Schafrik History of Mtls in Jet Engines 1

Materials in Jet Engines:Past, Present, and Future

Robert SchafrikGeneral Manger, Materials & Process Engineering

GE Aircraft Engines

Overview

Introduction

Highlights of Key Developments

Materials in Aero Engines

Future Directions

Summary and Take Aways

INTRODUCTION

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

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

Power Jets Whittle W-1A

Commercial High By-Pass Ratio Engine

Low Pressure Turbine

High Pressure Turbine

Combustor

High Pressure Compressor

Fan

CoreAir

Low Pressure Compressoror Booster

Drivers for Advancing AeroTurbine Technology

Modern World Expectation: Freedom to Travel

Anywhere

• Quickly

• Inexpensively

• Safely

National Defense Needs

• Push limits of technology

• High Reliability

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

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

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

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

Important Developments

Vacuum Melting

Nickel-based Superalloys

Titanium

Investment Casting

Forging

Vacuum Melting

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

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

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

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

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

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

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

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

Why Titanium?

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

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

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

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

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

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

Extrinsic Melt Related DefectsExtrinsic Melt Related Defects

High Density Inclusion(W rich inclusion)

Hard Alpha(N rich inclusion)

Investment Casting

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

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

Processing Advantage

GE90 Turbine Rear Frame

Castability and Weldability of Alloy 718 enables application of complex cast structures

Turbine Air Foil Casting Processes

Equiaxed (EQ) Dir. Sol. (DS) Single Xtal (SX)

Complexity of Airfoil Castings

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

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

Ladish 10,000 Ton Isothermal Press

High Reliability

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

Premium Quality Melting of Nickel Alloys

VIM ESR VAR

Remove Inclusions

Control Macrostructure

Formulate Composition

Triple Melt Key for High Reliability Components

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

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

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

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

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

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

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

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!

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

Vision

Today Future

Iden

tifie

d R

isk

Res

ourc

esTime

Iden

tifie

d R

isk

Res

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Evolutionary Materials

Advanced Materials

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

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!!!

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

Schafrik History of Mtls in Jet Engines 1

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