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Electrochemical machining

By Anthony Christopher Baxter


The thesis describes an investigation into the fundamental phenomena governing the electrochemical machining process. It excludes a detailed investigation of the electrochemistry of the anode surface, work on which is in hand at the University of Nottingham.\ud \ud Photographs have been obtained of both electrode surfaces during machining in a two dimensional channel, showing the important role of gas evolution both at currents below the limit and in limiting the current density achievable. The distribution of electrical potential across the gap has been measured, clearly showing that the limiting current phenomenon is governed by a process occurring very close to the cathode. Measurements have been made of the streamwise current distribution; the distribution is essentially uniform at low currents, but as the limit is approached the current at the downstream end falls, and this fall then propagates upstream to fill about two-thirds of the channel. It has been found that the limiting current is proportional to (absolute pressure)^1/3, that the size of bubbles produced is inversely proportional to (absolute pressure)^1/3, and that reduction of the surface tension of the electrolyte leads to a marked fall in limiting current. The efficiency of the process has been investigated by a technique involving the measurement of the gas evolved during machining.\ud \ud An analysis of these results leads to the formulation of an explanation of the cell voltage-current characteristic, a hypothesis to explain the current limiting process, and a suggestion of the detailed mechanism of the latter.\ud \ud The cell voltage-current curve (above) can be explained as follows: - AB is equilibrium dissolution with etching, BC is caused by the formulation of a solid ( impure oxide? ) film on the anode surface. The rise in current from C to D is caused by the anodic evolution of a gas (oxygen? ), causing better mixing conditions in the diffusion layer near the anode and hence a higher metal dissolution rate. The ratio of current used for metal dissolution to current used for gas evolution appears to be a constant for this region. This process would be expected to continue along DE, but the current is limited by the achievement of a maximum rate of cathodic hydrogen evolution which brings about the reduction in current to F.\ud \ud This limiting current crisis has been analysed in terms of the mechanics of bubble formation, and a detailed explanation in terms of various mechanisms has been attempted. The experimental data is fitted by a model in which the hydrodynamic conditions give a velocity at which bubbles can be removed from a fixed number of nucleation sites. The limiting current is then predicted to be proportional to (surface tension)^2 x (absolute pressure)^1/3.\ud \ud Several proposals are made for further experiments to investigate these proposals, and for data needed to extend the industrial application of the theory developed

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