team meeting 7.5.08 - sparks in edm 1 / 36 all about sparks in edm centre de recherches en physique...
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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)
2 / 36Team meeting 7.5.08 - Sparks in EDM
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
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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
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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
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Examples of parts machined with EDM
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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
7 / 36Team meeting 7.5.08 - Sparks in EDM
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
8 / 36Team meeting 7.5.08 - Sparks in EDM
Machining device
electrode
workpiece
dielectricshower
optical fibre
EDM pulsegenerator
EDM machine
control
dielectric circuit
motor
pump
verticaldisplacement
9 / 36Team meeting 7.5.08 - Sparks in EDM
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
10 / 36Team meeting 7.5.08 - Sparks in EDM
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
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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
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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
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Imaging of the process
(Cu / steel, 50 s, 8 A, water)
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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)
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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
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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
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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)
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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
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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
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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)
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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
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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
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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)
24 / 36Team meeting 7.5.08 - Sparks in EDM
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 :
25 / 36Team meeting 7.5.08 - Sparks in 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 !
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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
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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
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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 ?
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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
30 / 36Team meeting 7.5.08 - Sparks in EDM
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)
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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
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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)
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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
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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 ?
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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 ???
36 / 36Team meeting 7.5.08 - Sparks in EDM
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)
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Streamer propagation
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