**2. Experimental set-up**

The experiments performed for the study were made by means of equipment that allows accuracy and ease of handling of tools and parts to be achieved. **Figure 1** shows a sketch of this equipment.

The equipment for the experiments rests on an anti-vibrations Table TMC, which provides a floating bench that prevents the tool and the part from oscillating. The position of the recipient is controlled by a three-dimensional (3D) nanometric positioning system based on a piezoelectric technology and with a resolution of 1 nm. There is a system of recirculation for the electrolyte, which flows constantly through the cell to a tank from which it is pumped to the cell after passing through a filter. Thus, the particles that appear in the cell are constantly being removed from the electrolyte. Experiments were performed in a solution of NaNO3 at 2% in weight as the electrolyte.

The material of the workpiece is AISI 304 stainless steel and the tool is made of Tungsten, 99.7% high purity. The tools are pins with a very small tip, measuring about 5 μm in diameter. The tool tip is sharpened by means of anodic dissolution in which the tungsten pin is used as the anode and the sheet of stainless steel as the cathode. The electrolyte used for this process is a solution of KOH at 5% in weight. **Figure 2** shows a picture of the equipment used for the process. In **Figure 3**, a microtool used for the process is shown.

In order to apply the voltage pulses to the system, a Function Generator Agilent 33,250 A is used, which generates voltage signals of several types and a broad range of frequency, up to 100 MHz, which corresponds to a width of 10 ns in the

**Figure 1.** *Sketch of the equipment used for experiments.*

**243**

**Figure 4.**

*2: current (mA).*

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

shown in **Figure 4**.

adequate for the process.

**Figure 3.**

*Microtool used for the process.*

voltage pulses. The signal applied by the generator passes through a developed pulse amplifier that provides the necessary current for the process corresponding to the voltage amplitude. The amplifier is fed by a DC power source Keytheley 2220G-30-1 which provides a current limiting system, so that the amplifier is not overloaded. The graphs of voltage and current between electrodes for a machining process are

In this graph, the current rises from zero to the stationary value going through a transient period of about 50 ns. Taking the criterion that the time constant is the time taken by the system to reach 63% of the total amount of change, a value of 25 ns for the time constant is deduced for a value of IEG = 1 μm and the other conditions described above. Therefore, values of on-time pulse above 50 ns are

The electrochemical process is observed by means of a Supereyes USB Portable Digital Microscope B008 connected to a computer in which the amplified image

*Signals of voltage and current between the electrodes in the machining process. Signal 1: voltage (V) and signal* 

**Figure 2.** *Equipment for electrochemical micromachining.*

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

*Nanofibers - Synthesis, Properties and Applications*

microtool used for the process is shown.

cathode. The electrolyte used for this process is a solution of KOH at 5% in weight. **Figure 2** shows a picture of the equipment used for the process. In **Figure 3**, a

33,250 A is used, which generates voltage signals of several types and a broad range of frequency, up to 100 MHz, which corresponds to a width of 10 ns in the

In order to apply the voltage pulses to the system, a Function Generator Agilent

**242**

**Figure 2.**

**Figure 1.**

*Sketch of the equipment used for experiments.*

*Equipment for electrochemical micromachining.*

voltage pulses. The signal applied by the generator passes through a developed pulse amplifier that provides the necessary current for the process corresponding to the voltage amplitude. The amplifier is fed by a DC power source Keytheley 2220G-30-1 which provides a current limiting system, so that the amplifier is not overloaded. The graphs of voltage and current between electrodes for a machining process are shown in **Figure 4**.

In this graph, the current rises from zero to the stationary value going through a transient period of about 50 ns. Taking the criterion that the time constant is the time taken by the system to reach 63% of the total amount of change, a value of 25 ns for the time constant is deduced for a value of IEG = 1 μm and the other conditions described above. Therefore, values of on-time pulse above 50 ns are adequate for the process.

The electrochemical process is observed by means of a Supereyes USB Portable Digital Microscope B008 connected to a computer in which the amplified image

**Figure 3.** *Microtool used for the process.*

#### **Figure 4.**

*Signals of voltage and current between the electrodes in the machining process. Signal 1: voltage (V) and signal 2: current (mA).*

of the tool and the area of the part being machined can be seen. This microscope is also helpful to set the approach of the tool to the workpiece in order to establish the reference of distance.

The voltage applied to the cell as well as the current passing through it is measured by means of a digital oscilloscope Tektronic DPO 4104, which allows several signals with up to 3 GHz to be visualised by using a maximum sample rate of 5 Gs/s. It also permits mean signal values to be measured, applying filters and making mathematical operations with signals, such us obtaining Fourier Transforms.

In order to observe and measure the dimensions of the features machined, as well as the tip of the tools, a scanning electron microscope and an optic microscope were used.

The reference for the position of the tool is taken as the point of value 0 for the IEG. That position was found by electrical contact between the tool and the workpiece. It is observed that, when using an active tool, the current does not change significantly as IEG decreases. However, when there is electrical contact, the current increases suddenly to a very high value. This phenomenon allows the reference to be found with a very slow movement of the tool and, therefore, a brusque impact is avoided, which could damage the tool tip.

## **3. Results and discussion**

PECMM works on the principle of Faraday's laws of electrolysis. The process consists of applying a potential difference between the tool and the workpiece so that an electrochemical reaction arises, which removes material from the workpiece. The metal is detached atom by atom from the anode surface and appears in the electrolyte as ions (Fe2+). These ions result in the precipitation of ferrous hydroxide Fe(OH)2. Simultaneously, hydrolysis causes the water molecules to gain electrons from the cathode and they separate into free hydrogen gas and hydroxyl ion [22]. The reactions can be summarised in the following equations:

$$Fe \to Fe^{2+} + 2e^- \tag{2}$$

$$H\_zO + 2e^- \rightarrow H\_z \uparrow + 2OH^- \tag{3}$$

$$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe}(\text{OH})\_z \tag{4}$$

**245**

**Figure 5.**

*Slots machined in experiments R-01 to R-04.*

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

lower and there is no bright area around them.

surface roughness and process speed.

**(μm)**

**Voltage (V)**

*Conditions of the experiments for assessing current confinement and surface roughness.*

**Experiment IEG** 

**Table 1.**

several values of pulse on-time and maintaining a constant voltage and period. By observing the size of the machined area, an assessment of the confinement of current can be achieved. The conditions for the experiments are shown in **Table 1**. The average current is seen to decrease as the pulse on-time becomes lower, since the current only flows in the voltage pulse periods. A photograph of the holes machined in experiments R-01 to R-04 described in **Table 1** is presented in **Figure 5**. All the slots were machined with the same tool, which had a tip diameter of 10 μm. However, the width of the slot decreases with the pulse on-time from 150 to 70 μm approximately, as can be seen in the image. This is a consequence of the spreading of the current, which will be higher with an increased pulse on-time. In addition, the roundness of the edges can clearly be seen to be higher when the pulse on-time is augmented. A bright area can be observed around the slots, which suggests that the current also spread outside the hole and hence some material was removed from that area. It can therefore be deduced that when an active tool is used there is always a spreading of current outside the area under the tool tip, even if the edge of the hole is sharp. The holes machined with an isolated tool are shown in **Figure 6**. The current has clearly spread over a much smaller area since the diameter of the holes is much

Regarding surface roughness, the relationship between conditions and results are similar to those in mechanical machining, since a high MRR produces high surface roughness and vice versa. Therefore, a compromise must be achieved between

R-01 1 16 120 370 **26** R-02 1 16 110 370 **22** R-03 1 16 100 370 **16.1** R-04 1 16 90 370 **10.3**

**Pulse width (ns)**

**Period (ns)**

**Average current (mA)**

#### **3.1 Current confinement and surface roughness**

In order to achieve precision in the machining the process must take place only under the tool tip, so that the cavity obtained in the workpiece is exactly the one determined by the profile of the tool. Therefore, current through the sides of the tool must be avoided, since it would remove material from other areas far from the tool tip. There are two methods of attaining this goal. The first one is isolating the side surface of the tool and using DC voltage as the process signal. The other one is using ultrashort voltage pulses and a very low IEG. The second method is used by several researchers [8, 11, 12] due to its ease of use if a function generator is available.

The confinement of the current can be assessed by observing the edge of the hole machined. If there is confinement, the contour of the hole will be sharp; otherwise the edge will be rounded. This phenomenon was studied by machining slots with

#### *Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

several values of pulse on-time and maintaining a constant voltage and period. By observing the size of the machined area, an assessment of the confinement of current can be achieved. The conditions for the experiments are shown in **Table 1**.

The average current is seen to decrease as the pulse on-time becomes lower, since the current only flows in the voltage pulse periods. A photograph of the holes machined in experiments R-01 to R-04 described in **Table 1** is presented in **Figure 5**. All the slots were machined with the same tool, which had a tip diameter of 10 μm. However, the width of the slot decreases with the pulse on-time from 150 to 70 μm approximately, as can be seen in the image. This is a consequence of the spreading of the current, which will be higher with an increased pulse on-time. In addition, the roundness of the edges can clearly be seen to be higher when the pulse on-time is augmented. A bright area can be observed around the slots, which suggests that the current also spread outside the hole and hence some material was removed from that area. It can therefore be deduced that when an active tool is used there is always a spreading of current outside the area under the tool tip, even if the edge of the hole is sharp. The holes machined with an isolated tool are shown in **Figure 6**. The current has clearly spread over a much smaller area since the diameter of the holes is much lower and there is no bright area around them.

Regarding surface roughness, the relationship between conditions and results are similar to those in mechanical machining, since a high MRR produces high surface roughness and vice versa. Therefore, a compromise must be achieved between surface roughness and process speed.


#### **Table 1.**

*Nanofibers - Synthesis, Properties and Applications*

is avoided, which could damage the tool tip.

**3.1 Current confinement and surface roughness**

**3. Results and discussion**

reference of distance.

were used.

of the tool and the area of the part being machined can be seen. This microscope is also helpful to set the approach of the tool to the workpiece in order to establish the

The voltage applied to the cell as well as the current passing through it is measured by means of a digital oscilloscope Tektronic DPO 4104, which allows several signals with up to 3 GHz to be visualised by using a maximum sample rate of 5 Gs/s. It also permits mean signal values to be measured, applying filters and making mathematical operations with signals, such us obtaining Fourier Transforms. In order to observe and measure the dimensions of the features machined, as well as the tip of the tools, a scanning electron microscope and an optic microscope

The reference for the position of the tool is taken as the point of value 0 for the IEG. That position was found by electrical contact between the tool and the workpiece. It is observed that, when using an active tool, the current does not change significantly as IEG decreases. However, when there is electrical contact, the current increases suddenly to a very high value. This phenomenon allows the reference to be found with a very slow movement of the tool and, therefore, a brusque impact

PECMM works on the principle of Faraday's laws of electrolysis. The process consists of applying a potential difference between the tool and the workpiece so that an electrochemical reaction arises, which removes material from the workpiece. The metal is detached atom by atom from the anode surface and appears in the electrolyte as ions (Fe2+). These ions result in the precipitation of ferrous hydroxide Fe(OH)2. Simultaneously, hydrolysis causes the water molecules to gain electrons from the cathode and they separate into free hydrogen gas and hydroxyl

In order to achieve precision in the machining the process must take place only under the tool tip, so that the cavity obtained in the workpiece is exactly the one determined by the profile of the tool. Therefore, current through the sides of the tool must be avoided, since it would remove material from other areas far from the tool tip. There are two methods of attaining this goal. The first one is isolating the side surface of the tool and using DC voltage as the process signal. The other one is using ultrashort voltage pulses and a very low IEG. The second method is used by several researchers [8, 11, 12] due to its ease of use if a function generator

The confinement of the current can be assessed by observing the edge of the hole machined. If there is confinement, the contour of the hole will be sharp; otherwise the edge will be rounded. This phenomenon was studied by machining slots with

→ + <sup>2</sup>+ − *Fe Fe e* 2 (2)

− − *H O e H OH* 2 2 + → ↑+ 2 2 (3)

( ) + − <sup>2</sup> + → <sup>2</sup> *Fe OH Fe OH* <sup>2</sup> (4)

ion [22]. The reactions can be summarised in the following equations:

**244**

is available.

*Conditions of the experiments for assessing current confinement and surface roughness.*

**Figure 5.** *Slots machined in experiments R-01 to R-04.*

**Figure 6.** *Holes machined with an isolated tool.*

Electrochemical machining has been observed to cause tiny craters in the workpiece surface, as a result of the localised current flowing through the electrolyte at the points of least electrical resistance. Therefore, if the current intensity is lower, the craters will be less deep and the resultant surface will be smoother. This can be seen clearly in **Figure 4**, which shows that the roughness is increasingly higher in the holes corresponding to R-04, R-03, R-02 and R-01, that is, when the pulse on-time grows. Therefore, it can be concluded that a good result is achieved by applying a voltage of 16 V and a pulse on-time of 80 ns and both confinement and surface roughness worsen when more aggressive values are used.

#### **3.2 Material removal rate (MRR)**

Material removal rate is a crucial variable in machining, since it determines the productivity of the process. This variable depends on the overpotential *η*, according to the principles of electrochemistry [9]. Therefore, the amplitude of the voltage signal determines the current intensity. Nevertheless, as the voltage signal applied to the cell consists of pulses, what determines MRR is the mean value of the current, according to Faraday's law of electrolysis:

$$MRR = \dot{m} = \frac{A \cdot I}{Z \cdot F} \tag{5}$$

**247**

**Figure 8.**

**Figure 7.**

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

of the stainless steel so that the MRR is maximum.

*Average current density as a function of voltage amplitude and pulse on-time.*

*Average current as a function of voltage amplitude and pulse on-time.*

considered was the current density, which provides more information regarding the performance of the process than the current intensity, as it takes the tool tip size into account. The results can be observed in **Figure 7**, which shows the variation of the average current density as a function of the voltage for every value of the pulse on-time.

in a constant manner as the voltage increases, as can be seen in **Figure 8**. This effect determines that, in order to achieve good machining without dispersion of the current, the voltage value must be high beyond the passive area

As can be seen in the graph, from 7 to 11 V, there is a decrease in current density as the voltage increases. This is due to the passivation phenomenon which occurs on the stainless steel surface. At a value of 12 V, the current density increases dramatically and then remains approximately constant. The range beyond 12 V is therefore the transpassive area, in which the voltage of the tool is enough to dissolve the passive layer under the tool tip and to remove material locally. As the current density was calculated by dividing the total current by the area of the tool tip, the sudden increase in the current density in that area does not involve a significant increase in the current as whole. Therefore, the average current grows

where *A* is the gram atomic weight, *Z* is the valence of dissolution, *F* is Faraday's constant and *I* is the average current. In turn, the average current depends on the ratio between the period and the pulse on-time of the signal. Therefore, the main parameters which determine MRR are the pulse amplitude and the ratio between pulse on-time and period.

In order to determine the value of the parameters to attain a maximum of MRR several experiments were performed, setting the combination of parameters by means of an experiment design in which the voltage varied between 7 and 16 V and the pulse on-time from 50 to 80 ns, keeping the period constant at 370 ns. The output variable

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

*Nanofibers - Synthesis, Properties and Applications*

Electrochemical machining has been observed to cause tiny craters in the workpiece surface, as a result of the localised current flowing through the electrolyte at the points of least electrical resistance. Therefore, if the current intensity is lower, the craters will be less deep and the resultant surface will be smoother. This can be seen clearly in **Figure 4**, which shows that the roughness is increasingly higher in the holes corresponding to R-04, R-03, R-02 and R-01, that is, when the pulse on-time grows. Therefore, it can be concluded that a good result is achieved by applying a voltage of 16 V and a pulse on-time of 80 ns and both confinement and surface

Material removal rate is a crucial variable in machining, since it determines the productivity of the process. This variable depends on the overpotential *η*, according to the principles of electrochemistry [9]. Therefore, the amplitude of the voltage signal determines the current intensity. Nevertheless, as the voltage signal applied to the cell consists of pulses, what determines MRR is the mean value of the current,

<sup>⋅</sup> = = <sup>⋅</sup> *A I MRR m*

where *A* is the gram atomic weight, *Z* is the valence of dissolution, *F* is Faraday's constant and *I* is the average current. In turn, the average current depends on the ratio between the period and the pulse on-time of the signal. Therefore, the main parameters which determine MRR are the pulse amplitude and the ratio between

In order to determine the value of the parameters to attain a maximum of MRR several experiments were performed, setting the combination of parameters by means of an experiment design in which the voltage varied between 7 and 16 V and the pulse on-time from 50 to 80 ns, keeping the period constant at 370 ns. The output variable

*Z F* (5)

roughness worsen when more aggressive values are used.

**3.2 Material removal rate (MRR)**

*Holes machined with an isolated tool.*

**Figure 6.**

according to Faraday's law of electrolysis:

pulse on-time and period.

**246**

considered was the current density, which provides more information regarding the performance of the process than the current intensity, as it takes the tool tip size into account. The results can be observed in **Figure 7**, which shows the variation of the average current density as a function of the voltage for every value of the pulse on-time.

As can be seen in the graph, from 7 to 11 V, there is a decrease in current density as the voltage increases. This is due to the passivation phenomenon which occurs on the stainless steel surface. At a value of 12 V, the current density increases dramatically and then remains approximately constant. The range beyond 12 V is therefore the transpassive area, in which the voltage of the tool is enough to dissolve the passive layer under the tool tip and to remove material locally. As the current density was calculated by dividing the total current by the area of the tool tip, the sudden increase in the current density in that area does not involve a significant increase in the current as whole. Therefore, the average current grows in a constant manner as the voltage increases, as can be seen in **Figure 8**.

This effect determines that, in order to achieve good machining without dispersion of the current, the voltage value must be high beyond the passive area of the stainless steel so that the MRR is maximum.

**Figure 7.** *Average current density as a function of voltage amplitude and pulse on-time.*

**Figure 8.** *Average current as a function of voltage amplitude and pulse on-time.*

The method of assessing the real MRR is to observe the volume of material removed, which is determined by the geometry of the hole made in every experiment. The volume removed can be represented as a function of the voltage applied and the pulse on-time. These graphs are shown in **Figure 9**.

The graph shows that the volume removed increased with the voltage applied for every value of the pulse on-time. According to this tendency, the best value of the voltage to achieve a good MRR is the highest possible one. On the other hand, the increase is observed to be faster when the pulse on-time grows, so the value of this parameter should be as high as possible while maintaining the conditions of confinement. This graph can be presented in a 3D format in order to show the join effect of voltage and pulse on-time, as it can be seen in **Figure 10**.

#### **3.3 Efficiency**

The efficiency of the electrochemical machining can be obtained by comparing the theoretical value of material removed with the real one. The theoretical value is given by Faraday's law (5) and can be calculated from the current in the process.

**Figure 9.**

*Volume of material removed as a function of voltage amplitude and pulse on-time.*

**249**

**Figure 12.**

**Figure 11.**

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

*Efficiency of the process as a function of voltage amplitude and pulse on-time.*

*3D representation of the efficiency of the process as a function of voltage amplitude and pulse on-time.*

explained in Section 3.2.

appropriate parameters.

The real value can be calculated from the geometry of the machined feature, as

This characteristic of the process is of great significance to the cost of the process, most of all, at an industrial level, and should be optimised by choosing the

In order to assess the efficiency of the process the results of the experiments made for observing the MRR were used. The ratio between the volume removed and the theoretical volume corresponding to the current was obtained and represented in **Figure 11**.

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

*Nanofibers - Synthesis, Properties and Applications*

**3.3 Efficiency**

and the pulse on-time. These graphs are shown in **Figure 9**.

effect of voltage and pulse on-time, as it can be seen in **Figure 10**.

*Volume of material removed as a function of voltage amplitude and pulse on-time.*

*3D representation of the volume removed as a function of voltage amplitude and pulse on-time.*

The method of assessing the real MRR is to observe the volume of material removed, which is determined by the geometry of the hole made in every experiment. The volume removed can be represented as a function of the voltage applied

The graph shows that the volume removed increased with the voltage applied for every value of the pulse on-time. According to this tendency, the best value of the voltage to achieve a good MRR is the highest possible one. On the other hand, the increase is observed to be faster when the pulse on-time grows, so the value of this parameter should be as high as possible while maintaining the conditions of confinement. This graph can be presented in a 3D format in order to show the join

The efficiency of the electrochemical machining can be obtained by comparing the theoretical value of material removed with the real one. The theoretical value is given by Faraday's law (5) and can be calculated from the current in the process.

**248**

**Figure 10.**

**Figure 9.**

The real value can be calculated from the geometry of the machined feature, as explained in Section 3.2.

This characteristic of the process is of great significance to the cost of the process, most of all, at an industrial level, and should be optimised by choosing the appropriate parameters.

In order to assess the efficiency of the process the results of the experiments made for observing the MRR were used. The ratio between the volume removed and the theoretical volume corresponding to the current was obtained and represented in **Figure 11**.

**Figure 11.** *Efficiency of the process as a function of voltage amplitude and pulse on-time.*

In these graphs, very low values of efficiency can be seen, because the maximum efficiency is lower than 12%. This is a consequence of the dispersion of the current, as the results presented in Section 3.1 show. According to the graphs, the higher the voltage and pulse on-time, the better the confinement of the current in the area under the tool tip and hence the higher the efficiency. The join effect of voltage and pulse on-time in efficiency is shown in the 3D of **Figure 12**.

These results can be analysed along with those presented in Section 3.2. Observing those graphs and **Figure 5** it can be deduced that the reason why the volume removed increases so drastically for voltage values higher than 14 V is not the increased current, but the clear increase in the efficiency of the process for those values. So, in order to achieve the best efficiency along with a good value of material removed, the highest possible value of voltage amplitude must be used along with the widest pulse maintaining the confinement and surface roughness within acceptable values.
