Electrochemical Micromachining of Bulk Metallic Glasses

Abstract

Owing to unique structural and functional properties bulk metallic glasses (BMG) are very promising materials. Their outstanding durability aligned with high mechanical strength and/or soft magnetic properties make them suited for micro­-electro­mechanical systems (MEMS), diminutive spring-mass systems, micronsized yokes in positioning systems, magnetic cores or medical gadgets. For shaping the required micro-sized devices thermo-mechanical forming is so far applied, but this is only applicable to selected glassy alloys with appropriate thermal characteristics. An upcoming alternative which allows operating at room temperature is electrochemical micromachining (ECMM). As new technique voltage pulsed electrochemical micromachining with a micro-tool electrode is introduced and a specially designed four-electrode setup for machining glassy plates was developed. The multi-component and reactive nature of metallic glasses demands a strict control of the local electrochemical dissolution process based on the choice of a suitable electrolyte and the identification of the effects of the ideal ultra-short pulse voltage parameters. First studies focused on Zr-based BMGs. Their high susceptibility for pitting corrosion limits the choice of electrolytes to halide-free ones. The standard electrolyte for industrial ECM is a concentrated (250 g/l) sodium nitrate solution. When conducting machining experiments with ultra-short voltage pulses in the transpassive regime, it could be demonstrated that the electrochemical dissolution of the glassy alloy can be focused on the microscale. But machined structures were irregularly shaped and covered with corrosion products and no clear dependence from the machining parameters was given. The underlying reaction mechanism and the role of the amorphous alloy structure and of the alloy constituents are discussed [1,2]. Recent studies were conducted on selected Fe-based glasses. Surface structuring of the soft magnetic glassy alloy Fe65.5Cr4Mo4Ga4P12C5B5.5 (at.-%) using an electrolyte based on a methanolic sulfuric acid solution was successful. The transpassive dissolution process was utilized for a defined, localized material removal. By applying nanosecond pulses between a work piece and a tool electrode, micro-holes of high aspect ratio and depth of up to 100 µm can be machined. But methanolic solutions are highly toxic and flammable and thus, undesirable in industrial mass production. Water-based electrolytes are the better choice, but the BMG is forming a passive layer which is very stable throughout the machining process. The addition of an oxidizing agent (iron sulphate) is successfully introduced to reduce this passivation process. Further, the Fe(III) ion reduction dominates the W-tool cathode reactivity and thus, suppresses disturbing hydrogen evolution. The influence of the applied oxidizing agents is analyzed and robust parameters for a fast and repeatable forming process without corrosion products are evaluated. The impact of the crucial process parameters as voltage, heading velocity or pulse-pause-ratio on the surface structuring is discussed. Optimum hole and line micro-structures were machined on BMG surfaces in 0.1 M sulphuric acid solution with 0.1 M iron sulphate at parameters: pulse on/off voltage = 6.0/0.5 V, pulse period/length = 750/75 ns, machining velocity = 0.1 µm/s (deep hole) and pulse on/off voltage = 6.0/1.0 V, pulse period/length = 2400/300 ns, machining velocity = 0.05 µm/s (line) [3-5]. This work is funded by the German Research Foundation (DFG) under project numbers GE 1106/10 and 1106/11. [1] J. A. Koza, R. Sueptitz, M. Uhlemann, L. Schultz, A. Gebert: Electrochemical micromachining of a Zr-based bulk metallic glass using a micro-tool e lectrode technique, Intermetallics 19 (2011) 437-444 [2] A. Gebert, P.F. Gostin, R. Sueptitz, S. Oswald, S. Abdi, M. Uhlemann, J. Eckert: Polarization studies of Zr-based bulk metallic glasses for electrochemical machining , Journal of the Electrochemical Society 161 (4) (2014) E66-E73 [3] R. Sueptitz, K. Tschulik, C. Becker, M. Stoica, M. Uhlemann, J. Eckert, A. Gebert: Micro-patterning of Fe-based bulk metallic glass surfaces by pulsed electrochemical micro-machining (ECMM), Journal of Materials Research 27 (2012) 3033-3040 [4] R. Sueptitz, P. Dunne, K. Tschulik, M. Uhlemann, J. Eckert, A. Gebert : Electrochemical micromachining of passive electrodes, Electrochimica Acta 109 (2013) 562-569 [5] S. Horn, A. Freidank, M. Uhlemann, M. Stoica, J. Eckert, A. Gebert, Electrochemical micromachining of passive Fe-based bulk metallic glasses in aqueous solutions, Proceedings INSECT 2014, Saarbrücken, Germany (in press)