J. Cent. South Univ. (2014) 21: 151−159 DOI: 10.1007/s11771-014-1926-3

Mechanism of ultrasonic-pulse electrochemical compound machining based on particles

ZHANG Cheng-guang(张成光), ZHANG Yong(张勇), ZHANG Fei-hu(张飞虎)

School of Mechanical and Electrical Engineering, Harbin Institute of Technology, Harbin 150001, China

© Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: The electric double layer with the transmission of particles was presented based on the principle of electrochemistry. In accordance with this theory, the cavitation catalysis removal mechanism of ultrasonic-pulse electrochemical compound machining (UPECM) based on particles was proposed. The removal mechanism was a particular focus and was thus validated by experiments. The principles and experiments of UPECM were introduced, and the removal model of the UPECM based on the principles of UPECM was established. Furthermore, the effects of the material removal rate for the main processing parameters, including the particles size, the ultrasonic vibration amplitude, the pulse voltage and the minimum machining gap between the tool and the workpiece, were also studied through UPECM. The results show that the particles promote ultrasonic-pulse electrochemical compound machining and thus act as the catalyzer of UPECM. The results also indicate that the processing speed, machining accuracy and surface quality can be improved under UPECM compound machining. Key words: ultrasonic; pulse electrochemical machining (PECM); cavitation catalysis; removal mechanism; particles; electric double layer

1 Introduction

Ultra-precise machining technique plays an important role, not only in the improvement of the function and quality of machine and electric appliances and the development of high-tech products, but also in demonstrating the manufacturing level of a country and supporting the development of advanced science and technology [1].

In the vast and complex field of electrotechnologies, pulse electrochemical machining technique (PECM) represents a relatively new and important method of advanced material processing, where the traditional machining technologies become unable. PECM is based on removing metal by anodic dissolution, and is characterized by some indices of performance such as higher dimensional precision, higher productivity, reduced tool wear, no residual stress in the workpiece, comparative with the conventional machining techniques. The PECM techniques allow to accomplish some difficult machining operations (complex shaping, boring, turning, milling, polishing, etc), without a direct contact between the tool and the workpiece, with high stock removal rates, regardless of the mechanical properties of the workpiece. The workpiece can be done

from various materials such as alloys, metal-ceramic composites, characterized by improved strength, wear, corrosion and heat resistance [2].

The use of ultrasound energy in a series of industrial applications is related to the characteristic features of ultrasonic waves, relatively small wave-length, very high acceleration, leading, focusing and spreading facilities, as well as the specific interaction with the propagation/working environment. The benefits of ultrasonic intensification of electrochemical processes have been pointed out by a lot of theoretical approaches and experimental investigations. Ultrasonic assistance of ECM process is based on the effects on properties of workpiece material and working media, resulting in two specific interactions, which leads to an increase of surface dissolution and electrochemical reaction rate [3−5]. As presented in Ref. [6], ultrasonic vibrations have the significant influence on the kinetics of electrode process conditions and increase the rate of electro- chemical dissolution.

According to the theory of electric field, more electric charges will gather at the convex surface during pulse electrochemical machining (PECM), while less electric charges will gather at the concave surface, which shows a non-uniform distribution of power lines [7−8]. Therefore, a larger current density on the convex surface

Foundation item: Project(51275116) supported by the National Natural Science Foundation of China; Project(2012ZE77010) supported by the Aero Science

Foundation of China; Project(LBH-Q11090) supported by the Postdoctoral Science Research Development Foundation of Heilongjiang Province, China

Received date: 2012−07−02; Accepted date: 2013−09−18 Corresponding author: ZHANG Fei-hu, Professor, PhD; Tel: +86−451−86413657; E-mail: zhangfh@hit.edu.cn

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correlates with a higher metal corrosion rate. In accordance, the corrosion rate would reduce with the decrease of convex curvature. Indeed, a lower current density on the concave surface of the workpiece is shown to correlate with a lower metal corrosion rate, which promotes strong appeal for obtaining a smooth surface [9−10]. ZHOU and LIANG [11] reported that the ultrasonic machining causes ultrasonic energy generated by the ultrasonic generator to pass the particles through the solution, causing the particles to subsequently attack the surface of workpiece. Although the attack of the particles is stochastic, the probability of an attack on a convex surface is higher than that for a concave surface. The machining process is thus uniform and soft. Therefore, ultrasonic machining and pulse electro- chemical corrosion machining can be coordinated with each other.

It is evident that a high quality smooth surface can be obtained through ultrasonic machining. As ultrasonic and pulse electrochemical compound machining are both consonant, the combination of ultrasonic and pulse electrochemical compound machining is possible and feasible [12]. Consequently, several studies have analyzed ultrasonic-pulse electrochemical compound machining. In our previous research, the process and system of ultrasonic-pulse electrochemical compound machining (UPECM) were developed [12−13]. In this work, a cavitation catalysis removal model of UPECM based on particles was developed. 2 Principle of UPECM

UPECM utilizes the cavitation energy of ultrasonic and electrochemical dissolution of pulse electric current. The machining systems employ a machining tool as cathode and a workpiece as anode, resulting in high-speed flowing electrolyte between the tool and workpiece. The schematic diagram of the machining system is shown in Fig. 1. The main principle of UPECM is minimal erosion of metal under the action of pulse electric current, simultaneously accompanied by ultrasonic machining to improve the speed and quality of machining. This could be divided into two procedures, pulse electrochemical minimal erosion machining and ultrasonic machining.

Due to the interval and step transition of pulse electric current in pulse electrochemical machining, the electrolyte between tool and workpiece appears at oscillation and generates a pressure wave. The agitate action of the pressure wave greatly improves the flow condition of electrolyte, accelerates the update of electrolyte and eliminates the non-uniform distribution of the electric conductivity in electrolyte. This thus improves the machining precision and surface quality. It

Fig. 1 Schematic diagram of UPECM system: 1—Ultrasonic

generator; 2—Transducer; 3—Amplitude transformer; 4—Tool

head; 5—Pulse power supply; 6—Liquid cell; 7—Particle; 8—

Workpiece; 9—Anode dissolution area; 10—Electrolyte; 11—

Filter; 12—Electrolyte tank; 13—Pump; 14—Throttle; 15—

Flow meter; 16—Manometer; 17—Pipe can also promote the movement and uniform distribution of the particles in electrolyte, improving the manufacturing quality of ultrasonic machining. Furthermore, oxidation films can be formed on the anode workpiece surface with a non-uniform distribution. According to the theory of electric field, the microcosmic sags and crests of the oxidation films can result in non-uniform distribution of the power line and cause different metal electrochemical dissolution speeds at convex and concave surfaces, promoting pulse electrochemical machining.

During ultrasonic machining, the particles attack the surface of workpiece repeatedly and continuously under the action of ultrasonic. This removes the passive films on the workpiece surface and activates workpiece metal surface [12]. On the other hand, it could generate strong cavatition and hydraulic shock in the processing region. This could have two outcomes. The first result would be the strengthening of machining through high frequency and alternating plus−minus shock wave. The second result would accelerate the update and flow of electrolyte, improve the flow field of electrolyte and strengthen the activation of workpiece surface. This would promote the electrochemical reaction and strengthen pulse electrochemical minimal erosion machining [14].

3 Cavitation catalysis mechanism based on

particles

The mechanism of ultrasonic-pulse electrochemical compound machining is very complex and is yet to be understood [12−13]. Ultrasonic cavitation refers to the micro gas nucleus in the liquid (nucleus) in the field of

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ultrasound, vibration under the action of growth and collapse of closed process in the field of ultrasonic. The particles hit the oxidation films and workpiece under the action of the ultrasonic. Consequently, the oxidation films are worn at high speed and the electrochemical reaction is improved. As a result, the workpiece surface is not protected by oxidation films and becomes completely naked in the electrolyte. The anodic electrochemical dissolution of the workpiece is thus increased. The particles can be regarded as the catalyzer of UPECM. 3.1 Transmission of particles

In ultrasonic cavitation, acoustic streaming and shock wave can cause system macroscopic turbulence and particles to bump at high speed. Not only can they penetrate through the diffusion layer, but they can also penetrate the electric double layer by mass transfer. As demonstrated in Fig. 2(a) [15], the theory of electric double layer suggests that a layer of water molecules hold tightly to the surface of the workpiece in the compact double layer. Anion transmission is therefore hindered and the processing speed is limited. As seen in Fig. 2(b), the particles with ions pass through the diffusion layer and the compact double layer directly to the anode workpiece surface. Consequently, the oxidation films of the anode workpiece surface are destroyed and there is a subsequent increase in anions at the metal substrate surface, thus accelerating electrochemical corrosion. The particles play the role of chemical function and also play the role of mechanical function. The particles perform a catalysis role and improve the corrosion rate of workpiece in UPECM compound machining.

3.2 Mechanical energy of particles inducing formation of oxidation films The particles get mechanical energy under the

cavitation energy of ultrasonic, and the particles hit the surface of workpiece repeatedly and continuously at a high speed. It was proven that mechanical energy played an important role in oxidation films formation in the case of electrochemical−mechanical experiments [16]. The Kar-Liang equation was proposed with the addition of a mechanical energy term (ε) to Arrhenius-Eyring equation as

( ) /b( )e G RTk Tk

h

(1)

where k is the oxidation rate constant (s−1), R is the gas constant, kb is the Boltzmann’s constant, T is the absolute temperature, h is the Plank’s constant,

G is the Gibbs energy of activation and ε is the mechanical energy. This means that the mechanical energy of particles overcomes the metal surface activation energy to trigger the mechanochemical reaction. The experiment results also show that applying mechanical energy could induce the formation of oxidation states of metal.

Therefore, the particles with mechanical energy hit the surface of workpiece under the ultrasonic, so the mechanical energy (ε) of particles overcomes the metal surface activation energy to form the next oxidation films of metal. 3.3 Acceleration in electrochemical corrosion

The workpiece surface can form oxidation films very quickly in ultrasonic-pulse electrochemical compound machining [17]. The adhesive force of the

Fig. 2 Electric double layer without (a) and with (b) particle transmission

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passive films on the workpiece is weak. The particles with mechanical energy strike oxidation films and workpiece repeatedly and continuously under the ultrasonic (Fig. 3). As a result, the oxidation films are broken off by the particles. On the other hand, ultrasonic cavitation energy generates high temperature and high pressure locally. This causes molecular bond rupture and molecular activation, which promotes the electro- chemical reaction and intensifies the electrochemical machining. 3.4 Particles removal machining

The erosion rate may be expressed as the ratio of the mass of workpiece material removed to the mass of impacting particles, so the material removal rate Erate in a certain period of time can be estimated as [18]

Erate=KP/H (2) where P is the particle impact force, H is the material hardness and K is the coefficient. The micro jet, which produces ultrasonic cavitation energy, then peels off and erodes the oxidation films and the workpiece surface.

As a result, the workpiece surface creates a new active surface and forms new oxidation films. Generally, the hardness of the oxidation films is lower than that of the workpiece. At the same time, the adhesive force of oxidation films in the workpiece surface is also small.

The binding energy of workpiece surface decreases under the effect of electrochemical corrosion, so the material hardness of workpiece surface decreases. Therefore, the material removal rate is improved. Consequently, the processing speed is accelerated. 3.5 Increase in corrosion current density

The system of UPECM is in equilibrium and thus in a steady state. According to the electrochemical corrosion reaction rate theory [19], the removal rate of the workpiece is expressed with the following corrosion current density:

0

corr exp( )nF

J J ERT

(3)

where n is the quantivalency of material, F is the Faraday constant, R is gas constant, T is the absolute temperature, J0 is the iron exchange current density at the corrosion

state, β is the anodic electron transfer coefficient at the corrosion state, ∆E is the potential difference between corrosion potential and equilibrium potential in the system.

Therefore, if the other condition is invariable, the iron corrosion rate will increase with the increase of β value.

As a result of ultrasonic cavitation and the transition of polarization curves from a passivation state to a corrosion state, the oxidation films in ultrasonic-pulse electrochemical machining are worn at a high speed. The ultrasonic cavitation may promote the solid−liquid mass transfer process. Therefore, the diffusion can be strengthened, and the electronic transfer coefficient β can move from a smaller value of passivation state to a bigger value of corrosion state. Thus, the corrosion current density Jcorr is increased, accelerating the electrochemical corrosion. 3.6 Reduction in electrochemical diffusion layer

thickness During ultrasonic cavitation, acoustic streaming and

shock wave can cause system macroscopic turbulence and high speed vibration of particles. The diffusion layer can attenuate and the mass transfer rate can increase. At the same time, the high speed movement of particles can damage the surface diffusion layer and enhance mass transfer rate of reactant and product, thus improving the corrosion rate and accelerating the rate of UPECM.

Together, the electrolyte, the workpiece and tool head form a fluid system in ultrasonic-pulse electrochemical compound machining, and the diffusion layer is formed on the workpiece surface. The diffusion layer hinders mass transfer of the electrolyte, and the inhibition increases with the increase of diffusion layer thickness. Therefore, the diffusion layer should be eliminated or reduced, which enhances mass transfer during UPECM. The average thickness of the boundary layer and diffusion layer on the workpiece surface obeys the theory of fluid dynamic [20], which is explained as follows:

0

3

2L

u L

(4)

Fig. 3 Particle impact on surface of oxide and metal

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1 21 3 1 6 1 2

0D L u (5) where L is the length of the machining area, is the dynamic viscosity of electrolyte, ρ is the density of electrolyte, u0 is the tangential velocity of electrolyte, and D is the diffusion coefficient.

From Eqs. (4) and (5), it can be seen that diffusion layer thickness decreases with a decrease in electrolyte dynamic viscosity and also decreases with the increase of electrolyte flow rate during UPECM. Therefore, decreasing dynamic viscosity and increasing flow rate can reduce the thickness of diffusion layer, enhance the mass transfer process and increase the processing rate.

The material removal rate of ultrasonic-pulse electrochemical machining (UPECM), pulse electrochemical machining (PECM) and ultrasonic machining can vary with machining time (Fig. 4). The material removal rate in ultrasonic machining and pulse electrochemical machining is relatively small. However, the material removal rate in UPECM is about three times larger than the material removal rate of PECM. In fact, the material removal rate of UPECM is greater than the sum of the material removal rates of PECM and ultrasonic machining. This phenomenon is well explained by the cavitation catalysis mechanism model that cavitation catalysis improves the processing speed.

Fig. 4 Variation of material removal of different techniques

with time

4 Model of UPECM for hard and brittle

metals

During UPECM, pulse current flowing from workpiece to tool by electrolyte forms a current loop. As a result, electrochemical dissolution arises from the metal workpiece surface. The removal metal quantities in the workpiece surface would obey Faraday’s law. Ultrasonic cavatition is also considered to be based on catalysis removal mechanism as follows:

p

p 0

tZtM I

nF t t

(6)

where MI is the erosion quantity of workpiece surface, Z is the relative atomic mass of material, F is the Faraday constant, n is the quantivalency of material, I is the amplitude of the rectangular wave pulse current, t is work time, tp is the width of pulse, t0 is the interval of pulse, η is current efficiency and λ is the efficiency of electrolyze erosion.

In UPECM, Ohm’s voltage drop of electrolyte is the machining gap voltage UR, and it is given as

UR=RehI (7) where Re is the electrical resistivity of electrolyte and h is the machining gap between tool and workpiece.

With simultaneous equations Eqs. (6) and (7), the quantity of electrochemical erosion IM in unit time is obtained:

p R

Ip 0 e( )

t UZM

nF t t R h

(8)

In UPECM, there is the machining gap h between

the tool head and the workpiece. As seen in Fig. 5, the particle hits the workpiece surface under ultrasonic cavitation energy. The maximum force on workpiece surface P can be explained in Eqs. (9) and (10), respectively [11].

Fig. 5 Scheme of single particle impacting surface of

workpiece

2 5

2 21 2 4 51 2

11 2

1 15π

4R u

E E

(9)

1 5

2 22 51 2

11 2

1 15π

4r Ru

E E

(10)

where E1, E2 and ν1, ν2 are the elastic moduli and Poisson ratios of the particle and workpiece, respectively, ρ1 is the density of particle, u is the particle velocity in the

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electrolyte liquid, and R is the radius of particle. Therefore, the material removal is approximately

obtained by a particle impingement in the workpiece: 2 2

2 2 21 21

1 2

1 15π π

4V r R u

E E

(11)

where V is metal material removal quantity, ς is material removal coefficient in practice ultrasonic machining. By Eq. (11), the exponent of velocity is 2, which also associates with particle mass and property of workpiece and particles. This model is consistent with FINNIE’s model [21], so this impingement erosion model is right. However, our model is based on particles impingement wear action, while FINNIE’s ductile model is based on particles micro-cutting wear action. Therefore, the two models are completely different.

By Eq. (11), the material removal depends on particle velocity, particle density, particle size, and elastic modulus and Poisson ratio of workpiece and particles.

The particle velocity also depends on ultrasonic in the electrolyte:

3 3 0

2 2

4 e hf V Au

V

(12)

where ρ2 and ρ3 are the densities of electrolyte liquid and tool head, respectively, V2 and V3 are the ultrasonic velocities in the electrolyte liquid and in the tool head, respectively, f is the frequency of the ultrasonic vibration, A0 is the vibration amplitude of the tool head and α is the attenuation coefficient of ultrasonic in electrolyte.

The quantity of material removal by ultrasonic in unit time is described as

22 22 3 3 01 2

II 11 2 2 2

4 e1 15π

4

hf V AM N R

E E V

(13) where IIM is the quantity of material removal in unit time, N is the effective number of particles in unit time. N is constant when the electrolyte flow rate, particles and machining gap are determinate.

To simplify Eq. (13), we make ξ equal to

5π

4

22 23 31 2

1 2 2 2

41 1,

f V

E E V

ξ is constant when ultra-

sonic power, electrolyte, particles and workpiece material are determinate:

2 2II 1 0( e )hM N R A (14)

The total removal quantity M in UPECM compound

machining from Eqs. (8) and (14) is as follows: p R 1 2 2

1 2 1 0p 0 e

( e )( )

ht UZM K h K N R A

nF t t R

(15)

where K1 and K2 are the proportionality factors. According to cavatition catalysis removal mechanism, PECM and ultrasonic machining mutually promote and interact, which can be an integrated unity and effectual combination, so K1>1 and K2>1.

Equation (15) is the removal model based on the cavatition catalysis removal mechanism UPECM on hard and brittle metal materials. From Eq. (15), it can be seen that the material removal rate increases in the process of UPECM due to the complementarities between PECM and ultrasonic. 5 Experimental

The experimental equipment was designed based on the principle of UPECM, which has monitoring and control systems such as automatic tracking and testing. The series of experiments were carried out using silicon nitride as the particles and special treatment steel as workpiece. The experiments were performed to attain the effects of the main processing parameters including the particle size, the ultrasonic vibration amplitude, the pulse voltage, and the minimum machining gap between the tool head and workpiece. 6 Results and discussion 6.1 Effect of particles size

As can be seen from Fig. 6, the material removal rate increases with an increase of particle size in ultrasonic-pulse electrochemical compound finishing.

Fig. 6 Effects of particle size on material removal rate

As the size of the particles decreases, the particles

become more and more uniform. As a result, larger particles and higher density are seen on the same workpiece surface, and hence the impulse force creats better uniformity on the whole workpiece surface. At the same time, decreasing particles size correlates with

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increasingly softer impulse force, and thus, higher workpiece surface could be obtained in UPECM. The free movement of the particles in the machining gap is restricted by larger particles. The uniformity of the particles is reduced through UPECM, which in turn causes non-uniformity in the workpiece removal rate. The processing speed and the processing quality are contradictory between the material removal rate and surface roughness. It is therefore vital that the appropriate particle size is selected in UPECM. 6.2 Effect of ultrasonic vibration amplitude

As demonstrated in Fig. 7, the material removal rate increases with the increase of the ultrasonic vibration amplitude. At larger ultrasonic vibration amplitude, where the sound pressure increases, the machining gap between the tool head and the workpiece is usually small. This not only causes chaos among the particles, but also results in a non-uniform situation and an energy exchange between the particles. Energy loss and non-fluency of work solution appear and the impact force of the particles is reduced. Therefore, the material removal rate and surface roughness are decreased. As the ultrasonic vibration amplitude decreases, a decrease in the sound pressure is also seen. This results in lower impact force and no percussion action. As a result, it could not form effective machining. It is therefore essential to choose the best ultrasonic vibration amplitude.

Fig. 7 Effects of ultrasonic vibration amplitude on material

removal rate

6.3 Effect of minimum machining gap

The minimum machining gap between the tool head and workpiece has a clear effect on ultrasonic-pulse electrochemical compound finishing. As can be seen from Fig. 8, decreasing size of the minimum machining gap results in an improvement in the processing speed. The material removal rate increases with the decrease of the minimum machining gap due to the interreaction of

pulse electrochemical machining and ultrasonic machining. On one hand, a smaller minimum machining gap means that the work solution could not enter or lessen the gap. The effective particles therefore decrease in the machining gap and the particles non-uniformly hit the workpiece surface. On the other hand, the electrochemical corrosion removal increases with the decrease of the minimum gap. Consequently, the material removal rate of UPECM rapidly increases under the action of pulse electrochemical machining and ultrasonic machining. As the minimum machining gap increases, it causes directly the loss of ultrasonic cavitation energy and reduction of electrochemical corrosion. As a result, the material removal rate of UPECM rapidly increases. An increase in the minimum machining gap also results in an increase in the randomness of the particles, an increase in the randomness of the impact force of the particles and a decrease in the surface roughness.

Fig. 8 Effects of minimum machining gap between tool and

workpiece on material removal rate

6.4 Effect of pulse voltage

From Fig. 9, it can be seen that the material removal rate of UPECM is approximately proportional to the pulse voltage and decreases with the pulse voltage in accordance. On one hand, the electrochemical corrosion rate of the material increases with the increase of current density. However, it generates stray corrosion and non-uniformity of partial dissolution. It reduces the surface roughness. On the other hand, electric spark is generated during machining, which improves the sharpness of the particles and enhances the material removal rate. The sharpness of the particles generates micro cutting to the workpiece. It enhances the surface roughness. It could be seen to contradict itself for the surface roughness of UPECM. Therefore, this illustrates the importance to attain the material removal and the surface roughness in proper control of the pulse voltage.

In addition, the duty cycle of the pulse current has a great effect on the machining efficiency. The frequency

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Fig. 9 Effects of pulse voltage on material removal rate

of the current is invariable, and the electrochemical corrosion is reduced with a decrease in the duty cycle. It attains micro-corrosion and improves surface quality, yet the efficiency and the velocity decrease. Although the electrochemical corrosion increases with the increase of the duty cycle, it also rapidly eliminates the electrochemical products and affects surface quality. It is therefore essential that the current density and duty cycle are chosen carefully.

RUSZAJ et al [22] reported that ultrasonically assisted ECM process in mixture of electrolyte and abrasive grains made it possible to obtain surface roughness parameter Ra in the range of 0.05−0.1 μm. Figure 10 shows the surface roughness of the workpiece measured by an atomic force microscopy (AFM). The roughness Ra of the machining surface is about 0.05 μm in UPECM. This is consistent with their results.

Fig. 10 AFM roughness before (a) and after (b) UPECM

7 Conclusions

1) The cavitation catalysis mechanism model of UPECM is correct and proper. The high processing speed is very well explained by the cavitation catalysis mechanism model. It is clear that the particles promote ultrasonic-pulse electrochemical compound machining. The particles also act as a catalyzer of UPECM.

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(Edited by FANG Jing-hua)