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Electrochemical Machining and
MicromachiningSummer school on electrochemical
engineering, Palic, Republic of Serbia
Prof. a.D. Dr. Hartmut Wendt, TUD
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The fundamentals of electrochemical surface treatment• Electrochemical surface treatment is based on
anodic metal dissolution.• Metal dissolution by a) active dissolution,
b)transpassive dissolution• Dimensional resolution is mainly determined by
current and potential distribution around a cathodic matrix
• Forced convection removes bubbles (by H2 and O2 evolution) and oxidic and hydroxidic debris e.g. Fe(OH)3 and other oxidation-solvolysis products
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Schematic current voltage curve with active and transpassive metal dissolution
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Electrochemical shaping of metals
Active dissol. and mass transfer
Transpassive dissol. With fast sweep
Transpassive dissolution with mechanical scraping
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Some examples of electrochemical machining of hard metals: Primary Current density distribution
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Current density distributions
• Primary: neglects charge transfer kinetics and influence of mass transfer.Decisive: only distributions of pure Ohmic resistances
• Secondary: Adding charge transfer resistances to purely Ohmic resistances
• Tertiary: Mainly determined by mass transfer conditions
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Primary current density distribution between two parallel plates and at the electrode edge
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Addition of electrolyte resistances Rp and Rv add to charge transfer resistance to give primary and secondary current distributions
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Even current density distribution
At rotating disc electrode under mass transport limited condition ( limiting current density) is a typical tertiary c.d. distribution
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Electropolishing
Mass transfer controlled transport of dissolution products through a thin, statistically fluctuating layer of debris generates the polishing effect
Current densities amount from hundred to several hundred mA cm-2
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Electropolishing electrolytes
• Composition given in lecture manuscript
• Almost all contain phosphoric acid
• Almost all – exception electropolishing W – are strongly acidic
• Some contain organic cosolvents
• Are obtained and optimized by trial and error
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Electrochemical machining electrolytes
• Are neutral (neither basic nor acidic) with the exception of basic electrolyte for molybdenum
• Most of them contain sodium nitrates or perchlorate
• Current densities amount to several A cm-2
• Copious exchange of electrolyte must be secured to remove Joule`s heat and all debris
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Electrochemical micromachining
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During double layer charging: primary current distribution
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The surface of the workpeace which is farther away charges more slowly than next to the tool
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l x ρ x Cspec = ح
with l = length of current line
ρ = specific resistance of electrolyte (approximately 10 Ω cm)
ζ is charging time; as potential changes exponentially with time:
Φo – Φ = (Φo – Φ )t=(1-exp(-t/ ح)
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Improve the resolution of anodic dissolution from millimetres to
micrometres
Applying pulses in nanoseconds instead of direct currents
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Example from L.Cagnon, V. Kirchner, M. Kock, R. Schuster, G. Ertl, Th. Gmelin and H. Kueck, Z. Phys. Chem. 217, (2003), 299 - 313
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Example from M. Kock, V. Kirchner and R. Schuster, Electrochim. Acta 48, (2003) 3213 - 3219
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The LIGA – Process for building Micro-structures by x-ray-assisted masking
and cathodic metal deposition
• Resolution is determined by precision of masks and their copy on photo-resist – hence x-ray copying
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Summary• With maximal cutting rates corresponding to
several A cm-2 electrochemical machining is too slow to be generally applicable instead of mechanical machining
• But ultrahard alloys can only be treated by electrochemical machining which usually gives also a good polishing finish
• Applying nanosecond pulses increases the dimensional resolution, so that also micrometer structures can be produced – it is still an open question how to utilize these possibilities in commercial processes.