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Markus Aicheler, Ruhr-University Bochum and CERN

Markus Aicheler, Ruhr-University Bochum and CERN “Surface phenomena associated with thermal cycling of copper and their impact on the service life of particle accelerator structures”. Outline of the talk. Introduction into the project in the frame of CLIC Main goals of the PhD thesis

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Markus Aicheler, Ruhr-University Bochum and CERN

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  1. Markus Aicheler, Ruhr-University Bochum and CERN “Surface phenomena associated with thermal cycling of copper and their impact on the service life of particle accelerator structures”

  2. Outline of the talk • Introduction into the project in the frame of CLIC • Main goals of the PhD thesis • Experimental: Material and Fatigue devices • Discussion of 3 results • Hardening threshold of Cu [100] single crystal • Orientation dependent cyclic roughening • Orientation dependent cyclic hardening/roughening • Summary and Conclusion

  3. Introduction: CLIC surface heating phenomenon CLIC (Compact Linear Collider) two beam scheme: Electron – positron collider at center-of-mass energy of 3 TeV (LHC: 7 TeV but nonelementar head on collisions)

  4. Introduction: CLIC surface heating phenomenon • CLIC accelerating structure (AS): • Shape accuracy ± 2.5 µm • Roughness Ra 0.02 µm • Very high conductivity material bolting Assembly by: brazing

  5. Introduction: CLIC surface heating phenomenon a) b) Surface a) magnetic and b) electric field distribution in CLIC AS cell • • Pulsed magnetic field induces currents (200 ns, repetition rate 50 Hz) • • Superficial Joule heating for electrical conductivity of copper: ΔT ≈ 60 K • cyclic heating- and cooling phases (biaxial strain) • thermal fatigue with σ ≈ 0 MPa to 150 MPa (comp.) • skin depth several µm • surface roughness degrades operation conditions  “functional fatigue” Estimated CLIC life time 2 x 1010cycles @ 50Hz (= 20 years of operation) => No mean to test a “real” structure under “real” conditions for whole life time!

  6. Main goals of the thesis • understand the basic mechanism of fatigue observed when low loads induced by very superficial cyclic heating are applied to copper alloys • put them in relation with the conventional fatigue induced by bulk cyclic loads • determine if superficial pulsed laser and bulk ultrasonic fatigue tests may be extrapolated for selection of a best candidate material for the application to CLIC structures “Study of surface thermo-mechanical fatigue phenomena applied to materials for CLIC accelerating structures” PhD program, Markus Aicheler

  7. Experimental: Observation material • C10100 (OFE Copper) • Reference material • Well known • Results comparable to other researchers • Supplementary fatigue data needed (CuZrwell tested by predecessor) • 40% cold worked • Round bar cold rolled Ø40 mm and Ø100 mm • Yield Strength: Rp0.2 = 316 MPa • Ultimate tensile strength: Rm = 323 MPa • Average grain size: Ø110 µm • Relevance: state with best properties • Brazed • Heat treatment in vacuum furnace: • 300 K/h -> 795 °C; 60 min hold • 100 K/h -> 825 °C; 6 min hold • Natural cooling in vacuum • - Yield Strength: Rp0.2 ≈ 72 MPa • Ultimate tensile strength: Rm = 270 MPa • Average grain size: Ø400 µm • Relevance: state after brazing assembly • 2h@1000 °C • Heat treatment in vacuum furnace: • 300 K/h -> 1000 °C; 120 min hold • Natural cooling in vacuum • - Yield Strength: Rp0.2 ≈ 72 MPa • Ultimate tensile strength: Rm = 257 MPa • Average grain size: Ø1400 µm • Relevance: state after bonding assembly

  8. Experimental: Conventional fatigue test (CVF) • - Mechanical fatigue; R = -1 (R = σmin /σmax) • UTS electro-mechanical universal-test machine • Repetition rate 0.5 Hz • Tested in loads up to +/-250 MPa; stress controlled • Sample shape conform ISO 12106 • 3-5 samples for one data point • Damage criterion: rupture 2 mm

  9. Experimental: Ultrasound swinger device (USS) • - Mechanical fatigue; R = -1 (R = σmax/ σmin) • Piezoelectric resonant attenuator • Repetition rate 24 kHz • Cycles: 2 x 1010 • σmax = +/-60MPa ε = 6 x 10-4 • Samples: special designed sonotrodes

  10. Experimental: Laser fatigue device (LAF) • - Thermal fatigue through irradiation • OPTEX Excimer Laser; λ = 248 nm • Repetition rate 200 Hz • Pulse length: 40 ns • - 5 x 104 shots @ 0.3 J/cm2 • ΔT = 280 K εtot = 7 x 10-3 • Round disc diameter 40 mm • 25 discrete spots per disc

  11. Experimental: SLAC RF heating device (Stanford) • - Thermal fatigue due to RF heating • Mushroom cavity @ 11,4 GHz • Repetition rate 60 Hz • Pulse length 1.5 µs • 1 x 107Pulses @ 50 MW • ΔTmax = 110 K  εtot= 1.8 x 10-3 • Round disc diameter 100 mm • Continuous radial distribution of ΔT ΔT r

  12. 1st result: Hardening threshold of Cu [100] single crystal ΔT r • 107 Pulses • ΔTmax = 110 K  εtot = 3.13 x 10-3 • Radial micro hardness distribution

  13. 1st result: Hardening threshold of Cu [100] single crystal Threshold of cyclic temperature rise for hardening (58 K) Courtesy of KEK

  14. 1st result: Hardening threshold of Cu [100] single crystal ΔH / Δεcycl.max= 1.83 x 104 HV/1 Thresholdofcyclicstrainforhardening 1.7 x 10-3

  15. 2nd result: Orientation dependent surface roughening - 5 x 104 shots @ 0.3 J/cm2 -ΔT = 180 K -εtot,cycl = 5.13*10-3

  16. 2nd result: Orientation dependent surface roughening [1 0 0] [1 1 1]

  17. 2nd result: Orientation dependent surface roughening - 5 x 104 shots @ 0.3 J/cm2 -ΔT = 180 K -εtot,cycl = 5.13*10-3

  18. 2nd result: Orientation dependent surface roughening [1 0 0] [1 1 0]

  19. 2nd result: Orientation dependent surface roughening true surface projected surface Surface index = Rz

  20. 2nd result: Orientation dependent surface roughening 1. Isotropic thermal expansion causes different shear stresses (anisotrope moduli) 111/ 100 = 1.51 (Thesis Reiner Mönig) 110/ 100 = 1.60 maximum resolved shear stress as a function of out-of-plane grain orientation in Cu due to an equibiaxial in-plane strain of 0.1% and zero out-of-plane stress 2. Different Schmid factor configurations on slip systems (local strain) [1 0 0]: 8 Systems active [1 1 1]:6 Systems active [1 1 0]: 4 Systems active with Smax = 0.408 with Smax = 0.272 with Smax = 0.408 σ τ • High number of slip systems • lower local strain a) Straining of a body with ΔL. Illustration of local strain in slip system with b) low and c) high Schmid factor • High Schmid factor • lower local strain Schmid factor S=τ/σ

  21. 3rd result: Orientation dependent hardening/roughening - 5 x 104 shots @ 0.3 J/cm2 -ΔT = 180 K -εtot,cycl = 5.13*10-3 [1 1 0] [1 0 0] Micro hardness indents Micro hardness indents in fatigued surface irradiated area non irradiated area Hardness increase: [1 0 0]: 49 HV -> 58 HV (+17%) [1 1 1]: 49 HV -> 65 HV (+32%) [1 1 0]: 47 HV -> 68 HV (+44%)

  22. 3rd result: Orientation dependent hardening/roughening • Initially similar roughness and sligthly different hardness • Same notch free surface • Very different roughening / hardening behaviour • The rougher, the harder! • Linear relation of hardening and roughening • Indication of fundamental link between both mechanisms • Offset of hardness • Indication of microstructural activitybefore roughness detectable on surface • Hardness more sensitive criteria

  23. Summary and Conclusion Laser fatigue RF fatigue USS fatigue • Summary of Thesis • Test campaign on different states of OFE copper with 4 different fatigue devices • Phenomenon of orientation dependent roughening/hardening identified • Influence of grain boundaries identified (not shown here) • Influence of initial hardness identified (not shown here) • Results obtained and phenomena observed allowed to compare different fatigue techniques and to make a suggestion for the best material candidate for CLIC accelerating structures.

  24. Summary and Conclusion • Conclusions • Grain boundaries start to play important role in fine structures (grain sizes 1 µm - 5 µm). High local stresses arising from the effect of anisotropy of moduliare averaged out. • The [1 0 0] crystallographic orientation of surface grains shows the smallest amount of surface roughening and sub-surface hardening. • Copper materials with high initial hardness show no further cyclic strengthening, while significant cyclic hardening accompanied cycling of soft material states. • Results obtained by mechanical techniques cannot be directly related to thermal fatigue data. • Possible material candidates for the CLIC accelerating structure: • 1) A strongly textured and fine grained OFE copper, e.g. equal-angular-channel-pressed (ECAP) OFE copper (currently fabricated up to Ø 50 mm) • 2) A strongly [1 0 0] orientation textured pure copper thin film (observed and looks promising!)

  25. Acknowledgements • Prof. Eggeler and Dr. Sgobba • Prof. Theisen • CERN andespeciallythe CLIC study • All my collegues and friends at RUB and CERN • My parents • My better half: Anne-Laure

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