Aluminium powder mixed rotary electric discharge machining (PMEDM) on Inconel 718.
The electrical discharge machining (EDM) process has been commonly used for machining hard metals and alloys for aerospace and die industries. Its applications are also extended in pressure casting dies, forging dies, powder metallurgy and injection moulds (Guu 2012).
In powder mixed EDM, suitable metal powders such as Aluminium, copper and silicon are mixed into the dielectric fluid. After application of suitable gap voltage, powder particles fill up the spark gap. The high electric field across the gap energises powder particles and the particles act as conductors forming chains at different places under sparking area. It bridges the gap between the tool electrode and workpiece material (Mahammadumar 2014).
Rotary electrical discharge machining is a new technique of machining where the electrode can be rotated at desired speed. Few researchers have investigated the effect of rotary tool on machining characteristics in EDM
Soni and Chakraverti (1994) analysed the effect of rotary electrode tool on the EDM of titanium alloy. They found that the rotary motion of the tool increases the MRR and electrode wear rate (EWR) in all levels of current and pulse on time. Wang and Yan (2000) used rotary EDM concept to drill blind holes in [Al.sub.2][O.sub.3]/6061 Al composite.
Ghoreishi and Atkinson (2002) studied the influences of vibration and rotation of electrode on machining characteristics in three levels of machining pulse energy. Mohan and Rajadurai (2002) conducted the experimental study on Al-SiC composite material. They showed that the rotary electrode improves the MRR and reduce the surface roughness (SR). Kuppan and Rajadurai (2008) investigated the effects of various rotational speed of electrode on Inconel 718. Results show that the increasing of the rotational speed is an effective factor in low discharge energy. Saha and Choudhury (2009) applied the rotation of the electrode on dry EDM
Chattopadhyay (2008) has established that rotary electrical discharge machining with a polarity reversal magnetic field delivers better machining output than machining in a non-magnetic field. It benefited the PMEDM process by reducing the machining costs and by producing better geometrical trueness on work pieces, as MRR is increased and TWR is decreased, better geometrical accuracy and closer dimensional tolerances were attained. Generation of non-homogeneous magnetic lines at the workpiece aids in transporting of the debris out of the spark zone, and reduces sparking chances and spark zone contamination.
Gurule and Nandurkar (2012) investigated the potential of PMEDM for enhancing material removal rate (MRR) of Die steel with a rotary tool. Taguchi methodology was adopted to plan and analyse the experimental results. Experimental results indicate that the current, ON time, tool material, tool rpm and powder concentration significantly affect MRR.
Figure 1. Process parameters and performance measures. PMEDM Process Process Parameters Performance Measures Sparking Gap MRR Peak Current TWR Pulse on time SR Duty Factor HD Tool Rotation-300 RPM SEM Slurry Concentration EDX
Present authors have tried to investigate the issues related to MRR and SR by controlling the input process parameter of EDM machine along with tool electrode rotation.
The process parameters and performance measures are shown in Figure 1.The primary parameters of this study are sparking gap, peak current, pulse ON time, duty cycle, slurry concentration and tool rotation. Major performance measures in PMEDM are MRR, Tool Wear Rate (TWR), SR and HAZ (Figure 2).
2. Experimental setup
The experiments were conducted on Joe Mars Electric Discharge Machine (JM320-AZ50). Figure 3 shows the experimental set-up. A servomechanism controls the downward movement of the tool holder during machining. Commercial grade kerosene oil is used as the dielectric medium. The dielectric tank has a capacity of 240 l. The main objective of the experimental design is studying the relations between the response as a dependent variable and the various parameter levels. In present work, experiments were conducted using Taguchi's L18 orthogonal array. Experimentation parameters are listed in Table 1 (Figure 4).
2.1. Experimental set up for powder mixed in the dielectric
2.1.1. Dielectric circulation tank
In this experiment, a dielectric tank is having stirrer for better circulation of powder while machining. The nozzle is also a part of the dielectric tank for flushing the debris produced while machining process.
The inside dimensions of the Powder mixed tank are 580 mm x 440 mm x 250 mm (length x breadth x height). A stirrer has been provided for keeping the powder in suspension. On the completion of each reading, the powder mixed dielectric is siphoned off into a separate container using a rubber tube. After changing the workpiece and machine setting, the tank is filled with this powder mixed dielectric again.
2.1.2. Rotary electrode attachment
To perform the rotation of the electrode, a rotary electrode attachment unit has been developed in-house. This rotary unit has been mounted on the Z-NC EDM machine. The rotary electrode EDM unit comprises a spindle shaft supported by two supporting frames (upper and lower supporting frames-plate)which in turn support a pair of roller bearings. The shaft can rotate relative to the machine head. A tool holder (hand drill chuck) is mounted at the other end for holding electrode tools. The motor for spindle rotation is mounted on the support frame and power is transmitted from the motor through a belt-pulley system. The rotary electrode EDM attachment can be functionally divided into six units. Rotation of the shaft is provided by a 15 V DC motor. The motor is mounted on the upper support frame; power is transmitted from the motor to the spindle through a belt-pulley arrangement.
2.1.3. Powder material
Aluminium is used as powder material in the experiments. Aluminium is chosen for its low density, intermediate level of Electrical resistivity and high level of thermal conductivity. Smaller Aluminium particles (average particle size is 325mesh) are used, as it is believed to result in the smooth surface effect. Table 2 shows some properties of Aluminium powder.
3. Results and discussion
Table 3 and Figure 5 represent experimental outcomes during pure dielectric and Table 4 and Figure 6 represent experimental outcomes during Al powder-mixed dielectric. A new factor called slurry concentration, varying at three levels, has been added to study (Figures 7-10).
3.1. Analysis of MRR
The addition of particles alters the material removal mechanism in the EDM process. As seen in Figure 4, the modified material removal mechanism for the addition of powders during normal single electrical discharge is the combined effect of mechanical thrust driven by the gas explosion mainly from the working fluid evaporation and the impact by the suspended particle. The materials removed by the grinding effect of suspended particles within the interspace are negligible. It is worth noting that the weaker gas explosion in the interspace after powder addition might lead to a reduction in the MRR during the normal single electrical discharge process. To enhance the machining efficiency of the whole EDM process, the striking effect of the particles and the discharge transitivity, therefore, play a decisive role.
The removal of material from the workpiece surface is attributed to three mechanisms (1) Melting, (2) Evaporation and (3) Thermal spalling (Drof and Kusiak 1994).
The MRR values are then inputted to the Minitab 16 software for further analysis according to the steps outlined for Taguchi design without performing any transformation on the responses. The ANOVA details for the MRR response is shown in Table 4. It is concluded that the estimated model coefficient for means since its respective P value is greatly less than 0.9. It is revealed that all the input parameters such like as, sparking gap, peak current, slurry concentration, ON time and duty cycle. The lower the P value, the more significant the parameter. Most effective parameters are peak current, duty cycle and sparking gap.
MRR = 25.617 + 1.2027A - 14.9968 * B1 - 2.2724 * B2 + 2.2405 * E1 - 1.0704 * E2 - 0.4078 * C1 - 0.3375 * C2 - 2.746 * D1 + 0.265 * D2
The experimental results evidence that increasing peak ampere increases the MRR. In EDM process, the MRR is a function of electrical discharge energy. The increase in peak current generates high-energy intensity and due to this energy melts more materials from the workpiece. Thus, MRR increased with peak current is an increase. It increases with a pulse on time at all duty cycles. In general, the powder of the spark and frequency defined by the number of pulse per second determined the process performance.
The MRR behaviour during aluminium powder mixing with dielectric fluid. MRR is very low at 9 amp and higher at 28 amp. Sparking gap is also one of the main effective parameters of MRR. MRR is decreased from lower sparking (50 V) gap to higher sparking gap (62 V). MRR is lower at 50 [micro]s and a minor increase is evidenced at 150 [micro]s. Duty cycle has no more effects on MRR. MRR is lower at 0.4 duty cycle. There is no change in MRR from 0.5 to 0.6 duty cycle. There are not many changes on MRR when the slurry concentration increases from 0.5 to 1.5 gm/l.
3.2. Effect of TWR
In this section, we have discussed the TWR during powder-mixed EDM during electrode tool rotation at 300 rpm. TWR obtained from experimental observations are recorded, and subsequent calculations are plotted in Table 3.
TWR depends on peak current. It is increasing from 9 to 28 amp. TWR is lower at 9 amp and it is higher at 28 amp. The sparking gap is also one of the main effective parameters of TWR. TWR decreased from lower sparking (50 V) gap to higher sparking gap (62 V). TWR also depends on ON time; TWR is higher at 50 [micro]s, after it reduced at 100 [micro]s, and a minor increase is evidenced at 150 [micro]s. Duty cycle also affects TWR.TWR is lower at 0.4 duty cycle. TWR increased from 0.5 and a minor decrease from 0.5 to 0.6 duty cycle is observed.
TWR obtained from experimental observations are recorded and subsequent calculations are plotted in Table5.
TWR = 0.105316 + 0.020561 * A - 0.069339 * B1 - 0.014414 * B2 + 0.001207 * E1 - 0.035433 * E2 + 0.117584 * C1 - 0.080759 * C2 - 0.053034 * D1 + 10.039801 * D2
As per Table 5, it is concluded that the estimated model of coefficient for means, its respective P value is greatly less than 1. The calculated F values along with the corresponding P values are shown in Table 6. The most effective parameter is ON time.
The TWR depends on slurry concentration ratio and sparking gap. TWR is found negligible under different parameteric conditions. We cannot predict which parameters are mostly affected by the TWR.
3.3. Analysis of SR
In this section, we have discussed the SR during powder-mixed EDM when electrode tool rotation takes place.
The most effective factor of SR is peak current. SR has smooth surface at 9 amp and rough surface at 28 amp. It increases from 9 to 28 amp. SR is smoother at lower sparking gap (50 V) and has a rough surface at higher sparking gap (62 V). SR depends on ON time; SR is smoother at 50 [micro]s. SR starts increasing from 50 [micro]s and a minor increase is again evidenced at 150 [micro]s. The effect of duty cycle on SR results in more changes. SR is higher at 0.4 duty cycle. It is increasing at 0.5 duty cycle. It decreases from 0.5 to 0.6 duty cycle. From the observation given in Table 6, the most effective parameters are peak current and ON time.
SR = 6.61333 - 0.17556 * A - 0.84167 * B1 - 0.06 * B20.14667 * E1 - 0.07833 * E2 - 0.95667 * C1 + 0.45333 * C2 + 0.18 * D1 + 0.29667 * D2
SR is a primary requirement of the machined surface. It directly gets affected due to the powder concentration. SR is found very good at low peak current (9 amp) and powder concentration (1 gm/l) and re-deposits machined surface at maximum peak current (28 amp.) and time.
3.4. Analysis of hole depth
In this section, we have discussed the hole depth (HD) during powder-mixed EDM when electrode tool rotation takes place. HD is higher when sparking gap is at 50 V and it is decreasing at 62 V. The peak current is an effective factor of HD at 9 [micro]s and it is decreasing at 17 [micro]s and again increases at 28 [micro]s. Slurry concentration and duty factor are the most effective factors of HD. Slurry concentration and duty factor are total mirror from each other. HD is higher at 0.5 and it is decreased at 1 when lower HD was at 0.4 duty factor and maximum HD at 0.6 duty factor and no more effect of ON time.
In our experimental strategies, we have fixed 0.3 mm HD in order to study parametric affects on HD. The minimum HD was found at low voltage and peak current and higher HD was found at maximum voltage and duty cycle.
Hole Depth = 0.305294 + 0.001817 * A + 0.000839 * B1 - 0.001661 * B2 + 0.014206 * E1 - 0.008761 * E2 - 0.000628 * C1 - 0.000944 * C2 - 0.003244 * D1 - 0.009828 * D2
The Table 7 shows the ANOVA. From the observation given in Table 6, the most effective parameters are Slurry concentration ratio and duty cycle.
3.5. Analysis of SEM micrograph
In order to study the topography of a machined surface, the working specimens have been revealed under scanning electron microscopy. As a sample, a micrograph of specimen machined at 28 amp with 1 gm/l powder concentration is shown in Figure 11. It is evident from Figure 11 that powder particles are deposited on a machined surface. However, no burning of particles is observed.
Overview and study of all images help us to interpret that uniformity in surface topography is comparatively while machining in presence of aluminium powder at different-different input parameters. This may be attributed to the distribution of spark energy over the entire surface of a workpiece due to the presence of powder particles.
Few oxide particles and interphases are clearly visible in images. Randomly distributed solid particles are resolidified and remain attached to the eroded surface. The SEM images display that the particles formed by electrical discharge are solid spheres and range from 3 to 30 [micro]m in diameter. The solid behaviour attributed to the discharge channel temperature is higher than that of molten particles ejected from the crater and nucleation generally starts internally. The perfectly spherical particles were apparently resolidified from the gaseous state, whereas irregularly recast layers were solidified from the liquid state.
Very less or negligible amount of powder deposition is observed. A Microcracking which is observed in the images may be attributed to thermal factors.
3.6. Analysis of energy-dispersive X-ray spectroscopy (EDX)
EDX (Energy-dispersive X-ray Spectroscopy) is an analytical technique used for the elemental analysis or chemical characterisation of a sample. It relies on an interaction between some source of X-ray excitation and a sample. Its characterisation capabilities are due in large part to the fundamental principle that each element has an unique atomic structure allowing unique set of peaks on its X-ray emission spectrum (Corbari et al. 2008; Goldstein 2003).
Figure 12 shows the weight and the atomic percentage. The Table 8 shows the maximum element weight and atomic percentage of Ni. Elements weight and atomic percentage are 24.92 and 55.60 %, respectively. Reminder elements are Cr, Fe, O, Nb, Ti, Al, S, Si and Cl in decrement order.
The following conclusions can be derived based on the experimental results and discussions for the Inconel 718 material, with a rotary electrode at 300 rpm. Though powder concentration does not have a significant effect on the response parameters, machining parameters play a vital role in improving machining efficiency.
(1) The MRR is highly significant in order to depend on peak current and duty cycle.
(2) TWR depends on Peak current, slurry concentration, duty cycle and Voltage.
(3) Whereas the individual high-order effect of peak current and ON time and duty cycle on SR is evidenced. The SR is finer at lower range of parameters and coarse at higher range of parameters in SR.
(4) The individual high-order effect of factors on HD, such as slurry concentration and duty cycle is evidenced.
(5) The atomic weight is decreased with increase in powder concentration ratio. Experimental results conclude that the higher powder concentration ratio retards the deposition of the [Al.sub.2]O.sub.3] particles on the workpiece.
The authors wish to thank the management of Charotar University of Science & Technology-Changa for facilitating them with the resources.
No potential conflict of interest was reported by the authors.
Notes on contributors
Sagar Patel is a Utility and Maintenance engineer at GSP Crop Science Pvt. Ltd. Nandesari (Vadodara), a reputed agrochemicals company. He is in charge of all the utility machines like steam boiler, R.O plant, brine plant, chilling plant and cooling tower. He is also responsible for preventive and breakdown maintenance of machines and keeping records of power consumption, water consumption and fuel consumption. Earlier, he has served as an assistant professor in Mechanical Engineering Department at K J Institute of Information and Technology, Savli (Vadodara). Later, he joined Sardar Patel College of Engineering, Bakrol (Vallabh-Vidhyanagar) for the same post. During this service, he has guided several undergraduate Engineering projects on Electro Discharge Machine. Furthermore, He has published papers in international reputed journals and international conferences.
Dignesh Thesiya is an assistant professor (Faculty) in Manufacturing Engineering (HLC) in Central Institute of Plastic Engineering & Technology (CIPET-A Govt of India Enterprise), Ahmedabad, India since Dec 2016. He is teaching Product Design and Development and Non-conventional machining processes. Earlier, he has worked as postgraduate intern in Indian Space Research Organization (ISRO-SAC),
Ahmedabad where he has designed and proved prototype Foldable and Deployable Mechanism for Space telescope. He has published several articles on Electrical Discharge Machining. He is currently working on a novel foldable mechanism for space applications.
Avadhoot Rajurkar is an assistant professor in Production Engineering in Vishwakarma Institute of Technology, Pune, India where he teaches Metal Cutting and Tool Design, Non conventional machining processes, Design for Manufacturing and Finite Element Method and CAD. He has published several articles on unconventional machining of novel materials in journals and conferences and a book on Development of 2 Dimensional Nesting Algorithm. He is currently researching on development of technology for drilling of microholes in aerospace materials for Vikram Sarabhai Space Centre (VSSC), ISRO, Trivandrum.
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Soni, J. S., and G. Chakraverti. 1994. "Machining Characteristics of Titanium with Rotary Electro-discharge Machining." Wear (1-2): 51-58. http://www.scirp.org/journal/PaperInformation.aspx?PaperID=20771.
Wang, C. C., and B. H. Yan. 2000. "Blind-hole Drilling of [Al.sub.2.sub.O.sub.3]/6061 Al Composite using Rotary Electrodischarge Machining." Journal of Materials Processing Technology: 90-102: (1-3). doi:10.1016/s0924-0136(99)00423-9.
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Gurule N. B., and K. N. Nandurkar. 2012. "Effect of Tool Rotation on Material Removal Rate during Powder Mixed Electric Discharge Machining of Die Steel." International Journal of Emerging Technology and Advanced Engineering 2 (8). http://www.ijetae.com/files/Volume2Issue8/IJETAE_0812_57.pdf
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Sagar Patel (a), Dignesh Thesiya (b) and Avadhoot Rajurkar (c)
(a) GSP Crop Science Private Limited, Vadodara, India; (b) Department of Mechanical & Manufacturing Engineering, Central Institute of Plastics Engineering and Technology (CIPET), Ahmedabad, India; (c) Department of Industrial & Production Engineering, Vishwakarma Institute of Technology, Pune, India
CONTACT Dignesh Thesiya firstname.lastname@example.org
Received 25 February 2016
Accepted 24 January 2017
Table 1. Experimentation parameters. Parameter Description Work piece material Inconel 718 Work piece size 2.5 x 2.5 x 0.5 cm Electrode material (s) Copper tungsten (Cu-W) Electrode diameter 10 mm Electrode polarity (P) Positive Dielectric fluid Kerosene oil with Aluminium powder (Al) Flushing pressure (Inlet) 3.5 kgf/cm2 Flushing pressure (outlet) 2 kgf/cm2 Slurry concentration 0.5, 1, 1.5 gm/l Voltage (V) 50,62 V Peak current (A) 9,17,28 ([I.sub.p]) Pulse on time ([T.sub.on]) 50,100, 150 ([micro]s) Duty cycle (DO) 0.4,0.5, 0.6 Rotary electrode speed 300 rpm Table 2. Properties of aluminium powder. Thermal Electrical Density conductivity resistivity Melting Powder (g/cm) (w/cm [degrees]c) (pQ cm) point ([degrees]c) Al 2.70 2.38 2.45 660 Specific heat Powder (cal/g [degrees]c) Al 0.215 Table 3. Experimental result outcomes of Inconel 718 with Cu-W electrode in Al powder-mixed dielectric with Rotary electrode. Sparking gap Peak current Slurry conc. Sr No (V) (amp) (gm/l) ON time ([micro]s) 1 50 9 0.5 50 2 50 9 1.0 100 3 50 9 1.5 150 4 50 17 0.5 50 5 50 17 1.0 100 6 50 17 1.5 150 7 50 28 0.5 100 8 50 28 1.0 150 9 50 28 1.5 50 10 62 9 0.5 150 11 62 9 1.0 50 12 62 9 1.5 100 13 62 17 0.5 100 14 62 17 1.0 150 15 62 17 1.5 50 16 62 28 0.5 150 17 62 28 1.0 50 18 62 28 1.5 100 MRR ([mm.sup.3]/ TWR ([mm.sup.3] Sr No Duty cycle min) /min) SR ([micro]m) 1 0.4 10.2768 0.09290 5.63 2 0.5 11.2050 0.03059 6.21 3 0.6 12.0990 0.00000 6.31 4 0.5 22.8274 0.32320 5.86 5 0.6 21.5007 0.00000 6.56 6 0.4 22.6757 0.00000 6.11 7 0.4 43.4884 0.00000 7.51 8 0.5 46.9615 0.11430 8.22 9 0.6 50.3428 0.57190 5.53 10 0.6 11.7978 0.00000 5.59 11 0.4 8.8800 0.06140 4.89 12 0.5 9.4627 0.03097 6.00 13 0.6 33.6996 0.00000 7.03 14 0.4 19.5843 0.07361 7.53 15 0.5 19.7802 0.14860 6.23 16 0.5 45.0553 0.22304 8.94 17 0.6 39.1484 0.13940 5.80 18 0.4 32.3206 0.08578 9.09 Table 4. Analysis of variance for means of MRR. Source DF Seq SS Adj SS Adj MS F value P value Sparking gap 1 26.04 26.04 26.04 1.09 0.326 Peak current 2 3169.75 316975.00% 1584.87 66.52 0 Slurry conc. 2 45.21 45.21 22.61 0.95 0.427 On time 2 5.01 5.01 2.51 0.11 0.901 Duty cycle 2 82.6 82.6 41.3 1.73 0.237 Residual error 8 190.61 190.61 23.83 - - Total 17 3519.21 - - - - Table 5. Analysis of variance for Means of TWR. Source DF Seq SS Adj SS Adj MS F value P value Sparking gap 1 0.007609 0.007609 0.007609 0.55 0.481 Peak current 2 0.072183 0.072183 0.036091 2.6 0.135 Slurry conc. 2 0.01457 0.01457 0.007285 0.52 0.611 On time 2 0.130225 0.130225 0.065112 4.68 0.045 Duty cycle 2 0.027431 0.027431 0.013716 0.99 0.414 Residual error 8 0.111261 0.111261 0.013908 - - Total 17 0.363279 - - - - Table 6. Analysis of variance for means of SR. Source DF Seq SS Adj SS Adj MS F value P value Sparking gap 1 0.5548 0.5548 0.55476 1.07 0.331 Peak current 2 9.15 9.15 4.57502 8.84 0.009 Slurry conc. 2 0.1939 0.1939 0.09695 0.19 0.833 On time 2 8.2444 8.2444 4.1222 7.97 0.012 Duty cycle 2 2.0857 2.0857 1.04287 2.02 0.195 Residual error 8 4.1384 4.1384 0.5173 - - Total 17 24.3672 - - - - Table 7. Analysis of variance for means for HD. Source DF Seq SS Adj SS Adj MS F value P value Sparking gap 1 0.000059 0.000059 0.000059 0.14 0.718 Peak current 2 0.000025 0.000025 0.000012 0.03 0.971 Slurry conc. 2 0.001849 0.001849 0.000925 2.18 0.176 On time 2 0.000023 0.000023 0.000011 0.03 0.974 Duty cycle 2 0.001668 0.001668 0.000834 1.96 0.202 Residual error 8 0.003399 0.003399 0.000425 - - Total 17 0.007022 - - - - Table 8. Elements percentage. Element Weight% Atomic% C 24.92 55.60 O 6.95 11.64 Al 0.84 0.84 Si 0.55 0.52 S 0.69 0.58 Cl 0.41 0.31 Ti 0.89 0.50 Cr 14.06 7.25 Fe 12.45 5.98 Ni 34.23 15.63 Nb 4.00 1.15 Total 100.00 100.00
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|Author:||Patel, Sagar; Thesiya, Dignesh; Rajurkar, Avadhoot|
|Publication:||Australian Journal of Mechanical Engineering|
|Date:||Mar 1, 2018|
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