11-12-2012, 02:08 PM
Material removal rate and electrode wear ratio study on the powder mixed electrical discharge machining of cobalt-bonded tungsten carbide
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Abstract
In this article, a material removal rate (MRR) and
electrode wear ratio (EWR) study on the powder mixed
electrical discharge machining (PMEDM) of cobalt-bonded
tungsten carbide (WC-Co) has been carried out. This type
of cemented tungsten carbide was widely used as moulding
material of metal forming, forging, squeeze casting, and
high pressure die casting. In the PMEDM process, the
aluminum powder particle suspended in the dielectric fluid
disperses and makes the discharging energy dispersion
uniform; it displays multiple discharging effects within a
single input pulse. This study was made only for the
finishing stages and has been carried out taking into
account the four processing parameters: discharge current,
pulse on time, grain size, and concentration of aluminum
powder particle for the machinability evaluation of MRR
and EWR. The response surface methodology (RSM) has
been used to plan and analyze the experiments. The
experimental plan adopts the face-centered central composite
design (CCD). This study highlights the development of
mathematical models for investigating the influence of
processing parameters on performance characteristics.
Introduction
The electrical discharge machining (EDM) process is an
electrical spark-erosion process which removes workpiece
material by means of a series of recurring electrical spark.
The discharging sparks occur between the electrode and
electro-conductive workpiece flushed with or submerged in a
dielectric fluid. The EDM process has the advantage of
spark-erosion in order to machine the hard-to-cut material
and then easily achieve the required shapes and sizes with
enhancing machining productivity and better dimensional
accuracy [1, 2]. Since it has more versatile benefits, the
EDM has been widely applied in the modern metal industry
for producing complex cavities in moulds and dies, which
are difficult to manufacture by conventional machining.
However, the material removal mechanism is achieved by
the melting and evaporation of workpiece material at each
electrical discharge spot, which are then ejected and flushed
away by the dielectric fluid. Due to the rapid high
temperature melting and cooling process, subsurface defects
such as cracks, spalling, porosity, residual stress, metallurgical
transformations, and heat affected zones (HAZ) are easily
found on the machined surface of the workpiece [3–5]. To
restore the machined surface properties and remove the
surface detects, the technique of fine powder mixed into the
dielectric fluid of EDM, called the powder mixed EDM
(PMEDM), is thus proposed.
Description of the experiments
Equipment used in the experiments
A series of experiments was performed on a die-sinking
CNC EDM machine of type OMEGA-CM43 with a
dielectric cycling system. Figure 1a shows a photograph
of this equipment. Different grain sizes and different
concentrations of aluminum powder particles were suspended
in the dielectric fluid. A dielectric cycling system
maintained a uniform distribution of the added powder
particles by using the cycling capability. Commercial-grade
mineral oil (TOTAL EDM44) with a flash point of 85°C
was used as the dielectric fluid and the side injection of
dielectric fluid was adopted. In Fig. 1b, a jet flushing
system was employed to assure adequate flushing of the
EDM process debris from the gap zone. The electrolytic
copper with diameter 25 mm was used as an electrode. The
physical and mechanical properties of electrolytic copper
are a melting point of 1,360 K, density of 8.94 g/cm3,
thermal conductivity of 226 W/mK, and electrical resistivity
of 17.1 nΩm.
Material used in the experiments
The workpiece material used in this study is cobalt-bonded
tungsten carbide (94WC-6Co; Protool Industrial Co.,
USA), which is composed of approximately 94% tungsten
carbide (WC) and 6% cobalt (Co). This material is
frequently used for cutting tools due to its excellent
hardness properties (HRA 92.8). Although the major
machining application of this type of cemented tungsten
carbide is for cutting tools, the scope of alternative
applications is quickly growing such as moulding material
for metal forming, forging, squeeze casting, and high
pressure die casting. Furthermore, it possesses a high
compressive strength (5.7 kN/mm2), as well as good
resistance of wear and oxidation at high temperature status.
Results and discussion
The 30 experimental runs were conducted in duplicate, and
the average values of MRR and EWR along with the design
matrix are given in Table 3.
5.1 The effect of powder particles on discharge mechanism
Under a series of recurring electrical spark, the powder
particles located between the electrode and workpiece get
energized and behave in a zigzag fashion. Due to the
interlocking between the different powder particles, these
particles will display in the form of a chain at different
places under the sparking area [8–15]. This chain formation
improves in bridging the gap between the electrodes and
decreases the gap voltage and the insulating strength of
dielectric fluid. As a result, the “series discharge” within a
discharge easily takes place and disperses a discharge
energy effect under the sparking area.
Conclusions
In this article, quantitative analyses of machinability of
cobalt-bonded tungsten carbide within the process of
PMEDM were carried out. The quadratic models of MRR
and EWR have been evaluated to correlate the four
processing parameters: discharge current, pulse on time,
grain size, and concentration of aluminum powder particle.
The response surface methodology (RSM) has been used to
plan and analyze the experiments. The experimental plan
adopts the face-centered central composite design (CCD).