Trends and Issues - Titanium Alloy use in Gas Turbines Professor Dave Rugg

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1 Trends and Issues - Titanium Alloy use in Gas Turbines Professor Dave Rugg Corporate Specialist Compressor and Nuclear Applications Royal Society Industrial Fellow Rolls-Royce plc 2010 The information in this document is the property of Rolls-Royce plc and may not be copied or communicated to a third party, or used for any purpose other than that for which it is supplied without the express written consent of Rolls-Royce plc. This information is given in good faith based upon the latest information available to Rolls-Royce plc, no warranty or representation is given concerning such information, which must not be taken as establishing any contractual or other commitment binding upon Rolls-Royce plc or any of its subsidiary or associated companies.

2 Talk Structure 2 History Load regimes / material limitations Elastic regimes Elastic-plastic regimes Plastic regimes The way forward Process / structure relationships Structure / property relationships Conclusions

3 Engineering Challenge - The Gas Turbine 3

4 Rolls-Royce Ti overview Ti Alloys account for 1/3 of gas turbine weight Over 2000 tonnes per year used by RR >$100M PA 4 Conventional alloys Form a significant part of research Improved component lifing Introduction via improved design and improved manufacturing processes Seeking improvements in buy to fly ratio (near nett shape technology) Academic interaction Extensive collaboration with Academia Direct contract Via nationally funded programmes

5 Disc alloy temperature capability Temperature Capability (C) 600 Near-α Alloys 5 High strength α-β Alloys Aerofoil fire line 350 α-β Alloys

6 Titanium in Discs 3 shaft history 6 Ti685 Ti829 Ti834 HP Increasing temperature Decreasing space = Increased Ni alloy use IP Fan

7 7 If titanium was your child - (global perspective) Approx 50 years to mature Half the typical growth rate (with respect to adoption through the engine) Cost 2,000,000 per year whilst maturing (or 50,000 per mm of growth)

8 Load regimes and material limitations 8

9 Elastic properties (Texture) 9 Fan blade untwist -arise from centrifugal loads -blades manufactured with over-twist -running tip deflection > 15mm -Tip angle controlled to < 1 Shape = Mass flow = Thrust RB211 derivative1 (8%) Trent xyz (3.5%) Trent xyz (2.8%) Trent xyz (1.8%) Engine type (property differential L to T %) Trent xyz (1.4%) Parallel rolling

10 Elastic Properties - Resonance 10 Resonance -blade design to minimise exposure and magnitude of HCF stress. -design to 10 9 cycles is common. -criteria based on stress threshold, not cyclic life. Modal frequency = HCF load = Product integrity IP compressor blade - high order resonant mode stress contour plot. Resonant frequency 18 KHz

11 Hollow Fan Technology 11 MILITARY Trent Cross-Section of Hollow Blade Matls070289P

12 Elastic Plastic properties 12 Cold Dwell Fatigue Notch Fatigue Macrozone vs Effective Structural unit size

13 Plastic properties the great divide? UTS (High rate) Stress UTS (Quasi static) Room temp props are dictated by Dislocation creep. Failure Yield 13 tensile yield relaxation Notes; In all regimes strength level is controlled by degree of micro-structural refinement. Many alloys have similar cyclic yield strength. cyclic yield Low (Hours / days) Medium (seconds) Strain rate High (milliseconds)

14 Plastic Properties contd. Medium Bird 1.5lb 5min Run-on 4lb Large Bird RB211-22B & 524 Medium Bird 1.5lb 30min Run-on 4lb Large Bird RB E4 Medium Bird 2.5lb 20min Run-on 4lb Large Bird TRENT 700 Medium Bird 2.5lb 20min Run-on 8lb Large Bird TRENT 800 Medium Bird 2.5lb 20min Run-on 8lb Large Bird TRENT Medium Bird 2.5lb - 20min Run-on 8lb Large Bird, AND new 5.5lb Large Flocking Bird 20min Run-on TRENT Also consider containment, FOD, trailing blade integrity, Surface treatments (peening) etc.

15 Titanium other criteria Disc / blade contact fatigue (crushing stress) 15 Adaibatic shear (structural unit size) Compressor blade fire (Temp / mass flow criteria) Residual stress (section size / total stress consideration) Repair (microstructural control) Compressor disc oxidation (Temp limit alpha case cracking)

16 The way forward 16

17 17 Experimental techniques and models now available (Imperial, Manchester,Oxford,Swansea) F Dunne M Bache M Preuss, J Fonseca Stress yy (MPa) time=0.01s time=0.02s Position (micron) D Dye A Wilkinson

18 Local Microtexture - cross rolled plate; 18 AX

19 Understanding and modelling properties - overview 19 What / how to measure? How accurate Do the measurements Have to be? Poly crystal representation When does strain / Slip reversal become A crack? Databank? Measure Represent Model Validate Use / interface Length scales Misorientation distribution texture SAW EBSD Beam line Aspect ratio Topography GB type / area fraction CRSS Strain rate Elastic props Failure criteria Hydrostatic stress Coherent framework is required to allow adoption by the end user. A similar logic can be applied to modelling TMP. Boundary conditions From continuum FE

20 Structural variables real world issues 20 Prior Beta Grain Billet* preheat, temperature, time and ramp rate, transfer time to press, strain and strain rate during forging, press time and hold periods * Billet itself sets starting bulk chemistry, initial partitioning, macro/microstructure and crystallography Grain Boundary Morphology Forge temperature, strain and strain rate, transfer time and media for post forge cooling Secondary Fine Alpha Transfer time and media for post solution heat treatment cooling, Ageing temperature and time Retained Beta Transfer time and media for post solution heat treatment cooling, Ageing temperature and time Primary Alpha Laths Transfer time and media on post forge cooling, Solution heat treatment temperature and time

21 Symmetry variables real world issues E = 145 Elastic and plastic asymmetry; from single crystal to real engineering (complex) structures. 21 E = 100 σ Slip Slip Slip Slip transmission function of: (i) Relative orientation of α Ι and β and β to α ΙΙ giving favourable or unfavourable alignment of preferred slip systems. (ii) Orientation angle, θ α ΙΙ (iii) Stress, either macro applied or local due to crystal plasticity. α Ι β θ (iv) (v) (vi) Length scale and slip planarity Loading rate Local chemistry, driven by alloy partitioning, changing CRSS for individual slip systems σ

22 Understanding and modelling the evolution of texture, structure and properties. 22 Important for the material supplier AND end user; Process route optimisation (large gains possible) Assessing manufacturing change and concessions Design for local area properties Advanced micro-structural standards are coming... Possible options; Physics based Statistics based Neural network Phase field In reality, a combination of techniques is probable..

23 Conclusions. 23 Some limits for titanium application defined; Max temperature in discs Max temp in aerofoils Significant gains still to be had; Process improvements Improved component lifing new manufacturing practice and repair All require detailed fundamental understanding combined with the appropriate modelling framework Joined up programmes and extensive interaction with Academia and supply chains are critical in achieving these gains.