electrochemical processing
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1 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Advanced electrochemical processing of tungsten components for He-cooled divertor application
Outline:
• Motivation for advanced W machining technology
• Electrochemistry of tungsten
• Review of ECM processing methods
• Surface finishing by S‐ECM process
• C‐ECM process development for W structuring
• Conclusions
W. Krauss, N. Holstein, J. Konys
2 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
He-cooled modular divertor design
He600°C10 MPa
700°C
He600°C10 MPa
700°C
He600°C10 MPa
700°C
Tungsten tileHT brazing
He600°C
10 MPa ODS EUROFERstructure
W-alloy thimble
700°C
HEMSHEMJ
Motivation for Electro-Chemical Methods in He-cooled W divertor
Castellation
Typical W-parts
Risk: Actual processing technology easily introduces defects
Requirements:• Adopted surface machining• Microstruturing• No processing introduced defects• Processes with cost control
Thimble
e.g. slotted flow promoter
3 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Motivation for Electro-Chemical MethodsTypical defects and failures in W parts by conventional machining
Secondary failures, growing under HHF load
Primary machining defects
HHF results define needs for W-processing• Smooth surfaces• No defects from machining• No sharp edges
Innovative soft machining methods required
Crack growth at machining defect
Post HHF analyses of mock-up. Machining defect caused failure
Mock up # 4
4 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
In theory:Cathodic tungsten should be a accessible material. In praxis (state-of-the-art galvano-technology):Passivation by electrochemically generated oxide layer (d >> 1 μm)
Investigation and development of newelectrolyte systems necessary
Electrochemistry of tungsten towards ECMAdvantages of electrochemistry and tungsten specific requirements
Standard EC reduction potentials Tungsten :E0 = -0,1 2V: WO3 + 4 H+ + 4 e- → W + 2 H2OE0 = -0,0 9V: WO3 + 6 H+ + 4 e- → W + 3 H2O E0 = -0,04 V: W2O5 + 2 H+ 2 e- → W + 3 H2OE0 = -0,03 V: WO3 + 2 H+ + 2 e- → W2O5 + H2O
H HO
H H
tungsten metal
OO
O W
O
O W
WO3 WO3 WO3 WO3 WO3WW
H-H
-
By anodic dissolution metal removal takes place without any mechanical pressure and at low temperatures.
No gradients ΔT, Δp (and resulting forces) between• electrolyte medium and metal surface • metal surface and metal bulk→ no local heating as in EDM working→ no mechanical load
Me0
e-
liquid phase
solid metal phase
p = AtmT = RT...100癈
pliquid = psolidTliquid = Tsolid bulk
Δp = 0ΔT = 0
Δp = 0ΔT = 0
Me+
Me+Me+
e-
5 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
0,0001
0,001
0,01
0,1
1
10
100
1000
-0,5 -0,25 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5Potential E [V vs. NHE]
i [m
Acm
-2]
pH = 12 pH = 11 pH = 10 pH = 9 pH = 8 pH = 7 pH = 6 pH = 3 pH = 1
Linear Scanning Voltametry (LSV)Sytem: W / TCEE / PtScan rate: 1 mV/sec
Electrochemical investigations: Potentiostatic Linear Scanning Voltametry
Electrochemistry of tungstenInvestigation / development of ECM electrolytes
W + 3 H2O → WO3 + 6 H+ + 6 e-
W + 2 H2O + 2 OH-→ WO42- + 6 H+ +6 e-
W → W3+ + 3 e-
Does not take place !!!
W0 -> W6+
tungsten metal
H HO
H H
H He-
e-H H
OH H
tungsten metal
OO
O W
O
O W
WO3 WO3 WO3 WO3 WO3WW
H+ H+
H+ H+ H+
H+H+ SEM
H HO
H H
tungsten metal
OO
O W
O
O W
WO3 WO3 WO3 WO3 WO3WW
H-H
-
H H
tungsten metal
W W
WO42-
H-
H-
WO42-
WO42-
WO42-
6 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Electrochemical reactions
Anodic reaction:W dissolution
Surface treatment
Electro-polishingStructuring
ECMW - Substrate Deposition e.g. W
Cathodic reaction:Electro-chem. deposition
Deposition of layers forBrazing e.g. W-Steel
ProticNi/W
(water based)
AproticW
Electrochemical behavior of tungsten
0,0001
0,001
0,01
0,1
1
10
100
1000
-0,75 -0,5 -0,25 0 0,25 0,5 0,75 1 1,25 1,5 1,75 2 2,25 2,5 2,75 3 3,25E / V (Hg/HgSO4)
i / m
Acm
-2
pH = 12,4
pH = 12
pH = 11
pH = 10
pH = 9
pH = 9
pH = 8
pH = 7
pH = 7
pH = 6
pH = 5
pH = 3
pH = 1
i = f(pH)System: W / TCEE / Pt
v = 1 mV/s, 1000 U/min, d = 16 mm
Applications
Basic investigations
Primary machiningdefects
Ni
WTungsten
7 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
+
e-
e-
-
Inert Cathode
Current
e-
- +
Anodic Connexion
Tungsten-Substrate
Cathodic Connexion
Electro-Chemical-Machining (ECM) of tungstenECM processes and concept definition
Anodization under cath. propulsion
Form cathode Workpiece = Anode
Regioselective dissolution without mask
C-ECM
Application:Surface finishing in micrometer scale
Application:Bulk structuring (Processes M-ECM and C-ECM)
• Basic W-behavior analyzedDifferent ECM concepts defined, evaluated and selected S-ECM, M-ECM and C-ECM (S = surface, M = mask, C = cathode)
Aim:Gaps with high aspect ratios
Aim:Defect free surfaces
cathode = ECM working tool, negative imagevertically mobile
anode = workpieceelectrolyte
agitation by microstep-motor
frameworkcathode guiding
++
--
8 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Electrochemical tungsten processing S-ECMS-ECM in high aspect ratio
W surface, S-ECM
W surface, EDM
Standard conditions5 min Sono (degasing)S-ECM: 200 mA/cm2, min. 30 min (150 µm)
In high aspect ratio (HAR)-geometries no
convection in deeper region
Diffusion limited current iD
If iECM > iD: electrolyte decomposition
incl. gas evolution
Optimization of process parameters
Tile withcastellation
S-ECM finishingof castellation slots
9 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
cathode = ECM working tool, negative imagevertically mobile
anode = workpieceelectrolyte
agitation by microstep-motor
frameworkcathode guiding
++
--
ECM process variants M‐ECM and C‐ECM:
ECM Requirements
• Electrolyte development • M‐ECM: anode mask process• C‐ECM: cathode tool
ECM Advantages
• No cracks by ECM process• Surface polishing• residue‐free metal removal
M‐ECM , W disk coated by UV‐mask Novolak
M‐ECM: C‐ECM Technique conventional installation complex facilityParameters current × time = charge charge + distance + step rate + convectionCathode passive counter electrode active shaping component (by step motor)Tool design 2‐dim. mask (positive) 3‐dim. electrode (negative)Transformation +2‐dim g +3‐dim ‐3‐dim g +3‐dim
Main differing features
C-ECM scheme
C‐ECMtool
workpiece
tool
workpiece
Processed workpiece
Cathode = ECM working tool
Anode = workpiece
-
+
-
+
-
+
Processed workpieceProcessed workpiece
Cathode = ECM working tool
Anode = workpiece
--
++
--
++
--
++
2 H+ + 2e- → H2
Me → Me2+ + 2e-
M‐ECM
Mask processing UV-lithography Electrochemical Etching Structured workpiece
hν 365 nm
++
--
Anodic W dissolution – bulk shapingVariants of local metal dissolution by ECM
Slotted W diskDepth 800 µm
Restriction by mask stability
10 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
C-ECM of tungsten
Physical parameters for accurate shaping by ECM
Cathode as negative moldNo mask or resists necessaryMoveable 3-D cathode
Potentials EDiss in gap lower as on surface:Less resistance, higher currents, better dissolution.Result: local selectivity in high aspect ratios
Current densityStep rate Gap widthConvectionPulse profile
p = AtmT = RT...100癈
pliquid = psolidTliquid = Tsolid bulk
Δp = 0ΔT = 0
Δp = 0ΔT = 0
Me+ Me+
e-
Me+Me+
Me+
e-
solid metal phase
Me+
Me0
e-
Me0
liquid phase
solid metal phase
Me0
e-
Side gap reaches boundary limit:Side dissolution ceases
Front gap must be fully synchronized with step rate:Constant maximal dissolution rate
Propagation: step rate
Parameters:
11 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
-1100
-1000
-900
-800
-700
-600
-500
-400
-300
-200
-100
0
100
200
300
-2500250500750100012501500175020002250250027503000Abstand AE-GE / μm
E / m
V (H
g/H
gSO
4)
200 mA/cm2 100 mA/cm2 150 mA/cm2 50 mA/cm2
E = d(AE-GE)System: W / SL1 / Pt
i = 200 mA/cm2, 1500 U/min, d0 = 3 mm
C-ECM of tungstenParameter: Gap width
C-ECM: Local selectivity by distance; Reaction zone in a gap of max. 50 μm
Determination of optimal gap distance in dependence of current density
12 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Without any forced electrolyte flow
Electrolyte flow
Electrolyte flow: 20 and 100 cm3/h
Hydraulic diameter2 mm
Too high electrolyte flow: 500 cm3/h
Convection
C-ECM of tungstenParameter: Electrolyte flow
Forced electrolyte flow
13 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
0,4 mm
ν = 100 HzpH = 10
ν = 0 HzpH = 10
ν = 10HzpH = 10
Hz: 10000 1000 500 100 10 0
C-ECM:Edge steepness as
Function of pulse frequency νDesired
flank profile
C-ECM of tungstenParameter: Pulse current effects
ti tp
Pulse scheme t
i
W + 3 H2O → WO3 + 6 H+ + 6 e-
WO3 + 2 OH- → WO42- + H2O
ν = 10 kHzpH = 10
14 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
0 Hz . . .
. . . 1000 Hz
C-ECM of tungstenParameter: Pulse current effects
cathode = ECM working tool, negative imagevertically mobile
anode = workpieceelectrolyte
agitation by microstep-motor
framework cathode guiding
+
-
Influence of electric currentPulsed DC current
with ti = tp
ti tp
Pulse scheme t
i
W + 3 H2O → WO3 + 6 H+ + 6 e-
WO3 + 2 OH- → WO42- + H2O
Cathode – anode gapD = 50 µm
15 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
C-ECM of tungstenDemonstrators by C-ECM
C-ECM demonstrator cathodes
Generated W-structures
Parallel grooved structure
Angled structure
16 W. Krauss; ISFNT‐9, Dalian, China, October 11‐16, 2009
Conclusions
Electro‐chemical behavior of tungsten analyzed.
Suitable electrolytes developed for electro‐chemical machining (ECM) of tungsten.
Three ECM processes successfully developed for different main applications in tungsten machining.
‐ S‐ECM for surface finishing.
‐ C‐ECM and M‐ECM for W shaping.
Dependencies of C‐ECM on process parameters analyzed and optimized for C-ECM processing.
Demonstrators successfully fabricated by ECM.
Cooperation with industries started for W processing by C‐ECM.
Benefit from basic electrochemical work on W for deposition of W scales.
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