Study of surface integrity using the small area EDM process with a copper–tungsten electrode

Materials Science and Engineering A364 (2004) 346–356

Study of surface integrity using the small area EDM process with a copper–tungsten electrode Hwa-Teng Lee∗ , Fu-Chuan Hsu, Tzu-Yao Tai Department of Mechanical Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 14 March 2003; received in revised form 23 August 2003

Abstract The present study performs the small area electro-discharge machining (EDM) process with a low wear-rate copper–tungsten electrode of diameter 1.5 mm to establish the influence of the EDM parameters on various aspects of the surface integrity of AISI 1045 carbon steel. The residual stress induced by the EDM process is measured using the Hole-Drilling Strain-Gage Method. The experimental results reveal that the values of material removal rate (MRR), surface roughness (SR), hole enlargement (HE), average white layer thickness (WLT), and induced residual stress tend to increase at higher values of pulse current and pulse-on duration. However, for an extended pulse-on duration, it is noted that the MRR, SR, and surface crack density all decrease. Furthermore, the results indicate that obvious cracks are always evident in thicker white layers. A smaller pulse current (i.e. 1 A) tends to increase the surface crack density, while a prolonged pulse-on duration (i.e. 23 ␮s) widens the opening degree of the surface crack, thereby reducing the surface crack density. The EDM hole drilling process induces a compressive residual stress within the workpiece. A linear relationship is identified between the maximum residual stress and the average white layer thickness. It is determined that the residual stress can be controlled effectively by specifying an appropriate pulse-on duration. © 2003 Elsevier B.V. All rights reserved. Keywords: Copper–tungsten electrode; White layer; Residual stress; Hole-Drilling Strain-Gage Method

1. Introduction Electro-discharge machining (EDM) is a fundamental manufacturing process, which has been used extensively in the tool and dies industry, and which is now commonly applied in the fabrication of Micro-Electro Mechanical Systems (MEMS) [1–3]. In the EDM process, the electric sparks generate a high temperature which melts the workpiece material, i.e. material removal is achieved primarily by electro-thermal mechanisms. The EDM surface is formed by a series of discrete discharges between the electrode and workpiece, and consequently, an inspection of the machined surface reveals the presence of many craters. This machining technique is applicable to a wide variety of conductive materials irrespective of their mechanical properties, e.g. their hardness, strength, or toughness, etc. Furthermore, since no direct contact occurs between the electrode and the workpiece, the EDM process is suitable ∗ Corresponding author. Tel.: +886-6-2757575x62154; fax: +886-6-2745698. E-mail address: [email protected] (H.-T. Lee).

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.08.046

for the machining of brittle materials such as ceramic, and for those materials which are not readily machined using traditional machining methods. According to an investigation performed by Luo [4], small erosion area (SEA) machining (0.09–0.80 cm2 ) is one of the most problematic forms of EDM since the integrity of the EDM surface is degraded by the unstable arcing which always occurs during the machining process. The quality of an EDM product is evaluated in terms of its surface integrity, which is characterized by the surface roughness, and by the presence of a white layer, surface cracks, and residual stress. The roughness of the EDM surface is associated with the distribution of the craters formed by the electric sparks. Lim et al. [5] has reported that only 15% of the molten workpiece material is flushed away by the dielectric. The remaining material re-solidifies on the EDM surface due to the fast cooling rate generated by the dielectric. This recast layer is referred to as the white layer since it is very difficult to etch and because its appearance when observed through an optical microscope is white. It has been suggested previously that the electric sparks generated during machining decompose the kerosene dielectric, and that the resulting carbon penetrates

H.-T. Lee et al. / Materials Science and Engineering A364 (2004) 346–356

in to the machined surface [6,7]. Consequently, an inspection of the white layer reveals that it is densely infiltrated with carbon elements. The formation of surface cracks can be attributed to the differentials of contraction stress within the white layer, i.e. molten material with a higher carbon ingression contracts to a greater degree than the remaining molten material on the EDM surface, and when the contraction stress exceeds the material’s ultimate tensile stress, cracks will form [8,9]. According to Rebelo et al. [10] and Merdan and Arnell [11], residual stress within the EDM surface is the result of phase transformation of the metal and the high temperature gradient established during the EDM process. In terms of the mechanical properties of the white layer, it has high hardness and good wear-resistance due to the quenching effects of the dielectric and the carbon penetration. However, the cracks and residual stress which usually appear in this brittle white layer may lead to the failure of the EDM product, particularly under conditions of fatigue and impact loading [12,13]. Consequently, if the quality of the EDM product is to be improved, it is essential to develop a thorough understanding of the relationship between the EDM machining parameters and the resulting surface roughness, white layer, surface cracks, and residual stress. However, the surface integrity of an EDM product is dependent on the electrode material. Hence it is worthwhile exploring the EDM surface integrity associated with the use of a Cu–W electrode. Electrode miniaturization allows the EDM technique to be applied within MEMS fabrication processes. A review of the published literature reveals the use of a variety of electrode materials in the micro-EDM process, including copper [14–19], tungsten [19–22], and tungsten carbide [18,23,24]. However, the use of a small electrode increases the electrical discharge energy per unit area and this results in excessive electrode wear. Therefore, it is difficult to control the depth of the machined hole precisely, and consequently, the goal of high-precision machining is not readily attained. Two basic approaches exist to minimize the electrode wear. Firstly, the values of the EDM parameters can be specified such that the discharge energy per unit area is minimized. However, this approach prolongs the machining time. A second method is simply to use an electrode such as a copper–tungsten electrode, which has higher wear-resistance properties. The thermal properties of copper–tungsten alloy render it suitable for use as an EDM electrode, i.e. its Cu component provides a high thermal conductivity, while its W component enhances the spark erosion resistance, has a low thermal expansion coefficient, and possesses a high melting temperature [25]. Due to its high tungsten content and the existence of Cu (i.e. Cu–75 wt.% W), the Cu–W electrode has good wear-resistance properties and well thermal conductivity, and is suitable for the machining of small holes with flat bottoms, where high-precision machining is the main consideration. According to ASTM Standard E837 for residual stress measurement by the Hole-Drilling Strain-Gage Method [26], the depth of the drilled hole must be controlled precisely.

347

Hence the Cu–W electrodes are adopted in the present investigation. Since Cu–W electrodes are more expensive than the traditional copper or graphite electrodes, they tend not to be used in commercial EDM processes, and the experimental results for small area EDM machining using such electrodes are seldom published. Thus, the present investigation employs Cu–W electrodes in the machining of AISI 1045 carbon steel in order to explore how the EDM parameters influence the machined surface integrity. The present results provide a valuable source of reference for the use of copper–tungsten electrodes in the small area EDM process.

2. Experimental design and method 2.1. EDM parameters setting and procedure Previous studies by Lee et al. [27–29] using the Taguchi approach revealed that the EDM surface integrity is influenced primarily by the pulse current and the pulse-on duration. This present study also considers these two principal EDM parameters, and combines different values of each parameter to design a total of 16 different EDM conditions. The indices chosen to evaluate the surface quality of the EDM process included the material removal rate (MRR), the surface roughness (SR), the hole enlargement (HE), the white layer thickness (WLT), the presence and type of surface cracks, and the residual stress. It is known that the presence of alloying elements such as Cr, Mo, and V in the workpiece will cause deformation of its crystalline lattice and will induce residual stress within the material. Hence, the AISI 1045 carbon steel is used as the workpiece since it has a low alloying element content and is straightforward to eliminate internal residual stresses within this material by means of heat treatment. In this study, the AISI 1045 material is first fully annealed, and then stress relieved, in order to obtain a non-stressed state. In this way, the noise on the residual stress measurement is reduced, and therefore, the calculated residual stress obtained from the Hole-Drilling Strain-Gage Method is a result of the EDM hole drilling process only. Full factorial experiments were carried out using a copper–tungsten electrode (Cu–75 wt.% W) to machine AISI 1045 carbon steel (0.45 wt.% C, 0.57 wt.% Mn, 0.16 wt.% Si). The experimental results enable the influence of the EDM parameters on the surface integrity to be established for the particular case of the small area EDM process with a copper–tungsten electrode. Table 1 summarizes the EDM parameter settings adopted in the present study. It can be seen that the pulse current ranges from 1 to 12 A, and that the pulse-on durations (τ ON ) and the pulse-off durations (τ OFF ) are 9, 12, 18, and 23 ␮s. Each experiment specified an open voltage value of Vp = 200 V and a duty factor (τ ON /(τON + τOFF )) equal to 0.5. Furthermore, the working time for each EDM condition was 3 min, and kerosene was used as the dielectric liquid throughout. Machining was performed using an M-style 21-Series CNC EDM machine

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Table 1 The setting of EDM parameters EDM parameters

Setting condition

Pulse current (A) Pulse-on duration (␮s) Pulse-off duration (␮s) Open voltage (V) Duty factor

1, 4, 8, 12 9, 12, 18, 23 9, 12, 18, 23 200 0.5

produced by YAWJET Inc. in Taiwan. ASTM Standard E837 for residual stress measurement [26] specifies that the diameter of the drilled hole should be in the range of 1.5–2.6 mm. Therefore, a cylindrical electrode of diameter 1.5 mm is selected in the current study. It is noted that machining using an electrode of this diameter falls between the small erosion area machining process and the micro-EDM process. Tensile testing reveals that the yield stress of AISI 1045 after annealing is 291 MPa, and that the Poisson’s ratio and Young’s modulus are 0.3 and 205 GPa, respectively. In the machining investigation, appropriate values are initially specified for the open voltage, pulse current, pulse-on duration, and pulse-off duration parameters, and the cylindrical Cu–W electrode then moves automatically towards the workpiece under the control of a CNC controller. When the open voltage breaks down the dielectric insulation, an electric plasma channel passes through the dielectric, and the resulting electric spark forms a crater on the EDM surface. In order to establish the material removal rate, the weight of the workpiece is measured both before and after the EDM process using a micro-level balance. Meanwhile, a scanning electron microscope (SEM) is used to analyze the surface cracks and the hole-enlargement characteristics of the machined surface. The white layer is revealed by standard metallurgical procedures using cutting, grinding, polishing, and etching (3% Nital) techniques. A TAMAYA Area Instrument is used to measure the white layer thickness and the surface crack density. The value of the average white layer thickness is determined on the basis of five randomly selected photos taken at a magnification of 1000×, and the surface crack density is defined as the crack length per unit area. The surface roughness is measured using a Surfcoarder SE-30AK instrument. After analyzing the results of the 16 experiments, the Hole-Drilling Strain-Gage Method is used to carry out a further investigation of the residual stress measurement for four specific EDM conditions. 2.2. Method for residual stress measurement The Hole-Drilling Strain-Gage Method is a semi-destructive measurement method which was first proposed by Mathar [30] in 1934. This technique involves drilling a hole in the center of a rosette strain gage attached to the surface of the workpiece. According to elastic mechanics theory, the stress within a component can be calculated by

measuring the strain released on the specimen surface as the hole is drilled vertically into the workpiece. Following substantial investigation and calibration [31–33], this method is now a mature technique which has been widely applied throughout industry due to its intrinsic advantages of a smaller drilled hole and its non-interference with the workpiece functionality. This method has been quoted in ASTM Standard E837 since 1981 [26]. In accordance with ASTM Standard E837 [26], the electric-resistance rosette gage is first attached to the surface of the non-stressed AISI 1045 workpiece. The strain gage used throughout the current experiments is the TEA-06062RE device manufactured by Measurements Group Inc. The diameter of the gage circle (D) is 5.13 mm. The ratio of the drilled hole diameter (Do ) to that of the gage circle (D) should be in the range of 0.3–0.5, i.e. the diameter of the drilled hole (Do ) should lie between 1.5 and 2.6 mm. Furthermore, the depth of the drilled blind hole should be equal to 0.4D, which is equivalent to a depth of approximately 2.0 mm. In the current investigation, the center of the Cu–W electrode is aligned with the center of the gage circle, and a hole is then drilled progressively into the AISI 1045 workpiece in 10 discrete drilling operations until the depth of 2.0 mm has been attained. The relieved strain caused by the drilling operation is detected by the rosette strain gage and displayed on a P-3500 Strain Indicator manufactured by Measurements Group Inc. The three values of measured strain, i.e. ε1 , ε2 , and ε3 , are then substituted into Eq. (1) in order to calculate the minimum and maximum principal residual stresses within the workpiece. σmin , σmax

ε3 + ε1 = ± ¯ 4A




(ε3 − ε1 )2 (ε3 + ε1 − 2ε2 )2 ¯ 4B (1)

¯ and B¯ are calibration coefficients. where A Schajer [32,33] overcome the area effect of the strain gage and used finite-element calculation to determine the values of these calibration coefficients. The numerical solutions of ¯ and B¯ can be expressed as: A ¯ = − 1 + υ × a¯ A 2E B¯ = −

1 × b¯ 2E

(2a)

(2b)

where a¯ and b¯ are dimensionless coefficients, E is Young’s modulus, and ν is Poisson’s ratio.The values of the dimensionless coefficients a¯ and b¯ are virtually material-independent and are correct to within 1% for a Poisson’s ratio between 0.28 and 0.33. The exact values of the two coefficients depend upon the ratio of Do /D and are presented in ASTM Standard E837 [26]. If the material ¯ properties E and ν are known, the calibration coefficients A ¯ and B are given by Eqs. (2a) and (2b), respectively.

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5.0

349

Plasma channel 12A

8A 4A

MRR (mg/min)

4.0

1A

3.0 2.0 1.0

D 0.0 0

5

10

15

20

25

30

Pulse-on duration (µs) Fig. 1. Relationship between the MRR and EDM parameters.

3. Experimental results 3.1. Material removal rate, surface roughness, and hole enlargement Fig. 1 shows the relationship between the material removal rate and the pulse-on duration for four different values of pulse current. It can be seen that the MRR increases with increasing pulse current for a constant pulse-on duration. Further, it is noted that a continuous lengthening of the pulse-on duration does not necessarily improve the MRR. This is particularly evident for pulse currents of 8 and 12 A, where it is observed that the MRR decreases dramatically between 18 and 23 ␮s. Marafona and Wykes [34] also discovered that an extended pulse-on duration tends to reduce the material removal rate. This phenomenon is attributed to the extension of the electric plasma channel associated with an excessive pulse-on duration. When the plasma channel becomes large, its energy density decreases, and the heat energy absorbed by the workpiece per unit area is reduced. Thus, the energy provided by the plasma channel melts the material, but is insufficient to produce a high exploding pressure of the dielectric which can spray the molten metal away from the EDM surface. Consequently, the debris formed from the molten metal is not swept away by the circulative dielectric system, and hence the MRR decreases [35]. Fig. 2 illustrates the effect of the electric plasma channel on the EDM surface morphology. It is noted that D represents the crater diameter. The EDM surface is characterized by the presence of many craters, which are caused by the successive impacts of discrete electrical discharges. It is observed that the shape of the EDM drilling area presented in Figs. 3 and 5 resemble the original cylindrical shape of the electrode, where the symbol “D” also means the crater diameter. The diameter of these machined areas are approximately 1.5 mm, i.e. they are of the same diameter as the Cu–W electrode. Fig. 3 presents the morphologies of the EDM surface associated with the four pulse-on durations considered in the present study. As the pulse-on duration is extended, it

Workpiece Fig. 2. Illustration of electric plasma channel and EDMed surface morphology, D: crater diameter.

can be seen that the craters become larger and flatter under the influence of the expanding plasma channels discussed above. From these results, it is clear that the MRR cannot be enhanced simply by lengthening the pulse-on duration, i.e. it is necessary to consider the pulse-on duration and the pulse current parameters in combination. From Fig. 1, it is observed that the pulse-on duration does not affect the MRR significantly when pulse currents of 1 and 4 A are specified. Moreover, for pulse currents of 8 and 12 A, the optimum MRR results are obtained when a pulse-on duration between 12 and 18 ␮s is established. These EDM conditions avoid the presence of expansion craters on the machined surface, and result in a deeper machining depth. Fig. 4 illustrates the relationship between the SR and the pulse-on duration for different values of pulse current. It is clear that for a constant pulse-on duration, the SR increases with increasing pulse current. This phenomenon is similar to that observed in Fig. 1 for the case of the MRR, and therefore, it is possible to infer that a higher MRR will result in a rougher EDM surface. For pulse currents of 4, 8, and 12 A, the SR decreases dramatically during the pulse-on duration of 18–23 ␮s. This result may also be attributed to the expansion of the plasma channels. Fig. 5 shows the EDM morphology which results from an extended plasma channel at 4 A/23 ␮s/200 V. It can be seen that the morphology is similar to that presented in Fig. 3(d) for an EDM condition of 8 A/23 ␮s/200 V. The results indicate that both sets of EDM conditions lead to an expansion in the size of the crater. Since these large, smooth craters are distributed uniformly with an approximately circular shape on the machined surface, the probe track of the surface roughness instrument always falls on their surface. Hence, as shown in Fig. 4, the SR values at the EDM conditions of 4 A/23 ␮s/200 V and 8 A/23 ␮s/200 V are almost identical, and are significantly less than those values recorded at 4 A/18 ␮s/200 V and 8 A/18 ␮s/200 V, respectively. The HE evaluation index, which is also referred to as “clearance,” indicates the difference between the diameter of the EDM-drilled hole and that of the electrode. Fig. 6

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Fig. 3. The influence of increasing pulse-on duration on EDMed surface morphology and crater diameter, D. Material: AISI 1045, electrode: Cu–W, Ip = 8 A, Vp = 200 V (SEM).

presents the relationship between the HE and the EDM parameters. It can be seen that the HE increases with rising pulse current and lengthening pulse-on duration. According to Luo [4], the number of sparks discharged in the feed direction of the electrode is higher than the number discharged from the side of the electrode since the gap size between the electrode and the workpiece is small. Regarding the circular electrode used in the present study, the electric spark occurs more readily at the bottom of the electrode. The value of the HE is initially determined by the area affected by the elec-

Surface roughness Ra (µm)

6

tric spark. Increasing either the pulse current or the pulse-on duration extends the affected area of the electric spark bombardment of the workpiece, and this results in an increased HE value. From Fig. 6, it is clear that the extension of the plasma channel caused by a prolonged pulse-on duration of 23 ␮s increases the value of HE significantly. The HE analysis performed in the current study enables the optimum size of the copper–tungsten electrode to be determined, and thus represents a significant step towards the goal of achieving high-precision machining.

12A 8A 4A 1A

5 4 3 2 1 0

0

5

10 15 20 Pulse-on duration (µs)

25

30

Fig. 4. Relationship between the surface roughness and EDM parameters.

Fig. 5. The large and flat craters caused by the expanding of electric plasma channel. D: crater diameter, material: AISI 1045, electrode: Cu–W (SEM, 4 A/23 ␮s/200 V).

H.-T. Lee et al. / Materials Science and Engineering A364 (2004) 346–356 Table 2 Relationship between average WLT and EDM parameters

0.20

Hole Enlargement (mm)

12A

Ip (A)

8A

0.16

4A

1A

0.08

5

10

15

20

25

1.93 2.09 2.72 4.06

4

9 12 18 23

1.88 3.07 3.49 11.20

8

9 12 18 23

1.67 3.19 7.17 8.57

12

9 12 18 23

2.27 2.93 6.65 13.92

30

Pulse-on duration (µs) Fig. 6. Relationship between the hole enlargement and EDM parameters.

3.2. White layer thickness and crack formation Fig. 7 illustrates the relationship between the average WLT and the EDM parameters. Note that Table 2 provides a tabulation of the corresponding data. The average WLT is seen to increase with increasing pulse-on duration (τ ON ) for a constant pulse current. Since a larger pulse-on duration allows the electro-discharge energy to penetrate deeper into the material, the thickness of the molten metal increases, which ultimately results in a greater white layer thickness. Mamlis et al. [35] introduced the concept of melting isothermals. When the pulse-on duration increases, the melting isothermals penetrate further into the interior of the material, i.e. the molten zone extends further into the material, and this results in a greater white layer thickness. Additionally, increasing the pulse current enables the temperature of the machined surface to reach the melting point of the metal more readily. Therefore, the average WLT is also seen to increase with increasing pulse current.

WLT (␮m)

9 12 18 23

0.04

0

τ ON (␮s)

1

0.12

0.00

351

Ip: pulse current (A); τ ON : pulse-on duration (␮s); WLT: white layer thickness (␮m).

The unusually high value of WLT which occurs at an EDM condition of 4 A/23 ␮s/200 V is caused by the insufficient exploding pressure of the dielectric. Under this condition, the vaporization pressure is insufficient to spray the molten metal away from the surface of the workpiece, and so the cooling effect of the dielectric causes the metal to re-solidify smoothly on the machined surface. Therefore, as shown in Fig. 8, the WLT becomes extremely thick. Furthermore, since the molten metal is not removed by the dielectric, the MRR at this particular EDM condition is lower, as shown in Fig. 1 (4 A/23 ␮s/200 V). Fig. 9(a) shows the extremely wide cracks evident at extended pulse-on durations. Meanwhile, Fig. 9(b) demonstrates that many micro cracks exist on the machined surface under lower pulse current conditions. Fig. 9(c) and (d) reveals that the surface crack extends as far as the

14 12 10 8 6 4 23

2 18 12

N

1

(µs

)

0 4

9

8

Pulse current (A)

τO

Average white layer thickness (µm)

16

12

Fig. 7. Relationship between the average white layer thickness and EDM parameters.

Fig. 8. The thicker white layer re-solidified on the EDMed surface. Material: AISI 1045, electrode: Cu–W (SEM, 4 A/23 ␮s/200 V).

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Fig. 9. The morphology of EDMed surface by Cu–W electrode on AISI 1045 (SEM): (a) extremely open crack (low surface crack density, 12 A/23 ␮s/200 V), (b) micro crack (high surface crack density, 1 A/18 ␮s/200 V), (c) penetrating crack in the cross section (12 A/23 ␮s/200 V), (d) penetrating crack in the cross section (1 A/18 ␮s/200 V), and (e) no crack (4 A/9 ␮s/200 V).

white layer under EDM conditions of 12 A/23 ␮s/200 V and 1 A/18 ␮s/200 V, respectively. Finally, Fig. 9(e) shows an EDM surface with no cracks (4 A/9 ␮s/200 V). Fig. 10 presents the types of cracks associated with the 16 EDM conditions considered in the present study, and provides a qualitative analysis of the crack distribution. The results show that cracks readily occur for low values of pulse current (e.g. 1 A) or at longer pulse-on durations (e.g.23 ␮s). For a pulse current of 1 A, the cracks are in the form of mi-

cro cracks. If the pulse-on duration is increased to 23 ␮s, the severity of the crack opening increases accordingly. Fig. 10 suggests that the cracks may be suppressed by increasing the pulse current and reducing the pulse-on duration. Specifically, the results indicate that the cracks are eliminated if the pulse current is specified to be greater than 8 A and a pulse-on duration of no longer than 12 ␮s is applied. The current study adopts a parameter referred to as the surface crack density (defined as the crack length per unit

Pulse-on duration (µs)

20

Pulse-on duration (µs)

H.-T. Lee et al. / Materials Science and Engineering A364 (2004) 346–356

CCL

15

10

16

H13

Zone

D2

12

8 4

No Crack Zone

Extremely open crack

5

Crack existence

0

No crack

5

10

15

Fig. 10. The type of crack and the location of critical crack line (CCL) under different EDM conditions.

area) to provide a quantitative analysis of the cracks associated with the various EDM conditions. The relationship between the surface crack density and the two EDM parameters is presented in Fig. 11, from which it can be observed that for a constant pulse current, the crack density initially increases as the pulse-on duration is extended. However, it is also clear that an excessive pulse-on duration decreases the crack density. This phenomenon is due to the severe cracks noted previously at a pulse-on duration of 23 ␮s. In terms of the energy released during machining, a more severe crack opening indicates a greater energy release. Furthermore, with a greater severity of crack opening, a lower crack density is sufficient to release the unstable energy and to achieve an equilibrium state under a longer pulse-on duration. As shown in Fig. 9(b), an excessively small pulse cur-

1.40 1.20 1.00 0.80 0.60 0.40 23

0.20

(µs )

18

4

9 8

τ

0.00

ON

12 1

4

12 16 8 Pulse current (A)

Fig. 12. Modified crack prediction map of tool steel D2 and H13. [36] Material: D2 and H13 tool steel, electrode: Copper.

Pulse current (A)

Surface Crack density (cm/cm 2 )

Crack

353

12

Pulse current (A) Fig. 11. Relationship between the surface crack density and EDM parameters.

rent of 1 A results in an unstable machining process, which results in an abundance of micro cracks on the EDM surface. The experimental results for crack formation may be generalized by establishing the critical crack line (CCL) shown in Fig. 10. Increasing the pulse current allows the pulse-on duration to be extended without resulting in cracking of the EDM surface. Lee and Tai [36] have previously investigated the crack formation on D2 and H13 tool steels with copper electrodes. A modified crack prediction map of these tool steels is shown in Fig. 12. Their experimental results also revealed that increasing the pulse-current and shortening the pulse-on duration can reduce the occurrence of cracks. Thus, different electrode materials do not appear to affect the tendency of crack formation under different EDM conditions. 3.3. Investigation of EDM residual stress For the particular rosette strain gage used in this study, the drilled hole (Do ) should lie between 1.539 and 2.565 mm (0.3–0.5D) [26]. Meanwhile, the diameter of the copper–tungsten electrode is 1.5 mm. In considering the HE evaluation index, it is noted that three of the 16 EDM conditions, i.e.1 A/9 ␮s/200 V, 1 A/12 ␮s/200 V, and 1 A/18 ␮s/200 V, result in a low value of HE (see Fig. 6). Therefore, the drilled hole diameter falls below the minimum value of 1.539 mm, and consequently these three machining conditions are inappropriate. However, the EDM hole diameters machined under the other 13 conditions satisfy the requirements of ASTM Standard E837 [26]. The EDM conditions of 4 A/9 ␮s/200 V, 4 A/23 ␮s/200 V, 12 A/9 ␮s/200 V, and 12 A/23 ␮s/200 V were selected for further residual stress investigation. Fig. 13 shows the relationship between the residual stress induced by EDM hole drilling and the corresponding EDM conditions. The residual stresses are derived from Eq. (1) and are based on the strain values ε1 , ε2 , and ε3 measured by the rosette strain gage during the EDM drilling of the hole. The relative values of the measured strains and calculated stresses are tabulated in Table 3. It is noted that the

Induced compressive residual stress (MPa)

354

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σmax σmin

200

150

100

50

0 4A/9µs

12A/9µs

4A/23µs

12A/23µs

EDM working conditions

Fig. 13. The induced residual stress under different EDM working conditions. Table 3 Measured strain and calculated stress under different EDM conditions EDM conditions

Measured strain (␮ε)

Calculated stress (MPa)

ε1

ε2

ε3

σ max

σ min

4 A/9 ␮s 12 A/9 ␮s 4 A/23 ␮s 12 A/23 ␮s

0 5 120 180

19 33 124 183

25 37 151 205

−26.2 −39.0 −153.0 −200.3

−7.2 −13.6 −131.9 −184.0

residual stresses presented in Fig. 13 are of the compressive type. It is clear that increasing the pulse current or the pulse-on duration results in a greater compressive residual stress. With a pulse-on duration of 9 ␮s, the maximum principal stresses induced by pulse currents of 4 and 12 A, are 26.2 and 39.0 MPa, respectively. When the pulse-on duration is increased to 23 ␮s, the maximum principal stress in-

duced by a pulse current of 4 A is 153.0 MPa, which is approximately 53% of the material’s yielding stress. Moreover, the maximum principal stress induced at an EDM condition of 12 A/23 ␮s is 200.3 MPa, which corresponds to approximately 69% of the yielding stress. The experimental results indicate that there is a significant increment in the residual stress as the pulse-on duration is extended. Therefore, the magnitude of the induced residual stress can be controlled effectively by specifying an appropriate pulse-on duration value. The relationship between the induced residual stress and the average WLT can be evaluated for each of the EDM conditions. The results are presented in Fig. 14. It is observed that the average WLT and the maximum compressive residual stress increase for higher values of pulse current and pulse-on duration. At an extended pulse-on duration (i.e. 23 ␮s), the average WLT and maximum compressive residual stress increase significantly. This suggests that there is a strong dependency between these two characteristics. In fact, Fig. 14 shows there to be a linear relationship between the maximum compressive residual stress (σ max ) and the average WLT. Statistical analysis determines that they are related by the expression σmax = −14.15 × WLT. Thus, the thicker the white layer, the greater the magnitude of the induced residual stress. As shown in Fig. 10, EDM conditions of 4 A/23 ␮s/200 V and 12 A/23 ␮s/200 V result in extremely open cracks on the EDM surface. Furthermore, the WLT is also found to be thicker at these two EDM conditions. Therefore, it would appear that the contribution made by cracks to a relaxation of the induced residual stress is limited. From the residual stress analysis of this study, the linear relationship between the average WLT and the maximum principal stress (σ max ) induced by EDM hole drilling can be described as σ max (MPa) = −14.15 × WLT (␮m). It

Average white layer thickness (µm) 0

5

10

15

20

Maximum induced residual stress (MPa)

0 EDM conditions σ max (MPa)

-50

-100

4A/9µs 12A/9µs 4A/23µs 12A/23µs

-26.2 -39.0 -153.0 -200.3

WLT (µm) 1.88±0.62 2.27±0.86 11.20±2.43 13.92±3.41

σ max = -14.15 x WLT R2 = 0.996

-150

-200

-250 Fig. 14. Relationship between the average white layer thickness and residual stress induced by EDM process.

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can be concluded that the EDM residual stress noted previously by Lee and Hsu [37] was the result of a brittle white layer caused by rapid re-solidification on the EDMed surface, which prevented stress release after completion of the EDM hole drilling process.

4. Discussion The results of the 16 experiments allow the influence of the EDM parameters upon the average WLT, the type of crack, and the surface crack density to be evaluated for the small area EDM process with a 1.5 mm diameter copper–tungsten electrode (i.e. Figs. 7, 10, and 11, respectively). It is found that obvious cracks always appear in white layers with a thickness exceeding 8 ␮m. Conversely, when the WLT is less than 3 ␮m, virtually no cracks are evident. In other words, if extremely open cracks are identified on the EDM surface, it may be inferred that the white layer is thick. Similarly, an absence of cracks implies a thin white layer. When a pulse current exceeding 8 A is applied for a pulse-on duration greater than 18 ␮s, a thick white layer will be formed, which will exhibit obvious cracks. If the pulse-on duration is increased further, the crack opening will become large, and the surface crack density will tend to decrease. For smaller EDM parameters, e.g. a pulse current lower than 4 A and a pulse-on duration of less than 12 ␮s, the white layer will be thinner, and if cracks exist, they will be of the micro crack type. If the pulse current is further reduced, the surface crack density will increase. Therefore, it can be concluded that the pulse current affects the surface crack density, while the pulse-on duration influences the degree of crack opening, i.e. the crack density increases when the pulse current is too small, and the crack opening widens when the pulse-on duration is too long. This study reveals that the residual stress induced by EDM hole drilling is proportional to the average WLT. Thus, if the thickness of the white layer is known, then it is possible to predict the value of the induced residual stress. Bormann [38] stated that the stressed layer of the EDM surface extends beyond the heat-affected zone. Furthermore, it is known that the thicker the WLT, the deeper the heat-affected zone. Therefore, it can be concluded that a thicker white layer will increase the magnitude and depth of the EDM residual stressed layer. Since obvious cracks always appear in thick white layers, reducing the thickness of the white layer is not only effective in decreasing the magnitude and depth of the induced residual stressed layer, but also avoids the occurrence of cracks. In other words, reducing the WLT enhances the quality of the EDM surface.

5. Conclusions This study has investigated the small area EDM process using a copper–tungsten electrode, and has determined the

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relationship between the EDM parameters and various aspects of the surface integrity, namely the MMR, the SR, the HE, the WLT, the presence of cracks, and the degree of induced residual stress. The main conclusions of this study may be summarized as follows: 1. In general, the values of the MRR, SR, and HE evaluation indices increase for higher values of pulse current. However, an excessive pulse-on duration results in an expansion of the electric plasma channel, which results in a reduction in both the MRR and the SR. 2. Obvious cracks always appear in thick white layers, i.e. in excess of 8 ␮m. Conversely, when the WLT is less than 3 ␮m, virtually no cracks are evident. Increasing the pulse current (i.e. larger than 8 A) while shortening the pulse-on duration (i.e. smaller than 12 ␮s) is effective in reducing surface crack. 3. The pulse current affects the surface crack density, while the pulse-on duration influences the degree of crack opening. When the pulse current is too small (i.e. 1 A), the surface crack density increases. When the pulse-on duration is excessive (i.e. 23 ␮s), the severity of the crack opening increases. Furthermore, a significant crack opening is associated with a smaller crack density. 4. EDM hole drilling induces residual compressive stress, whose value increases at higher values of pulse current and pulse-on duration. The linear relationship between the maximum principal stress (σ max ) and the average WLT is given by: σ max (MPa) = −14.15 × WLT (␮m). Adjusting the pulse-on duration is an effective means of controlling the magnitude of the residual stress.

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