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Experimental investigation into micromilling of microgrooves on titanium by electrochemical micromachining
Journal of Manufacturing Processes 28 (2017) 285–294
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Experimental investigation into micromilling of microgrooves on titanium by electrochemical micromachining Sandip S. Anasane a,∗ , B. Bhattacharyya b a b
Department of Production Engineering and Industrial Management, College of Engineering, Pune, India Production Engineering Department, Jadavpur University, Kolkata, India
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 27 April 2017 Accepted 24 June 2017 Keywords: Microgroove EMM Titanium Layer-by-layer milling Disc microtool
a b s t r a c t Titanium possesses excellent physical and chemical properties such as higher strength to weight ratio, greater biocompatibility and outstanding corrosion resistance. Therefore, titanium is highly demanding material in aerospace to biomedical applications from macro to micro scale levels. However, machining of titanium in macroscopic or microscopic domain is a complex task either by conventional or nonconventional machining processes. Electrochemical micromachining (EMM) could be one of the alternatives for machining of titanium in microscopic area. The study elaborated in this paper demonstrates successful micromilling of through microgrooves on pure commercial titanium utilizing maskless electrochemical micromachining. Layer-by-layer micromilling strategy has been successfully employed with the help of in-situ fabricated disc shape microtool for micromilling of through microgrooves. Influence of various EMM process parameters such as machining voltage, pulse frequency and microtool feedrate on machining accuracy of through microgrooves were also investigated and established most suitable EMM process parameters for successful micromilling of through microgrooves on pure titanium. This paper also demonstrated successful fabrication of few complex geometry microgrooves by electrochemical micromilling for practical applications. Micromilling of microgrooves on pure titanium by EMM may find vital applications for MEMS, micro fluidics, microsensors, micro engineering and biomedical systems. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Demand of miniaturization is rapidly increasing in almost all fields, which drives researchers and engineers to explore potentials of various micromachining techniques as well as materials in order to establish novel solutions to microengineering applications. Incorporating advanced materials in micro domain will create larger impact to the functional aspects of microstructures. Titanium is a material known for its versatile physical and mechanical properties such as high strength to weight ratio, high compressive and tensile strength, low density, high fatigue resistance in air and seawater, and exceptional corrosion resistance [1–3]. This made titanium prominent material in a variety of applications such as aerospace, biomedical, sporting goods, marine, military and chemical industries. However, titanium machining either by conventional or non conventional methods is an always a concern to the researchers and engineers due to its poor machinability. In the conventional subtractive machining techniques, problems such as shorter tool life and higher tool wear reported because of higher
∗ Corresponding author. E-mail address: [email protected] (S.S. Anasane).
cutting forces and excessive heat generation due to poor thermal properties of the titanium as well as chatter, deflection and rubbing of cutting tool owing to low modulus of elasticity with its ability to maintain high strength at elevated temperatures [4]. Titanium is very chemically reactive hence; it has a tendency to weld to the cutting tool during machining leading to chipping and premature tool failure [5]. Even in non conventional machining process machining of titanium is an intricate task, Electric Discharge Machining (EDM) and Ultrasonic Machining (USM) has been applied to the machining of titanium and its alloys during recent times. In EDM, formation of thermal stresses in a small heat-affected zone is a serious issue which can lead to micro-cracks, decrease in strength and fatigue life and possibly catastrophic failure of the component [6]. Ultrasonic machining (USM), lower MRR compared with other processes and serious tool wear that usually affects machining precision [7]. Laser beam machining (LBM) can be applied for machining of titanium, but even this process has its own problems of heat affected zone and re-solidification of layers as well as probability of formation of hot spot due to thermal effect [8]. Electrochemical machining (ECM) has proved its compatibility for machining of titanium in macro scale level. In ECM, material removal carried out by anodic dissolution principle hence, this process can machine materials irrespective of their hardness.
http://dx.doi.org/10.1016/j.jmapro.2017.06.016 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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When ECM is employed for fabrication of features in microscale level i.e. microfeatures in the range from 1 to 999 m then ECM becomes electrochemical micromachining (EMM) [9]. EMM emerged as potential micromachining technology due to its several advantages such as high MRR, enhanced precision and control and environmentally tolerable in addition to that it also machine chemically resistant materials i.e. titanium, stainless steel and super alloys [10]. Therefore, micromachining of titanium by electrochemical process could be one of the competent alternative techniques for machining of titanium. Machining of titanium in macroscopic domain utilizing Electrochemical machining (ECM) has been reported [11,12]. However, in microscopic domain, most of the work has been reported in the area of micropatterns generation intended for surface structuring applications either by electrochemical etching, or electrochemical polishing (EP) [13–16]. All these techniques were utilized expensive and complex sophisticated processes such as through mask electrochemical micromachining (TMEMM) or with confined etchant layer technique or by oxide film laser lithography. But very few or almost no attempts have been reported based on fabrication of complex microfeatures on pure titanium utilizing maskless electrochemical micromachining (EMM). Major challenge in fabrication of microfeatures on titanium is to achieve controlled anodic dissolution of titanium in micro domain by overcoming passivity of oxide layer present on surface of the titanium. Hence, in this paper systematic experimental investigation has been carried out to identify optimum process parameters for generation of microfeatures such as through microgrooves by achieving controlled anodic dissolution of titanium in microscopic level. The work elaborated in this paper also demonstrated layer by layer electrochemical micromilling strategy with the aid of in-situ fabricated cylindrical disc shape microtool for successful micromilling of complex shape through microgrooves utilizing indigenously developed maskless electrochemical micromachining set up. 1.1. Anodic dissolution of titanium by EMM Anodic dissolution of pure titanium utilizing electrochemical micromachining (EMM) is complex task compared to EMM of other metals. Titanium possesses good corrosion resistance because of presence of surface oxide film. This protective tenacious oxide film is formed on the surface of titanium, when titanium is depicted to oxygen containing media such as air, water etc [17]. When titanium undergoes electrochemical actions, the role of transfer of Ti2+ and O2− is contribute for development of anodic film. Formation of passive oxide layer on titanium surface during electrochemical process with aqueous environment is initiated by reacting Ti2+ with hydroxide ions (OH− ) ionized from aqueous solution. Following electrochemical reactions represents stable titanium oxide (TiO2 ) [18]. At the interface of anode workpiece and electrolyte, the reactions taking place are: Ti → Ti2+ + 2e−
(1)
Ti2+ + H2 O → TiO2+ + H2 ↑
(2)
TiO2+
The oxocation, is acidic in nature and subsequently reacts with OH− to form stable TiO2 [19]. Following chemical reaction represent the formation of stable TiO2 : TiO2+ + 2OH− → TiO2 + H2 O
(3)
Throughout the anodic dissolution process, development of oxide layer with the help of titanium and hydroxide ions has been accelerated by the application of electric field.
This thin oxide film is highly passive in nature, causes anodic dissolution of titanium difficult. The controlled anodic dissolution of titanium is difficult by EMM process parameters generally utilized for micromachining of other metals especially in terms of machining voltage and type of electrolyte due to the presence of passive oxide layer. Hence, to attain uniform transpassive dissolution of titanium, the oxide film that obstructs the controlled dissolution of pure titanium in the passive potential region must lose its passivation phenomenon. Removal of oxide film is possible when the applied potential is adequately high [20]. The passive oxide film develops linearly with potential until a significant value is attained and the breakdown of the film takes place from random pitting at higher current densities and then shape controlled dissolution begins [21]. An additional factor, which plays vital role in rupturing this passive oxide film, is the electrolyte type i.e. Electrolytes containing bromide ions have been effective in breakdown of oxide film [22]. Electrolyte solution includes bromide ions can promote anodic dissolution of titanium with valance of 4 i.e. Ti → Ti4+ [23]. The use of aqueous based electrolytes for EMM of titanium can create the possibility of formation of passive oxide layer or increase the thickness of already existing oxide film formed due to atmospheric oxygen. Use of aqueous based electrolytes could be altered with non-aqueous based electrolytes to minimize the chances of formation of passive oxide layer [24]. The accuracy of the micro machined product in EMM is highly influenced by process parameters e.g. applied machining voltage, duty ratio, pulse frequency, concentration of electrolyte, inter-electrode gap (IEG) and micro tool feed etc. Therefore, appropriate selection along with controlling of all these EMM parameters play a vital role in attaining the preferred results during titanium micro machining utilizing EMM. 2. Experimental setup Experimentation for micromachining of titanium has been executed on indigenously developed electrochemical micromachining set up as shown in Fig. 1. The EMM experimental set up consist of different subsystems i.e. mechanical machining unit, desktop computer, DC pulsed power supply, digital storage oscilloscope (Tektronix) as well as measuring microscope etc. Indigenously fabricated electrolyte chamber from Perspex material having work holding arrangement has been employed for the experimentation. Mechanical machining unit includes three linear travel stages designating X, Y, and Z-axis. The stepper motors of each linear stage with resolution of 0.1 m/step are controlled by CNC controller unit, which is interfaced with desktop computer. Different feeds can provide to all the three stepper motors at a time with the help of position controller software through desktop computer. DC pulsed power supply has been used for generation of required nature of pulse power intended for micro machining operation. Digital storage oscilloscope has been used for observation of pulse waveform. The digital multimeter (Agilant U1252A) was connected in series to the circuit for the measurement of current values. Stereo type microscope (Leica S6D) was used for observation, measuring microscope (Leica DM2500) were utilized for measurement of various criteria of machined microfeature. To augment the supply of electrolyte into the narrow inter electrode gap (IEG) during EMM operation by providing longitudinal vibrations to the microtool, Piezoelectric transducer (PZT) has been utilized. 3. Experimental planning In order to investigate the influence of various EMM process parameters on various responses such as accuracy of microgroove in terms of width overcut (WOC), length overcut (LOC) and taper, all the experiments were systematically planned and executed by
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Fig. 1. Experimental setup.
In this work, to minimize the effect of stray machining and to reduce the effect of overcut, in-situ fabricated disc type microtool of disc diameter 118 m was utilized. The downward movement of microtool is maintained by 20 m at the end of each stroke length. Hence, microtool advances with layer thickness obtained at 20 m depth at each layer milling. In this experimentation stroke length of microtool along X-Y plane is restricted to 1000 m, which is total desired length of the through microgroove. 3.2. Development of disc type microtool
Fig. 2. Layer-by-layer micromilling.
varying one parameter at a time. Lower and upper limit of various process parameters, such as machining voltage, duty ratio, pulse frequency and microtool feed rate have been decided based on results generated by conducting extensive trial experiments. Layer-by-layer milling strategy is adopted for micromilling of microgrooves with the help of in-situ fabricated disc type microtool. 3.1. Layer-by- layer electrochemical micromilling In the Layer-by-layer micromilling technique, initially cylindrical microtool advances vertically i.e. along z-axis into the anode workpiece then it advances linearly along the length of microgroove. The initial depth of penetration of microtool in the anode workpiece is the initial layer thickness with this layer thickness microtool travels to and fro along the length of microgroove and mills the microgroove throughout the entire length. Fig. 2 represents the layer-by-layer milling process. In this layer-by-layer milling, layer depth gradually increases during downward feed movement at the end of each milling stroke length and continues till the desired depth is achieved. Dissolution of material takes place from the front face as well as side face of the microtool. In order to maintain the good shape precision i.e. width at the start and end location as well as along the length of microgroove. It is recommended to maintain milling layer thickness less than or equal to diameter of microtool [25].
Microtools are one of the important factor in the electrochemical micromachining process because microtool, which acts as a cathode produces its negative shape in the anode i.e. workpiece. Hence, microtool shape, size, and surface quality directly affect the dimensional accuracy, precision, and surface quality of the machined microfeature. Therefore, precise microtool of micron scale size with good surface finish is crucial for the machining of microfeatures. Microtool insulation made of polymer and resin dissolved in isopropyl alcohol as solvent [26] were also tested for EMM of titanium. However, during trial experiments it has been observed that insulation coating applied on the microtool was peeled off from the microtool surface due to higher heat, generated within the machining zone because of higher range of machining voltage utlized for titanium micromachining compared range of machining voltage utilized for other than titanium metals. Machining of microfeature by using un-insulated straight cylindrical microtool generates effect of stray machining which causes higher taper formation along the sidewalls of the structure due to the uniform electric field distribution,as illustrated in Fig. 3(a). Use of disk microtool electrode with suitable shank diameter and disc diameter minimizes the problem of taper formation [27] because of non-uniform electric field distribution i.e. reduced field area along the shank of disc microtool, as shown in Fig. 3 (b). Hence, in this experimentation disc shape microtool has been employed to minimize stray current effect. Disc micro tool was fabricated from solid cylindrical tungsten rod of Ø200 m. The tungsten rod of Ø 200 m is machined to a straight shaft of diameter of 118 m utilizing EMM process with the help of developed experimental setup. In this process, tungsten rod was made as anode whereas copper ring of 1 mm inner hole diameter, was made as cathode. The tungsten rod was placed centrally with respect to copper ring, immersed in the electrolyte. The etching of tungsten rod from
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Parameter
Value
1 2 4 5 6
Applied Voltage (V) Pulse Frequency (kHz) Tool feed rate (m/s) Duty ratio (%) Electrolyte concentration (Sodium bromide with ethylene Glycol) Inter electrode Gap (m) Microtool vibration frequency (Hz) Electrolyte concentration (mol/L) Microtool vibration amplitude (m)
10–16 140–200 0.5–2 30 1 mol/L
7 8 9 10
10 85 1 2
Fig. 3. Microfeature profile generation by (a) Cylindrical and (b) Disc microtool.
Fig. 5. Through microgroove terminology. Fig. 4. Fabricated disc tool.
Ø200 m to 118 m had been carried out in 1 mol/L aq. NaOH solution up to a period of 4.32 min with process parameters fixed as, machining voltage 4 V, pulse frequency 5 MHz and pulse ON time of 160 ns against total pulse period of 200 ns. thereafter, fine coating of enamel had been carefully applied on the end face as well as adjacent cylindrical surface of the etched tungsten rod. The height of insulation is carefully controlled to achieve desired disc thickness. Enamel coating is allowed to dry at room temperature for up to 1hr. thereafter; tungsten rod was further etched with EMM process as mentioned above for the period of 1.25 min. The portion under enamel insulation remains un-etched whereas, bare portion etched and reduced to average diameter of 68 m. The insulated coating had been removed by applying acetone. Disc micro tool of disc diameter 118 m with disc thickness of 84 m and average shank diameter of 68 m with total shank length of 724 m had been successfully fabricated. Front end of the disc micro tool was carefully rubbed against fine grade emery paper to make it perfectly flat as well as control the disc thickness. Fig. 4 shows fabricated disc type micro tool used for the experimentation. 3.3. Experimental methodology Pure commercial titanium of grade-1 sheet of 100 m thickness and 20 mm in length and 10 mm width were used. Titanium workpiece is thoroughly cleaned in ultrasonic cleaner and then rinsed with acetone to remove grease and dirt. During trial experiments, it has been observed that applied potential of 10 V with the aid of micro tool vibration improves controlled anodic dissolution of titanium. Hence, machining voltage of 10 V and above has been selected for the analysis. Pulse frequency has been maintained in the range of 140–200 kHz. Ratio of pulse ON to OFF time i.e. duty ratio has been set in such a way that, proper charging of double layer capacitor should be achieved during pulse ON time as well as pulse OFF is sufficient to flush out sludges and reaction products from the narrow machining zone. Microtool feed rate has been selected with minimum tool feed of 0.2 m/s as lower limit and varied with 0.2 m/s up to upper limit of 0.8 m/s.
During trial experiments, it was observed that utilization of aqueous base electrolytes for EMM of titanium accelerates formation of passive oxide layer which results in random pitting with uncontrolled anodic dissolution. Therefore, to achieve controlled anodic dissolution of titanium use of non-aqueous base electrolyte has been preferred. Hence pure ethylene glycol (99% pure laboratory grade) of very less water content (0.3%) was selected [18]. Sodium bromide of concentration 1 mol/L is mixed with ethylene glycol because solution containing bromide ions can dissolve titanium with valence of four as shown in Eq. (4) [18]. Ti + 4Br− → TiBr4 + 4e− .
(4)
To achieve better localization of the anodic dissolution process it is preferable to maintain lower inter-electrode gap (IEG) [28]. Hence, in this experimentation inter-electrode gap of 10 m is maintained. Table 1 represented all EMM process parameters used for experimentation. In order to investigate the suitable range of process parameters to breakdown passive oxide layer and to achieve controlled anodic dissolution of pure titanium all the experiments were systematically planned. The machining voltage has been varied with the interval of 2 V starting from 10 V to 16 V. Microtool vibration along the axis of microtool of 2 m amplitude with frequency of 85 Hz was applied with the help of piezoelectric transducer (PZT). Microtool vibration facilitates the proper circulation of fresh electrolyte in the narrow machining zone [29] leads to enhance the dissolution process by increasing current density through augmenting convective mass transport, results in better diffusion rate and improved conductivity [30] Fig. 5 shows terminology of through microgroove. The average width at the entry and exit of the microgrooves were evaluated by measuring width at different location at entry and exit side respectively with the help of measuring microscope. Based on the average width of entry and exit, width overcut (WOC) as exhibited in Fig. 6, were calculated with the help of Eq. (3) and conicity i.e. taper angle of microgroove has been find out by Eq. (4).
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Fig. 6. Overcut and taper in through microgroove.
Fig. 7. Effect of machining voltage on (a) width overcut (WOC) (b) Length overcut (LOC).
The width overcut (WOC) of micro hole is given by Eq. (5) Wt − Dt width overcut (WOC) = 2
(5)
Where, Wt is the average width of micrgroove measured at entry and exit separately, Dt is the microtool diameter. The length overcut (LOC) as shown in Fig. 6, is calculated utilizing Eq. (6). Length Overcut = Lact − (1000 + Dt)
(6)
Where, Lact is actual machined length of microgroove as shown in Fig. 6 and Dt is microtool diameter. The conicity has been derived from calculating taper angle () of microgroove with the help of Eq. (7). Taper angle() = tan−1
Wt − Wb 2h
(7)
Where, Wt is the average width at the top and Wb is the average width at the bottom, h is the height of microgroove or thickness of workpiece as shown in Fig. 6 4. Result and discussion for micromilling of through microgroove on titanium Microgrooves were machined by varying one parameter at a time i.e machining voltage, pulse frequency and microtool feed rate. The influence of each process parameter on the width overcut (WOC), length overcut (LOC) and taper of microgrooves has been investigated and discussed hereunder: 4.1. Influence of machining voltage on width overcut (WOC), length overcut (LOC) and taper Machining voltage plays important role to break down passive oxide layer and initiates smooth anodic dissolution of titanium. Based on the trial experiments it was observed that current density induced at machining voltage of 10 V is sufficient to break down
passive oxide layer of titanium and convert it into transpassive state to initiate dissolution process of titanium from random pitting to shape controlled dissolution. Hence, machining voltage of 10 V and above has been selected with the aid of microtool vibration for anodic dissolution of titanium keeping other EMM process parameters fixed as pulse frequency 140 KHz, pulse duty ratio 30%, microtool feed rate as 0.5 m/s and electrolyte concentration of 1 mol/L of sodium bromide in ethylene glycol. Microgroove milled with machining voltage of 10 V as well as with applied voltage of 12, 14 and 16 V respectively. Average width obtained at entry of microgroove, machined with 10 V is 304 m and at exit 278 m respectively. Average width overcut (WOC) of microgroove machined with lowest machining voltage of 10 V has been calculated with the help of Eq. (5) is, 93 m and 80 m at entry and exit of the microgroove. This has been plotted with respect to machining voltage and represented in Fig. 7. Microgroove machined with highest machining voltage of 16 V has average width generated at entry and exit is 584 m and 433 m respectively. Average width overcut (WOC)s are 236 m and 159 m at the top and bottom side of the microgroove. From Fig. 7(a) it has been clearly identified that, as the machining voltage increases, width overcut (WOC) increases. The average width overcut (WOC) at exit of microgroove obeys pure linear trend. Increase in machining voltage tends to rise in machining current. According to Faraday’s law material removal increases with machining current and hence, higher material removed at higher machining voltage. With the increase in machining current in the narrow machining zone Joule heating effect generated which leads to elevation in the temperature in the narrow machining zone. This causes variation in electrolyte conductivity results in non-uniform current distribution in the inter electrode gap. Hence, reduces localization of current flux flow leads to random material removal with higher stray machining. Therefore, higher stray current flows in the micromachining zone causes more material removal from the larger area of workpiece, results in an increase in overcut [31]. However, in this micromachining process use of disc type microtool minimizes the
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achieved at the exit of microgroove is lesser than entry thus taper formation takes place. From Fig. 9, it has also been observed that the change in width taper from 12 V to 14 V is due to higher stray machining at the entry of microgroove at 14 V machining voltage compared to 12 V machining voltage and this can also visible from SEM micrograph shown in Fig. 8. This phenomenon can be minimized by overtravel of microtool by few microns beyond the exit point of microgroove however this may create stray machining effects at exit edge. Based on the analysis of various responses such as width overcut (WOC), length overcut (LOC) and taper, machining voltage of 10 V is found to be suitable for keeping various values of responses at minimum. Hence, machining voltage of 10 V is utilized for further analysis. 4.2. Influence of pulse frequency on width overcut, length overcut (LOC) and taper Fig. 8. SEM micrograph of microgrooves machined with 10–16 V machining voltage.
effect of stray machining only up to the disc height hence, overall stray machining is controlled compared to use of conventional straight cylindrical microtool. Fig. 8 exhibits SEM micrograph of microgrooves machined at 12 V–16 V machining voltage. From SEM micrographs represented in Fig. 8 it is clearly observed that width overcut (WOC) increases as machining voltage increases as well as effect of stray machining is predominant at microgrooves machined at higher machining voltages. Length overcut of microgroove is also one of the important criteria in terms of machining accuracy. The microgroove machined with lowest machining voltage of 10 V has attained length of 1232 m and 1200 m at entry and exit respectively. Length overcut i.e. difference between actual machined length and total microtool travel considering distance between centre points of microtool at both the ends, is calculated with the help of Eq. (6). Length overcut at entry and exit of microgroove machined with 10 V machining voltage are obtained as 232 and 200 m respectively. Fig. 7(b) shows the trend of length overcut (LOC) plotted against the machining voltage. From Fig. 7(b) it is confirmed that length overcut (LOC) increases with machining voltage due to the effect of higher material removal at higher machining voltage as discussed earlier. Another influencing factor responsible for length overcut (LOC) is microtool stay time at the end of every scanning stroke i.e. when microtool has to move vertically downwards to machine next layer depth, during this period additional increase in length of the microgroove takes place. Another criterion of investigation of machining accuracy is formation of taper in microgroove which is also greatly influenced by machining voltage. As discussed earlier increase in machining voltage material removal rate increases with stray machining this causes higher width dimension at top side or entry of the microgroove. Taper angle of microgroove along the width as well as along the length has been calculated with the help of Eq. (3) and (7). Fig. 9 exhibits the variation of taper along the width as well as along the length. From Fig. 9 it has been revealed that taper along the width is obtained as 7.4◦ at 10 V machining voltage which further increases to 8.2◦ at 12 V machining voltage and it rises drastically up to 40◦ at machining voltage of 14 V and it again minimizes slightly to 38◦ at 16 V machining voltage. Whereas, taper along the length follow almost linear trend up to 14 V from 10 V machining voltage with taper of 9◦ , 18◦ and 21◦ and increases rapidly up to 34◦ at machining voltage of 16 V. This phenomenon of taper formation can be understood as the microtool further advances into the workpiece with each layer of milling depth of microgroove increases and with increase in depth, machining zone becomes narrower, circulation of fresh electrolyte become crucial results in lower material removal compared to initial machining at the entry hence, width
Microgrooves were further machined with variable pulse frequency in the range of 140–200 KHz keeping other process parameters fixed as machining voltage at 10 V, duty ratio of 30% and tool feed rate maintained at 0.5 m/s with electrolyte concentration remain unchanged. The response parameters are width and length overcut (LOC) at entry and exit and taper at width as well as along the length. The effect of pulse frequency on width and length overcut (LOC) of microgroove is clearly shown in Fig. 10. From Fig. 10(a), it can be observed clearly that width overcut (WOC) of microgroove remarkably reduced from 93 m to 44 m as well as from 80 m to 35 m at entry and exit of microgroove as pulse frequency increases from 140 kHz to 200 kHz respectively. The same trend is followed by decreasing overcut along the length of microgroove, from 114 m to 78 m at entry and from 82 m to 63 m at exit of microgroove as shown in Fig. 10(b). Pulse frequency in EMM directly influences pulse period, as the pulse frequency increases total pulse period reduces and hence pulse ON time is also proportionately reduced. The pulse ON time plays a vital role in charging and discharging of double layer capacitance. In an ideal electrochemical reaction, the total current available during the ON time of pulse period is the sum of non faradic current and faradic current. Non faradic current charges and discharges the double layer capacitance and faradic current governs the material dissolution rate. During pulse ON time of each cycle, initially charging of double layer capacitance takes place and the rest of the ON-time i.e., faradic time, current flows and actual anodic dissolution performed. Hence, material removal takes place only during faradic time of every cycle, which is much smaller. If pulse frequency increases faradic time lowers down with pulse ON time and hence, less material removal per cycle takes place. Therefore, the amount of sludge and gas bubbles are also lesser and the same is flushed out completely from the narrow inter electrode gap during OFF time. Hence, it also helps to create completely clean machining zone to achieve more controlled machining. Therefore, controlled and lesser material removal achieved which in turn improves the geometric shape of the microfeature. The microgrooves machined with 180 kHz and 200 kHz pulse frequency is shown in SEM micrograph represented in Fig. 11(a) and (b). From the SEM micrograph, it can be clearly visible that geometry and overcut of the microgroove has been improved compared to microgrooves machined in lower frequency parameter settings. The taper or conicity has also reduced as pulse frequency increases. Fig. 12 shows the effect of pulse frequency on taper of microgrooves. The lowest taper angle of 5.14◦ along the width and 5.5◦ along the length has been achieved at 200 kHz pulse frequency. As discussed earlier higher pulse frequency enhances machining localization hence, lower difference between entry and exit width as well as length has been produced, which in turn reduces taper. Based on lowest width and length overcut (LOC), pulse frequency
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Fig. 9. Effect of machining voltage on taper.
Fig. 10. Effect of pulse frequency on (a) width overcut (WOC) (b) Length overcut.
Fig. 11. SEM micrograph of microgroove machined with (a) 180 kHz and (b) 200 kHz pulse frequency.
of 200 kHz has been selected for further experimental analysis of through microgroove on pure titanium. 4.3. Influence of microtool feedrate on width overcut (WOC), length overcut (LOC) and taper Microgrooves machined with varying microtool feed rate in the range of 0.5 m/s to 2 m/s in the interval of 0.5 m/s. Frequent short circuit were experienced during trial experiments when microtool federate increases beyond 2 m/s hence, upper
value of federate was restricted to 2 m/s. Other process parameters were fixed based on previous set of experiments as discussed earlier. Machining voltage was kept at 10 V, pulse frequency at 200 kHz with duty ratio 30% and electrolyte concentration remains unchanged at 1 mol/L sodium bromide in ethylene glycol. In an ideal condition the advancement of micro tool into the anode workpiece is such that the tool feed should synchronize with material removal rate i.e. it should not be too fast or too slow. Faster tool feed may reduce the IEG distance with respect to the material removal rate and hence, less time is available to remove the sludge particles and
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Fig. 14. SEM micrograph of microgroove machined at 2 m/s microtool feedrate.
Fig. 12. Effect of pulse frequency on taper.
flow of fresh electrolyte within the machining zone that result in frequent short circuits leads to unstable machining process. Slower tool feed increases the relative tool presence time in the vicinity of machining zone which removes more material leading to overcut and tapering effect in the micro hole which ultimately affect the accuracy. The optimum tool feed rate should always maintain the constant IEG during travel to achieve localized and controlled machining. Effect of microtool feederate on width overcut (WOC) as well as length overcut (LOC) is shown in Fig. 13. From Fig. 13 it can be observed that the significant reduction in entry as well as exit overcut is achieved. As the microtool federate increases relative time of stay of microtool decreases and hence material dissolution synchronizes with machining gap results in controlled material removal with lesser overcut. The width overcut (WOC) of microgroove reduces from 44 m to 32 m at entry as well as at exit it reduces from 38 m to 26 m at highest microtool federate of 2 m/s. The average width achieved at 2 m/s microtool federate is 182 m at entry and 171 m at exit side of microgroove. The microtool feederate of 2 m/s proves to be smoother and optimum microtool feederate to obtain minimum width overcut (WOC) as shown in Fig. 13(a). Minimum overcut of 28 m is obtained along the length of microgroove at entry side. At the exit side length overcut (LOC) is reduced to 16 m with 2 m/s microtool federate as shown in Fig. 13 (b). The SEM micrograph of microgroove micro-
Fig. 15. Effect of microtool feedrate on taper.
machined at 2 m/s is shown in Fig. 14. From this SEM micrograph, controlled nature of machining with improved geometry can be clearly observed. The influence of microtool federate on taper of microgrooves has also been studied. Fig. 15 demonstrates the change in taper in microgroove with respect to tool feed rate. Fig. 15 exhibits that the taper angle reduced from 5.14◦ to 3.12◦ along the width and 5.5◦ to 3.43◦ along length of microgroove, machined at 2 m/s microtool feedrate.
Fig. 13. Effect of microtool feedrate on (a) width overcut (WOC) (b) Length overcut.
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Fig. 16. Complex geometry microgrooves on titanium (a) Square spiral shape (b) “S” shape.
Based on the criteria of minimum width overcut (WOC) and comparatively lower taper angle, microtool feedrate of 2 m/s has been recommended as optimum microtool federate. 5. Generation of complex geometry microgrooves on titanium After investigating the EMM process parameters such as machining voltage, pulse frequency and microtool feedrate on various responses such as width overcut (WOC), length overcut (LOC) and taper of microgroove machined on pure titanium, the best suitable EMM process parameters were obtained as machining voltage as 10 V, pulse frequency as 200 kHz and microtool federate as 2 m/s keeping other process parameters such as duty ratio 30% and electrolyte concentration at 1 mol/L, as constant parameters. Based on these process parameters settings various complex shaped microgrooves have been machined by layer by layer milling process on pure commercial titanium, which are exhibited in Fig. 16. Square spiral shaped microgroove as shown in Fig. 16(a) has total dimension of 853 m X 853 m with average groove width of 188 m. The “S” type microgroove machined has dimension of 1113 m along length and 776 m along height with groove width of 178 m as exhibited in Fig. 16(b). In Fig. 16(b) the “S” shaped microgrooves also generated two micro-cantilevers of length 881 m and 958 m with width of 181 m and 169 m respectively. The total machining time required for fabrication of square spiral shape microgroove was 1hr 46 min and for “S” shaped microgroove total machining time was 1 h 53 min. These complex microfeatures has been analyzed for the machining accuracy i.e. WOC, LOC and taper. The average WOC and LOC as well as taper for square spiral shaped microgroove is obtained as 35 m and 24 m with average taper 4◦ , for “S” shaped microgrooves, 18 m and 26 m respectively with average taper 3◦ . These complex geometry microgrooves on pure titanium have potential applications in the area of MEMS and micro engineering. This successful micromachining of different complex micro features establishes the competence of EMM for micromachining of titanium. 6. Conclusions Micromachining of microgrooves on pure commercial titanium has been demonstrated by employing layer-by-layer micromilling method utilizing maskless EMM process. This study also established most suitable EMM process parameters for successful micromilling of through microgrooves on pure commercial tita-
nium by analyzing the effect of various EMM process parameters on machining accuracy in terms of width overcut (WOC), length overcut (LOC), taper of through microgrooves. The experiments have been carried out by varying one parameter at a time i.e. machining voltage, pulse frequency and microtool feed rate keeping other process parameters constant with micromachined disc microtool. Various findings of this experimentation are:
(i) The machining voltage is one of the major influencing parameter to overcome passive oxide layer of titanium to initiate smoother anodic dissolution during EMM. Width as well as length overcut (LOC) is directly proportional to the machining voltage. Machining voltage of 10 V is found suitable for controlled micromachining of titanium through electrochemical micromachining process. Stray machining is predominant as the voltage increases hence; to reduce stray machining effect, lower machining voltage is recommended. (ii) Pulse frequency is another important parameter to achieve better machining accuracy by localizing material removal process. Overcut is almost linearly reduced as pulse frequency increases. width overcut (WOC) of 93 m and 80 m is obtained at entry as well as exit of through microgroove with initial pulse frequency of 140 kHz, which is further minimizes to 44 m and 38 m at 200 kHz pulse frequency. Length overcut was also reduced from 114 m to 78 m and 82 m to 63 m at entry as well as exit side of through microgroove. taper of microgroove has also reduced from 7.4◦ to 5.14◦ along width and from 9.09◦ to 5.5◦ along the length of microgroove. (iii) Microtool feed rate directly governs the material removal rate and hence, controls the localization effect. In this study microtool federate of 2 m/s is found to be optimum feed rate. Width overcut reduces from 44 m to 32 m at entry as well as at exit it reduces from 38 m to 26 m and length overcut (LOC) reduces to 28 m and 16 m at entry and exit side of through microgroove with minimum degree of taper 3.12◦ obtained with optimum microtool federate of 2 m/s. (iv) This work also demonstrated successful micromachining of complex shaped through microgrooves such as square spiral shape and “S” shape through microgrooves on pure commercial titanium utilizing established optimum EMM process parameters. These complex microfeatures are first of its kind generated on pure commercial titanium by layer by layer micromilling strategy utilizing maskless electrochemical micromachining process.
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The most suitable EMM process parameters are established by this experimental investigation for electrochemical micromilling of pure titanium. This findings can be utilized for generation of various other microfeatures on pure titanium such as micro channels, microslots as well as complex micro features intended for several important applications in the area of micro electro-mechanical systems (MEMS), micro engineering and bio-medical applications. Acknowledgement Authors acknowledge the support from the University Grants Commission, New Delhi India, for carrying out the research under Centre of Advanced Study (CAS) program for Production Engineering Department, Jadavpur University, Kolkata, India. References [1] Farthing T. Application of titanium in the chemical industry. Chem Age India 1979;30(2). [2] Kumar KV. Grinding titanium. Aerosp Eng 1991;11(9):17–9. [3] Yamashita Y, Takayama I, Fuji H, Yamazaki T. Applications and features of titanium for automotive industry. Nippon Steel Tech Rep 2002;85(11–14):151–66. [4] Yang Xiaoping, Liu CRichard. Machining titanium and its alloys’. Mach Sci Technol 1999;3(1):107–39. [5] Ezugwu EO, Wang ZM. Titanium alloys and their machinability a review. J Mater Process Technol 1997;68:262–74. [6] Yadav Vinod, Jain Vijay K, Dixit Prakash M. Thermal stresses due to electrical discharge machining. Int J Mach Tools Manuf 2002;42:877–88. [7] Lin Yan Cherng, Yan Biing Hwa, Chang Yong Song. Machining characteristics of titanium alloy (Ti6Al4V) using a combination process of EDM with USM. J Mater Process Technol 2000;104:171–7. [8] Choi KH, Meijer J, Masuzawa T, Kim D-H. Excimer laser micromachining for 3D microstructure. J Mater Process Technol 2004;149(1–3):561–6. [9] Jain VK, Subodh Kalia, Ajay Sidpara, Kulkarni VN. Fabrication of micro-features and micro-tools using electrochemical micromachining. Int J Adv Manuf Technol 2012;61:1175–83. [10] Bhattacharyya B, Malapati M, Munda J. Experimental study on electrochemical micromachining. J Mater Process Technol 2005;169:485–92. [11] Dyaminov RD, Mal’tsev AN, Kargin GV. Electrochemical machining of titaniumrotor cast blades for vortex pumps. Chem Petrol Eng 1977;13(9):817–8. [12] Dhobe Shirish D, Doloi B, Bhattacharyya B. Surface characteristics of ECMed titanium work samples for biomedical applications. Int J Adv Manuf Technol 2011;55:177–88. [13] Kern P, Vehand J, Michler J. New developments in through-mask electrochemical micromachining of titanium. J Micromech Microeng 2007;17:1168–77.
[14] Piotrowski O, Madore C, Landolt D. The mechanism of electropolishing of titanium in Methanol-Sulfuric acid electrolytes. J Electrochem Soc 1998;145(7). The Electrochemical Society, Inc. [15] Jiang LM, Li W, Attia A, Cheng ZY, Tang J, Tian ZQ, et al. A potential method for electrochemical micromachining of titanium alloy Ti6Al4V. J Appl Electrochem 2008;38:785–91. [16] Chauvy PF, Hoffmann P, Landolt D. Electrochemical micromachining of titanium using laser oxide film lithography: excimer laser irradiation of anodic oxide. Appl Surf Sci 2003;211:113–27. [17] Aladjem A. Anodic Oxidation of titanium and its alloys. J Mater Sci 1973;8:688–704. [18] Anasane SS, Bhattacharyya B. Experimental investigation on suitability of electrolytes for electrochemical micromachining of titanium. Int J Adv Manuf Technol 2016;86:2147–60. [19] Cotton FA, Wilkinson G, Murillo CA, Bochmann M. Advanced inorganic chemistry. 6th edn New York: Wiley; 1999. [20] Zinger O, Chauvy PF, Landolt D. Scale-resolved electrochemical surface structuring of titanium for biological applications. J Electrochem Soc 2003;150(11):B495–503. [21] Landolt D, Chauvy PF, Zinger O. Electrochemical micromachining, polishing and surface structuring of metals: fundamental aspects and new developments. Electrochem Acta 2003;48:3185–201. [22] Bannard J. On the electrochemical machining of some titanium alloys in bromide electrolytes. J Appl Electrochem 1976;6:477–83. [23] Madore C, Landolt D. Electrochemical micromachining of controlled topographies on titanium for biological applications. J Micromech Microeng 1997;7:270–5. [24] Sjöström Terje, Su Bo. Micropatterning of titanium surfaces using electrochemical micromachining with an ethylene glycol electrolyte. Mater Lett 2011;65:3489–92. [25] Yong Liu, Di Zhu, Linsen Zhu. Micro electrochemical milling of complex structures by using in situ fabricated cylindrical electrode. Int J Adv Manuf Technol 2012;60:977–84. [26] Rathod V, Doloi B, Bhattacharyya B. Sidewall insulation of microtool for electrochemical micromachining to enhance the machining accuracy. Mater Manuf Processes 2014;29:305–13. [27] Vijaysing Rathod, Doloi B, Bhattacharyya B. Parametric investigation into the fabrication of disk microelectrodes by electrochemical micromachining. J Micro- Nano-Manuf 2013;1/041005-1(December). [28] De Silva AKM, Altena HSJ, McGeough JA. Precision ECM by process characteristic modeling. Ann CIRP 2000;49(1):151–6. [29] Chikamori K. Possibilities of electrochemical micromachining. Int J Jpn Soc Precis Eng 1998;32(1):37–8. [30] Ghoshal B, Bhattacharyya B. Influence of vibration on micro-tool fabrication by electrochemical machining. Int J Mach Tools Manuf 2013;64:49–59. [31] Bhattacharyya B. Electrochemical Micromachining for Nanofabrication, MEMS and Nanotechnology. William Andrew publications (Imprint of Elsevier); 2015.
Contents lists available at ScienceDirect
Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro
Experimental investigation into micromilling of microgrooves on titanium by electrochemical micromachining Sandip S. Anasane a,∗ , B. Bhattacharyya b a b
Department of Production Engineering and Industrial Management, College of Engineering, Pune, India Production Engineering Department, Jadavpur University, Kolkata, India
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 27 April 2017 Accepted 24 June 2017 Keywords: Microgroove EMM Titanium Layer-by-layer milling Disc microtool
a b s t r a c t Titanium possesses excellent physical and chemical properties such as higher strength to weight ratio, greater biocompatibility and outstanding corrosion resistance. Therefore, titanium is highly demanding material in aerospace to biomedical applications from macro to micro scale levels. However, machining of titanium in macroscopic or microscopic domain is a complex task either by conventional or nonconventional machining processes. Electrochemical micromachining (EMM) could be one of the alternatives for machining of titanium in microscopic area. The study elaborated in this paper demonstrates successful micromilling of through microgrooves on pure commercial titanium utilizing maskless electrochemical micromachining. Layer-by-layer micromilling strategy has been successfully employed with the help of in-situ fabricated disc shape microtool for micromilling of through microgrooves. Influence of various EMM process parameters such as machining voltage, pulse frequency and microtool feedrate on machining accuracy of through microgrooves were also investigated and established most suitable EMM process parameters for successful micromilling of through microgrooves on pure titanium. This paper also demonstrated successful fabrication of few complex geometry microgrooves by electrochemical micromilling for practical applications. Micromilling of microgrooves on pure titanium by EMM may find vital applications for MEMS, micro fluidics, microsensors, micro engineering and biomedical systems. © 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
1. Introduction Demand of miniaturization is rapidly increasing in almost all fields, which drives researchers and engineers to explore potentials of various micromachining techniques as well as materials in order to establish novel solutions to microengineering applications. Incorporating advanced materials in micro domain will create larger impact to the functional aspects of microstructures. Titanium is a material known for its versatile physical and mechanical properties such as high strength to weight ratio, high compressive and tensile strength, low density, high fatigue resistance in air and seawater, and exceptional corrosion resistance [1–3]. This made titanium prominent material in a variety of applications such as aerospace, biomedical, sporting goods, marine, military and chemical industries. However, titanium machining either by conventional or non conventional methods is an always a concern to the researchers and engineers due to its poor machinability. In the conventional subtractive machining techniques, problems such as shorter tool life and higher tool wear reported because of higher
∗ Corresponding author. E-mail address: [email protected] (S.S. Anasane).
cutting forces and excessive heat generation due to poor thermal properties of the titanium as well as chatter, deflection and rubbing of cutting tool owing to low modulus of elasticity with its ability to maintain high strength at elevated temperatures [4]. Titanium is very chemically reactive hence; it has a tendency to weld to the cutting tool during machining leading to chipping and premature tool failure [5]. Even in non conventional machining process machining of titanium is an intricate task, Electric Discharge Machining (EDM) and Ultrasonic Machining (USM) has been applied to the machining of titanium and its alloys during recent times. In EDM, formation of thermal stresses in a small heat-affected zone is a serious issue which can lead to micro-cracks, decrease in strength and fatigue life and possibly catastrophic failure of the component [6]. Ultrasonic machining (USM), lower MRR compared with other processes and serious tool wear that usually affects machining precision [7]. Laser beam machining (LBM) can be applied for machining of titanium, but even this process has its own problems of heat affected zone and re-solidification of layers as well as probability of formation of hot spot due to thermal effect [8]. Electrochemical machining (ECM) has proved its compatibility for machining of titanium in macro scale level. In ECM, material removal carried out by anodic dissolution principle hence, this process can machine materials irrespective of their hardness.
http://dx.doi.org/10.1016/j.jmapro.2017.06.016 1526-6125/© 2017 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.
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When ECM is employed for fabrication of features in microscale level i.e. microfeatures in the range from 1 to 999 m then ECM becomes electrochemical micromachining (EMM) [9]. EMM emerged as potential micromachining technology due to its several advantages such as high MRR, enhanced precision and control and environmentally tolerable in addition to that it also machine chemically resistant materials i.e. titanium, stainless steel and super alloys [10]. Therefore, micromachining of titanium by electrochemical process could be one of the competent alternative techniques for machining of titanium. Machining of titanium in macroscopic domain utilizing Electrochemical machining (ECM) has been reported [11,12]. However, in microscopic domain, most of the work has been reported in the area of micropatterns generation intended for surface structuring applications either by electrochemical etching, or electrochemical polishing (EP) [13–16]. All these techniques were utilized expensive and complex sophisticated processes such as through mask electrochemical micromachining (TMEMM) or with confined etchant layer technique or by oxide film laser lithography. But very few or almost no attempts have been reported based on fabrication of complex microfeatures on pure titanium utilizing maskless electrochemical micromachining (EMM). Major challenge in fabrication of microfeatures on titanium is to achieve controlled anodic dissolution of titanium in micro domain by overcoming passivity of oxide layer present on surface of the titanium. Hence, in this paper systematic experimental investigation has been carried out to identify optimum process parameters for generation of microfeatures such as through microgrooves by achieving controlled anodic dissolution of titanium in microscopic level. The work elaborated in this paper also demonstrated layer by layer electrochemical micromilling strategy with the aid of in-situ fabricated cylindrical disc shape microtool for successful micromilling of complex shape through microgrooves utilizing indigenously developed maskless electrochemical micromachining set up. 1.1. Anodic dissolution of titanium by EMM Anodic dissolution of pure titanium utilizing electrochemical micromachining (EMM) is complex task compared to EMM of other metals. Titanium possesses good corrosion resistance because of presence of surface oxide film. This protective tenacious oxide film is formed on the surface of titanium, when titanium is depicted to oxygen containing media such as air, water etc [17]. When titanium undergoes electrochemical actions, the role of transfer of Ti2+ and O2− is contribute for development of anodic film. Formation of passive oxide layer on titanium surface during electrochemical process with aqueous environment is initiated by reacting Ti2+ with hydroxide ions (OH− ) ionized from aqueous solution. Following electrochemical reactions represents stable titanium oxide (TiO2 ) [18]. At the interface of anode workpiece and electrolyte, the reactions taking place are: Ti → Ti2+ + 2e−
(1)
Ti2+ + H2 O → TiO2+ + H2 ↑
(2)
TiO2+
The oxocation, is acidic in nature and subsequently reacts with OH− to form stable TiO2 [19]. Following chemical reaction represent the formation of stable TiO2 : TiO2+ + 2OH− → TiO2 + H2 O
(3)
Throughout the anodic dissolution process, development of oxide layer with the help of titanium and hydroxide ions has been accelerated by the application of electric field.
This thin oxide film is highly passive in nature, causes anodic dissolution of titanium difficult. The controlled anodic dissolution of titanium is difficult by EMM process parameters generally utilized for micromachining of other metals especially in terms of machining voltage and type of electrolyte due to the presence of passive oxide layer. Hence, to attain uniform transpassive dissolution of titanium, the oxide film that obstructs the controlled dissolution of pure titanium in the passive potential region must lose its passivation phenomenon. Removal of oxide film is possible when the applied potential is adequately high [20]. The passive oxide film develops linearly with potential until a significant value is attained and the breakdown of the film takes place from random pitting at higher current densities and then shape controlled dissolution begins [21]. An additional factor, which plays vital role in rupturing this passive oxide film, is the electrolyte type i.e. Electrolytes containing bromide ions have been effective in breakdown of oxide film [22]. Electrolyte solution includes bromide ions can promote anodic dissolution of titanium with valance of 4 i.e. Ti → Ti4+ [23]. The use of aqueous based electrolytes for EMM of titanium can create the possibility of formation of passive oxide layer or increase the thickness of already existing oxide film formed due to atmospheric oxygen. Use of aqueous based electrolytes could be altered with non-aqueous based electrolytes to minimize the chances of formation of passive oxide layer [24]. The accuracy of the micro machined product in EMM is highly influenced by process parameters e.g. applied machining voltage, duty ratio, pulse frequency, concentration of electrolyte, inter-electrode gap (IEG) and micro tool feed etc. Therefore, appropriate selection along with controlling of all these EMM parameters play a vital role in attaining the preferred results during titanium micro machining utilizing EMM. 2. Experimental setup Experimentation for micromachining of titanium has been executed on indigenously developed electrochemical micromachining set up as shown in Fig. 1. The EMM experimental set up consist of different subsystems i.e. mechanical machining unit, desktop computer, DC pulsed power supply, digital storage oscilloscope (Tektronix) as well as measuring microscope etc. Indigenously fabricated electrolyte chamber from Perspex material having work holding arrangement has been employed for the experimentation. Mechanical machining unit includes three linear travel stages designating X, Y, and Z-axis. The stepper motors of each linear stage with resolution of 0.1 m/step are controlled by CNC controller unit, which is interfaced with desktop computer. Different feeds can provide to all the three stepper motors at a time with the help of position controller software through desktop computer. DC pulsed power supply has been used for generation of required nature of pulse power intended for micro machining operation. Digital storage oscilloscope has been used for observation of pulse waveform. The digital multimeter (Agilant U1252A) was connected in series to the circuit for the measurement of current values. Stereo type microscope (Leica S6D) was used for observation, measuring microscope (Leica DM2500) were utilized for measurement of various criteria of machined microfeature. To augment the supply of electrolyte into the narrow inter electrode gap (IEG) during EMM operation by providing longitudinal vibrations to the microtool, Piezoelectric transducer (PZT) has been utilized. 3. Experimental planning In order to investigate the influence of various EMM process parameters on various responses such as accuracy of microgroove in terms of width overcut (WOC), length overcut (LOC) and taper, all the experiments were systematically planned and executed by
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Fig. 1. Experimental setup.
In this work, to minimize the effect of stray machining and to reduce the effect of overcut, in-situ fabricated disc type microtool of disc diameter 118 m was utilized. The downward movement of microtool is maintained by 20 m at the end of each stroke length. Hence, microtool advances with layer thickness obtained at 20 m depth at each layer milling. In this experimentation stroke length of microtool along X-Y plane is restricted to 1000 m, which is total desired length of the through microgroove. 3.2. Development of disc type microtool
Fig. 2. Layer-by-layer micromilling.
varying one parameter at a time. Lower and upper limit of various process parameters, such as machining voltage, duty ratio, pulse frequency and microtool feed rate have been decided based on results generated by conducting extensive trial experiments. Layer-by-layer milling strategy is adopted for micromilling of microgrooves with the help of in-situ fabricated disc type microtool. 3.1. Layer-by- layer electrochemical micromilling In the Layer-by-layer micromilling technique, initially cylindrical microtool advances vertically i.e. along z-axis into the anode workpiece then it advances linearly along the length of microgroove. The initial depth of penetration of microtool in the anode workpiece is the initial layer thickness with this layer thickness microtool travels to and fro along the length of microgroove and mills the microgroove throughout the entire length. Fig. 2 represents the layer-by-layer milling process. In this layer-by-layer milling, layer depth gradually increases during downward feed movement at the end of each milling stroke length and continues till the desired depth is achieved. Dissolution of material takes place from the front face as well as side face of the microtool. In order to maintain the good shape precision i.e. width at the start and end location as well as along the length of microgroove. It is recommended to maintain milling layer thickness less than or equal to diameter of microtool [25].
Microtools are one of the important factor in the electrochemical micromachining process because microtool, which acts as a cathode produces its negative shape in the anode i.e. workpiece. Hence, microtool shape, size, and surface quality directly affect the dimensional accuracy, precision, and surface quality of the machined microfeature. Therefore, precise microtool of micron scale size with good surface finish is crucial for the machining of microfeatures. Microtool insulation made of polymer and resin dissolved in isopropyl alcohol as solvent [26] were also tested for EMM of titanium. However, during trial experiments it has been observed that insulation coating applied on the microtool was peeled off from the microtool surface due to higher heat, generated within the machining zone because of higher range of machining voltage utlized for titanium micromachining compared range of machining voltage utilized for other than titanium metals. Machining of microfeature by using un-insulated straight cylindrical microtool generates effect of stray machining which causes higher taper formation along the sidewalls of the structure due to the uniform electric field distribution,as illustrated in Fig. 3(a). Use of disk microtool electrode with suitable shank diameter and disc diameter minimizes the problem of taper formation [27] because of non-uniform electric field distribution i.e. reduced field area along the shank of disc microtool, as shown in Fig. 3 (b). Hence, in this experimentation disc shape microtool has been employed to minimize stray current effect. Disc micro tool was fabricated from solid cylindrical tungsten rod of Ø200 m. The tungsten rod of Ø 200 m is machined to a straight shaft of diameter of 118 m utilizing EMM process with the help of developed experimental setup. In this process, tungsten rod was made as anode whereas copper ring of 1 mm inner hole diameter, was made as cathode. The tungsten rod was placed centrally with respect to copper ring, immersed in the electrolyte. The etching of tungsten rod from
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Parameter
Value
1 2 4 5 6
Applied Voltage (V) Pulse Frequency (kHz) Tool feed rate (m/s) Duty ratio (%) Electrolyte concentration (Sodium bromide with ethylene Glycol) Inter electrode Gap (m) Microtool vibration frequency (Hz) Electrolyte concentration (mol/L) Microtool vibration amplitude (m)
10–16 140–200 0.5–2 30 1 mol/L
7 8 9 10
10 85 1 2
Fig. 3. Microfeature profile generation by (a) Cylindrical and (b) Disc microtool.
Fig. 5. Through microgroove terminology. Fig. 4. Fabricated disc tool.
Ø200 m to 118 m had been carried out in 1 mol/L aq. NaOH solution up to a period of 4.32 min with process parameters fixed as, machining voltage 4 V, pulse frequency 5 MHz and pulse ON time of 160 ns against total pulse period of 200 ns. thereafter, fine coating of enamel had been carefully applied on the end face as well as adjacent cylindrical surface of the etched tungsten rod. The height of insulation is carefully controlled to achieve desired disc thickness. Enamel coating is allowed to dry at room temperature for up to 1hr. thereafter; tungsten rod was further etched with EMM process as mentioned above for the period of 1.25 min. The portion under enamel insulation remains un-etched whereas, bare portion etched and reduced to average diameter of 68 m. The insulated coating had been removed by applying acetone. Disc micro tool of disc diameter 118 m with disc thickness of 84 m and average shank diameter of 68 m with total shank length of 724 m had been successfully fabricated. Front end of the disc micro tool was carefully rubbed against fine grade emery paper to make it perfectly flat as well as control the disc thickness. Fig. 4 shows fabricated disc type micro tool used for the experimentation. 3.3. Experimental methodology Pure commercial titanium of grade-1 sheet of 100 m thickness and 20 mm in length and 10 mm width were used. Titanium workpiece is thoroughly cleaned in ultrasonic cleaner and then rinsed with acetone to remove grease and dirt. During trial experiments, it has been observed that applied potential of 10 V with the aid of micro tool vibration improves controlled anodic dissolution of titanium. Hence, machining voltage of 10 V and above has been selected for the analysis. Pulse frequency has been maintained in the range of 140–200 kHz. Ratio of pulse ON to OFF time i.e. duty ratio has been set in such a way that, proper charging of double layer capacitor should be achieved during pulse ON time as well as pulse OFF is sufficient to flush out sludges and reaction products from the narrow machining zone. Microtool feed rate has been selected with minimum tool feed of 0.2 m/s as lower limit and varied with 0.2 m/s up to upper limit of 0.8 m/s.
During trial experiments, it was observed that utilization of aqueous base electrolytes for EMM of titanium accelerates formation of passive oxide layer which results in random pitting with uncontrolled anodic dissolution. Therefore, to achieve controlled anodic dissolution of titanium use of non-aqueous base electrolyte has been preferred. Hence pure ethylene glycol (99% pure laboratory grade) of very less water content (0.3%) was selected [18]. Sodium bromide of concentration 1 mol/L is mixed with ethylene glycol because solution containing bromide ions can dissolve titanium with valence of four as shown in Eq. (4) [18]. Ti + 4Br− → TiBr4 + 4e− .
(4)
To achieve better localization of the anodic dissolution process it is preferable to maintain lower inter-electrode gap (IEG) [28]. Hence, in this experimentation inter-electrode gap of 10 m is maintained. Table 1 represented all EMM process parameters used for experimentation. In order to investigate the suitable range of process parameters to breakdown passive oxide layer and to achieve controlled anodic dissolution of pure titanium all the experiments were systematically planned. The machining voltage has been varied with the interval of 2 V starting from 10 V to 16 V. Microtool vibration along the axis of microtool of 2 m amplitude with frequency of 85 Hz was applied with the help of piezoelectric transducer (PZT). Microtool vibration facilitates the proper circulation of fresh electrolyte in the narrow machining zone [29] leads to enhance the dissolution process by increasing current density through augmenting convective mass transport, results in better diffusion rate and improved conductivity [30] Fig. 5 shows terminology of through microgroove. The average width at the entry and exit of the microgrooves were evaluated by measuring width at different location at entry and exit side respectively with the help of measuring microscope. Based on the average width of entry and exit, width overcut (WOC) as exhibited in Fig. 6, were calculated with the help of Eq. (3) and conicity i.e. taper angle of microgroove has been find out by Eq. (4).
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Fig. 6. Overcut and taper in through microgroove.
Fig. 7. Effect of machining voltage on (a) width overcut (WOC) (b) Length overcut (LOC).
The width overcut (WOC) of micro hole is given by Eq. (5) Wt − Dt width overcut (WOC) = 2
(5)
Where, Wt is the average width of micrgroove measured at entry and exit separately, Dt is the microtool diameter. The length overcut (LOC) as shown in Fig. 6, is calculated utilizing Eq. (6). Length Overcut = Lact − (1000 + Dt)
(6)
Where, Lact is actual machined length of microgroove as shown in Fig. 6 and Dt is microtool diameter. The conicity has been derived from calculating taper angle () of microgroove with the help of Eq. (7). Taper angle() = tan−1
Wt − Wb 2h
(7)
Where, Wt is the average width at the top and Wb is the average width at the bottom, h is the height of microgroove or thickness of workpiece as shown in Fig. 6 4. Result and discussion for micromilling of through microgroove on titanium Microgrooves were machined by varying one parameter at a time i.e machining voltage, pulse frequency and microtool feed rate. The influence of each process parameter on the width overcut (WOC), length overcut (LOC) and taper of microgrooves has been investigated and discussed hereunder: 4.1. Influence of machining voltage on width overcut (WOC), length overcut (LOC) and taper Machining voltage plays important role to break down passive oxide layer and initiates smooth anodic dissolution of titanium. Based on the trial experiments it was observed that current density induced at machining voltage of 10 V is sufficient to break down
passive oxide layer of titanium and convert it into transpassive state to initiate dissolution process of titanium from random pitting to shape controlled dissolution. Hence, machining voltage of 10 V and above has been selected with the aid of microtool vibration for anodic dissolution of titanium keeping other EMM process parameters fixed as pulse frequency 140 KHz, pulse duty ratio 30%, microtool feed rate as 0.5 m/s and electrolyte concentration of 1 mol/L of sodium bromide in ethylene glycol. Microgroove milled with machining voltage of 10 V as well as with applied voltage of 12, 14 and 16 V respectively. Average width obtained at entry of microgroove, machined with 10 V is 304 m and at exit 278 m respectively. Average width overcut (WOC) of microgroove machined with lowest machining voltage of 10 V has been calculated with the help of Eq. (5) is, 93 m and 80 m at entry and exit of the microgroove. This has been plotted with respect to machining voltage and represented in Fig. 7. Microgroove machined with highest machining voltage of 16 V has average width generated at entry and exit is 584 m and 433 m respectively. Average width overcut (WOC)s are 236 m and 159 m at the top and bottom side of the microgroove. From Fig. 7(a) it has been clearly identified that, as the machining voltage increases, width overcut (WOC) increases. The average width overcut (WOC) at exit of microgroove obeys pure linear trend. Increase in machining voltage tends to rise in machining current. According to Faraday’s law material removal increases with machining current and hence, higher material removed at higher machining voltage. With the increase in machining current in the narrow machining zone Joule heating effect generated which leads to elevation in the temperature in the narrow machining zone. This causes variation in electrolyte conductivity results in non-uniform current distribution in the inter electrode gap. Hence, reduces localization of current flux flow leads to random material removal with higher stray machining. Therefore, higher stray current flows in the micromachining zone causes more material removal from the larger area of workpiece, results in an increase in overcut [31]. However, in this micromachining process use of disc type microtool minimizes the
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achieved at the exit of microgroove is lesser than entry thus taper formation takes place. From Fig. 9, it has also been observed that the change in width taper from 12 V to 14 V is due to higher stray machining at the entry of microgroove at 14 V machining voltage compared to 12 V machining voltage and this can also visible from SEM micrograph shown in Fig. 8. This phenomenon can be minimized by overtravel of microtool by few microns beyond the exit point of microgroove however this may create stray machining effects at exit edge. Based on the analysis of various responses such as width overcut (WOC), length overcut (LOC) and taper, machining voltage of 10 V is found to be suitable for keeping various values of responses at minimum. Hence, machining voltage of 10 V is utilized for further analysis. 4.2. Influence of pulse frequency on width overcut, length overcut (LOC) and taper Fig. 8. SEM micrograph of microgrooves machined with 10–16 V machining voltage.
effect of stray machining only up to the disc height hence, overall stray machining is controlled compared to use of conventional straight cylindrical microtool. Fig. 8 exhibits SEM micrograph of microgrooves machined at 12 V–16 V machining voltage. From SEM micrographs represented in Fig. 8 it is clearly observed that width overcut (WOC) increases as machining voltage increases as well as effect of stray machining is predominant at microgrooves machined at higher machining voltages. Length overcut of microgroove is also one of the important criteria in terms of machining accuracy. The microgroove machined with lowest machining voltage of 10 V has attained length of 1232 m and 1200 m at entry and exit respectively. Length overcut i.e. difference between actual machined length and total microtool travel considering distance between centre points of microtool at both the ends, is calculated with the help of Eq. (6). Length overcut at entry and exit of microgroove machined with 10 V machining voltage are obtained as 232 and 200 m respectively. Fig. 7(b) shows the trend of length overcut (LOC) plotted against the machining voltage. From Fig. 7(b) it is confirmed that length overcut (LOC) increases with machining voltage due to the effect of higher material removal at higher machining voltage as discussed earlier. Another influencing factor responsible for length overcut (LOC) is microtool stay time at the end of every scanning stroke i.e. when microtool has to move vertically downwards to machine next layer depth, during this period additional increase in length of the microgroove takes place. Another criterion of investigation of machining accuracy is formation of taper in microgroove which is also greatly influenced by machining voltage. As discussed earlier increase in machining voltage material removal rate increases with stray machining this causes higher width dimension at top side or entry of the microgroove. Taper angle of microgroove along the width as well as along the length has been calculated with the help of Eq. (3) and (7). Fig. 9 exhibits the variation of taper along the width as well as along the length. From Fig. 9 it has been revealed that taper along the width is obtained as 7.4◦ at 10 V machining voltage which further increases to 8.2◦ at 12 V machining voltage and it rises drastically up to 40◦ at machining voltage of 14 V and it again minimizes slightly to 38◦ at 16 V machining voltage. Whereas, taper along the length follow almost linear trend up to 14 V from 10 V machining voltage with taper of 9◦ , 18◦ and 21◦ and increases rapidly up to 34◦ at machining voltage of 16 V. This phenomenon of taper formation can be understood as the microtool further advances into the workpiece with each layer of milling depth of microgroove increases and with increase in depth, machining zone becomes narrower, circulation of fresh electrolyte become crucial results in lower material removal compared to initial machining at the entry hence, width
Microgrooves were further machined with variable pulse frequency in the range of 140–200 KHz keeping other process parameters fixed as machining voltage at 10 V, duty ratio of 30% and tool feed rate maintained at 0.5 m/s with electrolyte concentration remain unchanged. The response parameters are width and length overcut (LOC) at entry and exit and taper at width as well as along the length. The effect of pulse frequency on width and length overcut (LOC) of microgroove is clearly shown in Fig. 10. From Fig. 10(a), it can be observed clearly that width overcut (WOC) of microgroove remarkably reduced from 93 m to 44 m as well as from 80 m to 35 m at entry and exit of microgroove as pulse frequency increases from 140 kHz to 200 kHz respectively. The same trend is followed by decreasing overcut along the length of microgroove, from 114 m to 78 m at entry and from 82 m to 63 m at exit of microgroove as shown in Fig. 10(b). Pulse frequency in EMM directly influences pulse period, as the pulse frequency increases total pulse period reduces and hence pulse ON time is also proportionately reduced. The pulse ON time plays a vital role in charging and discharging of double layer capacitance. In an ideal electrochemical reaction, the total current available during the ON time of pulse period is the sum of non faradic current and faradic current. Non faradic current charges and discharges the double layer capacitance and faradic current governs the material dissolution rate. During pulse ON time of each cycle, initially charging of double layer capacitance takes place and the rest of the ON-time i.e., faradic time, current flows and actual anodic dissolution performed. Hence, material removal takes place only during faradic time of every cycle, which is much smaller. If pulse frequency increases faradic time lowers down with pulse ON time and hence, less material removal per cycle takes place. Therefore, the amount of sludge and gas bubbles are also lesser and the same is flushed out completely from the narrow inter electrode gap during OFF time. Hence, it also helps to create completely clean machining zone to achieve more controlled machining. Therefore, controlled and lesser material removal achieved which in turn improves the geometric shape of the microfeature. The microgrooves machined with 180 kHz and 200 kHz pulse frequency is shown in SEM micrograph represented in Fig. 11(a) and (b). From the SEM micrograph, it can be clearly visible that geometry and overcut of the microgroove has been improved compared to microgrooves machined in lower frequency parameter settings. The taper or conicity has also reduced as pulse frequency increases. Fig. 12 shows the effect of pulse frequency on taper of microgrooves. The lowest taper angle of 5.14◦ along the width and 5.5◦ along the length has been achieved at 200 kHz pulse frequency. As discussed earlier higher pulse frequency enhances machining localization hence, lower difference between entry and exit width as well as length has been produced, which in turn reduces taper. Based on lowest width and length overcut (LOC), pulse frequency
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Fig. 9. Effect of machining voltage on taper.
Fig. 10. Effect of pulse frequency on (a) width overcut (WOC) (b) Length overcut.
Fig. 11. SEM micrograph of microgroove machined with (a) 180 kHz and (b) 200 kHz pulse frequency.
of 200 kHz has been selected for further experimental analysis of through microgroove on pure titanium. 4.3. Influence of microtool feedrate on width overcut (WOC), length overcut (LOC) and taper Microgrooves machined with varying microtool feed rate in the range of 0.5 m/s to 2 m/s in the interval of 0.5 m/s. Frequent short circuit were experienced during trial experiments when microtool federate increases beyond 2 m/s hence, upper
value of federate was restricted to 2 m/s. Other process parameters were fixed based on previous set of experiments as discussed earlier. Machining voltage was kept at 10 V, pulse frequency at 200 kHz with duty ratio 30% and electrolyte concentration remains unchanged at 1 mol/L sodium bromide in ethylene glycol. In an ideal condition the advancement of micro tool into the anode workpiece is such that the tool feed should synchronize with material removal rate i.e. it should not be too fast or too slow. Faster tool feed may reduce the IEG distance with respect to the material removal rate and hence, less time is available to remove the sludge particles and
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Fig. 14. SEM micrograph of microgroove machined at 2 m/s microtool feedrate.
Fig. 12. Effect of pulse frequency on taper.
flow of fresh electrolyte within the machining zone that result in frequent short circuits leads to unstable machining process. Slower tool feed increases the relative tool presence time in the vicinity of machining zone which removes more material leading to overcut and tapering effect in the micro hole which ultimately affect the accuracy. The optimum tool feed rate should always maintain the constant IEG during travel to achieve localized and controlled machining. Effect of microtool feederate on width overcut (WOC) as well as length overcut (LOC) is shown in Fig. 13. From Fig. 13 it can be observed that the significant reduction in entry as well as exit overcut is achieved. As the microtool federate increases relative time of stay of microtool decreases and hence material dissolution synchronizes with machining gap results in controlled material removal with lesser overcut. The width overcut (WOC) of microgroove reduces from 44 m to 32 m at entry as well as at exit it reduces from 38 m to 26 m at highest microtool federate of 2 m/s. The average width achieved at 2 m/s microtool federate is 182 m at entry and 171 m at exit side of microgroove. The microtool feederate of 2 m/s proves to be smoother and optimum microtool feederate to obtain minimum width overcut (WOC) as shown in Fig. 13(a). Minimum overcut of 28 m is obtained along the length of microgroove at entry side. At the exit side length overcut (LOC) is reduced to 16 m with 2 m/s microtool federate as shown in Fig. 13 (b). The SEM micrograph of microgroove micro-
Fig. 15. Effect of microtool feedrate on taper.
machined at 2 m/s is shown in Fig. 14. From this SEM micrograph, controlled nature of machining with improved geometry can be clearly observed. The influence of microtool federate on taper of microgrooves has also been studied. Fig. 15 demonstrates the change in taper in microgroove with respect to tool feed rate. Fig. 15 exhibits that the taper angle reduced from 5.14◦ to 3.12◦ along the width and 5.5◦ to 3.43◦ along length of microgroove, machined at 2 m/s microtool feedrate.
Fig. 13. Effect of microtool feedrate on (a) width overcut (WOC) (b) Length overcut.
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Fig. 16. Complex geometry microgrooves on titanium (a) Square spiral shape (b) “S” shape.
Based on the criteria of minimum width overcut (WOC) and comparatively lower taper angle, microtool feedrate of 2 m/s has been recommended as optimum microtool federate. 5. Generation of complex geometry microgrooves on titanium After investigating the EMM process parameters such as machining voltage, pulse frequency and microtool feedrate on various responses such as width overcut (WOC), length overcut (LOC) and taper of microgroove machined on pure titanium, the best suitable EMM process parameters were obtained as machining voltage as 10 V, pulse frequency as 200 kHz and microtool federate as 2 m/s keeping other process parameters such as duty ratio 30% and electrolyte concentration at 1 mol/L, as constant parameters. Based on these process parameters settings various complex shaped microgrooves have been machined by layer by layer milling process on pure commercial titanium, which are exhibited in Fig. 16. Square spiral shaped microgroove as shown in Fig. 16(a) has total dimension of 853 m X 853 m with average groove width of 188 m. The “S” type microgroove machined has dimension of 1113 m along length and 776 m along height with groove width of 178 m as exhibited in Fig. 16(b). In Fig. 16(b) the “S” shaped microgrooves also generated two micro-cantilevers of length 881 m and 958 m with width of 181 m and 169 m respectively. The total machining time required for fabrication of square spiral shape microgroove was 1hr 46 min and for “S” shaped microgroove total machining time was 1 h 53 min. These complex microfeatures has been analyzed for the machining accuracy i.e. WOC, LOC and taper. The average WOC and LOC as well as taper for square spiral shaped microgroove is obtained as 35 m and 24 m with average taper 4◦ , for “S” shaped microgrooves, 18 m and 26 m respectively with average taper 3◦ . These complex geometry microgrooves on pure titanium have potential applications in the area of MEMS and micro engineering. This successful micromachining of different complex micro features establishes the competence of EMM for micromachining of titanium. 6. Conclusions Micromachining of microgrooves on pure commercial titanium has been demonstrated by employing layer-by-layer micromilling method utilizing maskless EMM process. This study also established most suitable EMM process parameters for successful micromilling of through microgrooves on pure commercial tita-
nium by analyzing the effect of various EMM process parameters on machining accuracy in terms of width overcut (WOC), length overcut (LOC), taper of through microgrooves. The experiments have been carried out by varying one parameter at a time i.e. machining voltage, pulse frequency and microtool feed rate keeping other process parameters constant with micromachined disc microtool. Various findings of this experimentation are:
(i) The machining voltage is one of the major influencing parameter to overcome passive oxide layer of titanium to initiate smoother anodic dissolution during EMM. Width as well as length overcut (LOC) is directly proportional to the machining voltage. Machining voltage of 10 V is found suitable for controlled micromachining of titanium through electrochemical micromachining process. Stray machining is predominant as the voltage increases hence; to reduce stray machining effect, lower machining voltage is recommended. (ii) Pulse frequency is another important parameter to achieve better machining accuracy by localizing material removal process. Overcut is almost linearly reduced as pulse frequency increases. width overcut (WOC) of 93 m and 80 m is obtained at entry as well as exit of through microgroove with initial pulse frequency of 140 kHz, which is further minimizes to 44 m and 38 m at 200 kHz pulse frequency. Length overcut was also reduced from 114 m to 78 m and 82 m to 63 m at entry as well as exit side of through microgroove. taper of microgroove has also reduced from 7.4◦ to 5.14◦ along width and from 9.09◦ to 5.5◦ along the length of microgroove. (iii) Microtool feed rate directly governs the material removal rate and hence, controls the localization effect. In this study microtool federate of 2 m/s is found to be optimum feed rate. Width overcut reduces from 44 m to 32 m at entry as well as at exit it reduces from 38 m to 26 m and length overcut (LOC) reduces to 28 m and 16 m at entry and exit side of through microgroove with minimum degree of taper 3.12◦ obtained with optimum microtool federate of 2 m/s. (iv) This work also demonstrated successful micromachining of complex shaped through microgrooves such as square spiral shape and “S” shape through microgrooves on pure commercial titanium utilizing established optimum EMM process parameters. These complex microfeatures are first of its kind generated on pure commercial titanium by layer by layer micromilling strategy utilizing maskless electrochemical micromachining process.
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