Experimental investigation of process parameters in WEDMof Al 7075 alloyKingshuk Mandal1,*, Dipankar Bose2, Souren Mitra1, Soumya Sarkar1
1 Production Engineering Department, Jadavpur University, Kolkata 700032, India2 Mechanical Engineering Department, NITTTR-Kolkata, Kolkata 700106, India
* e-mail: m
This is anO
Received: 18 March 2020 / Accepted: 13 July 2020
Abstract. Process parameters selection is a critical task in wire electro-discharge machining (WEDM).The parameter settings would vary when a material to be machined is changed. In the present study,experimental investigation has been carried out for appropriate selection of process parameters in WEDM of Al7075. It has been perceived that pulse on time (Ton), arc on time (Aon) pulse off time (Toff), arc off time (Aoff),servo sensitivity (Sc), wire tension (Wt) and servo voltage (Sv) are the major influencing factors on machiningspeed (Vc), corner error (Ce) and surface roughness (Ra) for this alloy. It is also observed that selected sevenprocess parameters are essential for effective machining of Al 7075 alloy. Finally, three-dimensional (3D) surfacetopography has been analysed to determine the characteristics of the machined surface.
Al 7075, commonly known as a light-metal alloy, is a newgeneration metal alloy that has excellent mechanicalproperties and exhibits high strength, high toughnessand good ductility, and it also renders good resistance tofatigue and corrosion. Al 7075 is widely used in aircraftengine cylinder, frames and intricate vehicle sprocket inautomobile industries [1]. The dimensional accuracyincluding surface finish is extremely important aspectswhile machining of such beneficial materials. However, atthe same time Al 7075 is difficult to machine byconventional technique while maintaining the requiredprecision, surface finish and dimensional tolerances. Wireelectro-discharge machining (WEDM) is a potentialsolution for machining such high strength electricallyconductive material. WEDM is the process where a movingelectrode (wire) continuously transforms electrical energyinto thermal energy in the form of visible spark formachining materials [2,3]. Due to this nature, any kind ofelectrically conductive materials, metal matrix composites(MMCs) and low conductive ceramics can be efficientlymachined regardless to their mechanical or chemicalproperties [4,5]. Furthermore, WEDM is capable toproduce any kind of complex shape with high precision
penAccess article distributed under the terms of the CreativeComwhich permits unrestricted use, distribution, and reproduction
and good quality of surface finish [6,7]. In this researchwork, influence of the input factors in wire electro-discharge machining (WEDM) of Al 7075 alloy has beenreported. Machining speed, corner accuracy and surfacefinish are considered as performance characteristics. Infact, these response parameters determine the quality andproductivity of the manufacturing components in WEDM.
A wide range of the past research works in the field ofWEDM revealed the effect of input parameters on materialremoval rate (MRR) and the surface characteristics [8].A few number of research works have been reported on wirelag phenomenon during rough cutting operation inWEDM, that is an important and deciding factor toexplore the corner accuracy [9]. However, a very limitednumber of research works are available on the influence ofprocess parameters and optimization on dimensionalaccuracy [10]. Nevertheless, this response parameter isextremely important for achieving high precision job inWEDM. A small number of research works have beenreported in WEDM of Al 7075 based MMCs [11,12].Furthermore, the effect of arc on time (Aon), servosensitivity (Sc) and arc off time (Aoff) on machiningperformance, especially on corner error (Ce) and otherresponses have not been reported so far in the field ofWEDM for this alloy. The objective of the present researchwork is to carry out the experimental investigation ofdifferent input factors, namely pulse on time (Ton), arc ontime (Aon), pulse off time (Toff), arc off time (Aoff), servo
monsAttribution License (https://creativecommons.org/licenses/by/4.0),in any medium, provided the original work is properly cited.
Fig. 1. Pictorial view of WEDM table unit (Model: EX 40).
Table 1. Input factors and their levels.
Parameters Notations Units Levels
1 2 3
Pulse on time Ton ms 0.5 0.7 0.9Pulse off time Toff ms 14 24 34Arc on time Aon ms 0.2 0.3 0.4Arc off time Aoff ms 10 30 –
Servo voltage Sv V 10 15 20Wire tension Wt kg 0.8 1.2 1.6Servo sensitivity Sc – 1 2 3
2 K. Mandal et al.: Manufacturing Rev. 7, 30 (2020)
sensitivity (Sc), servo voltage (Sv) and wire tension (Wt) onmachining speed (Vc), corner error (Ce) and surfaceroughness (Ra) during single pass cutting operation.
2 Experimentation
2.1 Experimental set-up and procedure
The experimentations have been carried out in EX-40WEDM system. Brass coated wire electrode with diameterof 250mm is used to perform the experiments. Deionizedwater has been used as dielectric and its temperaturemaintained at 21 °C. Formaximumdielectric flow, both theupper and lower flushing nozzle are kept fully open.The pictorial view of WEDM table unit is shown inFigure 1.
The quality of the product manufactured by WEDM isaffected by the machining parameters. Genichi Taguchi isdeveloped a three-stage design methodology, to under-stand the importance of design of experiment [13]. In thepresent study, Taguchi based design technique has beenused to perform the experiments. L18 orthogonal array(mixed) design technique have been chosen for theexperiments. Here, three level design is used for sixparameters and two level design for one parameter. The fullfactorial design for seven factors with three levels is L2187(37) orthogonal array (OA). However, in this case L18 OAfractional factorial design is used [14]. Based upon somepreliminary investigation and past literature survey, pulseon time (Ton in ms), arc on time (Aon in ms), pulse off time(Toff in ms), arc off time (Aoff in ms), servo sensitivity (Sc),servo voltage (Sv in volt) and wire tension (Wt in kg) havebeen chosen as controllable factors. Nevertheless, in thiscase arc on time is found to be the least significant processparameter on corner error and surface roughness duringtrial run. At higher arc on time, frequently wire breakageand unstable machining are observed. For that reason, twolevel design of this factor is considered. The ranges andcorresponding levels of input parameters are shown inTable 1. The performance measures after the machiningare assessed in terms of corner error (Ce in mm),
surface roughness (Ra in mm) and machining speed(Vc in mm/min). The design of experiments (DOE) forrespective input and output are given in Table 2.
2.2 Measurements
Corner error (Ce) of the machined surface along the cuttingdirection is measured by CV-3200 (Mitutoyo, Japan) highprecision contour measuring instrument. Measurement ofthe corner profile has been taken along the bisector (45°) ofthe actual corner angle (90°). Schematic representation ofthe corner error measurement and the path produced bywire electrode are shown in Figure 2. The surface roughnessvalue is measured by SJ-410 (Mitutoyo, Japan) contacttype surface measuring instrument. 3D surface topographyof the machined surface is captured by high-resolution CCI(Taylor & Hobson, UK) microscope. Machining speed isobserved from the monitor of the machine and averagevalue is considered for analysis. Corner error and surfaceroughness measurement have been repeated five times tominimize the error and average value is taken. The requiredmeasurements have been taken in vibration free andstable temperature ambience to avoid the measurementerror.
3 Results and analysis
3.1 ANOVA for machining speed, corner errorand surface roughness
A statistical approach for estimation of the relativeinfluence of each variable on overall measured responseis analysis of variance (ANOVA). In general, the relativeimportance of individual factor is often represented bythe term of F-value [13]. The F-value in analysis of varianceplying most significant role for input factors. In this work,ANOVA has been employed to analysis the significance ofseven input factors on machining speed, corner error andsurface roughness. ANOVA results for Vc, Ce and Ra aregiven in Tables 3–5.
From the ANOVA tables, it has been observed thatpulse parameters (i.e. Ton & Toff) and arc on time (Aon) arethe most dominating factors (i.e. larger the F-value) onmachining speed (Vc), corner error (Ce) and surfaceroughness (Ra). Apart from the above parameters, servosensitivity (Sc) playing a vital role to determine the
Table 2. L18 experimental plan matrix and results.
Sl. No. Input factors Responses
Ton (ms) Toff (ms) Aon (ms) Aoff (ms) Sv (V) Wt (kg) Sc Vc (mm/min) Ce (mm) Ra (mm)
Fig. 2. Schematic representation of corner profile.
K. Mandal et al.: Manufacturing Rev. 7, 30 (2020) 3
machining speed (F-value=120.23) and wire tension (Wt)is playing a significant role on corner error (F-value=114.21). Servo voltage (Sv) is another important factorthat have a moderate influence on machining speed(F-value=15.09), surface roughness (F-value=15.34)and high impact on corner error (F-value=62.05). In thiswork, analysis has been carried out in 95% confidence level.
3.2 Modelling of the process parameters
In this study, relations between input factors and outputresponses have been developed using regression technique.
In regression technique, the mathematical form of correla-tion between output and input are as follows:
y ¼ fðTon;Toff ;Aon;Aoff ; Sv;Wt; ScÞ ð1Þwhere y defined, the output and f represent the outputfunction. In the process of analysis, estimate of y has beencalculated by using quadratic equation.
Mathematical model for corner error (Ce), surfaceroughness (Ra) and machining speed (Vc) are the functionof WEDM input parameters, in terms of pulse ontime (Ton), arc on time (Aon), pulse off time (Toff), arcoff time (Aoff), servo sensitivity (Sc), servo voltage (Sv) andwire tension (Wt). The relation between inputs and outputsare established based on the experimental results. Insignifi-cantprocessparameters (P-value>0.05)areeliminated fromthemodel for better accuracy. Themathematical models forVc, Ce and Ra are shown in Equation (2)–(4). The influenceof the process parameters on output have been evaluatedbased on these mathematical models.
4 K. Mandal et al.: Manufacturing Rev. 7, 30 (2020)
Ra ¼ 2:759� 0:473Ton þ 0:00665Toff � 0:326Aon
þ 0:0202Sv þ 0:354Wt � 0:0012Sc þ 0:711T2on
� 0:000214T2off þ 0:27A2
on � 0:000539S2v ð4ÞTo verify the developed mathematical model, another
set of confirmatory experiments have been carried out andresults are given in Table 6. Percentage of prediction errorshown in the Table 7 is defined as follows:
% prediction error
¼ Experimental value� Predicted value
Experimental value
��������
� �� 100
ð5Þ
It has been perceived that the predicted results areclosed to the experimental values. This proves the fact thatthe devolved mathematical models for machining of Al7075 are suitable and effectively used for parametricanalysis and optimization.
3.3 Parametric study on machining speed, corner errorand surface roughness3.3.1 Pulse on time (Ton) and arc on time (Aon)
From Figure 3a and 3b, it is observed that the corner error(Ce) machining speed (Vc), and surface roughness (Ra) allare increasing significantly with increase in pulse on time
K. Mandal et al.: Manufacturing Rev. 7, 30 (2020) 5
(Ton) and arc on time (Aon). With increase of pulse on timeand arc on time, energy per pulse increases and as a result,more material is removed per pulse. This discharge energydirectly decided by pulse parameter settings (i.e. pulse ontime and arc on time). These results in increasedmachiningspeed (Vc) and also increase the gap force (force induceddue to spark). At higher pulse on and arc on time, sparkingoccurred frequently and due to this frequent sparking, rateof melting material increases. This molten metal partially
Table 6. Verification experiments for mathematicalmodel.
Fig. 3. Effect of (a) arc on time (b) pulse on time and o
flushed out due to the high-pressure dielectric flushing andremaining materials stacked on the work surface. As aresult lumping of molten metal takes place which formsdifferent types of globule and making bigger crater in themachining zone. For that reason, the corner error (Ce) andsurface roughness (Ra) both are increasing with theincrease of pulse on time (Ton) and arc on time (Aon).
3.3.2 Pulse off time (Toff) and arc off time (Aoff)
At the same time from Figure 4a and 4b, it is seen thatcorner error (Ce), machining speed (Vc), and surfaceroughness (Ra) all are decreasing with increase in arc offtime (Aoff) and pulse off time (Toff). Increase in pulse offand arc off time results in less aggressive pulse parametersettings and as such, these results in decreased machiningspeed (Vc) and surface roughness (Ra). Due to lessaggressive pulse parameters setting, the gap force as wellas wire deflection reduces, which in turn helps in reducingthe corner error (Ce). Similarly, smaller sizes craters areinduced within the machining zone; as a result, surfaceroughness (Ra) decreases. It is good strategy of machiningat higher pulse off parameter (i.e. Toff & Aoff) setting forbetter corner accuracy and surface finish, but it will cause
n machining speed, corner error and surface roughness.
Fig. 4. Effect of (a) arc off time (b) pulse off time and on machining speed, corner error and surface roughness.
Fig. 5. Effect of (a) servo voltage and (b) wire tension on machining speed, corner error and surface roughness.
6 K. Mandal et al.: Manufacturing Rev. 7, 30 (2020)
frequently wire breakage and unstable machining. Themaximum possible upper limits of pulse off time and arc offtime for this material have been set to avoid these kinds ofproblems.
3.3.3 Servo voltage (Sv)
It is observed from Figure 5a that with increase in servovoltage (Sv) both machining speed (Vc) and corner error(Ce) decreases but surface roughness (Ra) increases. Thiscan be explained by the fact that, with increasing servovoltage (i.e. desired gap voltage) the inter electrode gapvoltage also increases, which in turn increases the gapbetween two electrodes. Due to this increased interelectrode gap, the proportion of pulse energy absorbedby the dielectric also increases. This results in less efficientutilization of pulse energy and consequently less machiningspeed. On the other hand, during sparking the gap force
generated between two electrodes are transferred to theworkpiece, wire and dielectric. With increasing interelectrode gap, amount of force transferred to dielectrictends to be more compared to that of wire. Due to thisphenomenon, there will be less wire deflection andconsequently less corner error (Ce) due to increased servovoltage (Sv). As already mentioned that increased servovoltage (Sv) results in increased gap voltage, which in turnspromote spark discharge at higher voltage, which containsmore energy. These again produce bigger crater andconsequently higher surface roughness (Ra) value.
3.3.4 Wire tension (Wt)
Figure 5b it is observed that with increase in wire tension(Wt) corner error (Ce) reduces significantly, the wiredeflection due to gap force reduces and as such, corner errorreduces.However, no such significant effecthasbeenobserved
K. Mandal et al.: Manufacturing Rev. 7, 30 (2020) 7
on machining speed (Vc) and surface roughness (Ra)during machining of Al 7075 alloy. Nevertheless, it isrequired to give a limiting value of wire tension to avoidthe wire breakage phenomenon under stable machiningcondition.
3.3.5 Servo sensitivity (Sc)
It is observed from Figure 6 that with increase in servosensitivity (Sc) both machining speed (Vc) and corner error(Ce) increases but surface roughness (Ra) decreases. Theservo sensitivity is the intensity with which the WEDMcontroller reacts (in respect of the machining speed) inresponse to the change in error voltage between the desiredgap voltage (servo voltage) and actual voltage. Thus, withincrease in servo sensitivity (Sc) for same error voltage (i.e.for same parameters setting)machining speed (Vc) tends tobe more to correct the error in gap voltage which in turnreduces the gap voltage and consequently the inter-electrode gap (IEG). Thus, the spark discharge takeplace at comparatively lower voltage that will contain
Fig. 7. Corner profile traced by CV 3200 high precis
Fig. 6. Effect of servo sensitivity on machining speed, cornererror and surface roughness.
comparatively less energy and thus results in smaller cratersize. Hence, with increase in servo sensitivity (Sc) there is areduction in surface roughness (Ra) value. At the time dueto this decreased inter-electrode gap, the ratio of the gapforce shared between the wire electrode and dielectricincreases, thus the net force transferred to the wireelectrode increases. Due to this increase in net gap forcewire deflection as well as corner error (Ce) increases withincrease in servo sensitivity (Sc).
It is finally observed that, all parameters except wiretension (Wt), influence machining speed (Vc) and cornererror (Ce) in a similar manner i.e. the parameter thatincreases the corner error also increases the machiningspeed. From the present experimental investigation, it isobserved that higher value of Wt is always desirable forachieving a better corner accuracy but at the same time, itmay be pointed out that there is an upper limit of this valueto avoid wire breakage. A similar trend is observed forsurface roughness (Ra) value in respect of Ton, Aon, Toff andAoff. Thus, it is observed clearly that it is impossible toachieve the best corner accuracy, surface finish along withthe best machining speed. Thus, it is evident that to get thedesired outcome, appropriate trade-off between all theseresponse parameters is extremely essential.
4 Corner error and surface characteristicsanalysis
Figures 7(a&b) and 8(a&b) show the corner profiles andCCI images of WEDM machined surface for two differentpulse on time (Ton) setting. From Figure 7a and 7b, it isclear that the corner error (Ce) of the profile largelydepends on pulse parameter settings, and graduallyincreases as the pulse on time (Ton) increases. Figure 8aand 8b exhibit the topography of Al 7075machined surface.Full of scratch and deep craters, high concentration ofmolten metal (nuggets) and straits have been clearlyvisualised on the machined surface. The concentration ofmolten metal, diameter and depth of the craters depend onthe energy per pulse i.e. on WEDM pulse parametersetting. Corner error and surface roughness both can be
ion contour scope (a) Ton=0.5ms (b) Ton= 0.9ms.
Fig. 8. High resolution CCI image of the machined surface (a) Ton= 0.5ms (b) Ton=0.9ms.
8 K. Mandal et al.: Manufacturing Rev. 7, 30 (2020)
controlled by adjusting pulse parameter setting (i.e. eitherdecreasing pulse on and arc on time or increasing pulse offand arc off time).
5 Conclusions
Single pass rough cutting operation in WEDM of Al 7075has been carried out in this study. The influence of processparameters e.g. pulse on time(Ton), arc on time (Aon), pulseoff time (Toff), arc off time (Aoff), servo voltage (Sv), wiretension (Wt) and servo sensitivity (Sc) on machining speed(Vc), corner error (Ce), and surface roughness (Ra) havebeen investigated. The conclusions of this research workare summarised as follows:
– Pulse on time, pulse off time and arc on time are the maindominating factors on machining speed, corner error andsurface roughness in the operating rage of input factors.Wire tension playing a crucial role on corner error. It isobserved that corner error significantly decreases withthe increase of wire tension. In case of servo sensitivity,machining speed (Vc) and corner error (Ce) are increasesand surface roughness decreases when sensitivity in-creased.
–
The main aim is to find out the best combination ofprocess parameters as known the quality of WEDM,which depends heavily on the input factors. From theexperimental investigation it is observed that in order toget the best corner accuracy, the parameters settingshould be Ton=0.5ms, Toff = 34ms, Aon= 0.1ms, Aoff =30ms, Sv= 20V,Wt=1.6 kg, Sc= 1. To achieve themaximum possible machining speed, parameters valueshould be Ton=0.9ms, Toff = 14ms, Aon= 0.5ms, Aoff =10ms, Sv= 10V,Wt=1.6 kg, Sc= 5 and for minimum
It is impossible to achieve simultaneously the best corneraccuracy and surface finish along with the maximummachining speed. Thus, a systematic trade-off betweenthe response parameters is extremely essential.
–
Majority of the input parameters influences the responseparameters in a similar manner i.e. the change in inputparameters that tends to improve the machiningspeed also deteriorates the corner accuracy and surfacefinish.
This research study will be extremely useful forproducing high precision job with better surface qualityin wire electro-discharge machining of Al 7075 in modernmanufacturing industries.
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