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Surface modication of AlZnMg aluminum alloy using thecombined process of EDM with USM

Yan Cherng Lin, Biing Hwa Yan*, Fuang Yuan HuangDepartment of Mechanical Engineering, National Central University, Chung-Li 32054, Taiwan, ROC

Received 12 October 1999

Abstract

This paper presents a novel technique to improve the machined surface of AlZnMg aluminum alloy by a combined machining process

of electrical discharge machining (EDM) with ultrasonic machining (USM). In these experiments, SiC particles were added into the

dielectric uid to produce a particle-reinforced mechanism and solid solution strengthened structure to form a modied layer on the

machined surface in a short working time.

The experimental investigation also discusses the inuence of machining polarity on the modied layer of aluminum alloy. Furthermore,

quantitative analysis of element content has been carried out by electron probe micro-analyzer (EPMA) to determine the thickness of the

modied layer and the distribution of silicon and carbon in the cross-section of the machined zone. Moreover, micro-hardness tests and

wear tests were also conducted to evaluate the effects of surface modication by the combined process. From the experimental results, it can

be seen that the combined process of EDM with USM can achieve an 80 mm deep modied layer on the machined surface of aluminum

alloy in 240 s, and that such layer can signicantly improve the hardness and wear resistance of the machined surface. # 2001 Elsevier

Science B.V. All rights reserved.

Keywords: Electrical discharge machining; Ultrasonic machining; Combined process; Surface modication

1. Introduction

In general, aluminum alloys provide relatively light-

weight and good specic strength that can minimize the

weight and increase the potential usage of the components.

Recently, aluminum alloy has become an important mate-

rial, especially in the aeronautic and automobile industries.

Although aluminum alloy has the previously mentioned

advantages, its weakness is that its surface property cannot

withstand wear under circumstances of high load. This may

cause serious damage, especially of sliding components in

direct contact. For this reason, it becomes increasingly more

importance to develop a technique that can improve the wear

resistance of the machined surface of aluminum alloy at

lower cost and with a simpler operating process.

Surface modication by electrical discharge machining

(EDM) has been examined by some researchers. For

example, Shunmugam and Philip [1] used a sintered elec-

trode made of WC powder in EDM to transfer WC to

the machined surface and form a modied layer on the

workpiece surface, improving the surface wear resistance.

Mohri et al. [2] used EDM with a green compact electrode of

AlTi alloy to produce a modied layer that contained rich

TiC on the machined surface. Tsunekawa et al. [3,4] found

that the titanium from an electrode and the carbon decom-

posed from the dielectric uid were deposited on the

machined surface during EDM. This result revealed that

the wear property of the machined surface was improved

signicantly after EDM since the effect of electric discharge

alloying was occurring on the machined surface. Ogata and

Mukoyama [5] examined carbonization and de-carboniza-

tion phenomena on the EDMed surface. They found that a

carbonization layer was formed when using kerosene as the

dielectric uid, but that a de-carbonization layer was

obtained when using distilled water as the dielectric uid.

Although the contents of the sintered electrode could be

transferred to the machined surface of the workpiece, form-

ing a modied layer during EDM, a high wear rate of the

electrode is necessary to provide sufcient modied agents

that would seriously affect the machining precision.

The combined process of EDM with ultrasonic machining

(USM) has been proven to facilitate the circulation of

dielectric uid, thus avoiding the clustering of debris and

improving the machining stability of EDM [6,7]. In addition,

Journal of Materials Processing Technology 115 (2001) 359366

*Corresponding author. Tel.: 886-3-4267353; fax: 886-3-4254501.

E-mail address: [email protected] (B. Hwa Yan).

0924-0136/01/$ see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 1 0 1 7 - 2


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USM can also assist with material removal from the work-

piece. Therefore, the combined process of EDM with USM

can promote overall machining efciency [6,7]. In the USM

process, abrasive particles such as SiC, Al2O3 and B4C, etc.

were added to the dielectric uid to create striking and

hammering actions by ultrasonic vibration, leading to micro-

chips and cracks on the workpiece surface for materialremoval [8].

The purpose of this research is to integrate the machining

mechanisms of EDM with USM as a combined process to

produce a modied layer on the machined surface. The

thickness of this modied layer can be easily and efciently

controlled by varying the discharge current and gap voltage

of the EDM conditions. Furthermore, the shape and dimen-

sions of the workpiece can also be well maintained during

this surface modication process.

2. Experimental method

2.1. Experimental procedure

To determine the effects of the combined process of EDM

with USM on the modied layer of AlZnMg alloy, this

research integrated EDM with USM for a series of related

experiments. This method used a conventional EDM

machine with transistor circuit and attached a magneto-strict

ultrasonic transducer as integration. The copper electrode

was fastened on a step horn that acted as an amplier and

was rmly connected to the transducer by a screw. The

ultrasonic transducer was actuated by a resonant frequency

of 17.522.5 kHz to provide USM power. Four liters ofcommercial dielectric uid used for EDM was employed in

each experiment. In addition, abrasive particles (SiC) of

20 mm diameter were added to the dielectric uid and

maintained at a concentration of 35 wt.% in the dielectric.

The dielectric uid was recycled by a pump and circulated

by an agitator to ensure that the abrasive particles were

uniformly suspended in the dielectric uid. The experi-

mental set-up is shown in Fig. 1. The working conditions

listed in Table 1 were varied to determine the inuence on

surface modication. Qualitative and quantitative analysis

of the modied layer was obtained by EPMA (using a Jeol

JXA-8800M EPMA) to determine the modied layer's

thickness and to explore the distribution of silicon and

carbon in the cross-section of the machined surface. In

addition, SEM and OM were used to observe the micro-

structure of the machined surface and the modied layer.

The hardness distribution in the cross-section of the

machined surface was obtained by using a Vickers micro-

hardness test instrument. Furthermore, a wear test was

performed using a Falex Model No. 6 wear device to

evaluate the wear resistance of the modied layer. The upper

counter-face specimen was ADI, made in accordance with

ASTM D3702. For each 30 m sliding distance, the specimen

was taken down and cleaned in an ultrasonic bath. After that,

the specimen was dried and weighed using a precise electric

balance to evaluate the wear resistance of the modied layer

by weight loss.

2.2. Experimental material

The material used for experiments was AlZnMg alloy,

the chemical composition of which was obtained using an

inductively coupled plasma atomic emission spectrometer

(ICP-AES) and is shown in Table 2. The workpiece dimen-sions were f38mm 10 mm for the wear test andf19mm 10 mm for the other tests. The electrode materialwas electrolytic copper of dimensions f8 mm 43mm, and

Fig. 1. Schematic diagram of the experimental equipment.

Table 1

Experimental conditions

Work condition Symbol Description

Discharge current (A) Ip 327

Pulse duration (ms) tp 1001250

Gap voltage (V) Eg 40110

Duty factor DF 0.55

Polarity Positive/negative

Abrasive size (mm) 20

Ultrasonic amplitude (mm) pp 30

Abrasive concentration (wt.%) 35

Working time (s) tw 20

Table 2

Chemical composition of the aluminum workpiece (obtained by ICP-AES)

Element (wt.%)

Zn Mg Si Fe Cu Al

1.65 0.85 0.20 0.21 0.57 Remainder

360 Y. Cherng Lin et al. / Journal of Materials Processing Technology 115 (2001) 359366


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all electrodes were drilled with a f3 mm hole through

the center to conduct the dielectric uid and make theabrasive particles disperse uniformly in the gap between

the workpiece and the electrode. The added abrasive parti-

cles were SiC (99.5 wt.%) to provide appropriate particles

for dispersed reinforcement and elements for solid solu-

tion strengthening in the modied layer. The mechanical

properties and chemical composition of SiC are shown in

Tables 3 and 4.

3. Results and discussion

3.1. Effect of polarity

Fig. 2 shows the comparison of a single crater created by

positive and negative machining polarity. From this gure, it

is found that the center part of the single crater has obvious

differences between the positive and negative polarity

machining. With positive polarity, some melted material

was solidied on the crater center to form a bulging droplet,

whilst for negative polarity, most of the melted material in

the crater center was ushed away to form a deeper hollow

in the center and a higher ridge around the crater. Fig. 3

shows the comparison of surface roughness and a typicalmachined surface for positive and negative machining

polarity. From the SEM micrographs, it can be seen that

the roughness of the surface machined by negative polarity

Ra 8:8 mm is coarser than that machined by positivepolarity Ra 5:1mm. The reason for this may be that thecathode distributes more energy to produce a larger imping-

ing force by ions in the discharge column and thus creates

more melted material on the machined surface. Then, the

melted material is ushed away from the center of the crater

by impulsive force due to the burnt and vaporized dielectric

uid. Finally, most of the melted material is splashed away

and solidies around the crater to form a higher ridge.

Therefore, the negative polarity secured a greater materialremoval rate (MRR) and produced a worse surface rough-

ness than for positive polarity.

Since improvement of MRR is not the main purpose of

this subject, i.e. concern to obtain a dispersed particle-

reinforced mechanism and solid solution strengthening

structure on the modied layer, the positive polarity machin-

ing method was adopted in the following experiments.

Figs. 4 and 5 show the distribution of silicon (Si) and

carbon (C) in the machined cross-section by the combined

process of EDM with USM under positive and negative

polarity in order to discuss the inuences of polarity on the

modied layer. Fig. 4 shows that the silicon content pro-duced by positive polarity is more than that produced by

negative polarity. As measured 10 mm from the machined

surface, the silicon content is 1.9 wt.% from positive polar-

ity, but 1.0 wt.% from negative polarity. In addition, the

Table 3

Properties of SiCa

Crystal form Hexagonal system

Density 3.20

Hardness 22002800 (Knoop)

9.09.5 (Mohs)

Maximum usable temperature About 20008C

a Source: Showa Denkou, 1990.

Table 4

Chemical composition of SiC (wt.%)a

SiC >99.48

Si SiO2 0.18FC 0.08Fe2O3 0.1

a Source: Showa Denkou, 1990.

Fig. 2. Influence of machining polarity on a single crater (Ip: 12 A; tp: 500 ms; Eg: 100 V).

Y. Cherng Lin et al. / Journal of Materials Processing Technology 115 (2001) 359366 361


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depth of the silicon content in the substrate using positive

polarity (approximately 80 mm) is deeper than that for

negative polarity (approximately 60 mm). From Fig. 4, it

is found that the silicon content was increased in the

machined zone by both positive polarity and negative polar-

ity using the combined process of EDM with USM, indicat-

ing that the combined process both inserted SiC particles

into the melted zone of the workpiece and also transferred

melted SiC to the machined surface. Therefore, the com-bined process of EDM with USM is a good method to

improve the machined surface. Furthermore, positive polar-

ity can secure greater silicon content, to a greater deeper

depth, when compared with negative polarity.

Fig. 5 shows the distribution of carbon in the cross-

section. The results show that the carbon content from

positive polarity is greater than that from negative polarity.

The carbon content from positive polarity was about

15 wt.% but from negative polarity was about 7 wt.% at

10 mm from the machined surface. Moreover, the solubility

of carbon in aluminum is about 7.4 wt.% at 20508C [9]. This

means that some carbon was dissociated into a free state in

the modied layer. Further, the layer with carbon contentproduced by positive polarity (about 50 mm) is thicker than

that produced by negative polarity (about 30 mm). Since

there is more complete material removed by negative polar-

ity, a thinner modied layer is obtained.

Fig. 3. Typical SEM micrographs of a machined surface (Ip: 12 A; tp: 500 ms; Eg: 100 V).

Fig. 4. Distribution of silicon content in the cross-section of the machined

surface.

Fig. 5. Distribution of carbon content in the cross-section of the machined

surface.



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3.2. Modified mechanisms of the combined process

The SiC abrasive particles were passed into the gap

between the electrode and the workpiece during the com-

bined process of EDM with USM, stimulated by ultrasonic

vibration of the electrode to create hammering and striking

effects on the workpiece, thus increasing the MRR. Since the

discharge column between the electrode and the workpiece

becomes ionized plasma, the high heat from the plasma will

melt the surface of the workpiece and the electrode, thus

driving the ionized particles to impact the melted surface.

After the melted substrate has cooled down, an alloying

layer is formed on the machined zone.However, some SiC particles around the discharge col-

umn were cracked into sub-particles by thermal energy and

inserted into the aluminum alloy substrate to form particle

dispersing reinforcement. Fig. 6 shows typical SiC dispersed

in the modied layer and silicon mapping obtained by

EPMA. From this gure, it can be seen that the SiC dispersed

in the modied layer can improve the mechanical properties

of the machined surface to achieve the effect of reinforce-

ment. Thus, the combined process of EDM with USM is an

effective way to obtain surface modication.

3.3. Variation of chemical composition in

modified layer

Fig. 7 shows the comparison of silicon distribution in the

cross-section of the machined surface by quantitative ana-

lysis of EPMA. The various processes chosen are: (a)

conventional EDM using commercial dielectric uid; (b)

conventional EDM with 35 wt.% SiC added to the dielectric

uid; (c) the combined process of EDM with USM using

35 wt.% SiC added to the dielectric uid.

As this gure demonstrates, the silicon content from the

combined process reached a peak value of 1.9 wt.% at a

depth of 10 mm from the machined surface. Below a depth of

approximately 80 mm, the silicon content approached thesubstrate value of 0.2 wt.%. Moreover, the conventional

EDM with 35 wt.% SiC added to the dielectric uid had

the same effect as the combined process. However, the

increased silicon content in the modied layer from the

conventional process with a peak value of 0.6 wt.% at 10 mm

from the machined surface was signicantly less than that

from the combined process. In addition, the silicon content

had reduced to be the same as that of the substrate at a depth

close to 70 mm. The variation in silicon content obtained by

conventional EDM using commercial dielectric uid is very

slight. The above results show that SiC added to the dielec-

tric uid will be transferred to the machined zone duringEDM. Therefore, the combined process will both transfer

SiC into the machined zone and disperse the SiC within the

gap more uniformly by ultrasonic vibration. Moreover, the

SiC is inserted into the machined layer by ultrasonic vibration

Fig. 6. Characteristic X-ray images in the cross-section by electron probe microanalysis (Ip: 12 A; tp: 500 ms; Eg: 100 V).

Fig. 7. Distribution of silicon content in the cross-section by various

processes.



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to produce surface modication. In addition, the cavitation

caused by ultrasonic vibration generates a micro-agitation in

the melted zone, so the added SiC can penetrate more deeply

and uniformly into the modied layer.

3.4. Influence of discharge energy on modified

layer depth

Figs. 8 and 9 depict the relationship of modied layer

depth versus discharge current and gap voltage. The modi-

ed layer depth is dened as the depth within which thesilicon content is higher than that of the substrate of alu-

minum by quantitative analysis of EPMA from the machined

surface. Fig. 8 shows the inuence of discharge current on

the modied layer depth, indicating that the silicon content

increased with the discharge current. The maximum mod-

ied layer depth was produced with a discharge current of

12 A. When the current was more than 12 A, the modied

layer depth showed a slight reduction. This may because the

increased discharge current resulted in a deeper melted zone,

so that the modied layer depth of the machined surface was

increased. However, with a longer pulse duration (500 ms)

the higher discharge current (above 12 A) will remove the

melted material more completely. Therefore, there will be

less melted material solidied on the machined zone, thus

decreasing the modied layer depth.

Fig. 9 shows the effect of gap voltage on the modiedlayer depth. From this gure, the modied layer depth

increased with the gap voltage, reaching a peak when the

voltage was about 80100 V. Then, the modied layer depth

slightly reduced as the gap voltage increased. The increased

modied layer depth may be due to the increasing gap

Fig. 8. The influence of the peak current on the depth of the modified

layer.

Fig. 9. The influence of the gap voltage on the depth of the modified layer.

Fig. 10. Vickers hardness in the cross-section of the modified layer for

different processes.

Fig. 11. Weight losses versus sliding distance for different processes.



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voltage expanding the gap distance to include more SiC

particles in the gap, thus leading to a thicker modied layer.

However, when the gap distance is too large, the constriction

in discharge gap will degrade and reduce the impulsive force

caused by EDM. Therefore, the modied layer depth showed

a slight decrease when the gap voltage exceeding the optimal

quantity (100 V).

3.5. Micro-hardness test

Fig. 10 depicts the distribution of micro-hardness in the

cross-section of the surface machined by the combined

process of EDM with USM and machined by conventional

EDM. As shown in the gure, a softened layer of about

80 mm was observed to be adjacent to the zone machined by

conventional EDM. Since the aluminum alloy is a precipi-

tated strengthening material, the precipitated reinforcement

phase would dissolve in the matrix, resulting in over-aging

by local high temperature due to EDM [10]. This could leadto a softened layer in the machined zone. In contrast, a

hardened layer of about 40 mm was found in the layer

machined using the combined process, with a softened layer

adjoining the hardened layer. Because the zone machined

by the combined process could produce both particles

Fig. 12. Comparison of the wear surface for different pre-processes.



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reinforced and solid solution strengthened in the modied

layer, the hardness could be signicantly improved by the

combined process and a higher hardness obtained in the

machined zone.

3.6. Wear test

Fig. 11 is the wear test result of the workpiece machined

by the following processes: (a) abrasive machining; (b)

conventional EDM; (c) combined process of EDM with

USM.

The device for this test was Falex Model No. 6. The initial

surface roughness values after the above-mentioned pro-

cesses are all the same at 40 mm (Rmax). As shown in this

gure, the wear resistance for the combined process is the

best and that for the conventional EDM is the worst. This

may be because the machined zone obtained by the com-

bined process will have strengthening by particles reinfor-

cement and AlSi solid solution strengthening structure in

the modied layer and effectively improve its wear resis-tance. Moreover, the local high temperature results from

conventional EDM will lead to over-aging and re-dissolution

of the precipitated reinforcement phase, which could soften

the machined surface of the aluminum alloy and reduce its

wear resistance. Fig. 12 shows the micrographs of SEM for

the specimen achieved from: (a) abrasive machining; (b)

conventional EDM; (c) the combined process of EDM with

USM. To compare the wear resistance of the machined

surfaces from the three processes, all the samples have been

wear tested. As shown in the gure, it is clear that the contact

scars of the specimen after the wear test are different

between the combined process and conventional EDM.The scars of the specimen from conventional EDM are

the deepest and those from the combined machined are

the attest, and maintaining smooth surface integrity after

the wear test. This evidence shows that the combined process

is able to improve the wear resistance of the machined

surface.

4. Conclusions

A series of experiments for surface modication has been

conducted using the combined process of EDM with USM.

The results as compared with conventional EDM in terms of

surface improvement are as follows:

1. The combined process of EDM with USM is a feasible

and effective process for surface improvement. The

machined surface can develop a modified layer of SiC

particles within the machined zone, as well as an AlSi

solid solution strengthening structure. This reinforcedmodified layer can be obtained in a very short time.

2. The silicon and carbon content in the machined zone

developed by positive polarity EDM is greater than that

by negative polarity, this indicates that positive polarity

machining could provide significantly better surface

modification.

3. The combined process can provide a hardened layer in

the machined zone, which is very different from the

layer provided by conventional EDM.

4. The wear resistance of the aluminum alloy could be

effectively improved by the combined process, thus

overcoming the softened layer of aluminum alloy causedby conventional EDM.

Acknowledgements

The authors would like to thank the National Science

Council of the Republic of China for nancially supporting

this research under Contract No. NSC 88-2212-E-008-004.

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[5] I. Ogata, Y. Mukoyama, Int. J. Jpn. Soc. Prec. Eng. 27 (3) (1993)

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[6] B.H. Yan, M.D. Chen, J. Jpn. Inst. Light Met. 44 (5) (1994) 281285.

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