study of recast layers and surface roughness on al-7075 metal matrix composite during edm machining

International Journal of Recent advances in Mechanical Engineering (IJMECH) Vol.3, No.1, February 2014

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STUDY OF RECAST LAYERS AND SURFACE ROUGHNESS

ON AL-7075 METAL MATRIX COMPOSITE DURING

EDM MACHINING

Rajesha S

1, C.S. Jawalkar

2, Radha Raman Mishra

1, A.K. Sharma

3, Pradeep

Kumar3

1Department of Mechanical Engineering, JSS Academy of Technical Education, Noida,

India 2Department of Production Engineering, PEC University of Technology, Chandigarh,

India 3Department of Mechanical and Industrial Engineering, Indian Institute of Technology,

Roorkee, India

ABSTRACT

Demand for low weight to strength ratio for exclusive application in automobile and aerospace components

created scope for advance studies on Metal Matrix Composites (MMCs). However, machining of MMCs

challenges for conventional machining processes due to uneven hardness, matrix of composites and

undesired tool life. Electric Discharge Machining (EDM) is one of the unconventional material removal

process, which offers alternatives for machining of MMCs because, being a non-contact process it can be

machine excessive harder material, intricate shape with better surface finish and tool life. The evolved heat

during EDM process and subsequent flushing of machined debris and rapidly resolidified layer is formed

by the re-solidification of residual molten material on the machined surface during the EDM process leaves

behind recast layers which is extremely hard and brittle. In the presents study optimization of process

parameters to minimize the surface roughness of the rapidly resolidified layer of Al 7075 MMC while

machining using EDM process is carried out using the Taguchi tequniues. The gap current, pulse on-time

and pulse off-time were considered as process parameters. The formation of resolidified layers is

investigated in terms of the surface roughness as response variables inside the hole and near the hole while

EDM drilling.

KEYWORDS

MMCs, EDM, Recast layer, Taguchi Technique.

1. INTRODUCTION

In EDM process material removal is due to melting and vaporization of workpiece material,

however presence of dielectric flushing flied created rapid cooling close to machined surface

produces the resolidification of the molten material as layers on the machined surface. The

thickness of the recast layer is varies with machining parameters such as peak current and pulse

on-time [1–3]. The recast layer thickness increases with increasing peak current, pulse on-time

and effect of dielectric flushing [1]. The average recast layer thicknesses increases with increase

in pulse energy, higher discharge current and longer duration of pulse-on. The micro cracks on

recast layers excessive amount of carbon presence, while using hydrocarbon-based dielectric fluid

[2]. The changes in pulse on-time durations is more significant factor than changes in the peak

current for variation in the recast layer thickness and it is observed that the majority of cracks


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terminated within the recast layer or at the interface of the recast layer [3]. The thickness of

layers observed about 10 µm when parameter like pulse current of 8 Amps and pulse on-time of 8

micro second [4]. Use of brass electrode shows the better material removal rate then the copper

electrode while EDMing 2124 Al alloy matrix reinforced with SiC in volume Fractions of 15%

and 25%. The EDM is a feasible process causing surface softening at lower cutting speeds, but at

higher speed caused micro damage in the surface and subsurface areas [5]. The effect of process

parameters on metal removal rate, re-cast layer and surface finish was studied with statistical

models of the EDM process and the pulse current is critical factor affect the surface finish [6].

During the study of machining of Al/SiC MMC's, it was observed that EDM process was slow

but suitable for machining the composite under specified conditions with a crater-like surface and

the size of the crater increased with an increase in discharge energy [7]. The increase in Material

Removal Rate (MRR), Tool Wear Rate (TWR) and better surface finish with the increase in the

flushing pressure, reported that improve MRR [8]. During machining of 6061 Al-MMC with 10%

SiC reinforcement the MRR and TWR were higher for larger pulse current and pulse on-time

levels, at the same time it leads to taper, radial overcut and poor surface finish [9]. The tungsten

carbide-cobalt (WC-Co) composite material, having very high hardness and excellent resistance

to shock wear, it was reported that the increase in pulse on-time increases the machining

instability due to higher rate of melting and solidification and leads to poor surface finish [10].

While investigating machining characteristics, surface integrity and material removal mechanisms

of advanced ceramic composite (Al2O3–SiC–TiC) with EDM process show that the surface

roughness and recast layer thickness increases with increase in discharge current and pulse on-

times and also rough EDMing leads to thermal spell and poor surface integrity [11]. Surface

integrity of EDM processed materials needs further investigation.

In the present study, Al 7075 MMC is selected as a workpieces for machining using EDM process

for processing the hole. The experiment is designed with Taguchi methods and gap current; pulse

on-time and pulse off-time were selected as control variable and surface roughness as a response.

The surface roughness of recast layer is investigated inside and near the hole surface. The

ANOVA analysis was carried out to analyze the surface roughness inside and near the hole

surface. The optimization has been carried out on process variables.

2. MATERIALS SELECTION Material selection is one of the impotent processes for any investigation based on the recent

development and there end applications. The Al-7075 MMCs of hardness 120.9 HV at load

100gms are considered for present investigation due to exclusive use of automobile components.

The composition of Al-7075 is shown in Table 1. The spark fusion oil rated 450 is used as a

dielectric fluid for EDM process and electrolytic copper of 6 mm diameter as an electrode for

present investigation.

Table1. Composition of Al-7075

3. EXPERIMENTATION

The present investigation EDM machine (Model No. EMS-5030, compact type) is used for

machining Al 7075 metal matrix composite. The die casted Al 7075 is initially cut in to

50x20x5mm (Length x width x thickness) and fixed into machine table. The electrolytic copper


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electrode is fixed in to Z-axis and auto positioning means placing the electrode and workpiece in

a predefined position with respect to each other. The depth of cut has been fixed as 2mm. The

experiments were planned and conducted as per Taguchi’s standard orthogonal array L-9 and

were repeated twice for adequacy. To determine the optimal set of the process parameters like

pulse current, pulse on-time and pulse off-time, that result in the minimum recast layer surface

roughness during machining of Al 7075 metal matrix composites, the process parameters and

their levels are shown in Table 2. The average values of surface roughness were measured using a

perthometer both inside the holes and near the holes EDM machined surface and results are

tabulated in Table 3.

Table 2. Experimental parameters and their levels

Level Gap current (I) Pulse-On time (Ton) Pulse-Off time (Toff)

Amp µs µs

1 5 10 3

2 15 15 6

3 25 20 9

• Supply voltage - 420 V, 3-phase, 50Hz

• Open gap voltage - 140±5% tolerance

• Electrode - Electrolytic copper, cylindrical shape in 6mm dia

• Dielectric - spark fusion oil Rated 450,

• Dielectric pressure - 250 N/m2

• Depth of cut - 2mm

• Gap width - 0.05mm

Table 3. Experimental Results for Recast Surface Roughness (Ra values) for both Inside and Outside Hole

3.1. Analysis of Recast Layer Surface Roughness Inside the Hole

ANOVA has been carried out for effect of process parameters on surface finish inside the hole.

The analysis shows that pulse off-time has the maximum effect of 39.47%, followed by gap

current 17.35% and pulse on-time 11.78% shown in the Figure 1.


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Table 4. ANOVA S/N Ratio (Inside the Holes)

Figure 1. Effect of parameters on the recast layer surface layer roughness inside the hole

The effect of gap current on surface roughness is shown in the Figure 2. The initial trend shows

the roughness marginally increases with increasing gap current. The current exceed 15A the

roughness decreases, due to high energy in the gap provides the stable condition for material

removal of all alloys of the composites. This is also evident in increasing the pulse on-time

(15µs) marginally improves the surface finish (Figure 3) indicate that high energy requirement,

while machining Al-7075 metal matrix composites. At low energy status many alloys of the

composites may not be evaporate or melt and flushed out. Lower the level of on-time shows

better the surface finish due to higher flushing time removes the debris and minimal recast layers

formation. Lower level of off-time imply higher on-time, quality of the surface finish decreases

(Figure 4) due to high melting rate and decrease of flushing time leads to high rate of recast layer

formation.

Figure 2. Gap Current Vs Ra and S/N ratio


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Figure 3. Pulse on-time Vs Ra and S/N ratio

Figure 4. Pulse off-time Vs Ra and S/N ratio

3.2 Analysis of Recast Layer Surface Roughness Near the Hole

ANOVA and signal to noise ratio and mean effect on surface finish near the hole is tabulated in

Table 5. It is shows that Pulse off-time has the maximum effect (35.34%), Pulse on-time (16.21%

) and Gap current (9.19%) contributions respectively on surface finish as shown in the Figure 5.

Table 5. ANOVA S/N Ratio (near the holes)

SOURCE SS DOF V P F-Ratio

Gap Current 2.9625 2 1.4811 9.1907 0.23414

Pulse On time 5.2259 2 2.6129 16.2152 0.41307

Pulse Off time 11.3903 2 5.6951 35.3419 0.90039

ERROR 12.6515 2 6.3257 39.2539

Total 32.2299 8 100

SS= Sum of squares, DOF=Degree of Freedom, V= Variance, P=Percentage,

SST: 101.3865, T: 86.05813, CF:822.8891


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Figure 5. Effect of parameters on the recast layer surface layer roughness near the hole

The effect of process parameters near the hole were analysed, the gap current increases the

surface quality decline (Figure 6) due to spell out of hot debris from the hole and solidifies

quickly nearby surface. Duration of pulse on-time increases surface roughness decline marginally

and further increase of pulse on-time quality of the surface improves (Figure 7). Increase of pulse

off-time above 6µs there is more time to solidified near hole and roughness increases (Figure 8).

Figure 6. Gap Current Vs Ra and S/N ratio

Figure 7. Pulse on-time Vs Ra and S/N ratio


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Figure 8. Pulse off-time Vs Ra and S/N ratio

3.3. Confirmation Experiments

Three confirmation experiments were conducted at the optimum levels of the process parameters.

The mean value of recast layer surface roughness inside hole and near hole by the optimal setting

of the process parameters was found within the confidence interval of the predicted optima of

quality characteristics. Process optimization for inside the hole and near the hole is tabulated in

Table 6.

Table 6. Optimum value of Process Parameters for both Inside and Near the Hole

4. RESULT AND DISCUSSION

The relative effect of control parameters on recast layer and surface roughness inside the hole and

near the hole during machining of Al 7075 metal matrix composite while machining using EDM

process. It shows that all the three parameters have strong influence on recast layer surface

roughness in both the cases.

Inside the hole and near the hole, average value of ‘Ra’ of recast layer is the minimum at gap

current 25 Amps and 5 Amps. It may be due to recasting of removed material in the form of

layers after being machined from the metal matrix composite more in the hole in comparison to

near the hole surface which leads to ineffective material removal in the hole ineffective flushing

of dielectric fluid, so at higher gap current poor surface finish is produced at near hole surfaces

due to high material removal rate.

The average value of ‘Ra’ of recast layer with pulse on-time has the minimum value 10 µs for

inside the hole and 20 µs for near the hole. As the pulse on time increases, the energy available

for spark increases. Inside the hole, increase in pulse on-time increases the Ra value because of

increase in material removal rate but further Ra value decreases as no fresh material removal takes

place and only recast layer is being removed again and again. Near the hole, the increase in pulse

on-time reduces the effective material removal drastically because of recasting of removed

material more in comparison of inside the hole.


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The pulse off time is having minimum Ra value in both the cases (inside and near the hole) at

6µs. Initially it was observed that surface roughness decreases by decreasing the pulse off-time, it

may be because the material melted from metal matrix composite had not got adequate time to get

recasted in large amount but after 6 µs there was sufficient time for recasting so surface

roughness started increasing with increase in pulse off-time.

Since, among all three parameters pulse on-time has least influence on recast layer surface

roughness inside the hole and gap current has least influence on recast layer surface roughness

near hole surface, therefore nominal level of pulse on-time and gap current for optimal recast

layer surface layer is justifiable.

5. CONCLUSION

The recast layer surface roughness of Al 7075 metal matrix composites has been investigated

using the Taguchi method. Effect of process parameters like gap current, pulse on-time and pulse

off-time on surface roughness have been studied and optimization were carried out. The

following conclusions were drawn from the investigation:

1. The optimum levels of the gap current (25A), pulse on-time (20µs) and pulse off-time

(6µs) has been established for inside the hole getting minimum recast layer surface

roughness during machining of Al 7075 metal matrix composite during EDM machining.

2. The optimum levels of the gap current (5A), pulse on-time (15µs) and pulse off-time

(6µs) has been established for near the hole getting minimum recast layer surface

roughness during machining of Al 7075 metal matrix composite during EDM machining.

3. The minimum recast layer surface roughness is found with gap current 15 Amps, pulse

on-time 10 µs and pulse off-time 6 µs inside the hole. The current 15 Amps, pulse on-

time 15 µs and pulse off-time 6 µs for near the hole case.

4. Roughness profiles indicate that minimum value of Ra and Rz is found at run 4 (both

near the hole and inside the hole).

5. The Surface Roughness initially increases rapidly with an increase in pulse off-time and

then decreases slowly with an increase in pulse off-time.

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Authors

First Author Name – Dr. Rajesha S

Job title – Professor

Organization/institution – JSS Academy of Technical Education, Noida, India

Biographical note – Presently working as a Professor and Head in Department of

Mechanical Engineering at JSS Academy of Technical Education, Noida. I have

received my B Tech, Mysore University M Tech, from VTU Belgam and Ph.D, from

Indian Institute of Technology, Roorkee. My research interests are including Advance Machining

Processes,

Composite Material and Hybrid Machining Process.

Second Author Name – Dr. C.S. Jawalkar

Job title – Assistant Professor

Organization/institution – PEC University of Technology, Chandigarh, India

Biographical note – Presently working as an Assistant Professor in the Department of

Production Engineering at PEC University of Technology, Chandigarh, India. I have

received my Ph.D, from Indian Institute of Technology, Roorkee. My research interests are including

Advance Machining Processes, Composite Material and Hybrid Machining Process.

Third Author Name - Radha Raman Mishra

Job title – Assistant Professor

Organization/institution – JSS Academy of Technical Education, Noida, India

Biographical note – Presently working as an Assistant Professor in Department of

Mechanical Engineering at JSS Academy of Technical Education, Noida. I have received

my M. Tech degree from Indian Institute of Technology, BHU, and Varanasi. My research interests are

including Composite Materials, Advance Machining Processes and Microwave Material Processing.

Fourth Author Name – Dr. A K Sharma

Job title – Associate Professor

Organization/institution – Indian Institute of Technology Roorkee, Roorkee, India

Biographical note – Presently working as an Associate Professor in Department of

Mechanical and Industrial Engineering at Indian Institute of Technology Roorkee, Roorkee,

India. I have received my B Tech, M Tech, and PhD from Indian Institute of Technology,

Madras, Chennai, India. My research interests are including Advanced Manufacturing Methods, Hybrid

Finishing Processes, Microwave Material Processing, and Surface Engineering.


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Fifth Author Name – Dr. Pradeep Kumar

Job title – Professor

Organization/institution – Indian Institute of Technology Roorkee, Roorkee, India

Biographical note – Presently working as a Professor in Department of Mechanical and

Industrial Engineering at Indian Institute of Technology Roorkee, Roorkee, India. I have received my B

Tech, M Tech, and PhD from Indian Institute of Technology, Roorkee, India. My research interests are

including Quality Engineering, Supply Chain Management, Advanced Manufacturing Processes, Metal

Casting, Industrial Engineering, Supply Chain Management, Quality Engineering, and Advanced

Manufacturing Process.