1. INTRODUCTION:

EDM is having unique feature of using thermal energy to machine electrically conductive parts regardless of hardness of wok piece. Low machining efficiency and poor surface finish restrict its further application.To overcome these problems, one relatively new technique used to improve the efficiency and surface finish is EDM in the presence of powder in the dielectric fluid. This new hybrid material removal process is named as powder mixed EDM (PMEDM)[1].

In the dielectric fluid powder is mixed either in the same tank or in a separate tank. As soon as a voltage of 80-300 V is applied to both the electrodes, an electric field in the range 10² to 10⁴ KV/m is created. The spark gap is filled up with additive particles, and the gap distance between tool and the work piece increases from 25 μm to 50 μm to many times larger. The powder particles get energized and behave in a zigzag fashion. The grains come close to each other under the sparking area and gather in clusters. Under the influence of electric forces, the powder particles arrange themselves in the form of chains at different places under the sparking area (Figure 1). Gap between both the electrodes is bridged due to formation of chain. The gap voltage and insulating strength of the dielectric fluid decreases due to bridging effect. Early explosion in the gap is caused due to easy short circuit. Thus, the ‘series discharge’ starts under the electrode area. Due to the increase in the frequency of discharging, the faster sparking within a discharge takes place, which causes faster erosion from the work piece surface[2].

At the same time, modification happens the plasma channel due to the added powder. The plasma channel gets enlarged. The electric density decreases resulting in uniform distribution of sparking among the powder particles. Which will result, even and more uniform distribution of the discharge, which causes uniform erosion (shallow craters) on the work piece. This results in improvement in surface finish[4].

Figure 1: Principle of PMEDM Process [2]

2. LITERATURE REVIEW:

Kansal et al. studied the effect of Silicon powder mixed into the dielectric fluid on machining characteristics of AISI D2 die Steel. Peak current, concentration of the Silicon powder, pulse-on time, pulse-off time, and gain significantly affect the material removal in PMEDM.Peak current and concentration of Silicon powder are the most influential parameters for causing material removal. The suspension of Silicon powder into the dielectric fluid of EDM appreciably enhances material removal rate as shown in Figure 2[2].

Figure 2: Effect Powder Concentration on MRR [2]

Kansal at al. have studied the process parameters of powder mixed electrical discharge machining (PMEDM). They have found that when Silicon powder was suspended into the dielectric fluid of EDM then MMR and surface roughness increases. The MRR increases with the increase in the concentration of the silicon powder. At higher concentration level of silicon powder improvement in MRR is expected.

Singh et al. have made an attempt to study the effect of Aluminium powder mixed in the dielectric fluid of Electric Discharge Machining on the machining characteristics of Hastelloy. Process input parameters were Concentrations of Aluminium powder and grain size of powder. They have observed that Aluminium powder suspended in the dielectric fluid affected MRR. Too low and too high concentration of Aluminium powder in EDM oil reduces MRR of Hastelloy[4].

Chatha et al. have studied impact of added titanium dioxide in dielectric of edm on surface characteristics and MRR. Work was done with an objective to modify the surface characteristics like surface roughness, material removal rate, and hardness by adding different concentrations of TiO2 into the dielectric fluid of EDM. They have found that MRR increases with the increase of TiO2 powder concentration into the dielectric. This trend is valid up to a certain limit i.e. 7gm/L, further increase of titanium dioxide concentration leads to decrease of MRR [5].

Singh et al. conducted experiments on ASTM A681 D3 die steel work piece using copper electrodes with Al2O3 and TiC mixed EDM oil (dielectric) at different powder concentrations and pulse time settings. With the addition of the powders in the dielectric, material removal rate has been increased and tool wear rate has been reduced. TiC gives better results in terms of Material removal rate and tool wear rate than Al2O3 powder.

3. EXPERIMENTAL METHODOLOGY:

3.1 WORKPIECE MATERIAL:

Inconel-718 has been selected as the workpiece material for experimental work. Inconel-718 is a high strength, temperature resistant (HSTR) nickel -based super alloy. It is extensively used in aerospace applications, such as gas turbines, rocket motors, and spacecraft as well as in nuclear reactors, pumps and tooling. Because of its poor thermal properties, presence of highly abrasive carbide particles, high hardness, high toughness , high work hardening rate and strong tendency to weld to the tool to form build up edge makes Inconel-718 is difficult to machine.

3.2 ELECTRODE MATERIAL:

Electrode for EDM are usually made up of Graphite, Brass, Copper, copper – tungsten alloy, Al alloy and silver-tungsten. Considering wear ratio, Material removal rate, fabrication and cost Copper is widely used electrode.

3.3 DIELECTRIC FLUID:

The most common dielectric fluids are mineral oils. Although kerosene, distilled water and deionised water are used in specialized applications. Kerosene is easily available and is used as dielectric.

3.4 PROCESS PARAMETERS AND THEIR LEVELS:

In this research work, the behavior of three process parameters such as powder media, current, pulse on time and duty cycle are studied. Process parameters and their levels are depicted in Table 1. Kerosene is used as dielectric medium.

Table 1:Process Parameters and their Levels

Process Parameters	Levels
Process Parameters	1	2	3
Powder Media	Sic	Graphite	Al₂O₃
Current, I (Amp)	5	10	15
Pulse on-time, (μ- sec.)	5	10	30
Duty cycle (%)	80	85	90

3.5 DESIGN OF EXPERIMENT (DOE):

The experiments are performed according to design of experiments (DoE). Compared to all the available methods, Taguchi method is one of the most used DoE methods.It is widely recognized in many fields particularly in development of new products and processes in quality control. Experiments are performed with L₂₇orthogonal array (Shown in Table 2) of Taguchi method[3].

Table 2: L₂₇orthogonal array

Dielectric Media	Current (A)	POT (µsec)	Duty Cycle (%)	MRR (gm/min)
Kerosene + SiC	5	5	90	0.0202
Kerosene + SiC	5	10	85	0.0198
Kerosene + SiC	5	30	80	0.0182
Kerosene + SiC	10	5	85	0.0491
Kerosene + SiC	10	10	80	0.0625
Kerosene + SiC	10	30	90	0.0425
Kerosene + SiC	15	5	80	0.0567
Kerosene + SiC	15	10	90	0.0513
Kerosene + SiC	15	30	85	0.0860
Kerosene + Graphite	5	5	85	0.0306
Kerosene + Graphite	5	10	80	0.0261
Kerosene + Graphite	5	30	90	0.0227
Kerosene + Graphite	10	5	80	0.1333
Kerosene + Graphite	10	10	90	0.1156
Kerosene + Graphite	10	30	85	0.0921
Kerosene + Graphite	15	5	90	0.1394
Kerosene + Graphite	15	10	85	0.1362
Kerosene + Graphite	15	30	80	0.0949
Kerosene + Al₂O₃	5	5	80	0.0212
Kerosene + Al₂O₃	5	10	90	0.0194
Kerosene + Al₂O₃	5	30	85	0.0221
Kerosene + Al₂O₃	10	5	90	0.0567
Kerosene + Al₂O₃	10	10	85	0.0611
Kerosene + Al₂O₃	10	30	80	0.0506
Kerosene + Al₂O₃	15	5	85	0.0727
Kerosene + Al₂O₃	15	10	80	0.0671
Kerosene + Al₂O₃	15	30	90	0.0549

3.6 MATERIAL REMOVAL RATE (MRR):

The Material removal rate is calculated with measuring the weight of workpiece before and after machining divided by time required for machining.

Where

Ww1=weight of work piece before machining (gm)

Ww2= weight of work piece after machining (gm)

T= time for machining (minute).

4. EXPERIMENTAL RESULTS AND ANALYSIS:

Work piece plates after performing the final experiments is shown in Figure 3.

Figure 3 : Workpiece after Machining on EDM (Final)

Analysis of material removal rate is performed using MINITAB 16 software. Analysis of variance (ANOVA) for material removal rate (gm/min) is given in Table 3.It shows that current and powder media are the significant parameters for material removal rate.

Table 3: ANOVA for Material Removal Rate (gm/min)

Source	DF	Seq SS	Adj MS	F	P
Powder Media	2	0.01043	0.00521	14.88	0.000
Current (A)	2	0.01983	0.00991	28.30	0.000
POT(µsec)	2	0.00056	0.00028	0.81	0.462
Duty Cycle (%)	2	0.00014	0.00007	0.2	0.820
Error	18	0.00630	0.00035
Total	26	0.03728
S = 0.0187214 R-Sq = 83.08% R-Sq(adj) = 75.56%

The main effect plot shown in Fig. 4 indicates that with increase in current from 5 Amp to 15 Amp, the material removal rate increases.The increase of peak current will increase the pulse discharge energy channel diameter and hence an increase in the crater diameter and depth which in turn can improve the metal removal rate[6]. MRR is higher in case of Graphite powder. This is because current carrying capacity of any material depends on its electric conductivity. Here Graphite is having highest electric conductivity than Al₂O₃ and SiC[7].

Figure 4: Main Effects Plot of SN Ratios for Material Removal Rate (gm/min)

5. CONCLUSION:

· Based on the experiments performed on Inconel-718 material using EDM process it is observed that MRR mostly depends on Current as compared to pulse on time (POT), Powder media and duty cycle.

· Graphite powder added to kerosene gives better material removal rate (MRR) as compared to Silicon carbide and Alumina.

· Higher material removal rate (0.1394 gm/min) is obtained at a high peak current of 15 A, pulse on time of 5 μsec, duty cycle of 90 % and Graphite as powder media

6. REFERENCES:

1. V. Phule and L Khose, 2017, “Comparative Study of Dielectric Fluid in Powder Mixed Electro– Discharge Machining” International Journal for Scientific Research and Development, vol. 5, pp 338-340.

2. H.K. Kansal, Sehijpal Singh and Pradeep Kumar, 2007, Effect of Silicon Powder Mixed EDM on Machining Rate of AISI D2 Die Steel”, Journal of Manufacturing Processes, Vol. 9, pp. 13-22

3. Jagtap and Dabade, 2010, “Effect of Process Parameters during Electrical Discharge Machining of Inconel-718”, Proceeding of National Conference on Recent Advances in Manufacturing (RAM 2010), pp. 139-142.

4. Paramjit Singh, Anil Kumar, Naveen Beri and Vijay Kumar, 2010, “Some Experimental Investigation on Aluminum Powder Mixed EDM on Machining Performance of Hastelloy Steel”, International Journal of Advanced Engineering Technology 1, pp. 28-45.

5. Sukhpal S. Chatha, Rakesh Bhatia, Hazoor S. Sidhu and Buta S. Sidhu,2010, “Impact of Added Titanium Dioxide in Dielectric of EDM on Surface Characteristics and MRR”, National Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering, pp. 115-121.

6. Saurabh Sharma, Anil Kumar and Naveen Beri, 2011, “Study of Tool Wear Rate during Powder Mixed Edm of Hastelloy Steel”, International Journal of Advanced Engineering Technology, vol. 2, pp. 133-139.

7. P. Pecas and E. Henriques, 2003, “Influence of Silicon Powder-Mixed Dielectric on Conventional Electrical Discharge Machining”, International Journal of Machine Tools and Manufacture 43, pp. 1465-1471

Received on 20.11.2018 Accepted on 08.06.2019

Int. J. Tech. 2018; 9(1):08-12.

DOI: 10.5958/2231-3915.2019.00003.8