Thermal Stress Analysis of Electro Discharge Machining

*
Corresponding author e-mail: mehulgmehta@yahoo.co.in Journal access: www.ijesft.com
Tel.: +91 0000000000 © 2015 A D Publication. All rights reserved
ID:IJESFT20150101005 January 2015
International Journal of
Engineering Science and Futuristic Technology
A Peer-reviewed journal
ISSN : 2454-1125
IJESFT 01 (2015) 034-042
Thermal Stress Analysis of Electro Discharge Machining
Prof. Mehul G Mehta *,1
, Dr. Jeetendra A Vadher2
, Prof. Bharat M Trivedi3
,Dr. Hemant S Trivedi4
1,2,3Department of Mechanical Engineering, S.S. Engg .College, Bhavnagar, Gujarat, India
4 Department of Production Engineering, S.S. Engg .College, Bhavnagar, Gujarat, India
A B S T R A C T : Procedures and results of experimental work to find thermal stress analysis in electric
discharge machined surfaces are presented. In this study, an axisymmetric thermo-physical FEA
model for the simulation of single sparks machining during electrical discharge machining (EDM)
process is shown. This model has been solved using ANSYS 14.0 software. A transient thermal
analysis assuming a Gaussian distribution heat source with temperature-dependent material
properties is used to investigate the stress analysis based on temperature distribution. The effect
on significant machining parameters (Gap current – Gap voltage) on aforesaid responses had been
investigated and found that the stresses sharply changes with the parameters [1]
.
The FEA model is used to study the relation between these parameters and maximum
temperature attended at the end of cycle which is further used to find residual stress produced at
the end of cooling cycle. To find residual stresses in the work piece during EDM, the temperature
distribution at the end of pulse duration in the work piece has to be estimated [2]
High residual thermal stresses are developed on the surfaces of electric discharge machined
parts because of the high temperature gradients generated at the gap during electrical discharge
machining (EDM) in a small heat-affected zone. These thermal stresses can be responsible for
micro-cracks, decrease in fatigue life and strength and possibly catastrophic failure. The results of
the analysis show high temperature gradient zones and the regions of large stresses where,
sometimes, they exceed the material yield strength. A transient thermal analysis assuming a
Gaussian distribution heat can be used to investigate the Stress analysis.
© 2015 A D Publication. All rights reserved
Keywords:
Electrical discharge machining (EDM), Finite Element Method (FEM), Material removal rate (MRR),
Temperature distribution, Thermal stresses
1. Introduction
Electrical discharge machine (EDM) is an important ‘non-traditional manufacturing method’, developed in the
late 1940s and has been accepted worldwide as a standard processing manufacture of forming tools to produce
plastics moldings, die castings, forging dies and etc.
It provides an effective manufacturing technique used for the production of parts made of hard materials with
complicated geometry that are difficult to produce by other machining processes. Because of its ability to control
the process parameters to get the required dimensional accuracy and surface finish it is placed at a prominent
position in industrial applications and effectively used in a wide range of industries such as die and mould-making,
aerospace, automotive, medical, micromechanics, etc. [3]
The complex nature of the process involves simultaneous interaction of thermal, mechanical, chemical and
electrical phenomena, which makes process modelling very difficult. An electrical discharge machining (EDM) is
based on the eroding effect of an electric spark on both the electrodes used. It is actually is a process of utilizing
the removal phenomenon of electrical-discharge in dielectric. The main mode of erosion is caused by the local
thermal effect of an electric discharge.

35 M G Mahta et al/ International Journal of Engineering Science and Futuristic Technology
Nomenclature
EDM Electrical discharge machine
HSS High Speed Steel
Ub Breakdown (discharge) voltage
I Current
Rw Energy partition to the work piece
T Temperature
t Time
ρ Density
k Thermal conductivity
C Specific heat capacity of workpiece material in solid state
The erosion by an electric discharge involves phenomena such as heat conduction, energy distribution,
melting, evaporation, ionization, formation and collapse of gas bubbles in the discharge channel [19].
EDM is a thermal process where thermal energy is generated in a discharge channel, called plasma channel.
The tool is made cathode and work piece is anode. As the voltage across the gap becomes sufficient it
discharges through the gap and the spark is generated in interval of 10 micro seconds and positive ions and
electrons are accelerated, which produces a discharge channel that becomes conductive. It is just at this point
when the spark jumps there are collisions between ions and electrons and a channel of plasma is created. A
sudden drop of the electric resistance of the this channel allows that current density to reach at very high level
producing an increase of ionization so it creates a powerful magnetic field [4,5].At this moment sufficient pressure
develops between work and tool as a result of which a very high temperature is reached and at such high
pressure and temperature that some metal is melted and vaporised.
Heat generated in the plasma channel in each spark, causes the work-material to melt. Extremely high
temperature resulted due to transient heat flux, induces thermal stresses within the heat-affected zone, which is
the most potential zone of initiation of network of micro-cracks [18]. Microscopic studies reveal multi-layered heat-
affected zone including a hardened layer that reduced fatigue strength and possesses high brittleness.
In this context, analysis of temperature and thermal stress profiles in the workpiece are of considerable
interest to study.
1.1 important parameters of EDM
There are different parameters like spark on time, spark off time, breakdown voltage, gap current, duty cycle
etc. Which are play very vital role in erosion of material [6] are presented below.
1. Spark On-time (pulse time or Ton): The duration of time (μs) for which the current is allowed to flow per
cycle. Material removal is proportional to the amount of energy given during this on-time.
2. Spark Off-time (pause time or Toff): The duration of time (μs) between the sparks (that is to say, on-time).
During this time the molten material solidifies and washed out of the arc gap. The speed and the stability of the
cut depend on this parameter.
3. Discharge current (current Ip): It is the Current measured in amp per cycle. Discharge current is directly
proportional to the MRR
4. Arc gap (or gap): It is distance between the tool electrode and workpiece during the process of EDM. It
may be called as spark gap. Servo mechanism is used to maintain this gap.
5. Duty cycle (τ): It is a ratio of the on-time to the total cycle time. This parameter is calculated by dividing the
on-time by the total cycle time i.e. on-time plus off time.
6..Over cut : It is a clearance per side between the electrode and the workpiece after the marching operation.
1.2 Tool material
Tool material should be selected such that it should undergo minimum wear due to impingement of positive
ions so there would be less material removal or tool wear per cycle so less dimensional loss or inaccuracy for the
same heat load and same tool wear by weight. It should have high electrical and thermal conductivity. [7]
There should be less localized temperature rise by properly choosing its properties or even when

M G Mahta et al/ International Journal of Engineering Science and Futuristic Technology 36
temperature increases, there would be less melting. Further, the machining of tool should be easy as intricate
shaped geometric features are machined in EDM.
1.3 Work Material
Geometrically complex and hard material components which are precise and difficult to machine such as tool
steels, composites, super alloys, ceramics, carbides, heat resistant steels etc can be machined with EDM.
It has been accepted worldwide as a standard processing manufacture of forming tools to produce plastics
moldings, die castings, forging dies and etc.
At present , Electro discharge machine (EDM) is one of the most widely used technique in industry for high
precision machining of all types of conductive materials such as metals, metal alloys, graphite, or some ceramic
materials of any hardness [8,9]. Their suitability comes from their distinctive hardness, resistance to abrasion, their
ability to hold a cutting edge, and/or their resistance to deformation at elevated temperatures (red-hardness). Tool
steel which are generally used in a heat-treated state made to a number of grades for different applications. In
general, the edge temperature under expected use is an important determinant of both composition and required
heat treatment. The higher carbon grades are typically used for such applications as stamping dies, metal cutting
tools, etc.
1.4 Dielectric fluid
Material is mainly removed because of thermal evaporation and melting. There can be poor surface
conductivity (electrical) of the work piece because of oxidation which hindering further machining [4] so The
machining is required to be carried out in absence of oxygen so that the process can be controlled without
oxidation. Hence, dielectric fluid should provide an oxygen free machining environment. It should also have strong
dielectric resistance so that it does not breakdown electrically so easily but at the same time ionize when
electrons collide with its molecule. Dielectric medium is generally flushed around the spark zone The electrode
wear rate, metal removal rate and other operation characteristics are also influenced by the dielectric fluid. [10].
The dielectric fluid has the following functions [11]:
(a) It initiates discharge by serving as a conducting medium when ionized, and conveys the spark.
(b) It helps in quenching the spark, cooling the work, tool electrode and enables arcing to be prevented.
(c) It carries away the solidified EDM debris along with it.
(d) It acts as a heat transfer mediuma in quenching the sparks.
1.5 EDM process
In this process the metal removal takes place from the work piece due to erosion cause by rapidly recurring
spark discharge taking place between the electrode and work piece. The tool is made cathode and work piece
is anode. When the voltage across the gap becomes sufficiently high it discharges through the gap in the form of
the spark in interval of 10 micro seconds and positive ions and electrons are accelerated, producing a discharge
channel that becomes conductive. It is just at this point when the spark jumps causing collisions between ions and
electrons and creating a channel of plasma. A sudden drop of the electric resistance of the previous channel
allows that current density reaches very high values producing an increase of ionization and the creation of a
powerful magnetic field. The moment spark occurs sufficiently pressure developed between work and tool as a
result of which a very high temperature is reached and at such high pressure and temperature that some metal is
melted and eroded.
Common methods of evaluating machining performance in EDM operation are based on the performance
characteristic like: MRR, SR, and EWR [12]. Basically, this characteristics’ are correlated with the machining
parameters such as work piece polarity, pulse on time, duty factor, and open discharge voltage, discharges
current and dielectric fluid. Proper selection of the machining parameters can obtain higher material removal rate,
better surface roughness, and lower electrode wear ratio [13]. The discharge pulse gap is relatively small, thus the
accuracy of components or parts manufactured by EDM is very high. EDM is a thermo electrical material removal
process, in which the tool electrode shape is reproduced mirror wise into a work material, with the shape of the
electrode defining the area in which the spark erosion will occur [14]

Fig.1 EDM process
2 Study of industrial EDM operation
The Experiment has been carried out to gain the in depth knowledge of the EDM and to computationally
simulate the process close to realistic applications. In Sureliya industries, Rajkot job work is conducted using the
EDM, Model S-25, SPARKONIX (I) PVT. LTD (Spark erosion) the polarity of the electrode was set as positive
while that of workpiece was negative. The dielectric fluid used was EDM oil (specific gravity- 747.7167 kg/m3).
The EDM consists of the following parts:
-Dielectric reservoir, pump and circulation system.
-Power generator and control unit.
-Working tank with work holding device.
-X-Y working table
-The tool holder
-The servo system for feeding the tool.
Fig.2: Set up of work piece and tool
2.1 Selection of work piece for experiment
The work piece material is High speed steel. It is capable of cutting metal at a much higher rate than carbon
tool steel and continues to cut and retain its hardness even when the point of the tool is heated to a low red
temperature. Tungsten is the major alloying element which gives red hardness but it is also combined with
chromium, molybdenum, vanadium and cobalt in different amounts. At many places it is replaced by cemented
carbides but it is still widely used for the manufacture of taps, dies, twist drills, reamers, saw blades and other

cutting tools[15,16].
Table 1 thermal and structural properties of material
SR. NO. PROPERTIES VALUE
1 k (W/mK) 40.2
2 ρ (kg/m3) 8691
3 C (J/kg K) 419
4 αt (/K) 11.7 × 10-6
5 E (GN/m2) 208
6 Tm (°C) 1965
7 ν 0.3
2.2 Tool design
The tool material used in EDM can be of a variety of metals like copper, brass, aluminium alloys, silver alloys
etc. The tool material used in this experiment is copper and workpiece as a HSS. The work piece is in the shape
of a rectangular having a dimension 25 mm × 15 mm × 10 mm.
Table 2 RESULTS OF EXPERIMENT
Run No Ip (A) ton (μs) toff (μs) Spark Radius (µm)
1 10 32 9 0.025
2 12 38 9 0.031
3 14 48 9 0.036
1 10 32 9 0.025
2 12 42 9 0.031
3 14 52 9 0.036
2. Simulation and discussion of temperature analysis
The simulation of the EDM in ANSYS APDL involves the development of a simulation model to simulate the
EDM for temperature and thermal stress distribution [14]. The work piece material for the present work is High
Speed Steel (HSS). The mechanical and thermal material properties are in general temperature dependent.
It can withstand higher temperatures without losing its temper (hardness). This property allows HSS to cut
faster than high carbon steel, hence the name high speed steel
To find out the spark radius the following equation is used [17]:
R = (2.04 × 10-3). I0.43 .ton0.44 (1)
Heat flux was found by using following equation
qw(r) = (2)
Governing equation
Heating of work piece due to a single spark is assumed to be axisymmetric [20], governed by the following
thermal diffusion differential equation
ρc = (3)
r and z are coordinate axes Boundary conditions

A small cylindrical portion of the workpiece around the spark is used as the domain [20]. Energy transferred to
the workpiece as heat input serves as the thermal boundary condition on the top surface Γ1 (Fig. 3). The heat
loss to the coolant on the surface Γ1 is modeled using the convective boundary condition. Further, the boundaries
Γ2 and Γ3 are at such distances that there is no heat transfer across them.
Fig-3 An axisymmetric model for the EDM process simulation
K(
(4)
Simulation of EDM is done by using ANSYS APDL [17]. ANSYS Parametric Design Language, or APDL
which utilizes concepts and structures very similar to common scientific programming languages such as BASIC,
FORTRAN,C etc. Using the APDL, the user can create (a) an Input File to solve a specific problem and (b) Macro
File(s) that act as special functions, accepting several arguments as input. In either case, each line consists of a
single command, and the lines are executed sequentially.
Fig-4 Grid created using ANSYS

4. Results and discussions
Table 3 Input used for simulation work
Sr. No. Gap Voltage (V) Gap Current (A)
1 40
5
10
15
2 50
5
10
15
3 60
5
10
15
Fig-5 temp. Analysis for Gap current 5A and 40V
Figure shows the stress analysis for the input data gap current 5A, gap voltage 40 V and Partition
coefficient to work piece is 0.08. The maximum temperature of about 2782.04°C is developed due to a single
spark of the metal which is far more than the melting point. The material will be removed due to the high
temperature value developed. Yield strength of material is 3610 MPa. So the maximum stress is below yield point
of the material and the highly sheared material will be removed.

Fig-6 Temp. Analysis for Gap current 15A and 60V
Figure shows the stress analysis for the input data gap current 15A, gap voltage 60 V and Partition
coefficient to workpiece is 0.08.The maximum temperature of about 12313.7°C is developed due to a single spark
of the metal which is far more than the melting point. From the Ansys graph shown in the above figure it can be
seen that maximum stress with Gap current of 15A and Gap voltage of 60V reaches above the yield point of
material.
Conclusion
From the above experimentation it can be seen that the temperature changes from 2782.04 oC to
12313.7 oC for different gap currant and gap voltage. Temperature produce here is much higher than melting
point of material. Because of such high temperature in small heat affected zone thermal stresses exceed the yield
strength of the workpiece mostly in an extremely thin zone near the spark. These thermal stresses can lead to
micro-cracks, decrease in fatigue life and strength and possibly catastrophic failure. These high tensile residual
stresses are increase from the surface and reaches to their maximum value. Many times This maximum value is
around the ultimate tensile strength of the material. From this temperature analysis we can study the pattern of
residual stress distribution and check crack network. So, it is required to determine the surface quality of the work
piece before making a practical application of the component during material removal process so it is necessary
to know thermal stress distribution.
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