THE EXPERIMENTAL ANALYSIS OF SURFACE …THE EXPERIMENTAL ANALYSIS OF SURFACE CHARACTERISTICS OF INCONEL-718 USING ELECTRICAL DISCHARGE MACHINING ... current, pulse on time, duty factor

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Int. J. Mech. Eng. & Rob. Res. 2012 D Sudhakara et al., 2012

THE EXPERIMENTAL ANALYSIS OF SURFACECHARACTERISTICS OF INCONEL-718 USING

ELECTRICAL DISCHARGE MACHINING

D Sudhakara1*, B Venkataramana Naik1 and B Sreenivasulu2

*Corresponding Author: D Sudhakara, [email protected]

Electric Discharge Machining (EDM) provides an effective Manufacturing technique that enablesthe production of parts made of hard materials with complicated geometry that are difficult toproduce by conventional machining processes. Its ability to control the process parameters toachieve the required dimensional accuracy and surface finish has placed this machining operationin a prominent position in industrial applications. This project reports on the experimentalinvestigation of machining of Inconel-718 using EDM process. The parameters such as peakcurrent, pulse on time, duty factor were chosen to study the machining characteristics. Anelectrolytic rectangular copper block of 12 8 mm was selected as a tool electrode. The outputresponse was measured were Material remove rate, Avg. Surface Roughness, Hardness. Theresults are revealed that how material removal rate, surface roughness and hardness areinfluenced by peek current, duty factor and pulse-on time. The surface crack lengths are alsoidentified.

Keywords: Electric discharge machining, INCONEL-718, MRR, Surface roughness, Hardness

INTRODUCTIONElectric Dischare Machining (EDM)

Electric Discharge Machining (EDM) is oneof the most extensively used nonconventionalmaterial removal processes. Its unique featureof using thermal energy to machine electricallyconductive parts regardless of hardness has

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Int. J. Mech. Eng. & Rob. Res. 2012

1 Department of Mechanical Engineering, Vignana Bharathi Institute of Technology, Proddatur 516361, YSR Kadapa (Dist.), AndhraPradesh, India.

2 Department of Mechanical Engineering, Madanapalle Institute of Technology & Science, Madanapalle 517325, Chittor (Dist.),Andhra Pradesh, India.

been its distinctive advantages in the

manufacture of mould, die, automotive,

aerospace and surgical components. In

addition, EDM does not make direct contact

between electrode and the work piece

eliminating the mechanical stresses, chatters

and vibration problems during machining.

Research Paper

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Today, an electrode as small as 0.1 mm canbe used to drill holes into curved surfaces atthe steep angles without drill wander (Kalpajianand Schmid, 2003).

The basis of EDM can be traced as for backas 1770, when English Chemist JosephPriestly discovered the erosive effect ofelectrical discharges or sparks (Webzell,2001). However, it was only in 1943 at theMoscow University where LazarenKo andLazarenko (Anonymous, 1965) exploited onthe destructive properties of electricaldischarges for constructive use. Theydeveloped a controlled process of machiningdifficult to machine metals by vaporizingmaterial from the surface of the metal. TheLazarenko EDM system used resistancecapacitance type of power supply, which waswidely used at the EDM machine in the 1950’sand later served as model for the successivedevelopment in EDM (Livshits, 1960).

There have been similar claims made atabout the same time when three Americanemployees came up with the notion of usingelectrical charges to remove broken taps anddrills from hydraulic valves. Their work becomethe basis for the vacuum tube EDM machineand an electronic circuit servo system thatautomatically provided the proper electrode towork piece spacing (spark gap) for sparking,without the electrode contacting the workpiece (Jameson, 2001). It was only in 1980’swith the advent of Computer Numerical Control(CNC) in EDM that brought about tremendousadvances in improving the efficiency of themachining operation. CNC has facilitated totalEDM, which implied an automatic andunattended machining from inserting theelectrodes in the tool changer to a finished

polished cavity or cavities (Houman, 1983).These growing merits of EDM have since thenbeen intensively sought by the manufacturingindustries yielding enormous economicbenefits and generating keen researchinterests.

The widely accepted principle of theprocess based on thermal conduction ispresented as a process over-view togetherwith the applications (Figure 1). The core ofthe paper identifies the major EDM academicresearch area with the headings of EDMperformance measures, EDM operatingparameters along with electrode design andmanufacture. The final part of the paperdiscusses these topics and suggests futuredirection for the EDM research.

Figure 1: EDM Principle

Tool

Work

EDM Process

The material erosion mechanism primarilymakes use of electrical energy and turns itinto thermal energy through series discreteelectrical discharges occurring between theelectrode and work piece immersed in adielectric fluid (Figure 2) (Tsai et al., 2003).The thermal energy generates a channel ofplasma between the cathode and anode

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(Shobert, 1983) at a room temperature in therange of 8000° to 12000° centigrade(Boothroyd, 1989) or as high as 20000°centigrade (McGeough, 1988) initialing asubstantial amount of heating and melting ofmaterial at the surface of each pole. Whenthe pulsating the direct current supplyoccurring at the rate of approximately 20,000-30,000 Hz (Krar and Check, 1997) is turnedoff, the plasma channel breaks down. Thiscauses a sudden reduction in the temperatureallowing the circulating dielectric fluid toimplore the plasma channel and flush themolten material from the pole surfaces in theform of microscopic derbies.

This process of melting and evaporatingmaterial from the work piece surface is incomplete contrast to the conventionalmachining processes, as chips are notmeasured mechanically produced. The volumeof material removed per discharge is typicallyin the range of 10–6 to 10–4 mm3 and theMaterial Removal Rate (MRR) is usuallyshaped electrode defines the area in which

the spark erosion will occur, the accuracy ofthe part produced after EDM is fairly high. Afterall, EDM is a reproductive shaping process inwhich the form of the electrode is mirrored inthe work piece (Konig et al., 1988).

Most of the EDM operations conducted withelectrodes (work and tool) immersed in a liquiddielectric, for example paraffin, and themechanism of sparking is similar to thatdescribed above except that the dielectric iscontaminated with conductive particles.Furthermore, the particles, removed from theelectrodes due to the discharge fall in theliquid, cool down and contaminate the areaaround the electrodes by forming colloidalsuspension of metal. These suspensions,along with the product of decomposition of theliquid dielectric are drawn into the spacebetween the electrodes during the initial partof the discharge process and are distributedalong the electric lines of force, thus formingcurrent ‘bridges’, Discharge then occurs onlyone of these bridges as a result of ionization,described earlier.

Figure 2: Working Diagram of EDM and its Components

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LITERATURE SURVEYIt is essential to understand the past andpresent status of the EDM process to suggestfuture areas of work. Extensive literaturesurvey has been carried out to find the state ofart at EDM process. Electric DischargeMachining (EDM) provides an effectivemanufacturing technique that enables theproduction of parts made of hard materialswith complicated geometry that are difficult toproduce by conventional machiningprocesses. Its ability to control the processparameters to achieve the requireddimensional accuracy and surface finish hasplaced this machining operation in anprominent position in industrial applications.

EDM can be described as a process foreroding and removing material by transientaction of electric sparks on electricallyconductive materials one being the workpieces the other being the electrode immersedin a dielectric liquid and separated by a smallgap. The main mode of erosion is caused bythe local thermal effect of an electric discharge.The charge induced between electrodes by apower supply creates a strong local electricfield. This field is strongest where theelectrodes are closest to each other.Molecules and ions of dielectric fluid arepolarized and orientation between these twopeaks. When the dielectric strength of the liquidin the gap exceeds a natural limit, a lowresistance discharge channel is formed dueto electron avalanche striking to anode andcathode. This collision process transforms andreleases the kinetic energy of electrons andions in the form of heat and pressure in thesolid body. The amount of generated heatwithin the discharge channel is predicted to

be as high as 1017 W/m2 and thus, could riseelectrode temperatures locally up to 20000°K even for short pulse durations (Kalpajian andSchmid, 2003). No machining process isknown to exist for which similar hightemperatures can be obtained even in suchsmall dimensions. The pressure increases inthe plasma channel forces the dischargechannel boundaries to expand and decreasethe current density across the inter-electrodegap. Most of the time, the pressure increaseis so high that it prevents evaporation of superheated material on the electrode surfaces.Appling consecutive spark discharges anddriving one electrode towards the other erodesthe work piece gradually in a formcomplementary to that of the tool electrode.The violent nature of the process leads a uniquestructure on the surfaces of the machinedparts. The microscopic observations haveshown that unusual phase changes occur sincehigh temperatures are attained during themachining process. The top most layers are arecast layer formed by resolidification of themolten metal at the base of the craters afterthe discharge.

This layer is found to be heavily alloyed withthe paralysis products of the cracked dielectricwhen pure iron and ferrous alloys are used aswork piece materials; the recast surface layeris often saturated with carbon from the crackeddielectric, as well as other alloying elementsintroduced via the tool electrode. The materialsurface is found to be fairly resistant to etchingby conventional metallographic reagents. Forthis reason, the recast layer on the ferrousalloys is often referred as unetchable whitelayer. Micro hardness have shown that forferrous alloys, the recast layer generally has a

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hardness value much higher than that of theunderlying matrix and may exceed thatattainable by normal quenching techniques(Livshits, 1960; Anonymous, 1965; Houman,1983; Shobert, 1983; Boothroyd, 1989;Jameson, 2001; Webzell, 2001; and Tsaiet al., 2003). Weng and Her (2002) carriedout several successful experiments involvingan electrode of 50 microns diameter and amulti electrode for the batch production microparts. The proposed method is reduces theproduction time and costs of fabricating boththe electrodes and parts. The recent trend inreducing the size of products has given micro-shafts as 5 microns in diameter but alsocomplex 3D micro cavities (Rajukar and Yu,2000).

Konig et al. (1988), McGeough (1988) andKrar and Check (1997) also made severalattempts to producing micro parts such asmicro pins, nozzles, cavities using micro-EDM.In addition a study of applying micro EDM asan alternative method for producing photo-masks used in IC industry has been conducted.The application of CNC to EDM has helped toexplore the possibility of using alternative typesof tooling to improve the MRR. The EDMcommonly employs 3D profile electrodes,which are costly and time consuming tomanufacture for the sparking process.However, experimental work has beenperformed with a frame electrode generatinglinear and circular swept surfaces by meansof controlling the electrode axial motion (Leeand Lau, 1991). These techniques eliminatethe need to utilize the 3D electrode to performthe roughing operation by replacing the simpleelectrode to remove unwanted material in ancomplete block improving the machiningefficiency and MRR.

Lok and Lee (1997) are worked on the twoadvanced Ceramics with EDM, the two typesof ceramics are sialon and Al

2O

3-TiC. They find

out the machining performance in terms ofmaterial removal rate and surface finish underdifferent cutting conditions. The removal ratesof these ceramics were very low and rangingfrom 4.20 to 6.00 mm3/min for that sailon 501and 2.49 to 7.53 mm3/min for that of SG-4.When similar currents setting were used tomachine an alloy steel grade SKD-11, aremoval of 28 to 120 mm3/min was achieved.As far as the sailon 501 ceramics isconcerned, the current setting have little effecton the metal removal rate, as the differencefrom maximum rate and minimum rate is only1.8 mm3/min, which is negligible amount (Lokand Lee, 1997). Rozenk and Kozak (2001) areworked on the metal metal composites andinvestigate the effect of machining parameterscurrent, pulse-on-time, pulse off time on themachine feed rate and surface finish with wireEDM. They conclude that the cutting feed rateand surface roughness for AlSi

7Mg/20% Al

2O

3

composite are increased with increasedischarge current. With increase in pulse ontime the feed rate and surface rough ness isalso increases (Rozenk and Kozak, 2001).Hocheng et al. (1997) are worked on the MetalMatrix Composite (MMC) SiC/Al with EDM.These materials have increasingly widenedtheir use due to the merits of processing ofhigh specific strength and modulus of elasticitywhile carrying good deformability andconductivity comparison to other metals. Thefundamental analysis starts from the metalremoval of MMC by single spark current. Theycorrelates between the major machiningparameters current, on-time, crater sizeproduced by the single spark. The metal

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removal rate is proportional to the appliedcurrent, and on-time. For effective EDM ofSiC/Al large current and short on-time arerequired (Hocheng et al., 1997).

The development of different moderncomposite materials in the list dacade hasled to an expansion of EDM applications. Yanet al. (2000) surveyed the various machiningprocesses performed on the Metal MatrixComposites (MMC) and experimented withmachining Al

2O

3/6061Al composite using

rotary EDM coupled with a disk like electrode.The feasibility of machining of ceramic-metalcomposite steel plate coated with WC-COusing plasma spraying was also examined(Mamalis et al., 1992). Muller and Monaghan(2000) compared the m of particle EDMreinforced MMC with their non-conventionalmachining processes such as laser beammachining and abrasive jet machining. It wasfound that EDM is suitable for machiningparticle reinforced metal matrix compositeswith a relatively small amount of sub-surfacedamage but MRR was very less. O A Abu Zeidworked on the AISI TI High Speed Steel withElectrical Discharge machine with atransistorized pulse generator to study theeffect of voltage pulse off time, at constantvalues of pulse-on–time. A copper electrodewith a positive polarity, kerosene dielectric,side flushing and current setting of 31.2 Ampwere used through out experiment. Heconclude that the metal removal rate is notvery much sensitive to off-time changes at alower pulse-on times corresponding to surfacefinish and show a higher sensitivity to thesechanges at higher on times corresponding torough machining. It is recommended to use ofshort off times when finish machining AISI TI

High speed steels, as this can result inhigher metal removal rate, less electrodewear and good surface finish. On the otherhand, long pulseoff -t ime values arerecommended that when rough machiningto secure same effect.

Kuppan et al. (2008) worked on the Inconel718 by making deep hole drilling with EDM.The parameters peak current, pulse-on-time,duty factor and electrode speed were chosento study behavior. The out put responses weremetal removal rate, depth of average surfaceroughness. The experimented were plannedusing central composite design. The resultsrevealed that metal removal rate is moreinfluenced by peak current, duty factor andelectrode rotation, and MRR is increased withincrease in current and duty factor andelectrode speed, where as depth of averagesurface roughness is increased with increasein peak current , electrode speed and pulseon time (Kuppan et al., 2008).

Electrode Design and Manufacture

The design and manufacture of an electrodehas progressed along with the technologicaladvancement made in the various computer-aided systems. A CAD system is capable ofcreating the electrode and holder designs fromthe work piece 3D geometry and identifyingany undesirable s harp corners on the designs,which are difficult to produce, by measuringthe surface angle along the edges (Soni andChakravarthi, 1996). The recent developmentin CAD/CAM systems and communicationscontrols has also provided a throughintegration towards the design andmanufacture electrodes by selecting essentialmachining parameters prior to the machiningoperation (Sato et al., 1986). A Computer-

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Aided Process Planning (CAPP) system forelectrode design has also been built usingfuture based work Piece decryption (Soni,1994). In view of growing concern for greenmanufacturing (Soni and Chakravarthi, 1994)developed an environmentally friendly processplanning system using a multi-objectiveanalysis of the EDM process. The systemtakes both the environment impact, such asprocess energy and waste, and traditionalmanufacturing measures, such as productionrates and quality, into account when performingthe process planning.

In this way a lot of research is going on thedifferent materials for machining metals, alloys,composites, super alloys for high MRR, goodsurface Finish. But in case of Inconel 718 veryless research is done. And Inconel-718 is aHigh Strength, Temperature Resistant (HSTR)Nickel based super alloy. It is extensively usedin aerospace applications such as gasturbines, rocket motors, spacecrafts, pumpsand tooling. Inconel-718 is difficult to machine,because of its poor thermal properties, high

toughness, high hardness, high workhardening rate, presence of highly abrasivecarbide particles and strong tendency to weldto the tool to form build up edge. Because ofthis wide area of applications in various fields,it is better to know the behavioral propertiesof Inconel-718 with EDM.

Selection of Work Material

Inconel 718 is a High Strength, TemperatureResistant (HSTR) Nickel based super alloy.It is extensively used in aerospaceapplications such as gas turbines, rocketmotors, spacecrafts, pumps and tooling.Inconel-718 is difficult to machine, becauseof its poor thermal properties, high toughness,high hardness, high work hardening rate,presence of highly abrasive carbide particlesand strong tendency to weld to the tool to formbuild up edge (Soni and Chakravarthi, 1994).Because of this wide area of applications invarious fields, it is better to know thebehavioral properties of Inconel-718 withEDM (Table 1).

Ni Mo Ti C Si Cu Cr Nb(+Ta) Al Mn Co B

50-55 2.8-3.3 0.65-1.15 0.08 0.35 0.3 17-21 4.75-5.5 0.2-0.8 0.35 1 0.006

Table 1: Chemical Composition of Inconel-718

Preration of Samples

These blocks were cut from the ingots. They

were cut in to 27 pieces by tool and turret

machine with dimensions of (20 8 1) mm.

The experiments were conducted on the

material with copper electrode.

Selection of Tool Material

The choice of the electrode depends upon the

performance criteria required (MRR, surface

roughness machining stability) and also upon

the electrode manufacturing constraints. A goodelectrical conductor will be selected first in orderto create the discharges. This material musthave high melting point and vaporizingtemperature as well as high thermal diffusivityto ensure the geometrical stability of theelectrode. There are 3 types materials areavailable in market with following compositions.

Toll Material

It has been experienced that certain materialsare more suitable than others depending upon

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the materials to be machined and the type ofgenerator used. The characteristic of toolmaterial should be such that the wear ratio,i.e., ratio of wear rate/metal removal rate is asmuch less as unity and its hardness does notallow any deformation of the toll during themachining process since in that case themachined surface shape will be damaged. Itis interesting to note that the wear ratio forbrass work is 0.5, for hardened plain carbonsteel work 1.0 and for tungsten carbide workis 3.0. Wear ratio has been reduced to 0.1 byusing graphite anode with a pulse generatormachine.

The performance of electrode materialscan be gauged by its material removal rate,low wear, and ability to be accuratelymachined or formed. The tool electrode forEDM constitutes the most important part andaccounts for major cost. Commercially EDMtool electrodes are made of any of thefollowing three categories of materials, viz.,metallic (electrolytic copper, tellurium orchromium copper, copper tungsten brass,tungsten carbide, aluminum, etc.), non-metallic (graphite), and combination ofmetallic and non-metallic (copper graphite).

Copper

This is the first choice as EDM tool electrode.It can be produced by casting or machining.Copper electrodes with very complex featuresare formed by chemical etching orelectroforming. Copper was the best toolmaterial used for EDM machine (Figures 3 and4) because of the following properties:

Good Electrical conductivity, high meltingand vaporizing temperatures and Lowcoefficient of thermal expansion

EXPERMENTAL DETAILSA number of experiments were conducted tostudy the effects of the various machiningparameters on EDM process. These studieshave been under taken to investigate theeffects of current, pulse on time and duty factoron the MRR, Surface roughness, hardness andcrack propagation. The Inconel-718 metal ismachined with the copper tool. Kerosene isthe dielectric medium.

Expermental Setup

Machine Specifications

The experiments were conducted on GRACEV6040 Die Sinking Electro Discharge

Figure 3: Copper Electrode (8 12) mm

Figure 4: Work Piece and Tool

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Machine. The specifications are Single- door,large work tank, Large table traverse, Work tank:800 600 400, X Y Z: 600 300 300,Maximum electrode weight: 100 Kg, MaximumJob weight: 350 Kg and Filter: Paper Filter.

Maximum Current: 50 amps the purpose ofdielectric section is to store dielectric, to feedit to the machine in adequate quantity and toremove dirt from the dielectric. MAHATHOLTRANSOL EDM Oil is formulated from highparaffin base stocks having high ViscosityIndex. This Oil undergoes repeated treatmentand filtrations and as a result, the EDM Oil ishaving high degree of clarity and is totallyodorless. Because of the low viscosity of thisEDM Oil, settlement of dust, metal particlesand carbon becomes easy.

Di-Electric Selection

Mahathol Transol EDM Oil has high fluiditywhich enables speedy settlement and is userfriendly because of its odorless nature, ascompared to other petroleum mineral Oils.Usage of this Oil enhances the filter life andalso improves the finish and efficiency of theelectrodes.

The following Equipments are used for themeasurement of Surface Roughness, Imageanalysis and hardness (Figure 5).

RESULTS AND DISCUSSIONIn the present work the influence of current,pulse on-time, duty factor on the performanceindices of EDM such as MRR, SurfaceRoughness, and Hardness is evaluated atdifferent experimental conditions. In thischapter we discuss and analyze theexperimental data obtained (Table 2). Relationbetween input variables and performancesmeasures are depicted in various figures.

Figure 5: Electro Discharge Machine

1. 4 50 50.00 0.3707 4.2620

2. 5 20 66.66 0.4061 4.2730

3. 6 100 33.33 0.5698 4.3845

4. 4 50 50.00 1.4245 5.4250

5. 5 20 66.66 1.6687 5.7056

6. 6 100 33.33 2.0757 5.9250

7. 4 50 50.00 0.3256 3.9725

8. 5 20 66.66 0.7326 4.1765

9. 6 100 33.33 1.1803 4.2565

Table 2: Variation of MRR at ConstantPulse-On Time and Duty Factor

Exp.No

Current(Amps)

Ton(µs)

MRRmm3/min

SR(µm)

DutyFactor

(%)

Figure 6 shows the relationship betweenmetal removal rate (mm3/min) and current.From the figure, we can observe that, whenthe current is increase at constant pulse on timeand Duty factor, the metal removal rate isincreases. When the current is increased from4A to 6A the metal removal rate is increasedwith current. This means that, when current arehigher, melting starts earlier, i.e., low machininginitiation time. Higher It can be attributed thatmetal removal rate is proportional to theproduct of energy and pulse frequency.Increasing the pulse current at a constantfrequency increases the energy of the pulse

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and ultimately higher metal removal rate (Soniand Chakravarthi, 1994).

The Figure 7 shows the relationshipbetween Surface Roughness and current.From the figure we can observe that, thesurface roughness is increased with increasein current. When the current is increased, themetal removal rate is also increased. As

normal when the metal removal rate increasesautomatically the surface roughness isincreased. When the discharge current is high,the spark intensity and discharge power ismore, subsequently causing a large craterdepth on the surface of the work piece, whichresulted in high surface roughness value (Soniand Chakravarthi, 1994).

Figure 6: Variation of MRR with Current

Figure 7: Variation of Surface Roughness with Current

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Figure 8: Variation of Hardness with Current

1. 4 50 33.33 0.6250 4.225

2. 4 50 50.00 0.8750 4.260

3. 4 50 66.66 1.2405 5.700

4. 5 20 33.33 0.9703 5.225

5. 5 20 50.00 1.3227 5.450

6. 5 20 66.66 1.6687 5.700

7. 6 100 33.33 1.1803 5.742

8. 6 100 50.00 2.6548 5.860

9. 6 100 66.66 3.0606 6.730

Table 4: Variation of MRR with ConstantCurrent and Pulse-On Time

Exp.No

Current(Amps)

Pulse onTime (Ton)(Micro sec)

MRR(mm3/min)

SR(µm)

DutyFactor

(%)

1. 4 20 66.66 425.6

2. 5 20 66.66 989.8

3. 6 20 66.66 765.4

Table 3: Variation of Hardnesswith Current

Exp.No.

Current(Amps)

Pulse on Time(Ton) (Micro sec)

DutyFactor

(%)

Hardness(Hv)

Figure 8 shows the relationship betweenHardness and current at constant pulse on timeand duty factor. From that figure we canobserve that the Vickers hardness valueincreases from 4A to 5Aand then decreasedto words the 6A. Due to the fact that, whencurrent increased from 4A to 5A the carbonparticles are deposited on machined surface

due to the carbon deposition, the hardness ofthe machined surface is increased, whereasin case of 6A current, due to the high current,the carbon deposited on the machined surfaceis flushed out. So that, no carbon depositiontakes place and the Vickers hardness valuedecreased at 6A (Table 3)

Figure 9 shows the relationship betweenmetal removal rate and duty factor at constantcurrent and pulse on time. As the duty factor isincreased, increase in percentage ofmachining time (or pulse on time) and increasein total current occurs. Due to the increase inmachining time, the metal removal rate on workpiece is also increases. when the duty factoris increased from 33.33 to 66.66 at 6 Amps

and 100 sec pulse on time the metal removalrate is increased drastically where as at the

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current 4 and 5 Amps and pulse on time 20,50 sec MRR is increased gradually. Forhigher MRR the duty should be high.

The Figure 10 shows the relationshipbetween surface roughness and Duty factorat constant current and pulse on time .It cannoted from figure that, the surface roughnessis increased with increase in duty factor from33.33.to 66.66. This is due to the fact that athigher duty factor (Table 4), Increase in percent

of machining time and increase in total currentthen automatically metal removal rate isincreased rapidly. Due to faster metal removalrate, surface roughness is increased. So thatat lower duty factor, we can get good surfacefinish rather than at higher duty factor.

The Figure 11 shows the relationshipbetween Vickers hardness and duty factor atconstant current and pulse on time. When theduty factor increases from 33.33% to the 50%

Figure 9: Variation of MRR with Duty Factor

Figure 10: Variation of Surface Roughness with Duty Factor

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1. 6 100 33.33 425.80

2. 6 100 66.66 987.50

3. 6 100 50.00 725.66

Table 5: Variation of Hardnesswith Duty Factor

Exp.No.

Current(Amps)


DutyFactor

(%)

Hardness(Hv)

the hardness value also increases due thedeposition of carbon and then, the hardnessvalue decreases at 66.66% due to the highduty factor (Table 5). Because high duty factorthe carbon deposition layer is flushed out fromthe work piece. So when duty factor increasesbeyond the 50%, the hardness valuedecreased.

Figure 11: Variation of Hardness with Duty Factor

1. 4 20 50.00 0.5087 5.165

2. 4 50 50.00 0.3707 4.829

3. 4 100 50.00 0.3561 4.260

4. 5 20 66.66 1.6687 5.870

5. 5 50 66.66 1.5453 5.675

6. 5 100 66.66 1.3270 5.105

7. 6 20 33.33 1.9332 6.750

8. 6 50 33.33 1.7533 6.425

9. 6 100 33.33 1.0415 5.824

Table 6: Variation of MRR with ConstantCurrent and Duty Factor

Exp.No

Current(Amps)

Pulse onTime (Ton)(Micro sec)

MRR(mm3/min)

SR(µm)

DutyFactor

(%)

The Figure 12 shows the relationshipbetween metal removal rate and pulse on time

at constant current and Duty factor. The metalremoval rate is decreased with increase inpulse on time. When the pulse on time isincreased from 20 sec to 100, the metalremoval rate is decreased. Because the shortpulses cause less vaporization, where as longpulse duration cause the plasma channel toexpand. The expansion of plasma channelcause less energy density on the work piece,which is insufficient to melt and/ or vaporizethe work piece material (Soni andChakravarthi, 1994). The metal removal rateis maximum at around 50 pulse on time (insec) (Soni and Chakravarthi, 1994).

The Figure 13 shows the relationshipbetween Surface Roughness and Pulse on

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Figure 12: Variation of MRR with Pulse on Time

Figure 13: Variation of Surface Roughness with Pulse on Time

time at constant current and duty factor. Fromthe figure we can observe that, the surfaceroughness is increased with increase in pulseon time from 20 micro sec to 100 microseconds (Table 7). At low pulse on time we willget good surface finish when compared withhigh pulse on time. At constant current and dutyfactor setting ,increase in pulse on time resultsin proportional increase in spark energy andconsequently melting boundary becomesdeeper and wider, and increases the surface

1. 4 20 4 1326

2. 4 50 4 1550

3. 4 100 4 2013

Table 7: Variation of Hardnesswith the Pulse-On Time

Exp.No.

Current(Amps)


DutyFactor

(%)

Hardness(Hv)

roughness value (Soni and Chakravarthi,1994).

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The Figure 14 shows the relationship betweenthe Vickers Hardness Value (HV) and Pulse ontime at constant current and duty factor. Fromthe figure we are observing that, the Vickershardness value increase with increase in pulseon time from 20 sec to 100 sec.This is due tothe fact that, along pulse duration causes the

Figure 14: Variation of Hardness with Pulse on Time

plasma channel to expand. The expansion ofplasma channel causes less energy density onthe work piece, which is insufficient to melt thework piece material and at same time it won’tdeplete the deposition of carbon layer on workpiece. So that Vickers hardness value increasedwith increased pulse on time.

Crack Propagation

When Inconel metal undergone EDMoperation, high temperature will be generatedat the machining surface. When the currentincreases, the spark intensity also increases.Due to the increase in intensity of spark thetemperature of the machining surface will alsoincreases, so that when the current increasesthe crack length and crack width alsoincreases. When the duty factor increases, themachining time and spark intensity alsoincreases due to which crack length and widthare also increases. When pulse-on timeincreases, the crack length and widthdecreases because of low intensity of plasmachannel. So that low temperature will begenerated at the machining surface. Finally,we will get low crack length and width withhigher pulse-on time (Figure 15).

Figure 15: Variation of Cracks Lengthand Crack Width

(a) Ip = 4A, DF = 33.33%, Ton = 50 µsec

(b) Ip = 6A, DF = 66.66%, Ton = 20 µsec

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Recast Layer

When Inconel metal undergone EDMoperation, high temperature will be generatedat the machining surface. When the currentincreases, the spark intensity also increases.Due to the increase in intensity of spark thetemperature of the machining surface will alsoincreases, so that when the current increasesthe recast layer also increases.

When the duty factor increases, themachining time and spark intensity alsoincreases due to which recast layer is alsoincreases. When pulse-on time increases, therecast layer decreases because of lowintensity of plasma channel. So that lowtemperature will be generated at themachining surface. Finally, we will get lessrecast layer on surface of work piece withhigher pulse-on time (Figure 16).

CONCLUSION• When current increases, the MRR also

increases. The higher the current, intensityof spark is increased and results in highmetal removal will takes place.

• When the current is increased, surfaceroughness is also increased. Because dueto increase in current, the spark intensity isalso increases. So the MRR per minuteincreases. Finally the surface roughness isincreases.

• When current is increases, hardness willdecreased. Because due to increasecurrent, the intensity of spark increases.Due high spark intensity the carbon layerwill depleted. So that the hardness isdecreased.

• When current is increased, the crack length,crack widths are also increased due to thehigh temperature generation at highcurrents.

• When duty factor is increases, the MRR isalso increases. The higher the duty factor,intensity of spark and machining time isincreased and results in high metal removalwill takes place.

• When the Duty factor is increased, surfaceroughness is also increased, because dueto increase in duty factor, the spark intensity,machining time is also increases. So theMRR per minute increases. Finally thesurface roughness is increases.

• When Duty factor is increases, hardness willdecreased. Because due to increase Dutyfactor, the intensity of spark increases. Duehigh spark intensity, the carbon layer willdepleted. So that the hardness isdecreased.

Figure 16: Variation of Recast Layersof Inconel-718

(a) Ip = 4A, DF = 33.33%, Ton = 50 µsec

(b) Ip = 6A, DF = 66.66%, Ton = 20 µsec

387


• When duty factor is increased, the cracklength, crack widths are also increased dueto the high temperature generation at highduty factors.

• When pulse on time is increases, the MRRis decreased. The higher the pulse on time,intensity of spark is decrease dueexpansion of plasma channel and results inless metal removal will takes place.

• When the Pulse on time is increased,surface roughness is decreased, becausedue to increase in pulse on time, the sparkintensity is also decreases due to theexpansion of plasma channel. So the MRRper minute decreases. Finally the surfaceroughness is decrease.

• When the Pulse on time is increases,hardness will increased. Because increasein pulse on time, the intensity of sparkdecreases due to the expansion of plasmachannel. Due low spark intensity, the carbonlayer will deposited, so that the hardness isincreased.

• When pulse on time is increased, the cracklength, crack widths are increased due tothe low temperature generation at high pulseon time due the expansion of plasmachannel.

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THE EXPERIMENTAL ANALYSIS OF SURFACE …THE EXPERIMENTAL ANALYSIS OF SURFACE CHARACTERISTICS OF INCONEL-718 USING ELECTRICAL DISCHARGE MACHINING ... current, pulse on time, duty factor