Investigation on machinability of nodular cast iron by WEDM ...

3. ULUSLARARASI İLERİ TEKNOLOJİLER SEMPOZYUMU, 18-20 AĞUSTOS 2003, ANKARA
<strong>Investigation</strong> on machinability of nodular cast iron by WEDM
a Niyazi ÖZDEMİR, b Cebeli ÖZEK, a Nuri ORHAN
nozdemir@firat.edu.tr, cozek@firat.edu.tr, norhan@firat.edu.tr
a Department of Metal, Faculty of Technical Education, University of Fırat,
23190, Elazığ, Turkey
b Department of Machine, Faculty of Technical Education, University of Fırat,
23190, Elazığ, Turkey
Abstract
Wire electric discharge machining (WEDM) is suitable especially for the
materials, which can not be machining with conventional machining methods. In this
study, the machinability of standard GGG40 nodular cast iron by WEDM using
different parameters (machining voltage, current, wire speed and pulse duration) was
investigated. From the results, the increase in surface roughness and cutting rate
clearly follows the trend indicated with increasing discharge energy as a result of
increase of current and pulse -on time, because the increased discharge energy will
produce larger and deeper discharge craters. Three zones were identified in rough
regimes of machining for all samples: decarburised layer, heat affected layer and
bulk metal. High machining efficiency can be obtained when selected the proper
electrical parameters, but, whether high energy or the low energy is used, a coarse
surface is always obtained.
Keywords: Wire electric discharge machining (WEDM), nodular cast iron
1.Introduction
Nodular cast iron is becoming popular for many engineering applications on
account of their potantiel advantages (i.e. having high strength and toughness, good
fatigue and wear resistance). Due to the metallurgical nature of these materials, the
machining of these materials with the conventional machining techniques such as
milling and turning are problematic and difficult [1,2]. Wire electric discharge
machining (WEDM) is suitable especially for the materials which can not be
machining with conventional machining methods[3]. WEDM is an electrothermal
process where the material removal mechanism is achieved by electrical discharges
occurring between an anode (usually the tool electrode) and a cathode (the worcpiece)
submerged in a fluid dielectric. These electrical discharges melt and vaporize minute
amounts of the work material, which are then ejected and flushed away by the
dielectric[4,5]. Application of electrical pulse creates an intense electrical field at the
point where surface irregularities provide the narrowest gap between the workpiece
and electrode and results in the formation of high-conductivity bridge in the medium
across the gap[6]. The increase of the voltage or decrease of the gap between the
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3 RD INTERNATIONAL ADVANCED TECHNOLOGIES SYMPOSIUM, AUGUST 18-20, 2003, ANKARA
workpiece and the electrode results in vaporization and ionization of dielectric in the
high-conductivity bridge and forms a spark channel between the two surfaces[7,8].
The structural changes of electro-discharge machined surfaces and the
influences of machining input parameters on the performance of WEDM have been
widely reported in the literature, for tool steels. But the influences of electrical
parameters on the surface microstructure of machined workpiece in the WEDM
process have not been studied for cast irons. The main aim of this research was to
investigate effect of machining input parameters (machining voltage, current and wire
speed) on the surface roughness and surface microstructure of nodular cast iron by
WEDM.
2.Materials and experimental procedures
In this experiment, standard GGG40 nodular cast iron was used as the
machining workpiece. Chemical composition and physical properties of nodular cast
iron are given Table 1 and 2 respectively. The samples were machined on a A300
Fine Sodick Mark XI EDM electro discharge machine with isopulse generator and
pure water was used as the dielectric liquid. In all the experiments, dielectric flow
pressure of 1 bar, tension of the wire of 1133 g, diameter of the brass wire of 0,25 mm
are kept as constant. Nine of the tests were performed using a range of WEDM
conditions shown Table 3. After machining, the following experimental techniques
were used to evaluate the surface microstructure and surface roughness of machined
workpieces. For the surface roughness measurements of the machined workpieces, a
portable device called “Mitutoyo Surftest 211” was used. Microstructural changes of
samples after the each machined process were observed by optical microscopy. The
perpendicular surfaces to the machined surfaces of the specimens were polished with
200-1200 mesh emery paper and cleaned in an acetone bath and etched with 2 % nital
after polishing 3 µm diamond paste.
Figure 1. Schematic diagram of the cutting process and examination surfaces.
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3. ULUSLARARASI İLERİ TEKNOLOJİLER SEMPOZYUMU, 18-20 AĞUSTOS 2003, ANKARA
Table 1. Chemical composition of the test material
Alloy
Element
C Si S P Mn Ni Cu Sn Mg Cr Ti Mo
Wt %
3,588 1,918 0,006 0,014 0,261 0,507 0,271 0,094 0,054 0,642 0,007 0,613
Experimental
series no
Table 2. Physical properties of the test material
Tensile strength (N/mm 2 ) 420
Elongation (%) 26
Modulus of elasticity (N/mm 2 ) 175
Hardness (BHN) 215
Density (g/cm 3 ) 7,2
Thermal conductivity (W/m.°K) 35
Electric resistivity (µ.Ω.m) 0,35
Circuit
voltage
(V)
Table 3. Experimental conditions.
Current
(A)
Wire
speed
(m/min)
3
Factors
Machining
time (min)
Surface
roughness
Ra(µm)
Cutting
rate
(mm 2 /min)
S1 80 5 5 60 1,26 8,33
S2 80 8 5 56 1,32 8,92
S3 80 12 5 53 1,70 9,43
S4 100 5 10 48 1,30 10,41
S5 100 8 10 39 1,70 12,82
S6 100 12 10 28 2,09 17,85
S7 270 5 5 25 1,90 20,00
S8 270 8 10 16 2,15 31,25
S9 270 12 15 10 2,38 50,00
3. Results and discussion
3.1. Effect of the machining parameters on the surface roughness
Table 1 shows the results obtained for machining of standard GGG40
nodular cast iron at different discharge voltage, discharge current, wire speed and
discharge duration for a discharge(spark) pulse. The effect of the discharge voltage on
the surface roughness is illustrated in Fig.2. It can be seen that the surface roughness
increases with the increase of the voltage. The increase in surface roughness and

3 RD INTERNATIONAL ADVANCED TECHNOLOGIES SYMPOSIUM, AUGUST 18-20, 2003, ANKARA
cutting rate clearly follows the trend indicated with increasing discharge energy as a
result of increase of current and pulse-on time, because the increased discharge
energy will produce larger and deeper discharge craters Fig.2, 3 and 4. The reason
why the discharge energy plays an important role is that there is very high probability
of “blind feeding” under small pulse duration. The increase of the voltage means that
the electric field becomes stronger and the spark discharge takes place more easily
under the same gap and a coarse surface is always obtained. Additionally, the rise of
voltage enables the discharge energy to increase, which is beneficial in the removal of
the dielectric grains.
Surface roughness Ra(µm)
Surface roughness Ra(µm)
2,5
2
1,5
1
0,5
0
2,5
2
1,5
1
0,5
0
0 100 200 300
Discharge voltage (V)
Figure 2. The effect of voltage on the surface roughness.
0 5 10
Current (I)
15 20
Figure 3. The effect of the current on the surface roughness.
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3. ULUSLARARASI İLERİ TEKNOLOJİLER SEMPOZYUMU, 18-20 AĞUSTOS 2003, ANKARA
Surface roughness Ra(µm)
2,5
2
1,5
1
0,5
0
0 5 10
Wire speed (m/min)
15 20
Figure 4. The effect of the wire speed on the surface roughness.
3.2. Effect of machining parameters on the microstructure
In this study, optical microscopy was used to identify the microstructural
changes during the WEDM process. From the results and analysis of microstructure,
among the specimens no significant differences were observed between the
microstructure Fig. 5. Massive pearlitic structure was seen around graphite nodules on
the outer surfaces of all the specimens. In higher magnifications a very thin
decarburised layer was seen in all specimens. As an example, the top surfaces and
cross-sections of WEDM S3 sample are shown in Fig. 5(b). There is no an uneven,
non-etchable layer, namely ‘White Layer’ in specimens as seen in steel after WEDM
Fig. 5(a). This is attributed to the higher heat capacity and consequent slow cooling
rate of nodular cast iron not enough to induce microstructural transformations relative
to steel. The heat affected zone is not visible. The most important features of the topmost
surface layer the spaces left by the broken graphites nodular and some under
surface cracks. This effect increases the surfaces roughness.
Nodular graphite space
Figure 5. Optical micrograph of machining surface by WEDM: a: S1, b: S3
5
Decarburized layer
a b

3 RD INTERNATIONAL ADVANCED TECHNOLOGIES SYMPOSIUM, AUGUST 18-20, 2003, ANKARA
4. Conclusions
From the investigation of the machinability of standard GGG40 nodular cast
iron by WEDM using different electrical parameters, the following results can be
made:
1. The results of the experiment shows that the machining input parameters
(machining voltage, current and wire speed) have an important effect on the high
machining efficiency for nodular cast iron.
2. The discharge energy increases with the increase of the voltage and the
current, resulting in the removal of the metal under the action of the discharge force,
the cutting rate increases. When the voltage increase up to 80, 100, 270 V
respectively, the machining stability is improved and the cutting rate substantially
increases.
3. In this study, due to the heat properties of nodular cast iron the processes
parameters was not effective on the microstructure of the specimens during WEDM
except broken nodules increasing surface roughness.
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