Material removal mechanisms of cemented carbides machined by ultrasonic vibration assisted EDM in gas medium

Material removal mechanisms of cemented carbides machined by ultrasonic vibration assisted EDM in gas medium

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1742–1746 journal homepage: www.elsevier.com/locate/jma...

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1742–1746

journal homepage: www.elsevier.com/locate/jmatprotec

Material removal mechanisms of cemented carbides machined by ultrasonic vibration assisted EDM in gas medium M.G. Xu a,∗ , J.H. Zhang b , Y. Li a , Q.H. Zhang b , S.F. Ren c a b c

Department of Precision Instruments and Mechanology, Tsinghua University, Beijing 100084, China School of Mechanical Engineering, Shandong University, Jinan 250061, China School of Mechanical Engineering, Jinan University, Jinan 250022, China

a r t i c l e

i n f o

a b s t r a c t

Article history:

A new electrical discharge machining (EDM) technology named tool electrode ultrasonic

Received 16 August 2006

vibration assisted electrical discharge machining in gas medium (UEDM in gas) is proposed

Received in revised form

and its principle is introduced. Relevant experimental equipment was designed by which a

6 April 2008

series of machining experiments of cemented carbide material were carried out. The mech-

Accepted 12 April 2008

anisms of cemented carbide material removal are discussed in detail through observing and analyzing the microstructures of machined surface. Five material removal mechanisms of cemented carbides machined by UEDM in gas were proposed, which are melting and

Keywords:

evaporation, oxidation and decomposition, spalling, the force of high-pressure gas and the

Material removal

affection of ultrasonic vibration.

Cemented carbide

© 2008 Elsevier B.V. All rights reserved.

Ultrasonic vibration EDM Gas

1.

Introduction

Cemented carbide materials used as tools and structural components play an important role in modern industry by virtue of their unique combination of hardness, strength and wear resistance (Casal et al., 2006). And the application of cemented carbide for cutting die and material-deforming tools wins in the industrial employment an increasing importance (Juhr et al., 2004). But it has high specific strength and cannot be processed easily by conventional machining techniques (Lee and Li, 2001). This kind of materials are generally machined using electrical discharge machining (EDM) and diamond grinding due to their high hardness, and EDM is the



only machining method in some cases (Koshy et al., 1997; Li, 1989). It is generally considered that dielectric liquid medium is necessary in the process of EDM. However, some unwanted gases are always generated in the machining process, which will pollute the environment and do harm to the operator’s health. Health, safety and environment are important ˜ aspects, particularly when hydrocarbon oil is used (Leao and Pashby, 2004). So the green method of EDM without pollution aimed to protect environment has become a hot studying subject in the word recently. The trend of electrical discharge machining in the world is to develop green EDM technology with high efficiency, low waste and

Corresponding author. Tel.: +86 10 62796337. E-mail addresses: [email protected] (M.G. Xu), [email protected] (J.H. Zhang), [email protected] (Y. Li), [email protected] (Q.H. Zhang), [email protected] (S.F. Ren). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.04.031

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Fig. 2 – The lathe structure of UEDM in gas (with tool electrode ultrasonic vibrate). (1) Impulse power; (2) lathe bed; (3) ultrasonic generator; (4) compressor; (5) headstock; (6) ultrasonic transducer; (7) tool electrode; (8) worktable. Fig. 1 – The principle of UEDM in gas.

no pollution (Pandit and Rajurkar, 1981; Ho and Newman, 2003). Gas medium electrical discharge machining is a new technology, which was proposed by Kunieda and Yoshida (1997). The pollution decreases in this method because the EDM is achieved in gas medium instead of kerosene-based oils or mineral oils. Moreover, the electrode erosion rate is very low when selecting appropriate gas medium and rarely affected by impulse-on time. While process instability and arcing failure in electrical discharge machining in gas are still serious problems in practice especially when high erosion rates are attempted. To improve the machining efficiency and quality, ultrasonic vibration assisted EDM in gas was proposed (Zhang et al., 2002; Zhang, 2003; Xu et al., 2006). In this new technology, the tool electrode is formed to be tubular, which vibrates with ultrasonic frequency and rotates with the axis synchronously, as Fig. 1 shows. The high pressure and velocity gas is supplied through the internal hole of the electrode and flow over the discharging gap, which can enhance the removal of molten and evaporated workpiece material. Also, it cools and solidifies the removed material and prevents them from adhering onto the surface of the tool electrode. Furthermore, during the pulse-off time, the gas with a high velocity blows off the plasma formed by the previous discharge guaranteeing the recovery of the dielectric strength of the gap, and decreases the temperature of the discharge spot on the tool electrode and the workpiece due to heat transfer. All these endow the technology with high machining efficiency and wide machining range.

Cemented carbide material was machined by ultrasonic vibration assisted electrical discharge machining (UEDM) in gas medium in this work. Experimental results show that UEDM in gas is suitable for machining cemented carbides. Five material removal mechanisms of cemented carbides machined by UEDM in gas are proposed, which are melting and evaporation, oxidation and decomposition, spalling, the force of high-pressure gas and the affection of ultrasonic vibration.

2.

Experimental conditions

Relevant experimental equipment was designed to study the technology of UEDM in gas. The equipment was specially designed for UEDM in gas, as shown in Fig. 2. (1) Discharge power: output voltage range is from 100 to 300 V and the maximum output current is 40 A. (2) Gas medium: generated by a common air compressor and the maximum output pressure is 5 kPa. (3) The frequency of ultrasonic vibration is 20 kHz. (4) The amplitude of ultrasonic vibration is 12 ␮m. The material of tool electrode is copper and workpiece material is YT15 cemented carbide. The components and physical properties of YT15 cemented carbide are shown in Table 1.

3.

Results and discussions

Experimental results showed that material removal rate (MRR) could be increased greatly when ultrasonic vibration was

Table 1 – Components and physical properties of YT15 cemented carbide Components (%)

Physical properties

WC

TiC

Co

79

15

6

Density

(g/cm3 )

11.0–12.7

Hardness (HRA)

Thermal conductivity (W/m K)

91

33.49

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Fig. 3 – The MRR comparison of traditional EDM in gas and ultrasonic vibration assisted EDM in gas.

introduced. The MRR is defined as the volume (mm3 ) of material removed, divided by the machining time (min). Fig. 3 is the MRR comparison of traditional EDM in gas and ultrasonic vibration assisted gas medium EDM when machining cemented carbides. There are five types of materials removal mechanisms during machining cemented carbides, which are melting and evaporation, spalling, oxidation and decomposition, the force of high-pressure gas as well as material removal in the affection of ultrasonic vibration. The first material removal process is melting and evaporating, which was aroused by the thermal energy in the discharge channel of EDM process. This process depends on electrical

discharge parameters, such as discharge voltage, peak current, pulse-on time, etc. and on the components and properties of cemented carbides, such as melting point, thermal conductivity, hardness, etc. Figs. 4 and 5 show the top surface and cross-section of cemented carbide machined by UEDM in gas medium. As can be seen from Fig. 4, there are many microholes and small droplets on the surface, indicating that the material was melted first and then blown by gas medium with high pressure and velocity. Also, it can be seen from Fig. 5 that the recast layer is thin and full with many microholes and microcracks. The second mechanism of material removal is oxidation and decomposition. The temperature of plasma is very high (which can up to 10,000 K) and the O2 in high pressure and velocity air can easily react with some components of the cemented carbide. For example, O2 can react with TiC of cemented carbide: TiC + 2O2 ⇒ TiO2 + CO2

(1)

Therefore, the oxidation reaction can increase the MRR. The third mechanism is spalling (Lauwers et al., 2004; Trueman and Huddleston, 2000). Spalling is another main material removal mechanism of cemented carbide by which a series of small volumes were separated from the other material, as can be seen from Figs. 5 and 6. Spalling often occurs

Fig. 4 – Top surface and cross-section of cemented carbide machined by UEDM in gas.

Fig. 5 – Cross-section of cemented carbide machined by UEDM in gas.

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Fig. 6 – Microcracks on cross-section of cemented carbide machined by UEDM in gas.

in large pulse-on time during UEDM in gas medium. This is because the larger the pulse-on time, the higher the discharge energy. Also, the spalling depends on the cemented carbide’s properties and EDM parameters imposed on the surface of the material. Moreover, the amount of energy required to generate sub-surface cracks and remove material from the surface by flake detachment is clearly less than that required to remove material directly through the mechanism of melting, evaporation or dissociation, which makes material removal through controlled spalling an attractive proposition (Trueman and Huddleston, 2000). The fourth mechanism is the force of high-pressure gas. The gas with a high velocity and high pressure can blow off the small drops and small chips that produced in the discharging process. Experimental results showed that the MRR increases with the increase of gas pressure. The last mechanism is the effect of ultrasonic vibration. Experimental results showed that the MRR increased whether workpiece ultrasonic vibrates or tool electrode vibrates assisted. The strength model of a molten drop was shown in Fig. 7, which can be considered as a fraction of a sphere, where fc is inertia force aroused by ultrasonic vibrating and fg is gravity, which is so small compared to fc that can be ignored and fz is surface tension of liquid drops.

The displacement between a liquid drop and the ultrasonic vibration node can be expressed as formula (2) during ultrasonic vibration assisted gas medium electrical discharge machining: y(t) = A sin

 2f  c

x

sin(2ft)

(2)

The speed equation and acceleration equation can be calculated by the following formulae: y (t) = 2fA sin

 2f  c

y (t) = 42 f 2 A sin

x

cos(2ft)

 2f  c

x

sin(2ft)

(3)

(4)

So, the maximum acceleration of a certain molten drop is showed as ymax = 42 f 2 A

(5)

And the maximum inertia force of the drop is Fmax = mamax = mymax = 4m2 f 2 A

(6)

The crater’s volume of a single pulse can be calculated by formula (7) and the section plane model of craters is showed in Fig. 7. Therefore, the maximum inertia force can be calculated: Fmax

  2

D 2 = H3 f 2 3 3 2

 +H

2

(7)

where f is the frequency of ultrasonic vibration, A is the amplitude of ultrasonic vibration, D is the diameter of the discharge crater and H is the depth of the discharge crater. And the surface tension of the liquid drops can be reckoned by formula (Zhang et al., 1996) (8): a = 100M/Tm /C1 /C2

Fig. 7 – The strength model of molten drops.

(8)

where M is a coefficient,  is the density of the metal, Tm is the melting point of the metal, C1 is adjusting coefficient of density and C2 is adjustment coefficient of melting point.

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It is obvious that the melting liquid drops can be easily shot off in the effect of inertia force aroused by ultrasonic vibrating. Also, EDM is a thermal process that can lead great temperature on workpiece’s surface. Great temperature gradients result in large nonuniformity in the thermal expansion of workpiece material that can lead to high thermal stress (Yadav et al., 2002). And microcracks occurs when the thermal stress beyond the tension intensity of the material. It can be seen from Fig. 6 that there are many microcracks on the surface and cross-section. And these cracks can be divided into two types: horizontal cracks and vertical cracks. Also, most of the horizontal cracks were periodically intersected by vertical cracks. It is noted that some of the vertical cracks running through recast layer showing no extension beyond sub-surface of machined surface. The separated material by horizontal and vertical cracks was easily shot off in the affection of ultrasonic vibration.

4.

Conclusions

Five types of cemented carbide material removal mechanisms, which are melting or evaporation, spalling, oxidation, the force of high-pressure gas and ultrasonic vibration affection, were discussed in detail in the work. Moreover, the formation mechanisms of microcracks were investigated. The microcracks depend not only on electrical discharge parameters, such as discharge voltage, peak current, pulse-on time, etc, but also on properties of cemented carbide that was machined, such as melting point, thermal conductivity, fracture toughness, etc.

Acknowledgements The work is supported by Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20030422014) and The National Natural Science Foundation of China (Grant No. 50575128).

references

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