15.10 mrs Liu
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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
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– 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…
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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)
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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
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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
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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
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• 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
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– 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
µEDM ceramics: process-material interaction
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• 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!
µEDM ceramics: process-material interaction
MRM Vs. Pulse Parameter
25
30
Non-Foamy
Mixture
Foamy
u
0 i
u
0
i
0Iso static pulse
1 µs
10 µs
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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
µEDM ceramics: process-material interaction
Themadag Mikrocentrum: microvonken 11
• 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
µEDM ceramics: process-material interaction
Spalling
Microcracks
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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
µEDM ceramics: process-material interaction
Themadag Mikrocentrum: microvonken 13
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
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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
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• Max. stress: 485 MPa
choice of material:Si3N4-TiN
Application: Ultra miniature gasturbine
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
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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
Application: Ultra miniature gasturbine
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• 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
Application: Ultra miniature gasturbine
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• 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:
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– 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:
Themadag Mikrocentrum: microvonken 20
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
Themadag Mikrocentrum: microvonken 21
– 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
Themadag Mikrocentrum: microvonken 22
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
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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
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• 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?
Themadag Mikrocentrum: microvonken 25
Questions?