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    International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976

    6340(Print), ISSN 0976 6359(Online) Volume 4, Issue 1, January - February (2013) IAEME

    54

    INVESTIGATION ON PROCESS RESPONSE AND PARAMETERS IN

    WIRE ELECTRICAL DISCHARGE MACHINING OF INCONEL 625

    Rodge M. K1, Sarpate S. S2, Sharma S. B3

    1&2Research scholars,

    3Professor

    Production Engineering Dept., SGGSIE&T, Nanded, India.

    ([email protected])

    ABSTRACT

    Continuous research in the field of material science leads to production of very

    hard, tough, high temperature and corrosion resistant materials which are difficult-to

    machine with conventional methods. Advanced manufacturing processes play an

    important role in production of complicated profiles on such difficult-to-machine

    components. Inconel 625 is one of the recent materials developed to have high

    strength, toughness and corrosion resistant. The high degree of accuracy, fine surfacequality and good productivity made wire electrical discharge machining (WEDM) a

    valuable tool in todays manufacturing scenario. The right selection of the machiningconditions is the most important aspect to take into consideration in the processes

    related to WEDM. As electrode wire is not reused, its wear is generally ignored.

    However, it is interesting to study wire wear as it may have an effect on kerf width

    and surface quality of the product. The present study is focused on investigation of the

    effect of process parameters on multiple performance measures such as cutting width,electrode wear and hardness during WEDM of Inconel 625. The control factors

    considered are: pulse-on time, pulse-off time, upper flush, lower flush, wire feed andwire tension. The relationships between control factors and responses are established

    by means of regression analysis. The study demonstrates that, there is a goodagreement between experimental and predicted (theoretical) values of performance

    measures.

    Keywords: Orthogonal array (OR), Signal-to-noise (S/N) ratio, Taguchis design of

    experiment (DOE), Wire Electrical Discharge Machining (WEDM)

    INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERINGAND TECHNOLOGY (IJMET)

    ISSN 0976 6340 (Print)ISSN 0976 6359 (Online)

    Volume 4 Issue 1 January- February (2013), pp. 54-65 IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2012): 3.8071 (Calculated by GISI)

    www.jifactor.com

    IJMET I A E M E

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    1. INTRODUCTIONAs newer and more exotic materials with requirement of complex shapes are

    developed, conventional machining operations will continue to reach their limitations. Wireelectrical discharge machining (WEDM) is an extremely potential (thermoelectric) process

    having capacity to machine parts made up of conductive materials regardless of their

    hardness, toughness and geometry. In WEDM, a series of discrete electrical sparks between

    the work and tool electrodes immersed in a liquid dielectric medium melt and vaporize

    minute amounts of the work material which is then ejected and flushed away by the dielectric

    fluid. Latest WEDMs are assisted by CNC table to produce any complex two and three

    dimensional profiles on work. Due to high process capability, this method is widely used in

    manufacturing of car wheels, special gears, various press tools, dies and similar complex and

    intricate shapes. Hence, the increased use of the WEDM in manufacturing will continue to

    grow at an accelerated rate.

    Wire electrical discharge machining manufacturers and users emphasize on achievement of

    better stability, higher machining productivity along with desired accuracy and surfacequality. However, due to involvement of large number of variables, even a highly skilled

    WEDM operator is rarely able to achieve the optimal performance. Proper selection of

    process parameters for best process performance is a challenging job. An effective way to

    attempt this problem is to establish the relationship between performance measures of the

    process and its controllable input parameters. Optimization of process parameters can play a

    important role in this regard. In WEDM, the commonly affecting process parameters are

    ignition pulse current, time between two pulses, pulse duration, servo voltage, wire speed,

    wire tension and dielectric fluid injection pressure. Any slight variation in one of the

    parameters can affect the production quality and economics of the process. The parameter

    settings given by manufacturers are only applicable for commonly used steel grades and

    alloys.

    The important performance measures in WEDM are metal removal rate (MRR), cutting width(kerf) and surface quality. In WEDM operations, MRR determines the economics of

    machining and rate of production where as kerf denotes degree of precision and dimensional

    accuracy. The internal corner radius to be produced is limited by the kerf. The gap between

    the electrode wire and work usually ranges from 0.025 to 0.075 mm and it is constantly

    maintained by a computer controlled positioning system. In setting the machining parameters,

    particularly in rough cutting operation, the goal is twofold: the maximization of MRR and

    minimization of kerf.

    Konda R. et al. [1999] classified the various potential factors affecting the WEDM

    performance measures into five major categories: the different properties of workmaterial,

    dielectric fluid, machine characteristics, adjustable machining parameters and component

    geometry. They have applied the design of experiments (DOE) technique to study and

    optimizethe possible effects of variables and validated the experimentalresults using noise-to-signal (S/N) ratio analysis. Different areas of WEDM research identified by Ho K. H. et al.

    [2004] are: such as process optimization, process monitoring and control. The settings for the

    various process parameters play a crucial role in producing an optimal machining

    performance. The application of adaptive control systems to the WEDM is vital for the

    monitoring and control of the process. The authors have investigated the advanced

    monitoring and control systems including the fuzzy, the wire breakage and the self-tuning

    adaptive control systems used in WEDM process. Cabanes I. et al. [2008] analyzed new

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    symptoms that allow us to predict wire breakage. Symptoms may be increase in discharge

    energy, peak current, increase/decrease in ignition delay time. They have proposed a novel

    wire breakage monitoring and diagnostic system with virtual instrumentation system (VIS)

    that measures relevant magnitudes and diagnostic system (DS) that detect new quality cuttingregimes and predicts wire breakage. Almost in 80% of total wire breakage cases, the

    anticipation time longer than 50 ms has been detected. Efficiency of supervision system has

    been quantified to 82%.

    Parashar Vishal et al. [2010] analyzed kerf width of wire cut electro discharge machining of

    SS304L steel using DOE technique. They have used statistical methods and regression

    analysis for finding kerf width. Mixed OR of L32 is used for experimentation. ANOVA is

    used to find out the variables affecting kerf width more significantly. Results show that pulse-

    on time and dielectric flushing pressure are the most significant factor to the kerf width.

    Theoretical and experimental results appeared to be in good agreement. Mohammadreza

    Shabgard et al. [2011] used 3D finite element for prediction of the white layer thickness, heat

    affected zone (HAZ) and surface roughness (SR) of electro discharge machined AISI H13

    tool steel. They carried out experimental investigations to validate the numerical results. Bothnumerical and experimental results show that increasing the pulse-on time leads to a higher

    white layer thickness, depth of HAZ and surface roughness and increase in the pulse current

    slightly decrease the white layer thickness and depth of HAZ with increase in SR.

    Experimental and numerical results are closer to each other. Manoj Malik et al. [2012] have

    carried out optimization of process parameters of WEDM using Zinc-coated brass wire for

    MRR, electrode wear rate (EWR) and SR. They observed that, for minimum EWR, pulse-on

    time and pulse peak current should be high. For EWR, pulse peak current is the most critical

    factor and duty cycle time is the least significant parameter.

    From the literature it is found that, most of the authors have studied the effect process

    parameters on different response variables while WEDM of commonly used materials like,

    die steel, EN31, AISI H13 steel, etc. Studies on WEDM of Inconel 625 are scantly available.

    Inconel 625 is recently coming up as one of best candidate materials which is extensivelyused in various applications including: marine, aerospace, chemical processing, nuclear

    reactors and pollution control equipments. It is used in any environment that requires

    resistance to heat and corrosion retaining mechanical properties. It is an alloy having

    excellent corrosion resistance in a wide range of corrosive media being especially resistant to

    pitting and crevice. It is a favorable choice for sea water applications. Therefore, it is

    interesting to study the effect of control parameters on work as well as electrode materials.

    The studies about effect of control factors on hardness of work material is important as it is

    one of the most critical parameter in relation to wear and tear of components made out of

    WEDM.

    2. METHODOLOGY

    2.1 Taguchi MethodGenerally, the machine tool builder provides information in the form of tables to be

    used for setting machining parameters. The selection of process parameter relies heavily on

    the experience of the operators. With a view to alleviate this difficulty, a simple but reliable

    method based on statistically designed experiments is suggested for investigating the effects

    of various process parameters and to get optimal process settings. This new experimental

    strategy proposed by Genichi Taguchi is called Taguchi method. It is a powerful

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    experimental design tool which uses simple, effective and systematic approach for deriving

    the optimal levels of machining parameters. This approach efficiently reduces the effect of

    the sources of variation. It requires minimum experimental cost due to reduction in number of

    experiments required to meet the specific requirements in terms of quality and reliability. Ituses specially constructed tables known as orthogonal arrays (OA). Each row represents a set

    of parameters for a particular experiment. This is a best way to study the effect of large

    number of variables on desired quality characteristics with small number of experiments.

    2.2 Design of Experiment

    To evaluate the effects of machining parameters on performance characteristics (kerf,

    wire wear and hardness of machined surface) a specially designed experimental procedure is

    required. In this study, the Taguchi method, a powerful tool for parameter design of the

    performance characteristics is used to determine optimal machining parameters for minimum

    of kerf, minimum wire wear and higher hardness of the machined surface. Six control factors

    with five levels each and three response variables are used to get qualitative results. The

    control factors represent stability in design of manufacturing process whereas the noisefactors denote all factors that cause variation. Table 1 shows six parameters with five levels

    each. This may increase number of experiments to be carried out. However, it may help in

    getting a good relationship between input and output parameters. Based on Taguchi method

    we use L25 orthogonal array (obtained using Minitab 16 software) as shown in Table 2. The

    experiments are performed as per orthogonal array which has 25 rows indicating 25 of

    experiments. The results are shown in Table 2. The kerf is measured with the help of a

    microscope. The loss in weight of electrode wire per meter length is determined by

    subtracting final weight from initial weight of the wire. The hardness of machined surface is

    measured with the help of hardness tester.

    Table 1: Parameters and their levels

    Factors level 1 level 2 level 3 level 4 level 5 Unit

    Pulse-on time (Ton) 3 4 5 6 7 s

    Pulse-off time (Toff) 3 4 5 6 7 s

    Wire Feed (WF) 6 7 8 9 10 mm/s

    Upper Flush (UF) 6 7 8 9 10 kg/cm2

    Lower Flush (LF) 6 7 8 9 10 kg/cm2

    Wire Tension (WT) 60 70 800 900 1000 kg

    2.3 Experimental set-upThe inputs used in the present study are chosen through review of literature,

    experience and some preliminary investigations. Each time an experiment was performed, a

    particular set of input parameters was chosen. The work piece is a block of Inconel 625 with

    length 100 mm width 30 mm thickness 10 mm. The workpiece material composition is

    tested at ICS, Pune. Its percentage composition is: C - 0.035, Mn - 0.130, Si - 0.247, S -

    0.010, P - 0.0064, Cr - 22.86, Ni 58.1, Mo - 8.55, W -

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    depth along the length of the work are taken. The experiments are performed on Maxicute

    WEDM (Figure 1). The machine allows operator to choose input parameters according to

    geometry and material of electrodes from a manual provided by the WEDM manufacturer.

    Figure 1: Maxicut-e WEDM

    2.4 Signal-To-Noise RatioIn signal-to-noise (S/N) ratio signal represents the desirable value (mean for output

    parameters) and noise represents undesirable value (the square deviation of output

    parameters). Thus, it is the ratio of mean to square deviation. It is designated by symbol

    with unit of dB. The characteristic for which the lower value represents better performance,

    the S/N ratio should be smaller the better (SB) and the characteristic for which the large value

    represents better performance, the S/N ratio should be larger the better (LB). In this study the

    parameters kerf and wire wear should have lower values and hardness of the machined

    surface should have larger values.

    The loss function (L) for kerf width, wire wear and hardness is defined as:LSB =

    kerf

    LSB =

    ww

    LLB =

    1/ hv

    where Ykerf, Yww and Yhv are the responses for kerf width, wire wear and hardness

    respectively and n denotes the number of experiments. The S/N ratios can be calculated as a

    logarithmic transformation of the loss function as shown below.

    S/N ratio for kerf width = -10 log10 (LSB) i)

    S/N ratio for wire wear = -10 log10 (LSB) ii)

    S/N ratio for hardness = -10 log10 (LLB) iii)

    The analysis is done using the popular software specifically used for design of experimentapplications known as MINITAB 16. The S/N ratio for kerf width, wire wear and hardness is

    computed using Eqs. i), ii) and iii) respectively for each treatment as shown in Table 2. Then,

    overall mean for S/N ratios of kerf width, wire wear and hardness are calculated as average of

    all treatment responses for each level (Table 3, 4 and 5). The graphical representation of the

    effect of the six control factors on kerf width, wire wear and hardness is shown in Figure 2, 3

    and 4 respectively.

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    Table 2: Orthogonal array, experimental results for kerf width (KW), wire wear (WW) and

    hardness (HV) of WEDMed surface along with S/N ratios

    S.N Ton Tof U L W WT KW S/N WW S/N HV S/N1 3 3 6 6 6 600 0.33 9.6297 0.01 40.000 310 49.827

    2 3 4 7 7 7 700 0.32 9.8970 0.01 40.000 312 49.883

    3 3 5 8 8 8 800 0.33 9.6297 0.03 30.457 286 49.127

    4 3 6 9 9 9 900 0.34 9.3704 0.01 40.000 301 49.571

    5 3 7 10 10 10 100 0.35 9.1186 0.03 30.457 331 50.396

    6 4 3 7 8 9 100 0 31 10.172 0.01 40.000 325 50.237

    7 4 4 8 9 10 600 0.30 10.457 0.01 40.000 325 50.237

    8 4 5 9 10 6 700 0.35 9.1186 0.03 30.457 295 49.396

    9 4 6 10 6 7 800 0.30 10.457 0.02 33.979 276 48.818

    10 4 7 6 7 8 900 0.31 10.172 0.02 33.979 311 49.855

    11 5 3 8 10 7 900 0.30 10.457 0.03 30.457 325 50.237

    12 5 4 9 6 8 100 0.29 10.752 0.02 33.979 341 50.65513 5 5 10 7 9 600 0.28 11.056 0.03 30.457 296 49.425

    14 5 6 6 8 10 700 0.34 9.3704 0.04 27.958 290 49.248

    15 5 7 7 9 6 800 0.30 10.457 0.02 33.979 325 50.237

    16 6 3 9 7 10 800 0.29 10.752 0.03 30.457 301 49.571

    17 6 4 10 8 6 900 0.28 11.056 0.01 40.000 309 49.799

    18 6 5 6 9 7 100 0.28 11.056 0.02 33.979 299 49.513

    19 6 6 7 10 8 600 0.33 9.6297 0.03 30.457 311 49.855

    20 6 7 8 6 9 700 0.33 9.6297 0.02 33.979 309 49.799

    21 7 3 10 9 8 700 0.30 10.457 0.05 26.020 297 49.455

    22 7 4 6 10 9 800 0.29 10.752 0.01 40.000 310 49.827

    23 7 5 7 6 10 900 0.29 10.752 0.01 40.000 298 49.48424 7 6 8 7 6 100 0.32 9.8970 0.01 40.000 295 49.396

    25 7 7 9 8 7 600 0.31 10.172 0.02 26.020 290 49.248

    Table 3: Response Table forS/N ratios (smaller the better) for kerf width

    Level Ton Toff UF LF WF WT

    1 9.529 10.294 10.196 10.244 10.032 10.189

    2 10.076 10.583 10.182 10.355 10.408 9.6950

    3 10.419 10.323 10.014 10.081 10.128 10.410

    4 10.425 9.7450 10.033 10.360 10.196 10.362

    5 10.406 9.9100 10.429 9.8150 10.090 10.199

    Delta 0.8960 0.8380 0.4150 0.5450 0.3760 0.7150

    Rank 1 2 5 4 6 3

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    Table 4: Response Table forS/N ratios (smaller the better) for wire wear

    Level Ton Toff UF LF WF WT

    1 36.18 33.39 35.18 36.39 36.89 34.98

    2 35.68 38.80 36.89 34.98 34.48 31.68

    3 31.37 33.07 34.98 34.48 30.98 33.77

    4 33.77 34.48 33.77 34.80 36.89 36.89

    5 36.00 33.28 32.18 32.37 33.77 35.68

    Delta 4.82 5.73 4.70 4.02 5.91 5.20

    Rank 4 2 5 6 1 3

    Table 5: Response Table forS/N ratios (larger the better) for hardness

    Level Ton Toff UF LF WF WT

    1 49.76 49.67 49.65 49.72 49.73 49.72

    2 49.71 50.08 49.94 49.63 49.54 49.563 49.96 49.39 49.76 49.53 49.78 49.52

    4 49.71 49.38 49.69 49.80 49.77 49.79

    5 49.48 49.91 49.58 49.94 49.79 50.04

    Delta 0.48 0.70 0.36 0.41 0.25 0.52

    Rank 3 1 5 4 6 2

    Figure 2: Graphs for kerf width

    Figure 3: Graphs for wire wear

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    Figure 4: Graphs for hardness of WEDMed surface

    Table 6: Experimental and Predicted values of kerf width (KW), wire wear (WW) andhardness (HV)

    where E = Experimental, P = Predicted values from regression analysis

    S. N. EKW PKW EW PW EH PHV

    1 0.33 0.3138 0.01 0.016

    310 304.00

    2 0.32 0.3166 0.01 0.020

    312 308.00

    3 0.33 0.3258 0.03 0.024

    286 311.10

    4 0.34 0.3300 0.01 0.028

    301 314.20

    5 0.35 0.3378 0.03 0.032

    331 317.30

    6 0.31 0.3000 0.01 0.017

    325 322.24

    7 0.30 0.3200 0.01 0.036

    325 309.44

    8 0.35 0.3200 0.03 0.026

    295 306.44

    9 0.30 0.3100 0.02 0.027

    276 298.7410 0.31 0.3200 0.02 0.020

    311 308.00

    11 0.30 0.2800 0.03 0.020

    325 317.00

    12 0.29 0.2900 0.02 0.021

    341 309.88

    13 0.28 0.3100 0.03 0.040

    296 297.00

    14 0.34 0.3200 0.04 0.033

    290 306.68

    15 0.30 0.3200 0.02 0.023

    325 303.00

    16 0.29 0.2880 0.03 0.034

    301 308.00

    17 0.28 0.2900 0.01 0.024

    309 305.00

    18 0.28 0.3000 0.02 0.017

    299 314.00

    19 0.33 0.3210 0.03 0.036

    311 302.00

    20 0.33 0.3100 0.02 0.037 309 294.0021 0.30 0.2800 0.05 0.037

    297 303.00

    22 0.29 0.2900 0.01 0.030

    310 313.16

    23 0.29 0.2800 0.01 0.031

    298 305.00

    24 0.32 0.2900 0.01 0.021

    295 302.00

    25 0.31 0.3100 0.02 0.040

    290 289.66

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    The regression analysis is used for modeling the responses in terms of process variables. The

    regression equations are obtained by using Minitab software.

    Regression equation for kerf width

    KW = 0.318 - 0.00760Ton + 0.00580Toff - 0.00100UF +0 .00320LF + 0.00040WF-0.000024WT

    Regression equation for wire wear

    WW = - 0.0098 + 0.00200Ton + 0.00140Toff + 0.00220UF + 0.00060LF + 0.00280WF

    -0.000030 WT

    Regression equation for hardness of machined surface

    HV = 286 -2.06Ton - 2.16Toff - 1.30UF + 2.16LF + 1.22WF + 0.0318WT

    From these equations, the predicted (theoretical) values of kerf width, wire wear and

    machined surface hardness are determined manually for all set of parameters. Table 6 shows

    experimental and predicted values for above said process responses.

    3. RESULTS AND DISCUSSION

    The purpose of the experimentation is to identify the factors which have strong effects

    on the machining performance. From mean of S/N ratios (Table 3) for kerf width, it is found

    that pulse-on time has highest rank 1. Therefore, it has most significant effect on kerf width.

    As pulse-on time increases the kerf width increases significantly. The wire feed has least

    effect on kerf width. The order of other influencing parameters for kerf width is: pulse-off

    time, wire tension, lower flush and upper flush. Also, from mean of S/N ratios (Table 4) for

    wire wear, it is observed that, the wire feed has highest rank 1 and therefore, it affects wire

    wear significantly. The wire wear initially decreases significantly with increase in wire feed;

    however it increases with further rise in wire feed. This may be due the combined effect of

    other factors. The lower flush has least effect on wire wear. The order of other influencing

    parameters for wire wear is: pulse-off time, wire tension, pulse-on time and upper flush.

    Table 5 shows that, for hardness of the machined surface, the pulse-off time has highest rank1 and hence, it affects hardness of the machined surface most significantly. However, the

    effect of increase in pulse-off time on hardness of the machined surface has no fixed nature.

    The hardness first increases and then decreases significantly. Again it increases. The wire

    feed has least effect on hardness. The order of other influencing parameters of hardness is:

    wire tension, pulse-on time, lower flush, upper flush and upper flush.

    From Table 3, the optimal combination of process parameters for minimum kerf width is

    found to be: A1B4C3D5E1F2. The symbols A, B, C, D, E and F represents process

    parameters: Ton, Toff, UF, LF, WF and WT respectively and numbers represents the levels.

    This means, to have minimum kerf width, Ton should be set on level 1, Toff on 4, UF on 3,

    LF on 5, WF on 1 and WT on 2. Similarly from Table 4, it is observed that, the optimal

    combination of process parameters for minimum wire wear is: A3B3C5D5E3F2. This means,

    to have minimum wire wear, Ton should be set on level 3, Toff on 3, UF on 5, LF on 5, WFon 3 and WT on 2. It is to be noted that the optimal levels of factors differ widely for both the

    objectives (for minimum kerf width and minimum wire wear). From Table 5, the optimal

    combination of process parameters for hardness of wire electrical discharge machined surface

    is:A3B2C2D5E5F5. The hardness of the machined surface should be more for better working

    performance. This means to have high hardness Ton should be set on level 3, Toff on 2, UF

    on 2, LF on 5, WF on 5 and WT on 5.From Figure 5, it is observed that, the predicted values for kerf width determined from regressionanalysis are in agreement to experimental values. However, wire wear during the WEDM operations

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    is sensitive to many operational parameters other than current characteristics of the system such aswire tension, wire feed, flushing conditions which must have attributed toward the non uniformity in

    experimental wire wear reading with respect to predicted one (Figure 6). Lower wire wear generally

    results in higher kerf width and the present experimental study also depicts the similar results. Figure

    7, shows good agreement between experimental and predicted hardness values reasonably. Thehardness of the machined surface is first decreased and then improved when lower flush, wire feedand wire tension is increased. Whereas it first increases and then decreases when pulse-on, pulse-off

    and upper flush is increased. Thus, the recommended values are of the combined effect of the processparameters.

    Figure 5: Comparison of experimental and predicted values of kerf width

    Figure 6: Comparison of Experimental and predicted values of wire wear

    Figure 7: Comparison of Experimental and predicted values of hardness

    0.25

    0.27

    0.29

    0.31

    0.33

    0.35

    0.37

    1 3 5 7 9 11 13 15 17 19 21 23 25

    Kerfwidth

    EKW

    PKW

    0

    0.01

    0.02

    0.03

    0.04

    0.05

    0.06

    1 3 5 7 9 11 13 15 17 19 21 23 25

    Wirewear

    EWW

    PWW

    240

    260

    280

    300

    320

    340

    360

    1 3 5 7 9 11 13 15 17 19 21 23 25

    HARDNESSHV

    EHV

    PHV

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    4. CONCLUSIONS

    WEDM process parameters optimization is responsive not only to the process

    variables but also the work materials. Therefore, for quality machining performance of amaterial, parameter optimization is essential to result cost effective usages of the material for

    the given application. The present investigation revealed that pulse-on ranks high in terms of

    machining performance of Inconel 625 and it has a predominant effect on kerf width. During

    machining of Inconel, as pulse-on increases, the kerf width increases which resulted

    relatively lower wire wear. The wire wear initially decreases significantly with increase in

    wire feed; however it increases with further rise in wire feed. This may be due the combined

    effect of other factors. With regard to hardness of machined components, pulse-off causes

    significant variation. Optimized process parameters could be used as guideline for WEDM of

    Inconel 625.

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