15.10 mrs Liu

Micro Electrical Discharge Machining of Ceramic materials

Themadag Mikrocentrum: microvonken

Kun LiuProf. dr. Bert Lauwers

Prof. dr. Dominiek ReynaertsAfd. PMA, Department of mechanical engineering

K.U. Leuven, [email protected]

Motivation

• Requirements on micro manufacturing processes– Wide spectrum of functional materials– High variety of shapes– High efficiency at single and small batch

production– High accuracy

1 mm

Themadag Mikrocentrum: microvonken 2

– High accuracy

• Advantages of micro EDM– Independent of mechanical properties– High geometric flexibility– Low process forces

1 mm

Innovation driven micro EDM

• Process conditions: Limitations from – Environment: temperature, vibrations…– Machine tool: positioning accuracy,

stiffness and high dynamics, clamping systems…

– Processes: dielectric fluid, hydrodynamic forces, electrostatic forces…


forces, electrostatic forces…– Technology: thermally introduced

stresses, wear…

• Increasing requirements:– Edge and corner radii– Low surface roughness Ra < 0.1 µm– Tolerance and shape deviation < 1 µm

tool

workpiece

Micro EDM fabrication complex shapes

Z

ωDielectricsupply Rotating clamping

device

Electrode (W / WC / Cu)


Wide material choice– Metals– Highly-doped semiconductors– Conductive ceramics & WC

WEDG: wire electrode discharge grinding

– Accurate and axi-symmetrical shaped electrodes

– High aspect ratio: up to 50

100 µm

Micro EDM contouring

• Similar as die-sinking– Shaped electrodes by WEDG– Multiple electrodes follow same

tool path until reaching the final shape


Micro EDM milling

• Similar as conventional micro milling• Electrode:

– Rods or tubes– No WEDG required

• Thin layers electrode geometry retained• Require accurate tool length compensation• Possible staircase effects on sidewalls


EDM of ceramics

• Material should be electrically conductive– Guideline value: ρ < 100 Ω·cm– For non-conductive ceramics

• Additional of conductive secondary metallic phase, such as:

– TiB2, TiN, or TiC

• Increased hardness and strength


• Increased hardness and strength• Toughness remains however

modest– Available commercial electro-

conductive ceramics• Commercially: Si3N4-TiN, SiSiC,

TiB2, B4C…• Lab-scale: Al2O3-TiN, ZrO2-TiN,

Si3N4-TiB2, ZrO2-WC…

Si3N4-TiN (Kersit®, Saint-Gobain)

• µEDM machining performance is discharge pulse shape dependable

• Relaxation pulses:– Short duration: ns-range– Machining speed:

0.4 mm3/min; ≈ 3x stainless steel– Tool wear ratio:

Oxidized droplets

µEDM ceramics: process-material interaction


– Tool wear ratio: ~1.8 %; ≈ 10x less comparing to steel

– Material removal mechanisms:Mainly chemical reactions• Decomposition: both Si3N4 and TiN• Oxidation: particularly water dielectric

– Surface quality • Limited achievable Ra ~0.7 µm• Foamy and porous surface topography

– Generation of large amount of N2 gas bubbles• Regardless dielectric material

• Iso static discharge pulses– Longer duration: tens of µs or ms

• Medium MRR, ~0.3 mm3/min• Higher TWR, ~5.6 %

– Surface quality:• More regular craters• No trace of porous or foamy layer• Micro cracks

33.6 µm30.6 µm



• Micro cracks– Material removal mechanisms:

• Melting and evaporation• Surface Optimization

– Reduced energy input• Limitations on machine parameter modification

• Smaller splash crater size

– Surface quality improvement to an extent– Minimum obtainable Ra is 0.55 µm

30.6 µm

• Further modification of discharge pulse:– Reduced ie– Prolonged te– Dramatically reduced Ra: 0.25 µm is achievable!


MRM Vs. Pulse Parameter

25

30

Non-Foamy

Mixture

Foamy

u

0 i

u

0

i

0Iso static pulse

1 µs

10 µs


0

5

10

15

20

0 0.5 1 1.5 2 2.5 3 3.5 4

discharge duration te (µs)

disc

harg

e cu

rren

t ie

(A)

Foamy i 0

Relaxation pulse

Modified relaxation pulse

2 µs

u

0

i

0

Silicon infiltrated silicon carbide (SiSiC, Saint-Gobain)

Sintered silicon carbide (SSiC, FCT)

• High electrical resistivity:– SiSiC: 10 Ω·cm– SSiC: 330 Ω·cm

• Narrow process window:

uo

0 ie

0

400 ns

(a)

ue

uo=200 V, ie=8A, te=0.25 µs



• Narrow process window:– High open gap voltage (>150 V)– Sufficient large discharge current (>3

A)– Long discharge duration (>0.2 µs) and

interval• Particularly for Sintered SiC

– Voltage drop: elevated discharge voltage

– Prolonged discharge duration– Reduced average discharge current

2 µs

uo

0

ie

0 (b)

ue

uo=200 V, ie=0.7A, te=6 µs

For SiSiC:• Machining performances:

– High MRR up to 0.57 mm3/min with TWR of 19%

– Minimum Ra 0.4 µm for MRR 0.03 mm3/min and reduced TWR of 12%

– Higher ie is more likely inducing a rougher surface


Spalling

Microcracks


surface

• Material removal mechanisms:– High discharge energy unstable

process: • Spalling• Thermal shock• Large amount of micro cracks along

boundaries of grains– Melting and evaporation are dominant

For sintered SiC:– Lower conductivity voltage drop

consumption of energy – Machining performances:

• Greatly decreased MRR to 0.12 mm3/min • Comparable tool wear ratio as SiSiC (24%

at roughing and 11% at finishing)

20 µm



at roughing and 11% at finishing)• Smoothest surface of 0.20 µm Ra or 0.8 µm

at higher discharge energy

– Material removal mechanisms:• Spalling: large temperature gradient• High electrical resistance additional Joule

heating• Reduced energy: melting & evaporation

100 µm

Spalling

Micro EDM of ceramic composites

• Machining performances comparison

MaterialDielectric/

toolEnergy

Scale (µJ)

MRR (mm 3/min

)TWR (%)

Ra (µm)

MRMs

SSiC24.5 × 10³ 0.12 24 0.82 Spalling

320 0.03 11 0.2Melting and evaporationSiSiC

400 0.65 19 1.5


Oil/WC

evaporationSiSiC12 0.05 12 0.43

Kersit(Si3N4-

TiN)

215 0.36 1.8 2.45 Foamy surface; chemical reaction8 0.05 6 0.7

6000.32 5.4 2.1 Non-foamy surface;

melting and evaporation8 0.04 8.3 0.54

16 0.003 ~20 0.25Non-foamy; modified

generator

Application: Ultra miniature gasturbine

• Output 1-2 kW• Total size: Ø 95 x 120 mm• Impellers: Ø 20 mm• Speed: 500,000 rpm• Pressure ratio: 3• Temperature > 1200 K• Max. stress: 485 MPa


• Max. stress: 485 MPa

choice of material:Si3N4-TiN


Mac

hini

ng S

tep

Pul

se ty

pe

Electrode

Mac

hini

ng

time/

cavi

ty

• Developed machining strategy: die sinking– Rouging and semi-finishing:

• Relaxation pulse type• Faster speed, lower wear

– Finishing: • Iso-energetic pulse• Better surface finish


Mac

hini

ng S

tep

Pul

se ty

pe

Reg

ime

Mac

hini

ng

time/

cavi

ty(m

in)

Und

ersi

ze

(µm

)

No.

of

elec

trod

e

1 Relax. Rough I 150 4 45

2 Relax. Semi-finish II 100 8 60

3 Iso. Finish 25 8 40

Total 145

• Electrodes– Graphite– Kern 3-axis milling

• 60min/electrode– No. of electrodes

• 1 for roughing/cavity• 1 for finishing /cavity



• Quality control– Moderate obtained surface roughness

• ~0.82 µm Ra

– CMM measurements (Mitutoyo FN 905)• 1200 measuring points per cavity

(pressure, suction and hub surface)• Fully symmetrical• Small error at the tip of shroud and



• Small error at the tip of shroud and suction surface

• Testing– Simplified set-up

• No generator• No combustion chamber• Driven by compressed air

– Cold spin already at 240,000 rpm• No defects so far

Application: Ø20 mm Turbine Impeller

• Micro EDM milling: Sarix– WC rod electrode– Layer-by-layer milling (3 ~ 8 µm)

• Properties:– No electrode preshape required– Slow EDMing:


– Slow EDMing: 20 hours/cavity

– More accurate (< 2 µm)– Lower Ra achievable with

further modified generator

Application: SiC micro structures

200 µm

• Ø 0.5 mm hemisphere by micro-EDM milling– Roughing tool Ø 0.18 mm, 3 µm cutting depth– Finishing tool Ø 0.05 mm; 2 µm cutting depth

• 25 µm thin wall:


20 µm

20 µm

20 µm

• Micro-EDM drilling:– Ø 65 µm, Aspect ratio 20– Min. Ø 30 µm, fair accuracy

and surface integrity

• 25 µm thin wall:– Aspect ratio 25– No deformation of geometry observed

Application: SiSiC heat exchanger

• Heat exchanger:– Ribs, deep cavity, and

chamfers– 2 electrodes for roughing, 1 for

semi-roughing and finishing each

– Small corner radius


– Small corner radius– Features are all within

tolerance of ± 0.1 mm– Total machining times ~ 72

hours

Applications: other examples

B4C nozzle with a spray

Ø 1 mm miniature gear wheel in AlN-TiN


B4C nozzle with a spray hole of Ø 0.7 mm

Ø 6 mm ZrO2-TiN aerodynamic thrust

bearing

Ø 5 mm turboshaft in Si3N4-TiN

Ø 6 mm Si3N4-TiN journal air bearing with Ø 0.2 mm air feeing

hole; rotates at more than 200,000 rpm

Conclusion

• Micro EDM has proved to be a versatile production technique for the machining of micro structures– Accurate– Cost effective

• For ceramic micro EDM, it is important to understand the “process-material” interaction to achieve the most


the “process-material” interaction to achieve the most optimal results

• Ceramic oriented modifications on EDM machines are necessary:– Pulse generators– Knowledge database– Low or non-conductive ceramic materials

On-going research

• Micro EDM (milling)– Broadened ceramic materials:

• Al2O3-based, ZrO2-based, TiB2, …

– Pulse analyze on ceramic composites– Factors contribute to the wear compensation

• On-line correction/modification


• On-line correction/modification• Improving the machining efficiency without losing

accuracy

• Macro EDM (die sinking)– Follow-up of PowerMEMS project

• SiC turbine impellers by die-sinking for further testing

– Developing ceramic materials for EDM

Thank you for your attention.

Questions?


Questions?

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15.10 mrs Liu