Electrochemical Machining and

MicromachiningSummer school on electrochemical

engineering, Palic, Republic of Serbia

Prof. a.D. Dr. Hartmut Wendt, TUD

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

Schematic current voltage curve with active and transpassive metal dissolution

Electrochemical shaping of metals

Active dissol. and mass transfer

Transpassive dissol. With fast sweep

Transpassive dissolution with mechanical scraping

Some examples of electrochemical machining of hard metals: Primary Current density distribution

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

Primary current density distribution between two parallel plates and at the electrode edge

Addition of electrolyte resistances Rp and Rv add to charge transfer resistance to give primary and secondary current distributions

Even current density distribution

At rotating disc electrode under mass transport limited condition ( limiting current density) is a typical tertiary c.d. distribution

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

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

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

Electrochemical micromachining

During double layer charging: primary current distribution

The surface of the workpeace which is farther away charges more slowly than next to the tool

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)

Improve the resolution of anodic dissolution from millimetres to

micrometres

Applying pulses in nanoseconds instead of direct currents

Example from L.Cagnon, V. Kirchner, M. Kock, R. Schuster, G. Ertl, Th. Gmelin and H. Kueck, Z. Phys. Chem. 217, (2003), 299 - 313

Example from M. Kock, V. Kirchner and R. Schuster, Electrochim. Acta 48, (2003) 3213 - 3219

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

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.