Team meeting Sparks in EDM 1 / 36 All about sparks in EDM Centre de Recherches en Physique des Plasmas (CRPP), Ecole Polytechnique Fédérale de Lausanne (EPFL) Charmilles Technologies SA, Meyrin Antoine Descoeudres, Christoph Hollenstein, Georg Wälder, René Demellayer and Roberto Perez (and links with the CLIC DC spark test)

Team meeting Sparks in EDM 2 / 36 Outline of the presentation I.Introduction II.Experimental setup and diagnostics III.Some results about the EDM spark IV.Links with the DC Spark Test

Team meeting Sparks in EDM 3 / 36 Electrical Discharge Machining (EDM) EDM = successive removal of small volumes of workpiece material, using the eroding effect of electric discharges on electrodes time V I ~ 200 V ~ 20 V ~ 10 A ~ 100  s Breakdown Discharge End of the discharge Post-discharge Pre-breakdown electrode workpiece dielectric liquid

Team meeting Sparks in EDM 4 / 36 Two types of EDM machines die-sinking machines The electrode keeps its form (asymmetry wear/erosion) Production of injection moulds wire-cutting machines The electrode is a travelling wire Production of steel cutting dies and extrusion dies

Team meeting Sparks in EDM 5 / 36 Examples of parts machined with EDM

Team meeting Sparks in EDM 6 / 36 Motivations and purpose of the work improvements in machining accuracy (  -machining) and surface roughness improvements in material removal rate and reduction of wear reliable numerical models to optimize the performances of EDM Systematic investigation of the EDM plasma important lack of knowledge about basic EDM phenomena –complex phenomena –experimental difficulties –stochastic nature empirical optimization numerical models with empirical parameters We want … But … We need a better fundamental understanding of the EDM discharge and of its interaction with the electrodes

Team meeting Sparks in EDM 7 / 36 Outline of the presentation I.Introduction II.Experimental setup and diagnostics III.Some results about the EDM spark IV.Links with the DC Spark Test

Team meeting Sparks in EDM 8 / 36 Machining device electrode workpiece dielectric shower optical fibre EDM pulse generator EDM machine control dielectric circuit motor pump vertical displacement

Team meeting Sparks in EDM 9 / 36 Diagnostics workpiece (steel) electrode (Cu, C, W, Zn) dielectric (water, oil, liq-N 2 ) plasma EDM pulse generator G Electrical measurements voltage probe V current probe Light intensity optical fibre to spectrograph Optical emission spectroscopy optical fibre to photomultiplier Imaging endoscope to camera

Team meeting Sparks in EDM 10 / 36 Optical Emission Spectroscopy Characteristic lines  identification of emitting atoms and ions in the plasma Relative intensities of Cu lines (+ LTE)  electron temperature measurements Stark broadening and shift of the H  line  electron density measurements OES = analysis of the emitted light with a spectrograph (dispersion of the light by a grating) gratings optical fibre spectrograph spectrum computer CCD camera

Team meeting Sparks in EDM 11 / 36 Experimental difficulties Small size (gap 10 – 100  m) In a liquid environment Weak light emission for spectroscopy Short duration (~  s, ~ ns for breakdown phenomena) Electrical interferences Poor reproducibility of the discharges Few diagnostics available, and difficult to apply The list is almost the same with our DC sparks… Ultra High Vacuum

Team meeting Sparks in EDM 12 / 36 Outline of the presentation I.Introduction II.Experimental setup and diagnostics III.Some results about the EDM spark IV.Links with the DC Spark Test

Team meeting Sparks in EDM 13 / 36 Imaging of the process (Cu / steel, 50  s, 8 A, water)

Team meeting Sparks in EDM 14 / 36 Plasma imaging Evolution of the plasma light intensity Typical plasma image breakdown phase / discharge phase Excited region broader than the gap Slight growth with time Diameter increases with the discharge current (Cu / steel, 100  s, 24 A, oil) (8  s, 4 A, water)

Team meeting Sparks in EDM 15 / 36 Beginning of the discharge: fast imaging 200 to 250 ns 250 to 300 ns 50 to 100 ns 100 to 150 ns 150 to 200 ns time [  s] 0 1 I / 8 [A] V/200 [V] exposure (6 A, water) the plasma develops very fast (< 50 ns) afterwards : stability

Team meeting Sparks in EDM 16 / 36 End of the discharge and post-discharge The plasma disappears as soon as the current is shut down Corresponding spectrum blackbody : incandescence of the eroded particles 2300 K : molten metal Images obtained directly after a discharge : blackbody fit 2300 K Weak light emission during the post-discharge

Team meeting Sparks in EDM 17 / 36 Typical spectrum Characteristic atomic lines : dielectric cracking and contamination Cu / steel, water water : H, O oil : H, C, C 2 liquid nitrogen : N Dielectric HH O O copper : Cu graphite : C, C 2 tungsten : W zinc : Zn, Zn + Electrode material Cu Cr Fe C Workpiece material steel : Fe, Cr, C (12  s, 12 A)

Team meeting Sparks in EDM 18 / 36 Effect of the discharge on-time Extremely high electron density at the beginning of the discharge (Cu / steel, water, 12 A) Increase in the H  FWHM and shift Increase in the continuum

Team meeting Sparks in EDM 19 / 36 Time-resolved spectroscopy Continuum due to the merging of spectral lines (12 A, water, time res. 200 ns) t = 0 : breakdown The plasma is very dense during the first microsecond

Team meeting Sparks in EDM 20 / 36 Time-resolved spectroscopy of H  : electron density n e reaches cm -3 at the beginning n e decreases with time (plasma expansion) The plasma is created from a LIQUID ! (16 A, water, time res. 1  s)

Team meeting Sparks in EDM 21 / 36 Time-resolved spectroscopy: electron temperature No ionic spectral lines Cu lines T e  0.7 eV (8'100 K) Two-line method with Cu lines: cold plasma

Team meeting Sparks in EDM 22 / 36 Spatially-resolved spectroscopy Spatial sampling with endoscope + fibre bundle spatial asymmetry of the contamination Spatial resolution : ~ 20  m (typical gap : ~ 100  m) (100  s, 6 A, water) Plasma contamination: Cu line (from electrode) Cr line (from workpiece) Cu line Cr line to steel electrode (-) to copper electrode (+) plasma light analysis in different zones

Team meeting Sparks in EDM 23 / 36 Spatially-resolved spectroscopy Electron temperature vertical profile Electron density vertical profile T e homogeneous n e slightly higher in the center (50  s, 12 A, water)

Team meeting Sparks in EDM 24 / 36 Plasma coupling parameter   << 1: ideal plasma  1: weakly non-ideal plasma  > 1: strongly coupled plasma T n EDM plasma: weakly non-ideal (dense & cold) EDM n e  cm -3 T e  0.7 eV   0.33 EDM :

Team meeting Sparks in EDM 25 / 36 Summary: physical properties of EDM plasmas Composition: dielectric cracking + electrodes contamination T e  0.7 eV (cold) n e  cm -3 (dense) p  10 bar Weakly non-ideal Fairly ionized Small dimensions High electric fields Intense during the first  s Numerous terms in the energy balance Relatively insensitive to most of the discharge parameters schematic ! 

Team meeting Sparks in EDM 26 / 36 Outline of the presentation I.Introduction II.Experimental setup and diagnostics III.Some results about the EDM spark IV.Links with the DC Spark Test

Team meeting Sparks in EDM 27 / 36 Are we talking about the same thing ? EDM time V I ~ 200 V ~ 20 V ~ 10 A ~ 100  s current constant duration is chosen (2  s – 1ms) low voltage brkd spark energy < 0.01 J DC spark test V time I ~ 10 kV ~ 300 A ~ 2  s discharge of a capacitor high voltage brkd, high current spark energy ~ 1 J

Team meeting Sparks in EDM 28 / 36 Are we talking about the same thing ? EDM DC spark test sparks in VACUUM sparks in LIQUID both plasmas are sparks And what about the breakdown mechanism ?

Team meeting Sparks in EDM 29 / 36 Breakdown mechanisms in vacuum BANG! material fatigue material break-up FE current outgassing presence of vapour heating of the emission site avalanche of electrons vapour is ionized by FE current Vapour ( = a medium ‘‘more dense than vacuum’’…) is needed for the propagation of the avalanche

Team meeting Sparks in EDM 30 / 36 Outgassing in the DC spark test gas is released in a few attempts before breakdown Example: Release of hydrogen gas, Mo electrodes (from Trond)

Team meeting Sparks in EDM 31 / 36 Breakdown mechanism in dielectric liquids (and, to some extent, in high-pressure gases) A single electron avalanche can not propagate far away in a liquid  needs a medium less dense than a liquid : streamer breakdown ! Initiation : a vapour bubble (pre-existing, or by FE) BANG! primary avalanche in this low-density region, bubble growth streamer growth and propagation ( m/s) back streamer (‘‘return stroke’’) = enormous electron avalanche, reverse ionizing front ~ 10 7 m/s formation of a streamer (= thin weakly ionized channel) the streamer bridges the gap creates highly ionized channel, heats up surrounding gas, shockwave

Team meeting Sparks in EDM 32 / 36 Streamers in dielectric liquids in oil time in  s gap 1.3 cm 82 kV sharp needle J.C. Devins et al., J. Appl. Phys (1981)

Team meeting Sparks in EDM 33 / 36 Streamers in dielectric liquids structure and propagation speed depend on polarity (NB: can start from the cathode or the anode!) J.C. Devins et al., J. Appl. Phys (1981) Common fact about breakdowns in vacuum, gas or liquid : the electron avalanche takes place in a gaseous medium ~ 100 m/s ~ km/s

Team meeting Sparks in EDM 34 / 36 Light emission at breakdown EDM 500 ns light peak DC spark test 50 ns light peak (sometimes!) Also a dense plasma at the very beginning ? Rapid melting / vaporization of a small protrusion ?

Team meeting Sparks in EDM 35 / 36 Light emission at breakdown EDM 500 ns light peak DC spark test 50 ns light peak (sometimes!) line emission by excited species continuous emission due to high density ???

Team meeting Sparks in EDM 36 / 36 Conclusion Electron avalanches leading to breakdown need a gaseous medium to propagate (‘‘low-density region’’)  This is true in vacuum, gases, dielectric liquids and solids EDM discharges produce dense plasmas, especially during the first microsecond  They are created from a liquid First light measurements suggest that plasmas of the DC spark test could also be dense immediately after the breakdown  The ‘‘melting of a protrusion’’ scenario is probable in this case These plasmas have some similarities in the early stage of their life (probably cousins, but not twin brothers)

Team meeting Sparks in EDM 37 / 36

Team meeting Sparks in EDM 38 / 36 Streamer propagation