03-07-2012, 04:35 PM
Thermal stresses due to electrical discharge machining
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Abstract
The high temperature gradients generated at the gap during electrical discharge machining (EDM) result in large localized thermal
stresses in a small heat-affected zone. These thermal stresses can lead to micro-cracks, decrease in strength and fatigue life and
possibly catastrophic failure. A finite element model has been developed to estimate the temperature field and thermal stresses due
to Gaussian distributed heat flux of a spark during EDM. First, the developed code calculates the temperature in the workpiece
and then the thermal stress field is estimated using this temperature field.
Introduction
Electrical Discharge Machining (EDM) is undeniably
a thermal process where thermal energy is generated in
a discharge channel. Heat generated in the channel,
causes the work material to melt and even evaporate.
High temperature generated due to high-density thermal
energy discharge leads to not only thermal erosion but
also to formation of recast layer with micro-cracks on
machined surface. The presence of multi-layered heat
affected zone and brittleness of the hardened layer has
been reported to reduce the fatigue strength of electrodischarge
machined components [1,2]. The computation
of temperature and thermal stress fields in the workpiece,
therefore, are of considerable interest as far as surface
integrity is concerned.
Mathematical modeling
In EDM, two electrodes namely workpiece and tool,
are submerged in liquid dielectric (kerosene) and physically
separated by a gap, called inter-electrode gap. Due
to the random and complex nature of EDM, the following
assumptions are made to make the problem mathematically
tractable.
Results and discussion
A FEM based code named as STRESS-EDM has been
developed for the stress analysis of the workpiece in
EDM. To validate the code, first results on temperature
distribution are compared with numerical results of
Shankar et al. [12]. Here the present model is used with
the same process conditions as taken in reference [12].
Gaussian heat flux distribution with energy partition (Rw)
value of 0.42 is used for the calculation of temperature
distribution. The material properties and process parameters
used are given in Table 1. The dielectric used is
kerosene for which the convective heat transfer coefficient
is taken as 10,000 W/m2 K.
Conclusions
Numerical simulations have been performed to predict
thermal stress fields in HSS workpiece by developing a
finite element based code. The results obtained serve to
illuminate the damaging nature of the thermal stresses
as they develop during EDM. It is observed that, after
one spark, substantial compressive and tensile stresses
develop in a thin layer around the spark location. It is
also found that the thermal stresses exceed the yield
strength of the workpiece mostly in an extremely thin
zone near the spark.