Development of Electrochemical Machining (ECM) for Tungsten

INSTITUTE FOR APPLIED MATERIALS - MATERIALS PROCESSES IAM-WPT | CORROSION DEPARTMENT

www.kit.eduKIT – University of the State of Baden-Württemberg andNational Research Center of the Helmholtz Association

Development of Electrochemical Machining (ECM) for Tungsten

J. Konys, W. Krauss, N. Holstein

2 W-Based Materials for Fusion Applications | UCSB, February 13-15, 2012

Contents:

Motivation for electrochemical machining (ECM)

Electrochemistry of tungsten

ECM of tungsten

S-ECM

C-ECM

Conclusions and recommendations


Motivation for Electrochemical Processing (ECM) of TungstenTypical defects and failures in W parts by conventional machining

Primary machining defects

HHF results define needs for W-processing

Smooth surfacesNo defects from machiningNo sharp edges

Conventional machining is expensiveInnovative soft machining methods required

Crack growth at machining defect

Post HHF analyses of

mock-up EFREMOV.

Machining defect

caused failure

Mock up # 4

Secondary failures, growing under HHF load


Electrochemical Reactins of TungstenW dissolution: EC oxidation

Basic Investigation

Anodic reactionW-dissolution

Application Cathodic reactionDeposition

Surface treatmentElectro-polishing,cleaning

Scale depositionbrazing e.g. W byNi protic

MachiningECM

Depositionaprotice.g. W or Ta

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 3,25

E / V (Hg/HgSO4)

i /

mA

cm

-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


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 for Slot array : UV-mask Novolak

cathode =

ECM working tool,

negative imagevertically mobile

anode = workpieceelectrolyte

agitation by microstep-motor

frameworkcathode guiding

++

--

C-ECM scheme

Processed workpiece

= ECM

Anode = workpiece

-

+

-

+

-

+

Cathode = ECM working tool

Anode = workpiece

--

++

--

++

--

++

C-ECM

2 H+ + 2e- → H2

Me → Me2+ + 2e-

MAIN DIFFERING FEATURES

M-ECM, S-ECM C-ECM

Technique Conventional installation Complex facility

Parameters Current x time = charge Charge + distance + steprate + convection

Cathode passive counter electrode active shaping component (by step motor)

Tool design 2-dim. mask (positive) 3-dim. electrode (negative)

Transformation +2-dim →+3-dim resp. +0-dim -3-dim → +3-dim

ECM for 3-D structuring | M-ECM => S-ECM + C-ECM

M-ECM

Mask processing UV-lithography Electrochemical Etching Structured workpiece

hn365 nm

++

--

M-ECM without resist masks = S-ECM


ECM of W towards W/refractory-alloys

Dissolution ability of W-alloys by ECM electrolyte

-50

0

50

100

150

200

250

300

350

400

450

500

550

-1 -0,8 -0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2 2,2 2,4 2,6 2,8 3

E / V (Hg/HgSO4)

i /

mA

cm

-2

W, pH = 12

WL10, pH = 12

Mo, pH = 12 b

Ta, pH = 12

Al, pH = 12

i = f(pH)

System: Me / 1,25 M N+NH3 / Pt

Me = W, Wla10, Mo, Ta, Al

v = 1 mV/s, 1000 U/min

Mo improves machinability / dissolution rate

Stable oxide forming elements (Ta) are less favorable for the electrolyte N-NH3


Electrochemistry of tungstenInvestigation and development of ECM electrolytes

W0 -> W6+

tungsten metal

H HO

H H

H He-

e-

H HOH H

tungsten metal

OO

O W

O

O W

WO3 WO3 WO3 WO3 WO3WW

H+ H+

H+ H+ H+

H+H+

SEM0,0001

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 2,5

Potential E [V vs. NHE]

i [m

Ac

m-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 / Pt

Scan rate: 1 mV/sec

W + 3 H2O → WO3 + 6 H+ + 6 e-

W → W3+ + 3 e-

Does not take place !!!

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-

Electrochemical investigations: Potentiostatic Linear Scanning Voltammetry

W + 2 H2O + 2 OH-→ WO42- + 6 H+ + 6 e-


Electrochemical Tungsten Processing S-ECMS-ECM of surfaces

…on EDM-cut rod pieces

W as machined

S-ECM 15 min, 400 mA/cm2

In SEM

In SEMVIS


Electro-chemical tungsten processing S-ECMS-ECM of surfaces

Single crystal pieces

Mechanical mirror

polishing

S-ECM, rising current densities

elaboration on z-orientation

Effect on sc-orientation: parameters of S-ECM mirror polishing

on one orientation cannot be applied unchanged to other orientation

Wittmann‘sche Etching structures rough, without any reflection Plain, partial reflection Plain, black = total reflection


-50

0

50

100

150

200

250

300

350

400

450

500

1 10 100 1000 10000 100000

Frequency /Hzd

elt

a

%

delta upper gap width

delta ground width

delta flank width

HF-Pulses in ECM of tungsten

Dependencies of measures from frequence

Extrapolation to HF = 00

Extrapoliert

depth

upper gap width

Flank width

ground gap width

Development of techniques to

generate high HF currents

C-ECM of tungsten Impact of pulse current effects

νννν = 10 Hz

pH = 10

0,4 mm

νννν = 1 kHz

pH = 10


C-ECM of tungstenFabrication of 3-dim demonstrators

Parallel grooved structure Angled structures

Tool Tool

Cathode tool

Cathodic potential: no corrosion + mechanically contactless - no pressure

Working tool (every material, every design, every current) without any chemical,

elctrochemical,thermal, mechanical abrasion

TOOLS

Demonstrators:

Generated W-structures

12 W-Based Materials for Fusion Applications | UCSB, February 13-15, 201212 | J. Konys | IMF III/KOR | 10.03.2009

Anodic dissolution by ECM

Hz: 10000 1000 500 100 10 0

C-ECM:

Edge steepness as

Function of

pulse frequency νDesired

flank

profile

-1100

-1000

-900

-800

-700

-600

-500

-400

-300

-200

-100

0

100

200

300

-2500250500750100012501500175020002250250027503000Abstand AE-GE / µµµµm

E / m

V (

Hg/H

gS

O4)

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

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 / Pt

Scan rate: 1 mV/sec

Quality improvement

W �� WL 10, WTaMoX

Higher frequency of pulsed DC powerMobility control / reduction

Conductivity of electrolyte

Parameter dependencies

Smaller distance

tool – work piece


Development of C-ECM tool for shaping

of micro-structured workpieces:

e.g. Castellation:

6-fold star,

Projected depth 6 mm, width 0,1 mm;

aspect ratio = 60

Design of cathode tool and

facility for castellation-shaping

C-ECM Castellation tool

Castellation cathode tool:

first prototype (brass foils)


C-ECM European Patent EP 467


Conclusions

Electro-Chemical Machining (ECM) of tungsten:

W can be successfully machined by ECM without chemical passivation

Three ECM branches selected for special applications (S-ECM, C-ECM M-ECM)

ECM shows strong dependency on DC pulse frequency and gap distance

High frequency technology in DC pulses is required for good accuracy

S-ECM can polish and optimize fine structured surfaces

Demonstrators fabricated by S-ECM and C-EC

MGeneral statement :

ECM is a well investigated technology for tungsten

Achievements (work progress) in agreement with work plan

ECM has an established status for industrial W-shaping

Achieved technical status allows tt to industrial scale, AND /OR….

Achieved patent EP 467 allows commercial exploitation


Open questions

Actual and future work:

Accuracy improvement by high frequency (> 10 kHz range)

Analyses of conductivity impact on long range dissolution suppression

Application of not symmetrically pulse pause ratios of DC profiles

Investigations of new non-aqueous electrolyte systems (e. g. EMIM, IL)

Analyses of ECM application to W-Ta by improved electrolytes

Response / needs from discussions with industry

Testing of newest electro-mechanical tools and forced flow

Electrolyte regeneration

Integration of industries into ECM of W under patent application

Documents

Development of Electrochemical Machining (ECM) for Tungsten