Investigation on Machinability of Nodular Cast Iron by WEDM

8/10/2019 Investigation on Machinability of Nodular Cast Iron by WEDM

1/6

3. ULUSLARARASI LERTEKNOLOJLER SEMPOZYUMU, 18-20 AUSTOS 2003, ANKARA

1

Investigation on machinability of nodular cast iron by WEDM

aNiyazi ZDEMR, bCebeli ZEK, aNuri [email protected],[email protected], [email protected]

a

Department of Metal, Faculty of Technical Education, University of Frat,23190, Elaz, Turkey

bDepartment of Machine, Faculty of Technical Education, University of Frat,

23190, Elaz, Turkey

Abstract

Wire electric discharge machining (WEDM) is suitable especially for thematerials, which can not be machining with conventional machining methods. In thisstudy, the machinability of standard GGG40 nodular cast iron by WEDM usingdifferent parameters (machining voltage, current, wire speed and pulse duration) wasinvestigated. From the results, the increase in surface roughness and cutting rate

clearly follows the trend indicated with increasing discharge energy as a result ofincrease of current and pulse -on time, because the increased discharge energy willproduce larger and deeper discharge craters. Three zones were identified in roughregimes of machining for all samples: decarburised layer, heat affected layer andbulk metal. High machining efficiency can be obtained when selected the properelectrical 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, goodfatigue and wear resistance). Due to the metallurgical nature of these materials, themachining of these materials with the conventional machining techniques such asmilling and turning are problematic and difficult [1,2]. Wire electric discharge

machining (WEDM) is suitable especially for the materials which can not bemachining with conventional machining methods[3]. WEDM is an electrothermalprocess where the material removal mechanism is achieved by electrical dischargesoccurring between an anode (usually the tool electrode) and a cathode (the worcpiece)submerged in a fluid dielectric. These electrical discharges melt and vaporize minuteamounts 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 thepoint where surface irregularities provide the narrowest gap between the workpieceand 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


2/6


3/6


3

Table 1.Chemical composition of the test material

Alloy

ElementC 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

Table 2.Physical properties of the test material

Tensile strength (N/mm2) 420

Elongation (%) 26

Modulus of elasticity (N/mm2) 175

Hardness (BHN) 215

Density (g/cm3) 7,2

Thermal conductivity (W/m.K) 35

Electric resistivity (..m) 0,35

Table 3. Experimental conditions.

FactorsExperimental

series noCircuitvoltage

(V)

Current(A)

Wirespeed

(m/min)

Machiningtime (min)

Surfaceroughness

Ra(m)

Cuttingrate

(mm2/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 anddischarge 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 roughnessincreases with the increase of the voltage. The increase in surface roughness and


4/6

3RD INTERNATIONAL ADVANCED TECHNOLOGIES SYMPOSIUM, AUGUST 18-20, 2003, ANKARA

4

cutting rate clearly follows the trend indicated with increasing discharge energy as aresult of increase of current and pulse-on time, because the increased dischargeenergy 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 probabilityof blind feeding under small pulse duration. The increase of the voltage means thatthe electric field becomes stronger and the spark discharge takes place more easilyunder the same gap and a coarse surface is always obtained. Additionally, the rise ofvoltage enables the discharge energy to increase, which is beneficial in the removal ofthe dielectric grains.

0

0,5

1

1,5

2

2,5

0 100 200 300

Discharge voltage (V)

Surfaceroughness

Figure 2. The effect of voltage on the surface roughness.

0

0,5

1

1,52

2,5

0 5 10 15 20

Current (I)

Surfaceroughness

Figure 3. The effect of the current on the surface roughness.

Ra

(m)

Ra

(m)


5/6


5

0

0,5

1

1,5

2

2,5

0 5 10 15 20

Wire speed (m/min)

Surfacer

oughness

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 microstructuralchanges during the WEDM process. From the results and analysis of microstructure,among the specimens no significant differences were observed between themicrostructure Fig. 5. Massive pearlitic structure was seen around graphite nodules on

the outer surfaces of all the specimens. In higher magnifications a very thindecarburised layer was seen in all specimens. As an example, the top surfaces andcross-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 WEDMFig. 5(a). This is attributed to the higher heat capacity and consequent slow coolingrate 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 top-most surface layer the spaces left by the broken graphites nodular and some undersurface cracks. This effect increases the surfaces roughness.

Figure 5. Optical micrograph of machining surface by WEDM: a: S1, b: S3

Ra

(m)

Decarburized layerNodular graphite space

a b


6/6

3RD INTERNATIONAL ADVANCED TECHNOLOGIES SYMPOSIUM, AUGUST 18-20, 2003, ANKARA

6

4. Conclusions

From the investigation of the machinability of standard GGG40 nodular castiron by WEDM using different electrical parameters, the following results can bemade:

1. The results of the experiment shows that the machining input parameters

(machining voltage, current and wire speed) have an important effect on the highmachining efficiency for nodular cast iron.

2. The discharge energy increases with the increase of the voltage and thecurrent, 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 Vrespectively, 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 WEDMexcept broken nodules increasing surface roughness.

References

[1]. Ghani, A., K.; Choudhury., I., A.,; Husni; Study of tool life, surface roughnessand vibration in machining nodular cast iron with ceramic tool, Journal of MaterialsProcessing Technology, 127, 17-22 , (2002).

[2]. Rozenek., M.; Kozak, J.; Dabrowski, L; Lubkowski, K; Electrical DischargeMachining Charecteristics of Metal Matrix Composites, Journal of MaterialsProcessing Technology, 109, 367-370, (2001).[3]. Guo, Z., N.; Wang, X.; Huang, Z., G.; Yue., T., M.; Experimental InvestigationInto Shapping Particle-Reinforced Material by WEDM-HS, Journal of Materials

Processing Technology, 129, 56-59, (2002).[4]. Prohaszka., J.; Mamails., A., G.; Vaxevanidis., N., M.; The Effect of ElectrodeMaterial on Machinability in Wire Electro-Discharge Machining, Journal ofMaterials Processing Technology, 69, 233-237, (1997).[5]. Snoeys., R.; Staelens., F.; Dekeyser., W.; Current Trends in Non-Conventional

Material Removal Processes, Annals CIRP 35 (2), 467, (1986).[6]. Mamails., A., G.; Vaxevanidis., N., M.; Karafillis., A., P.; Surface Integrity andFormability of Steel Sheet, Fortschritt-berichte VDI-Z, Reihe 2, Nr. 197, VDI

Verlag, Dusseldorf, 219, (1990).[7]. Luo., Y., F.; Rupture Failure and Mechanical Strength of The Electrode WireUsed in Wire EDM, Journal of Materials Processing Technology, 94, 208-215,(1999).

[8]. Tosun., N., Cogun., C; An Investigation on Wire Wear in WEDM, Journal ofMaterials Processing Technology, 134, 273-278, (2003).

Investigation on Machinability of Nodular Cast Iron by WEDM

Documents

Transcript of Investigation on Machinability of Nodular Cast Iron by WEDM