Post on 03-May-2017
Materials in Jet Engines:Past, Present, and Future
Robert SchafrikGeneral Manger, Materials & Process Engineering
GE Aircraft Engines
Slide 1
Overview
Introduction
Highlights of Key Developments
Materials in Aero Engines
Future Directions
Summary and Take Aways
INTRODUCTION
Slide 3
We Have Come a Long Way!
GE90-115B
Engine SpecificationsBore: 4 inches
Stroke: 4 inches
Displacement: 201 cubic inches
Compression Ration: 4.7:1
Horsepower: 25 hp
Cooling: Liquid Circulated by thermo-siphon and radiator
Lubrication: Splash system, circulation by pump and gravity
Dry Weight: 180 pounds
SpecificationsThrust Class (lb) 115,300Length (in) 218Bypass Ratio 7.1Pressure Ratio 42.2
Slide 4
Jet Propulsion Beginnings
Sir Frank Whittle• Original Patent on Jet Engine filed January, 1929• First flight engine: Power Jets W-1
– Flew in British Gloster G-40, May 15, 1941• Came to GE to scale-up jet engines
Hans von O’Hain• Worked in secret for German military• First demo engine: S-1, 1937, burned hydrogen gas• First flight engine: Heinkel S-3B
– Flew in Heinkel 178 airplane, Aug 27, 1939
Slide 5
Power Jets Whittle W-1A
Slide 6
Commercial High By-Pass Ratio Engine
Low Pressure Turbine
High Pressure Turbine
Combustor
High Pressure Compressor
Fan
CoreAir
Low Pressure Compressoror Booster
Slide 7
Drivers for Advancing AeroTurbine Technology
Modern World Expectation: Freedom to Travel
Anywhere
• Quickly
• Inexpensively
• Safely
National Defense Needs
• Push limits of technology
• High Reliability
Slide 8
50 years … of turbine engine improvements
Flight Safety(accidents per MFH)
1940 1960 1980 2000
90%Improvement
Thrust to Weight
1940 1960 1980 2000
350%Increase
1940 1960 1980 2000
Fuel Efficiency(SFC)
45%Improvement
1940 1960 1980 2000
Engine Noise(cum db’s)
35 dbDecrease
Slide 9
Conceptual Cycles and Temperatures
Cruise
ClimbHSCT
(Future)HSCT
(Future)
Land
Take-off
Existing Sub-sonicExisting Sub-sonic
Cruise
ClimbT41
Time
HIGHLIGHTS OF KEY M&P DEVELOPMENTS
Slide 11
Improved Engine Materials
Proc
ess
Con
trol
& N
DE
NewNewMaterialsMaterials
Mat
eria
ls
mpo
sitio
n
Proc
essi
ng
Co
Ref: Prof James C. Williams, Ohio State University
Improving Engine Materials RequiresMuch More Than Alloy Development
Slide 12
Interplay of Process and Alloy Development
Titanium
Stainless Steel
CobaltNickel Superalloys
Polymer Matrix Composites
Thermal Barrier Coatings
Vacuum Induction Melting
Arc Melting
Investment Casting of Complex Shapes
Powder Metal Superalloys
Turbine CoatingsTIM
E
Directionally Solidified and Single Crystal Airfoils
Multiple Vacuum Melting Cycles
Intermetallics
Ceramic Matrix Composites
1950
s19
60s
1970
s19
80s
1990
s20
00s
EB-PVDLarge Structural Castings
Iso-Thermal Forging
SiC Melt Infiltration
Laser Deposition
Slide 13
Important Developments
Vacuum Melting
Nickel-based Superalloys
Titanium
Investment Casting
Forging
Vacuum Melting
Slide 15
Vacuum Melting
Superalloy age really commenced with Vacuum Induction Melting about 1950• Commercial pumps able to sustain 10µ vacuum level
• Vacuum sealing technology greatly improved leak-down rate
Eliminated detrimental trace and minor elements• Allowed addition of reactive elements to the melt
Slide 16
52100 Bearing SteelVIM replaced air melt electric furnace
• Steel properties had varied widely due to oxide inclusions
– Led to many bearing failures
But VIM 52100 suffered from rarely occurring,
randomly distributed exogenous ceramic inclusions
• Early failure in a few bearings -> infant mortality
• Source: erosion of furnace liner, weir, and gating
Important lesson learned:
• Exceptionally deleterious defects occurring at low frequency
Slide 17
VARFirst disclosed as process for melting in 1839
Became follower of VIM in premium quality nickel and iron alloy formulation • Unique chemistry control best in VIM
VAR ingots have higher bulk density than VIM• Macrostructure managed via solidification control
Premium Quality 52100• VIM-VAR dispersed exogenous inclusions
• Eliminated infant mortality problem
Slide 18
Development Risk Assessment Map
A D
Impact of Defect Occurrence
Prob
abili
ty o
f Def
ect
Occ
urre
nce
Example: Forging grain size slightly out of specification
Example: Hard alpha in wrought titanium
Example: Quench cracking of hardenable superalloy
HIGHLOW
LOW
Example: Low angle grain boundaries in single crystal castings
Defects that occur sporadically, causing negligible harmAccommodate by changes to design practice and/or specifications
Defects that occur very infrequently, and are exceptionally deleterious to component performanceRigorous attention to all elements of the process, or an entirely new process, is required
process
Defects that occur frequently, causing slight component detrimentReduce frequency to Zone A by process control changes
B CH
IGH
Defects that occur often, and are quite deleterious to component performanceReduce or eliminate by process control changes or change to an improved
Nickel-based Superalloys
Slide 20
Overview
First Jet Engines Employed Stainless Steels
• Temperature Limitations of these materials led many to
question commercial viability of jet propulsion
Success: Several excellent heat resistant alloy families
implemented during the 1950s
• Nimonic Series in Great Britain
• Tinidur Alloys in Germany
• Inconel Alloys in US
Slide 21
Early Nickel-based Superalloys
Superalloys truly enabled efficient, practical gas turbines• Outstanding strength…tensile, creep, fatigue
• Excellent ductility and toughness
• High Temperature Capability, to 0.75 solidus temperature
1950s• Chemistry changes and melting improvements
– Derivatives of oxidation resistant rotor stainless steels
• Addition of Al and Ti opened age of superalloys
– Gamma-prime (γ', [Ni3Al]) highly effective strengthener
– Stable at high temperature
– Coherent precipitate
Slide 22
Highlight: Alloy 718
IN718 introduced by Huntington Alloys in 1960
• Key precipitation phase: γ" [Ni3Nb]
– Effective strengthener, high tensile strength
– Not quite temperature capability of γ‘ alloys
– Slower precipitation kinetics allowed improved processing
and welding
– Excellent balance of properties, reasonable cost, readily
castable and forgeable
Slide 23
Superalloy Progress: 1970s, 1980s, 1990sProgress often chaotic and undisciplined• Much work done in secret, proprietary fashion
Excessive alloying additions led to precipitation phase instability• Gradual but persistent TCP formation during service exposure
Need separation between hardening phase solvus and alloy MP• At least 30ºC
• Permits re-solutioning and re-precipitation of the strengthening phase
• Limits amount of strengthening elements that can be added
Alloys can be tailored for specific environments, such as oxidation resistance• Trade-off for some other desirable property
Alloy compositions possessing the “best” properties not always producible in the required shape due to processing limitations
Titanium
Slide 25
Why Titanium?
Slide 26
Fan Blades and DisksProperties Considered• Tensile strength
– Load carrying capability…disk burst strength
• High cycle fatigue– Blade resistance to airflow stimulus
• Low cycle fatigue– Life capability of blade dovetail and disk critical locations
• Impact Strength– Airfoil Foreign Object Damage (FOD) resistance
• Damage tolerance…crack growth rate & threshold– Ability to accommodate metallurgical/mechanical anomalies
• Elastic modulus– Blade deflection & HCF stimulus
• Density– Strength to weight ratio
• Environmental resistance– Erosion
Alloy Ti Al V Cr Mo Zr SnTi-64 Bal. 6 4Ti-17 Bal. 5 4 4 2 2Ti-811* Bal. 8 1 1 * Blades Only
Chemical Composition of Fan Disk/Blade Alloys
Slide 27
Processing Temp Effect - Ti-17Processing Temp Effect - Ti-17
•Higher tensile duct.•Higher toughness•Better LCF Life•Lower Crack Growth
β transus -25 ºC > β transus
Slide 28
Challenges of TitaniumHydrogen Embrittlement• Brittle fracture at less than design load minimum
– Caused by migration of occluded hydrogen to tensile stress concentration• Mitigation: designing chemical and thermal processes to prevent introduction of
hydrogen into titanium componentsAnode “drop in”• Introduction of tungsten into the melt during non-consumable VAR• Mitigation: Consumable electrode VAR
Hot Salt Stress Corrosion• Alloys with high alpha phase content most susceptible• Mitigation: Avoid use of susceptible alloys at elevated temperatures
Alpha Case formation• Formation of brittle oxygen-rich surface layer• Mitigation: Heat treat titanium in vacuum or chemical mill after heat treatment to
remove the contaminated layerDwell Time Fatigue• Creep-fatigue interaction that substantially reduces fatigue life
– Occurs at sustained (dwell) loads at relatively low temperatures (200°C)– Susceptible alloys: Creep-resistant, forged alloys with highly textured alpha
phase– Mitigation: Modify thermo-mechanical processing to avoid textured alpha
phase microstructure
Slide 29
Melting TitaniumMolten titanium is very reactive• Cannot be melted in a VIM furnace
– Reacts with refractory lining– Cannot be contained in metal crucibles
Melting and synthesis of titanium made practical arc melting in a water-cooled copper crucible• Molten titanium is contained by a thin layer of titanium that solidifies on the
cooled copper wallsInfrequent undermining and spalling of tungsten non-consumable electrode caused a Zone D defect• Abated by Radiographic inspection• Eventually eliminated by consumable electrode VAR
– Electrode made from the material being meltedCold Hearth Melting…an important new process technology• Increased residence time of the input material in the molten pool
– Dissolving high interstitial defects (nitrogen, oxygen , or carbon- rich)– Trapping high density inclusions in the skull– Producing an ingot with minimum solute segregation– CHM is currently followed by a final VAR step to remove various process-
related conditions – Initial VAR melts are typically followed by 2 additional VAR melts, each done
under somewhat different processing conditions to provide additional refining capability and to improve the macrostructure of the ingot
Slide 30
EBM (Electron Beam Cold Hearth Melting)EBM (Electron Beam Cold Hearth Melting)Electron Beam
Power Input
Ingot MoltenPool
MeltingHearth
RefiningHearth
Ingot Being
Withdrawn
Acknowledgement:THT patented hearth design
Slide 31
Challenges of TitaniumType I Defect • High Density Inclusions--Stabilized hard, brittle particles
– Result from reactivity of titanium: Titanium nitride, tungsten carbide
• Mitigation: Cold Hearth Melting and Ultrasonic inspection
Type II Defect• Segregation of elements during solidification
– Reduce fatigue life
• Mitigation: Improved process control during melting & Ultrasonic inspection
Self-sustaining Titanium Fires• Fires ignited by high contact stress rub against a titanium structure
– Occurs under conditions of elevated temperature and pressure, and high mass flow
• Mitigation
– Coating titanium structure in susceptible regions to minimize effect of a rub
– Development of improved burn-resistant alloys
Slide 32
Extrinsic Melt Related DefectsExtrinsic Melt Related Defects
High Density Inclusion(W rich inclusion)
Hard Alpha(N rich inclusion)
Investment Casting
Slide 34
Investment Casting
Casting found extensive application
• Reduce manufacturing cycle time and cost
• Acceptable quality and strength levels
• Enabled design of components with:
– Lower weight and part count
– Eliminating welds and associated preps, inspections
Slide 35
Progress in Investment CastingFirst application of a casting on a rotating part occurred in the 1950s
when a solid turbine airfoil was investment cast
• Required processes to reduce casting defects that limited strength
• Driving force for casting was increased complexity in airfoil design
– Internal cooling air passages
– Later, it was discovered that airfoils could be cast as single crystals
Improved casting of large structural components
Challenges
• Maintaining thermodynamic stability of complex superalloys
• Accommodating ductility trough (650ºC – 760ºC ) during processing
Slide 36
Processing Advantage
GE90 Turbine Rear Frame
Castability and Weldability of Alloy 718 enables application of complex cast structures
Slide 37
Turbine Air Foil Casting Processes
Equiaxed (EQ) Dir. Sol. (DS) Single Xtal (SX)
Slide 38
Complexity of Airfoil Castings
Slide 39
Thermal Barrier Coatings
Key TBC Features:• Columnar structure in top coat for spall resistance• Oxidation resistant and adherent bond coat• Bond coat compatible with alloy substrate
Ceramictop coat
Bond coat
Turbine blade
Hot Gas
Forging
Slide 41
Progress in Forging
Evolution from hammer forging to press forging
• Enabled forging of large, complex shapes (Disks)
– Part-to-Part uniformity of properties
Isothermal forging (Superplastic)
• High strength superalloy powder billets
– Eliminate strain-induced cracking
• “Clean” powder
• Molybdenum TZM die material
• Controlled slow strain rate forging
Slide 42
Ladish 10,000 Ton Isothermal Press
High Reliability
Slide 44
Elements of High ReliabilityNon Destructive Evaluation• Locate defects• Surface NDE Methods
– Visual, Smoothness, Replication, Dye Penetrant• Near-Surface NDE Methods
– Eddy current, Magnetic particle• Sub-surface NDE Methods
– Radiography, Ultrasonic Life Prediction• Estimate component life based on aircraft engine mission profile and
material damage mechanisms– Low cycle fatigue, thermal fatigue, oxidation, hot corrosion, inter-
diffusion, creep, plus interactions of these mechanismsPremium Quality Melting• Multiple melting steps required to eliminate defects
– Reproducible properties require defect-free metal
Slide 45
Premium Quality Melting of Nickel Alloys
VIM ESR VAR
Remove Inclusions
Control Macrostructure
Formulate Composition
Triple Melt Key for High Reliability Components
Slide 46
Premium Quality Melting of Titanium Alloys
CHM VAR
-Dissolve High Interstitial Defects-Trap High Density Inclusions-Minimize Solute Segregation
Remove Various Process-related
Conditions
USE of NICKEL in AEROENGINES
Slide 48
Alloy 718 Introduction1950’s
Turbine manufacturers primarily relied upon: • Precipitation-strengthened stainless steels (i.e., A286) • γ′−strengthened Ni-base superalloy, such as René 41
Late 1950'sReached limits of the stainless steelsFabrication limits of René 41
1960Huntington Alloys-INCO introduced Alloy 718• Significantly improved ease of manufacture • Mechanical properties that approach René 41• Interest from several GE engine programs
Slide 49
Material Usage
Relative Input Weights For a CF6 Engines
Al-base8%
CF6 Material By Finished Weight
71834%
Other Ni-base13%
Ti25%
Fe-base16%
Powder0%
Composites4%
Forging82%
Sheet12%
Cast6%
CF6 Material By Form
Slide 50
Metals Used in All Forgings for CY 2000
Alloy 71856%
Other Ni18%
PM5%
Titanium9%
Aluminum5%
Fe-base6%
Co-base1%
Alloy 718 represents 56%of the forgings at GEAE
Future Directions, Summary and Take
Aways
Slide 52
Large Fan Blades
Hollow Ti and PMC’s in use• Necessary for large engines
• Both presently in service
• New PMC’s make this possible
• PMC’s gain benefit with size
• Both expensive to make
• Life cycle costs may differ
• Both different than solid blades
Neither TechnologyPossible 15 Years Ago
Slide 53
Composite and Titanium Fan Ducts
Composite
Ti alloy
Composite Duct• Carbon fiber + PMR15• Filament wound Tow-preg• Wt: 23% less than Ti• Cost: 28% less than Ti
Ti Duct• Ti-6Al-4V• Wrapped & welded• Chemical milled grid
New Manufacturing Technology Makes PMC Ducts Attractive
Slide 54
Future Directions to Improve M&P
Alloy development• Disk alloys
– Higher T capability, better damage tolerance– E.G., alloy with more temperature capability than 718
• Turbine blade alloys– Higher T capability
• Layered structures, hybridized components
Processing• Reduce variation in processing• Closer marriage of materials and process technology• Improved process control to eliminate rework and scrap • Reduced cost
Slide 55
Typical Development Times for MaterialsI. Modification of an existing material for a non-critical component
– Approximately 2-3 years
II. Modification of an existing material for a critical structural component– Up to 4 years
III. New material within a system that we already have experience– Up to 10 years
– Includes time to define the chemistry and the processing details
IV. New material class– Up to 20 years, and beyond
– Includes the time to – Develop design practices that fully exploit the performance of the material
– Establish a viable industrial base
GRAND CHALLENGEDrastically Reduce Development Times for New Materials
…While Reducing Risk!
Slide 56
Fundamental Challenge
How can the Materials Community best contribute to achieving an improved aero-engine in a timely way
New Materials Development
Business NeedRisk
Development Cost Technical Maturity
Slide 57
Vision
Today Future
Iden
tifie
d R
isk
Res
ourc
esTime
Iden
tifie
d R
isk
Res
ourc
es
Time
Evolutionary Materials
Advanced Materials
Slide 60
SummaryMaterials have enabled progress in aero-engines
• Materials and Design engineers have both benefited from
ongoing game of “leapfrog”
High introductory cost of new M&P offset by compelling
customer benefit
Continuing challenge of exceptionally deleterious
defects occurring at very low frequencies
• Significantly influences M&P development of high integrity
structural materials
Slide 61
Summary
Each gain in an alloy property is often tempered by a
corresponding debit
• Material property trade-offs
Materials modeling and simulation will revolutionize
materials development
• Not just a matter of doing faster…doing it much better
Still Lots of Exciting Materials Challenges!!!
Slide 62
Take AwaysSprague Laws
1. The first information you hear about a new material
– Usually its the best thing you’ll ever hear about it
2. Any fool can melt it
– Getting it to solidify properly is what counts
3. Materials scientists still believe that microstructure controls properties
– Materials engineers understand that defects actually control the usable properties