Realizing Potential of Graphite Powder in Enhancing Machining Rate in AEDM

AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010

Realizing Potential of Graphite Powder in Enhancing Machining Rate in AEDM of Nickel Based Super Alloy 718 Anil Kumar1, Sachin Maheshwari2, Chitra Sharma3, Naveen Beri1 1

Research scholar, University School of Engineering and Technology, GGSIPU, Delhi, India (ak_101968@yahoo.com) 2 Division of Manufacturing Process & Automation Engineering, NSIT, Dwarka, New Delhi, India 3 Department of Mechanical and Automation Engineering, IGIT, Kashmiri Gate, Delhi, India Email: {ssaacchhiinn@redifmail.com, chitrabisht@rediffmail.com, nav_beri74@yahoo.co.in} Abstract— Additive mixed electric discharge machining (AEDM) is a recent innovation for enhancing the capabilities of electrical discharge machining process. The objective of present study is to realize the potential of graphite powder as additive in enhancing machining capabilities of AEDM on Inconel 718. Taguchi methodology has been adopted to plan and analyze the experimental results. L 36 Orthogonal Array has been selected to conduct experiments. Peak current, Pulse on time, duty cycle, gap voltage, retract distance and concentration of fine graphite powder added into the dielectric fluid were chosen as input process variables to study performance in terms of material removal rate. The ANOVA analysis identifies the most important parameters to maximize material removal rate. The recommended best parametric settings have been verified by conducting confirmation experiments. From the present experimental study it is found that addition of graphite powder enhances machining rate appreciably. Machining rate is improved by 26.85% with 12g/ l of fine graphite powder at best parametric setting.

Figure 1. Principal of AEDM [4] TABLE I PROCESS PARAMETERS AND THEIR LEVELS

Index Terms— Electrical discharge machining, Additive mixed electrical discharge machining, Machining rate, Spark gap, Graphite powder concentration, Taguchi methodology

I. INTRODUCTION conventional EDM process [3-4]. In AEDM when a voltage of 80-320V is applied across work piece and electrode electrical intensity in the range of 105 to 107V/m is generated [3]. Under the influence this electric intensity additives powder particles get energized and behave in a zigzag fashion. These additives particles arrange themselves in the form of chain at different places under the sparking area (Fig.2). The chain formation helps in bridging the gap between both the electrodes. This bridging effect results in lowering the breakdown strength of the dielectric fluid which causes early explosion in the gap. As a result, the series discharge starts under the electrode area. Due to increase in frequency of discharging, faster erosion takes place from the work piece surface. Therefore gap contamination facilitates ignition process and increases gap size thereby improving process stability. The absence of debris may results in arcing due to absence of precise feeding mechanism with highly position resolution. However excessive contamination may increase spark concentration i.e. arching leading to unstable and inefficient process [6].

Electrical discharge machining (EDM) is a common nonconventional material removal process. This technique has been widely used in modern metal working industry for producing complex cavities in dies and moulds, in press tools, aerospace, automotive and surgical components manufacturing industries which are otherwise difficult to create by conventional machining methods [1-2]. This machining process involves removal of material through action of electrical discharges of short duration and high current density between tool electrode and the work piece.

However, its low machining efficiency and poor surface finish restricted its further applications [3]. To overcome these problems, one relatively new innovation used to improve the efficiency of EDM in the presence of additives suspended in the dielectric fluid. This new hybrid machining process is called additive mixed electrical discharge machining (AEDM) [4-5]. Fig.1. depicts principal of AEDM. The machining mechanism of AEDM is different from Corresponding author: Anil Kumar, Department of Mechanical Engineering, Beant College of Engineering & Technology Gurdaspur 143521, Punjab, India.

© 2010 AMAE

DOI: 01.IJPIE.01.01.51

48

AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010 Electronica make electrical discharge machine; model SMART ZNC (S50). The dielectric flow system was modified for circulation of aluminum powder suspended dielectric medium in small quantities to prevent contamination of whole of dielectric fluid. Inconel 718 (specimen 65mm X 25mm X 4mm) was selected as work piece. Cylindrical copper electrode ( 8.0 mm) was used as an electrode. Based on literature survey and preliminary investigations, the parameters chosen as inputs are polarity, peak current, types of electrodes, pulse on time, gap voltage duty cycle, retract distance and concentration of graphite additive powder. The Inconel 718 work piece was eroded with fine graphite powder (325 mesh) in AEDM. The machining process parameters set up as shown in Table 1 keeping all other parameters constant. Preliminary experiments were conducted in the given range of different input parameters to select their levels shown in Table 1. L36 (21×36) orthogonal array has been used which contains 36 experimental runs at various combinations of eight input variables [13]. Machining rate is measured in term of volume of material eroded from work piece per minute by weight loss method as per following equation.

Figure 2. Mechanism of AEDM.(a) Bridge formation, (b) Spark initiation because of breakage of chain, (c) Explosion leading to zigzag particle motion, (d) Re- bridging [4]

Erden and Belgin [7] were the first who studied the effect of impurities (copper, aluminium, iron and carbon) in electrical discharge machining of brass steel and copper steel pair and obtained increase in machining rates with increase in concentration of impurities. It was further observed that machining becomes unstable at an excessive powder concentration due to the occurrence of short-circuits.

Thereafter Jeswani [8] investigated the effect of adding fine graphite powder in dielectric of EDM and reported that addition of about 4g of fine graphite powder (10 µm in average size) per liter of kerosene improved machining stability thereby increasing metal removal rate (MRR) by 60% and tool wear rate (TWR) by 15%. The WR was reduced by about 28%. This effect was attributed to increase in interspaces for electric discharge initiation and reduction in the breakdown strength of dielectric fluid. Later many researchers Zhao, Meng and Wang [9], Singh, Maheshwari and Pandey [10] Kumar, Maheshwari, Sharma and Beri [11,12] investigated the process capabilities of additive powder mixed EDM. They reported that that machining efficiency of powder mixed EDM can be increased by selecting proper discharging parameters. Very little research work is reported on effect of machining parameters on machining characteristics in AEDM of Inconel 718. Nickel based super alloy are extremely useful in gas turbine, space vehicles, aircraft, nuclear reactors, submarine, petrochemical equipments and other high temperature applications. The electrical parameters (like pulse frequency, duty cycle, pulse on time, spark gap, current and gap voltage, polarity etc.), material properties of electrode, work piece and dielectric fluid, properties of additive powders (like melting point, specific heat, thermal conductivity, grain size and concentration etc.) are the main factors which influence additive mixed electrical discharge machining. Hence there is need to investigate machining parameters to obtain optimum machining rate. Therefore, promoting the quality of the process by developing a thorough understanding of the relationship between these parameters for better machined surfaces has become a major research concern [6, ­12]. The aim of the present research work was to identify the significant parameters affecting AEDM process and find best parametric settings to maximize MR of Inconel 718 super alloy.

Machining rate = (1) Weight loss from work piece (gms) Density of work piece (g/mm3) × machining time (min) III. RESULTS AND DISCUSSIONS Taguchi methodology has been used for the design and analysis of the experiment [13, 14]. The Taguchi method uses signal-to-noise (S/N) ratio to quantify the variation in data. Higher response characteristics i.e MR is desirable, so The selected response characteristic in the present work is machining rate and will be considered as ‘higher the best’.

(2) The results are analyzed using MINITAB 15.1.1 software.

The S/N ratios of the MR for each trial run through have been calculated from experimental data and response table for larger the better are summarized in Table 2 giving relative importance of each parameter on desired response. Higher the delta value, higher is contribution of parameter on response. From the Table 2 it is clear that polarity, peak current, and concentration of powder significantly affect the process performance. The main effect plots are plotted in Fig.3. Although cryogenic treatment of copper electrode increases conductivity however its effect on machining rate is negligiblePulse on time affects the machining rate significantly. Increased pulse on time produces crater of higher diameter, thereby increasing machining rate. Middle value of duty cycle 0.8 i.e 80% of available energy produces higher machining rate. At higher duty cycle there is possibility of accumulation of more debris within sparking area leading to unfavorable flushing conditions, thereby reducing machining rate. A gap voltage of 80V produces higher machining rate.

II. EXPERIMENTAL SET UP AND PROCEDURE In the present study experiments were carried out on © 2010 AMAE

DOI: 01.IJPIE.01.01.51

49

AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010 TABLE I. RESPONSE TABLE FOR SIGNAL TO NOISE RATIOS (LARGER THE BETTER)

Higher retract distance leads to better flushing conditions.

Increase in concentration of graphite fine powder in dielectric fluid increase material removal rate. This observation suggests that addition of an appropriate amount of additives into the dielectric fluid of EDM causes greater erosion of the material. The reason for the enhancement of MR is mainly attributed to reduction in breakdown strength of the dielectric fluid leading to early spark, and increase in frequency of sparking within the discharge. Fig.3 suggest that +ve polarity, copper electrode , 6 amps peak current , 150 Âľs pulse on time , 0.8 duty cycle, 80V gap voltage, 3mm retract distance and 12g/l concentration of graphite powder produce the maximum machining rate in Inconel 718. ANOVA analysis performed on the S/N data suggest that polarity , peak current , pulse on time and graphite as additive powder in dielectric medium significantly affect response characteristic of machining rate at 5% level of significance.

Figure 3. Main Effect plot for S/N ratios for machining rate

TABLE II. ANALYSIS OF VARIANCE FOR S/N DATA (LARGER THE BETTER )

following conclusions are drawn: 1. Selected EDM process parameters and fine graphite powder concentration affects the machining rate in AEDM. 2. Additive mixed EDM enhance machining rate appreciably.

3. Peak current contribute maximum to machining rate i.e 81.3% 4. Contribution of fine graphite powder is 2.4% 5. Overall machining rate after addition of graphite powder improves by 26.85%.

6. In the present study within the experimental conditions the best parametric settings are A1B1C3D3E2F3G3H3.

7. 12g/l of graphite in EDM medium produces maximum MR.

To verify the improvement in MR using the best parametric settings of cutting parameters, three confirmation experiments were performed, and the data from the confirmation runs and their comparisons with the predicted value for MR is listed in Table 5. The error is less than 10 % and the maximum improvement of MR at best parametric settings is about 26.85%.

IV. CONCLUSIONS The result of the present work identifies the significant process parameters and optimizes the machining conditions in the presence of graphite powder in the dielectric fluid toget maximum MR from Inconel 718 super alloy. Within the range of parameters selected for the present work, the Š 2010 AMAE

DOI:1.IJPIE.01.01.51

50

AMAE Int. J. on Production and Industrial Engineering, Vol. 01, No. 01, Dec 2010 [4]. A. Kumar, S. Maheshwari, C. Sharma and N. Beri, “Research Developments in Additives Mixed Electrical Discharge Machining (AEDM): A State of Art Review”, Materials and Manufacturing processes, (In press 2010).

[5]. K.P. Rajurkar, Handbook of Design Manufacturing and Automation Chapter 13: Nontraditional Manufacturing Processes, Wiley, USA, 1994.

[6]. Y.F. Tzeng and C. Lee, “Effects of Powder Characteristics on Electro discharge Machining Efficiency”. International Journal of Advanced Manufacturing Technology,vol. 17(8), pp. 586–592,2001.

[7]. A. Erden and S. Bilgin, “Role of impurities in electric discharge machining”, Proceedings of the 21th International Machine Tool Design and Research Conference, Macmillan, London, pp. 345-350, 1980.

[8]. M.L. Jeswani, “Effect of the addition of graphite powder to kerosene used as the dielectric fluid in electrical discharge machining” Wear, vol.70, no.2, pp.133-139, 1981.

[9]. W.S. Zhao, Q.G. Meng and Z.L. Wang, “The application of research on powder mixed EDM in rough machining”. Journal of Materials Processing Technology, vol. 129(1-3), 30-33.

2002.

[10].S. Singh, S. Maheshwari and P.C. Pandey, “An experimental investigation into Additive Electrical Discharge Machining (AEDM) of Al2O3 particulate reinforced Al-based metal matrix composites”, Journal of Mechanical Engineering, vol. 57, no.1, pp.13-33, 2006.

[11].A. Kumar, S. Maheshwari, C. Sharma and N. Beri, “Performance evaluation of silicon additive in electrical discharge machining of EN 24 steel based on Taguchi method”, Journal of Mechanical Engineering,vol.60, no.(56), pp. 289-304, 2009.

[12].A. Kumar, S. Maheshwari, C. Sharma and N. Beri, “A study of multi-objective parametric optimization of silicon abrasive mixed electrical discharge machining of tool steel”, Materials and Manufacturing processes, (In press 2010).

[13].Montgomery, D.C. (2001). Design and analysis of experiments. Fifth edition. John Wiley & Sons, New York.

[14].Lin, Y.C.; Wang, A.C.; Wang, D.A.; Chen, C.C. Machining Performance and Optimizing Machining Parameters of Al2O3TiC Ceramics Using EDM Based on the Taguchi Method.

Materials and Manufacturing Processes,vol. 24(6), 667 – 674, 2009.

TABLE III ANALYSIS OF VARIANCE FOR S/N DATA (LARGER THE BETTER

ACKNOWLEDGEMENT The authors would like to acknowledge the support of department of Mechanical Engineering, Beant College of Engineering and Technology, Gurdaspur, Punjab, India and All India Council for Technical Education New Delhi, India for supporting and funding the research work under research promotion scheme (F.No.: 8023/BOR/RID/RPS-129/20072008, F.No.: 8023/BOR/RID/RPS-144/2008-2009 and 8023/ BOR/RID/RPS-86/2009-10) in this direction.

REFERENCES [1]. N.M. Abbas, D.G. Solomon and M.F. Bahari, “A review on current research trends in electrical discharge machining (EDM)”. International Journal of Machine Tools and Manufacture, vol. 47, no. (7-8), pp. 214-1228, 2007.

[2]. K.H. Ho and S.T. Newman, “State of the art electrical discharge machining (EDM”. International Journal of Machine Tools and Manufacture, vol. 43, pp. 1287-1300, 2003.

[3]. H.K. Kansal, S. Singh and P. Kumar, “Technology and research developments in powder mixed electric discharge machining(PMEDM)”, Journal of Materials Processing Technology, vol. 184, pp. 32-41, 2007.

© 2010 AMAE

DOI:1.IJPIE.01.01.51

51