Journal of Scientific & Industrial Research

Vol. 77, June 2018, pp. 325-329

Electric Discharge Drilling of Hybrid Metal Matrix Composites using Different

Tool Electrodes

S Kachhap1, A Singh

1 and K Debnath

2*

1Department of Mechanical Engineering, National Institute of Technology Patna, Patna – 800 005, India 2Department of Mechanical Engineering, National Institute of Technology Meghalaya, Shillong – 793 003, India

Received 27 May 2017; revised 17 November 2017; accepted 18 March 2018

In this paper, the machinability characteristics of hybrid metal matrix composites (HMMCs) (Al6063/SiC/Gr/Al2O3)

have been experimentally evaluated during electro-discharge drilling (EDD) with solid and hollow tool electrodes made of

copper and brass. The influence of different parameter such as (i) discharge current, (ii) tool material, and (iii) tool geometry

on (i) material removal rate (MRR) and (ii) tool wear rate (TWR) has been experimentally investigated. The end face wear

of the tool electrode has also been presented with the help of microscopy imaging. The results of the experimental

investigation indicated that the MRR is improved with hollow brass tool electrode and TWR is reduced with solid copper

tool electrode during EDD of developed composites.

Keywords: HMMCs, EDD, MRR, TWR, SEM

Introduction MMCs have multifunctional properties such as higher

stiffness, toughness, and low-density as compared to

conventional monolithic metals1. But precise machining

of these materials due to the presence of hard

reinforcement is quite challenging and thus, necessitates

the development of cost-effective machining methods.

The EDM method has been developed to machine

complex and precise geometrical features in electrically

conductive difficult-to-machine materials2,3

. An extensive

study has been carried out in the context of EDM of

advanced materials. But a few investigations have been

conducted in the area of machining of MMC using

different tool electrodes by means of EDD. Mohanty

et al.4 evaluated the machinability characteristics of

Inconel 718 super alloy in terms of MRR and surface

quality during EDM. The statistical analysis of the

experimental results showed that the (i) tool material,

(ii) discharge current, and (iii) pulse-on-time are the

important parameters influence the MRR and surface

quality. Gopalakannan and Senthilvelan5 studied the

influence of major machining parameters in EDM of

aluminium/SiC MMC using copper tool electrode. The

main objective of the study was to identify the

significance of machining parameters on

(i) MRR, (ii) electrode wear ratio, and (iii) surface

roughness. Dewangan et al.6 found that the surface

roughness obtained with copper and graphite electrodes

was better than the brass electrode during EDM of AISI

P20 tool Steel. Khanna et al.7 adopted Taguchi method

to optimize the different input parameters for better

MRR and TWR during EDM of Al 7075. The statistical

analysis showed that the MRR and TWR were

significantly affected by the pulse-on and pulse-off time.

Kuriachen and Mathew8 studied the influence of powder

mixed dielectric on MRR and surface modification

during micro-EDM of Ti-6Al-4V using tungsten carbide

electrode. Another research reported on near-dry-EDM

of HSS9. It was concluded that the TWR is negligible in

near-dry-EDM. The better surface finish and thinner

recast layer can also be obtained by near-dry-EDM

process when compared one-on-one with conventional

EDM process. Singh et al.10

investigated the influence of

various operating parameters on TWR during powder-

mixed EDM of Al 6061/SiC MMC. It was observed that

the powder-mixed EDM of developed composites

provides a significant reduction (51.12%) in TWR as

compared to basic EDM process. Rahul et al.11

studied

the influence of cryogenically treated copper tool

electrode on surface integrity and metallurgical

characteristics during EDM of Inconel 825. The

performance of rotary-tool-electrode during EDM

process has also been studied for hard materials such as

Inconel, tool steel, and polycrystalline diamond12-14

.

From the present discussion, it can be inferred that the

___________

*Author for Correspondence

E-mail: debnath.iitr@gmail.com

J SCI IND RES VOL 77 JUNE 2018

326

EDM process and it’s variants have been applied to

study the machinability characteristics of verity of

materials such as conventional monolithic metals, alloys,

MMCs etc. But the performance of EDM process has

not been studied for HMMCs. Thus, in the present work

an attempt has been made to investigate the influence of

different parameters during EDD of HMMCs. The

influence of discharge current, tool electrode material

and geometry on MRR and TWR has been

experimentally investigated.

Experimental details

The experiments were conducted on ZNC-EDM

machine (Sparkonix India Pvt. Ltd., Pune). A rotary

setup was designed and developed to provide rotation

to the tool electrode at different levels.

This setup is then retrofitted with the EDM machine.

The rotary setup consists of a universal motor, v-pulley,

v-belt, shaft, shaft cashing, aluminium plate, and tool

holder, as shown in Figure 1(a). Highly conductive

materials were considered to fabricate different

components of the rotary setup. Two different types of

tool electrodes such as hollow and solid tool electrode of

copper and brass have been used for making through

holes of 8 mm in diameter in developed HMMCs. The

dimensions of the tools are shown in Figure 1(b). The

different parameters considered for the purpose of

investigation are shown in Table 1. The basic principal

of EDD is similar to EDM. The power supply in the

machine is controlled by RC circuit. The spark gap of

0.05 mm is maintained by a computer controlled

positioning system. The HMMCs was developed using a

standard stir and squeeze casting process.

Results and Discussion

Analysis of material removal rate

The variation of MRR with discharge current for

solid cylindrical copper and brass tool electrodes is

shown in Figure 2(a). The other parameters are kept

constant as shown in Table 1. It is evident in the

figure that MRR increases with a gradual increase in

discharge current for both tool electrodes. The

discharge current is a significant parameter of EDD

process as it is a function of spark energy. The spark

energy increases with discharge current which leads

to formation of higher volume of crater. The localized

temperature is increased at high discharge current

which results higher rate of melting and vaporization.

It was also observed that the MRR with brass tool

electrode was more than the copper tool electrode

with increasing pulse current. The plausible reason is

that the brass tool electrode has relatively high

specific resistance as compared to the copper tool

electrode. This results in an increase in spark intensity

at the inter-electrode gap. The discharge drilling

operation is more stable and uniform spark energy is

distributed on the work surface at a lower value of

discharge current. The debris of smaller size is

removed from the work surface and thus low MRR.

The spark energy becomes quite unstable as the

discharge current increases. The material melts and

vaporizes rapidly and bigger size debris is removed

Table 1 — Parameters of EDD process

Parameters Values

Discharge current 6-30 amps

Pulse-on-time 120 µs

Pulse-off-time 45 µs

Gap voltage 50 V

Tool rotational speed 600 RPM

Depth of machining 8 mm

Tool electrode materials Copper and brass

Tool electrode geometry Solid and hollow cylindrical

Workpiece

Hybrid Metal Matrix Composites

(Matrix: Al6063 and Reinforcement:

SiC (5%), Gr (2.5%), Al2O3 (2.5%),

remaining Al 6063)

Fig. 1 — Experimental setup (a) electro-discharge drilling setup and (b) dimensions of the tool electrodes

DEBNATH et al.: ELECTRIC DISCHARGE DRILLING OF HYBRID METAL MATRIX COMPOSITES

327

from the work surface and thus results in increased

MRR. The debris formed during EDD of HMMC

consists of non-conductive reinforcement particles.

These particles left loose after vaporization of the

matrix material which may also results in erratic

discharge. Figure 2(b) illustrates the influence of the

tool electrode geometry (solid and hollow) on MRR

during EDD of HMMC. It was observed that the

machining with hollow tool electrodes (both copper

and brass) resulted in higher MRR as compared to

solid tool electrodes. The cross-sectional area of the

hollow tool electrode is smaller than the solid tool

electrode. Thus, minimum area is available for

discharge of current. The material is only removed by

the peripheral end face of the hollow tool electrode

and the remaining material is removed in the form of

cylindrical rod. This leads to higher removal of

material from the work surface while machining is

performed with the hollow tool electrodes.

Analysis of tool wear rate

The influence of discharge current on TWR for

different tool electrode materials chosen for the purpose

of investigation is shown in Figure 2(c). It was observed

that discharge current has a direct effect on the TWR

i.e., TWR increases with discharge current for both tool

electrodes. It is also envisaged from the figure that the

wear of the brass tool electrode is substantially high as

compared to copper tool electrode. The tool electrode

wear depends on both melting point and thermal

conductivity of the tool electrode material. The thermal

and electrical properties of the tool electrode materials

are shown in Table 2. The copper tool electrode has

higher thermal and electrical conductivity as compared

to the brass tool electrode. Thus, a larger amount of heat

is produced between the tool and work surface as the

value of discharge current is increased. The tool

electrode wear can be minimized if the temperature at

the inter-electrode gap can be controlled. Figure 2(d)

Table 2 — Properties of tool electrode materials

Properties Copper Brass Unit

Electrical conductivity 58.5 15.9 10E6 Siemens/m

Electrical resistivity 1.7 6.3 10E-8 Ohm.m

Thermal conductivity 401 150 W/mK

Thermal expansion

coefficient

17 20 10E-6 K-1

Density 8.9 8.5 g/cm3

Melting point 1083 900 °C

Fig. 2 — Variation of MRR and TWR with discharge current and different tool electrodes (a) MRR vs. discharge current, (b) MRR vs.

different tool elcetrodes, (c) TWR vs. discharge current, and (d) TWR vs. different tool elcetrodes

J SCI IND RES VOL 77 JUNE 2018

328

shows the variation in TWR for different tool electrode

geometry (solid and hollow). It is evident in the figure

that the TWR for the hollow tool electrode is more than

the solid tool electrode. It is also pertinent to mention

that the wear of solid and hollow copper tool electrode is

relatively low as compared to the brass tool electrode.

The specific spark energy for hollow tool electrode is

relatively high as compared to the solid tool electrode.

At short-pulse duration, the spatial current density of the

hollow tool electrode is larger than the solid tool

electrode. The wear of the tool electrodes primarily

occurs at the corner and end face of the tool electrode, as

shown in Figure 3(a). This type of wear is more when a

tubular tool electrode is used. During EDD, the inner

and outer edges of the hollow tool worn out owing to

spark erosion and continued abrasion by the formed

debris. This causes higher TWR with hollow tool

electrode. Moreover, the melting point of brass tool

electrode is lower than that of the copper tool electrode.

The scanning electron microscopy examination studies

were conducted to understand the surface characteristics

of the eroded tool electrode. Figure 3(b) shows the

surface characteristics of the eroded end face of the

copper hollow tool electrode. It is quite clear that the

wear is more predominant at the edge and bottom face

of the tool electrode. The micrographs clearly indicate

the edge rounding of tool electrode. The electrolyte

flushes away the formed debris containing hard carbide

particles during pulse-off-time. The edges of the tool

electrode get rounded due to continuous abrasion

exerted by the formed debris particles. The corner wear

of tool electrode causes diffusion of heat. This type of

wear of tool electrode is more obvious when thermal

conductivity of the tool electrode is low. A substantial

damage in the form of cracks was also observed at the

tool end face due to quick heating and cooling that is

induced at higher discharge current.

Conclusions In the present study, the influence of discharge

drilling process parameter on the MRR and TWR for

different tool geometries made of copper and brass has

been investigated. The following inferences can be made

from the present investigation.

• Higher discharge current is suggested to obtain the

maximum MRR during EDD of HMMCs.

• The brass tool electrode is better than copper tool

electrode due to improved discharge current.

Machining with brass tool electrode resulted in

higher MRR as compared to copper tool electrode in

all condition.

• The performance of hollow tool electrode is better than

the solid tool electrode for achieving higher MRR.

• Minimum tool wear rate can be obtained by

reducing the discharge current. The wear of copper

tool electrodes is less as compared to the brass tool

electrodes. Also, wear of hollow tool electrodes are

more than the solid tool electrodes.

• The wear of the tool primarily occurs at the corner

point and bottom face of the tool electrodes. The

wear in the form of rounded edge, cracks etc. have

been observed during EDD of HMMCs.

Fig. 3 — Tool electrode wear (a) wears phenomenon and (b) SEM images of the worn surface

DEBNATH et al.: ELECTRIC DISCHARGE DRILLING OF HYBRID METAL MATRIX COMPOSITES

329

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