01-09-2012, 04:48 PM
Material removal rate and electrode wear study on the EDM of silicon carbide
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Abstract
In this work, a material removal rate (MRR) and electrode wear (EW) study on the die-sinking electrical discharge machining (EDM) of
siliconised or reaction-bonded silicon carbide (SiSiC) has been carried out. The selection of the above-mentioned conductive ceramic was
made taking into account its wide range of applications in the industrial field: high-temperature gas turbines, bearings, seals and lining of
industrial furnaces. This study was made only for the finish stages and has been carried out on the influence of five design factors: intensity
supplied by the generator of the EDM machine (I), pulse time (ti), duty cycle (η), open-circuit voltage (U) and dielectric flushing pressure
(P), over the two previously mentioned response variables. This has been done by means of the technique of design of experiments (DOE),
which allows us to carry out the above-mentioned analysis performing a relatively small number of experiments. In this case, a 25−1 fractional
factorial design, whose resolution is V, has been selected considering the number of factors considered in the present study. The resolution
of this fractional design allows us to estimate all the main effects, two-factor interactions and pure quadratic effects of the five design factors
selected to perform this study.
Introduction
Electrical discharge machining (EDM) is a non-traditional
manufacturing process based on removing material from a
part by means of a series of repeated electrical discharges
between a tool, called the electrode, and the part being machined
in the presence of a dielectric fluid. At present, EDM
is a widespread technique used in industry for high-precision
machining of all types of conductive materials such as: metals,
metallic alloys, graphite, or even some ceramic materials,
of any hardness [1].
The term, technical ceramic materials or advanced ceramic
materials, is a relatively newterm, which is applied to a
range of various materials generally obtained from inorganic
primary materials with a high grade of purity. These primary
materials are subjected to typical processes in powder metallurgy
technology and subsequently, to high-temperature sintering
processes.
Design of the experiments
In the present section, the design factors and response variables
selected for this work, as well as the methodology employed
for the experimentation, will be described.
Design factors selected
There are a large number of factors to consider within
the EDM process, but in this work the level of the generator
intensity (I), pulse time (ti), duty cycle (η), open-circuit
voltage (U) and dielectric flushing pressure (P) have only
been taken into account as design factors. The reason why
these five factors have been selected as design factors is that
they are the most widespread and used amongst EDM researchers.
The intensity (I) depends on the different power levels that
can be supplied by the EDM machine generator. It represents
the maximum value of the discharge current intensity. The
intensity values used in the EDM machine programming are
power levels of the generator, these corresponding with values
of the peak intensity (Ip), which is applied between the
electrode and the part to be EDMed.
Pulse time or on-time (ti) is the duration of time (in s) the
current is allowed to flow per cycle. On the other hand, duty
cycle (η) is the percentage of pulse time relative to the total
cycle time. This third factor is calculated by dividing pulse
time by the total cycle time (i.e., pulse time plus pause time),
where pause time or off-time (to) is the duration of time (in
s) between two consecutive sparks.
Results and analysis of MRR
A first-order model was proposed for the response variable
MRR, where this was rejected as a result of the values
obtained for the curvature test that can be seen in Table 3.
As in this case the P-value, which is equal to 0.0002, is
lower than 0.05, the null hypothesis that there are no pure
quadratic effects in the model is therefore rejected, accepting
that there is statistical evidence of curvature in the first-order
model, for a confidence level of 95%. Thus, that the proposed
first-order model is suitable for a significance level
α of 0.05 is rejected and so, the second-order model is
selected.
Table 4 shows the ANOVA table for the case of the
second-order model proposed, where now, the total number
of degrees of freedom is equal to 29. As can be observed
in this table, there are three effects with a P-value
less than 0.05, which means that they are significant for
a confidence level of 95%. These significant effects, arranged
in order of importance, are: the main effects of intensity
and voltage and finally, the interaction effect between
them.
Conclusions
The enormous interest in the study of die-sinking EDM
when machining conductive ceramics is a consequence of
the problems encountered when utilising other manufacturing
processes such as: laser machining, abrasive jet machining,
ultrasonic machining or grinding, among others.
In this work, a study on the influence of the most relevant
EDM factors over material removal rate (MRR) and electrode
wear (EW) has been carried out. The study has been
made for a conductive ceramic such as siliconised silicon
carbide (SiSiC) and only for the finish stages.