Development of DEEL (Deep Electrochemical Etching with Laser assistance) technology for low heat affected zone and high aspect ratio processing

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1 IWMF2014, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 2014, HONOLULU, U.S.A. / 1 Development of DEEL (Deep Electrochemical Etching with Laser assistance) technology for low heat affected zone and high aspect ratio processing Tsuneo Kurita 1, Taiki Yamane 1,2, Kiwamu Ashida 1 and Shinya sasaki 2 1 Advanced Manufacturing Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Asia, JAPAN 2 Name of the department, The Tokyo University of Science (TUS), Asia, JAPAN # Tsuneo Kurita / t.kurita@aist.go.jp, TEL: , FAX: KEYWORDS: Laser, ECM, Complex machining, Stent, Stainless steel Medical instruments such as catheters and stents require microscopic diameters to meet the needs of brain surgery. Despite the requirement for the medical applications, it has been difficult to machine microscopic metal tubes into complex shapes. One of the difficulties is removal of heat-affected zone (HAZ), such as resolidified material on the surface (especially inside surface) of tube after laser processing. If the processing technology, which can minimize the generation of HAZ, can be developed, the function loss caused by HAZ can be reduced. In this paper, The DEEL (Deep Electrochemical Etching with Laser assistance) is proposed as new precise machining technology for metal. DEEL has the potential to make small and precise parts without HAZ. Firstly, laser processing and Electrochemical machining (ECM) ware carried out respectively to find a suitable machining condition for DEEL. Secondly, DEEL processing was carried out with former machining conditions. Machining characteristics of DEEL were considered by measuring machining depth and observing the machined surface of DEEL. Finally, The low HAZ and high aspect ratio complex machining has been experimentally demonstrated with the DEEL. 1. Background A stent is a small mesh tube that is used to treat narrow or weak arteries. A stent expands the blood tube and improve blood flow by supporting the inner wall of the artery. A stent is basically made by laser rough machining and electrochemical machining (ECM) finishing. Medical instruments such as catheters and stents require microscopic diameters to meet the needs of brain surgery. Despite the requirement for the medical applications, it has been difficult to machine microscopic metal tubes into complex shapes. One of the difficulties is removal of resolidified material on the surface (especially inside surface) of tube after laser processing. The resolidified material inside of tube basically removed by mechanical cutting and grinding. It is hard to apply tool and grain to inside of the tube in microscopic diameter tube machining. The resolidified material on the surface of tube causes function loss such as adhesion of platelet and adsorption of plasma protein. A residual swarf and grain inside tube is also problem. If the processing technology which can minimize the generation of heat affected zone (HAZ), can be developed, the function loss caused by HAZ such as resolidified material, can be reduced. In this paper, The DEEL (Deep Electrochemical Etching with Laser assistance) is proposed as new precise machining technology for metal. DEEL has the potential to make small and precise parts without HAZ. The low HAZ and high aspect 272

2 IWMF2014, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 2014, HONOLULU, U.S.A. / 2 ratio complex machining has been experimentally demonstrated with the DEEL. 2. DEEL (Deep Electrochemical Etching with Laser assistance) complex machining ECM. Because DEEL has the process of coating of electrical insulating film and laser selective removal process of the film, the effect of isotropic etching in ECM can be decreased. High aspect ratio, low HAZ and low resolidified material processing would be carried out with DEEL. In order to carry out precise and complex shape machining on difficult to machine material and difficult to machine shape, authors proposing CM (Complex Machining). The CM has been defined as follows: High efficiency and precision machining technology, which is combined more than one machining method sequentially/simultaneously on the same machine tool, for hard-to-cut material and hard-to-cut shape [1]-[6]. The CM also defined that combined machining methods are carried out sequentially, is sequentially complex machining (SQCM), and complex machining that are carried out simultaneously or changed under 1 s, is simultaneously complex machining (SMCM). DEEL is categorized as ECM and Laser SQCM. Because the process contains laser processing and ECM and each processes are alternately repeated. Fig. 1 shows the process sequences of DEEL. DEEL has following processes. (1) Coating of electrical insulating film. (2) Selection of removal area by removing electrical insulating film with laser. (3) Material removal with ECM. (4) Coating the electrical insulating film on the surface of removal area. (5) Removal bottom surface of coated area with ECM. By repeating above-mentioned processes, we try to obtain following functions. (a) Low HAZ processing. Because material is mainly removed by ECM. (b) High aspect ratio processing. Because bottom area of processed shape mainly removed. While an ECM has a characteristic of low machining affected area because of material is electrochemically removed, it is difficult that the control of removal area and high aspect ratio machining, because material is isotropically etched in Fig. 1 Process sequences of DEEL Authors had been tried to apply a polyimide film to the Fig. 1 (1) process. The problem was found that the heat affected polyimide film made negative influence to ECM. Furthermore, there was a barrier to find simple coating method for Fig. 1 (4) process. In this study, authors propose to apply ECM process not only removal processes such as Fig. 1 (3) and (5), but also coating processes such as (1) and (4). In case that ECM is carried out to aluminum, chromium, titanium and each alloy, passivation film is generated on the surface of the material. The passivated surface layer is usually an oxide. The oxide film has characteristics of electrical insulation and resistance to environmental factors such as air and water. If the passivation film is applied, DEEL can be carried out more simply. Because the material removal and coating of insulating film processes can carry out simultaneously with ECM. 3. Laser and ECM complex machine tool Laser processing, ECM and DEEL were carried out with Laser and ECM complex machine tool. Laser and ECM complex machine tool had been manufactured on the concept of CM. In the CM system [7], a microscopic tube, which is difficult to machine mechanically, is machined into a complex shape by laser and ECM processes were combined within one 273

3 IWMF2014, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 2014, HONOLULU, U.S.A. / 3 system, and high-efficiency high-precision machining was carried out without detaching the work piece once it had been set in a machine. Fig. 2 shows the developed laser and ECM complex machine tool. By using the system, high-value added tubular devices of diameters of less than 300 µm can be fabricated efficiently, a level of work that other machining technologies cannot achieve. Because ECM and laser processing can carry out on the same machine tool, Laser and ECM complex machine tool was used in the DEEL CM. L/min. The laser power was set 36 mw, 73 mw, 120 mw and 163 mw. The color of processed surface became dark as the laser power increases. Because the black and brown surface assumed as oxide surface, it is thought that oxidation effect increase as the laser power increases. The oxidation effect of the processed surface in air was larger than that in argon atmosphere, because of the comparison of each surface color. Fig. 2 (b) Laser and ECM complex machine tool Fig. 3 Photo of laser processed surface 4. ECM result and laser processing result 4.1 Laser processing To study the suitable laser processing condition of DEEL, the processing experiment was carried out. The ultra violet laser wavelength of 349 nm, witch equipped in Laser and ECM complex machine tool, was used. A stainless steel (SUS304) tube, diameter of 5 mm and thickness of 500 m, was used as work piece material. The laser was irradiated to the work piece repetition rate of 3 khz. The laser power was changed from 36 mw to 163 mw. The laser was scanned spirally, which pitch was 10 μm and width was 100 μm, by rotating a work piece. The rotation speed was 114 rpm. The spiral was reciprocatory and the number of repeat was 5. Fig. 3 shows the photo of laser processed surface. The processing atmospheres were set air and argon volume of ECM To study the suitable ECM removal and electrochemical coating condition, ECM experiment was carried out. A stainless steel tube, same as the case of laser processing, was dipped in the 10w% NaNO 3 electrolyte. The work piece was faced an electrode, 5 mm of height and 10 mm of width. Each distance was 300 μm. The rotation speed of work piece was set to 30 rpm. ECM power was changed from 0.5 V to 5 V. After the 10 min ECM, diameter of work piece was measured. The ECM current was logged. The diameter difference between before and after machining was defined as machined diameter. The machining current defined as average current from start to the end of ECM. Fig. 4 shows the relationship between machined diameter and machining current as ECM power changes. The 274

4 IWMF2014, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 2014, HONOLULU, U.S.A. / 4 machining current increases rapidly from ECM power of 3 V. From the power, bubble was observed on the surface of work piece. On the other hand, the machined diameter is almost 0 until 4 V of ECM power. The machined diameter rapidly increases from 4 V of ECM power. The ECM power in DEEL was set under 4 V, because machining diameter did not increase. Fig. 5 Photo of DEEL processed surface Fig. 4 Relationship between machined diameter and ECM current 5. Experimental result and discussion about DEEL Fig. 5 shows photos of DEEL processed surface, that the laser power and atmosphere were changed. The laser processing and ECM ware repeated 10 times alternately. The DEEL processing is shown in Table 1. It is recognized from the comparison of Fig. 3 and Fig. 5, that the black and brown area on the processed surface with DEEL is smaller than that with the laser processing. Especially, there is little area in Fig. 5(b) that is supplied argon volume of 10 L/min. These show the black and brown (oxide) area on the processed surface can remove with the ECM. On the other hand, work piece cannot be machined at 3 V ECM power, which is shown in the chapter of 4.2. Therefore, it is assumed that process affected area such as oxide layer witch is generated by laser irradiation, can machine easier with ECM than the passivation layer made by ECM. Table 1 Processing condition in DEEL CM Processing condition Value Laser power mw Laser pulse repetition rate 3 khz Work piece rotation speed in laser 114 rpm Scan path Spiral (pitch 10 μm) ECM power 3 V ECM time 1 min Electrolyte 10 w% NaNO 3 Work piece rotation speed in ECM 30 rpm Atmosphere in laser processing Air, Argon Stainless steel passivation layer made by ECM contains more chromium than base material, because iron dissolves easier with ECM than chromium. Chromium is concentrated in stainless steel processing with ECM. On the other hand, chromium in oxide layer with laser processing does not concentrate, because iron and chromium is removed in the same proportion in laser processing. It is known that the concentration of chromium affects the strength of passivation layer. It is assumed that the difference between the concentrations of chromium causes the difference of machining capability of ECM. To consider about the processing effect in DEEL against the case that laser processing and ECM are carried out each, the 275

5 IWMF2014, 9 th INTERNATIONAL WORKSHOP ON MICROFACTORIES OCTOBER 5-8, 2014, HONOLULU, U.S.A. / 5 depth of DEEL processed grooves such as shown in Fig. 5 were measured and compared to the depth of laser processed grooves such as shown in Fig. 3. The depths of grooves were measured with confocal microscope. Fig. 6 shows the comparison of the machined depth in the DEEL and the machining depth in the laser processing only. Machining depth increase as the increase of laser power in all machining conditions of the experiment. The depth ratio of ECM to DEEL is also shown in Fig. 6. The ratio was calculated with following equation. (Machined depth with DEEL Machined depth with laser processing) / (Machined depth with DEEL) The depth ratios of ECM to DEEL are decreased as the increase of laser power in both atmospheres. This means the machined depth with laser increases as the increase of laser power, while machined depth with ECM has almost constant value, because same machining condition of ECM was used in the experiment. The difference between the depth ratios of ECM to DEEL in air and argon atmosphere has the trend to increase as the increase of laser power. It is considered that the oxide layer generated with laser processing blocks ECM. On the other hand, the depth ratio of ECM to DEEL in the condition of 73 mw of laser power and argon atmosphere is 100%. This means that there was no machined depth in laser processing and only laser-scanned area was machined with ECM. From the result, it is cleared that the DEEL processes, which were thought in Fig. 1, can be carried out. 6. Demonstration of DEEL and future work The optimal machining condition of DEEL was found by performing the DEEL CM experiments. Fig. 7 shows the photo of DEEL machining result in optimal condition. The precise glove width of 30 μm and depth of 300 μm. The aspect ratio of the groove was about 10. Furthermore, there was little resolidified material on the surface of material. It is cleared that the DEEL has the high machining function, because the shape shown on Fig. 7 is very difficult to machine with conventional machining processes such as mechanical marching, laser processing and etchings. On the other hand, while DEEL has the high machining function, the machining time is very long. The machining time to make the shape shown Fig. 7 is about 15 hour. One of the reason is the process was carried out the laser and ECM complex machine tool. In the machine tool, there was much loss in process flow of DEEL, because the machine tool did not be made for DEEL. Fig. 7 Photo of DEEL marching result 7. Conclusions DEEL (Deep Electrochemical Etching with Laser assistance) complex machining technology was newly proposed and demonstrated. The technology has the characteristics of low HAZ (Heat Affected Zone) and high aspect ratio machining. REFERENCES Fig. 6 Comparison of the machined depth in the DEEL and the machining depth of laser processing 1. T. Kurita, M. Hattori, Development of New-Concept Desk Top Size Machine Tool, INTERNATIONAL 276

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