Team meeting 7.5.08 - Sparks in EDM 1 / 36 All about sparks in EDM Centre de Recherches en Physique des Plasmas (CRPP), Ecole Polytechnique Fédérale de

1 / 36Team meeting 7.5.08 - Sparks in EDM

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)


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


Electrical Discharge Machining (EDM)

EDM = successive removal of small volumes of workpiece material, using the eroding effect of electric discharges on electrodes

time

VI

~ 2

00 V

~ 20 V

~ 1

0 A

~ 100 s

Breakdown

Discharge

End of the discharge

Post-discharge

Pre-breakdown

electrode

workpiece

dielectric liquid


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


Examples of parts machined with EDM


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



I. Introduction





Machining device

electrode

workpiece

dielectricshower

optical fibre

EDM pulsegenerator

EDM machine

control

dielectric circuit

motor

pump

verticaldisplacement


Diagnostics

workpiece (steel)

electrode (Cu, C, W, Zn)

dielectric (water, oil, liq-N2)

plasma

EDM pulsegenerator G

• Electrical measurements

voltageprobe

V

current probe

• Light intensity

optical fibre

to spectrograph

• Optical emission spectroscopy

optical fibre

to photomultiplier

• Imaging

endoscope

to camera


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

spectrographspectrum

computer

CCDcamera


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



I. Introduction





Imaging of the process

(Cu / steel, 50 s, 8 A, water)


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)


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

- 0.5 0.50 1

time [s]

0

1 I / 8 [A]V/200 [V]

exposure

(6 A, water)

• the plasma develops very

fast (< 50 ns)

• afterwards : stability


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 fit2300 K

• Weak light emission during the

post-discharge


Typical spectrum

Characteristic atomic lines : dielectric cracking and contamination

Cu / steel , water• water : H , O

• oil : H , C , C2

• liquid nitrogen : N

Dielectric

H

O

O

• copper : Cu

• graphite : C , C2

• tungsten : W

• zinc : Zn, Zn+

Electrode material

Cu

Cu

Cu Cr

Fe C

Workpiece material

• steel : Fe, Cr, C(12 s, 12 A)


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


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


Time-resolved spectroscopy of H: electron density

ne reaches 2 • 1018 cm-3 at the beginning

ne decreases with time (plasma expansion)

The plasma is created from a LIQUID !

(16 A, water, time res. 1 s)


Time-resolved spectroscopy: electron temperature

• No ionic spectral lines

Cu lines

Te 0.7 eV (8'100 K)

• Two-line method with Cu lines:

cold plasma


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 steelelectrode (-)

to copperelectrode (+)

plasma light analysis in different zones


Spatially-resolved spectroscopy

• Electron temperature vertical profile

• Electron density vertical profile

510.5515.3

521.8

Te homogeneous

ne slightly higher

in the center

(50 s, 12 A, water)


Plasma coupling parameter

T

n

Tak

eZ

B

3/1

0

22

4

energy" thermal"

n"interactio Coulomb"

<< 1 : ideal plasma 1 : weakly non-ideal plasma > 1 : strongly coupled plasma

T

n

EDM plasma: weakly non-ideal (dense & cold)

EDM

ne 1018 cm-3

Te 0.7 eV 0.33

EDM :


Summary: physical properties of EDM plasmas

• Composition: dielectric cracking + electrodes contamination

• Te 0.7 eV (cold)

• ne 1018 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 !



I. Introduction





Are we talking about the same thing ?

EDM

time

VI

~ 2

00 V

~ 20 V

~ 1

0 A

~ 100 s

• current constant

• duration is chosen (2s – 1ms)

• low voltage brkd

• spark energy < 0.01 J

DC spark test

V

time

I

~ 1

0 kV

~ 3

00 A

~ 2 s

• discharge of a capacitor

• high voltage brkd, high current

• spark energy ~ 1 J


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 ?


Breakdown mechanisms in vacuum

BANG!

material fatigue

material break-up

FE currentoutgassing

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


Outgassing in the DC spark test

• gas is released in a few attempts before breakdown

2700 2750 2800 2850 2900 29504,0x10-10

4,1x10-10

4,2x10-10

4,3x10-10

4,4x10-10

4,5x10-10

4,6x10-10

Ion

Cur

ren

t [A

]

Relative Time [sec]

0 500 1000 1500 2700 2800 2900 30000

1x10-9

2x10-9

3x10-9

4x10-9

5x10-9

6x10-9

7x10-9

8x10-9

2,0x10-8

4,0x10-8

6,0x10-8

8,0x10-8

1,0x10-7

1,2x10-7

Pre

ssur

e H

2 [m

bar]

389,341 MV/m

372,037 MV/m

341,755 MV/mIon

Cur

rent

[A

]

Relative Time [sec]

Hydrogen GasExample: Release of hydrogen gas, Mo electrodes

(from Trond)


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 (102 - 104 m/s)

back streamer (‘‘return stroke’’)= enormous electron avalanche, reverse ionizing front ~ 107 m/s

formation of a streamer(= thin weakly ionized channel)

the streamer bridges the gap

creates highly ionized channel, heats up surrounding gas, shockwave


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. 52 4531 (1981)


Streamers in dielectric liquids

• structure and propagation speed depend on polarity

(NB: can start from the cathode or the anode!)

J.C

. Dev

ins

et a

l., J

. App

l. P

hys.

52

4531

(19

81)

Common fact about breakdowns in vacuum, gas or liquid :the electron avalanche takes place in a gaseous medium

~ 100 m/s ~ 1 - 10 km/s


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 ?


Light emission at breakdown

EDM

• 500 ns light peak

DC spark test

• 50 ns light peak (sometimes!)

line emission by excited species

continuous emissiondue to high density ???


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)



Streamer propagation

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Team meeting 7.5.08 - Sparks in EDM 1 / 36 All about sparks in EDM Centre de Recherches en Physique des Plasmas (CRPP), Ecole Polytechnique Fédérale de