SOME EXPERIMENTAL INVESTIGATION ON ALUMINUM POWDER MIXED EDM ON MACHINING PERFORMANCE OF HASTELLOY STEEL

Research Article SOME EXPERIMENTAL INVESTIGATION ON ALUMINUM POWDER MIXED EDM ON MACHINING PERFORMANCE OF HASTELLOY STEEL Paramjit Singh 1, Anil Kumar* 2, Naveen Beri 3, Vijay Kumar 4 Address for correspondence 1,4 Dept. of Mechanical Engineering, ACET Amritsar, (Punjab.) India. *2,3 Dept. of Mechanical Engineering, BCET Gurdaspur, (Punjab.) India. Email ak_101968@yahoo.com ABSTRACT In this paper, an attempt has been made to study the effect of aluminium powder mixed in the dielectric fluid of Electric Discharge Machining on the machining characteristics of Hastelloy. Concentrations of aluminium powder and grain size of powder are taken as process input parameters. Material removal rate, tool wear rate, %age Wear Rate, surface roughness are taken as output parameters to measure process performance. The experimental investigations are carried out using copper electrode. Nine experiments are performed on Hastelloy using Electronica make smart ZNC EDM machine. Relationships are developed between various input and output parameters. The study indicates that both the input parameters strongly affect the machining performance of Hastelloy. The addition of aluminium powder in dielectric fluid increases MRR, decreases TWR and improves surface finish of Hastelloy. KEYWORDS Powder mixed EDM, material removal rate, tool wear rate, %age wear rate, and surface roughness. INTRODUCTION The development of super tough electrical conductive materials such as carbides, stainless steels, hastalloy, nitralloy, waspalloy, nomonics, etc., arisen the requirement of non-traditional manufacturing processes. These materials are very difficult to machine by conventional methods. Many of these materials find applications in industry where high strength to weight ratio, hardness and heat resisting qualities are required. Electric discharge machining (EDM) is one of the most extensively used non conventional machining processes. It uses thermal energy to machine all electrical conductive materials of any hardness and toughness for applications like manufacturing of dies, automobile components and aerospace parts. Since there is no direct contact between work piece and tool electrode in EDM, machining problems like mechanical stresses, chattering and vibrations dose not arise during machining. In spite of remarkable advantages of the process, disadvantages like poor surface finish and low volumetric material removal limits its use in the industry. In the past few years, powder mixed Electric Discharge Machining (PMEDM) emerges as new technique to

enhance process capabilities [31]. In PMEDM, a suitable material (aluminum, chromium, copper, silicon carbide, etc.) in powder form is mixed into the dielectric fluid used in EDM. When a voltage of 80 320 V is applied between the tool electrode and the work piece placed close to each other, an electric field in the range of 10 5-10 7 V/m is generated [10]. The additive particles fill up the spark gap. It increases the spark gap. These high electric field energies powder particles. These particles act as conductors. These conductive particles form chains at different places under sparking area, which bridges the gap between tool electrode and work piece material. Due to this bridging effect, the gap voltage and insulting strength of dielectric fluid reduces which facilitates easy short circuiting and hence early explosion in the gap between tool electrode and work piece material. Due to this, series discharges takes place within the gap. Due to increase in number of discharging per unit time, rapid sparking takes place that causes faster erosion from work piece surface. At the same time the added powder particles enlarged the plasma channel [33]. Due to this, electric density decreases and hence uniform distribution of sparking takes place. This leads to uniform erosion on work piece which results in improvement in surface finish. Very little literature is available on PMEDM. Researchers have done work to improve surface finish and other machining output parameters for various hard and tough materials by adding powders of different materials in the dielectric of EDM during machining. In addition, some work has been done to optimize the machining input parameters for yielding maximum process output for different work materials. However, No work has been reported on machining characteristics of hestelloy. The major applications of hestelloy are in making of pressure vessels of nuclear and chemical reactors, pipes and valves in chemical industry, aerospace engine parts etc. The objective of present research work is to study the variation of material removal rate, tool wear rate, percentage wear rate and surface roughness with the variation of two process input parameters i.e. the concentration of aluminium powder and grain size of powder in the dielectric fluid of EDM. LITERATURE REVIEW Erden A., et al. [4] found that machining rate of mild steel work piece increases by the addition of powder particles (carbon, iron, aluminium & copper) in the dielectric

fluid of EDM during machining. They noticed the improvement in breakdown characteristics of dielectric fluid due to the suspended powder particles. However, after certain critical concentration of powder in dielectric fluid, the problem of shortcircuiting takes place, which results in poor machining. Jeswani M.L. [7] found 60% increase in MRR & 28% reduction in wear ratio by the addition of fine graphite powder into kerosene oil during machining of tool steels. Concentration of 4g/L of fine graphite powder results in increasing the inter space of electric discharge initiation & lowering of breakdown voltage. Takawashi T., et al. [28] observed that surface finish obtained is greatly affected by the nature of carbon (free or combined form) present in work piece material. Narumiya H., et al. [18] reported that under properly controlled machining conditions, aluminium & graphite powders yields better surface finish as compare to the surface finish obtained by suspending silicon powder in dielectric. Mohri N., et al. [17] studied machining characteristics of H- 13 die steel by adding silicon powder in dielectric fluid. Under properly controlled machining conditions, the fine & corrosion resistant surfaces having surface roughness of 2µm were produced. Kobayashi K., et al., [14] reported the improvement in surface finish of SKD-61 material by suspending silicon powder in the dielectric fluid. Yan B.H., et al. [32] reported the improvement in machining rate of SKD11 and Ti-6Al-4V by the use of silicon carbide & aluminium powders. They noticed that surface roughness also increases with the increase in machining rate. Ming Q.Y., et al. [16] reported the reduction in surface roughness, reduction in tool wear rate and improvement in machining rate by the addition of additives (conductive & inorganic oxide particles). They further noticed that these changes are quite appreciable in mid finish machining and finish machining phases. Uno Y., et al. [29] observed that silicon powder mixed in kerosene fluid enhances the surface finish. It was argued that lesser impact force acts on work piece which results in small craters. This results in stable machining. Uno Y., et al. [30] investigated that surface of aluminium bronze components modify by using nickel powder mixed dielectric fluid. Nickel powder deposits a layer on EDM surface and makes it abrasion resistant. Wong Y.S., et al. [31] investigated that graphite powder particles enhances machining rate of SKH-54 tool steel. Chow H.M., et al. [2] investigated that addition of both aluminium and silicon

carbide powder into kerosene oil increases gap distance between tool electrode and work piece material thus increases material removal rate. Tzeng Y.F., et al. [27] studied the effect of various powder characteristics by using SKD-11 material. Tani T., et al. [25] found that the non-conductive materials like glass and Si3N4 ceramics can be finely machined by PMEDM. Pecas P., et al. [23] reported that by the addition of 2g/lt silicon powder in dielectric fluid, the operating time reduces and surface finish increases when compared with the same for conventional EDM. The average surface roughness depends upon machining area and machining time. Tzeng F.Y., et al. [26] applied the optimization strategy to reduce the functional variability of EDM process. Kansal H.K., et al. [9] tried to optimize the process parameters of PMEDM by using response surface methodology technique. Kansal H.K., et al. [11] establishes optimum process conditions for PMEDM of Al 10%SiC P Metal Matrix Composites by an experimental investigation using Response Surface Methodology. Aluminium powder was suspended into the dielectric fluid of EDM. Kansal H.K., et al. [10] investigate the effect of silicon powder mixed into the dielectric fluid of EDM on machining characteristics of AISI D2 steel and reported the appreciably enhancement of material removal rate of AISI D2 steel. Singh S. et al. [24] compares machinability study carried out on stir-casted 6061Al/Al 2 O 3P /20p work specimens with copper electrode tools by using plain dielectric fluid and Silicon Carbide (SiC) abrasive powder-suspended dielectric fluid. It was found experimentally that abrasive particle size, abrasive particle concentration and pulse current are the most significant parameters that affect the surface characteristics. Kansal H.K., et. al. [12] develops an axisymmetric two-dimensional model for PMEDM using the finite element method to predict the thermal behavior and material removal mechanism in PMEDM process. Peças P., et al. [21] tried to compare the performance of PMEDM technology with conventional EDM when dealing with the generation of high-quality surfaces. The results achieved evidenced a significant performance improvement when the powder mixed dielectric is used. Chiang K.T. [1] proposes mathematical models for the modeling and analysis of the effects of machining parameters on the performance characteristics in the EDM process of Al 2 O 3 +TiC mixed ceramic. It was concluded that discharge current and the duty factor affects significantly the value of MRR. The discharge current and the pulse on time also

have statistical significance on both the value of the electrode wear ratio and the surface roughness. Dvivedi A., et al. [3] investigated the effect of pulse-on (T on ), pulse-off (T off ), pulse current (I p ), gap control setting and flushing pressure on EDM of cast Al 6063-SiC p MMC by using Taguchi's technique to obtain an optimal setting of the EDM process parameters. It was found that MRR increases with increasing I p and T on up to an optimal point. The effect of I p was predominant on MRR as compared to other parameters. Peças P., et al. [20] reported the improvement in the polishing performance of conventional EDM when used with a powder-mixed-dielectric. The analysis was carried out by varying the silicon powder concentration and the flushing flow rate. Positive influence of the silicon powder in the reduction of crater dimensions, white-layer thickness and surface roughness was reported. Prihandana G.S., et al. [22] presents a new method that consists of suspending micro-mos 2 powder in dielectric fluid and using ultrasonic vibration during µ-edm processes. It was observed that the introduction of MoS 2 micro-powder in dielectric fluid and using ultrasonic vibration significantly increase the MRR and improves surface quality. Furutani K., et al. [6] described the influence of the discharge current and the pulse duration on the titanium carbide (TiC) deposition process by EDM with titanium (Ti) powder suspended in working oil. It was noticed that Ti powder reacted with the cracked carbon from the working oil, then depositing a TiC layer on a work piece surface. Kung K.Y., et al. [8] analyzed MRR and electrode wear ratio in PMEDM of cobalt-bonded tungsten carbide by suspending aluminum powder in dielectric fluid and reported that the powder particles disperses and makes the discharging energy dispersion uniform. Kibria G., et al. [13] compares different dielectrics in micro- EDM machining operation and reported that the machining characteristics are greatly influenced by the nature of dielectric used during micro-edm machining. From the available literature, it is concluded that the machining characteristics of some hard and difficult to cut material can be studied by suspending powder of some material in the dielectric fluid of EDM. EXPERIMENTAL DETAIL Experiments are performed on Electronica make smart ZNC EDM machine. The working tank of ZNC machine has the dimensions of 800mm X 500mm X 350mm. It needs large amount of aluminum metal

powder for mixing in such large tank of electrode and work piece material facing EDM to obtain desired powder each other. About 10 inches distance is concentration in dielectric fluid for maintained between nozzle outlet and operation. Moreover, filter of machine might clog due to presence of powder particles and debris when using existing circulation powder mixed dielectric suction point to ensure proper circulation of powder in discharge gap. A permanent magnet of system of machine itself. So, new diameter 12cm is used in machining tank to experimental container was developed in workshop, which has the capacity of 6.5 liters of dielectric fluid. The made up of hold the fixture during operation and to separate the debris from dielectric fluid. Hastelloy is used as work material. A copper sheet metal, is called the machining electrode with a diameter of 8.14 mm is container. It is placed in the existing container of EDM machine and experiment was performed in this machining container. Special fixture was made in machine shop to used as tool electrode. The machining is performed in standard EDM oil. Aluminum powder is mixed in EDM oil as per the need of experiment. Mitutoyo SJ-201P surface hold the work piece. This fixture is placed in roughness tester was used for the machining tank and work piece is fixed in it. The machining tank is filled up by EDM oil. A small dielectric circulation pump is installed in machining tank for proper circulation of powder mixed dielectric fluid measurement of surface roughness of holes. Various experimental settings, chemical composition of Hastelloy, properties of aluminium powder, mesh size of powder are tabulated in table 1(a, b, c, d). into the discharge gap between tool Table 1(a) : Experimental settings Polarity Positive (+) Pressure of dielectric 0.5 Kg/cm 2 Machining time 20 min. Electrode lift time 0.2 sec. Table 1(b) : Chemical composition of Hastelloy Element Ni Co Cr Mo Fe Si Mn C Ti % 65 2 16 16 3 0.08 1 0.01 0.7

Powder Density Table 1(c): Properties of aluminium powder Thermal conductivity (300 K) Electrical resistivity (20 C) Melting point Specific heat capacity (25 C) Al 2.70 (g/cm 3 ) 237 W m 1 K 1 28.2 nω m 933.47 K 24.200 1 J mol 1 K Table 1(d) : Mesh size of aluminium powder Type Mesh size (µm) Fine 300 400 Medium 200 300 Coarse 100-200 EXPERIMENTATION namely the current, voltage, pulse on time, The process parameters namely the duty cycle, grain size of powder etc. are kept concentration of powder and grain size of constant. For the next four experiments, the powder are selected for measurement of value of grain size of powder changes for MRR, TWR, %WR and SF. Total nine each experiment and other input parameters experiments were performed. In the first five including concentration of powder remains experiments the concentration of powder unchanged. The necessary readings are values are changed at different intervals for taken after each experiment to calculate various experiments. During these five process output parameters. experiments the process other parameters Measurement of MRR: MRR = Work piece weight loss (g) Machining Time (min) Measurement of TWR: TWR = Work piece weight loss (g) Machining Time (min) Measurement of % age WR: %WR = TWR X 100 MRR Measurement of Surface Roughness

Various methods are available for measuring of surface roughness of work piece. The arithmetic surface roughness value (Ra) is using Mitutoyo SJ-201P Surface Roughness tester. The parametric variation chart and the used in present work to measure the surface observational data obtained for each finish. Various measurements of roughness experiment is shown in table 2 and table 3. are carried out at the bottom of holes by Table 2: Parametric Variation Chart Exp No. Current (A) Voltage (V) Pulse on time (µs) Duty cycle (µs) Concentration (g/lt) Type of powder 1 5 60 150 9 00 ---- 2 5 60 150 9 03 medium 3 5 60 150 9 06 medium 4 5 60 150 9 09 medium 5 5 60 150 9 12 medium 6 5 60 150 9 00 ---- 7 5 60 150 9 06 fine 8 5 60 150 9 06 medium 9 5 60 150 9 06 coarse Table 3: Observations in each experiment Work piece Electrode Time Exp. MRR TWR Ra weight loss weight of cut % WR No. (g/min) (g/min) (µm) (g) Loss (g) (min) 1 0.189 0.003 3 0.063 0.001 1.587302 3.2988 2 0.245 0.002 3.953 0.06198 0.000506 0.816327 3.14 3 0.215 0.002 3.218 0.06681 0.000622 0.930233 3.12 4 0.207 0.002 3.243 0.06383 0.000617 0.966184 2.845 5 0.245 0.002 4.091 0.05989 0.000489 0.816327 2.8883 6 0.189 0.003 3 0.063 0.001 1.587302 3.2988 7 0.255 0.001 4.569 0.05581 0.000219 0.392157 3.4016 8 0.215 0.002 3.218 0.06681 0.000622 0.930233 3.12 9 0.246 0.003 4 0.0615 0.00075 1.219512 2.7533

RESULTS AND DISCUSSION A set of experiments are performed on Hastelloy steel by using copper electrode in aluminium powder mixed EDM. At the end of each experiment, calculations are done Analysis of MRR for MRR, TWR, %age WR, SR. The final phase of experimental work is analysis and discussion of results. The variations of all the four output parameters are plotted against the variation of input parameters Figure 1: Graph of MRR (g/min) versus powder concentration (g/lt) Figure 2: Graph of MRR (g/min) versus grain size (µm) of powder Concentration between the facing surfaces of electrode MRR yielded by conventional EDM is and work piece. Due to this bridging, short low. With the addition of aluminium circuiting takes place. The process powder into the dielectric fluid, MRR becomes unstable and net reduction in slightly lowers down (figure 1). It is due to MRR. Thereafter with the addition of the reason that as soon as a little powder more powder in dielectric MRR starts enters into the spark gap, bridging occurs increasing at higher rate. Maximum MRR

is produced at 6g/lt concentration. It is supposed that best combination of particle striking and powder density takes place at this concentration. This enlarges and widens the spark gap size. However, with further increase in concentration, MRR lowers down. This may be due to short circuiting at higher powder density, which leads to arching. Arching causes instability and inefficiency in EDM process. This lowers down the material removal rate at high concentration. Grain size MRR is minimum by suspending fine grain size aluminium powder and maximum by suspending medium grain size aluminium powder in EDM oil at Analysis of TWR equal concentrations (Figure 2). It is due to the reason that the density of suspended fine particles is very much higher than the density of suspended medium particles. Due to higher density bridging effect occurs which leads short circuiting and hence reduction in MRR. It is assumed that density of medium particle is optimum to yield maximum MRR. On the other hand, MRR obtained by suspending coarse grain size aluminium powder in EDM oil is less than MRR yielded by pure EDM oil. It is because a little density of coarse grain size powder bridges the gap between electrodes. This leads to short circuiting and hence lowers MRR. Figure 3: Graph of TWR (g/min) versus powder concentration (g/lt)

Figure 4: Graph of TWR (g/min) versus grain size (µm) Concentration An ideal EDM tool electrode erodes maximum material from work piece and resists self erosion. The fast wear of tool is not a desirable feature. Electrical erosion resistance of the tool electrode is governed by various thermo physical and mechanical characteristics of tool material. Tool wear process is quite similar to material removal mechanism. During sparking, dielectric fluid precipitates carbon as a layer on tool surface. This layer affects the TWR. Electrode erodes at much higher rate in pure dielectric fluid (Figure 3). 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. This causes faster erosion from tool electrode. TWR decreases by adding 3g/l concentration aluminium powder in the dielectric fluid. The reason for this is that the powder particles come in the path of ions moving towards electrode surface. It reduces the momentum of striking ions with electrode surface. The ions with low energy strike the electrode and hence erode less material from electrode. TWR increases slightly by adding more powder in dielectric fluid up to 6g/l concentration. This concentration of powder may be the best for the ions to carry the powder particles along with them towards electrode surface. The powder particles at this concentration may hit the carbon layer more vigorously and erodes the tool surface at rapid rate. TWR starts decreasing at further higher concentrations. It may be due to the reason that much higher concentration of powder particles blocked the path of ions to hit the electrode surface. Grain Size From the trend of variation of cure in figure 4, it is observed that tool electrode

erodes at higher rate in pure dielectric fluid (EDM oil). 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. This causes faster erosion from tool electrode. It is further observed that keeping the same concentration and increasing the grain size of powder particles in dielectric fluid leads to increase TWR. It is due to the reason that fine powder particles have less mass and no sharp edges. They, when strikes to tool electrode with ions erodes less material from electrode surface. On the other hand coarse powder particles have sharp edges and more mass. They hit the tool electrode more vigorously with high momentum and striking energy. Hence more erosion of material from tool electrode takes place. Analysis of % WR Figure 5: Graph of %age WR versus powder concentration (g/lt) Figure 6: Graph of %WR versus grain size (µm) of powder Concentration The ratio of tool wear rate to material removal rate is termed as wear ratio. Wear ratio is very much for the plain dielectric fluid (Figure 5). It is due to the reason that tool wears at very fast rate and MRR is low for plain dielectric fluid. Percentage wear ratio decreases continuously by suspending aluminium powder in dielectric fluid up to a concentration of 3g/l. It is due to the reason that TWR decreases continuously up to this concentration. Increasing further concentration up to 9g/l leads to increase in %WR. This is because TWR increases at higher rate as compare to MRR up to this concentration. However, adding little more powder in EDM oil lowers down %WR. This is because of slow rate of

degradation of tool electrode at high concentration. Grain Size: The variation of curve in Figure 6 depicts that wear ratio is very much for the plain dielectric fluid. It is due to the reason that tool wears at very fast rate and MRR is low for plain dielectric fluid. It is observed from the figure that wear ratio lower down by suspending fine grain size powder. This is because of lower tool wear rate when fine grain size powder is suspended in dielectric fluid. %WR continuously increases with increase in grain size of powder at same concentration because of high TWR and low MRR yielded by coarse powder particles. Analysis of SR Figure 7: Graph of SR (µm) versus powder concentration (g/lt) Figure 8: Graph of SR (µm) versus grain size (µm) Concentration The curve of Figure 7 indicates that surface roughness lowers down by suspending aluminium powder in dielectric fluid. The slope of the curve indicates the rate of decrease of surface roughness. This improvement in surface quality is due to the reason that added powder particles enlarges and widens the discharge passage which facilitates easy evacuation of produced debris from the spark gap. The powder particles lead to uniform dispersion of discharge energy in all directions. This results in shallow and small craters on the machining surfaces. Due to this, surface roughness reduces. This decrease in surface roughness is up to a concentration of 9g/lt. However, with further increase in concentration, the surface roughness increases. It is due to

the reason that too much powder particles in spark gap hinders the discharge passage. Due to this, short circuiting occurs between the tool electrode and work piece. This makes the process instable. Black layers of carbon starts depositing on facing surfaces of both electrodes. As a result dull surfaces produced. Grain Size: Surface produced without suspension of powder in dielectric fluid have large roughness value. The variation of curve in Figure 8 indicates that more rough surfaces of Hastelloy steel produced by suspending fine grain size aluminium powder in dielectric fluid. It is due to the reason that too low a size of powder particles means highly dense powder in dielectric fluid. These particles come into the spark gap clogs the discharge passage. Due to this, short circuiting occurs between the tool electrode and work piece. This makes the process instable. Black layers of carbon starts depositing on facing surfaces of both electrodes. As a result dull surfaces produced. Keeping same concentration and increasing the grain size of powder particles improves surface quality. Coarse powder particles produce best quality surface of Hastelloy steel. This is because increasing grain size at same concentration has lesser density of powder particles in dielectric fluid. This density could be the best at which powder particles easily enlarges and widens 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. This results in shallow and small craters on the machining surfaces. Due to this, surface roughness reduces. CONCLUSIONS Powder mixing into the dielectric fluid is one of the recent developments in EDM that ensures better machining rates at desired surface quality. The result of the present study depicts the process performance with the variation of selected process parameters. Within the range of parameters selected for the present work, the following conclusions are drawn: 1. Aluminium powder suspended in the dielectric fluid affected MRR, TWR, %WR, SR. 2. Too low and too high concentration of aluminium powder in EDM oil reduces MRR of Hastelloy. 3. Too low and too high grain size of aluminium powder in EDM oil reduces MRR of Hastelloy.

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