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ScienceDirect Procedia CIRP 42 (2016) 766 – 771

18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)

Electrochemical, electrodischarge and electrochemical-discharge hole drilling and surface structuring using batch electrodes Grzegorz Skrabalaka*, Andrzej Stworaa a

Institute of Advanced Manufacturing Technology, ul. Wrocławska 37a, 30-011 Kraków, Poland

* Corresponding author. Tel.: +48-12-63-17-237; fax: + +48 12 63-39-490. E-mail address: [email protected]

Abstract According to the rapid development of new materials, and requirements of the industrial sector towards effective and efficient machining of holes with diameters smaller than 1mm, recent researches in the field of machining processes are aimed at investigations and optimization machining processes using multiple tools, multi-electrodes. Effective methods of machining difficult to cut materials are non-contact machining methods of electrochemical (ECM), electrodischarge (EDM) and hybrid electrochemical-discharge machining (ECDM). Both processes incorporating electrochemical dissolution, characterise with high material removal rates. They also do not induce high electrode wear ratios. In this paper, the results of experiments in the field of ECM, EDM and ECDM machining processes using batch electrodes will be presented. Batch electrodes used in the presented study consist of 9 square working tips of 2x2 mm, enabling flushing of the electrolyte through the electrode tip. Electrodes are prepared using additive manufacturing method – Selective Laser Sintering (SLS). The paper focuses on performance characteristics of machining processes for various materials used in the aircraft and plastic industry. There is also discussed (basing on the experimental results) influence of the working media supply method to the machining area (submerged or flushing) on the: machining accuracy, material removal rate and influence of stray currents on the surface of machined material. In the paper there is also presented comparison of results of various methods of contactless machining with tool electrodes prepared of using subtractive and additive manufacturing method. The comparison concerns machining time, machining accuracy and time necessary for manufacturing of tool electrodes. © 2016 The Authors. Authors. Published Published by byElsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). XVIII). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) Keywords: hybrid machining, mass production, material removal rate, relative tool wear, process, electrochemical machining, Selective Laser Melting

1. Introduction Requirements concerning accuracy, durability and mechanical properties of machined components, by various branches of industry, lead to the rapid development of materials and methods of manufacturing. Important problem which is often met, when modern materials are being applied is the accuracy and efficiency of machining / shaping final products. Contactless machining methods (non-conventional machining i.e.: electrical discharge machining process – EDM and electrochemical machining – ECM), allow to machine most of commonly used, current conducting materials independent of their hardness and brittleness. Research works on the development of EDM process are aimed at increasing productivity of the process, as the machining efficiency is limited in comparison to other

machining methods. Important way of increasing process productivity is application of multi-electrode machining – machining of components with many electrodes at the same time. Kunieda [1] described development of the process energy source enabling parallel sparks during machining of large surfaces. On the other hand there are also conducted works concerning development of methods of production of electrodes for simultaneous machining of holes and patterns with array/batch electrodes [2,3]. Application of array/batch electrodes allows to machine large number of holes or structure surface with standard EDM machining system. System incorporating application of multiple electrodes are already in use for industrial scale, i.e. electrical discharge systems developed for rolls surface texturing for rolling mills. Roll texturing machines allow to achieve desired roughness of the roll surface causing also local hardening of the surface layer.

2212-8271 © 2016 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 organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.316

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Grzegorz Skrabalak and Andrzej Stwora / Procedia CIRP 42 (2016) 766 – 771

Similar developments concern application of multiple electrodes for machining when using ECM machining process. Due to process characteristics and developments made for the needs of aviation and aerospace industries, multielectrode ECM process is already used in practice for machining of cooling holes in turbine blades. Machining method already finding its industrial application concerns machining of cooling holes using insulated electrodes and jet-ECM [4-6]. Also array / batch electrodes are prepared for ECM sinking process [7]. Important problem involved in preparation of tool electrodes for array / batch machining concerns methods of manufacturing and their limitations. The most commonly used method concerns machining with wire-EDM. However the method is very accurate, enabling achieving high surface finishing, it brings important limits concerning shape of the tips, which is limited to polygons (the same shape of produced holes / pattern is also limited). Another important aspect limiting the potential usage of this method of electrodes machining concerns problem of working medium supply to the interelectrode gap through the electrode. Machining of array / batch tool electrodes with subtractive machining method (wire EDM) limits the possible depth of holes produced with them. Proper supply of medium to the interelectrode gap is important for all contactless machining methods. It is especially important for evacuation of erosion / dissolution products from the interelectrode gap, when there is no electrode rotation applied (i.e. machining with array electrodes). Due to necessity of efficient gap flushing, for the needs of machining of high aspect ratio holes (hole dimensions vs hole depth), it is necessary to prepare hole for working medium supply in each of the electrode tool tips. Machining of holes in each tip of the batch electrode makes the process ineffective. Effective solution for production of electrodes for batch / array hole machining concerns application of additive manufacturing methods.

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Selective Laser Melting (SLM) of the array electrodes with square tips (2mm wall) and cylindrical tips (2mm diameter) with holes enabling flushing of the working medium through the tip (channel diameter – 1mm) – Fig. 2.

Fig. 1. Array electrode machined with the wire EDM technology (square wall – of 2mm).

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c 2. Experimental setup Performed researches concerned application of various noncontact machining processes and comparison of their effectiveness for: - passing through holes machining; - surface structuring to desired depth. Experiments were conducted for two types of material / metal plates of 5mm thickness prepared of: - INCONEL 617 (Ni - 54.12 %, Cr - 21.61 %, Co - 11.54 %, Mo - 9.62%, Fe - 1.40%, Al - 1.11%); - stainless steel 4H13 / 1.4034 (C – 0.45%, Si – 0.6%, Mn – 0.5%, Cr 13.5%, S – 0.015%, P – 0.04%, Mo – 0.5%, V – 0.2%). 2.1. Tool electrode preparation For the needs of experiments, several sets of electrodes were prepared. The methods of electrode preparation concerned: - wire EDM cutting of array electrode with square tips of 2 mm wall (Fig. 1);

Fig. 2. Array electrode machined with the SLM technology: (a) CAD project, (b) cross section, (c) machined electrode.

The SLM manufactured tool electrodes presented in this study were prepared of stainless steel 316L / 1.4404 (C – 0.03%, Ni – 11.5%, Mn – 2.0%, Cr – 17.5%, N – 0.11%, Mo – 2.3%). Material for preparation of the electrode tool resulted from the availability of the powder for the needs of SLM process. Electrodes made with SLM process were build using Renishaw AM250 machine (equipped with 400W fiber laser). Powder of 316L steel used for the study was of average diameter of 50ȝm. Technology for building parts of 316L steel was optimised, enabling achievement of low pores structures, where measured density is about 97-98% of the density of 316L rods. In order to perform additional comparison, array electrode (Fig.1) was prepared of copper block. Additional experiments with EDM and ECDM sinking with copper electrode were performed, as the electrical resistivity of copper is 44 times smaller than in case of 316L steel. Distance between the tips of the electrodes was the same as tip wall width – equal to 2 mm.

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2.2. Sinking machines setup Experiments of EDM sinking of holes with array electrodes were performed using the standard EDM sinking machine designed and built by Institute of Advanced Manufacturing Technology – MESO 30 CNC. As the dielectric medium kerosene was used. Workpiece was submerged in the kerosene during experiments. For the needs of ECM and ECDM processes, electrochemical CNC sinking machine (of Institute design) was used. In case of ECM process, stabilized DC power supplier was used enabling work with constant current up to 60A (with supply voltage up to 60VDC) or constant voltage working mode. For the needs of ECDM machining, process energy source designed by Otto von Guericke University in Magdeburg. The process energy source is a current supply (fixed adjustable current) with controllable/adjustable limited voltage (voltage supply). It enables following parameter setup: - working / pulse current: 0 – 50 A; - limiting voltage: 20 – 99 V; - pulse ON/OFF time: 10 – 2000 µs. In case of ECDM process, the gap control system had to be replaced as well. External controller, based on PC equipped with LabVIEW and two expansion cards (high speed digitizer – for acquisition of gap feedback signal; motion card – responsible for path generation) was used. Feedback signals used for process gap control were: interelectrode voltage and current. Both ECM and ECDM processes were performed with usage of water solution of sodium nitride, with temperature stabilised in the range 20-22°C. In case of ECDM process, basing on previous experiments [8] electrolyte concentration was 1% (density of 1,005 g/cm3), whilst during ECM process 5% water solution was used (density of 1,042 g/cm3). Low concentration of the electrolyte was used in order to reduce stray current effect on the surface of machined sample.

Øin – hole dimension at the inlet (on top surface of the workpiece);, Øout – hole dimension at the outlet (on bottom surface of the workpiece); h – workpiece thickness Į – taper angle

Fig. 3. Method of hole taper angle estimation.

One of the factors affecting performance of presented machining methods concerns wear of the working electrode. During performed researches, wear of the electrodes was measured on the electrode length – measurement was done before and after machining process in the reference point. Averaged results of machining are presented in Figures 4-7. Each experiment was performed 3 times all the obtained results were in the range of ±3% of presented averaged 3.1. Electrodischarge machining (EDM) Experiments concerning EDM machining with both types of electrodes were performed for various free-run voltages. Results of machining are presented in Figures 4-5.

3. Results of machining Presented results refer to the early stage of research works in the field of machining of holes using array / batch electrodes. They are aimed at determining possibility of application of electrodes produced with Selective Laser Melting (SLM) and Selective Laser Sintering (SLS) processes for non-contact machining methods. Performed experiments focused on the accuracy and efficiency (material removal rate) of machining with various methods and tool electrodes. At this stage of investigations, influence of the process parameters on the quality of surface layer was not investigated. In order to assess the accuracy of machining process, geometry of produced holes was measured. Investigated factor was the taper angle of produced hole. Method of taper angle measurement and calculation is presented in Fig. 3. Dimensions of the holes at inlet and outlet were measured using optical microscope.

Fig. 4. Taper angle and machining time using: (a) 316L – SLM manufactured, (b) 316L – wire EDM, (c) Cu – wire EDM electrode.

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b Fig. 5. Comparison of electrode tool wear on the length during EDM sinking.

Analysing results obtained during EDM sinking of array holes it may be stated the in case of machining holes with relatively small aspect ratios electrodes machined with wire EDM method from the solid material perform better than those manufactured with additive technologies. Even better flushing of the working gap did not help in the performance of SLM manufactured electrodes. Worse performance (not satisfactory machining efficiency) of SLM manufactured electrodes results mainly from the fact of much faster electrode wear. Although density of SLM sintered parts is similar to the solid material, higher wear ratio results from the weaker bindings between material particles in case of the SLM machined elements. Another important aspect of using 316L tool electrodes concerns the: lower thermal and electric conductivity in comparison to Cu electrodes. Similar physical properties of tool electrode and machined workpiece resulted in higher relative electrode tool wear, than in case when Cu tool electrode was used. Combination of higher sample porosity and physical properties of 316L SLM manufacture tool electrode resulted in increased relative tool wear. 3.2. Electrochemical discharge machining (ECDM) Hybrid electrochemical discharge machining method is one of the promising methods of machining. However it has many advantages, it is still under development and is not popular in the industrial practice in the means of hole machining / drilling. During performed experiments, in case of machining with EDM wire cut electrodes, workpiece was flushed from the side. When using SLM machined electrodes, electrolyte was flushed through the holes in electrode tool tips. For the needs of experiments, during initial investigations, optimal parameters were determined. In case of prepared machining setup, the best performance of the ECDM sinking process was achieved for pulse ON / OFF times equal to 40/60ȝs and limiting current of 30A. Results of machining for various gap voltages are presented in Figures 6.

a

c Fig. 6. Taper angle and machining time of ECDM hole sinking using: (a) 316L – SLM manufactured, (b) 316L – wire EDM, (c) Cu – wire EDM array electrodes.

During performed ECDM process of machining holes with array electrodes, wear of the electrodes on the length was not significant. In case of electrodes made of solid materials there was no electrode wear measured. When SLM manufactured electrode was applied the wear on the length did not exceed 0.2mm. Analyzing achieved results of ECDM, there can be observed significant increase of taper angle with the increase of gap voltage. This effect results from the high share of ECM dissolution in the material removal process, although the electrolyte used is low concentrated water solution. In case of ECDM machining, electrodes prepared with the SLM method perform similar to those machined of solid material. Slightly better efficiency of machining and better machining accuracy can be observed when SLM manufactured electrode is used. It results from the fact of better electrolyte flushing into the working gap, than in case of tool electrode prepared of solid 316L. Better performance of the copper electrodes results from better current conductivity of copper. 3.3. Electrochemical machining (ECM) Electrochemical sinking was performed at the same stand as ECDM process. Experiments of ECM sinking of holes using the array electrodes were conducted with moving electrode. Electrode speed was set depending on the applied interelectrode voltage – power supplier was working in the constant voltage regime. Electrode speed varied from 0.2mm/min (for 5V) up to 0.6mm/min (for 20V). Machining process started with speed of 0.1mm/min and initial gap was set at 0.1mm. Results of machining for prepared electrodes and various interelectrode gap voltages are presented in Figure 7. Analyzing the results presented in figure 7, it may be observed that together with the increasing inter-electrode voltage, increases taper angle of machined holes and time of machining shortens. It results from the increasing current during ECM machining process.

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a

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to electrode made of solid material. Big advantage of batch / array electrodes production in the additive way may be observed when complex shape is required or there is requirement for effective supply of working media to the interelectrode gap. Also for the needs of other, electrolyte based machining methods, array electrodes manufactured by SLS/SLM method may be useful for machining of large number of holes or surface structuring. Bigger taper angle of the holes machined by electrodes produced by SLM process results from the irregularities and shape error of the manufactured tool electrodes. Electrodes used for the above described processes, after SLM manufacturing looked like presented in Figure 8.

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Fig. 8. Array electrode after ECDM machining process: (a) top view, (b) side view.

c Fig. 7. Taper angle and machining time of ECM hole sinking using: (a) 316L – SLM manufactured, (b) 316L – wire EDM, (c) Cu – wire EDM array electrodes.

Similar to ECDM process, the fastest machining was obtained when copper electrode was used. This phenomena is connected (as it was mentioned before) with better current conductivity of copper in comparison to 316L steel. Machining is more effective due to smaller energy losses resulting from electrode’s resistance. Similar to ECM sinking process, also in this case of ECDM machining, there may be observed positive effect resulting from flushing electrolyte through the electrode tips. The better flushing of the working gap results in shorter machining time, and the same minimizing the measured taper angle of machined holes. 4. Conclusions Results of experiments presented in section 3 refer to the machining of INCONEL 617 alloy. Similar results, differing less than 5-10% from above presented were achieved for machining of stainless steel. The same characteristics of the investigated processes were observed for similar sets of parameters. Results of the experiments presented above prove, that electrodes manufactured by Selective Laser Melting / Sintering (SLM/SLS) processes might be effectively used for the needs of contactless machining methods. Unfortunately in case of EDM machining the wear of the electrode significantly increases when SLM manufactured tool is used, in comparison

Surface roughness of the electrode tool tips, after preparing for ECM / EDM / ECDM sinking differs in the range Ra = 2030ȝm. Tool electrodes produced with SLM technology characterize with worse finishing of the side walls surfaces in comparison to wire cut electrodes, what finally results in higher taper angle. Additive method of production of electrodes for contactless machining methods is very effective in comparison to wire cutting method. Total production process of the described electrodes using SLS/SLM method consists of following steps: 1. SLS/SLM process preparation - ~40 mins.; 2. Build of the electrode (in described case) - ~3 hours; 3. Finishing of the electrodes (sand blasting, levelling – EDM wire cutting of tips) - ~2 hours. Altogether it took around 6 hours to produce 45 electrodes for the needs of experiments conducted with usage of SLM manufactured electrode tools. The preparation process is effective, as all the electrodes were build at the same time. Also sand / abrasive blasting and levelling of the electrode tool tips were performed during single machining operations for the set of all electrodes, attached to the base plate of SLM system. In comparison, complete machining of single copper or solid 316L electrode concerned: 1. Milling of copper / 316L rod (~30mins, per electrode); 2. EDM Wire cutting of electrode tool tips - ~ 1 hour (for electrode consisting of 9 tips). For the needs of presented experiments, machining of 18 electrodes made of solid materials took altogether 27 hours. In case of manufacturing electrodes with more tool tips the time needed for wire EDM cutting operation from solid will take significantly longer: - 3x3 array - ~1 hour; - 6x6 array - ~ 2 hours; - 12x12 array - ~ 4 hours. In case of additive manufacturing the time necessary to produce electrodes with bigger number of tips will not increase significantly (in case of single array 12x12 tips 2x2mm, increase will be less than 5 minutes comparing to 3x3 array).


Significant difference would be observed when the height of the electrode would be doubled. On the other hand preparation of many electrodes at the same time allows to reduce time necessary for single electrode tool. Important aspect of machining electrode tools for needs of contactless machining methods, concerns material which might be used for their production. Material shall characterize with good electrical conductivity. Unfortunately the number of materials available on the market is limited mainly to the materials used for medical, and aerospace industries. Future research in the field of EDM, ECM and ECDM machining using array electrode tools prepared with additive manufacturing methods will be focused on comparison of materials with various electrical and thermal conductivities than the described 316L steel (i.e.: AlSi or brass based powders). Acknowledgements Presented works were supported by the National Centre for Research and Development in Poland under LIDER-4 Programme, Grant No.: LIDER/038/683/L-4/12/NCBR/2013.

References [1] Han F., Kunieda M., TDevelopment of parallel spark electrical discharge machining. Precision Engineering 28 (2004) 65–72. [2] Chen S.T., Liao Y.S., A novel approach for batch production of micro holes by micro EDM, Proceeding of the 3rd International Conference on MultiMaterial Micro Manufacture, Borovets, Bulgaria, 2007, 123-126. [3] Weng F.-T., Her M.-G., Study of the Batch Production of Micro Parts Using the EDM Process, Int J of Advanced Manufacturing Technology, February 2002, Volume 19, Issue 4, 266-270. [4] Wang M. H., Zhu D., Fabrication of multiple electrodes and their application for micro-holes array in ECM, Tnt J of Advanced Manufacturing Technology, March 2009, Volume 41, Issue 1, 42-47. [5] Kozak J., Łubkowski K., Rozenek M.: Elektrochemiczne drą-Īenie otworów chłodzących w łopatkach turbinowych. Zbiór prac VI Konferencji „Technologia przepływowych maszyn wirnikowych”, 319-326, Rzeszów 1988 (in Polish). [6] Kozak J., Some Aspects of Electro Jet Drilling, Proceed. Int. Conf. on PEDAC'89, Alexandria, 363-369,1989. [7] Kozak J., Rajurkar K., P., Balkrishna R., Study of Electrochemi-cal Jet Machining Process, Transactions of the ASME-Journal of Manufacturing Sciences and Engineering, vol.118, No.4, 490-498,1996. [8] Skrabalak G., Kozak J., Zybura M., Optimization of Electrochemical Discharge Machining Process, (2010), Proceedings of ISEM XVI, Shanghai, China, pp.: 491 – 496

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