Electrochemical Processing

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


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


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


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-


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


H+ H+

H+ H+ H+

H+H+ SEM

H HO

H H

tungsten metal

OO

O W

O

O W


H-H

-

H H

tungsten metal

W W

WO42-

H-

H-

WO42-

WO42-

WO42-


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


+

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

++

--


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





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


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:


-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


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


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


0 Hz . . .

. . . 1000 Hz

C-ECM of tungstenParameter: Pulse current effects




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


C-ECM of tungstenDemonstrators by C-ECM

C-ECM demonstrator cathodes

Generated W-structures

Parallel grooved structure

Angled structure


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.

Documents

Electrochemical Processing