Baxter, Anthony Christopher (1967) Electrochemical machining. PhD thesis, University of Warwick.
WRAP_THESIS_Baxter_1967.pdf - Requires a PDF viewer such as GSview, Xpdf or Adobe Acrobat Reader
Official URL: http://webcat.warwick.ac.uk/record=b1722565~S15
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.
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.
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.
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.
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.
Several proposals are made for further experiments to investigate
these proposals, and for data needed to extend the industrial application
of the theory developed.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||T Technology > TJ Mechanical engineering and machinery|
|Library of Congress Subject Headings (LCSH):||Electrochemical cutting|
|Institution:||University of Warwick|
|Theses Department:||School of Engineering|
|Supervisor(s)/Advisor:||Shercliff, J. A. (John Arthur)|
|Sponsors:||Science Research Council (Great Britain) (SRC) ; Shell International Petroleum Company|
|Extent:||94 leaves : ill., charts|
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