15.10 mrs Liu

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

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15.10 mrs Liu

  1. 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. 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 Themadag Mikrocentrum: microvonken 2
  3. 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 Themadag Mikrocentrum: microvonken 3
  4. 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 Themadag Mikrocentrum: microvonken 4
  5. 5. Micro EDM contouring • Similar as die-sinking – Shaped electrodes by WEDG – Multiple electrodes follow same tool path until reaching the final shape Themadag Mikrocentrum: microvonken 5
  6. 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 Themadag Mikrocentrum: microvonken 6
  7. 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… Themadag Mikrocentrum: microvonken 7
  8. 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 Themadag Mikrocentrum: microvonken 8
  9. 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 Themadag Mikrocentrum: microvonken 9
  10. 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 Themadag Mikrocentrum: microvonken 10
  11. 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 Themadag Mikrocentrum: microvonken 11
  12. 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 Themadag Mikrocentrum: microvonken 12
  13. 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 Themadag Mikrocentrum: microvonken 13
  14. 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 Themadag Mikrocentrum: microvonken 14
  15. 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 Themadag Mikrocentrum: microvonken 15
  16. 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 Themadag Mikrocentrum: microvonken 16
  17. 17. Application: Ultra miniature gasturbine Themadag Mikrocentrum: microvonken 17
  18. 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 Themadag Mikrocentrum: microvonken 18
  19. 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 Themadag Mikrocentrum: microvonken 19
  20. 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 Themadag Mikrocentrum: microvonken 20
  21. 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 Themadag Mikrocentrum: microvonken 21
  22. 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 Themadag Mikrocentrum: microvonken 22
  23. 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 Themadag Mikrocentrum: microvonken 23
  24. 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 Themadag Mikrocentrum: microvonken 24
  25. 25. Thank you for your attention. Questions? Themadag Mikrocentrum: microvonken 25

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