**3. General results of electrospark alloying**

Regarding the experimental results, following generalizations could be made, which essentially could change the approach to the ESA technology (Ribalko et al., 2005; Ribalko & Sahin, 2006). Nonlinear decrease in erosion of treating electrode during alloying of steel specimens after the first layer deposition was a fact. Thus, it is possible to ascertain that an additivity principle observed by B.R. Lazarenko in case of electroerosive dimensional machining, is not true in case of ESA after first layer of deposition (Ribalko et al., 2004b). The additivity principle is the stationary anodic erosion when the discharge energy between electrodes is constant. The observed decrease in anodic mass change in case of ESA was due to the decrease in power of cathodic stream, in turn, due to the change in surface properties of the substrate after alloying of its surface with treating electrode. The cathodic stream conduces to the anodic erosion.

In Tables 4, 5 and 6, average coating thickness obtained under recommended number of scanning is given. The analysis of this data shows that if pulse duration increases, the average thickness increases, in spite of the fact, that the number of recommended layers for

In case, if it is necessary to deposit only one layer, irregular coating (non uniform thickness) is inevitable. For example, ESA with pulse duration of 800 µs and pulse amplitude of 1000 А, the minimum thickness of the deposited spot due to a single spark discharge was 10 microns at the edge and 200 microns at the center. Thus, a layer of 1 cm2 area could be formed by only 60 spark discharges: 3 pulses per second for a total processing of 20 seconds (Table 6). Morphology of coating is similar to the one illustrated in Fig. 13 (cross-sectional

Fig. 13. A typical view of cross-section and surface of a coating could be obtained with high energy pulses. Pulse amplitudes and pulse durations could be from 600 A to 1000 A and 400

Regarding the experimental results, following generalizations could be made, which essentially could change the approach to the ESA technology (Ribalko et al., 2005; Ribalko & Sahin, 2006). Nonlinear decrease in erosion of treating electrode during alloying of steel specimens after the first layer deposition was a fact. Thus, it is possible to ascertain that an additivity principle observed by B.R. Lazarenko in case of electroerosive dimensional machining, is not true in case of ESA after first layer of deposition (Ribalko et al., 2004b). The additivity principle is the stationary anodic erosion when the discharge energy between electrodes is constant. The observed decrease in anodic mass change in case of ESA was due to the decrease in power of cathodic stream, in turn, due to the change in surface properties of the substrate after alloying of its surface with treating electrode. The cathodic stream

alloying decreases.

micrograph)

to 1000 µs respectively

conduces to the anodic erosion.

**3. General results of electrospark alloying** 

As it was shown experimentally, the phenomenon of coating destruction by its evaporation occurred during alloying in all ranges of pulse parameters investigated. Thus, to obtain high quality depositions, it is necessary to limit the processing time by the moment of the signs, indicating destruction of the deposited layer. The beginning of coating destruction could be determined by some features of ESA, namely: when mass loss of treating electrode ceases down to a minimum stable level, when mass gain of substrate becomes zero or even negative and the moment of a sharp decrease in mass transfer coefficient

Most precisely, the beginning of coating destruction could be defined as the moment of sharp decrease in mass transfer coefficient, which is the ratio between the mass gain of substrate to the corresponding mass loss of treating electrode between two successive measurements. The decision could be considered as a criterion to end the process.

The analysis of experimental results indicates that the number of superimposed layers prior to the beginning of coating failure, for the investigated range of pulse energies, depends on pulse energy and it could be between 1 and 3 (the latest one is for low pulse energy) (Fig. 14). A single-valued dependence of the number of these layers to the amount of pulse energy was not observed. The same number of layers could be obtained by the employment of different pulse energies which depend on pulse amplitude and pulse duration. As an example, for 3 layers of deposition, either a pulse amplitude of 400 A (pulse energy not more than 0.3 J), or a pulse amplitude of 800 A (pulse energy not more than 1.5 J) or pulse amplitude of 1000 A (pulse energy not more than 3 J) could be employed. It should be noticed that, although there is a ten fold increase in pulse energy, the maximum number of deposited layers prior to fracture does not change. This behavior was explained as follows.

Despite of the constant amount of electricity for the ESA processing (3 Coulomb), deposition time of the first layer strongly varies, and it was, for example, 20 seconds for a high pulse energy case and 70 seconds for a low pulse energy case. Examination of the surface morphology of coatings shows that size, e.g. average diameter, of the solidified splashes due to mass transferred by a single pulse for the each energy significantly varies. For the low pulse energy of 0.032 J (100 А, 25 μs) the time to scan the substrate area of 1 cm2 requires 80650 pulses (then the average splash area as a result of each spark discharge is roughly 0.00124 mm2) and for the high pulse energy of 9.47 J (800 А, 800 μs), the same area was totally scanned by the application of only 125 pulses (thus the average splash area is nearly 0.8 mm2 per pulse). That is, although pulse energy was increased 259 times, the average splash area was increased 645 times. Therefore, the growth of splash size is not directly proportional to the increase in pulse energy.

The erosive processes on treating electrode and substrate are determined not only by the electrical parameters of discharge and the chemical properties of the electrodes, but also, substantially by the rate of heat flow received by the electrodes (Namitokov, 1978; Butkevich et al., 1978). The amount of heat which could cause evaporation of substrate under the first layer of coating is not reached by the employment of long duration pulses, as in the case of pulses which have same energy but shorter duration. To reach dense heat flow in electrodes high enough to cause same kind of fracture, it is necessary to raise the energy of a long duration pulse by increasing its amplitude.

Since the number of superimposed layers prior to beginning the fracture of coating is limited, the amount of mass loss of the treating electrode and mass gain of the substrate are also limited. It is of interest to determine the amount of mass transfer, in turn coating thickness, quantitatively till fracture. As mentioned previously, it is a fact that the

Electrospark Deposition: Mass Transfer 499

experimental conditions of ESA (processing parameters e.g. properties of electrodes, medium and electrical pulse parameters), are the same only for the deposition of first layer. Therefore, the experimental data could be compared for the first layer of depositions only. For alloying time of 30 seconds, i.e. first layer of deposition, the change in mass of electrodes as a function of pulse energy was given in Fig. 15. Amplitudes of pulses were marked on the figure as well. The data were taken from Tables 1-6. As it is seen, the relation between the mass loss of treating electrode and pulse energy is not linear. For example, the mass loss of treating electrode to the pulse energy ratio is about 10.0 -12.0 mg/J for pulse energy of 0.25 J. This ratio is only 1.42 mg/J for the pulse energy of 1.0 J and it does not practically vary further with increasing energy. The nonlinearity of the treating electrode mass loss is

Apparently, the difference between these two ratios is significant. This result could be interpreted as; on the one hand, to use energy effectively, pulses with low energy should be used for processing, on the other hand, to obtain considerably thick coating, i.e. high erosion of treating electrode, it is necessary to use pulses with high energy under the condition of

Various combinations of pulse energy components, pulse amplitude and pulse duration did not produce a wide scatter in the values of mass loss of the treating electrode (Fig. 15). The voltage drop at the moment of spark discharge which is a requirement of ESA was constant at around 17 V for the present electrode couple (WC92-Co8 - steel 35). Using the experimental data about mass loss of treating electrode versus pulse energy till 5 J, an empirical relation given below was derived. For a given energy, the corresponding values of

where, ΔMa,sp. =(0.0089+0.0555 W – 0.0243 W2 + 0.0054 W3- 0.0004 W4) is the specific value of the mass loss of treating electrode for 1 Coulomb of electricity spent during 1 second alloying (mg/C· s); С is the amount of electricity of ESA processing (Coulomb); t is the time for the first layer deposition (s); W is the amount of pulse energy (J). The calculated curve and experimental average curve (solid dots) of the mass loss of the treating electrode are

However, for a given pulse energy, the mass gain of the substrate shows a wide scattering depending on the pulse amplitude (Fig. 15). For example, for a pulse energy of 1.5 J which could be obtained by the application of pulse amplitudes of 200 A, 400 A, 600 A and 800 А, the mass gains of substrate were 3.5 mg, 2.5 mg, 1.7 mg and 0.25 mg, respectively. The difference in mass gained by substrate is because of two concurrent mass actions. These are the mass transferred from treating electrode and the mass loss due to washing away of molten pool of substrate spot caused by the intensive gas dynamics phenomena in plasma channel at the beginning of each spark discharge. In the case of processing with high pulse amplitude, due to larger material washed away, the mass loss of substrate would be expected to be more. However, cross sectional thickness measurements show that the thicknesses of coatings were close to each other despite the large differences in the amount of mass gain (14 times). This shows that the mass gained by substrates were almost the same despite the differences in pulse amplitudes. So, the amount of mass gained by substrate

given in Fig. 16 for a 30 seconds of alloying by spending 3 Coulomb of electricity.

could be calculated by a simple relation, if the effect of washing away is negligible:

ΔMa = (ΔMa,sp· C· t ) mg (1)

originated from the decreasing rate of heat flow density in electrodes.

mass loss of treating electrode taken from Fig. 15, were averaged.

single layer scanning.

Fig. 14. The relationship between pulse energy and number of layers which could be superimposed without fracture of coating

Fig. 15. Mass loss of treating electrode (solid dots) and mass gain of substrate as a function of pulse energy. Processing time is 30 s

0 1 2 3 4 5 6 7 8 9 10 11 12

Fig. 14. The relationship between pulse energy and number of layers which could be

0 1 2 3 4 5 6 7 8 9 10 11 12

Fig. 15. Mass loss of treating electrode (solid dots) and mass gain of substrate as a function

Pulse energy, J

100A 100A 200A 200A 400A 400A 600A 600A 800A 800A 1000A 1000A

Pulse energy, J

100А 200А 400А 600А 800А 1000А

number of layers

1.0


of pulse energy. Processing time is 30 s

superimposed without fracture of coating

ΔM, mg

1.5

2.0

2.5

3.0

experimental conditions of ESA (processing parameters e.g. properties of electrodes, medium and electrical pulse parameters), are the same only for the deposition of first layer. Therefore, the experimental data could be compared for the first layer of depositions only.

For alloying time of 30 seconds, i.e. first layer of deposition, the change in mass of electrodes as a function of pulse energy was given in Fig. 15. Amplitudes of pulses were marked on the figure as well. The data were taken from Tables 1-6. As it is seen, the relation between the mass loss of treating electrode and pulse energy is not linear. For example, the mass loss of treating electrode to the pulse energy ratio is about 10.0 -12.0 mg/J for pulse energy of 0.25 J. This ratio is only 1.42 mg/J for the pulse energy of 1.0 J and it does not practically vary further with increasing energy. The nonlinearity of the treating electrode mass loss is originated from the decreasing rate of heat flow density in electrodes.

Apparently, the difference between these two ratios is significant. This result could be interpreted as; on the one hand, to use energy effectively, pulses with low energy should be used for processing, on the other hand, to obtain considerably thick coating, i.e. high erosion of treating electrode, it is necessary to use pulses with high energy under the condition of single layer scanning.

Various combinations of pulse energy components, pulse amplitude and pulse duration did not produce a wide scatter in the values of mass loss of the treating electrode (Fig. 15). The voltage drop at the moment of spark discharge which is a requirement of ESA was constant at around 17 V for the present electrode couple (WC92-Co8 - steel 35). Using the experimental data about mass loss of treating electrode versus pulse energy till 5 J, an empirical relation given below was derived. For a given energy, the corresponding values of mass loss of treating electrode taken from Fig. 15, were averaged.

$$
\Delta \mathbf{M}\_{\mathbf{a}} \equiv (\Delta \mathbf{M}\_{\mathbf{a}, \text{sp}} \cdot \mathbb{C} \cdot \mathbf{t}) \, \text{mg} \tag{1}
$$

where, ΔMa,sp. =(0.0089+0.0555 W – 0.0243 W2 + 0.0054 W3- 0.0004 W4) is the specific value of the mass loss of treating electrode for 1 Coulomb of electricity spent during 1 second alloying (mg/C· s); С is the amount of electricity of ESA processing (Coulomb); t is the time for the first layer deposition (s); W is the amount of pulse energy (J). The calculated curve and experimental average curve (solid dots) of the mass loss of the treating electrode are given in Fig. 16 for a 30 seconds of alloying by spending 3 Coulomb of electricity.

However, for a given pulse energy, the mass gain of the substrate shows a wide scattering depending on the pulse amplitude (Fig. 15). For example, for a pulse energy of 1.5 J which could be obtained by the application of pulse amplitudes of 200 A, 400 A, 600 A and 800 А, the mass gains of substrate were 3.5 mg, 2.5 mg, 1.7 mg and 0.25 mg, respectively. The difference in mass gained by substrate is because of two concurrent mass actions. These are the mass transferred from treating electrode and the mass loss due to washing away of molten pool of substrate spot caused by the intensive gas dynamics phenomena in plasma channel at the beginning of each spark discharge. In the case of processing with high pulse amplitude, due to larger material washed away, the mass loss of substrate would be expected to be more. However, cross sectional thickness measurements show that the thicknesses of coatings were close to each other despite the large differences in the amount of mass gain (14 times). This shows that the mass gained by substrates were almost the same despite the differences in pulse amplitudes. So, the amount of mass gained by substrate could be calculated by a simple relation, if the effect of washing away is negligible:

$$
\Delta \mathbf{M}\_{\mathbf{c}} = (\mathbf{k}\_{\text{if}} \cdot \Delta \mathbf{M}\_{\mathbf{a}}) \text{ mg} \tag{2}
$$

where, the mass transfer coefficient, ktf, is equal to 0.74-0.60 (less for higher pulse amplitude). The mass transfer coefficients given in Fig. 28 for the pulse amplitude beyond 200 A could not be used in Eq. 2, because the amount of mass loss of substrate due to washing away was not taken into account for the calculation of ktf .

Under this circumstance, the mass transfer coefficient should carefully be used to estimate the efficiency of mass transfer from treating electrodes to substrates. The following is an attempt to explain this point in detail.

The mass transfer coefficients as a function of pulse energy for the first layer of deposition were given in Fig. 17. The mass transfer coefficients corresponding to the short pulse durations were minimum for all pulse amplitudes ranging from 100 A to 1000 A. Since the substrate material was washed away due to the gas dynamics process at the beginning of a pulse, among these, the highest mass transfer coefficient belongs to smallest pulse amplitude which could not wash away molten substrate material.

The increase in pulse energy by means of increasing pulse duration which increases the mass transfer coefficient, because the mass gained by substrate from treating electrode is probably higher than the mass loss of original substrate as a result of washing away that occurs mainly at the beginning of the pulse. The amount of mass washed away during deposition of the first layer is not simple to measure, therefore the procedure to estimate the efficiency of mass transfer from treating electrode to substrate with the help of mass transfer coefficients is not quite correct.

Fig. 16. Calculated curve and experimental data of mass loss of treating electrode as a function of pulse energy

Electrospark Deposition: Mass Transfer 501

0 1 2 3 4 5 6 7 8 9 10 11 12

0 1 2 3 4 5 6 7 8 9 10 11 12

Fig. 18. Average coating thickness as a function of pulses energy

Fig. 17. Mass transfer coefficient as a function of pulse energy. Processing time is 30 s

Pulse energy, J

100А 200А 400А 600А 800А 1000А

Pulse energy, J

100А 200А 400А 600А 800А 1000А

ktf

0.0

Thickness, μm

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

where, the mass transfer coefficient, ktf, is equal to 0.74-0.60 (less for higher pulse amplitude). The mass transfer coefficients given in Fig. 28 for the pulse amplitude beyond 200 A could not be used in Eq. 2, because the amount of mass loss of substrate due to

Under this circumstance, the mass transfer coefficient should carefully be used to estimate the efficiency of mass transfer from treating electrodes to substrates. The following is an

The mass transfer coefficients as a function of pulse energy for the first layer of deposition were given in Fig. 17. The mass transfer coefficients corresponding to the short pulse durations were minimum for all pulse amplitudes ranging from 100 A to 1000 A. Since the substrate material was washed away due to the gas dynamics process at the beginning of a pulse, among these, the highest mass transfer coefficient belongs to smallest pulse

The increase in pulse energy by means of increasing pulse duration which increases the mass transfer coefficient, because the mass gained by substrate from treating electrode is probably higher than the mass loss of original substrate as a result of washing away that occurs mainly at the beginning of the pulse. The amount of mass washed away during deposition of the first layer is not simple to measure, therefore the procedure to estimate the efficiency of mass transfer from treating electrode to substrate with the help of mass transfer

0 1 2 3 4 5

Fig. 16. Calculated curve and experimental data of mass loss of treating electrode as a

Pulse energy, J

 calculation experiment

washing away was not taken into account for the calculation of ktf .

amplitude which could not wash away molten substrate material.

attempt to explain this point in detail.

coefficients is not quite correct.

ΔM , mg


function of pulse energy









0 a ΔMc = (ktf · ΔMa) mg (2)

Fig. 17. Mass transfer coefficient as a function of pulse energy. Processing time is 30 s

Fig. 18. Average coating thickness as a function of pulses energy

Electrospark Deposition: Mass Transfer 503

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One might estimate the range of coating thicknesses that could be obtained under the necessary restrictions for the number of superimposed layers. That is, substrates should be alloyed according to the condition given in Fig. 14. The average thickness of coating as a function of pulse energy was given in Fig. 18. For any chosen pulse amplitude, the coating thickness increases with increasing pulse energy, especially, for the low pulse energy range the rate of increase was the highest. Upon further increase of pulse energy, above 1 J, the growth of coating thickness levels off, that correlates in the course of mass loss of treating electrode as shown in Fig. 15.
