Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel. N. Annamalai1+, V. Sivaramakrishnan2, N.Baskar3 1+

Department of Mechanical Engineering, Mookambigai College of Engineering, Pudukkottai – 622502, Tamilnadu, India.E-mail: [email protected] 2

Department of Mechanical Engineering, Roever Engineering College,Perambalur – 621212,Tamilnadu, India.E-mail: [email protected]

3

Department of Mechanical Engineering, SaranathanCollege of Engineering, Tiruchirappalli – 620012, Tamilnadu, India. E-mail: [email protected] ABSTRACT

Electric Discharge machining is used to produce complex shapes that would be difficult to produce in conventional machine tools and also good surface finish can be obtained in EDM. The work material EN24 is machined by using copper as electrode. The EN24 contains nickel, chromium and molybdenum and it is used in automobile and aircraft transmission components .For this reason, the EN24 is experimentally investigated with the machining parameters for achieving maximum MRR and minimum electrode wear rate and surface roughness. The RSM is also used to identify the machining parameter responses on MRR, EWR and SR. The input parameters are peak current, pulse on time and pulse off time. The experimental design is done using Box Behnken design of RSM. Regression equations are formulated based on the experimental results. The effects of input parameters are analyzed on MRR, EWR and SR. Keywords: EDM, Response surface methodology, Material removal rate, Regression analysis

1. Introduction Electric Spark erosion is the methodology used to control the metal removal process in Electrical Discharge Machining. Very tiny amount of material is removed from the work piece. It is immersed in dielectric fluid and the pulse generator creates electric spark between the work piece and electrode tool. Heat resistant steels, super alloys, carbides, heat treated tool steels, composites and ceramics which are difficult to machine can be machined to attain geometrically complex shape in EDM for which the process parameters must be optimized. Optimum process parameters are to be followed while machining in EDM to obtain maximum Material Removal Rate (MRR), minimum Electrode Wear Rate (EWR) and Surface Roughness (SR). In the heat treated condition EN24 is capable of developing high strength. Its toughness and retaining good fatigue strength is commendable. Previously few researchers have considered optimizing material removal rate, tool wear and overcut with Taguchi methodology but not surface roughness [7]. So this work considers MRR and surface roughness. MRR is more influenced by duty factor and peak current while machining AISI 4140 grade alloy steel in EDM [1]. Pulse on time, duty cycle, peak

current and concentration of the silicon powder added into the dielectric fluid of EDM are used as process parameters to study the process performance in terms of material removal rate and surface roughness [1].The debris evacuation efficiency and low work piece conductivity posed a challenge of low material removal rate and using design of experiments peak current, duty ratio, gap voltage and pulse duration were studied and analysis of variance was conducted to achieve higher MRR [8]. Matrix nano composite of Al 7075 reinforced with 0.5 in weight%SiC nano particles is machined in electrical discharge machining with copper electrode using facecentered central composite design of response surface methodology and mathematical model was developed for MRR, EWR and SR.The experimental values fitted with a 95% confidence interval [9].The effects of discharge current, pulse ontime, duty factor and open discharge voltage were used to analyze the performance characteristics of material removal rate, electrode wear rate and surface roughness in the EDM process of Al2O3+TiC mixed ceramic[10].

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Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

2. Methodology Box Behnken Design of Response Surface Methodology is used to model the equation. Experimental investigation is done through design of experiments and the parameter influence and interaction effect on peak current, pulse on time and pulse off time are examined.

duty cycle, gap control potentiometer and sensitivity pot to vary the speed of Z axis. The die electric used is clean kerosene. Through pressure flushing kerosene under pressure of 0.80 kg/cm2 is admitted in the vicinity of the spark area and the debris is carried away. The response surface methodology is analyzed to maximize MRR and to minimize SR.

2.4 Measurement Procedure 2.1 Work Piece and Electrode Material EN 24 is the work material which is a low alloy medium carbon steel used for large size parts which requires high strength and toughness. The electrode material is copper.

2.2 Characteristics This nickel-chromium-molybdenum alloy possesses increased ductility and toughness and much deeper harden ability. EN 24 is ideal for all highly stressed parts in the most severe conditions because of its high fatigue strength. It has good wear resistance and used in both elevated and low temperature environments. Typical applications include aircraft landing gear, power transmission gears and shafts and other structural parts, high strength machine parts, heavy-duty shafting, high tensile bolts and studs, gears, axle shafts, crankshafts, boring bars and down-hole drilling components.

2.3 Experimental Setup Elektra Puls SE35 ZNC Electric Discharge machine manufactured by Electronica Machine Tools is the machine used to carry out this experiment. The input parameters considered for process optimization are Ton, Pulse on time, Ip,Peak current and Toff, Pulse off time. Weight of work piece and electrode is measured using electronic weighing scale before and after machining to measure material removal rate and electrode wear rate. Mathematical models are developed on the basis of experimental data. The table 1 shows the EDM input parameters and their levels. Table 1: Input parameters and their level Param eter Ip Ton Toff

Descriptio n Peak current Pulse on time Pulse off time

Unit

Level 1

Level 2

Level 3

amp

3

6

10

µsec

20

50

100

µsec

150

200

250

An electronic weighing scale is used to measure the weight of work piece before and after trial. The digital timer is used to measure the period of trial in minutes. SURFCORDER, a surface roughness measuring instrument is used to measure the surface roughness Ra in terms of µM.

2.4.1Measurement of MRR (wi − w f x1000 mm 3 MRR = min 7.8 xt

)

Where Wi = weight of work piece in grams before trial Wf= weight of work piece in grams after trial t = period of trial in minutes 7.8 = Density of steel in gms/cc

2.4.2Measurement of Surface Finish The surface finish is measured on a surface test recorder, SURFCORDER of Kosaka Laboratory Ltd,Japan. Stroke length: 4 mm Stylus Speed: 2 mm/sec Cut – off value: 0.8 mm Arithmatic mean of Surface Roughness Ra is recorded in terms of µM.

3. Mathematical Modeling Mathematical models are developed on the basis of experimental data. The experimental planning is done based on Design of Experiments. The Box Behnken design was used to find the quadratic response surfaces to construct the second order polynomial models. Design of Experiments (DOE) is a method used to obtain useful information about a process by conducting only minimum number of experiments. Each controllable variable (Ton, Ip, Toff) can be set on EDM machine at three consecutive levels and hence the design consisting of 17 experiments based on box behnken design was generated as shown in fig-1

The other standard settings are work time potentiometer Tw for the sparking amplitude, antiarc sensitivity pot

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5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India

As per the box behnken design of response surface methodology for 17 runs the various input parameters and the results obtained are shown in table 2. Regression analysis for MRR indicates that the individual and higher order effects of variablessuch as Ip, Toff and higher order effect of tonand Ip have significant contributions in MRR model since these Pvalues are less than 0.05. Regression analysis for Electrode Wear Rate indicates that the individual and higher order effects of variablessuch as Ton,Ip and higher order effect of Tonand Ip and interactive terms of Tonand Ip have significant contributions in Electrode Wear Rate model since these P-values are less than 0.05. Regression analysis for Surface Roughness indicates that the individual and higher order effects of variablessuch as Ton,Ip and higher order effect of Tonand Ip and interactive terms of Tonand Ip have significant contributions in Surface Roughness model since these P-values are less than 0.05. Figure -1 Work piece after experiment Theequationforcalculatingthe approximateMRRis

MRR = −13.05567 + 0.032718x T on + 2.84774x I p + 0.065230xT off + 2.91488E − 003x T on x I p 2

2

− 4.70741E − 005xT on x T off − 1.76239E − 003x I p x T off − 2.98260E − 004 x T on − 0.15895x I p 2

− 1.54043E − 004 x T off Theequation for calculating the approximate Electrode Wear Rate is

EWR = −3.02708- 6.26448E- 003xT on + 0.82753x I p + 0.018249xT off + 1.38117E − 003xT on x I p 2

2

+ 4.09994E − 005xT on xT off + 1.67736E − 004x I p xT off − 5.12159E − 005xT on − 0.054038x I p 2

− 6.54016E − 005xT off The equation for calculating the approximate Surface Roughness is

SR = −5.18238 + 0.027995x T on + 1.86190 x I p + 9.81413E − 003x T off + 4.63971E − 003x T on x I p 2

2

+ 2.48665E − 005 x T on x T off + 3.91892E − 004 x I p x T off − 3.40737 E − 004 x T on − 0.12854 x I p 2

− 2.53903E − 005x T off

4.1.1Effect of Peak Current on MRR : 4. Results and Discussions Pulse on time,Peak current and Pulse off time are the three parameters which are used as conrolling parameters.Its effect of Material removal rate,Electrode wear rate and Surface roughness are discussed.

The fig-2 shown below indicates that MRR is 0.323 mm3/min when the peak curent is 3 amp.It increases to 4.931 mm3/min when the peak current is 6 amps.It reaches 5.222 mm3/min when the peak current is increased to 10 amps.The pulse off time is kept constant as 200 µsec.

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Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

Figure -2 Effect of Peak Current on MRR

4.1.2 Effect of Pulse on time on MRR: The MRR increases from 0.2585 to 5.2222 mm3/min when the pulse on time increases from 20 µsec to 100 µsec. Fig-3 shown below indicates the change in MRR when the peak current is kept constant at 6.0 amp.

Figure -3 Effect of Pulse on Timeon MRR

4.1.3Effect of Pulse off time on MRR: There is no much changes on MRR when the pulse off time increases from 150 µsec to 250 µsec. Fig4shown below indicates the change in MRR when the pulse on time is kept constant at 60 µsec.

Table 2: Plan of Experiments and Output Responses Standard order 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Run 6 5 11 16 2 3 15 1 13 8 9 17 10 14 12 4 7

PULSE ON TIME (µsec) 20 100 20 100 20 100 20 100 50 50 50 50 50 50 50 50 50

PEAK CURRENT (amp) 3 3 10 10 6 6 6 6 3 10 3 10 6 6 6 6 6

PULSE OFF TIME (µsec) 200 200 200 200 150 150 250 250 150 150 250 250 200 200 200 200 200

MRR (mm3/min) 0.322996 0.311954 3.619058 5.222205 4.105849 4.342257 2.952335 2.957442 0.329195 5.133338 0.258504 3.74595 4.203447 4.930966 4.060337 4.102564 4.56246

EWR (mm3/min) 0.176922 0.136699 1.585879 2.288382 1.799192 1.902787 0.64686 1.295958 0.180318 1.68708 0.141596 1.641484 1.84196 1.08038 1.334436 1.797753 1.99928

SR (µM) 1.49 1.775 3.604 6.678 3.529 5.159 3.962 5.718 1.675 5.35 1.716 5.707 4.771 4.792 4.845 5.098 5.331

The fig-5 shown below indicates that EWR is 0.1769 mm3/min when the peak curent is 3 amp.It increases to 1.798 mm3/min when the peak current is 6 amps and reaches 2.288 mm3/min when the peak current is increased to 10 amps .The Pulse Off Time is kept constant as 200 µsec.

Figure -4 Effect of Pulse Off Time on MRR

4.2.1 Effect of Peak Current on EWR 150-4

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India

indicates the change in Surface Roughness while keeping pulse off time in 200 µsec as constant.

Figure -5 Effect of Peak Current on EWR

4.2.2 Effect of Pulse on time on EWR The EWR increases from 0.1367 to 2.2884 mm3/min when the pulse on time increases from 20 µsec to 100 µsec.The fig 6 shown below indicates the change in EWR when the peak current is kept constant at 6.0 amp.

Figure -8 Effect of Peak Current on Surface Roughness

4.3.2 Effect of Pulse on time on SR The surface roughness increases from 1.49 µM to 6.678 µM when the pulse on time increases from 20 µsec to 100 µsec.The fig 9 shown below indicates the change in surface roughness while keeping peak current as constant in 6.0 amp.

Figure -6 Effect of Pulse on Time on EWR

4.2.3 Effects on Pulse off time on EWR: There is no much changes on EWR when the pulse off time increases from 150 µsec to 250 µsec.The fig-7 shown below indicates the change in EWR when the pulse on time is kept constant at 60 µsec.

Figure -9 Effect of Pulse on Time on Surface Roughness

4.3.3Effect of Pulse off timeon SR The fig 10 shown below indicates that there is no impact or change on surface roughness when the pulse off time increases from 150 µsec to 250 µsec .

Figure -7 Effect of Pulse off Time on EWR

4.3.1 Effect ofPeak Current on SR The surface roughness is 1.49 µM when the peak current is 3 amps, increases to 5.098 µM when the peak current is 6 amps and finally reaches 6.678 µM when the peak current is 10 amps.The fig 8 below

Figure -10 Effect of Pulse off Time on Surface Roughness

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Response Surface Modeling of Electric Discharge Machining Process Parameters for EN 24 Low Alloy Steel.

5. Conclusions The following conclusions are derived based on the results and discussions done on machining EN24 in EDM . 1. The effect on MRR improves when peak current and pulse on time is increased whereas there is no much impact when pulse off time is increased.

2.

3.

The EWR increases when peak current and pulse on time is increasedwhereas there is no much change when pulse off time is increased. The surface roughness increases when peak current and pulse on time is increased whereas there is no much impact when pulse off time is increased.

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