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Adapting ECF to steels used for micro mould inserts
Multi-Material Micro Manufacture W. Menz, S. Dimov and B. Fillon (Eds.) © 2006 Elsevier Ltd. All rights reserved
309
Adapting ECF to steels used for micro mould inserts L. Staemmlera, K. Hofmannb, M.-H. Kimb, D. Warkentina, H. Kücka,b a
b
Hahn-Schickard-Institute for Micro Assembly Technology (HSG-IMAT), Stuttgart, Germany University of Stuttgart, Institute of Micro- and Precision Engeneering (IZFM), Stuttgart, Germany
Abstract Electrochemical machining with ultra short voltage pulses (ECF) is an innovative technique to machine electrochemically active materials particularly very hard materials at micrometer feature size. Since the ECF technique is an electrochemical process neither mechanical forces nor thermal load are applied to workpiece or tool. For that reason ECF it is an ideal technique for the production of microstructures. Especially the use of steel as the workpiece material makes the ECF technique a promising technique for the production of micro mould inserts, since steel is resistant against the wear that occurs due to the injection process. Therefore the abilities and the limits of the ECF process for different types of tool steels are shown and the process parameters have been optimised. Acetic acid has been proven to be a suitable electrolyte for the ECF-process of tool steels. In this electrolyte tool steels like 1.2767 or 1.2312 can be processed with the ECF technique at a high quality. But in lowalloyed steels like 1.1730 only a poor quality can be obtained. Keywords: ECF, micro mould, steel
1. Introduction The production of microstructures demand highly precise techniques due to the small size of the structures. Good results have been obtained by the LIGA technique. But also with high speed cutting (HSC) milling machines which can have an accuracy in the micrometer range microstructures can be achieved by using milling cutters with diameters of 80 µm and below. Also EDM is capable of producing structures in the 10 µm-range. Nevertheless the production of microstructures is extremely timeconsuming. It therefore makes sense to reproduce the microstructure, for example by injection moulding. With a single microstructure several thousand parts can be made. This demands a mould that withstands the abrasive properties of the injected polymer. The lifetime of nickel moulds which are made with the LIGA technique is often too short for an effective use of theses moulds. Hard materials like steel, which are therefore normally used for making these moulds, cannot be processed by the LIGA technique. Because EDM brings a huge thermal load to the workpiece and HSC is not capable of obtaining patterns with dimensions much below 100 µm, electrochemical milling with ultrashort voltage pulses (ECF, where F is the German acronym for milling) is a powerful alternative for the production of the microstructures in the mould [1]. Because it is an electrochemical process neither thermal load nor mechanical forces act on the tool, hence no tool wear arises. In the ECF process short voltage pulses in the range of 10 ns to 1000 ns are applied between tool and workpiece to dissolve the workpiece locally [2]. Therefor the workpiece as well as the tool are submerged in an appropriate electrolyte. In the idle state both the workpiece and the tool are hold at a constant cathodic potential to prevent corrosion. For the dissolution process the pulses are applied additionally to the existing potentials. An electrode that is submerged in an electrolyte forms a double
layer capacitance at its surface. During the pulse these capacitances are charged over the resistance of the electrolyte. The value of this resistance depends on the length of the path of the electric current in the electrolyte. Thus a large separation between tool electrode and workpiece leads to a slow charging of the capacitance whereas a small separation leads to a fast charging (fig.1). Since charging takes place only during the time of the pulse the double layer capacitance is charged to a potential high enough for an anodic dissolution of the workpiece only in a close proximity around the tool, the so called working distance. Surface areas of the workpiece that are further away from the tool are not affected by the ECF process. This leads to a confined milling with a high spatial resolution. Nevertheless the size of the working distance depends in a first approximation linearly on the pulse width. Hence by changing the pulse width the working distance can be adjusted.
Fig. 1. Sketch of the electrochemical cell in the ECF process.
310 Since the pulse polarizes only the workpiece anodically the tool itself is not dissolved by the process. If the feed rate is higher than the dissolution rate the tool comes into contact with the workpiece forming a short circuit, the so called contact. This is detected by the ECF machine and the motion of the tool is stopped. Because this event appears mainly at inhomogeneities of the steel, a strategy of tool movements is started to remove the contact and continue the ECF process. IZFM together with HSG-IMAT set up an ECF machine. Free form surfaces can be obtained with tools with diameters down to 20 µm and below. Good results have been achieved in stainless steel using an electrolyte containing hydrofluoric acid (s. fig. 2). The aim of the experiments shown here is to ECF-process tool steels that are typically used for producing mould inserts. It is also shown that acetic acid instead of HF is suitable for the ECF process. For these steel-electrolyte systems optimised sets of ECF parameters had to be found. The set of parameters that leads to a small number of contacts and at the same time to a small working distance is optimal for a fast and accurate ECF process. 2. Experimental All ECF experiments shown here were carried out on the named ECF machine. It consists of an xyz-stage (Walter Uhl, Aßlar, Germany) with a travel range of 100 mm in each direction. A PC software interprets CAD/CAM-data files and drives the stage accordingly. Due to the fact that the electrochemical parameters have to be adjustable during the ECF-process special CAD/CAM-codes are used to set the values via this software. The feed rate for all experiments shown here was set to 0.3 µm/s. The electrochemical cell is made from PTFE and it is provided with a silver/silver-chloride (Ag/AgCl) reference electrode and a platinum counter electrode. The workpiece is mounted to the bottom of the cell and connected from the outside. A feedback loop controls the level of the electrolyte using an attached photo sensor. The potentiostat and the pulse generator were especially developed for the ECF process (IZFM and ECMTEC, Holzgerlingen, Germany). The potentiostat is equipped with an input that stops the potential control keeping the potential constant while the tool is in contact with the workpiece. The pulse generator is designed to fit into the processing head of the ECF machine. This is necessary to keep the distance between pulse generation and tool as short as possible. The pulse on-time can be adjusted between 10 ns and 1600 ns and different periods can be set. The pulse generator also detects the contacts between tool and workpiece by measuring the current through the tool. All experiments were processed in an aqueous solution of 1 M CH3COOH. For the workpieces samples of tool steels 1.1730 1.2312 and 1.2767 with a diameter of 19 mm and a height of 5 mm were used. Each workpiece was prior to the experiment finished by wet-grinding to 2500 grit silicon carbide paper and rinsed with deionised water.
Fig. 2. Grooves and holes in stainless steel 1.4301 produced with the ECF technique in HF-based electrolyte.
The tools are made from tungsten wire. They are etched by ECM grinding to achieve a diameter of typically 40 µm [4]. Afterwards the tip shape is formed using the ECF process. This is done by applying an inverted pulse. This forming is done already in situ in the ECF machine prior to the processing of the workpiece. To find out whether a steel-electrolyte system is suited for the ECF technique a first test array was processed. The array is set up of 24 holes 30 µm deep milled by ECF technique with different sets of parameters. The parameters varied are the tool potential UTip, the pulse amplitude Um, the pulse width B and the current through the counter electrode ICE that keeps the workpiece at its cathodic potential. During the milling the number of contacts KT was counted for each hole. 3. Results Figure 3 shows an optical micrograph of an array processed into the tool steel 1.2767 using the CH3COOH-based electrolyte. In the first two rows a current ICE of 2 mA was used whereas in the second two rows ICE was 0.1 mA. In row one and three the pulse width B was set to 500 ns in row two and four B
Fig. 3. Array of 24 holes ECF processed into steel 1.2767 in CH3COOH-based electrolyte.
311
Fig. 4. Number of contacts as a function of the pulse amplitude for 1.2767.
was 800 ns. In the left three columns UTip was 100 mV, in the three right columns UTip was 300 mV. The pulse amplitude for both blocks of the three columns was set to 4 V, 6 V and 10 V, respectively. Even from this optical image (fig. 3) it can be seen that the ECF process is strongly dependent on the used set of parameters. This can also be seen in fig. 4. Here the number of contacts is plotted as a function of the pulse amplitude for all sets of parameters. It is remarkable that the number of contacts for a pulse amplitude of 6 V and above is negligible
Fig. 6. Working distance as a function of the pulse amplitude for a) 1.1730, b) 1.2312 and c) 1.2767.
Fig. 5. Number of contacts as a function of the pulse amplitude for a) 1.1730 and b) 1.2312.
whereas for an amplitude of 4 V it increases dramatically. This is true for all systems tested, even though not as obvious in the case of 1.2312 (fig. 5). No ECF processing was possible with a pulse amplitude of 4 V in the case of 1.1730. For that reason a set of 6 V, 8 V and 10 V was chosen. Only for 1.2312 for an amplitude of 10 V and a current ICE of 2 mA, a pulse width B of 500 ns and a tool potential UTip of 300 mV the number of contacts increases again. This may be due to a large unsolvable inclusion that hinders the ECF process. As can be seen in fig. 6 the working distance in all of the steel-electrolyte systems increases with respect to the pulse amplitude. While for the 1.1730 and 1.2312 these distances are in the same range, it is remarkable that for the 1.2767 the distance is up to a factor of two larger and much more dependent on the set of parameters. This may be explained with
312 the smaller amount of manganese in the 1.2767 (0.15 % compared to 0.8 % for the 1.1730 and 1.4 % for the 1.2312) leading to less MnS inclusions. This comparably higher working distance and the fact that the number of contacts at an amplitude of 4 V is still only two thirds of the number of contacts for the 1.2312 indicates that a smaller pulse amplitude would be adequate for the 1.2767. If one compares in fig. 6 the working distances for a single set of parameters it can be seen that the working distance for the 800 ns pulse width leads to an higher working distance as for the 500 ns pulse width at the same pulse amplitude. This is as expected and confirms the theory. In tab. 1 the sets of parameter that lead to the best results are listed. With these set the following experiments have been carries out. If fig. 7 nine holes are shown which are ECF processed with the optimal set of parameters for the 1.2312 (fig. 7 a) and the 1.1730 (fig. 7 b) respectively. It is obvious that in the case of the 1.2312 all holes have the same shape, i. e., the ECF process worked out reproducibly. This is not the case for the 1.1730 (fig 7 b). Here the shapes vary. Especially the hole at the left bottom is much smaller. This is due to the influence of inclusions, precipitations and grain boundaries in this low-alloyed steel. Although further experiments are needed to fully explain the role of inclusions in the case of 1.1730, good results are obtained in the case of 1.2312 and 1.2767.
Tab. 1. Sets of parameter leading to best results.
UTip Um ICE B
1.1730 300 mV 8V 2 mA 800 ns
1.2312 100 mV 6V 2 mA 800 ns
1.2767 300 mV 6V 2 mA 800 ns
a)
4. Conclusion The ECF process was applied to different steelelectrolyte systems. It can be observed, that quite a number of systems can be treated with the ECF process using acetic acid. Nevertheless does the set of the applied parameters have a large influence of the achieved results. For all shown steels in this electrolyte a pulse amplitude above 6 V leads to a negligible (for the 1.1730 at least reasonable) number of contacts whereas a pulse of 4 V and below makes ECF processing difficult if not impossible. It is shown that for all investigated systems the working distance increases with both the pulse width and the pulse amplitude. Even if the ECF processing of a steel-electrolyte system with an appropriate set of parameters is possible the resulting holes may have a not circular shape. This is probably due to the grain size of the steel and the composition of the grain boundary. Further experiments will give an answer to this phenomenon. Acknowledgements This research project (AiF FV-Nr.: 180 ZN) was aided by budget funds of the Bundesministeriums für Wirtschaft und Technologie through the Arbeitsgemeinschaft industrieller Forschungsvereinigungen „Otto von Guericke“ e. V. (AiF). We also like to thank R. Schuster (TU Darmstadt) and T. Gmelin (ECMTEC) for the helpful discussions and comments.
b) Fig. 7. Holes ECF processed in a) 1.2312 and b) 1.1730.
References [1] Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical Micromachining, Science 289 (2000) 98-101. [2] Kock M, Kirchner V, Schuster R. Electrochemical micromachining with ultrashort voltage pulses—a versatile method with lithographical precision, Electrochimica Acta 48 (2003) 3213– 3219 [3] Cagnon L, Kirchner V, Kock M, Schuster R, Ertl G, Gmelin WT, Kück H. Electrochemical Micromachining of Stainless Steel by Ultrashort Voltage Pulses. Z. Phys. Chem. 217 (2003) 299–313. [4] Hacker B, Hillebrand A, Hartmann T, Guckenberer R. Preparation and characterization of Tips for scanning tunneling microscopy of biological specimens, Ultramicroscopy 42-44 (1992) 1515–1518
309
Adapting ECF to steels used for micro mould inserts L. Staemmlera, K. Hofmannb, M.-H. Kimb, D. Warkentina, H. Kücka,b a
b
Hahn-Schickard-Institute for Micro Assembly Technology (HSG-IMAT), Stuttgart, Germany University of Stuttgart, Institute of Micro- and Precision Engeneering (IZFM), Stuttgart, Germany
Abstract Electrochemical machining with ultra short voltage pulses (ECF) is an innovative technique to machine electrochemically active materials particularly very hard materials at micrometer feature size. Since the ECF technique is an electrochemical process neither mechanical forces nor thermal load are applied to workpiece or tool. For that reason ECF it is an ideal technique for the production of microstructures. Especially the use of steel as the workpiece material makes the ECF technique a promising technique for the production of micro mould inserts, since steel is resistant against the wear that occurs due to the injection process. Therefore the abilities and the limits of the ECF process for different types of tool steels are shown and the process parameters have been optimised. Acetic acid has been proven to be a suitable electrolyte for the ECF-process of tool steels. In this electrolyte tool steels like 1.2767 or 1.2312 can be processed with the ECF technique at a high quality. But in lowalloyed steels like 1.1730 only a poor quality can be obtained. Keywords: ECF, micro mould, steel
1. Introduction The production of microstructures demand highly precise techniques due to the small size of the structures. Good results have been obtained by the LIGA technique. But also with high speed cutting (HSC) milling machines which can have an accuracy in the micrometer range microstructures can be achieved by using milling cutters with diameters of 80 µm and below. Also EDM is capable of producing structures in the 10 µm-range. Nevertheless the production of microstructures is extremely timeconsuming. It therefore makes sense to reproduce the microstructure, for example by injection moulding. With a single microstructure several thousand parts can be made. This demands a mould that withstands the abrasive properties of the injected polymer. The lifetime of nickel moulds which are made with the LIGA technique is often too short for an effective use of theses moulds. Hard materials like steel, which are therefore normally used for making these moulds, cannot be processed by the LIGA technique. Because EDM brings a huge thermal load to the workpiece and HSC is not capable of obtaining patterns with dimensions much below 100 µm, electrochemical milling with ultrashort voltage pulses (ECF, where F is the German acronym for milling) is a powerful alternative for the production of the microstructures in the mould [1]. Because it is an electrochemical process neither thermal load nor mechanical forces act on the tool, hence no tool wear arises. In the ECF process short voltage pulses in the range of 10 ns to 1000 ns are applied between tool and workpiece to dissolve the workpiece locally [2]. Therefor the workpiece as well as the tool are submerged in an appropriate electrolyte. In the idle state both the workpiece and the tool are hold at a constant cathodic potential to prevent corrosion. For the dissolution process the pulses are applied additionally to the existing potentials. An electrode that is submerged in an electrolyte forms a double
layer capacitance at its surface. During the pulse these capacitances are charged over the resistance of the electrolyte. The value of this resistance depends on the length of the path of the electric current in the electrolyte. Thus a large separation between tool electrode and workpiece leads to a slow charging of the capacitance whereas a small separation leads to a fast charging (fig.1). Since charging takes place only during the time of the pulse the double layer capacitance is charged to a potential high enough for an anodic dissolution of the workpiece only in a close proximity around the tool, the so called working distance. Surface areas of the workpiece that are further away from the tool are not affected by the ECF process. This leads to a confined milling with a high spatial resolution. Nevertheless the size of the working distance depends in a first approximation linearly on the pulse width. Hence by changing the pulse width the working distance can be adjusted.
Fig. 1. Sketch of the electrochemical cell in the ECF process.
310 Since the pulse polarizes only the workpiece anodically the tool itself is not dissolved by the process. If the feed rate is higher than the dissolution rate the tool comes into contact with the workpiece forming a short circuit, the so called contact. This is detected by the ECF machine and the motion of the tool is stopped. Because this event appears mainly at inhomogeneities of the steel, a strategy of tool movements is started to remove the contact and continue the ECF process. IZFM together with HSG-IMAT set up an ECF machine. Free form surfaces can be obtained with tools with diameters down to 20 µm and below. Good results have been achieved in stainless steel using an electrolyte containing hydrofluoric acid (s. fig. 2). The aim of the experiments shown here is to ECF-process tool steels that are typically used for producing mould inserts. It is also shown that acetic acid instead of HF is suitable for the ECF process. For these steel-electrolyte systems optimised sets of ECF parameters had to be found. The set of parameters that leads to a small number of contacts and at the same time to a small working distance is optimal for a fast and accurate ECF process. 2. Experimental All ECF experiments shown here were carried out on the named ECF machine. It consists of an xyz-stage (Walter Uhl, Aßlar, Germany) with a travel range of 100 mm in each direction. A PC software interprets CAD/CAM-data files and drives the stage accordingly. Due to the fact that the electrochemical parameters have to be adjustable during the ECF-process special CAD/CAM-codes are used to set the values via this software. The feed rate for all experiments shown here was set to 0.3 µm/s. The electrochemical cell is made from PTFE and it is provided with a silver/silver-chloride (Ag/AgCl) reference electrode and a platinum counter electrode. The workpiece is mounted to the bottom of the cell and connected from the outside. A feedback loop controls the level of the electrolyte using an attached photo sensor. The potentiostat and the pulse generator were especially developed for the ECF process (IZFM and ECMTEC, Holzgerlingen, Germany). The potentiostat is equipped with an input that stops the potential control keeping the potential constant while the tool is in contact with the workpiece. The pulse generator is designed to fit into the processing head of the ECF machine. This is necessary to keep the distance between pulse generation and tool as short as possible. The pulse on-time can be adjusted between 10 ns and 1600 ns and different periods can be set. The pulse generator also detects the contacts between tool and workpiece by measuring the current through the tool. All experiments were processed in an aqueous solution of 1 M CH3COOH. For the workpieces samples of tool steels 1.1730 1.2312 and 1.2767 with a diameter of 19 mm and a height of 5 mm were used. Each workpiece was prior to the experiment finished by wet-grinding to 2500 grit silicon carbide paper and rinsed with deionised water.
Fig. 2. Grooves and holes in stainless steel 1.4301 produced with the ECF technique in HF-based electrolyte.
The tools are made from tungsten wire. They are etched by ECM grinding to achieve a diameter of typically 40 µm [4]. Afterwards the tip shape is formed using the ECF process. This is done by applying an inverted pulse. This forming is done already in situ in the ECF machine prior to the processing of the workpiece. To find out whether a steel-electrolyte system is suited for the ECF technique a first test array was processed. The array is set up of 24 holes 30 µm deep milled by ECF technique with different sets of parameters. The parameters varied are the tool potential UTip, the pulse amplitude Um, the pulse width B and the current through the counter electrode ICE that keeps the workpiece at its cathodic potential. During the milling the number of contacts KT was counted for each hole. 3. Results Figure 3 shows an optical micrograph of an array processed into the tool steel 1.2767 using the CH3COOH-based electrolyte. In the first two rows a current ICE of 2 mA was used whereas in the second two rows ICE was 0.1 mA. In row one and three the pulse width B was set to 500 ns in row two and four B
Fig. 3. Array of 24 holes ECF processed into steel 1.2767 in CH3COOH-based electrolyte.
311
Fig. 4. Number of contacts as a function of the pulse amplitude for 1.2767.
was 800 ns. In the left three columns UTip was 100 mV, in the three right columns UTip was 300 mV. The pulse amplitude for both blocks of the three columns was set to 4 V, 6 V and 10 V, respectively. Even from this optical image (fig. 3) it can be seen that the ECF process is strongly dependent on the used set of parameters. This can also be seen in fig. 4. Here the number of contacts is plotted as a function of the pulse amplitude for all sets of parameters. It is remarkable that the number of contacts for a pulse amplitude of 6 V and above is negligible
Fig. 6. Working distance as a function of the pulse amplitude for a) 1.1730, b) 1.2312 and c) 1.2767.
Fig. 5. Number of contacts as a function of the pulse amplitude for a) 1.1730 and b) 1.2312.
whereas for an amplitude of 4 V it increases dramatically. This is true for all systems tested, even though not as obvious in the case of 1.2312 (fig. 5). No ECF processing was possible with a pulse amplitude of 4 V in the case of 1.1730. For that reason a set of 6 V, 8 V and 10 V was chosen. Only for 1.2312 for an amplitude of 10 V and a current ICE of 2 mA, a pulse width B of 500 ns and a tool potential UTip of 300 mV the number of contacts increases again. This may be due to a large unsolvable inclusion that hinders the ECF process. As can be seen in fig. 6 the working distance in all of the steel-electrolyte systems increases with respect to the pulse amplitude. While for the 1.1730 and 1.2312 these distances are in the same range, it is remarkable that for the 1.2767 the distance is up to a factor of two larger and much more dependent on the set of parameters. This may be explained with
312 the smaller amount of manganese in the 1.2767 (0.15 % compared to 0.8 % for the 1.1730 and 1.4 % for the 1.2312) leading to less MnS inclusions. This comparably higher working distance and the fact that the number of contacts at an amplitude of 4 V is still only two thirds of the number of contacts for the 1.2312 indicates that a smaller pulse amplitude would be adequate for the 1.2767. If one compares in fig. 6 the working distances for a single set of parameters it can be seen that the working distance for the 800 ns pulse width leads to an higher working distance as for the 500 ns pulse width at the same pulse amplitude. This is as expected and confirms the theory. In tab. 1 the sets of parameter that lead to the best results are listed. With these set the following experiments have been carries out. If fig. 7 nine holes are shown which are ECF processed with the optimal set of parameters for the 1.2312 (fig. 7 a) and the 1.1730 (fig. 7 b) respectively. It is obvious that in the case of the 1.2312 all holes have the same shape, i. e., the ECF process worked out reproducibly. This is not the case for the 1.1730 (fig 7 b). Here the shapes vary. Especially the hole at the left bottom is much smaller. This is due to the influence of inclusions, precipitations and grain boundaries in this low-alloyed steel. Although further experiments are needed to fully explain the role of inclusions in the case of 1.1730, good results are obtained in the case of 1.2312 and 1.2767.
Tab. 1. Sets of parameter leading to best results.
UTip Um ICE B
1.1730 300 mV 8V 2 mA 800 ns
1.2312 100 mV 6V 2 mA 800 ns
1.2767 300 mV 6V 2 mA 800 ns
a)
4. Conclusion The ECF process was applied to different steelelectrolyte systems. It can be observed, that quite a number of systems can be treated with the ECF process using acetic acid. Nevertheless does the set of the applied parameters have a large influence of the achieved results. For all shown steels in this electrolyte a pulse amplitude above 6 V leads to a negligible (for the 1.1730 at least reasonable) number of contacts whereas a pulse of 4 V and below makes ECF processing difficult if not impossible. It is shown that for all investigated systems the working distance increases with both the pulse width and the pulse amplitude. Even if the ECF processing of a steel-electrolyte system with an appropriate set of parameters is possible the resulting holes may have a not circular shape. This is probably due to the grain size of the steel and the composition of the grain boundary. Further experiments will give an answer to this phenomenon. Acknowledgements This research project (AiF FV-Nr.: 180 ZN) was aided by budget funds of the Bundesministeriums für Wirtschaft und Technologie through the Arbeitsgemeinschaft industrieller Forschungsvereinigungen „Otto von Guericke“ e. V. (AiF). We also like to thank R. Schuster (TU Darmstadt) and T. Gmelin (ECMTEC) for the helpful discussions and comments.
b) Fig. 7. Holes ECF processed in a) 1.2312 and b) 1.1730.
References [1] Schuster R, Kirchner V, Allongue P, Ertl G. Electrochemical Micromachining, Science 289 (2000) 98-101. [2] Kock M, Kirchner V, Schuster R. Electrochemical micromachining with ultrashort voltage pulses—a versatile method with lithographical precision, Electrochimica Acta 48 (2003) 3213– 3219 [3] Cagnon L, Kirchner V, Kock M, Schuster R, Ertl G, Gmelin WT, Kück H. Electrochemical Micromachining of Stainless Steel by Ultrashort Voltage Pulses. Z. Phys. Chem. 217 (2003) 299–313. [4] Hacker B, Hillebrand A, Hartmann T, Guckenberer R. Preparation and characterization of Tips for scanning tunneling microscopy of biological specimens, Ultramicroscopy 42-44 (1992) 1515–1518