Presentatie tijdens informatie & netwerkbijeenkomst bij het Mikrocentrum in Eindhoven over innovatie in EDM

1. Micro Electrical Discharge Machining
of Ceramic materials
Kun Liu
Prof. dr. Bert Lauwers
Prof. dr. Dominiek Reynaerts
Afd. PMA, Department of mechanical engineering
K.U. Leuven, Belgium
Kun.liu@mech.kuleuven.be
Themadag Mikrocentrum: microvonken

2. Motivation
• Requirements on micro manufacturing
processes
– Wide spectrum of functional materials
– High variety of shapes 1 mm
– High efficiency at single and small batch
production
– High accuracy
• Advantages of micro EDM
– Independent of mechanical properties
– High geometric flexibility
– Low process forces
1 mm
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3. 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…
– Technology: thermally introduced
stresses, wear…
• Increasing requirements:
tool
– Edge and corner radii
– Low surface roughness Ra < 0.1 µm
– Tolerance and shape deviation < 1 µm
workpiece
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4. Micro EDM fabrication complex shapes
Z
Dielectric ω
supply 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 100 µm
shaped electrodes
– High aspect ratio: up to 50
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5. 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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6. 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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7. EDM of ceramics
• Material should be electrically
conductive
– Guideline value: ρ < 100 ·cm
– For non-conductive ceramics
• Additional of conductive secondary
metallic phase, such as:
– TiB2, TiN, or TiC
• 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…
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8. µEDM ceramics: process-material interaction
Si3N4-TiN (Kersit®, Saint-Gobain)
Oxidized droplets
• 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:
~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
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9. µEDM ceramics: process-material interaction
• 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 33.6 µm
• Micro cracks 30.6 µm
– 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
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10. µEDM ceramics: process-material interaction
• Further modification of discharge pulse: u
– Reduced ie 0
– Prolonged te i 10 µs
– Dramatically reduced Ra: 0.25 µm is achievable! 0
MRM Vs. Pulse Parameter
Iso static pulse
30 u
Non-Foamy 0
25 Mixture 1 µs
i
discharge current ie (A)
Foamy
20
0
Relaxation pulse
15
u
10
0
2 µs
5 i
0
0
0 0.5 1 1.5 2 2.5 3 3.5 4
Modified relaxation
discharge duration te (µs)
pulse
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11. µEDM ceramics: process-material interaction
uo
Silicon infiltrated silicon carbide
(SiSiC, Saint-Gobain) ue
Sintered silicon carbide (SSiC, FCT) ie0
• High electrical resistivity: 400 ns
– SiSiC: 10 ·cm 0
(a)
– SSiC: 330 ·cm
uo=200 V, ie=8A, te=0.25 µs
• Narrow process window:
– High open gap voltage (>150 V) uo
– Sufficient large discharge current (>3 2 µs
A) ue
– Long discharge duration (>0.2 µs) and
interval 0
• Particularly for Sintered SiC
ie
– Voltage drop: elevated discharge
voltage 0
(b)
– Prolonged discharge duration uo=200 V, ie=0.7A, te=6 µs
– Reduced average discharge current
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12. µEDM ceramics: process-material interaction
Microcracks 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%
Spalling – Higher ie is more likely inducing a rougher
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
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13. µEDM ceramics: process-material interaction
For sintered SiC:
– Lower conductivity voltage drop
consumption of energy
20 µm
– Machining performances:
• Greatly decreased MRR to 0.12 mm3/min
• Comparable tool wear ratio as SiSiC (24%
at roughing and 11% at finishing)
• Smoothest surface of 0.20 µm Ra or 0.8 µm
at higher discharge energy 100 µm
– Material removal mechanisms:
Spalling
• Spalling: large temperature gradient
• High electrical resistance additional Joule
heating
• Reduced energy: melting & evaporation
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14. Micro EDM of ceramic composites
• Machining performances comparison
Energy MRR
Dielectric/ Ra
Material (mm3/min TWR (%) MRMs
tool Scale (µJ) (µm)
)
24.5 × 10³ 0.12 24 0.82 Spalling
SSiC
320 0.03 11 0.2
Melting and
400 0.65 19 1.5
SiSiC evaporation
12 0.05 12 0.43
215 0.36 1.8 2.45 Foamy surface;
Oil/WC 8 0.05 6 0.7 chemical reaction
Kersit 0.32 5.4 2.1 Non-foamy surface;
600
(Si3N4- melting and
TiN) 8 0.04 8.3 0.54 evaporation
Non-foamy; modified
16 0.003 ~20 0.25
generator
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15. 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
choice of material:
Si3N4-TiN
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16. Application: Ultra miniature gasturbine
• Developed machining strategy: die sinking
– Rouging and semi-finishing:
• Relaxation pulse type
• Faster speed, lower wear
– Finishing:
• Iso-energetic pulse
Electrode
Machining Step
• Better surface finish
time/cavity
Pulse type
Machining
• Electrodes
Regime
Undersize
electrode
(min)
No. of
(µm)
– Graphite
– Kern 3-axis milling
• 60min/electrode
1 Relax. Rough I 150 4 45
– No. of electrodes Relax. Semi-finish II
2 100 8 60
• 1 for roughing/cavity
3 Iso. Finish 25 8 40
• 1 for finishing /cavity
Total 145
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17. Application: Ultra miniature gasturbine
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18. Application: Ultra miniature gasturbine
• 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
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
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19. 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:
20 hours/cavity
– More accurate (< 2 µm)
– Lower Ra achievable with
further modified generator
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20. Application: SiC micro structures
• Ø 0.5 mm hemisphere by micro-EDM
milling
– Roughing tool Ø 0.18 mm, 3 µm cutting depth
200 µm – Finishing tool Ø 0.05 mm; 2 µm cutting depth
• 25 µm thin wall:
– Aspect ratio 25
– No deformation of geometry observed
20 µm
• Micro-EDM drilling:
– Ø 65 µm, Aspect ratio 20
– Min. Ø 30 µm, fair accuracy
20 µm 20 µm and surface integrity
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21. 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
– Features are all within
tolerance of ± 0.1 mm
– Total machining times ~ 72
hours
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22. Applications: other examples
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 6 mm Si3N4-TiN journal air
Ø 5 mm turboshaft in bearing with Ø 0.2 mm air feeing
Si3N4-TiN hole; rotates at more than
200,000 rpm
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23. 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
optimal results
• Ceramic oriented modifications on EDM machines
are necessary:
– Pulse generators
– Knowledge database
– Low or non-conductive ceramic materials
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24. 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
• 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
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25. Thank you for your attention.
Questions?
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