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15.10 mrs Liu
- 1. Micro Electrical DischargeMachining of Ceramic materials Kun Liu Prof. dr. Bert Lauwers Prof. dr. Dominiek Reynaerts Afd. PMA, Department of mechanical engineering K.U. Leuven, Belgium [email protected] Themadag Mikrocentrum: microvonken
- 2. Motivation • Requirements onmicro 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. Innovation driven microEDM • 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. Micro EDM fabricationcomplex 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. 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. 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. 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. µEDM ceramics: process-materialinteraction 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. µEDM ceramics: process-materialinteraction • 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. µEDM ceramics: process-materialinteraction • 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. µEDM ceramics: process-materialinteraction 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. µEDM ceramics: process-materialinteraction 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. µEDM ceramics: process-materialinteraction 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. Micro EDM ofceramic 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. Application: Ultra miniaturegasturbine • 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. Application: Ultra miniaturegasturbine • 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. Application: Ultra miniaturegasturbine Themadag Mikrocentrum: microvonken 17
- 18. Application: Ultra miniaturegasturbine • 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. Application: Ø20 mmTurbine 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. Application: SiC microstructures • 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. Application: SiSiC heatexchanger • 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. 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. Conclusion • Micro EDMhas 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. On-going research • MicroEDM (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. Thank you foryour attention. Questions? Themadag Mikrocentrum: microvonken 25