EFFECT OF ALUMINIUM POWDER ADDITION IN DIELECTRIC DURING ELECTRIC DISCHARGE MACHINING OF HASTELLOY ON MACHINING PERFORMANCE USING REVERSE POLARITY

Transcription

1 Research Article EFFECT OF ALUMINIUM POWDER ADDITION IN DIELECTRIC DURING ELECTRIC DISCHARGE MACHINING OF HASTELLOY ON MACHINING PERFORMANCE USING REVERSE POLARITY 1 Saurabh Sharma, * 2 Anil Kumar, 3 Naveen Beri 4 Dinesh Kumar Address for Correspondence 1,4 Dept. of Mechanical Engineering, S.S.C.E.T Badhani Pathankot, (Pb.) India. *2, 3 Dept. of Mechanical Engineering, BCET Gurdaspur, (Pb.) India. macsonu163@sify.com, ak_101968@yahoo.com,nav_beri74@yahoo.co.in, dinesh123badhan@yahoo.co.in ABSTRACT The addition of powder particles suspended in dielectric fluid of electrical discharge machining (EDM) modifies some process characteristics and creates the condition to achieve higher machining performance. In this paper attempt has been made to study the effect of aluminium powder on the machining performance of conventional EDM with reverse polarity. The machining performance is evaluated in terms of material removal rate, tool wear rate, percentage wear rate, surface roughness. Concentration and grain size of aluminium powder are taken as the input powder parameters and its effect are presented on machining performance. It is found experimentally that powder characteristics significantly affect machining characteristics. KEYWORDS: Electrical discharge machining, Powder-Mixed dielectric electrical discharge machining, Aluminium powder, material removal rate, tool wear rate, percentage wear rate, surface roughness and polarity. 1. INTRODUCTION metal powder (aluminium, chromium, copper, Since 1940 considerable research efforts have fostered a deep understanding, prediction and control of the electric discharge machining process (EDM). The development of the super tough electrical conductive material such as hastelloy, carbides, stainless steel, nitralloy etc. resulted in development of the nontraditional machining processes. These materials are difficult to machine by conventional machining process, & have wide range of applications in industry. EDM has been widely used is a removal process to manufacture parts, dies & moulds for long time now. In EDM thermal energy is used to machine all electrical conductive materials of any hardness & toughness. Since there is no direct contact between the tool electrode & work piece in EDM, machining defects like mechanical stresses, clattering & vibration do not create problem during machining. In the past few years, powder-mixed dielectric electric discharge machining (PMD-EDM) emerges as a new technique to enhance the process capabilities. In PMD-EDM a suitable silicon carbide etc.) is mixed into the dielectric used in EDM. When a voltage in the range of 80 to 320 V is applied between the tools and the work piece placed close to each other an electric field of the range V/m is generated. The additive particles suspended in the dielectric has important influence on the discharge process; increase both the gap distance & the discharging rate. The high electric field energises the conductive powder particles. These conductive particles form chains at different places under sparking area, which bridges the gap between tool electrode & work piece material. Due to this bridging effect, the gap voltage & insulting strength of the dielectric decreases, this causes easy shortcircuiting and hence early explosion in the gap between the electrode and the work piece. At the same time the suspended particles in the dielectric enlarged the plasma channel, because of which electric density decreases and hence uniform distribution of the sparking takes place. Very little literature is available on PMD-EDM on reverse polarity.

2 Researchers have done work to improve the surface finish and machining output Parameters for various hard and tough materials by adding different metal powder during machining. However no work has been reported on the machining characteristics of hastelloy using reverse polarity on EDM. The major applications of the Hastelloy are making of pressure vessels of nuclear and chemical industry and aerospace engine parts etc. The objective of the present research work is to examine the variation of material removal rate, tool wear rate and surface roughness with variation of the two input parameters i.e. grain size of the aluminium Powder particles and concentration of the aluminium powder in the dielectric fluid of EDM using the reverse polarity. 2. LITERATURE SURVEY Erden A., et al., [1] Reported during the machining of mild steel that the machining rate increases by the addition of powder particles (aluminium, copper, iron) in the dielectric fluid of dielectric machining. Here improvement in the Break Down characteristics of the dielectric fluid is observed with the addition of powder particles, but after a certain critical concentration of powder short circuiting take place which causes poor machining. Jeswani M.L., et al., [2] 1981 Study the effect of addition of graphite powder to kerosene used as dielectric fluid in the EDM. He concluded that addition of about 4gm/litre of fine powder having average size of particle as 10µm increases the MRR (Material Removal Rate) by 60% and TWR (tool wear rate) by 15% in electrical discharge machine. Wear ratio is also reduced by 30%. He concluded that there is 30% reduction in the breakdown voltage of kerosene at spark gap of 50µm was observed. Narumiya H., et al., [3] used silicon, aluminium and graphite as powder materials. The concentration range of the powder was between 2gm/l to 40gm/l. Their conclusion showed that the gap distance increases with the powder concentration and is larger for the aluminium powder but there is no direct relation between the surface roughness and the gap distance. The best results concerning the surface finish were achieved for low powder concentrations levels and that also for silicon and graphite powders. Kobayashi K., et al., [4] have concluded that silicon powder mixed in the dielectric improves the surface finish of SKD-61 tool steel. It has also been observed, however, that at specific machining conditions in the EDM of steel the aluminium and graphite powders generate better surface roughness than silicon powder. Wong Y.S., et al., [5] Study the powder mixed dielectric electric discharge machining (PMD-EDM) by employing a current of 1A and pulse on time as 0.75µs to produce a near mirror finish on SKH-54 tool steel. The conclusion was that the resulting machining surface was composed of well defined, uniformly sized, smoothly overlapped and shallow craters. The analysis was carried out by varying the silicon powder concentration and the flushing flow rate. Furutani K., et al., [6] Used titanium powder in dielectric fluid (Kerosene) and found that the layer of titanium carbide of hardness 1600HV (Vickers hardness number) on a carbon steel with negative polarized copper electrode, peak current 3A and 2 µs pulse duration. Titanium and titanium Carbide are found in X-Ray diffraction (XRD) analysis of machine surface. It was concluded that the breakdown of dielectric takes place and carbon came from it. Tzeng Y.F., et al., [7] examines the effect of powder characteristics on machining efficiency of electrical discharge machining. They reach to a conclusion that 70-

3 80nm powder suspended in dielectric produces the greatest material removal rate and least increase in the spark gap. Yan BH., et al., [8] studied the electric discharge machining with powder suspended working media and reported that the gap length become shorter regardless of a mixed powder with a decrease of the pulse duration at a duty factor of 0.5. Kozak J., et al., [9] Reported that the material removal rate and tool wear rate were decreased by addition of powder. Consequently the machined surface becomes smooth. Peças P, Henriques E., et al., [10] studied the relationship between the roughness and pulse energy under a few sets of the conditions in the removal process. However, the influence of the energy was not systematically analysed. Klocke F., et al., [11] Used HSFC high speed forming camera technique to find out that in comparison to standard electrode, the aluminium mixed dielectric forms larger plasma channel. It was concluded that discharge energy distribution is on the larger part on the work piece surface. The type and concentration of the powder mixed in the dielectric fluid also found to have direct effect on the machining performance output. Wu KL., et al., [12] Study the problem of powder settling by adding a surfactant with aluminium powder in dielectric fluid and observed that a surface roughness (Ra value) of less than 0.2µm. This is because of more apparent discharge distribution. It was also reported that negative polarity of the tool resulted in better hardness of the surface. Kansal H.K., et al., [13] reported that the addition of Silicon powder into the dielectric fluid of EDM and an enhanced rate of material removal and surface finish. Yeo S H., et al., [14] The experiments were conducted using dielectric with and without additive and at low discharge energies of 2.5µJ, 5µJ and 25µJ, and was observed that a considerable difference in crater morphology is seen between craters in dielectric with and without the powder at low discharge energy of 2.5µJ, 5µJ and 25µJ. More circular shapes with smaller diameters are produced with powder additive as compared to without powder additive. Craters with the additives are smaller and have more consistent depth than in dielectric without additive. They reported that dielectric with additive in it lower the amount of discharge flowing between the work piece and the tool electrode and slows down the rate at which these charges flow. Peças P., et al., [15] Study the effect of silicon powder particles suspended in dielectric fluid. The powder concentration and flushing flow rate are two input parameters. They reach to a conclusion that even for small level of powder concentration there is evident amount of reduction in crater depth, crater diameter and the white layer thickness. They reported that for a particular experimental configuration used, we can find the powder concentration that generates better surface morphology. It was observed that there is dielectric flow rate that minimises the surface roughness for each electrode area and for larger flow rates, no positive effect on the surface morphology. Furutani K., et al., [16] the conditions for deposition machining by Ti powder suspended EDM was investigated with respect to discharge current and pulse duration in this paper. They concluded that the discharge energy affected the deposit able condition range. TiC could be deposited in the case that both discharge energy and powder density was small. They reported that the hardness of the deposition achieved was 2000Hv. The matrix surface was also hardened. Kumar S., et al., [17] found that significant amount of material transfer takes place from the manganese powder suspended

4 in dielectric fluid to the machined surface under appropriate machining conditions which changes the surface composition and its properties. They reported that percentage of manganese increased to0.95% from 0.52% and that of carbon to 1.03% from 0.82% that result in increase in the micro hardness. For surface alloying favourable machining conditions were found to be low peak current (4 A), shorter pulse on-time (5µs), longer pulse offtime (85µs), and negative polarity of the tool electrode. avoid this problem a new container was developed with a capacity of 6.5liter for the EDM oil. The container was filled with EDM oil and placed in the empty working tank. Experiments were performed in that container. Hastelloy Steel was used as a work material and standard EDM oil was used. To hold the work piece a special type fixture was made. Container was filled with EDM oil and the fixture was placed in it with the work piece fixed on it. A small dielectric circulation pump was placed in the container to achieve the proper circulation of the powder mixed 3. EXPERIMENTAL SETUP dielectric through the gap between the work Experiments were conducted on smart ZNC piece and the electrode tool. Proper distance EDM machine Electronica make. The was maintained between the nozzle of the dimensions of the working tank of ZNC EDM pump and the discharge gap for proper are 800mm X 500mm X 350mm. Working circulation. A magnet was also placed in the tank contains the dielectric fluid and with container to hold the fixture with the work these dimension the tank contains large piece. Mitutoyo SJ-400 surface roughness amount of dielectric. So it requires large tester was used for the measurement of surface amount of aluminium powder to get the roughness of holes. desired amount of concentration of powder. To Table1. Chemical composition of Hastealloy steel Element Ni Co Cr Mo Fe Si Mn C Ti % Powder Density (g/cm 3 ) Table2. Chemical properties of aluminium metal powder Thermal conductivity (300 K) Electrical resistivity (20 C) Melting point Specific heat capacity (25 C) (25 C) W m 1 K nω m K J mol 1 K Table3. Grain size of the aluminium powder Type Mesh size (µm) Fine Medium Coarse

5 3.1 Experimentation The experiments were conducted with the Reverse Polarity on the EDM. Two input process parameters decided are concentration of aluminium powder and the grain size of the powder particles. Output parameters decided are MRR, TWR, %WR, SR. Table4. Experimental settings Polarity Negative (-) Machining time 20 min. Electrode lift time 0.2 sec. Total nine numbers of experiments were conducted. During the first five experiments Current, voltage, pulse on time, duty cycle and grain size was kept at a known constant value and the concentration of the powder was changed after certain intervals for different experiments. In the next four experiments the concentration of powder was kept constant along with current, voltage, and pulse on time, duty cycle and grain size of the powder is changed for each experiment. Output parameters were calculated accordingly by taking necessary observations. Measurement of Material Removal Rate: MRR = Work piece weight loss (g) Machining Time (min) Measurement of Tool Wear Rate: TWR = Work piece weight loss (g) Machining Time (min) Measurement of % age Wear Rate: %WR = TWR X 100/MRR Measurement of Surface Roughness: The arithmetic surface roughness value (Ra) was used to measure the surface finish. Various measurements of roughness were carried out at the bottom of holes by using Mitutoyo SJ-400 Surface Roughness tester. 4. RESULTS AND DISCUSSIONS Total nine numbers of experiments were performed on hastelloy steel with powder mixed EDM process using reverse polarity. At the end of each experiment; calculations were done for MRR, TWR, Percentage WR, and SR. The final phase of experimental work has been analysed and results have been discussed. The variations of all the four output parameters have been plotted against the variable input parameters Table 5.Parametric variation chart Exp Current Voltage Pulse on Duty cycle Concentration No. (A) (V) time (µs) (µs) (g/lt) Type of powder Without powder Medium Medium Medium Medium Without powder Fine Medium Coarse.

6 Table 6.Observation chart Exp. No. Work piece weight loss (g) Electrode weight Loss (g) Time of cut (min) MRR (g/min) TWR (g/min) % WR Ra (µm) Analysis of Material Removal Rate (MRR) Fig.1 Graph between concentration and MRR Fig.2 Graph between grain size of powder and MRR Concentration It was seen that material removal rate is very low in conventional EDM with reverse polarity. With the addition of aluminium powder in the dielectric fluid material removal rate increases sharply. It is because of the fact that with addition of powder particles in dielectric, the spark gap is filled up with additive particles. The powder particles reduces the insulating strength of dielectric

7 fluid and increases the spark gap distance between tool and work piece This increases the material removal rate with the increase in concentration. However, with further increase in concentration, MRR lowers down, it may be due to short circuiting with increase in powder density. Again at concentration of 12gm/lt the value of MRR rises it can be explained by the fact that with the more powder particles more erosion from the work piece. Grain Size of the powder Analysis of the Tool Wear Rate (TWR) It was observed that material removal rate is very low without any grain size of particles. However, with the addition of fine grain size of aluminium powder the value of MRR increases sharply. With future addition of medium and coarse grain size powder particles the value of MRR increases, but not as sharp. This may be defined by the reason that the density of suspended fine particles is higher than that of medium and coarse particles. Fig3. Graph between Concentration and TWR Fig4. Graph between Grain size of the powder and TWR

8 Concentration It was observed that the tool wear rate is more than the material removal rate with zero concentration of powder and tool wear rate increases with the addition of powder particles in the dielectric. This can be explained by the fact that during reverse polarity more heat is at the tool electrode than the work piece. With the addition of powder concentration in fluid due to erosion of electrode the tool wear rate increases. A little change in the trend may be due to the more erosion of tool at concentration of 9gm/lt. It may be due to the reason that higher concentrations of powder particles block the path of ions to hit the Analysis of percentage wear rate (%WR) electrode surface. Again at 12gm/lt concentration the TWR increases. It may be due to the reason that at higher concentration more particles hit the electrode during reverse polarity. Grain size of the powder As per (figure 4) graph between Grain size of the powder and TWR, it is observed that with the addition of powder with fine particles the tool wear rate increases. It is due to the reason that ions produced by the ionization of dielectric fluid, hits the tool electrode with high momentum and high energy during the reverse polarity setup. And hence more tool material is eroded. Fig5 Graph between Concentration and % wear rate Fig6. Graph between grain size of powder and %wear rate Concentration

9 It was observed that the value of percentage wear rate is high with no powder suspended; this can be explained that more heat is at the tool during reverse polarity. As the powder is suspended in the dielectric the value of MRR increases more than that of TWR and hence the value of percentage wear rate decreases sharply. A little change in trend can be explained by the fact that tool wear rate increase due to erosion of tool electrode due to more concentration of powder particles. Grain size of the powder With the change in the grain size of the powder particles the percentage wear rate decreases continuously. This can be explained by the fact that MRR increases with change in the grain size of the powder particles more than the increase in the tool wear rate. Hence decrease in the percentage wear rate.

10 Concentration From the graph drawn between the concentration of the powder & surface roughness figure7, it is clear that the value of surface roughness keeps on decreasing with increasing the concentration of the powder in dielectric. This may be explained by the fact that this improvement in surface quality is due to the reason that suspended powder particles enlarge and widen the discharge passage which helps easy evacuation of produced debris from the spark gap. The suspended powder particles lead to uniform dispersion of discharge energy in all directions, which results in shallow and small craters on the machining surfaces. Due to this, surface roughness reduces. Moreover during reverse polarity more heat is at the electrode and less heat is at the work piece. Reverse polarity helps to attain better surface quality. Grain size of powder From the graph drawn between the concentration of the powder & surface roughness figure8, it is seen that surface roughness increases when we add fine powder particles. This can be explained by the fact that densities of the fine powder particles are very high so these particles come in the spark gap and clog the discharge passage. Because of this short circuiting takes place and process became unstable. Moreover not easy evacuation of debris produced leads to somewhat rough surface. As we add powder with medium grain size particles and then coarse size powder particles the surface roughness decreases continuously. This can be explained by the reason that these particles are not as dense as the fine particles. So these powder particles easily enlarge and widen the discharge passage which further facilitates easy evacuation of produced debris from the spark gap and lead to uniform dispersion of discharge energy in all directions. Due to this, better surface finish is achieved. 5. CONCLUSION The experimental research work carried out on reverse polarity on EDM are intended to contribute to the generation of knowledge related to the effect of aluminium powder particles suspended in the EDM dielectric in the quality of the final surface. The input parameters have been taken as concentration of the powder and the grain size of the powder and the output parameters are MRR, TWR, Percentage wear rate, Surface roughness. Within the experimental range following conclusions can be drawn: 1. The surface roughness of the work material continuously decreases with the increase in the concentration of aluminium powder and with change in the grain size of the powder particles. 2. With the increase in the concentration of the powder, percentage wear rate decreases sharply. 3. With change in the grain size of the powder, the percentage wear rate decreases continuously. 4. With the increase in the concentration of additive powder in the dielectric fluid, the tool wear increases. 5. With the addition of aluminium powder in the dielectric fluid of EDM, the material removal rate increases.

11 6. With increase in the grain size of the aluminium powder particles in the dielectric, the material removal rate increases continuously. ACKNOWLEDGEMENTS The authors would like to acknowledge the support of the 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/ and F. No.: 8023/BOR/RID/RPS-144/ ) in this direction. REFERENCES 1. Erden A., Bilgin S. (1980) Role of impurities in electric discharge machining. Proceedings of 21st International Machine Tool Design and Research Conference, pp Jeswani ML (1981) Effect of the addition of Graphite Powder to Kerosene used as the dielectric fluid in electrical discharge Machining Wear Narumiya H,Mohri N, Saito N, Ootake H, Tsunekawa Y, Takawashi T, Kobayashi K (1989) EDM by powder suspended working fluid. Proc Int Sympo Electro- Machining IX Japan pp Kobayashi K., Magara, T., Ozaki, Y., and Yatomi, T.(1992) The present and future developments of electrical discharge machining. In Proceedings of the 2nd International Conference on Die and Mould Technology, Singapore pp Wong Y S, Lim L C, Rahuman I and Tee W M (1998) Near-mirror-finish phenomenon in EDM using powder-mixed dielectric. J. Mater. Process. Technol. (Switzerland) Furutani K, Saneto A, Takezawa H, Mohri N, Miyake H (2001) Accretion of titanium carbide by electrical discharge machining with powder suspended in working fluid. Precision Engg 25: Tzeng Y.F., Lee C.Y., (2001) Effects of powder characteristics on electro discharge machining efficiency. International Journal of Advanced Manufacturing Technology, Volume 17, 2001, pp Yan BH, Lin YC, Huang FY, Wang CH (2001) Surface modification of SKD 61 during EDM with metal powder in the dielectric. Mater Trans 42(12): Kozak J, Rozenek M, Dabrowski L (2003) Study of electrical discharge machining using powder-suspended working media. Proc Inst Mech Eng Pt B 217(B11): Peças P, Henriques E (2003) Influence of silicon powder-mixed dielectric on conventional electrical discharge machining. Int J Mach Tools Manuf 43(14): Klocke F, Lung D, Antonoglou G, Thomaidis D (2004) The effects of powder suspended dielectrics on the thermal influenced zone by electrodischarge machining with small discharge energies. Int J Mater Process Technol 149: Wu KL, Yan BH, Huang FY, Chen SC (2005) Improvement of surface finish on SKD steel using electro-discharge machining with aluminium and surfactant added dielectric. Int J Mach Tools Manuf 45: Kansal H.K.,Singh S, Kumar P. (2005) Parametric optimization of powder mixed electrical discharge machining by response surface methodology. Journal of Materials Processing Technology S H Yeo, P C Tan and W Kurnia (2007) Effects of powder additives suspended in dielectric on crater characteristics for micro electrical discharge machining. J. Micromech. Microeng. 17 N91 N Peças P, Henriques (2008) Effect of the powder concentration and dielectric flow in the surface morphology in electrical discharge machining with powder-mixed dielectric (PMD-EDM) Int J Adv Manuf Technol 37: Furutani K,Sato H & Suzuki M (2009) Influence of electrical conditions on performance of electrical discharge machining with powder suspended in working oil for titanium carbide deposition process. Int J Adv Manuf Technol 40: Kumar S & Singh R (2010) Investigating surface properties of OHNS die steel after

12 electrical discharge machining with manganese powder mixed in the dielectric. Int J Adv Manuf Technol DOI /s