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Journal of Materials Processing Technology 148 (2004) 362367

Experimental study of wire electrical dischargemachining of AISI D5 tool steel

Ahmet Hasalk, Ulas aydasDepartment of Manufacturing, Technical Education Faculty, University of Firat, Elazig, Turkey

Received 27 August 2003; received in revised form 20 February 2004; accepted 20 February 2004

Abstract

This paper presents an experimental investigation of the machining characteristics of AISI D5 tool steel in wire electrical discharge

machining process. During experiments, parameters such as open circuit voltage, pulse duration, wire speed and dielectric fluid pressurewere changed to explore their effect on the surface roughness and metallurgical structure. Optical and scanning electron microscopy,surface roughness and microhardness tests were used to study the characteristics of the machined specimens. Taking into consideration theexperimental results, it is found that the intensity of the process energy does affect the amount of recast and surface roughness as well asmicrocracking, the wire speed and dielectric fluid pressure not seeming to have much of an influence. 2004 Elsevier B.V. All rights reserved.

Keywords: WEDM process; Surface integrity; AISI D5 tool steel

1. Introduction

The quality of a machined surface is becoming more and

more important to satisfy the increasing demands of sophis-ticated component performance, longevity and reliability.When machining any component, it is necessary to satisfythe surface integrity requirements. Surface integrity has twoimportant parts. The first is surface texture, which governsprincipally surface roughness. The second is surface met-allurgy, which concerns to the nature of the surface layerproduced in machining. Surface integrity of a surface pro-duced by a metal removal operation includes the nature ofboth surface topography as well as surface metallurgy onthe mechanical and physical properties of a material in itschosen environment.

Electrical discharge machining removes electricallyconductive material by means of rapid, repetitive sparkdischarges from a pulsating direct-current power supplywith dielectric flow between the workpiece and the tool[1]. Each discharge melts or vaporizes a small area ofthe workpiece surface. This molten metal is then cooledin the dielectric fluid and solidifies into a small spheri-cal particle which is flushed away by sweep action of thedielectric.

Corresponding author. Tel.: +90-424-2370000/6544.E-mail address: [email protected] (A. Hasalyk).

Considering the challenges brought on by advanced tech-nology, the EDM process is one of the best alternativesfor machining an ever increasing number of high-strength,

non-corrosion and wear resistant materials. Wire-EDM isa special form of electrical discharge machining whereinthe electrode is a continuously moving conductive wire.Material removal is effected as a result of spark erosion asthe wire electrode is fed through the workpiece. Thermalnature of this process always produces a recast and under-lying heat-affected zone on the surface being machined anddevelops a residual tensile stress that often causes micro-cracks [2,3]. However, the thermal sensitivity or chemicalcomplexity of the material can also affect the surface in-tegrity. Optimum utilization of the capability of the WEDMprocess requires the selection of an appropriate set ofmachining parameters. Hence, the machining parameters,including pulse-on time, pulse-off time, table feed rate,flushing pressure, wire tension, wire velocity, etc. should bechosen properly according to workpiece properties so thatbetter performance can be obtained. However, the selectionof appropriate machining parameters is difficult and reliesheavily on the operators experience and machining param-eters tables provided by the machine-tool builder [4]. But,these alternatives completely do not satisfy the require-ments of both high efficiency and good quality. In orderto optimize machining conditions for materials with differ-ent thermal properties or advanced materials, experimentalinvestigations are still required [5].

0924-0136/$ see front matter 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jmatprotec.2004.02.048

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A. Hasalyk, U. aydas / Journal of Materials Processing Technology 148 (2004) 362367 363

Several studies have been carried out in order to deter-mine appropriate workmaterial/machining parameters com-bination from aspect surface integrity. These studies showthat the surface roughness of the process is closely dependenton the machining parameters [610]. The structural changesof EDMed surfaces of steels have been studied and found

that the top-most surface layer is non-etchable layer, namelythe white layer. Immediately beneath this layer, there is aheat-affected zone, where heat caused microstructural trans-formations [11,12]. Relationship between EDM parametersand surface crack formation for D2 and H13 tool steels wasstudied [13]. It was shown that crack formation and whitelayer thickness is related to the EDM parameters. An in-creased pulse-on duration will increase both the white layerthickness and the induced stress. Two conditions tend topromote crack formation.

The purpose of this paper is to determine effect of ma-chining parameters namely, open circuit voltage, pulse du-ration, wire speed and dielectric fluid pressure on surface

integrity of AISI D5 tool steel used in manufacturing dieand mold components by wire-EDM process.

2. Experimental

The composition of AISI D5 steel used in experiments isgiven in Table 1. One series of specimens were quenchedand tempered. Another series were annealed (Table 2).Specimens were prepared that cutting area is 8 mm 8 mmsquare shaped. The experiments were performed on a SodickA320D/EX21 WEDM machine. In experiments, different

settings of pulse duration, wire feed rate, dielectric flushingpressure and open circuit voltage were used (Table 3). Dif-ferent combinations of these machining parameters weretested using factorial experimental design method. Pulseinterval time (16s), table feed rate (7.6 mm/min) and wiretension (1800 g) were fixed. CuZn37 master brass wire with0.25 mm diameter was used in experiments. De-ionizedwater was used as the dielectric fluid. After machining, fol-

Table 1Chemical composition of specimens tested (wt.%)

C 1.53

Si 0.89Mn 0.46Cr 12Mo 1.00Ni 0.384Fe Balance

Table 2Initial state of specimens

Heat treatment Microhardness (HV) Notation

Annealed 380 AQuenched and tempered 750 B

Table 3Parameters of the setting

Input parameters Level 1 Level 2 Level 3

Pulse duration (ns) 300 500 700Open circuit voltage (V) 100 270 Wire speed (m/min) 5 10

Dielectric flushing pressure (MPa) 0.6 1.2

lowing experimental techniques were employed for assess-ing the surface integrity of the specimens. Average surfaceroughness (Ra) was obtained using Mitutoyo Surftest SJ-201portable device. Specimens were sectioned transversely andprepared under standard procedure for metallographic ob-servation. Etching was performed by immersing the speci-men in 5% Nital reagent. Microhardness from cross-sectionof machined surface was measured to determine hardnessvariation of heat-affected zone of specimens.

3. Experimental results and discussion

3.1. Surface morphology

The surface texture is composed of a random array ofoverlapping craters or cusps, as shown in Fig. 1, after ma-chining. During each electrical discharge, intense heat isgenerated, causing local melting or even evaporation of theworkpiece material. With each discharge a crater is formedon the workpiece. Some of the molten material produced bythe discharge is carried away by the dielectric circulation.

The remaining melt resolidifies to form an undulating ter-rain. Figs. 24 show typical cross-sectional view of A andB specimens, respectively. Four zones were identified con-sidering microhardnesses and micrographs in all specimensafter machining. Outermost surface is debris or recast layerwhich cooled too quickly to escape the gap and were re-cast to the material. Next layer commonly called the white

Fig. 1. A typical surface after wire-EDM.

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364 A. Hasalyk, U. aydas / Journal of Materials Processing Technology 148 (2004) 362367

Fig. 2. Cross-sectional view of A specimen (dielectric fluid pressure0.6 MPa, wire speed 5 m/min).

Fig. 3. Cross-sectional view of A specimen (dielectric fluid pressure1.2 MPa, wire speed 5 m/min).

Fig. 4. Cross-sectional view of B specimen (dielectric fluid pressure0.6 MPa, wire speed 5 m/min).

Fig. 5. Showing cross-sections of A specimen (dielectric fluid pressure0.6 MPa, wire speed 5m/min).

layer, has been heated to very high temperatures, taken toa molten state, and then rapidly cooled. This rapid heat-ing and cooling process causes a very brittle surface, whichis highly susceptible to thermal stress cracks. White lay-ers have been suggested to have an untempered martensiticstructure [1]. Below the white layer is an area which washeated and cooled more slowly. This created an annealedarea, softer than the parent material. Finally, we come to theparent material. As can be seen from figures, the thickness ofwhite layer of WEDMed surfaces increases with the increasepulse duration and open circuit voltage. This layer is alwayspresent to some degree. It is found that the wire speed anddielectric flushing pressure have not a significant influenceon the microstructure in applied conditions. However, in es-pecially high pulse duration and open circuit voltage value,wire breakage caused of carbides in the workpiece decreaseswith increase wire speed and dielectric flushing pressure.

As seen in Figs. 5 and 6, another primary feature onwire-EDMed surfaces is the existence of microcracks. These

Fig. 6. Showing cross-sections of A specimen (dielectric fluid pressure1.2 MPa, wire speed 5m/min).

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1

1,2

1,4

1,6

1,8

2

0 200 400 600 800

Pulse duration (ns)

Surfaceroug

hnessRa(m)

A;100V B;100V

A;270V B;270V

Fig. 7. Surface roughness Ra vs. pulse duration for various open circuitvoltage (dielectric fluid pressure 0.6 MPa, wire speed 5 m/min).

cracks are seen to radiate from, and also to circumvent, thecraters. Formation of the microcracks in the white layer nor-mally is promoted by the high carbon content in workpieceand excessive electric parameters [14]. The crater sizes in-crease with pulse energy, as does the density of surfacecracks (Fig. 6). Furthermore, the cracks can penetrate intothe heat-affected zone depending on pulse energy. The out-come of the study is in agreement with the study of Lee andTai [13], who pointed out that crack formation is related tothe EDM parameters. These microcracks can act as initia-tion points for failure.

3.2. The influence of machining parameters on surface

roughness

Figs. 7 and 8 show surface roughness versus pulse dura-tion and open circuit voltage after wire-EDM, respectively.According to these figures, the surface roughness increasedwhen the pulse duration and open circuit were increased.

1,3

1,4

1,5

1,6

1,7

1,8

1,9

2

50 100 150 200 250 300

Open circuit voltage (V)

SurfaceroughnessRa(m) A;300 ns

A;500 ns

A;700 ns

B;300 ns

B;500 ns

B;700 ns

Fig. 8. Surface roughness Ra vs. open circuit voltage for various pulseduration (dielectric fluid pressure 0.6 MPa, wire speed 5 m/min).

(m)

1,2

1,3

1,4

1,5

1,6

0,5 0,7 0,9 1,1 1,3

Dielectric fluid pressure (MPa)

SurfaceroughnessRa

A;300 ns A;500 nsA;700 ns B;300 nsB;500 ns B;700 ns

Fig. 9. Surface roughness Ra vs. dielectric fluid pressure for various pulseduration (dielectric fluid pressure 0.6 MPa, wire speed 5 m/min).

The surface roughness is obviously affected by the amountof discharge energy and on-time, since the greater thedischarge energy conducted into the machining zone, the

greater the melted depth of the workpiece that is created.Furthermore, greater discharge energy will produce a largercrater, causing a larger surface roughness value on theworkpiece. It can be seen in Figs. 7 and 8, that surfaceroughness values of annealed samples are greater than ofquenched/tempered samples. This can be explained by thehigher thermal conductivity of annealed samples whichpermits a rapid dissipation of the heat through the sampleinstead of being concentrated on the surface. As a con-sequence, different surface roughness can be expected forboth microstructures for identical discharge condition.

Fig. 9 shows surface roughness versus the dielectric fluid

pressure. As seen from this figure, the surface roughnessdecreased slightly with increasing the dielectric fluid pres-sure in chosen conditions. This result can be explained bythe cooling effect of increasing dielectric fluid flow on theworkpiece surface. Another possible reason may be that in-creased the dielectric fluid flow prevent the debris to adhereto surface. In addition, the cutting performance with increas-ing the dielectric fluid pressure has been also improved sincethe removed particles in the machining gap are evacuatedmore efficiently.

3.3. The influence of machining parameters on

microhardness

Figs. 10 and 11 show microhardness profile versus opencircuit voltage and pulse duration after wire-EDM, respec-tively. The shallow thermal impact of these processes is ac-companied by the very rapid quench rate from the heat sinkof the bulk of the material. These transient thermal wavesproduce a recast and/or resolidified layers on the white layerwith a heat-affected zone. As seen figures, the surface of allspecimens is harder than the bulk material because of whitelayer, while the heat-affected zone is softer in B specimensbecause of overtempered martensite, which was produceddue to heating and cooling slowly, respectively. Similar

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366 A. Hasalyk, U. aydas / Journal of Materials Processing Technology 148 (2004) 362367

400

Microhardness(HV)

0

200

600

800

1000

0 100 200 300 400

Depth (m)

A;300 ns B;300 nsA;500 ns B;500 nsA;700 ns B;700 ns

WLParent layerAnnealed layer

Fig. 10. Microhardness distribution below the surface for various pulseduration (dielectric fluid pressure 0.6 MPa, wire speed 5 m/min).

0

200

400

600

800

1000

0 50 100 150 200 250 300

Depth (m)

Microhardness(HV)

B;100 V B;270 V

A;100 V A;270 V

WLParent layerAnnealed layer

Fig. 11. Microhardness distribution below the surface for various opencircuit voltage (dielectric fluid pressure 0.6 MPa, wire speed 5 m/min).

observation has been reported by Lim et al. [15] and highhardness levels of surface on tool steels have been attributedto the rapid quenching and to increase in the carbon contentof the recast layer. It is found experimentally that microhard-ness distribution below the surface with increased dielectricfluid pressure is not changed but the hardness of surface in-creased slightly. It is observed that wire speed has not haveany effect on microhardness distribution of specimens.

4. Conclusions

In this experimental study, the effect of WEDM parame-ters such as open circuit voltage, pulse duration, wire speedand dielectric fluid pressure on machining characteristics ofAISI D5 steel was investigated. Summarizing the main fea-tures of the results, the following conclusions may be drawn:

1. The surface texture is composed of a random array ofoverlapping craters or cusps, after machining. Four zoneswere identified considering microhardness and micro-graphs in all specimens. Outermost surface is debris orrecast layer. Next layer is white layer. Below the whitelayer is an area which was heated and cooled more slowly.This created an annealed area, softer than the parent ma-

terial. Finally, parent material is present. The thicknessof the heat-affected zone or white layer on the surface isapproximately proportional to the magnitude of the en-ergy impinging on that surface.

2. The density of cracks in white layer increase with in-creased pulse duration and open circuit voltage. Further-

more, the cracks penetrate into the heat-affected zonedepending on pulse energy.3. The surface roughness increased when the pulse dura-

tion and open circuit voltage were increased. It appearsthat the surface roughness primarily depends on theseparameters, dielectric fluid pressure and wire speed notseeming to have much of influence.

4. The cutting surface of all specimens is harder thanthe bulk material because of white layer, while theheat-affected zone is softer in quenched and temperedspecimens because of overtempered martensite.

Acknowledgements

The support of the F.. Scientific Research Project De-partment is gratefully acknowledged.

References

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