Analysis of parametric effects on response characteristics and … · 5th International & 26 th All...

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

2014, IIT Guwahati, Assam, India

702-1

Analysis of parametric effects on response characteristics and

faults diagnosis during WEDM of Al/SiCp-MMCs

Harmesh Kumar1*, Alakesh Manna2, Rajesh Kumar3

1Deptt.of Mech. Engg., SLIET, Longowal-148106, Punjab (India), Email: [email protected]

2 Deptt.of Mech. Engg., PEC University of Technology Chandigarh-160012, (India), Email: [email protected]

3Deptt.of Mech. Engg., SLIET, Longowal-148106, Punjab (India), Email: [email protected]

Abstract

The paper analyzes the effects of WEDM parameters on response characteristics including cutting

speed, surface roughness, spark gap and wire breakage in wire electric discharge machining of Al/SiCp-

MMC. This study also examines and identifies the various faults occurred on the machined surface and

analyses their causes. The machined workpiece surface and wire electrode surface morphology have been

investigated during machining. Wire deflection, burning spots, band marks,craters formed etc. during

machining are the important faults which deteriorates the machined surface textures. The presence of

percentage of hard reinforced SiC particulates in MMCs and machining parameter pulse on time have great

effects on response characteristics. The wire breakage could be reduced by appropriate setting of parameters

pulse off time andspark gap set voltage. This research outcome suggested the range of parameters for stable

machining of Al/SiCp-MMC with minimum chances of wire breakage and wire deflection. This range can

be utilized to optimize the WEDM parameters for better machining performance. Keywords:WEDM parameters, Al/SiCp-MMC, Fault diagnosis

1 Introduction Metal matrix composites (MMCs) are materials

having the properties of light weight, high elastic

modulus, high specific strength, good wear

resistance, improved stiffness and a low thermal

expansion coefficient, etc. These materials have

gained importance in various fields like aerospace,

defense, automobile and sports.The effective

machining of MMCs is a challenge to the

manufacturing industries which mainly restrict the

wide spread applications of MMCs in

practice.Manna and Bhattacharyya (2005) explained

that Al/SiCp-MMCs are extremely difficult to

machine using traditional machining processes

because of severe tool wear due to the presence of

hard SiC reinforced particle.Many researchers have

tried advanced machining methods to machine the

MMCs and out of which wire electrical discharge

machining (WEDM) emerged as an effective

machining method. Tosun and Cogun (2003)

investigated the effect of machining parameters on

wire wear ratio based on the weight loss of wire in

WEDM and modeled statistically by using

regression analysis techniques. Yan et al. (2005)

used WEDM in the machining of Al2O3p/6061Al

composite. The results show that the cutting speed,

the surface roughness, and the width of slit of

cutting test material significantly depend on the

volume fraction of Al2O3 reinforcement particles.

Manna and Bhattacharyya (2006) used Taguchi L18

orthogonal array and Gauss elimination method for

the parametric optimization during machining of

Al/SiCp-MMCs.Patil and Brahmankar (2010)

experimentally analyzed the effect of electrical as

well as non-electrical machining parameters on

machining performance during WEDM of MMCs.

Pragyaet al. (2012) studied the effect of process

parameters on wire breakage frequency and

microstructure of cut surface during Eco cut WEDM

of SiCp/Al6061.

From the review of the available literature it is

clear that there is no quantity work has been

published for analyzing the effect of each parameter

on response characteristics including wire breakage

separately while machining of Al/SiCp-MMCs on

Sprintcut CNC WEDM. Moreover wire breakage

were found to pose limitation on cutting speed in

these materials but a very few work has been found

that suggests the range of parametric setting in

which effective machining of Al/SiCp-MMCs is

possible with minimum chances of wire breakage.

The present work is therefore an attempt to

experimentally investigate and analyze the effect of

various parameters on the response characteristics

during machining of Al/SiCp-MMCs on WEDM

and identify the suitable parametric range for

effective machining of such MMCs.

2 Experimentation A four-axis ELECTRONICA SPRINTCUT

CNC-wire cut EDM machine is used for

Analysis of parametric effects on response characteristics and faults diagnosis during WEDM of Al/SiCp-MMCs

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experimental investigation. Al matrix material,

Al/10wt%SiCp and Al/20wt%SiCp-MMCs are used

as workpiece materials. The MMCs for the present

investigation were made by stir casting process.

Thickness of all pieces was kept constant as 15mm.

Brass wire of 250µm diameter was used as wire

electrode. The deionized water at temperature in the

range 22– 25°C was used as dielectric fluid. The

peak voltage (i.e. 100V) and flushing pressure (i.e.

15kg/cm2) were kept at constant level during

experimentation.

The main objective of this research work is to

investigate the response characteristics of WEDM

during machining of MMCs (i.e. Al/10wt%SiC&

Al/20wt%SiC) and Al matrix materials. In this

research cutting speed (mm2/min), surface

roughness (Ra, µm), spark gap (mm) and wire

breakage are considered as response characteristics.

The spark gap (mm) is calculated from the

relationship, as follow: 2 x spark gap + diameter of

wire = desired size – actual size (measured). The

input parameters i.e. pulse on time (TON), pulse off

time (TOFF), servo voltage (SV), Peak current (IP),

wire feed (WF) and wire tension (WT) are

considered as controllable parameters and by

varying their setting values experiments are

conducted. This work also investigates the causes of

wire breakage during machining and suggests their

remedial actions.

3 Experimental results and analysis

3.1 Effect of pulse on time (TON) Fig.1 shows the variation of cutting speed with

pulse on time during machining of Al matrix

material and Al/SiCp-MMCs. These experimental

results indicate that average cutting speed increases

with increase in pulse on time. This is because the

discharge energy increases with the pulse on time

leading to faster cutting speed. The cutting speed

was found highest in Almatrix material any value of

pulse on time. Lower cutting speed in Al/SiCp-

MMCs is attributed to lower electrical and thermal

conductivities of these materials as compared to Al

matrix material. It was found that cutting speed was

extremely low when machining was performed at

0.3µs or less setting value of parameter pulse on

time. The frequent wire breakage did not permit to

cut the MMCs at higher level of Pulse on time i.e.

beyond 1.4µs. The broken wire part was found to be

almost burned during machining of Al/20wt%SiCp-

MMCs. Hence it is not recommended to machine

Al/SiCp-MMCs at higher setting value of pulse on

time i.e. beyond 1.4 µs. During machining it is also

found that frequency of wire breakages are more

while machining of Al/20%wtSiCp as compared to

Al/10wt%SiCp-MMCs for same span of machining.

The machining of Al matrix material did not exhibit

wire breakage even at higher level of pulse on

time.Fig.2 shows the effect of pulse on time

onsurface roughness. This figure depicts that high

level of pulse on time results in greater surface

roughness.

Figure 1 Influence of pulse on time on cutting

speed

This is because of high discharge energy in the gap

at higher pulse on time results increased depth of

craters. However in machining of MMCs, the size of

craters may not be the only reason of increased

roughness. The plot shows that at higher pulse on

time i.e. above 1µs, the surface roughness for

Al/20wt%SiCp have decreasing trend. Moreover,

for the Al/20wt%SiCp, it can be seen that machined

surface roughness is worst. These results may be

attributed to the fact that SiC particles in the

machined gap have influence on the surface

roughness. It is believed that the surface roughness

in machining of Al/SiCp-MMCs is determined by

the protruding particles on the machined surface,

depth made due to pullout of particles and due to

depth of crater formed.

1.61.31.00.70.4

5.0

4.5

4.0

3.5

3.0

2.5

2.0

Pulse on time (µs)

Surface roughness, Ra (µm)

SR (Al/SiCp/0%)

SR (Al/SiCp/20%)

SR (Al/SiCp/10%)

SV= 25 V

WT= 1200 grams

WF= 8 m/min

VP= 110 V (DC)

IP= 130 ampere

Toff= 26 µs

x- wire breakage

xx

Figure 2 Influence of pulse on time on surface

roughness

Fig.3 shows the variation of spark gap with

pulse on time. As can be seen, that spark gap

increases with increasing pulse on time. The

narrowest spark gap was obtained with

Al/20wt%SiCp material. Such results may be

attributed to the fact that the spark gap depends

upon discharge energy, thermal and electrical

conductivity of the workpiece material. The thermal

and electrical conductivity of Al/SiCp-MMCs

material are lower than that of Al matrix material.

This is because of reinforcement particles (SiCp)

that have very high melting point and also act as

insulator during machining, hence leading to the



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requirement of much more thermal energy per unit

volume to melt Al/SiCp-MMCs material.

1.61.31.00.70.4

0.055

0.050

0.045

0.040

0.035

0.030

0.025

Pulse on time (µs)

Spark gap (mm)

SG (Al/S iCp/20% )

SG (Al/S iCp/10% )

SG (Al/S iCp/0% )

SV= 25 V

WT = 1200 grams

WF = 8 m/min

VP = 110 V (DC)

IP = 130 ampere

Toff = 26 µs

x

x

x- w ire breakage

Figure 3 Influence of pulse on time on spark gap

3.2 Effect of Pulse off time (TOFF) Fig. 4 shows the variation of cutting speed with

pulse off time, depicting that cutting rate decreases

with the increase in pulse off time in a practically

straight line fashion.

Figure 4 Influence of pulse off time on cutting

speed

It is attributed to the lower pulse frequency at

high pulse off time. Too low pulse off time is not

desirable as the ejected workpiece material will not

be flushed away with the flow of the dielectric fluid

and the dielectric fluid will not be deionized. Hence

a minimum pulse off time is essential for

deionization of the gap. The wire breakage did not

permit to machine Al/SiCp-MMCswhen the pulse

off time set value at or less than 10µs. The

experimental results revealed that the wire breakage

frequency during machining of Al/SiCp-MMCs is

reduced to a great extent with the increase in pulse

off time.

3.3 Effect of spark gap set voltage (SV) The spark gap set voltage is a reference voltage

for the actual gap voltage between the wire

electrode and workpiece. Fig.5 plot cutting speed

verse SV, depicting cutting speed increases

significantly with the decrease in SV for Al/SiCp-

MMCs and Al matrix material. The wire breakage

did not permit to machine any material when SV set

value at or less than 15V. The wire breakage was

found maximum for Al/20wt%SiCp material at all

setting of SV. The result may be attributed to the

fact that too low value of SV results in small gap

between wire electrode and workpiece. Hence

flushing of molten metal is ineffective which may

contaminates the small gap. As a results theremay

be continuous discharge i.e. arcing between wire

electrode and workpieceresults subsequent wire

breakage. The experimental results revealed that

wire breakage is reduced with the increase in SV for

Al/SiCp-MMCs materials.

Figure 5Influence of spark gap set voltage on

cutting speed

Fig.6 shows the effect of SV on surface

roughness indicates that surface roughness

decreases with the increase in SV.


surface roughness

From Fig.7 it is clear that spark gap increases

with the increase in SV for Al/SiCp-MMCs and Al

matrix material as expected.

3.4 Effect of Wire feed (WF) The experimental investigation revealed that wire

feed did not play a vital roleduring machining of

Al/SiCp-MMCs on response characteristics.

However higher value of wire feed rate are required

for working with higher pulse power. It was found

that machining of Al/SiCp-MMCs materials are not

possible with feed rate less than 6m/min. The

frequency of wire breakage decreases with

theincrease in wire feed rate. To facilitate wire

breakage, discharge energy was increased with

different wire feed rate. It was found that

maximumcutting speed decreases from 90mm2/min

to 78mm2/minwhen wire feed rate decreases from

10m/min to 6m/min during machining

ofAl/10wt%SiCp. This effect is attributed to the fact

that reducing the wire speed enables the brass wire

to bear higher discharge energy per unit time,

causing the wire to break. Moreover when fresh

6050403020

4.4

4.0

3.6

3.2

2.8

2.4

2.0

Spark Gap Set Voltage (V)

Surface Roughness, Ra (µm) SR-(Al/SiCp/20%)

SR-(Al/SiCp/10%)

SR-(Al/SiC/0%)

WT = 1200 grams

WF = 8 m/min

VP = 110 V (DC)

IP = 130 ampere

Toff = 26 µs

Ton = 0.9 µs


702-4

wire is supplied, greater is the conductivity resulted

to stable machining without wire breakage.


spark gap

3.5 Effect of wire tension (WT) Test results shown that surface roughness and

spark gap slightly decreases with the increase in

wire tension. This is because of that higher wire

tension avoids the unintentional wire deflection

from its straight path. The wire deflection is caused

due to spark induced reaction forces and water

pressure. Additionally it was observed that

maximum cutting speed decreases as the wire

tension increases. The machining of Al/SiCp-MMCs

is not stable with wire tension set value less than

800 grams.

4 Analysis of surface morphology Fig.8shows the SEM micrographs of the

machined surfaces obtained during two sets of

experiments on Al matrix material, Al/10wt%SiCp

and Al/20wt%SiCp at 0.7µs and 1.3µs pulse on time

respectively by keeping other parameters remain

constant at base level. From these SEM micrographs

it is clear that craters are formed during machining

and which are associated with the discrete

discharges resulting a matt appearance. The craters

and ridge rich surfacesare formed;it may be formed

by the melted material. It is clear that increasing the

pulse on time generates high discharge energy

resulting widening and deeper craters formed on

workpiece surface, it is one of the causes of

deteriorated the surface integrity. The crater size and

hence surface roughness depends upon the discharge

energy. In addition the presence of SiC reinforcing

particles in Al/SiCp-MMCs makes it more distinct

from pure Al machined with WEDM.

Fig.9(a) shows black spots scattered all

over the machined surface. It is believed that these

spots might have occurred as a result of wire

breakage.One of the causes of wire breakage may be

arcing (i.e. continuous discharging energy) due to

contamination of spark gap and leave black spot on

the machined surface. These spot are found when

pulse off time, SV andwire tension set at low value.

Another interesting feature observed on

Al/SiCp/20% was wire shifting marks on the

machined surface as shown in Fig.9(b). These marks

are observed at low discharge energy, low SV and

low pulse off time but these features were generally

not observed at high SV, high discharge energy and

at high wire tension machining conditions. These

marks termed as band marks and adversely affect

the machined surface. It is believed that more SiC

particles within the discharge gap impeded the

advance of the brass wire along the machined path

causing the brass wire to shift as a result a clear

banding were observed on the machined surface

during machining of Al/20wt%SiC. The fluctuation

of actual gap voltage within 10% of SV set value

can be considered as a stable machining

[Technology Manual, Electronica Ltd.]. It was

observed during machining of Al/20wt%SiC, the

actual gap voltage which is indicating on the

machine, was very much fluctuating beyond the

10% limit which shows that the machining is not

stable. This phenomenon may be explained as that

the actual gap voltage sensor may not feedback the

real gap status as the gap is contaminated by SiC

particles. Therefore the system may feed the wire

very fast after a feedback signal of larger gap. This

fast forward speed of wire may cause the wire

electrode to collide with the workpiece material and

form a wire band mark. It is observed that SiC

particles are completely dislodged from the surface

at higher level of discharge energy with the matrix

material which melts and even partially evaporates

during electric discharges. This phenomenon may

be attributable due to differences in thermal

expansion coefficients of SiC particle and Al matrix,

even with the same temperature of both phases, high

thermal stresses develop, usually exceeding the

resistance of reinforcing grain bond with the Al

matrix. Fig9.(c) shows the depth created due to

particle dislodged from the matrix material. A

certain number of SiC particles may not completely

dislodged from the surface and still affixed to the

matrix thus protruding on the machined surface as

shown in Fig.9(d). It is observed that it happens at

lower level of discharge energy. At higher discharge

energy the SiC particles on machined surface were

very rare. At higher setting values of pulse on time,

a micro crack like appearance was observed on the

workpiece machined surface as shown in Fig.9

(e).These cracks may be unacceptable due to fatigue

resistance. This is because of very high tensile

stresses, exceeding the matrix cohesion strength

limit. Fig.9(f) shows the wire entrance mark on the

machined surface. Fig.10 shows the

SEMmicrographs of wire electrode surface after

WEDM on the Al matrix material and Al/SiCp-

MMCs. It is clear that the wire surface discharge

crater becomes deeper and irregular as the

percentage of reinforcing particles increased. In

particular, the wire electrode surface of machining

Al/SiCp/20% exhibited larger discharge craters due

to abnormaldischarge produced by WEDM. It is

6050403020

0.055

0.050

0.045

0.040

0.035

0.030

Spark Gap Set Voltage (V)

Spark gap (mm)

SG-(Al/SiCp/20%)

SG- (Al/SiCp/10%)

SG-(Al/SiCp/0%)

WT = 1200 grams

WF = 8 m/min

VP = 110 V (DC)

IP = 130 ampere

Toff = 26 µs

Ton = 0.9 µs

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


believed that dislodged SiC particles may come in

contact withtravelling brass wire during machining

a. Al Matrix (Ton=0.7 µs)

d. Al/10wt%SiC (Ton=1.3 µs)

Figure 8 SEM micrographs of machined surface with

a. Black spots/ Burning points

d. Protruding particle

Figure 9 SEM micrographs of mac

a. Al/SiCp/0% wt. (Ton = 1 µs)

Figure 10 SEM micrographs of the machined surface of wire electrode

%wt fraction of SiCp contents

Burning Points

Protruding particle

Wear on wire

surface

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12

believed that dislodged SiC particles may come in

contact withtravelling brass wire during machining

and due to abrasive nature of these particles they

produce

b. Al Matrix (Ton=1.3 µs) c. Al/10wt%SiC (Ton=0.7

e. Al/20wt%SiC (Ton=0.7 µs) f. Al/20wt%SiC (Ton=1.3

SEM micrographs of machined surface with %wt fraction SiCp content

b. Band marks or wire deflection c. Particle pullout cavity

e. Crack like appearance f. Wire entrance mark

SEM micrographs of machined surface of Al/SiCp/20% wt

b. Al/SiCp/10% wt (Ton = 1µs) d. Al/SiCp/20% wt (Ton = 1 µs)

SEM micrographs of the machined surface of wire electrode

p contents

Band Mark

Particle

pullout cavity

Crack like appearance

Wire entrance mark

Craters

Deeper and

All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th,

702-5

due to abrasive nature of these particles they

Al/10wt%SiC (Ton=0.7 µs)

Al/20wt%SiC (Ton=1.3µs)

SiCp contents

Particle pullout cavity

Wire entrance mark

hined surface of Al/SiCp/20% wt

d. Al/SiCp/20% wt (Ton = 1 µs)

SEM micrographs of the machined surface of wire electrode for various

Particle

pullout cavity

Wire entrance mark

Deeper and wider craters


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indents or scratches on wire electrode surface.

Hence the strength of wire decreased due to erosion

crater and indents formed on the wire surface which

leads to wire break.

Based on the experimental investigations

and thereafter analysis of the test results a set of

ranges of machining parameters can be

recommended for effective machining of Al/SiCp-

MMCs on Electronica Sprintcut WEDM. Table-

2represents various controllable parameters of

WEDM and their specific proposed ranges for

effective machining of Al/SiCp-MMCs with 250µm

dia. brass wire and at minimum wire breakage.

Table 1 Range of machine input parameters for

stable machining (Sprintcut Machine)

Input parameters Range

Pulse on time (TON) 0.4-1.2 µs

Pulse off time (TOFF) 14-26 µs

Peak current (IP 100-160 amp

Wire feed (WF) 6-10 m/min

Wire tension (WT) 850-1200 gm

Spark gap voltage (SV) 20-40V

Flushing pressure (WP) High (15kg/cm2)

5 Conclusions Based on the experimental investigation and

discussed on the test results during WEDM of

Al/SiCp-MMCs the following conclusions are

drawn as listed below.

1. Higher weight fraction ofhard reinforced

SiC particles in MMCs resulted into

reduced cutting speed; deteriorate surface

finish and increased frequency of wire

breakage. Lower cutting speed in MMCs

is attributed to lower thermal and electrical

properties of these materials as compared

to Al matrix. Moreoverthe machining is

not found to be stable in MMCs due to

presence of large no. of SiCp with the

debris in the gap which increases the

frequency of wire breakage and also

deteriorates the machined surface as

identified in Fig.8.

2. The parameter pulse on time is found to be

a great effect on response characteristics.

Higher setting value of pulse on time

resulted higher cutting speed, deteriorate

surface finish and greater spark gap. This

is because of high discharge energy in the

gap at higher pulse on time results increase

in cutting speed as well as spark gap but it

also produces larger and deeper craters on

the machined surface as identified (Fig.8)

hence increases surface roughness.

3. The setting of parametersPulse off time

and Spark gap set voltage are found to

have greatinfluenceon wire breakage.At

appropriate higher setting values of these

parameters it is found that the chances of

wire breakage are reduced to a great extent

during machining of Al/SiCp-MMCs. This

is due to better flushing of debris from the

gap and enough time for deionization of

dielectrichenceprevents arcing between

workpiece and wire electrode.

4. The surface roughness height, Ra varies

between 1.8 to 3.78 µmduring machining

of Al matrix material. It is in the range of

2.12 - 3.43 µm and 2.82 -4.2 µm when

machining operations are carried out

onAl/10wt%SiC and Al/20wt%SiC-

MMCs respectively.

5. The spark gap variesbetween 0.038 and

0.055mm during machining of Al matrix

material.It is in the range of 0.03-0.05mm

and 0.028-0.047mm whenmachining

operations are carried out on

Al/10wt%SiCp and Al/20wt%SiCp-

MMCs respectively.

6. Wire shifting and wire breakage are

foundthe causes of deteriorating the

machined surface. Wire breakage not only

poses limitation on cutting speed but it

also leaves burning spot (Fig.9a) on the

machined surface. Wire shifting produce

band marks (Fig.9b) which affect quality

of the machined surface.

References

Ho, K.H., Newman, S.T., Rahimifard, S. and Allen,

R.D. (2004), State of the art in wire electrical

discharge machining (WEDM), Int J Machine

Tools & Manuf , Vol.44, pp.1247–1259.

Manna, A., Bhattacharayya, B. (2005), Influence of

machining parameters on the machinability of

particulate reinforced Al/SiC–MMC, Int. J. Adv.

Manuf. Technol., Vol. 25, pp. 850–856

Manna, A. and Bhattacharya, B. (2006), Taguchi

and Gauss elimination method: a dual response

approach for parametric optimization of CNC wire

cut EDM of PR-Alsip-MMC, International Journal

of Advanced manufacturing Technology, Vol. 28,

pp. 67-75.

Patil, N.G. and Brahmankar, P.K. (2010), Some

studies into wire electro-discharge machining of

alumina particulate-reinforced aluminium matrix

composites, International Journal of Advanced

manufacturing Technology, Vol. 48, pp. 537-555.

Shandilya, P., Jain, P.K. and Jain, N.K. (2012), On

wire breakage and microstructure in WEDC of

SiCp/6061 aluminum metal matrix composites, Int.

J. Adv. Manf. Technol, Vol. 61, pp. 1199-1207.

Tosun, N. and Cogun, C. (2003), An investigation

on wire wear in WEDM, J Mater Process Technol,

Vol. 134(3), pp. 273–278.

Technology Manual (2004) for Wirecut EDM-

Sprintcut, Electronica Machine tools Ltd.



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Yan, B.H., Tsai, H.C., Huang, F.Y. and Lee, L.C.

(2005), Examination of wire electrical discharge

machining of Al2O3p/6061Al composites, Int J of

Machine tools & Manf, Vol. 45, pp. 251-259

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