- Email: [email protected]
Optimization of Pulsed Electrochemical Micromachining in Stainless Steel
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 426 – 431
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Optimization of pulsed electrochemical micromachining in stainless steel P. Rodrigueza*, D. Hidalgoa, J.E. Labargaa a
University of Leon, 24071 Leon, Spain
* Corresponding author. Tel.: 00-34-87291914; fax: 00-34-87291930. E-mail address: [email protected]
Abstract The machining of materials at micro and submicroscales is considered a key technology of the future and plays a role increasingly important in the miniaturization of complete machines, with important applications as bioengineering, MEMS, microsensors and microactuators. In this work the process of Pulsed Electrochemical Micromachining was studied. The experiments were carried out in a workpiece of Stainless Steel sunk in an electrochemical cell with an electrode of Tungsten with very small diameter (2-8 Pm). Pulses of voltage with very small width are introduced between the electrode and the part, so that the current is confined under the area of the tool tip. With this method microholes and microslots were obtained. It is observed that there is a minimum pulse-on time for each value of the voltage to make the current flow. Nevertheless, for values of voltage of 7 V and higher there is always material removal for any value of pulse-on time. A maximum ratio between pulse-on time and period of 1/3 and an interelectrode gap in the order of micrometers are required to provide a localized material removal under the tip of the tool. In these conditions, the confinement will decrease when the pulse on-time increases. The main parameter that determines the material removal rate in the process is the current intensity. Surface roughness increases with pulse-on time, as well as the average current of the process and hence the material removal rate. Therefore, there is a compromise between surface finish and material removal rate. 2018The The Authors. Published by Elsevier B.V. ©2018 © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Pulsed electrochemical micromachining; current confinement; optimum parameters
1. Introduction Microfabrication consists in obtaining products or parts with features at micro or submicroscale, which therefore requires very narrowly controlled material removal. Microfabrication has been widely used for the manufacturing of holes in injectors, fluidic microchemical reactors requiring microscale pumps, micromoulds, and many more applications [1]. Microfabrication plays an increasingly important role in miniaturization of components which expand from biomedical applications to manufacturing of sensors. Surfaces to be obtained are slots, complex surfaces, microholes, etc. Electrochemical micromachining is acquiring more importance due to its specific characteristics to avoid the problems of conventional processes, such as tool wear and breakage, inaccuracy due to low rigidity of the tool, etc. Electrochemical micromachining has been a process with high specialization to be used in aerospace industry. Today it is starting to be used in other industries, where difficult to
manufacture parts, complex surfaces and components in the microscopic scale are necessary. In an analogous way to conventional Electrochemical Machining (ECM), Pulsed Electrochemical Micromachining (PECMM) is a controlled process of anodic dissolution to remove material which takes place at high current densities, typically in the order of 105 A/m2 between the tool (cathode) and the workpiece (anode) through the electrolyte [2]. By causing the tool to move towards the workpiece, the material is removed under its tip, since the current density is higher at lower distance between tool and workpiece, and thus, the geometry of the tool is copied as a cavity in the workpiece. As compared with other processes, PECMM is a high precision technique to obtain holes of small diameter or crack-free microcomponents without residual stress. The use of ultrashort voltage pulses, usually shorter than 100 ns, allows achieving accuracy by confining the faradaic current density under the tool, since this current is the responsible for the anodic dissolution of the material. Confinement is due to the
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.090
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
incomplete charge of the double layer in areas far from the workpiece, through which a very low faradaic current flows. A very important advance has been made in the research of this process on many materials such as Aluminum, Titanium, Steel and Copper [3,4]. Stainless Steel is a very important material to be used in any type of microcomponents, but its dissolution is difficult since its chemical properties are not very suitable for this process. Some of the existent studies were performed specifically on Stainless Steel [5–7]. Nevertheless, the pulse on-time used in those works is too high to obtain a good confinement of the current. Furthermore, there are no studies in which the diameter of the tool is as small as a few microns. Therefore, there is a huge amount of work to do to characterize correctly this process regarding the values of the parameters in order to obtain a good result in terms of current confinement, surface roughness, material removal rate and experimental setup.
427
that appear in the cell are constantly being removed from the electrolyte. The electrolyte used in the experiments was an aqueous solution of NaNO3 at 2% in weight. The material of the workpiece is AISI 304 Stainless Steel and the tool is made of Tungsten with a purity of 99.7%. The tools are pins with a very small tip, of about 5 Pm in diameter. The tool tip is sharpened by means of anodic dissolution in which the tungsten pin is used as the anode and a 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 microtool used for the process.
In this work, experiments of PECMM with pulse on-time values in the order of ns and tools with diameter in the order of a few microns have been made to observe the results and to characterize the process more accurately.
2. Experimental setup The experiments performed for the study were made by means of an equipment that allows achieving accuracy and easy handling of tools and parts. Figure 1 shows a sketch of this equipment.
Fig. 2. Microtool used for the process
In order to apply the voltage pulses to the system, a Function Generator Agilent 33250 A is used, which provides voltage signals of several types and a broad range of frequency, up to 100 MHz, which corresponds to pulses with a minimum of 10 ns of pulse-on time. The signal applied by the Generator passes through a Pulse Amplifier that provides the necessary current for the process that corresponds to the voltage amplitude. The Amplifier is fed by a DC Power Source Keytheley 2220G-301 which provides a current limiting system, so that the Amplifier is not overloaded. In Figure 3 the graphs of voltage and current between electrodes for a machining process are shown.
Fig. 1. Sketch of the equipment used for the experiments
The equipment for the experiments rests on an antivibrations table TMC, which provides a floating bench that keeps the tool and the part from oscillations. The position of the recipient is controlled by a three-dimensional nanometric positioning system PI-Micos based on a piezoelectric technology with a resolution of 1 nm. There is a system of recirculation for the electrolyte, which flows constantly from the cell to a tank from which it is pumped again to the cell after being filtered. Thus, the particles removed from the workpiece
428
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431 Fig. 3. Signals of voltage and current between electrodes in the process
The electrochemical process is observed by means of a Supereyes USB Portable Digital Miroscope B008 connected to a computer in which the amplified image of the tool and the area of the part being machined can be seen. This microscope is also helpful to observe the process of setting the reference of distance between tool and workpiece. The reference of the position of the tool is taken when there is contact between the tool and the workpiece, i.e. when the interelectrode gap (IEG) is 0. That position is found by electrical contact between the tool and the workpiece. The voltage applied to the cell as well as the current that passes through it is measured by means of a digital oscilloscope Tektronic DPO 4104, which allows to visualize several signals with up to 3 GHz by using a maximum sample rate of 5 Gs/s. It also permits measuring mean values of signals, applying filters and making mathematical operations with them, such us 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.
3. Results and discussion In this section some results of the process are shown as well as the influence of some variables in the process, so that some conclusions can be drawn about its optimization.
In Figure 5 a circular microslot is shown. The conditions used were: voltage amplitude 10 V, period 370 ns, pulse-on time 70 ns, initial IEG 1 Pm, feed rate 3 Pm/s.
Fig. 5. Circular microslot machined on a stainless steel workpiece
The position control of the tool can be commanded by CNC software, also used for controlling the position of the tool in a mechanical machining process. This software can move the tool in 3 axes with controlled feed rate. By running the corresponding program a complex geometry can be obtained, as can be seen in Figure 6.
3.1. Results Several types of geometric features can be obtained with the process. Figure 4 shows a through microhole machined with the following conditions: voltage amplitude 12 V, period 370 ns, pulse-on time 80 ns, initial IEG 2 Pm, feed rate 10 Pm/min. As it can be seen, a very sharp edge was obtained with no material removed around the hole, which proves that the current confinement was very good.
Fig. 6. Celtic symbol machined by using CNC software
3.2. Thresholds of the process
Fig. 4. Through microhole machined on a stainless steel workpiece
In order to perform a good machining the conditions of the process must be as smooth as possible, so that the current spread outside the area under the tool tip is very low. This could occur if the voltage or the ratio pulse-on time to period is too high. It is usually considered that a ratio between pulse-on time and period of the voltage signal over 1/3 causes too much current spreading [4]. The current confinement will increase as this ratio decreases. On the other hand, if the variables are too low there will be no material removal. This is due to the incomplete charge of the double layer, which acts as a
429
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
capacitor. For every value of the voltage there is a value of the pulse-on time for which the capacitor does not acquire a complete charge and therefore faradaic current does not flow. A set of experiments was designed to find the minimum values of voltage and pulse-on time below which the process does not take place. Table 1 presents the results of those experiments. It can be observed that as the voltage amplitude increases the pulse-on time for which there is no process decreases. As it was explained above, the process consists in charging a capacitor under a certain voltage in each pulse-on period. This charging process will be faster if the voltage applied is higher, since the slope of the current will be higher. Table 1. Results of the experiments for finding the thresholds of the process Voltage (V)
Pulse-on time (ns)
Period (ns)
Machining
5
200
370
Yes
5
175
370
Yes
5
150
370
No
6
150
370
Yes
6
125
370
Yes
6
100
370
No
7
125
370
Yes
7
100
370
Yes
7
75
370
Yes
From the values of the table it can be deduced that for values of the voltage higher than 7 V the process always takes place, whereas below that value the occurring of the process depends on the pulse-on time. At 6 V the pulse-on time must be higher than 100 ns and at 5 V higher than 150 ns. According to these considerations, a good choice for the voltage would be 7 V if a good accuracy is needed, since there is process for any value of the pulse-on time and the current confinement is very good. If accuracy is not so important, a higher value of the voltage should be chosen to obtain a faster process.
Fig. 7. Detail of the circular microslot Table 2. Conditions of the experiments for assessing surface roughness Experiment
Voltage (V)
Pulse-on time (ns)
Average current (mA)
R-01
16
120
26.0
R-02
16
110
22.0
R-03
16
100
16.1
R-04
16
90
10.3
Figure 8 shows an overview of the machining performed by these experiments. The walls of the microslots are shown in detail in the photographs taken by the SEM corresponding to figures 9 to 12. It can be observed that the surface roughness of the bottom of the slots decreases from R-01 to R-04. Therefore, the surface generated by electrochemical machining, similarly to what happens in mechanical machining, is smoother when the process is slower.
3.3. Surface roughness The pictures of the workpieces machined show a high surface roughness on the walls and the bottom of the holes or slots obtained. Figure 7 shows a detail of the microslot shown in Figure 5. It can be observed that this roughness is higher when the variables of the process increase since the machining is then more aggressive. Some experiments were performed to assess the influence of the variables in the surface roughness. In those experiments, the voltage amplitude was 16 V and the period 370 ns, whereas the pulse-on time varied from 120 to 90 ns. The conditions of the experiments are shown in Table 2. It can be seen that the average current increases with the pulse-on time, as it was expected. The average current, according to Faraday’s law of electrolysis, is the variable that determines the material removal rate, so the process speed will increase with pulse-on time.
R-01
R-02
R-03
Fig. 8. Microslots of experiments R-01 to R-04
R-04
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P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
Fig. 9. Side wall and edge of microslot machined in R-01
Fig. 12. Side wall and edge of microslot machined in R-04
According to these results, there is a compromise between surface roughness and material removal rate. If a good surface finish is needed a low value of pulse-on time must be chosen, provided it is over the threshold value for the occurring of the process. A good result is achieved when using a voltage amplitude of 16 V and a pulse-on time of 90 ns with a IEG value of 2 Pm. Nevertheless, the material removal rate is too slow for some specific applications. Therefore, if surface finish is not important, then the pulse-on time chosen must be high, always under the value corresponding to a ratio between this time and the period of 1/3, so that no high spreading of the current takes place. A good choice for this situation is a pulseon time of 120-130 ns with a voltage of 16 V. 4. Conclusions Fig. 10. Side wall and edge of microslot machined in R-02
Fig. 11. Side wall and edge of microslot machined in R-03
A study of the pulsed electrochemical micromachining on stainless steel was carried out in which the influence of the main variables of the process has been analysed. The parameters used for the study are more fitting to the process of micromachining than those used for previous works, such as the pulse-on time and the gap. As the values of pulse-on time chosen are very low in order to attain current confinement, a threshold for the process is discovered, which was not known so far because in previous studies the values of this variable were higher. It is observed that the threshold of the process depends on the voltage amplitude. For each voltage there is a value for the pulse-on time under which no material is removed. However, if the voltage is set in 7 V, there will be process for any value of the pulse-on time. Surface roughness is a variable that attracted too little attention in previous works. In the present work, the compromise between material removal rate and surface finish is confirmed. Furhtermore, the best choice for the parameters in order to achieve a good surface finish is given, in view of the results of the experiments. A pulse-on time between 90 and 120 ns must be chosen for a voltage of 16 V, depending on the relevance of surface finish and material removal rate.
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
References [5] [1]
[2]
[3]
[4]
E. B. Brousseau, S. S. Dimov, and D. T. Pham, “Some recent advances in multi-material micro- and nano-manufacturing,” Int. J. Adv. Manuf. Technol., vol. 47, no. 1–4, pp. 161–180, 2010. E. L. Hotoiu, S. Van Damme, C. Albu, D. Deconinck, a. Demeter, and J. Deconinck, “Simulation of nano-second pulsed phenomena in electrochemical micromachining processes-Effects of the signal and double layer properties,” Electrochim. Acta, vol. 93, pp. 8–16, 2013. R. Schuster, V. Kirchner, P. Allongue, and G. Ertl, “Electrochemical Micromachining,” Science (80-. )., vol. 289, no. 5476, pp. 98–101, Jul. 2000. Bijoy Bhattacharyya, Electrochemical Micromachining for Nanofabrication, MEMS and Nanotechnology - Bijoy
[6]
[7]
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Bhattacharyya - Google Libros. Elsevier, 2015. L. Yong, Z. Yunfei, Y. Guang, and P. Liangqiang, “Localized electrochemical micromachining with gap control,” Sensors Actuators, A Phys., vol. 108, no. 1–3, pp. 144–148, 2003. S. H. Ahn, S. H. Ryu, D. K. Choi, and C. N. Chu, “Electrochemical micro drilling using ultra short pulses,” Precis. Eng., vol. 28, no. 2, pp. 129–134, 2004. B. H. Kim, C. W. Na, Y. S. Lee, D. K. Choi, and C. N. Chu, “Micro Electrochemical Machining of 3D Micro Structure Using Dilute Sulfuric Acid,” CIRP Ann. - Manuf. Technol., vol. 54, no. 1, pp. 191–194, 2005.
ScienceDirect Procedia CIRP 68 (2018) 426 – 431
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Optimization of pulsed electrochemical micromachining in stainless steel P. Rodrigueza*, D. Hidalgoa, J.E. Labargaa a
University of Leon, 24071 Leon, Spain
* Corresponding author. Tel.: 00-34-87291914; fax: 00-34-87291930. E-mail address: [email protected]
Abstract The machining of materials at micro and submicroscales is considered a key technology of the future and plays a role increasingly important in the miniaturization of complete machines, with important applications as bioengineering, MEMS, microsensors and microactuators. In this work the process of Pulsed Electrochemical Micromachining was studied. The experiments were carried out in a workpiece of Stainless Steel sunk in an electrochemical cell with an electrode of Tungsten with very small diameter (2-8 Pm). Pulses of voltage with very small width are introduced between the electrode and the part, so that the current is confined under the area of the tool tip. With this method microholes and microslots were obtained. It is observed that there is a minimum pulse-on time for each value of the voltage to make the current flow. Nevertheless, for values of voltage of 7 V and higher there is always material removal for any value of pulse-on time. A maximum ratio between pulse-on time and period of 1/3 and an interelectrode gap in the order of micrometers are required to provide a localized material removal under the tip of the tool. In these conditions, the confinement will decrease when the pulse on-time increases. The main parameter that determines the material removal rate in the process is the current intensity. Surface roughness increases with pulse-on time, as well as the average current of the process and hence the material removal rate. Therefore, there is a compromise between surface finish and material removal rate. 2018The The Authors. Published by Elsevier B.V. ©2018 © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: Pulsed electrochemical micromachining; current confinement; optimum parameters
1. Introduction Microfabrication consists in obtaining products or parts with features at micro or submicroscale, which therefore requires very narrowly controlled material removal. Microfabrication has been widely used for the manufacturing of holes in injectors, fluidic microchemical reactors requiring microscale pumps, micromoulds, and many more applications [1]. Microfabrication plays an increasingly important role in miniaturization of components which expand from biomedical applications to manufacturing of sensors. Surfaces to be obtained are slots, complex surfaces, microholes, etc. Electrochemical micromachining is acquiring more importance due to its specific characteristics to avoid the problems of conventional processes, such as tool wear and breakage, inaccuracy due to low rigidity of the tool, etc. Electrochemical micromachining has been a process with high specialization to be used in aerospace industry. Today it is starting to be used in other industries, where difficult to
manufacture parts, complex surfaces and components in the microscopic scale are necessary. In an analogous way to conventional Electrochemical Machining (ECM), Pulsed Electrochemical Micromachining (PECMM) is a controlled process of anodic dissolution to remove material which takes place at high current densities, typically in the order of 105 A/m2 between the tool (cathode) and the workpiece (anode) through the electrolyte [2]. By causing the tool to move towards the workpiece, the material is removed under its tip, since the current density is higher at lower distance between tool and workpiece, and thus, the geometry of the tool is copied as a cavity in the workpiece. As compared with other processes, PECMM is a high precision technique to obtain holes of small diameter or crack-free microcomponents without residual stress. The use of ultrashort voltage pulses, usually shorter than 100 ns, allows achieving accuracy by confining the faradaic current density under the tool, since this current is the responsible for the anodic dissolution of the material. Confinement is due to the
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.090
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
incomplete charge of the double layer in areas far from the workpiece, through which a very low faradaic current flows. A very important advance has been made in the research of this process on many materials such as Aluminum, Titanium, Steel and Copper [3,4]. Stainless Steel is a very important material to be used in any type of microcomponents, but its dissolution is difficult since its chemical properties are not very suitable for this process. Some of the existent studies were performed specifically on Stainless Steel [5–7]. Nevertheless, the pulse on-time used in those works is too high to obtain a good confinement of the current. Furthermore, there are no studies in which the diameter of the tool is as small as a few microns. Therefore, there is a huge amount of work to do to characterize correctly this process regarding the values of the parameters in order to obtain a good result in terms of current confinement, surface roughness, material removal rate and experimental setup.
427
that appear in the cell are constantly being removed from the electrolyte. The electrolyte used in the experiments was an aqueous solution of NaNO3 at 2% in weight. The material of the workpiece is AISI 304 Stainless Steel and the tool is made of Tungsten with a purity of 99.7%. The tools are pins with a very small tip, of about 5 Pm in diameter. The tool tip is sharpened by means of anodic dissolution in which the tungsten pin is used as the anode and a 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 microtool used for the process.
In this work, experiments of PECMM with pulse on-time values in the order of ns and tools with diameter in the order of a few microns have been made to observe the results and to characterize the process more accurately.
2. Experimental setup The experiments performed for the study were made by means of an equipment that allows achieving accuracy and easy handling of tools and parts. Figure 1 shows a sketch of this equipment.
Fig. 2. Microtool used for the process
In order to apply the voltage pulses to the system, a Function Generator Agilent 33250 A is used, which provides voltage signals of several types and a broad range of frequency, up to 100 MHz, which corresponds to pulses with a minimum of 10 ns of pulse-on time. The signal applied by the Generator passes through a Pulse Amplifier that provides the necessary current for the process that corresponds to the voltage amplitude. The Amplifier is fed by a DC Power Source Keytheley 2220G-301 which provides a current limiting system, so that the Amplifier is not overloaded. In Figure 3 the graphs of voltage and current between electrodes for a machining process are shown.
Fig. 1. Sketch of the equipment used for the experiments
The equipment for the experiments rests on an antivibrations table TMC, which provides a floating bench that keeps the tool and the part from oscillations. The position of the recipient is controlled by a three-dimensional nanometric positioning system PI-Micos based on a piezoelectric technology with a resolution of 1 nm. There is a system of recirculation for the electrolyte, which flows constantly from the cell to a tank from which it is pumped again to the cell after being filtered. Thus, the particles removed from the workpiece
428
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431 Fig. 3. Signals of voltage and current between electrodes in the process
The electrochemical process is observed by means of a Supereyes USB Portable Digital Miroscope B008 connected to a computer in which the amplified image of the tool and the area of the part being machined can be seen. This microscope is also helpful to observe the process of setting the reference of distance between tool and workpiece. The reference of the position of the tool is taken when there is contact between the tool and the workpiece, i.e. when the interelectrode gap (IEG) is 0. That position is found by electrical contact between the tool and the workpiece. The voltage applied to the cell as well as the current that passes through it is measured by means of a digital oscilloscope Tektronic DPO 4104, which allows to visualize several signals with up to 3 GHz by using a maximum sample rate of 5 Gs/s. It also permits measuring mean values of signals, applying filters and making mathematical operations with them, such us 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.
3. Results and discussion In this section some results of the process are shown as well as the influence of some variables in the process, so that some conclusions can be drawn about its optimization.
In Figure 5 a circular microslot is shown. The conditions used were: voltage amplitude 10 V, period 370 ns, pulse-on time 70 ns, initial IEG 1 Pm, feed rate 3 Pm/s.
Fig. 5. Circular microslot machined on a stainless steel workpiece
The position control of the tool can be commanded by CNC software, also used for controlling the position of the tool in a mechanical machining process. This software can move the tool in 3 axes with controlled feed rate. By running the corresponding program a complex geometry can be obtained, as can be seen in Figure 6.
3.1. Results Several types of geometric features can be obtained with the process. Figure 4 shows a through microhole machined with the following conditions: voltage amplitude 12 V, period 370 ns, pulse-on time 80 ns, initial IEG 2 Pm, feed rate 10 Pm/min. As it can be seen, a very sharp edge was obtained with no material removed around the hole, which proves that the current confinement was very good.
Fig. 6. Celtic symbol machined by using CNC software
3.2. Thresholds of the process
Fig. 4. Through microhole machined on a stainless steel workpiece
In order to perform a good machining the conditions of the process must be as smooth as possible, so that the current spread outside the area under the tool tip is very low. This could occur if the voltage or the ratio pulse-on time to period is too high. It is usually considered that a ratio between pulse-on time and period of the voltage signal over 1/3 causes too much current spreading [4]. The current confinement will increase as this ratio decreases. On the other hand, if the variables are too low there will be no material removal. This is due to the incomplete charge of the double layer, which acts as a
429
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
capacitor. For every value of the voltage there is a value of the pulse-on time for which the capacitor does not acquire a complete charge and therefore faradaic current does not flow. A set of experiments was designed to find the minimum values of voltage and pulse-on time below which the process does not take place. Table 1 presents the results of those experiments. It can be observed that as the voltage amplitude increases the pulse-on time for which there is no process decreases. As it was explained above, the process consists in charging a capacitor under a certain voltage in each pulse-on period. This charging process will be faster if the voltage applied is higher, since the slope of the current will be higher. Table 1. Results of the experiments for finding the thresholds of the process Voltage (V)
Pulse-on time (ns)
Period (ns)
Machining
5
200
370
Yes
5
175
370
Yes
5
150
370
No
6
150
370
Yes
6
125
370
Yes
6
100
370
No
7
125
370
Yes
7
100
370
Yes
7
75
370
Yes
From the values of the table it can be deduced that for values of the voltage higher than 7 V the process always takes place, whereas below that value the occurring of the process depends on the pulse-on time. At 6 V the pulse-on time must be higher than 100 ns and at 5 V higher than 150 ns. According to these considerations, a good choice for the voltage would be 7 V if a good accuracy is needed, since there is process for any value of the pulse-on time and the current confinement is very good. If accuracy is not so important, a higher value of the voltage should be chosen to obtain a faster process.
Fig. 7. Detail of the circular microslot Table 2. Conditions of the experiments for assessing surface roughness Experiment
Voltage (V)
Pulse-on time (ns)
Average current (mA)
R-01
16
120
26.0
R-02
16
110
22.0
R-03
16
100
16.1
R-04
16
90
10.3
Figure 8 shows an overview of the machining performed by these experiments. The walls of the microslots are shown in detail in the photographs taken by the SEM corresponding to figures 9 to 12. It can be observed that the surface roughness of the bottom of the slots decreases from R-01 to R-04. Therefore, the surface generated by electrochemical machining, similarly to what happens in mechanical machining, is smoother when the process is slower.
3.3. Surface roughness The pictures of the workpieces machined show a high surface roughness on the walls and the bottom of the holes or slots obtained. Figure 7 shows a detail of the microslot shown in Figure 5. It can be observed that this roughness is higher when the variables of the process increase since the machining is then more aggressive. Some experiments were performed to assess the influence of the variables in the surface roughness. In those experiments, the voltage amplitude was 16 V and the period 370 ns, whereas the pulse-on time varied from 120 to 90 ns. The conditions of the experiments are shown in Table 2. It can be seen that the average current increases with the pulse-on time, as it was expected. The average current, according to Faraday’s law of electrolysis, is the variable that determines the material removal rate, so the process speed will increase with pulse-on time.
R-01
R-02
R-03
Fig. 8. Microslots of experiments R-01 to R-04
R-04
430
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
Fig. 9. Side wall and edge of microslot machined in R-01
Fig. 12. Side wall and edge of microslot machined in R-04
According to these results, there is a compromise between surface roughness and material removal rate. If a good surface finish is needed a low value of pulse-on time must be chosen, provided it is over the threshold value for the occurring of the process. A good result is achieved when using a voltage amplitude of 16 V and a pulse-on time of 90 ns with a IEG value of 2 Pm. Nevertheless, the material removal rate is too slow for some specific applications. Therefore, if surface finish is not important, then the pulse-on time chosen must be high, always under the value corresponding to a ratio between this time and the period of 1/3, so that no high spreading of the current takes place. A good choice for this situation is a pulseon time of 120-130 ns with a voltage of 16 V. 4. Conclusions Fig. 10. Side wall and edge of microslot machined in R-02
Fig. 11. Side wall and edge of microslot machined in R-03
A study of the pulsed electrochemical micromachining on stainless steel was carried out in which the influence of the main variables of the process has been analysed. The parameters used for the study are more fitting to the process of micromachining than those used for previous works, such as the pulse-on time and the gap. As the values of pulse-on time chosen are very low in order to attain current confinement, a threshold for the process is discovered, which was not known so far because in previous studies the values of this variable were higher. It is observed that the threshold of the process depends on the voltage amplitude. For each voltage there is a value for the pulse-on time under which no material is removed. However, if the voltage is set in 7 V, there will be process for any value of the pulse-on time. Surface roughness is a variable that attracted too little attention in previous works. In the present work, the compromise between material removal rate and surface finish is confirmed. Furhtermore, the best choice for the parameters in order to achieve a good surface finish is given, in view of the results of the experiments. A pulse-on time between 90 and 120 ns must be chosen for a voltage of 16 V, depending on the relevance of surface finish and material removal rate.
P. Rodriguez et al. / Procedia CIRP 68 (2018) 426 – 431
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