Electrochemical micromilling

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As electrochemical micro-milling (ECF) is a 3D and 2D micromachining process for metals referred to. The process is based on the same electrochemical removal principle as the already known electrochemical removal processes such as electrochemical removal (ECM), precise electrochemical metalworking PEM / PECM and electropolishing . The ECF is a tool-free, non-contact, burr-free and thermally stress-free micro-machining process.

Procedure

The electrochemical micro-milling (ECF), like already established ECM processes, uses the property of metallic materials to dissolve in a suitable electrolyte by applying a voltage between the tool and workpiece electrodes. With classic ECM (electrochemical machining) processes, the speed at which the metal dissolves locally depends on the current density distribution in the electrolyte. The latter, however, depends heavily on the geometry of the tool electrode, so that in the end there is no homogeneous gap over the geometry. This results in contour-dependent molding errors that make it difficult to manufacture precise parts. In order to generally reduce the working gap, pulsed ECM processes (PECM / PEM) can be used, in which the pauses between the current pulses are mostly used in connection with oscillating tool movements for cooling and rinsing purposes in order to reduce gap contamination. This allows the working gap to be reduced to around 10 µm.

Since the working gap in adapted electrolyte systems depends almost linearly on the pulse width, ultra-short voltage pulses are used in electrochemical milling (ECF). In this way, the working gap can be preset to be particularly small. The electrochemical reaction rate and thus the working distance can be controlled directly by a targeted charge reversal of the so-called electrochemical double layer. This double layer is formed immediately when metallic bodies are immersed in an electrolyte at its phase boundary and can be viewed in simplified terms as a capacitor with a plate spacing in the order of a few nanometers. This can be represented in a substitute model by an RC element . While the specific resistance and the specific capacity are almost constant, only the working distance is included as a variable in the time constant. This means that when ultra-short voltage pulses are applied, only those areas are recharged quickly enough where the working distance is small enough. The local, sharp delimitation of the erosion is also favored by the exponential dependence of the reaction rate on the charge reversal voltage of the double layer.

The tools and tool movement are very similar to those of mechanical milling. Due to the force-free and tool wear-free material removal, rod-shaped micro-tools made of tungsten are mostly used. Complex structures are machined by sequential removal along a defined tool path. Since almost all tool types such as radius milling cutters, spherical milling cutters, end mills or engraving styluses can also be reproduced in the smallest dimensions for the ECF process, more complex structures can also be produced in hard materials.

In this process, almost every material requires a suitable electrolyte. CrNi steels, some tool steels and pure metals such as copper, nickel, gold or tungsten can be processed. The machining area of ​​the tool is limited by the pulse power, so this method is not suitable for machining large workpieces. Typical tool diameters are between 2 µm and 500 µm, the infeed speed is around 1 µm / s. The process can be used to produce or rework micro-bores, grooves or webs, such as those required in atomization systems, plug-in connections or medical-technical components.

The ECF method was developed by Rolf Schuster and Viola Kirchner under the direction of Gerhard Ertl at the Fritz Haber Institute of the Max Planck Society and published for the first time in 2000.

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