Average diameter at 5 min of machining.
concentration due to high viscosity and less surface tension but standard deviation (departure from cylindrical shape) increases. In the present research, 2 M is most suitable for the machining of micro tool. However, for the fabrication of special tool tip, high concentration electrolyte is preferable as machining is possible at low dipping length and diffusion layer can be confined within the front tip only, by utilizing vibration. Fig. 15 shows the tool fabrication at concentration of 3 M NaOH, 5 V applied voltage and 228 Hz frequency of vibration. Fig. 15(a) shows that, at high concentration and low amplitude of vibration i.e. 1 mm, transient cavitations effect is small at the front tip of the tool and this type of shape of micro tools are suitable for taper less drilling in micromachining. Comparing Fig. 15(b) and (c) it is obvious that transient cavitations effect increases with the increase of amplitude of vibration. Fig. 15(b) shows the fabricated micro tool which was machined at 3 mm amplitude of vibration and that in Fig. 15(c) was machined at 4 mm amplitude of vibration. Fig. 15(d) shows an important fact that the standard deviation decreases with the increase of immersion length of the tool, along with the appropriate amplitude of vibration. Machining was done at 5 mm amplitude of vibration of tungsten rod and dipping length was increased to 1500 mm and fabricated tool length was 1970 mm. Each applied voltage has corresponding amplitude of vibration for the uniform current density on the surface of the tool.
6. Fabrication of suitable micro-tool From the earlier analysis on test results for the fabrication of cylindrical micro tool with small deviation, the values of parameters chosen as 228 Hz frequency of vibration of tungsten rod,
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Fig. 15. (a) 1 mm amplitude (b) 3 mm amplitude (c) 4 mm amplitude (d) 5 mm amplitude. Table I Recommended applied voltage and appropriate amplitude of vibration for minimum diameter cylindrical tool fabrication. Volt (V) Amplitude of vibration(mm) Machining time (s)
10 6 60
9 5.5 70
8 4.5 100
7 4 118
6 3.5 137
5 3 158
4 2.5 208
3 2 283
2 1.5 305
350
Diameter (μm)
300 250 200 150 100 50
Fig. 17. Fabrication of micro tool in two steps.
0 0
20
40
60
80
100
120
140
160
180
Time (sec) Fig. 16. Diameter of cylindrical fabricated tool according to machining time.
2 M electrolyte concentration and dipping length 900. Fabrication was done at each and every voltage starting from 2 V to 10 V and appropriate amplitude of vibration for finding out minimum diameter cylindrical tool. Table I shows the recommended voltage and amplitude combination along with machining time for the present set of research. However, tools other than cylindrical shape are also very effective for particular type of machining some of which are also discussed in brief. Taking one combination from Table I, the machining time and corresponding diameters are shown in Fig. 16. Fabrication was done at 5 V and amplitude of vibration 3 mm. Minimum micro tool diameter of 10 mm was fabricated at 158 s of machining. The cylindrical tools of very small diameter are vulnerable to breakage in a single use if the length of the micro tool is high. To make the tool of small diameter and to maintain rigidity as well, the fabrication may be done in two or more than two steps. Fig. 17 depicts the fabrication of micro tool in two steps. Firstly, cylindrical
Fig. 18. Fabrication of micro tool in three steps.
tungsten tool of |80 mm was fabricated at 5 V and 3 mm amplitude of vibration within 2 min. Then fabrication continued at 1.5 V and without vibration for 5 min. Thus micro tool of length 150 mm, tip diameter of 1 mm and tip angle of 11.591 is fabricated. Fig. 18 exhibits the tool fabricated at three steps. Diameter reduced to 150 mm cylindrical in first step from 300 mm. Then, again reduced to
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Fig. 19. (a) Tool of 12 mm diameter (b) Entry side (c) Exit side.
Fig. 20. Blind micro channel of 50 mm depth.
|37 mm cylindrical followed by third step reduction in low voltage for conical shape generation having tip angle 261. This type of tools is suitable from rigidity point of view as bending stress will be less.
7. EMM application of micro tools A micro hole of entry diameter 24 mm and exit diameter 22.5 mm was drilled on a SS-304 plate of 35 mm thickness as shown in Fig. 19(b) and (c) with the tool of 150 mm long and 12 mm uniform diameter at the tip as shown in Fig. 19(a). EMM operation was performed at 5 MHz pulse power, pulse width 60 ns, average voltage 2.8 V, amplitude of vibration of micro tool 0.2 mm and frequency of vibration of micro tool 236 Hz. The total machining time for the generation of |22.5 mm hole was about 25 min. A blind micro channel of 1000 mm length, 135 mm width and 50 mm depth was machined on a SS-304 plate of 60 mm thickness as shown in Fig. 20. EMM was carried out at 5 MHz pulse power, pulse width 60 ns, average voltage 2.7 V, amplitude of vibration of micro tool 0.1 mm, frequency of vibration of micro tool 228 Hz and horizontal velocity of micro tool was 78 mm/s. The total machining time for the generation of micro channel was 55 min. The micro tool used for the machining is of reversed taper with tip diameter 60 mm. Such type of micro tool generates vertical side wall of micro channel.
8. Conclusions Fabrication of micro tools is essential for the manufacturing of micro components and features in micromachining area and preferably in EMM. In experimentation during fabrication of micro tools, some important observations can be drawn from the test results and analyses are as follows: (i) It is observed from the simulation of the machining zone that current density will be maximum at the front face and front side lateral face and also, at the interface between the electrolyte and air, when the diffusion layer is neglected.
(ii) Transient cavitations phenomena results in the increases the convective mass transport, disruption of the diffusion layer and replenishment of fresh electrolyte. Amplitude of vibration promotes the transient cavitations phenomena. Thus, volume removal rate increases with the increase of amplitude of vibration at a particular voltage and micro-tool fabrication is possible within the shortest time. (iii) Standard deviation increases with time both at low voltage and high voltage as the current density distribution varies with machining time. During the fabrication at 2 V, 228 Hz frequency and 1 mm amplitude of vibration, the std. deviation was 0.5 mm, after 90 s of machining and 4 mm, after 600 s of machining. During fabrication at 7 V, 240 Hz frequency and 2 mm amplitude of vibration, the std. deviation was 0.3 mm after 45 s of machining and 3 mm after 147 s of machining. (iv) Fabricated tool length increases with the increase of voltage, because of high rate of evolution of hydrogen, increase in reaction rate. Fabricated tool length was 1400 mm at 1.6 V and 3000 mm at 10 V, though dipping length was 900 mm in both the cases. (v) At high frequency nodes and antinodes are observed along with the reversed taper formation of the micro tool above 2 V applied voltage. Transient cavitations phenomena diminish and disappear as the frequency is raised, making the tool reversed taper. Nodes are formed due to acoustic streaming at the interval of half the wavelength. So both EMM and fabrication of micro tool should be done at low frequency i.e. less than 250 Hz.
Tools of various shape at the tip is very helpful for various purpose of micro machining. For micro hole generation reversed taper is essential for minimization of side gap. Similarly, micro tool tip with various end shape are also effective for channel machining or internal groove machining. Conical tools are useful for nozzle drilling as well as STM probe and stylus of various sensors. In this research paper, attempt has been made to establish the fact that micro tools of various shapes can be fabricated within shortest time utilizing amplitude of vibration. Thus, fabrication of complicated micro tools with the help of amplitude and frequency of vibration, along with the high electrolyte concentration may be possible in future and can be utilized for layer by layer machining and also, for free form micro machining.
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