EXPERIMENTATION AND ANALYSIS INTO MICRO-HOLE MACHINING IN EDM ON Ti-6AL-4V ALLOY USING BORON CARBIDE POWDER MIXED DE-IONIZED WATER

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1 Volume 1 Number 1 January-June 2012 IJMMD Academic Research Journals (India), pp EXPERIMENTATION AND ANALYSIS INTO MICRO-HOLE MACHINING IN EDM ON Ti-6AL-4V ALLOY USING BORON CARBIDE POWDER MIXED DE-IONIZED WATER G. Kibria*#, B. B. Pradhan* and B. Bhattacharyya** Mechanical Engineering Department, Sikkim Manipal Institute of Technology, Majitar, Rangpo, East Sikkim , India (#Corresponding author: prince_me16@rediffmail.com) ** Production Engineering Department, Jadavpur University, Kolkata , India * ABSTRACT: In micro-electrical discharge machining (micro-edm), dielectric plays a significant role during the machining process as different types of dielectrics have different chemical compositions, cooling rates and dielectric strengths. For this, dissimilar process responses are accounted when machining in EDM at micron level. The present paper investigates micro-edm characteristics such as material removal rate (MRR), tool wear rate (TWR), overcut (OC), taperness and machining time (MT) during micro-machining of through holes on Ti-6Al-4V superalloy employing de-ionized water based dielectric other than conventional hydro-carbon oil i.e. kerosene. The paper also includes the comparative study of the micro-edm machining characteristics employing boron carbide (B4C) powder as additive in deionized water dielectric at different discharge energies. The results shows that MRR and taperness of micro-hole is improved and TWR is reduced employing B4C powder than pure dielectric. Surface topography and recast layer formed during micro-hole machining by micro-edm has also been investigated based on optical and SEM micrographs. Keywords: Micro-EDM; Ti-6Al-4V; de-ionized water; micro-hole; B4C powder suspended dielectric, scanning electron microscopy, material removal rate, boron carbide. 1. INTRODUCTION With the trend of rapid development of micro-machining technology, the demand of precision miniaturized products has grown tremendously. Again, the innovation of hard-to-machine materials has thrown a challenging task to

2 18 International Journal of Materials, Manufacturing and Design (IJMMD) the manufacturing engineers to fabricate micro-components by micro-machining of these materials. Micro-EDM is one of the established and emerging advanced microfabrication processes, which are being utilized to manufacture micro-sized features in products made of hard materials like titanium superalloys, etc. Micro-EDM utilizes the spark erosion phenomenon for removing material from electrically conductive workpiece through a dielectric medium. As the material removal process is noncontact type between the micro-tool and workpiece, it produces no mechanical stresses in workpiece, thereby reducing chatter or vibration problems during machining. Since micro-edm process is performed into a dielectric fluid, the type of dielectric fluids influences the machining performances criteria. Typically, in EDM, generally hydrocarbon oil kerosene is used as the dielectric fluid. However, in micro-edm, the use of kerosene dielectric creates several problems. These problems includes deposition of carbide layer on workpiece surface that reduces material removal rate, adhesion of carbon particles on micro-tool surface that makes the discharge inefficient, formation of harmful vapours such as CO and CH4 that create toxic environment around the machining area, etc. (Zhang et al., 2006; Chung et al., 2007). To promote better micro-machining performances and safe machining environment, experimental investigations are going on employing different non-hydrocarbon based dielectric oils. The oxygen-based fluid, i.e. de-ionized water is one of them that can be applied as dielectric liquid during micro-machining in EDM. The oxygen-based dielectrics improve the electrical discharge phenomena in the machining gap and further results in better machining efficiency in EDM (Kunieda et al., 1991; Chen et al., 1999). Several experimental studies on powder-mixed EDM process have been performed by researchers across the globe to investigate their influences on electrical discharge machining performances (Kansal et al., 2007). The researches include powders of Al, Cr, Cu, graphite, silicon, silicon carbide etc in some of the dielectrics such as kerosene and de-ionized water (Kansal et al., 2007; Erden and Bilgin, 1980; Tzeng and Lee, 2001; Jeswani, 1981). Influence of electrically conductive additives such as Al, Cu, graphite etc. and non-conductive additives such as SiC, Al2O3 etc. have been investigated. These electrically conductive additives are soft in nature and cannot withstand at high temperature and SiC, Al2O3 which are hard but electrically nonconductive. However, experimental researches using powder additives in dielectrics have not been carried out at lower energy discharge scale in micro-edm. In the present experimental study, a new powder additive i.e. boron carbide (B4C) is mixed in deionized water and used as dielectric fluid to find out its effect on micro-edm process responses such as material removal rate (MRR), tool wear rate (TWR), overcut (OC) and taperness of micro-hole and also machined micro-hole quality and surface topography. This new electrically conductive additive, B4C powder has also chemical resistance and hardness ranking third behind diamond. Today, B4C powder is widely used in the area of lapping and polishing operation of most hard materials. In addition, boron carbide has excellent wear and abrasion resistance which assisting micro-

3 Experimentation and Analysis into Micro-Hole Machining in EDM on 19 machining along with micro-electrical discharge action, it may generate fine finish machined surface on hard-to-machine materials. On the other hand, Ti-6Al-4V super alloy is a well-established material for successful applications in various engineering fields such as aerospace, automotive, biomedical etc for its excellent mechanical and thermal properties, outstanding corrosion resistance and low modulus of elasticity (Ezugwu and Wang, 1997). To fulfill the need of research study for employing B 4C additive in micro-edm and also to improve machining characteristics such as MRR, TWR, OC, micro-hole taperness and machined micro-hole quality and surface topography, an attempt have been made in this investigation to study the influence of B4C powder as additive in de-ionized water during micro-hole machining on Ti-6Al4V superalloy by micro-edm process. 2. ROLE OF POWDER PARTICLES IN MICRO-EDM PROCESS The machining zone consists of dielectric fluid only. When additive particles is mixed with the working fluid and applied at the machining zone, due to the voltage applied between micro-tool electrode and work material, an electric field is created between them. The powder particles in the spark gap get energized and behave in a zigzag fashion. This electric field accelerated the charged particles, which behaves like conductors. Due to this, breakdown of the gap is occurred creating an increase in spark gap. The powder particles come close to each other and assemble themselves in the chain like structures between the electrodes. The chain formation helps in bridging the discharge gap between both the electrodes. Due to this, the dielectric strength of powder mixed working fluid decreases and causes early explosion in the gap. As a result, a series discharge starts under the micro-electrode area near smallest gap zone and material removal takes place from both the electrodes. Simultaneously, the added powder modifies the plasma channel and becomes enlarged and widened. The electrical discharges uniformly distributed among the powder particles at the machining zone and shallow craters are produced on the workpiece surface. The result is the improvement in surface finish of the micro-edmed surface employing powder mixed dielectric. In Figure 1, the schematic diagram of the discharge pattern between two electrodes is shown during micro-edm process when conductive additives are being employed in the dielectric fluid. During micro-hole machining in EDM employing powder particles mixed in dielectric, the discharge gap consists of additive powders as well as debris produced in erosion of electrode materials. Therefore, the debris concentration and its distribution in the dielectric, formation of bubbles, deionization and surface irregularities of the workpiece as well as the micro-tool electrode determines the stability of the machining process in micro-edm. The thermo-physical properties i.e. particle size, particle concentration, particle density, thermal conductivity, electrical resistivity, melting point, evaporation point, specific and latent heat etc. of the additive particles play a significant role in discharge transitivity, gap size and breakdown strength of the gap. Electrical and thermal conductive particles such as Al, Cu, graphite etc. can easily

4 20 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 1: Schematic Diagram Discharge Using Additive Mixed Dielectric in Micro-EDM distribute themselves uniformly in the discharge gap with the help of electrical field created between electrodes. On the other hand, non-conductive particles like SiC, Al2O3 etc. require higher gap voltage and current to arrange themselves in zigzag fashion to improve stable discharge condition. Therefore, sparking efficiency greatly depends on thermo-electrical properties of debris in the gap. 3. EXPERIMENTAL METHOD The EDM set-up used in this experimental study is Series 2000, EMS-5535-R50, ZNC EDM machine manufactured by Electronica Machine Tools, India. It uses hydrocarbon oil kerosene as dielectric fluid. The machining system is modified to perform micromachining in EDM employing de-ionized water. A separate dielectric chamber was developed and used with a pump combined with a pressure-regulating valve to circulate the de-ionized water without affecting dielectric supply system of the main machine. Figure 2 shows the schematic view of micro-edm system with dielectric supply units. Among the micro-edm process parameters, peak current (Ip) and pulseon-time (Ton) are most significant and effective parameters that control the EDM process responses in micro machining domain (Pradhan et al., 2009). The present experimentations have been conducted with a fixed peak current, Ip of 2 A and pulse-on-time, Ton was varied from 1 µs to 20 µs. As very low current density is not sufficient to melt and vaporize the workpiece for micromachining and high current density results in high thermal stress to the micro-tool material that causes more TWR, in turn reduces machining accuracy, so Ip was selected as 2 A for the experiments. The other process parameters i.e. flushing pressure (Pr), duty ratio (t), polarity etc. were

5 Experimentation and Analysis into Micro-Hole Machining in EDM on 21 kept constant. Boron carbide (B4C) powder was mixed as additive in de-ionized water dielectric during machining in micro-edm. Figure 2: Schematic Diagram of de-ionized Water Dielectric Circulating System Electrical resistivity of boron carbide is in the range of W-cm. As B4C abrasive lie in a transition zone between good conductors and isolators, the potential difference as well as the plasma channel produced in the micron sized inter electrode gap can make the abrasive to conduct thermoelectric power in the machining zone (Pierson, 1996; Luis and Puertas, 2007). Boron carbide is characterized by a relatively wide gap in its forbidden band, a low thermal conductivity, and a high thermoelectric power. These properties make it a potentially useful material for high-temperature thermoelectric energy conversion compared to silicon carbide as well as tungsten carbide abrasives. As micro-edm process uses discharge energy in the range of 5 150µJ, so there is a little chance to melt and evaporate the B4C particles due to discharge in the inter electrode gap. It was observed in the optical measuring microscope that the powder particles comprise of mixture of different shapes and sizes ranging from 8 to 20 µm. When, Boron carbide (B4C) powder-mixed de-ionized water was applied in the machining zone, most of the particles, which are more than 10 µm in size, get accumulated at the base of machining zone because of self-weight of particles even though a motor driven stirrer was applied to provide turbulence in powder

6 22 International Journal of Materials, Manufacturing and Design (IJMMD) mixed dielectrics in the machining tank. Hence, the effective average particle size that may involve in the machining phenomena at the micro-machining zone ranges from 8 Therefore, the average size of B4C particles which actively take part in machining is in the range of 8 to 10 µm. Boron carbide powder of concentration of 4 gm/lit was mixed as additive in de-ionized water dielectric during machining in micro-edm. This particular concentration has been selected based on past research studies on powder mixed dielectrics in conventional EDM (Kansal et al., 2007; Tzeng and Lee, 2001). t o 1 0 µ m d u r i n g m i c r o - E D M. The detailed machining conditions are tabulated in Table 1. The micro-tool electrode used was cylindrical shaped tungsten electrode of 300 µm diameter and work material used was Ti-6Al-4V plate of thickness 1 mm. The chemical composition and physical and mechanical properties of Ti-6Al-4V superalloy are listed in Tables 2 and 3, respectively. The thermo-physical properties of boron carbide (B4C) are shown in Table 4. Figs. 3 (a) and (b) show the cross-sectional shape of the micro-hole to be produced and after machining by micro-edm employing pure and powder mixed dielectrics. All the experiments have been conducted three times to make sure that the measured responses are reliable during experimentation. After each run of experimental settings, the micro-edm response criteria were measured as well as calculated for material removal rate, tool wear rate and the enlargement of machined micro-hole compared to micro-tool diameter etc. MRR and TWR were calculated by measuring initial and final weights using a high precision weighing machine (least count: 0.01 mg, make: Mettler Toledo, Switzerland). A measuring microscope manufactured by OLYMPUS, Japan was used to measure the diameters of entry and exit of micro-holes. Overcut and micro-hole taperness were calculated as per the following equations. Table 1 Experimental Conditions for through Micro-hole Machining on Ti-6Al-4V alloy Conditions Description Workpiece (+ve) 13 mm 15 mm 1 mm of Ti-6Al-4V plate Electrode (-ve) Tungsten micro-tool, 300 µm diameter Dielectric fluids Pure de-ionized water, B4C additive mixed de-ionized water Additive Boron Carbide (B4C) powder Average size of additive (µm) 8-10 Powder concentration (g/lit) 4 Peak current (A) 2 Pulse-on-time (µs) 1, 2, 5, 10, 20 Duty factor (%) 95 Flushing pressure (kg/cm ) 0.5 Resistivity of pure de-ionized water (megohm-cm) 4.2 2

7 Experimentation and Analysis into Micro-Hole Machining in EDM on 23 Table 2 Compositions of Ti-6Al-4V superalloy Element Percentage Aluminium Carbon Iron Hydrogen Nitrogen Oxygen Other Vanadium Titanium 5.5 to < to 4.5 Balance Table 3 Physical and Mechanical Properties of Ti-6Al-4V Physical Property Typical Value Density (g/cm ) Melting range (ºC±15 ºC) Specific heat (J/Kg C ) Thermal conductivity (W/m-K) Tensile strength (Mpa) Elastic modulus (GPa) Hardness Rockwell C Table 4 Thermo-physical Properties of Boron Carbide (B4C) Additives Thermo-physical Property Typical Value Density (g/cm3) 2.52 Melting point ( C) 2445 Electrical conductivity (at 25 C) (S) 140 Thermal conductivity (at 25 C) (W/m-K) Thermal neutron capture cross section (barn) 600 Young s modulus (Gpa) Hardness (Knoop 100g) (kg-mm-2) Specific heat ( J.K-1.kg-1 ) (at 25 C) 950 Overcut (OC ) = [ Diameter of entry hole ( D ) Diameter of tool electrode ( D)] / 2 t Taperness = Entry hole dia. ( D ) Exit hole dia. ( D ) t b 2 Thickness ( L) (1) (2)

8 24 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 3: Cross-sectional View of the Micro-hole (a) to be Machined and (b) after Machining SEM micrographs were also taken to study the machined surface topography. The effects of pure and powder mixed de-ionized water have been compared based on test results and micrographs during micro-edming of Ti-6Al-4V. 4. RESULTS AND DISCUSSIONS 4.1. Analysis Based on Test Results In this section, the experimental results for different micro-edm machining process performances such as material removal rate (MRR), tool wear rate (TWR), overcut

9 Experimentation and Analysis into Micro-Hole Machining in EDM on 25 (OC), taperness of micro-holes and machining time (MT) are shown and analyzed at varying pulse-on-time (Ton) for a fixed peak current (Ip) of 2 A. Figure 4 shows the variation of MRR with pulse-on-time, Ton for pure and boron carbide mixed deionized water for a fixed peak current, Ip. The plotted graphs show errors in measurements in MRR (4.8% error) for conducting three repetitions in each parametric setting during experimentation. It is seen from this figure that when powder additive is applied to de-ionized water, MRR is found to be more compared to pure de-ionized water throughout the considered range of pulse-on-time at constant peak current setting of 2 A. Also, as the pulse-on-time increases, MRR initially increases, then it decreases and at high pulse duration setting, it again increases for both pure and B 4C mixed de-ionized water dielectrics. The increase of MRR with the increase of pulse duration is due to increase of sparking discharge delivery time i.e. effective machining time per pulse. Also, the presence of carbide additive in de-ionized water further helps in uniform distribution of discharge energy and better conduction of discharge current through the B4C powder thereby enabling better machining condition. However, the decrease in MRR is found may be due to the formation of titanium oxide layer on the workpiece surface and adhesion of carbon particles on micro-tool electrode from the boron carbide powder dissociated at higher pulse discharge energy. However, with further increase in pulse-on-time (Ton), MRR is found to increase gradually due to the reduction in formation of oxide layer on the workpiece surface with the availability of more effective discharge energy at the machining zone. Higher MRR with B4C mixed dielectric is due to the ease of conduction of current as these B4C particles suspended in the dielectric helps in better conduction of pulse current in the machining gap. The better conduction environment implies of reaching of large amount of energy on the work surface thus acts as intensifier of spark energy. Figure 4: Variation of Material Removal Rate (MRR) with Pulse-on-time (Ton)

10 26 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 5 shows the variation of TWR varying pulse-on-time, Ton with fixed Ip of 2 A employing both dielectrics i.e. pure and B4C additive mixed de-ionized water. The error in measurements of TWR is 3% for three repetition experiments with same parametric setting. It is observed from this figure that TWR decreases for both pure and additive mixed dielectrics over the considered pulse-on-time settings. It is also found from the figure that the rate of decrement of TWR is much higher for B4C mixed de-ionized water as compared to pure dielectric. It is so because as pulse-on-time increases, more carbide powder particles are subjected to longer discharge energy which in turn decompose and produces carbon particles that adhere to the micro-tool surface and further reduces tool wear rate. On the other hand, as there is no such carbon deposition phenomena using pure de-ionized dielectric and oxygen based product is available at the machining zone and more thermal stress is developed on the micro-tool tip, which further increase the tool wear rate compared to B4C additive mixed dielectric. Figure 5: Variation of Tool Wear Rate (TWR) with Pulse-on-time (Ton) The variation of overcut (OC) of machined micro-hole for varying pulse-on-time, Ton is shown in Figure 6 for both pure and additive mixed dielectrics. For three repetition experiments with same parametric setting, the error in measurements of OC is 4.6%. It is found from this figure that B4C mixed de-ionized water results more overcut of the machined micro-hole compared to pure de-ionized water over the considered pulse-on-time range in this investigation. It is due to the fact that the machining zone is comprised of boron carbide powder as well as machining debris particles and their presence encourage more secondary discharge sparking occurs which further results in higher overcut. Again, as pulse-on-time increases, OC is

11 Experimentation and Analysis into Micro-Hole Machining in EDM on 27 Figure 6: Variation of Overcut (OC) with Pulse-on-time (Ton) slightly more for additive mixed de-ionized water. This is so because these additive particles strike on the inside wall of the micro-hole at the time of implosion and subsequently violent rush of dielectric with particles causing the peeling of oxide layer formed on the wall surface which finally contributed in hole enlargement. However, for pure dielectric, OC of micro-hole increases upto 10 µs and then it decreases. It may be due to the fact that more pulse duration results more time for discharge on the machining zone, which results more effective machining and larger debris generation and more secondary sparking phenomena. But at higher pulse duration i.e. 20 µs, effective and efficient discharge occurs between the micro-tool and work material, as a result the time to fabricate through micro-hole on workpiece is less and also less number of secondary sparking, which further reduces micro-hole OC which can be clearly seen in same figure. However, unlike the OC with pure de-ionized water the OC with B4C mixed de-ionized water was more uniform across the range of pulse-ontime and can be effectively utilized as overcut offset factor during design calculations. This uniform overcut might have resulted from improved distribution of electrical energy in the presence of B4C particles. In Figure 7, the variation of taperness of machined micro-hole is shown with varying pulse-on-time at constant Ip of 2 A utilizing pure and B4C additive mixed deionized water. Here, the error in measurements of taperness is 2.4% for considering three repeated experiments. It can be observed from this figure that more taperness is found when machining is done employing pure de-ionized water and it increases as pulse-on-time increases for both pure and B4C mixed de-ionized water. It is due to the fact that as pure dielectric results in more tool wear, so the tip of the micro-tool

12 28 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 7: Variation of Taperness with Pulse-on-time (Ton) becomes tapered one and this further result in more deviation in size at entry and exit side of micro-hole. However, it is found that the increment of OC is limited upto 10 µs of pulse-on-time for pure dielectric. The taperness of micro-hole then reduces in the range of 10 µs to 20 µs of pulse-on-time. It may be due to the fact that at higher pulseon-time, the material removal from the micro-hole side surface as well as bottom surface is uniform, which reduces the variance of micro-hole diameters at entry and exit i.e. taperness. Figure 8 shows the variation of machining time (MT) required to generate through micro-hole on Ti-6Al-4V plate of same thickness for different pulse-on-time using pure and B4C powder mixed de-ionized water. The plotted graphs show errors in measurements in machining time (6% error) for conducting three repetitions in each parametric setting during experimentation. From this figure, it is found that pure de-ionized water results in low machining time to generate through micro-hole on the workpiece plate compared to additive mixed dielectric. As boron carbide mixed de-ionized water delivers carbide particles at the machining zone, so TiC layer whose melting temperature is about 3100º C is formed on the workpiece surface that takes more time to melt and remove. Further, as boron carbide is conducting material and its presence in dielectric increases the gap size between the two electrodes, the distance traveled by the micro-tool is increased and it results higher machining time although additive particles distribute the discharge energy uniformly for better machining performance. The machining time is more in case of boron carbide mixed dielectric, which may be due to the results of higher overcut of micro-hole.

13 Experimentation and Analysis into Micro-Hole Machining in EDM on 29 Figure 8: Variation of Machining Time (MT) with Pulse-on-time (Ton) 4.2. Analysis Based on Optical and SEM Micrographs This section describes the effect of pure and B 4C additive mixed de-ionized water on quality of the micro-holes generated as well as machined surface integrity through some optical and SEM micrographs during micro-edm process for machining at different parametric settings. Figure 9 shows the SEM micrographs of machined through micro-holes for both the dielectrics i.e. pure and B 4C mixed de-ionized water at machining parametric setting of 2 A/10 µs. It is observed from this figure that quality of the micro-hole as well as the circularity at both ends i.e. entry and exit of micro-holes is better using B 4C mixed de-ionized water compared to pure deionized water. It is also found from this figure that the overcut of micro-hole is more using additive mixed dielectric than pure one which corroborates the experimental results of overcut as described earlier. The comparative machined surface topography of the micro-hole wall can be viewed in Figure 10 for machining parametric setting of 2 A/10 µs for these two dielectrics. This figure also revealed that the machined surface profile is smooth enough when boron carbide is mixed in de-ionized water than pure dielectric. Figure 11 shows comparative optical photographs of tip of micro-tool electrodes that were taken by a precision measuring microscope with 10X zoom lens. It is found that the degree of tool wear and in turn taperness of micro-tool tip is more for pure de-ionized water which results more taperness in micro-hole as also corroborated by the experimental result as discussed earlier. When enlarge SEM micrographs of the edge of micro-holes were taken, some

14 30 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 9: SEM Micrographs of Machined Micro-holes Using for Pure and B4C Mixed de-ionized Water Figure 10: SEM Micrographs of Surface Topography of Machined Micro-holes for Pure and B4C Mixed de-ionized Water Figure 11: Optical Photographs of Micro-tool Tip at Machining Setting of 2A/10µs for Both Dielectrics

15 Experimentation and Analysis into Micro-Hole Machining in EDM on 31 Figure 12: SEM Micrographs of Micro-hole Edge Showing White Layer on the Machined Surface amount of white or recast layer can be noticed which were generated during machining. Figure 12 shows the comparative SEM micrographs of recast layer employing both pure and B4C additive mixed de-ionized water at two machining parametric settings i.e. 2 A/ 5 µs and 2 A/ 10 µs. It is observed from these figures that additive mixed dielectric results in lower the thickness of recast layer than pure dielectric. Boron carbide particles in de-ionized water act as an agent to remove the melted portion of the work material from the craters on micro-hole wall and further restrict the formation of thicker recast layer on machined surface. Further, it is also found that higher pulse-on-time i.e. 20 µs machining setting produces thicker recast layer than lower pulse-on-time i.e. 5 µs for both the dielectrics i.e. pure and B 4C mixed de-ionized water. It is due to the fact that as the pulse duration increases the effective time of machining also increases, which results in more re-solidification of melted material on the machined surface. To further analyze the effect of various process parametric combinations on the machined micro-hole surface, energy dispersive spectroscopy (EDS) has been performed. Figures 13 (a), (b) and 14 (a), (b) show EDS graphs of micro-hole internal surface for the two machining parametric settings at 2A/5µs and 2A/10µs employing pure and boron carbide mixed deionized water. Comparing these figures, more percentage of oxygen (O) element is found at 10 µs experimental settings than 5 µs of pulse-on-time for both pure (3.52% and 1.93%) and B4C mixed de-ionized water (2.86% and 1.67%). These results clearly show that the formation of oxide layer is more for 10 µs settings than 5 µs, which have resulted lower MRR as already explained in Figure 4. Moreover, it is also seen that different percentage weight of tungsten is diffused from tool electrode to the micro-hole machined surface of Ti-6Al-4V material. It is also found that the percentage of carbon element is more using carbide additive mixed dielectric (C = 3.72% for 5 µs and C = 4.05% for 10 µs) compared to pure dielectric (C = 1.51% for 5 µs and 1.24% for 10 µs), which have resulted due to using boron carbide mixed additive as dielectric. Moreover, infusion of boron elements is also found on machined surface when carbide mixed dielectric is employed.

16 32 International Journal of Materials, Manufacturing and Design (IJMMD) Figure 13: Energy Dispersive Spectroscopy of Machined Micro-hole Surfaces at Parametric Combination of 2A/5µs for Using (a) de-ionized Water and (b) B4C Mixed de-ionized Water

17 Experimentation and Analysis into Micro-Hole Machining in EDM on Figure 14: Energy Dispersive Spectroscopy of Machined Micro-hole Surfaces at Parametric Combination of 2A/10µs for Using (a) de-ionized Water and (b) B4C Mixed de-ionized Water 33

18 34 5. International Journal of Materials, Manufacturing and Design (IJMMD) CONCLUSIONS In the present investigation, through the comparative study of various micro-edm characteristics such as MRR, TWR, OC, micro-hole taperness, machining time and also the surface topography, it is found that there is a great influence of boron carbide (B4C) powder mixed in de-ionized water for machining of Ti-6Al-4V super alloy at micro-level discharge energies. Summarizing the main features found during the experimental analysis of present research, the following conclusions can be drawn. (i) Boron carbide (B4C) mixed de-ionized water resulted higher material removal rate (MRR) compared to pure water for considered peak current and pulse-ontime parametric settings. The presence of carbide powder helps to improve the uniform and intense discharge phenomena and further it results in more material removal per discharge. (ii) The tool wear rate (TWR) is lower when using B4C additive in de-ionized water compared to pure de-ionized water. The additive decomposes at higher discharge energy and produces carbon particles which adhere on the surface of micro-tools and this phenomena subsequently results in less thermal stress in turn less wear on micro-tool. Moreover, higher micro-hole taperness is found when using pure de-ionized water compared to B4C mixed dielectric. High tool wear is occurred employing pure de-ionized water, which in turn produces more degree of taperness of micro-holes. (iii) Employing B4C powder in de-ionized water, the overcut of through micro-hole is higher due to more secondary discharge sparking at the machining zone compared to pure water. However, overcut is uniform throughout the range which can be utilized for precise design calculation of micro-hole size. (iv) SEM micrographs revealed that good surface quality can be generated employing B4C mixed de-ionized water compared to pure dielectric. Furthermore, less amount of recast layer is found when B4C is used in deionized water dielectric compared to pure one. B4C powder mixed de-ionized water dielectric can improved the surface quality of the micro-hole. EDS analysis of the machined surface shows infusion of tungsten element from tool electrode materials and also boron particle due to the use of powder mixed dielectric. In this paper, an attempt is made to show the influence of boron carbide (B 4C) in de-ionized water dielectric compared to pure de-ionized water. It can be concluded from this study that there is a great potential of using boron carbide powder mixed deionized water dielectric for micro-machining of Ti-6Al-4V super alloy in EDM with regards to generation of high accuracy and good surface quality through micro-holes. The present research study will open up challenging possibilities for exploring effective applications of micro-edm technology for machining titanium based super alloy in the field of high precision micro-engineering.

19 Experimentation and Analysis into Micro-Hole Machining in EDM on 35 Acknowledgements Authors acknowledge the assistance provided by CAS Ph-IV programme of Production Engineering Department of Jadavpur University, Kolkata under the University Grants Commission (UGC), New Delhi, India. References Chen, S. L., Yan, B. H., and Huang, F. Y. (1999), Influence of kerosene and distilled water as dielectrics on the electric discharge machining characteristics of Ti 6A1 4V, Journal of Materials Processing Technology, 87, Chung, D. K., Kim, B. H, and Chu, C. N. (2007), Micro electrical discharge milling using deionized water as a dielectric fluid, Journal of Micromechanics and Microengineering, 17, Erden, A., and Bilgin, S. (1980), Role of impurities in electric discharge machining, In: Proceedings of 21st IMTDR conference, Macmillan, London, Ezugwu, E. O., and Wang, Z. M. (1997), Titanium alloys and their machinability A review, Journal of Materials Processing Technology, 68, Jeswani, M. L. (1981), Effects of the addition of graphite powder to kerosene used as the dielectric fluid in electrical discharge machining, Wear, 70, Kansal, H. K., Singh, S. and Kumar, P. (2007), Technology and research developments in powder mixed electric discharge machining (PMEDM), Journal of Materials Processing Technology, 184, Kunieda, M., Furuoya, S., and Taniguchi, N. (1991), Improvement of EDM efficiency by supplying oxygen gas into gap, Annals of CIRP, 40 (1), Luis, C. J. and Puertas I. (2007), Methodology for developing technological tables used in EDM processes of conductive ceramics, Journal of Materials Processing Technology, 189, Pierson, H. O. (1996), Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing and Applications, Noyes Publications, Westwood, New Jersey, U.S.A. Pradhan, B. B., Masanta, M., Sarkar, B. R., and Bhattacharyya, B. (2009), Investigation of electro-discharge micro-machining of titanium super alloy, International Journal of Advanced Manufacturing Technology, 41, Tzeng, Y. F., and Lee, C. Y. (2001), Effects of powder characteristics on electrodischarge machining efficiency, International Journal of Advanced Manufacturing Technology, 17, Zhang, Q. H., Du, R., Zhang, J. H., and Zhang, Q. (2006), An investigation of ultrasonic-assisted electrical discharge machining in gas, International Journal of machine Tools and Manufacture, 46,