study on parameter’s optimization for the minimizing of

Scientia Iranica B (2021) 28(5), 2639{2654

Sharif University of TechnologyScientia Iranica

Transactions B: Mechanical Engineeringhttp://scientiairanica.sharif.edu

Study on parameter's optimization for the minimizingof the residual stress in powder mixed near-dry electricdischarge machining

S. Sundriyala, Vipina, and R.S. Waliab;�

a. Department of Mechanical Engineering, Delhi Technological University, Delhi, 110089, India.b. Department of Production and Industrial Engineering, Punjab Engineering College, Chandigarh, 160012, India.

Received 8 May 2020; received in revised form 7 November 2020; accepted 30 August 2021

KEYWORDSResidual stress;EDM;Powder;Near dry;X-ray method.

Abstract. The products processed by Electrical Discharge Machining (EDM) leave someundesirable e�ects, the most prominent of which is Residual Stress (RS). These RS a�ectfatigue behavior, dimensional stability, and stress corrosion. The latter is the cause offailure of processed products. It has been observed that products obtained by PowderMixing Near Dry Electric Discharge Machining (PMND-EDM) have relatively low RSvalues. The purpose of this research is to minimize the RS caused in the machined workpiece(EN-31) by optimizing the process parameters that signi�cantly a�ect the machiningcharacteristics. The selected parameters were tool diameter, mist ow rate, metal powderconcentration, and mist pressure, and the selected workpiece was EN-31 because it hasideal mechanical properties and is widely used in manufacturing. The minimum value ofRS at optimized values of process parameters was found to be 106.32 MPa.© 2021 Sharif University of Technology. All rights reserved.

1. Introduction

Electric Discharge Machining (EDM) was developedin 1943 by Russian scientist Lazarenko. EDM isone of the non-conventional machining methods whichutilize thermal energy for material removal from theworkpiece. Today, EDM is widely used in globalmanufacturing engineering. This technology makes itpossible to cut complex shapes on very hard materials.Further study of EDM by the researchers has led tothe development of various hybrid EDM machiningmethods such as Dry Electric Discharge Machining(D-EDM), Near Dry EDM (ND-EDM), Powder MixedEDM (PM-EDM), and Powder Mixed Near Dry EDM(PMND-EDM). These developed methods were more

*. Corresponding author. Tel.: +91 9717325233E-mail address: [email protected] (R.S. Walia)

doi: 10.24200/sci.2021.55837.4429

prominent in terms of machining e�ciency. ND-EDM uses gas mist and a small amount of dielectricoil as the dielectric, so unlike traditional EDM, thismachining method also eliminates a large amount ofEDM oil required [1]. The feasibility of the new EDMprocess which was known as PMND-EDM came intoexistence [2]. Gases such as nitrogen or air as adielectric medium instead of dielectric oil were usedin DEDM which makes processing eco-friendly as thisnew methodology produces less harmful fumes and atthe same time also eliminates the need for EDM oil [3].

Previous research has been conducted on PM-EDM for performance enhancements such as improvedMaterial Removal Rate (MRR), Tool Wear Rate(TWR), surface �nish (Ra), and morphology. A studyon machining e�ciency was performed for PMEDMand Taguchi L9 (OA) was utilized for optimization.The results show that PMND-EDM can reduce TWRand residual stress due to the stable arc conditions atthe gap between the electrodes [4]. Rotary tool elec-

2640 S. Sundriyal et al./Scientia Iranica, Transactions B: Mechanical Engineering 28 (2021) 2639{2654

trode was used as in NDEDM and there was enhancedMRR with improved surface �nish with reduced micro-cracks and debris accumulation [5]. Similarly, oxygengas was utilized as a dielectric medium in NDEDMand it was reported that more MRR was observed withthe increasing phenomenon of melting by rapid oxida-tion [6]. The experimental result and analysis showedthat PMND-EDM was a better machining method thanND-EDM because in the former technique, the MRRis increased by 45.04%, while surface and TWR arereduced by 45.33% and 60.60%, respectively [7]. It wasalso noticed that PMND-EDM resulted in producing amachined surface with better surface quality due to thepresence of metallic foreign particles in the dielectricmedium which leads to a stable arc at the machininggap [8].

The study has also been performed in the �eldof surface morphology such as improved microhardnessin PM-EDM. The presence of hard tungsten carbide(W2C) and cementite (Fe3C) increased the MH (�150%) on the machined surface [9]. The formationof carbides on the machined surface increased the mi-crohardness to 912 HV without sacri�cing the qualityof the machined surface [10]. Taguchi's philosophywas adopted to determine the suitable combination ofEDM parameters to improve machining performancesin terms of material removal and micro-hardness [11].

Products made by di�erent EDM methods havea variety of adverse e�ects on components, and theresidual stress generated on the working surface is themain one. The metallurgical transformation causedby the sharp temperature gradient is the cause ofthe residual stress in the mechanical parts [12]. Itis found that the strength of this residual stress isgreater than the yield point of the material, which willcause serious cleavage, twinning, and slippage in thecrystal structure [13]. The nature of residual stresswas investigated and it was revealed that high residualstresses were found at the immediate surface of themachined sample and then gradually falls to a lowvalue before giving a small way to compressive residualstresses [14]. Compared with the ND-EDM process, theresidual stress of the PMND-EDM machined surface isreduced by 56.09% [15]. A hard layer of zinc carbideis deposited on the surface of the processed sample,which led to a higher microhardness value [16]. Apeak value of residual stress in EDM was investigatedon the machined surface of the workpiece and it wasconcluded that residual stress with a peak value of590 MPa was found at 40 �m beneath the top surfaceof the workpiece and was reduced to zero value at200 �m depth [17]. EDM results in the generation ofresidual stresses of high value which increase from thesurface and reach the maximum value. This maximumvalue is around the ultimate tensile strength of thematerial [18]. Optimization of the residual stress

using X-ray cos� method in vibration-assisted hybridEDM process was performed over high carbon highchromium D2 tool steel workpiece. The results showthat discontinuous vibrations enhanced the MRR andreduced the residual stresses induced in the machinedworkpiece [19]. The �nite element method was usedto model the residual stress of the EDM workpiece. Itwas revealed by the study that the value of residualstresses at the subsurface of the workpiece was morethan the top surface due to high surface roughnesson the top surface and low discharge energy of thesparks was attributed to lower values of the residualstresses [20]. Finite element analysis of residual stressesin EDM was performed and techniques like nano-indentation technique and Raman spectroscopy wereadopted to measure the residual stress in the AISIH13 tool steel machined by EDM process and it wasrevealed that microcracks which were responsible forrelieving some part of residual stress [21]. A parametricstudy for residual stresses was performed in wire EDMand the Taguchi method of optimization was utilizedto minimize the residual stresses in the aluminumworkpiece [22].

There are many factors that are responsiblefor the residual stress such as heating, cooling, re-solidi�cation, melting, and phase transformations withvolume change. This induced residual stress resultsin a decrease in the reliability of the product andat the same time leads to uncertain failure of themanufactured products. Therefore, it is necessaryto study residual stresses in order to minimize thesestresses and improve product reliability. These residualstresses are generated in the specimens due to a heatingphenomenon and re-solidi�cation on the surface of theworkpiece. Thus this residual stress plays a vitalrole in determining product quality and reliability.Although studies on performance enhancements andoptimization techniques have been applied in PM-EDM still investigations related to residual stress andoptimization of critical parameters in PMND-EDMare still lacking therefore in this study contributionsand e�orts have been made to explore the importantmechanical property (RS) of the machined products.This research could help further in-depth studies in the�eld of residual stress optimization and could providethe ground for conducting research in the �eld ofPMND-EDM [16].

2. Experimentation

2.1. Experimental procedureThe experimental process parameters are dividedinto six categories: dielectric, metal powder,tools, electrical parameters, workpieces, and otherparameters (air pressure, oil pressure, and mist owrate). Series of experiments were performed according

S. Sundriyal et al./Scientia Iranica, Transactions B: Mechanical Engineering 28 (2021) 2639{2654 2641

Table 1. Residual stress experimental results according to Taguchi L9 OA.

Experimentno.

Parametericconditions

Residual Stress (RS)(MPa)

S/N(decibel)A B C D Run 1 Run 2 Run 3

1 2 5 2 0.4 565 558 530 {54.822 2 10 5 0.5 621 551 550 {55.193 2 15 8 0.6 510 659 678 {55.854 3 5 5 0.6 370 361 340 {51.055 3 10 8 0.4 145 169 186 {44.486 3 15 2 0.5 239 279 280 {48.517 4 5 8 0.5 251 214 311 -48.358 4 10 2 0.6 230 209 191 {46.469 4 15 5 0.4 556 501 587 {54.79

Total { { { { 3478 3501 3653 {Mean { { { { 386.44 389 405.88 {

Overall mean RS (RS) = 394.11 MPa

to Taguchi L9 (OA) for residual stress. Each series ofexperiments was carried out 3 times, and the resultsare shown in Table 1.

The experimentation methodology was derivedfrom previous work in the �eld of EDM [15,16]. Eachseries of experiments were carried out in three runs. Byconsidering the better principle given by S=N (Signal-to-Noise ratio) in Eq. (1), a total of 27 experimentswere carried out to analyze residual stress [16]:

S=N = � log10

1n

nXi=1

y2ij

!; (1)

where n is replications, and yij is response value.The standard deviation error bar between the

mean value of the obtained residual stress for di�erentparametric experimental conditions was plotted (totaltwenty-seven experiments: three repeatabilities foreach set of parametric conditions) and the error waswithin +=� 5% range as shown in Figure 1.

Residual stress measurement techniques can bedivided into di�erent categories, such as destructivetesting, semi-destructive testing, and Non-DestructiveTesting (NDT). NDT has several advantages over semi-destructive and destructive testing. NDT can be

Figure 1. Standard deviation error bar for residual stressfor di�erent set of experiments.

applied to a wide variety of materials and the accuracyof measurement is high. There are many NDT methodsfor measuring residual stress, such as X-ray di�rac-tion technology, Burkhausen noise method, neutrondi�raction method, and ultrasonic method. The X-raydi�raction method has several advantages over othermethods because the X-ray di�raction technique can beused to measure the residual stress of a variety of mate-rials. Another advantage of X-ray di�raction technol-ogy is that it can analyze microscopic and macroscopicresidual stresses. At the same time, the technology ishandheld and can be read in any direction.

The residual stress measurement is performed bya �X-360 pulsetec machine (made in Japan), as shownin Figure 2. It is mainly composed of three parts:computer, sensor unit, and power supply unit. Thesetup consists of the X-ray tube (30 KV) and 2-D X-ray sensor for visual analysis along with a power supplyunit to generate the X-ray.

The X-ray takes 90 seconds to take one readingof residual stress. The machine has a special func-

Figure 2. Portable XRD - residual stress analyzer inoperation.


tion of air cooling, which can achieve high-e�ciencyperformance. The X-ray tube current was 1 mA andthe sample distance from the X-ray focusing lens was38 mm. The incident angle for the X-ray was adjustedat 35 degrees while the information regarding the spec-imen to be analyzed were lattice constant, interplanarspacing and di�raction planes (h, k, l), and crystalstructure, Poisson's ratio, and Young's modulus. Themeasured output responses were residual stress, shearstress, and full width half maximum value. The FullWidth Half Maximum (FWHM) is the expression ofthe function range (pro�le peak di�raction), and themaximum and minimum peak di�raction are expressedas �-max and �-min, respectively. The procedure ofresidual stress measurement is divided into four stages.The �rst stage includes sample positioning and captureof the sample image. The second stage includes theincidence of the X-ray and its detection by a 2-Dsensor. The third stage includes the calculation ofresidual stress and the �nal stage includes output dataresults of residual stress and value of FWHM. Theoutput reading also calculates the standard deviationof the obtained readings to maintain the repeatabilityof reading. The working principle of residual stressis based on X-ray di�raction scattering and it is inagreement with Bragg law given by Eq. (2) [23]:

n� = 2d sin �; (2)

where n is the order of re ection; d is the interplanarspacing of the crystal; � is the wavelength of theincident X-ray; and � is the angle of incidence thatequals the angle of scattering (di�raction angle) asshown in Figure 3. X-ray di�raction occurs fromvarious orientations of the grain crystals that satisfyBragg's law as shown in Figure 4. The di�racted X-rayform a cone around the incident X-ray axis because ofthe variation in crystal's orientation which also resultsin the formation of a ring of residual stress known asthe Debye-Scherrer ring as shown in Figure 5. ThisDebye ring is formed by the single short-duration X-ray exposure. Determination of the residual stress is

Figure 3. Di�raction orientation of incident X-ray fromthe sample.

Figure 4. Formation of Debye-Scherrer ring.

Figure 5. Scattering phenomenon of di�racted X-ray.

achieved by accurately measuring the position of theDebye- Scherrer rings and their positions are a directmeasure of strain.

The magnitude of the strain is determined by theposition of the detected Debye-Scherrer ring and iscalculated by using the formula given in Eqs. (3) and(4). The residual stress and shear stress are calculatedby Eqs. (5) and (6):

"�1 =12f("� � "�+�) + ("�� "��)g ; (3)

"�2 =12f("� � "�+�) + ("�� "��)g ; (4)

�x = � E1 + #

� 1sin 2�

� 1sin 2'o

� @"�1@ cos�

; (5)

Txy = � E2(1 + #)

� 1sin 2�

� 1sin 2'o

� @"�2@ sin�

; (6)

where � is the Azimuth angle of Debye-Sherrer ring;"�1 is the strain measured in the vertical direction;"�+� is the strain measured in the direction of � +�; �x is the residual stress; �xy is the residual shear


Figure 6. Schematic diagram of the working principle of Powder Mixing Near Dry Electric Discharge Machining(PMND-EDM).

Figure 7. Indigenously developed setup for Powder Mixing Near Dry Electric Discharge Machining (PMND-EDM).

stress; � is Poisson's ratio; "�2 is the strain measuredin the horizontal direction; E is Young's modulus; �is the angle between Debye-Scherrer ring axis and thesample di�raction detector X-ray; and " is the Poisson'sratio.

3. Method and developed setup

The schematic diagram for the working principle ofPMND-EDM is shown in Figure 6. A newly designedsetup of PMND-EDM was made indigenously at theprecision engineering lab (DTU, Delhi) as shown inFigure 7 [4]. The setup developed for PMND-EDM

includes various technical features and componentsdesigned and manufactured according to the requiredexperimental conditions to achieve the desired outputresults. The setup body is made of stainless steeland consists of an EDM oil pressure regulator, an airpressure regulator, and a mist ow controller.

The pressure gauges were analogous in naturewith a limit up to 0.6 MPa. The mist ow regulatorwas mounted on the setup together with a transparentscale, and the scale was up to 20 ml min�1. It isas per the our setup developed and its speci�cation.A separate air compressor (built-in Nu-Air) with apower of 2 H.P (horsepower), a pressure limit of 0.8


Figure 8. Developed copper tool electrodes forexperiments (all dimensions are in mm).

MPa, and a capacity of 24 liters was integrated withsetup. The mixing chamber was equipped with asetup for obtaining a 3-� (phase) mixture of metalpowder, glycerin (a stabilizer), and EDM oil, andcompressed air supplied separately from an air com-pressor (Glycerin 5% dielectric oil, metal powder 2%dielectric oil). With the help of a exible plastictubes, the generated dielectric mist was provided at theEDM machining gap between the tool electrode andthe workpiece. The experiments were performed onthe Sparkonix EDM machine (Sparkonix limited Pune,35 A series). Customized copper tool electrodes ofdi�erent dimensions with special design features werefabricated and developed at the precision engineeringlab (DTU, Delhi) as shown in Figure 8. The concept ofthe design of tool electrodes was taken from a previousstudy in the �eld of NDEDM [24]. The detailed designfeatures of the hollow tool electrode are shown in Fig-ure 9. A exible tube of 6 mm is inserted into the inletend of the tool for conveying the pressurized mist ofthe dielectric medium. The electrode sizes of di�erenttools used in the experiment are shown in Table 2. Theexperimental conditions set for PMND-EDM are shownin Table 3, while the chemical and physical propertiesof the tool electrode are shown in Table 4.

The workpiece chosen for machining was EN-31

Table 2. Tool dimensions for experiments.

Tool 1 Tool 2 Tool 3

di = 2; do = 3 di = 3; do = 4 di = 4; do = 5

di= inner dia; do = outer dia; All dimensions are in mm.

grade due to its desirable physical properties such ashigh strength and hardness. The chemical compositionand physical properties of the EN-31 workpiece areshown in Table 5.

4. Working principle of PMND-EDM

The working principle of EDM is to convert electricalenergy into heat energy. When current was supplied tothe electrode, a plasma channel with high heat energyis generated at the working gap between the tool andthe workpiece or the electrode gap. The ionizatione�ect takes place due to the interaction of ions andmolecules at this gap which results in generating aspark that jumps from tool to workpiece in case ofstraight polarity and vice versa. This repetitive cyclegenerates thousands of sparks which results in theformation of a plasma of very high temperature. Thisplasma channel of high heat energy is responsible formelting and removing material from the workpiecefollowing which the erosion process starts. With thehelp of the dielectric oil supplied at the working gap,the erosion of the debris was ushed. The dielectric inPMND-EDM is composed of metal powder, compressedair, a very small amount of EDM oil, and glycerin usedas a stabilizer. The metal powder acts as an additive,helping to form a chain structure of powder particlesin the plasma channel, thereby reducing the dielectricstrength of the dielectric medium and contributingto rapid corrosion. The compressed air supplied bythe compressor helps to form the dielectric mist, andalso helps to stabilize the arc at the gap between theelectrodes due to the increase in the pressure of thesupplied mist. The glycerol liquid which has high

Figure 9. Design feature of the developed tool.


Table 3. Condition for experimental machining.

Parameter Description

Workpiece/tool EN-31 (30 mm �15 mm �15 mm)/copper

Dielectric medium Metallic powder + Air + oil (LL-221) + stabilizing agent

Current 12 A

Gap voltage 25 V

Pulse on/o� 500 �s/ 75 �s

Polarity +ve

Tool diameter (mm) 2, 3, 4

Concentration of metallic powder 2, 5, 8 (g l�1)

Flow rate of dielectric medium 5, 10, 15 (ml min�1)

Pressure of dielectric 0.4, 0.5, 0.6 (MPa)

Metallic powder Zinc

Grain size of powder 15 �m

Stabilizing agent Glycerol (5%)

Machining time 10 mins

Table 4. Chemical and physical properties of tool.

Properties ValuesBulk modulus (Gpa) 140Tensile strength (MPa) 33.3Hardness (Mohs) 2.5{3Thermal conductivity (Wm�1 K�1) 385Boiling point (�C) 2567Melting point (�C) 1083Density (kg m�3) 8960Atomic number 29Atomic weight 63.546

viscosity was added to prevent the powder particlesfrom settling down. This mixture is fed into themixing chamber from the inlet of the tank and thisheterogeneous mist is supplied at IEG (Inter ElectrodeGap or gap between the tool and the workpiece) 25 �mat high pressure with the help of tubes from the mixingchamber. The current is supplied to the electrodes as

soon as the experimental conditions are obtained. Thesparking phenomenon starts and the erosion cycle takesplace. The increased working gap due to the presenceof metal powder results in a more uniform energizedplasma. The reason for the increase in working gapand excited plasma is that the interlocking of powderparticles at the IEG leads to the formation of chains ofpowder particles, which leads to high-frequency rapidsparks. The schematic diagram of setup is given inFigure 10.

The three-phase dielectric is supplied by hollowcopper electrode tools in the �nal stage of the ma-chining area. The compressed air is supplied to themixing chamber. The compressed air is mixed withdielectric EDM oil and metal powder additives. Then,through the pressure control regulator, the dielectricmixture is supplied to the hollow tool electrode throughthe plastic tube. The dielectric is ejected from theother end of the electrode and starts to spark whenpower is supplied to the tool electrode. This method ofinjecting heterogeneous mist into the gap between the

Table 5. Chemical composition and physical properties of the workpiece (EN-31).

Chemical properties Physical properties

Element % Thermal conductivity (W m�1K�1) 44.5

Carbon 0.90{1.20 Hardness (HRC) 63

Silicon 0.10{0.35 Yield stress (MPa) 450

Manganese 0.30{0.75 Melting point (�C) 1540

Sulphur 0.050 Tensile strength (MPa) 750

Phosphorus 0.050 Density (kg m�3) 7850


Figure 10. Schematic diagram of setup for Powder Mixing Near Dry Electric Discharge Machining (PMND-EDM).

electrodes or the working gap is e�ective in machining,and it also reduces the need for EDM oil. Unlike thetraditional EDM method that requires a large amountof EDM oil, the consumption rate of EDM oil in thismethod was very small. Therefore, it is necessary tocustomize a new setup with speci�cations accordingto the speci�ed design to meet the research goals andobtain the expected results.

5. Results and discussion

5.1. Analysis of residual stressThe workpiece material (EN-31) chosen for residualstress analysis was B.C.C (Body-Centered Cubic) crys-tal structure with interplanar spacing (d) of 1.170�A, Poisson's ratio of 0.280, di�raction plane (h, k,l) of 2, 1, 1, and with di�raction angle of the X-ray at 156.39 degrees. Figure 11(a){(i) shows theresidual stress analysis of nine series of experiments byX-ray di�ractometer according to Taguchi L9 (OA).Figure 11 shows nine sets of experiments based onTaguchi L9 (OA) showing di�raction rings, residualstress, peak pro�le, and FWHM and their values.Among the experimental values obtained from nineseries of experiments, the minimum value of residualstress was found to be 145 MPa. Compared with thehigh residual stress obtained in the previous work, theobtained residual stress value was much lower [17].Due to the induced microcracks on the machinedsurface, the residual stress is reduced. These inducedmicrocracks as shown in Figure 12, were responsiblefor relieving some of the residual stress, another reasoncan be attributed to the uniform distribution of heat onthe machined surface and improved ushing conditionwhich ultimately relieves some part of the residual

stresses on the machined surfaces [22].The plasma at the machining gap is partially

energized by the metal powder concentration, whichmakes the surface have fewer induced microcracks, asshown in Figure 12. The ionization is improved byadding zinc powder to the dielectric uid because theadded powder expands and widens the discharge gapbetween the electrodes. This results in the easy removalof debris, which leads to an improvement in surfacequality. The results show that by adding the metalpowder, the discharge energy dispersion is improvedbecause the dielectric strength of the dielectric uiddecreases with the addition of the metal powder. Thesefavorable conditions improve the forming e�ect of themachined surface with improved surface characteristicsin absence of large cracks [15]. This proves that themetal powder has played an important role in changingthe plasma channel. This resulted in improved de-ionization e�ect and discharging conditions. All thesefactors increase the discharge frequency as the ushingconditions improve. A mixture of solid and liquid,such as a mist assisted by metal powder, changes theelectric �eld strength of the dielectric medium, therebyfacilitating the onset of discharge. Enlarged dischargegap results insu�cient heat dissipation, due to whichmore molten materials will be ejected by the explosiveforce resulting from the gasi�cation of solid and liquidphases. This phenomenon also restrains the excessiveexpansion of the discharge channel in its radius. Thusoptimal parameters achieve a surface with reducedresidual stress.

Table 6 shows the output results of residual stressanalysis of nine sets of experiments where � is residualstress, �1 is the standard deviation of residual stress,�xy is shear stress, �2 is the standard deviation of shearstress, FWHM is full width half maximum angle, �max


Figure 11. Residual stress analysis according to Taguchi L9 (OA).


Figure 11. Residual stress analysis according to Taguchi L9 (OA) (continued).

Table 6. Results obtained by X-ray di�raction testing according to Taguchi L9 (OA).

Exp.no

RS (�)(MPa)

�1

(MPa)�xy

(MPa)�2

(MPa)FWHM(degree)

�-max(degree)

�-min(degree)

1 530 20 {79 11 3.49 314.60 204.48

2 551 43 {61 26 3.80 20.88 241.20

3 659 49 13 33 3.90 83.52 200.16

4 370 66 23 53 4.42 34.56 216.72

5 145 64 {199 60 4.31 103.68 282.24

6 280 19 {133 23 3.95 39.60 216

7 251 51 {63 54 4.19 277.92 259.92

8 191 81 {115 44 4.21 161.28 110.88

9 280 19 {133 23 3.95 39.60 216


Figure 12. Induced micro-cracks in Powder Mixing NearDry Electric Discharge Machining (PMND-EDM).

is the maximum azimuth angle, and �min is minimumazimuth angle of debye scherrer ring.

Radial stress analysis on the machined surface wasconducted to study the e�ect of variation of residualstresses with respect to the radial distance from themachined area. The radial residual stress analysis wasperformed at di�erent radial distances from the centerof the machined area.

It was observed that the nature of variation ofstress was transforming from tensile to compressivevalue. Similar trend of residual stress was also observedin previous literature [12{14, 25]. There was a suddenreduction of temperature with respect to an increase

in radial distance due to low heat ux in the work-pieces. The phenomenon of phase transformation wasnegligible at a more radial distance from the center ofthe machined area due to which the grains were againrestrained back to compressive nature.

With the help of the residual stress measurementof the machining, shown in Figure 1, the test wasperformed on di�erent parts of the processed sample.As the radial distance increased further, the thermale�ect decreased to a very low value, and �nally becamenegligible at a distance of 1.7 mm, as shown inFigure 13. The average values of residual stresses, maine�ect values, and S/N ratios of di�erent levels L1, L2,and L3 were calculated, as shown in Tables 7 and 8.

The average values of residual stresses are shownin Figure 14. The parameters A, B, C, and D atlevels 2, 2, 1, 2 were most e�ective in reducing residualstress as shown in Figure14. The same trend for S/Nratios is shown in Figure 14. Parameter A at the 2ndlevel, parameter B at the 2nd level, parameter C at the1st level and parameter D at the 2nd level were moste�ective in reducing residual stress.

The residual stress induced in the workpiece ini-tially decreased with an increase in the inner diameterof the tool-tip at 2nd level (L2). Better ushingoccurred on the machined surface which provided abetter cooling condition for dissipation of heat on themachined surface. Therefore, less heat was generatedon the machined surface which resulted in reducing

Table 7. Average values and main e�ects: Residual Stress, RS (in MPa).Process

parameterLevel Tool

dia (A)Flow

rate (B)Powderconc (C)

Pressure(D)

Data type S/N Data raw S/N Data raw S/N Data raw S/N Data rawAveragevalues

(% RS)

L1 {55.29 580.22 {51.41 388.88 {49.93 342.33 {51.36 421.88L2 {48.01 263.22 {48.71 316.88 {53.68 493.00 {50.69 366.22L3 {49.87 338.88 {53.05 476.55 {49.56 347.00 {51.12 394.22

Main e�ects L2-L1 7.27 {317.00 2.69 {72.00 {3.74 150.66 0.67 {55.66L3-L2 {1.85 75.66 {4.34 159.66 4.11 {146.00 {0.43 28

(% RS)Di�erences

(L3-L2){(L2-L1){9.12 392.66 {7.04 231.66 7.86 {296.66 {1.11 83.66

Table 8. ANOVA pooled data for residual stress.

SourceSSraw

SSS/N

SS'raw

SS'S/N

DOFraw

DOFS/N

Varianceraw

VarianceS/N

F-ratioraw

F-ratioS/N

P%raw

P%S/N

Tool diameter 493368.66 85.63 489736 84.92 2 2 246684.30 42.81 135.81 121.16 62.67 58.54

Flow rate 115088.66 28.82 111456 28.11 2 2 57544.33 14.41 31.68 40.78 14.61 19.70

Powder conc. 132114.66 31.11 128482 30.40 2 2 66057.33 15.55 36.36 44.02 16.78 21.27

Mist pressure 13944.66 0.70 * - 2 { 6972.33 * 3.83 * 1.77 *

Error 32694 0.70 57536.67 2.82 18 2 1816.33 0.35 { { 4.15 0.48

Total 787210.66 146.27 787210.70 146.27 26 8 * { { { 100 100

*Signi�cant at 95% con�dence level, F-critical=3.55 (tabular value for raw data), SS: Sum of squares,

DOF: Degree Of Freedom, V: Variance, F-critical=19 (tabular value for S/N data), SS0: Pure sum of squares.


Figure 13. Residual stress (MPa) vs radial distance (mm).

Figure 14. E�ect of process parameters on Residual Stress (RS) (MPa).

residual stress to the lowest value at L2. At level 3there was a phenomenon of rapid solidi�cation. Thisrapid solidi�cation was responsible for an increase inthe value of residual stress [25]. Rapid solidi�cationled to the solidi�cation of sputtered melted liquid onthe workpiece surface as shown in Figure 15 (scanningelectron microscope image). It can be clearly seenfrom Figure 15 that there is non-homogenous solidi�edmaterial on the machined surface which was responsiblefor the change in a speci�c volume of the moltenmaterial and therefore the value of residual stresses washigh.

The RS value initially decreased with an increasein ow rate values due to better heat dissipation atL2 but with a further increase of ow rate, there wasa phenomenon of stress corrosion due to which the

RS value increased to its peak value at L3. Extremetemperature di�erences are developed on the surfaceof the machined samples due to high thermal sparkingat the inter-electrode gap in EDM. Therefore rapidcooling leads to high residual stresses on the EDM'edsurfaces [15].

The residual stress initially increases with in-creases in powder concentration but afterward de-creases. Due to the low discharge energy density, thespark gap is enlarged, and the addition of metal powderled to the reduction of tensile residual stress. Addinggraphite powder to the dielectric will reduce the tensilestress, which in turn also increases the fatigue strengthof the machined parts [26]. Due to the presence ofmetallic powder particles the plasma channel becomesmore energized and more heat was is generated on the


Figure 15. Rapid solidi�cation on the surface (SEMimage).

workpiece due to which high residual stress was foundon the machined surface at L2. As the concentrationof metal powder increased, the heat density or heat ux at the machining gap increased exponentially. Thebridging e�ect between the metallic powder particlesincreases and maintains conditions that are conduciveto spark generation, so the temperature of the ma-chined area becomes very high, which is the cause ofthe higher residual stress value. A further increasein the powder concentration leads to improper sparkdischarge, thereby reducing the thermal energy of theworkpiece. Excessive aggregation of metal powderparticles at the inter-electrode gap resulted in theformation of a small thermal plasma, so the thermalintensity at the machining gap is reduced. The lowertemperature value at the gap between the electrodesis the cause of the low metallurgical transformation ofthe machined surface (Scanning Electron Microscopeimage is shown in Figure 16), which leads to a decreasein the residual stress at L3.

The residual stress value decreased with the in-crease of the mist pressure and then increased withthe further increase of the pressure. The moleculardensity initially increased as the pressure of the mistincreased, thereby improving the deionization e�ect.These conditions lead to better heat dissipation whichreduces residual stress value at L2. The relaxation

time for residual stress gets decreased with an increaseof mist pressure which led to localized in-homogenousplastic deformation on the machined part as shown inFigure 17 (scanning electron microscope image). Theseconditions were responsible for the increase in the valueof residual stress at L3. Sudden pressure change of mistat L3 was very signi�cant in changing the structure ofthe grains of the surface layer of the machined part.

5.1.1. Residual stress performance characteristicsestimation

Response characteristic is determined by the followingequation [16]:

RS = �A2 + �B2 + �C1 + �D2 � 3RS: (7)

The con�dence interval to con�rm the experiments isgiven by Eq. (8) [16]:

CICE =

sF� (1; fe)Ve

�1

neff+

1R

�: (8)

The Con�dence interval of the population is given byEq. (9) [16]:

CIPOP =

sF� (1; fe)Ve

neff; (9)

where, F�(1; fe) is the F ratio at the con�dence levelof (1� �) against DOF 1.

F0:05(1; 18) = 3:5546 (tabulated)

neff =N

1 + DOF= 9:

DOF is associated in the estimation of the meanresponse. Also, treatment is 9; repetition is 3; N(totalnumber of experiments) is 27; Ve (error variance) is1816.33 (according to Table 8); fe (error DOF) is 18(according to Table 9); R (con�rmation experimentsfor sample size) is 3, and:

F =Variation between the samplesVariation within the samples

:

Table 9. Con�rmation experiments for Residual Stress (RS).

Tooldiameter

(mm)

Flow rate(ml min�1)

Metallicpowder

concentration(g l�1)

Mistpressure(MPa)

Experimentalresults

RS (MPa)

AverageRSa (MPa)

3 10 2 0.590.32110.39118.49

106.40

a: RS: Residual Stress.


Figure 16. SEM image of low metallurgicaltransformation (SEM image).

Figure 17. Localized in-homogenous plastic deformation(SEM image).

The F value is compared with the F limit for therespective DOF. If the F value is equal to or greaterthan the F limit value (Table 8), it can be said thatsigni�cant di�erences exist between the sample means.So:

RS = 106:32 MPa;

CICE = �53:56;

CIPOP = �26:78:

The predicted optimal range of CICE is:

Mean RS�CICE<RS(MPa) < mean RS+CICE ;

that is, 52.76 MPa < RS (MPa) < 159.88 MPa;

Mean RS�CIPOP <RS (MPa)<mean RS+CIPOP ;

that is, 79.54 MPa < RS (MPa) <133.10 MPa.

5.1.2. Con�rmation experiments for residual stressThree repetitions of con�rmation experiments for resid-ual stress were performed at optimized levels of processparameters. The residual stress con�rmation test

was carried out under the experimental conditionsof A2, B2, C1, and D2. The experimental residualstresses for three runs under optimized conditions werefound to be 90.32 MPa, 110.39 MPa, and 118.49 MParespectively. The mean experimental residual stresscalculated at the optimized process of parameters was106.40 MPa which was within the con�dence intervalof predicted residual stress. Thus, the range provedthat the predicted value of residual stress lies in theoptimum range as shown in Table 9.

6. Conclusion

Powder Mixing Near Dry Electric Discharge Machining(PMND-EDM) process is a hybrid method of machin-ing that was e�cient in machining rate with improvedquality characteristics such as reduced Residual Stress(RS) in the machined product. Optimization techniquewas utilized in PMND-EDM to study the e�ect onresidual stress. Con�rmation experiments were per-formed at the optimized experimental condition. Forthe developed settings, the following conclusions canbe drawn:

1. When experimenting with optimized process pa-rameter values, it is found that RS had the smallestvalue. Tool diameter at level 2 (3 mm), the mist ow rate at level 2 (10 ml min�1), metallic powderconcentration at level 1 (2 g l�1), and dielectricmist pressure at level 2 (0.5 MPa) were the mostin uential values which resulted in lowest RS valueof 106.32 MPa in the machined product;

2. The predicted optimal range of residual stress was52.76 MPa < RS (MPa) < 159.88 MPa. Predictedmean of residual stress at 95% con�dence intervalwas 79.54 MPa < RS (MPa) < 133.10 MPa. Thiscon�rms that the average residual stress was withinthe optimal range of residual stress prediction;

3. Residual stress analysis was performed at a di�erentradial distance from the machined area. It wasobserved that the residual stresses in the machinedarea are essentially tensile, but as the radial dis-tance increased, the nature of the residual stressbecame compressive. The residual stress becomesnegligible at a radial distance of 1.7 mm.

Nomenclature

EDM Electric Discharge MachiningDEDM Dry Electric Discharge MachiningNDEDM Near Dry Electric Discharge MachiningPM� EDM Powder Mixed Electric Discharge

MachiningPMND� EDM Powder Mixed Near Dry Electric

Discharge Machining


MRR Material Removal RateTWR Tool Wear RateRa Surface �nishRS Residual StressFWHM Full Width Half MaximumANOVA Analysis of VarianceCIce Con�dence interval of con�rmation

experimentsCIpop Con�dence interval of populationVe Error varianceMH Micro-HardnessHV Vickers Hardness number

References

1. Kao, C.C., Tao, J., and Shih, A.J. \Near dry electricaldischarge machining", Int. J. Mach. Tool. Manuf., 47,pp. 2273{2281 (2007).

2. Gao, Q., Zhang, Q.H., and Zhang, J.H. \Experimentalstudy of powder-mixed near dry electrical dischargemachining", Chin. J. Mech. Engg., 45, pp. 169{175(2009).

3. Shen, Y., Liu, H.Y., and Zhang, Y. \High-speed dryelectrical discharge machining", Int. J. Mach. Tool.Manuf., 93, pp. 19{25 (2015).

4. Sundriyal, S., Walia, R.S., and Vipin. \Powder mixednear dry electric discharge machining parameter op-timization for tool wear rate", Advances in Uncon-ventional Machining and Composites. Lecture Noteson Multidisciplinary Industrial Engineering, Springer,Singapore (2020).

5. Yadav, V.K., Kumar P., and Dvivedi, A. \Performanceenhancement of rotary tool near-dry EDM of HSSby supplying oxygen gas in the dielectric medium",Material. Manuf. Process, 34, pp. 1{15 (2019).

6. Yadav, V.K., Kumar, P., and Dvivedi, A. \E�ect oftool rotation in near-dry EDM process on machiningcharacteristics of HSS", Material. Manuf. Process., 34,pp. 779{790 (2019).

7. Sundriyal, S., Walia, R.S., and Vipin. \Near dry andpowder mixed near dry electric discharge machining",Int. J. Engg. Adv. Tech., 9, pp. 2021{2025 (2019).

8. Sundriyal, S., Walia, R.S., Vipin., and Tyagi, M.\Investigation on surface �nish in powder mixed neardry electric discharge machining method", MaterialsToday: Proceedings, 25, pp. 808{809 (2019).

9. Gill, A.S. and Kumar, S. \Investigation of micro-hardness in electrical discharge alloying of En31 toolsteel with Cu-W powder metallurgy electrode\, Arab.J. Sci. Engg., 43, pp. 1499{1510 (2018).

10. Chundru, V.R., Koona, R., and Pujari, S.R. \Surfacemodi�cation of Ti6Al4V alloy using EDMed electrodemade with nano- and micron-sized TiC/Cu powderparticles", Arab. J. Sci. Engg., 44, pp. 1425{1436(2019).

11. Rahul, Datta, S., and Biswal, B.B. \A novel sat-isfaction function and distance-based approach formachining performance optimization during electro-discharge machining on super alloy inconel 718", Arab.J. Sci. Engg., 42, pp. 1999{2020 (2017).

12. Lloyd, H.K. and Warren, R.H. \Metallurgy of spark-machined surfaces", J. Iron. Steel Inst., 203, pp. 238{247 (1965).

13. Rebelo, J.C., Dias, A.M., and Kremer, D. \In uenceof pulse energy on the surface integrity of martensiticsteels", J. Mat. Process. Technol., 84, pp. 90{96(1998).

14. Crookall, J.R. and Khor, B.C. \Residual stressesand surface e�ects in electro discharge machining",Proceedings 13th MTDR Conf., Birmingham, pp. 16{27 (1972).

15. Sundriyal, S., Vipin., and Walia, R.S. \Study on thein uence of metallic powder in near-dry electric dis-charge machining", Strojni�ski vestnik-J. Mech. Engg.,66, pp. 243{253 (2020).

16. Sundriyal, S., Vipin., and Walia, R.S. \Experimentalinvestigation of the micro-hardness of EN-31 die steelin a powder-mixed near-dry electric discharge machin-ing method", Strojni�ski vestnik-J. Mech. Engg., 66, pp.184{192 (2020).

17. Ekmekci, B., Elkoca, O., and Tekkaya, A.E. \Residualstress state and hardness depth in electric dischargemachining: De-ionized water as dielectric liquid",Mach. Sci. Technology., 9, pp. 39{61 (2007).

18. Ekmekci, B., Tekkaya, A.E., and Erden, A.D. \Asemi-empirical approach for residual stresses in electricdischarge machining (EDM) ", Int. J. Mach. Tool.Manuf., 46, pp. 858{868 (2006).

19. Kumar, S., Grover, S., and Walia, R.S. \E�ect ofhybrid wire EDM conditions on generation of residualstresses in machining of HCHCr D2 tool steel underultrasonic vibration", Int. J. Interact. Des. Manuf.,12, pp. 1119{1137 (2018).

20. Liu, J.F. and Guo, Y.B. \Residual stress modeling inElectric Discharge Machining (EDM) by incorporatingmassive random discharges", Procedia CIRP., 45, pp.299{302 (2016).

21. Shabgard, M., Seydi, S., and Seyedzavvar, M. \Novelapproach towards �nite element analysis of residualstresses in electrical discharge machining process", Int.J. Adv. Manuf. Technol, 82, pp. 1805{1814 (2016).

22. Rao, P.S., Ramji, K., and Satyanarayana, B. \E�ect ofwire EDM conditions on generation of residual stressesin machining of aluminum 2014 T6 alloy", Alex. Engg.J., 55, pp. 1077{1084 (2016).

23. Aruja, E. \Displacement of X-Ray Re exions\, Nature,154(53) (1944).

24. Shen, Y., Liu, Y., and Sun, W. \High-speed neardry electrical discharge machining", J. Mat. Process.Technol, 233, pp. 9{18 (2016).


25. Kruth, J.P. and Bleys, P. \Measuring residual stressescaused by wire EDM of tool steel", Int. J. Elect. Mach.,5, pp. 23{28 (2000).

26. Talla, G., Gangopadhyay, S., and Biswas, C.K. \In- uence of graphite powder mixed EDM on the surfaceintegrity characteristics of inconel 625", J. Part. Sci.Technol., 35, pp. 219{226 (2016).

Biographies

Sanjay Sundriyal is a PhD student at Delhi Tech-nological University of India. He graduated fromthe College of Engineering Roorkee and received hismasters from the National Institute of TechnologyWarangal. His research interests are advanced man-

ufacturing processes and optimization.

Vipin is a Professor at the Delhi Technological Univer-sity of India. He received his PhD degree from DelhiTechnological University. His area of interest includesadvanced manufacturing processes, optimization, andmodeling. He has authored many papers, book chap-ters, conferences, and references papers.

Ravinderjit Singh Walia is a Professor at PunjabEngineering College. He received his PhD degree fromthe Indian Institute of Technology (Roorkee). His areaof interest includes advanced manufacturing process,optimization, modeling, and industrial engineering. Heis the author of more than 100 papers, book chapters,conferences, and references papers.

study on parameter’s optimization for the minimizing of

Documents