Field of Invention:
The present invention relates to the development of improved WEDM process using Ni powdermixed
dielectric and investigation of CR and surface characteristics.
Background of Invention
Recently, a significant research has been carried by different researchers to process the
composites, aero-space and automobile alloys, hard materials and HSTR alloys (Hascalyk et al.,
2004; Hewidy et al., 2005; Yu et al., 2011; Khanna and Singh, 2013; Sharma et al., 2013,
Jangra et al., 2015; Kumar et al., 2016).WEDM consist numerous process parameters viz.:
pulse on-time, pulse off-time, servo voltage, peak current, water pressure, wire tension, wire feed
and servo feed. An optimum combination of process parameters becomes a challenging task to
investigate optimized value of response variables. Response variables can be cutting speed,
surface roughness, material removal rate, kerf width, overcut, recast layer thickness. Various
researchers selected different output parameters and optimize the various process parameters
(Kansal et al., 2007; Delgado et al., 2011;Ghodsiyeh et al., 2013; Bobbili et al., 2014;
Khanna and Singh, 2016).
William and Rajurkar (1991) investigated the effect of peak current on the machine stability
and productivity. Larger value of peak current deteriorates the surface quality. Same can be
revealed by scanning electron microscopy micrographs. Energy dispersive spectroscopy
unveiled that the some particles of wire deposited on the work-piece. Banerjee et al. (1993)
worked on the prevention of wire-rupture. As frequently wire-failure decreases the cutting speed.
The main reason of wire –failure is the improper selection of process parameters (i.e. input
power, pulse on-time, wire velocity and wire diameter. An optimum value of input process
parameters decreases thermal load on wire efficiently. Finite element modeling was used by
researchers to model the input parameters and investigate the thermal load. Speeding and Wang
(1997) optimized the process parameters using back propagation artificial neural network
(ANN). Input process parameters during their research are time between two pulses, pulse width,
wire feed speed and wire mechanical tension while cutting speed and surface waviness are the
response variables. ANN used to fabricate the models for cutting speed and surface roughness.
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Hsue et al. (1999)controlled geometrical accuracy and MRR by maintaining discharge angle and
other parameters. Increase in the gap voltage results into decrease in the MRR. During the corner
cutting a decrease in the MRR value has been observed. Simple formulae have been used to
compute the geometrical accuracy and MRR. At the specified settings of experiments were
carried out and results were found inline with the computational model.Puri and Bhattacharyya
(2003) unveiled the influence of 13 control factors on response variables viz.: surface finish,
cutting speed and geometrical inaccuracy. Most influencing factors in case of cutting speed are
pulse off time, on-time and peak current. Same control factors significantly affect the
geometrical inaccuracy due to wire lag. Surface quality significantly affected by peak current,
pulse on-time, wire offset, gap voltage and dielectric flow rate in their research. Yan and Huang
(2004) reduced the vibrations in the wire in the process of wire feed. 50% of geometrical errors
can be efficiently reduced by incorporating the dynamic absorber along with developed control
(closed) system. Newly developed system was used to reduce the transient response and steady
state error (approximately 7%). This system also improves the cliff edge and corner cutting by
40% and 50% respectively. Miller et al. (2005) investigated the applications of WEDM in
minimum MRR. Thin sections have been machined on different materials by complaint
mechanism. A finite element model has been proposed by the researchers with the composition
of electrostatic and thermal forces to make the miniaturize size products. Their research also
provides the procedure and guidelines for the development of miniature products by advanced
materials. Huang et al. (2006) evaluated the surface alloying behaviour of martensitic steel after
different passes of wire electrode onto the surface of steel. Initially steel was quenched at
1050°C, and then tempered at different temperature range from 200°C to 600°C. Wire electrode
moves upto 5 passes and alloying surface has been investigated to check the surface quality.
Polarization curve measures the anodic peaks and the presence of secondary peak dissolves the
copper contents from the workpiece. Prasad and Krishna (2009) selected the proper
combination of process parameters to minimize the SR and maximize the MRR collectively.
Pulse off-time, on-time, dielectric flow rate, wire tension and wire feed were the input process
parameters to machine AISI D3 material on WEDM. Integrated RSM and GA based approach
has been used to optimize the process parameters. Sadeghi et al. (2011) used
regressionmodelling and Tabu search algorithm to find out the best cutting conditions
considering quality and productivity. Sharma et al. (2015) worked on porous NiTi alloy and
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investigated the optimum combination for compromising dimensional shift, CR and SR.Khanna
and Singh (2016) compared the optimized settings for D3 and cryogenically treated D3 for
research purpose during rough cut.
WEDM is assumed as a variant of electric discharge machining (EDM) while other variants are
powder mixed EDM and die sinking EDM. Conductive powder in the dielectric decreases its
strength and increases the spark energy. This enhances the stability of EDM and finally improves
response variables (Kansal et al., 2007). In the present work conductive metal powder i.e. nickel
is blended in dielectric to improve the performance measure. CR and SR are measured after
varying the powder concentration from 2g/l to 6g/l in the dielectric of WEDM. The work-piece
selected during present work is pure titanium, which has many applications in aero-space and
medical industries.
US4363949A gives a travelling wire mechanism in EDM to process the work-piece. Two servo
motors are installed, one to support arm guide and another for removing the displacement in the
wire guides.
US5689427 gives a proper feed rate controlling method and apparatus of WEDM. Frequency of
abnormal and normal frequency has been measured with the help of a frequency counter.
US 20160039027A1 discloses a piezoelectric responsive coating on the metallic core of the
electrode wire. This type of piezoelectric wire increases the cutting speed of WEDM for any type
of material.
WO/1996/038252 (PCT/JP1996/001483) discloses a wire electric discharge machine for
subjecting a work to electric discharge machining by using a wire electrode extended in a tensed
state between first and second wire guides with a processing liquid packed in a space between
the wire electrode and the work, the machine being provided with a work tank (WT) holding a
work (WP) therein, a guide mechanism (N) provided in the work tank and adapted to guide a
movement of a first wire guide (14) in X-Y planes formed by an X-axis and a Y-axis which cross
each other at right angles, and a seal member (16) enclosing the guide mechanism (N) so as to
prevent the entry thereinto of the processing liquid supplied to the work tank (WT), pressurized
fluid supply units (401, 402, 403, A1, A2) for supplying a pressurized fluid to the interior of the
seal member (16) being connected to the same member (16), the pressurized fluid supply unit
5
being adapted to respond to a signal from a sensor (404) provided in the seal member (16) for
detecting a liquid pressure, an inner pressure of the seal member (16) being controlled to be
equal to a pressure of the processing liquid.
WO/2001/089750 (PCT/EP2001/004371) discloses an electrode for electric discharge machining
comprises a high strength pearlitic steel wire having a carbon content higher than 0.6% and a
tensile strength higher than 3000 N/mm2. The steel wire is coated with a copper free zinc or zinc
alloy coating. The electrode is in particular suitable for high precision performance applications.
WO/1996/003247 (PCT/JP1995/001477) discloses a wire electric discharge machining method
for improving a machining precision when machining a corner portion. A speed is gradually
reduced starting at point A until the machining of a corner portion C-D is initiated, and
machining is performed on the corner portion C-D at a constant feed speed. Then, the machining
feed speed is gradually increased starting at point D where the machining of the corner portion
has been completed until the original speed is restored at point F. In accordance with this change
in feed speed, the off-time of a voltage applied between a wire and a work is increased starting at
point B, reduced starting at point E after the machining of the corner portion is completed, and
restored to the original level at point F. Furthermore, the flow rate of processing liquid is also
reduced with the progress of machining of the corner portion. This process is controlled in
accordance with the radius curvature of the corner portion by automatically changing a feed
speed along the corner portion, off-time and flow rate of processing liquid. There is little
deflection of the wire electrode at the start and during the machining of the corner portion,
whereby the precision with which corner portion is machined is improved.
Any above references cited in the text does not explain the present invention. Present work or
invention differentiates from the prior work and references. The present invention discloses an
improved WEDM to machine pure titanium. Nickel powder is mixed with dielectric water in a
separate tank and used as a flushing water to modify the process as powder mixed WEDM.
Summary of the Invention
Wire-electric discharge machining (WEDM) is considered one of the most versatile nonconventional
machining process to machine hard-to-cut, conductive and high strength
temperature resistive (HSTR) materials. It is a spark-erosion non-traditional machining process,
6
in which a moving wire is used as an electrode to process the material. The gap between the wire
electrode and work-piece is maintained by the help of in-built micro-processors. The main
mechanism of material removal in WEDM is the localized heating by repetitive sparking in
between the gap of electrode wire and work-piece. With the help of micro-processors and servomechanism
it becomes easy to cut intricate shapes on any conductive material. Due to this
WEDM has become a commercialized in many industries. The main applications of WEDM
have been found in aero-space, automobile, die and punch industries etc.
A proper combination of process parameters in WEDM saves a lot of time and also the quality of
product manufactured. As there are number of process parameters in WEDM, so the
performance analysis efficiently evaluates the significant parameters. Depending upon the
performance measure the quality characteristics may be “higher the better” type or “lower the
better” type. MRR, CR, dimensional accuracy are the “higher the better” type and SR,
dimensional deviation, overcut, recast layer thickness, dimensional shift are the “lower the
better” type quality characteristics. Discharge energy in between the wire and work-piece
removes the material in the form of craters. These craters are the main cause of surface
roughnessin WEDM. A high value of discharge energy removes larger crater size and produces a
high value of SR.
A minimum value of SR is desirable during the machining for better quality. With rough
cut, minimum SR upto 3.84μm is obtained. Trim cut is assumed a better alternative to obtain a
good surface quality. During trim cut SR upto 1.5μm can be achieved, while to improve the
surface quality some modification in the process has been carried out. An additional powder
mixed dielectric tank is aided to improve the surface quality of pure-Ti. Powder mixed dielectric
is supplied separately onto the work-piece at low value of discharge energy.
Brief description of Drawings and Tables
Figure 1: Schematic Diagram of Improved WEDM (Powder Mixed WEDM)
Figure 2: Pictorial View of Improved WEDM
Figure 3: Work-profile
Figure 4:Basic Terminology of WEDM Process (Jangra et al., 2015)
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Figure 5:SR tester measuring the Ra value
Figure 6:CR at Different Trial Runs
Figure 7:SR at Different Trial Runs
Figure 8:(a) Rough cut (b) Trim Cut (c) Trim Cut with Powder (2g/L) (d) Trim cut with powder
(4g/L)
Table 1:Chemical Composition of Pure Ti
Table 2:Fixed Process Parameters
Table 3:Operating Conditions of Experimentations
Detailed Description
3.1 Set-up for Powder-mixed dielectric
There are two nozzles for supplying the dielectric fluid i.e. one upper nozzle and other is lower
nozzle (Figure 1). High pressure nozzle favours the rough cutting operation, where a high value
of discharge energy is generated in between the electrode wire and work-piece. This high
discharge energy produces large size of craters and rough surface. High pressure dielectric
removes the debris generated during WEDM.
It is very uneconomical to mix the powder in the main dielectric tank which contains
approximately 400liter dielectric. So, a separate tank of 50 liter capacity along with a stirrer and
pump has been used in present work (Figure 2). Powder mixed dielectric supplied separately to
avoid the upper nozzle clogging by powder. At the same time dielectric is also supplied at a very
slow rate (i.e. 1L/m) from upper nozzle to avoid the heating. If dielectric is not supplied then this
heating can be a cause of wire rupture.
3.2 Work-material and work-profiles
Pure titanium is used as a work material in the present research work. It is known for its high
strength to weight ratio, ability to withstand high temperature and corrosion resistance. Due to
these properties titanium is used in aero-space and space industries, medical industries etc. Due
8
to high value of strength it is difficult to process it with conventional machining method.
However it can be possible to process pure titanium by conventional techniques but generation of
build-up edge, sub-surfaces, metallurgical alternation and micro-cracks avoids its processing. So,
WEDM is considered a viable option to process pure titanium with good surface characteristics.
Table 1 representsthe chemical composition and mechanical properties of pure titanium. In
present work material available in form of square bar of 144.9mm×66mm×27mm. Square
punches of 10mm×10mm×27mm sizes are extracted from the bar. Figure 3 shows the workprofile
path traced by the wire.
3.3 Machine tool and Machining conditions
Experiments are performed on 5-axis ECOCUT (ELPUSE-40) WEDM machine tool made by
Electronica Machine tool Ltd India. There are number of process parameters in WEDM which
can be used to control the process. Some of the parameters are vary with in a range while
remaining parameters are kept fixed during the machining of pure titanium. Table 2 gives the
fixed process parameters during machining, while Table 3 gives the operating conditions for
rough cut, trim cut and trim cut with powder mixed dielectric.The basic terminology of WEDM
processing is represented in Figure 4. During rough cut dielectric is supplied at 12L/min, while
during trim cut dielectric is supplied at 6L/min. During the trim cut dielectric from upper nozzle
is supplied at 1L/min, while from the aided tank dielectric is supplied at a rate of 8L/min.
4. Results and Discussions
Experiments for trim cut, powder mixed trim cut and rough cut are performed according to the
settings suggested by Table 3.Response variables (i.e. cutting rate, surface roughness and surface
morphology) are measured according to Table 3. CR is recorded from the display panel of
WEDM while SR tester SJ-301P Mitutoyo make (Figure 5) is used to measure the SR value. The
effect of powder mixed dielectric after varying its concentration is also measured on the response
variables.
4.1 Influence on Cutting Rate (CR)
Cutting rate is measured in five conditions viz.: (i) at rough cut (ii) at trim cut (iii) Ni powder
mixed in dielectric at 2g/l (iv) Ni powder mixed in dielectric at 4g/L (v) Ni powder mixed in
9
dielectric at 6g/L.During the rough cut CR is maximum because during this pulse on time is
maximum (120 machine control units) and pulse off time is minimum (30 machine control
units). Maximum pulse on time means that current in a cycle is on for a larger period of time,
which generates a high amount of discharge energy. Lower value of pulse off time means that
current is off for a smaller duration of time i.e. high amount of discharge energy is librated. So a
large value of cutting rate (2.3mm/min) is obtained during rough cut.
During the trim cutting process parameters are adjusted in such a way that low value of discharge
energy is produced and is utilized for processing the titanium. The CR obtained during this
setting is higher (10.5mm/min) than the rough cut while material removal rate is lower than the
rough cut. This is due to the fact that trim cut is used only for finishing.
The powder mixed dielectric is supplied from the side flushing (aided) inspite of regular flushing
from upper nozzle. This is due to the fact that the Ni powder in the dielectric may clog the upper
nozzle and finally cause wire rupture. The lower nozzle kept closed during trim cutting by
powder-mixed dielectric cutting. Initially small amount of powder (2g/L) is mixed with dielectric
with the help of a stirrer operated by an electric motor.
With small amount (2g/L) of addition of powder the CR decreases in the present research work,
while during EDM the addition of powder in the dielectric reduces the insulating strength of
powder and hence increases cutting rate (Kansal et al., 2005). The addition of nickel powder in
between the electrode wire and work-piece decreases the sparking gap in WEDM and decreases
the CR. Figure 6 represent that addition of Ni powder decreases CR (8.6mm/min) as compared to
CR during trim cut. If the quantity of powder is increased (i.e. 4g/L) in the dielectric then CR
(6.1mm/min) found to be decreased. This is due to the fact that larger amount of powder addition
in the dielectric further decreases the sparking gap and hence reduces the spark intensity. This
reduces the CR of pure Ti on WEDM. Further addition of powder (6g/L) in dielectric reduces
CR (4.9mm/min) of pure Ti. Also wire rupture takes place 4 to 5 times during 40mm of profile
cutting. The main reason behind this may be short-circuit due to fill of the sparking gap between
electrode wire and work-piece. This reduces the productivity as most of the time wastes in
resetting and aligning of brass wire.
4.2 Influence on Surface Roughness (SR)
10
During the rough cut discharge energy between the wire and work-piece is high.So larger size of
craters are induced, which produces a high SR (Ra). Basically SR is a function of crater size
removed on the surface of work-piece. If size of crater is large then surface finish will be less
and if the crater size is shallow then surface roughness is less i.e. surface finish will be fine.
During the rough cut, the process parameters are set in such a way that high discharge energy is
induced as explained in CR section. Surface roughness in rough cut is achieved upto 3.216μm.
The flushing pressure in rough cut is also high to remove the debris from the spark gap
efficiently.
Wire offset plays a significant role during WEDM operation. As work-profile is processed by the
edge of diameter and the wire diameter is 250μm. So, to get an exact dimension an offset of
125μm is given during the machining of pure Ti. If the wire-offset is increased beyond this limit,
then the spark gap between the tool and work-piece increases, which also increases the crater
size and finally a larger value of surface roughness. During the trim cut the process parameters
produces a lower amount of discharge energy, so crater size in case of trim cut is shallow. Hence
SRupto 1.738μm can be obtained in this case. Also the flushing pressure in trim cut is adjusted in
such a way that only the shallow craters debris be removed from the spark gap and the surface
finish can be kept at its best value.
After the addition of powder in small quantity (2g/L) or an extra flushing of powder-mixed
dielectric the spark gap decreases upto some extent. Due to this decrement in spark gap the crater
size also reduces. Another factor is the process parameters setting during trim cut. As this setting
gives small discharge energy along with a smaller spark gap, so size of craters becomes smaller
than the trim cut. Hence a SR upto 1.071μm can be obtained during this set of experiment.
Further the addition of powder (4 g/L and 6g/L) can create the waviness on to the surface and
damage the surface quality (Figure 7). Again an increment in the SR has been found with the
addition of powder. The nickel powder present in the dielectric reduces the spark intensity due to
decrease in insulating strength.
4.3 Surface Morphology of Pure Ti
Surface morphology of pure Ti reveals that during rough cutting (Figure 8a) large size of craters
are found. Also at high discharge energy presence of subsurface and micro-surface are found.
11
The main reason behind the micro-cracks on the machined surface is the high value of thermal
stress. These cracks are not only depends upon process parameters but also on thermal
conductivity, coefficient of linear expansion, yield strength etc. The mechanism of crack is the
rapid heating and cooling. When due to heating the plastic deformation occurs then tensile
strength increases while when dielectric strikes on the surface, thermal changes occurs. This
thermal changes or thermal stresses create micro-cracks on the machined surface (Lee and Tai,
2003).
Figure 8b shows that after trim cutting surface morphology changed due to removal of subsurfaces.
A very small material removal rate occurs during trim cutting due to low value of
discharge energy. This small amount of discharge energy removes sub-surfaces produced in
rough cut. After the addition of Ni powder (2g/L and 4g/L) surface morphology found to be
changed. It improved marginally as the spark become stable. Nano- impingement shows that
surface is modified by Ni powder (Figure 8c and 8d). These impingement the basically the
powder marks as if the size of powder is large, then this nanosconverts into microns.So, the
addition of metal powder (Ni) improves surface morphology.
Conclusions
In the present research work CR, SR and surface characteristics have been evaluated. Initially
rough cut is applied on pure Ti during the processing on WEDM. In the second phase, trim cut is
used by adjusting the process parameters setting. After that Ni powder is mixed in dielectric and
powder-mixed dielectric is used at trim cut setting to process pure Ti. So, an improved version of
WEDM is presented and following conclusions have been drawn
1. During the rough cut CR is equal to 2.3mm/min. With changing the process parameters
according to trim cut the CR increases upto 10.5mm/min. MRR value during trim cut is
very low as compared to rough cut due to removal of sub-surfaces only. Addition of
smaller amount of Ni powder reduces the CR to some extent. Further the addition of
powder reduces the CR upto 4.9mm/min.
2. Maximum value of SR (3.216μm) is obtained during rough cut due to larger crater
removal. Surface quality of pure Ti increases after trim cutting and SR value reaches upto
1.738μm. After powder-mixed (2g/L) dielectric usage best surface quality of SR value
12
1.071μm has been achieved. Further addition of powder increases the SR i.e. decreases
surface quality. So, limited amount of powder increases surface quality and after that SR
increases.
3. Larger craters, micro-cracks and sub-surfaces are found after rough cut. Trim cut reduces
the quantity of all craters, cracks etc. After powder addition in the dielectric lumps,
cracks etc. are eliminated and surface morphology improved.
Prospective Work in this field
In future research, a systemic investigation of process parameters on responses of powder-mixed
WEDM using design of experiments can be considered. Other metallic powders (preferably nano
size) can be considered for the processing of advanced composite and alloys.
13
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16
We Claim;
1. A process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder,
comprising the steps of ;
a spark-erosion non-traditional machining process, in which a moving wire is used as an
electrode to process the material,
mechanism of material removal in said process performed by localized heating by
repetitive sparking in between the gap of electrode wire and work-piece,
cut intricate shapes on any conductive material with the help of micro-processors and
servo-mechanism.
2. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1, having lower and upper nozzles which are used for supplying the
dielectric fluid,
3. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1, wherein supply of dielectric is at 12L/min during rough cut,
4. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1, wherein mixture of Ni powder and dielectric supplied by additional
pipe on to the work-surface,
5. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1, wherein said mixture of Ni powder and dielectric 1 is used during
trim cut only,
6. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1,wherin the said process is used to process work material i.e. pure
Ti.
7. The process for improvement in WEDM for the Processing of Pure-Ti using Ni Powder
as claimed in claim 1, wherein said work material has an ability to withstand high
temperature, corrosion resistant and high strength to weight ratio alloy.