**Electrospark Deposition: Mass Transfer**

Orhan Sahin and Alexandre V. Ribalko

*Gebze Institute of Technology Turkey* 

#### **1. Introduction**

480 Mass Transfer - Advanced Aspects

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Electrospark alloying (ESA) is one of the surface modification methods to change physical and chemical properties of metal surfaces. It was developed by the Soviet scientists, B.R. Lazarenko and N.I. Lazarenko. The core of this method is the phenomenon of material erosion of both electrodes as a result of the electric discharge between them in a gaseous environment and subsequent mass transfer from one of them to the other, basically from anode to the cathode (Lazarenko, 1951; Lazarenko, 1976).

The anode (treating electrode) usually is a rod with several mm square cross sectional area. Comparing with treating electrode, the cathode (substrate) has significantly larger size and surface area. Both of them are electrically conductive. After the application of a current pulse, a spark discharge takes place between treating electrode and substrate. Following spark discharge, part of the tip of treating electrode and a corresponding spot on substrate melt. Molten spot on the substrate forms a swallow molten pool. Some of the molten tip of treating electrode material transfers to the substrate in the form of molten droplets, mixes with its molten pool and usually solidifies as in the form of splash. By scanning substrate surface, so many splashes could be deposited on it. As a Consequence of single or multiple scanning of substrate surface, a deposit having chemical composition same as treating electrode forms on the substrate.

According to Lazarenko (1951), the size and sign of electrical erosion at the electrodes, consequently, the mass transfer from the treating electrode to the substrate depends on chemical composition of electrode materials, environment between electrodes and parameters of the electrical pulse. It is obvious that for ESA in air, size of erosion depends basically on chemical composition of electrodes and pulse energy, in turn pulse parameters, pulse amplitude and pulse duration. Therefore, for a given pair of electrodes, mass transfer depends only on electrical parameters of pulses. It was experimentally shown that, the mass gain of substrate is limited, i.e. it is impossible to obtain thick electrospark coating. According to the author (Lazarenko, 1976), the limitation of mass gain of substrate depends on several factors. Lazarenko named them as: change in chemical properties of molten droplet during its transfer to the substrate; change in chemical properties of substrate surface due to mixing with molten droplet ejected from treating electrode and oxidation in air; radical changes arising in alloyed substrate surface - occurrence and accumulation of defects in crystal lattice preventing diffusion; occurrence of residual stress etc. She has also reported that even under the non-oxidizing gas environment–such as argon, helium, hydrogen, there was still limitation on coating thickness. In this case, the processing time till limitation was slightly longer than that of the processing in air.

Electrospark Deposition: Mass Transfer 483

L

<sup>C</sup> Tr

U R

(b1)

t

(b2) (b3)

(b4) Fig. 1b. Generating of a pulse by pulse-width modulation. The electrical discharge circuit (Tr is the transistor, U is the voltage of power supply, C, L and R are the capacitance, inductance and resistance (including the load) of the discharge circuit, respectively) (b1). A representative current pulse oscillogram (b2) could be formed by on-off switching of the transistor (b3). t and i are the time and current, respectively. The on-off switching of transistor was operated by programmed voltage pulses from pulse oscillator. A real current pulse oscillogram, pulse

The present installation is capable of forming pulse groups with various parameters. Power consumption of the installation is 1000 W. The stabilized output voltage of the converter is 40 V. Range of energy and duration of pulse is 0,25x10-3-15 J and 2-8000 µs respectively. Range of pulse frequency is 2-120000 Hz. The upper boundary of frequencies is used for

For a chosen voltage, taking into account the full resistance of discharge circuit, the average rate of increase of pulse first front (first slope of pulse) is 14,4 A/µs till to the amplitude of 1000 A. Continuous sliding of the processing electrode on the substrate surface back and forth, provided the possibility of discharge initiation with explosion of contacting micro roughness and formation of plasma channel at any moment of time (Rybalko et al., 2000).

i U

amplitude is 300 A and pulse duration is 200 μs (b4)

minimum pulse amplitude of 15 А.

t

Many years past since then; however, published scientific studies on ESA shows that, breakthrough in technology to form thick coating has not been achieved yet. This problem could partially be solved by using pulses providing maximum erosion (mass loss) of the treating electrode and maximum mass gain by the substrate. The process of mass transfer must be completed before any one of the factors described above could have sufficient time to be fully effective (Lazarenko, 1976).

Thus, in order to determine the condition of maximum mass transfer from treating electrode to substrate, it is necessary to study mass transfer characteristics of electrodes as a function of pulse durations. The literature review on this question shows that, the range of pulse durations is from 10-5 sec (short durations), to 10-3 – 10-2 sec (long durations). The pulse duration was limited by possibility of generating current pulses with amplitude of 100 А and voltage of 100 V at lower side of pulse duration range and change in polarity of mass transfer and increase in heat content of the substrate at upper side of pulse duration range. Thus, the possible interval of pulse durations for ESA is from 10 µs to 10000 µs.

Lazarenko (1957) investigated mass transfer characteristics of ESA process only in the range of 50 µs to 300 µs. For alloying, she used sinusoidal pulses generated by the discharge of capacitor on spark loading. According to the Lazarenko (1957), the optimal pulse duration for ESA is between 50 µs to 300 µs. The following years and practically at present, same pulse durations have been used for ESA, because above pulse range was considered as a base to fabricate ESA installations.

Zolotih (1957) investigated the dependence of erosion of treating electrode to pulse duration with reference to the electrospark dimensional machining (EDM) of metals. The range of sinusoidal pulse duration was 100 µs -1100 µs. It is interesting to notice that in EDM case the mass loss of treating electrode versus processing time curve has a maximum. The emergence of the maximum depends on pulse duration and physico-chemical properties of electrodes.

Dependence of erosion of treating electrode to the pulse shape (Rybalko et al., 2003a) revealed that, ESA with application of square pulses was more productive. Therefore, to form square pulses, an ESA installation has been developed (Rybalko et al., 2003b, 2003c) (see, Fig 1a and 1b). The current pulse shaper was executed on the basis of single-cycle electric generator with a transistor switchboard. It is capable to form a pulse with desired parameters by a method of pulse-duration modulation. Such approach allows producing a current pulse with various amplitude, duration and shape without changing the parameters of pulse forming circuit, U, C, L and R. The master generator of the pulse shaper regulate the duration of its pulses and pauses in steps of 200 nanoseconds. The pulse amplitude could be increased in the order of 3 А. For the case of forming more complicated pulse shape, the approach of pulse-duration modulation was used.

Fig. 1a. The diagram of the ESA installation

Many years past since then; however, published scientific studies on ESA shows that, breakthrough in technology to form thick coating has not been achieved yet. This problem could partially be solved by using pulses providing maximum erosion (mass loss) of the treating electrode and maximum mass gain by the substrate. The process of mass transfer must be completed before any one of the factors described above could have sufficient time

Thus, in order to determine the condition of maximum mass transfer from treating electrode to substrate, it is necessary to study mass transfer characteristics of electrodes as a function of pulse durations. The literature review on this question shows that, the range of pulse durations is from 10-5 sec (short durations), to 10-3 – 10-2 sec (long durations). The pulse duration was limited by possibility of generating current pulses with amplitude of 100 А and voltage of 100 V at lower side of pulse duration range and change in polarity of mass transfer and increase in heat content of the substrate at upper side of pulse duration range.

Lazarenko (1957) investigated mass transfer characteristics of ESA process only in the range of 50 µs to 300 µs. For alloying, she used sinusoidal pulses generated by the discharge of capacitor on spark loading. According to the Lazarenko (1957), the optimal pulse duration for ESA is between 50 µs to 300 µs. The following years and practically at present, same pulse durations have been used for ESA, because above pulse range was considered as a

Zolotih (1957) investigated the dependence of erosion of treating electrode to pulse duration with reference to the electrospark dimensional machining (EDM) of metals. The range of sinusoidal pulse duration was 100 µs -1100 µs. It is interesting to notice that in EDM case the mass loss of treating electrode versus processing time curve has a maximum. The emergence of the maximum depends on pulse duration and physico-chemical properties of electrodes. Dependence of erosion of treating electrode to the pulse shape (Rybalko et al., 2003a) revealed that, ESA with application of square pulses was more productive. Therefore, to form square pulses, an ESA installation has been developed (Rybalko et al., 2003b, 2003c) (see, Fig 1a and 1b). The current pulse shaper was executed on the basis of single-cycle electric generator with a transistor switchboard. It is capable to form a pulse with desired parameters by a method of pulse-duration modulation. Such approach allows producing a current pulse with various amplitude, duration and shape without changing the parameters of pulse forming circuit, U, C, L and R. The master generator of the pulse shaper regulate the duration of its pulses and pauses in steps of 200 nanoseconds. The pulse amplitude could be increased in the order of 3 А. For the case of forming more complicated pulse

Pulse generator

Thus, the possible interval of pulse durations for ESA is from 10 µs to 10000 µs.

shape, the approach of pulse-duration modulation was used.

Substrate

Fig. 1a. The diagram of the ESA installation

Layer <sup>+</sup>

Treating Discharge electrode

to be fully effective (Lazarenko, 1976).

base to fabricate ESA installations.

(b4)

Fig. 1b. Generating of a pulse by pulse-width modulation. The electrical discharge circuit (Tr is the transistor, U is the voltage of power supply, C, L and R are the capacitance, inductance and resistance (including the load) of the discharge circuit, respectively) (b1). A representative current pulse oscillogram (b2) could be formed by on-off switching of the transistor (b3). t and i are the time and current, respectively. The on-off switching of transistor was operated by programmed voltage pulses from pulse oscillator. A real current pulse oscillogram, pulse amplitude is 300 A and pulse duration is 200 μs (b4)

The present installation is capable of forming pulse groups with various parameters. Power consumption of the installation is 1000 W. The stabilized output voltage of the converter is 40 V. Range of energy and duration of pulse is 0,25x10-3-15 J and 2-8000 µs respectively. Range of pulse frequency is 2-120000 Hz. The upper boundary of frequencies is used for minimum pulse amplitude of 15 А.

For a chosen voltage, taking into account the full resistance of discharge circuit, the average rate of increase of pulse first front (first slope of pulse) is 14,4 A/µs till to the amplitude of 1000 A. Continuous sliding of the processing electrode on the substrate surface back and forth, provided the possibility of discharge initiation with explosion of contacting micro roughness and formation of plasma channel at any moment of time (Rybalko et al., 2000).

Electrospark Deposition: Mass Transfer 485

0,5 1,0 1,5 2,0 2,5 3,0 3,5

0,5 1,0 1,5 2,0 2,5 3,0 3,5

Fig. 3. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 200 A. Pulse

Fig. 2. Mass loss of treating electrode and mass gain of substrate (solid dots) as a function of pulse energy for a period of 3 minutes of processing. Pulse amplitude was 100 A. Pulse

t, min

 50 μs 50 μs 100 μs 100 μs 150 μs 150 μs 200 μs 200 μs 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs 2000 μs 2000 μs

t, min

 25 μs 50 μs 50 μs 100 μs 100 μs 150 μs 150 μs 200 μs 200 μs 300 μs 300 μs 400 μs 400 μs 600 μs 600 μs 800 μs 800 μs 1000 μs 1000 μs 2000 μs 2000 μs 4000 μs 4000 μs


duration was variable


duration was variable





0

2

4

6

ΔM, mg

The total electricity through the inter-electrode gap was kept constant at 3 Coulomb as a base for comparison of experimental results.
