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Designing High Strength Aluminium Alloys for
Aerospace Applications
H.Aourag
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Aluminium Alloys in Aerospace
Airbus A340
Despite competition from other materials, Al alloys still make up > 70% of structure of modern commercial airliner
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Design Requirements
Components must beLightweightDamage tolerantDurable (corrosion resistant)Cost effective
Requires careful balance of material properties
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Critical Material Properties
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Aluminium Alloys
Pure aluminium has Low density (
relative Al=2.7, Fe=7.9)
Readily available (Al is 3rd most abundant element in Earth's crust)
Highly formable (FCC crystal structure)
Low strength and stiffness (EAl=70GPa,
EFe
=211GPa)
Low melting point (Tm=660oC)
Alloy with other elements to improve strength and stiffness - results in alloys with properties well matched to aerospace requirements
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Aerospace Al-Alloys
Dominated by “high strength” wrought alloys Two main alloy series in particular
2xxx alloys (Al + Cu, Mg) UTS~500MPa 7xxx alloys (Al + Mg, Zn, (Cu)) UTS~600MPa
A) Slats - 2618B) D-Nose Skins - 2024C) Top Panel - 7150D) Bottom Panel - 2024
E) Spars / Ribs - 7010F) Flap Support - 7175G) Flap Track - 7075H) Landing Gear - 2024
A
B
C
E
D
FG
H
Alloys used in typical wing structure
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Next Generation Aircraft
Bigger....
...FasterBoeing sonic cruiser > Mach.95
Airbus A380 > 950 seats
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Goals
Next generation aircraft rely on advances in materials and assembly methods
Weight reduction is critical Alloy optimization
Increase strength and stiffness and/or reduce density whilst maintaining other properties
Assembly optimization Reduce weight associated with joints between
components
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Alloy Design
Traditionally, alloy and process development largely by trial and error based on metallurgical experience
Recently, emphasis has changed to designing alloys and processes to meet specific property goals Improved understanding of relationships between
processing, microstructure and properties Development of models to predict alloy microstructure
and performance
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Applications of Modelling
Models on a range of length scales Atomistic (nm)
Limited application as currently capable of dealing with only very small volumes of material
Microstructural (nm-m) Used to predict particle distributions, grain sizes etc.. as
function of alloy chemistry and processing conditions, often coupled to microstructure-property models
Macro-scale (>mm) Widely used to predict performance of components
during processing and service as a function of average material properties and stress, strain, temperature....
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Modelling Examples
Finite element modelling to optimize extrusion processing of aerospace Al-alloys
Thermodynamic modelling for the development of weldable aerospace aluminium alloys
Precipitation kinetics modelling for optimization of dispersoid particles in 7xxx alloys
Ma
cro
Mic
ro
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Finite Element Modelling of Extrusion
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Direct
The Extrusion Process
Extrusion is widely used to produce aerospace components
Direct Indirect
Ram DieBillet
Extrusion
Extruded shapes are often complex - design of die is critical
(Al alloy)
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Die Design
Die must be designed to ensure balanced metal flow to avoid bending of extrusion
Die shape influences metal temperature-aim to avoid cold or hot spots
Traditionally, die design based on past experience and modifications of existing dies
Alternative: Use finite element methods to model extrusion process and identify and test new die designs
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The Finite Element Method
Divide billet/extrusion into small, connected elements
Relate displacements/temperature changes in one element to those in surrounding elements using well established physical laws
2D finite element mesh for an extrusion
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Use of Finite Element Model
Use commercially available FE package to model metal flow and temperature during extrusion
New die design
Simulate extrusion process
Any problems?Unbalanced metal flowExcess temperature variation
Modify design
No
Yes
Make prototype die
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FE Model - Example Simulations
Example Simulations in 2D and 3D
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Weldable Aerospace Al-Alloys
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Joining Aerospace Al-Alloys Mechanical fasteners (rivets) are still the most
widely used method of joining airframe components
Riveted joints have a number of disadvantages
Problem: Most high strength Al-alloys suitable for aerospace are considered “unweldable”
Welded jointNo extra material (less weight)Process readily automated
Riveted jointExtra material requiredLabour intensive
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Difficulties with welding
One of the major metallurgical problems preventing the widespread application of welding to aerospace Al-alloys is solidification cracking
250 m
7075 TIG Weld
Cracks arise when the thermal stresses generated during cooling exceed the strength of the almost solidified metal
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Factors Influencing Solidification Cracking
1) Level of Thermal Stresses
3) Absolute Freezing Range
- alloys with a wide freezing range are susceptible to cracking
5) Volume Fraction of Low Melting Point Eutectic Phases
- if there is sufficient liquid at the end of solidification to flow around the dendrites, then any cracks might be healed
2) Grain Structure of Fusion Zone
- columnar grains vs equiaxed grains
4) Freezing Range for Dendrite Cohesion
- thought to occur at about 50-60% Solid (depend on grain structure)
?
?
?
Thermodynamic Modelling
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Thermodynamic Modelling For any alloy system, set of conditions and
configuration of the components there will be an associated free energy
Use computer models to calculate the free energy for complex systems (lots of elements) from data for simple systems (1,2 or 3 elements)
Calculate the equilibrium (minimum free energy) configuration and hence phase diagrams for complex systems Can be useful in the interpretation of real microstructures
Calculate phase fractions and compositions for certain other well defined non-equilibrium problems
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Simple Phase Diagrams
300
350
400
450
500
550
600
650
700
0 10 20 30 40 50 60
wt.% Mg
Tem
pe
ratu
re (
C)
Liquid
Liquid + -Al
-Al
-Al + -AlMg
-A
lMg
-AlMg
-A
lMg
Al-Mg System (Al-Rich)
300
400
500
600
700
800
900
1000
1100
1200
0 20 40 60 80 100
wt.% Mg
Tem
per
atu
re (
C)
Liquid
Liquid + Mg
CuMg2 + Mg
CuM
g2
L + CuMg2
L + -Cu
Laves - C15
-Cu +
Laves - C15
Liquid + Laves - C15
Cu-Mg System
Ternary Phases S - Al2CuMg, T - Mg32(Al,Cu)49, V - Al5Cu6Mg2, Q - Al7Cu3Mg6
Even for simple 2xxx alloy (Al-Cu-Mg), need data for 3 binaries and information about ternary phases
400
450
500
550
600
650
700
0 10 20 30 40 50 60wt.% Cu
Tem
per
atu
re (
C)
Liquid
Liquid + -Al
-Al
Liquid + -Al2Cu
-Al + -Al2Cu-Al2Cu
Al-Cu System (Al-Rich)
MTDATA predicted phase diagrams
Real, commercial Al-alloys may contain > 10 alloying elements!
Success of thermodynamic models relies on availability of sufficient, high quality, thermodynamic data
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Solidification Microstructures
Solidification occurs rapidly under non-equilibrium conditions
However, given certain assumptions, thermodynamic calculations and the equilibrium phase diagram can still be used to predict solidification microstructure
Scheil Solidication Model - Assumptions:
(i) Local equilibrium exists at the solid/liquid interface
(ii) No diffusion in the solid phases
(iii) Uniform liquid composition
(iv) No density difference between
solid and liquid
% Solute
T
Cliq1
Csol1Cliq2
Csol2Cliq3
Csol3
C0
Csol0
Liquid
Solid
Microstructure
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Predictions for Binary Al-Cu Alloy
Freezing Range
520 540 560 580 600 620 640 660 680 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Mas
s P
has
e F
ract
ion
Liquidfcc -Al
- Al2Cu
EutecticReaction
Temperature (C) fcc -Al dendrites
- Al2Cu eutectic fcc -Al
eutectic
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Predictions for Ternary Al-Cu-Mg alloy
470 490 510 530 550 570 590 610 630 650
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0M
ass
Ph
ase
Fra
ctio
n
Temperature (C)
Liquid
fcc -Al
TLTST
S - Al2CuMg
- Al2Cu
Ternary Eutectic Predicted at ~ 500ºC
Predictions for 2xxx (Al-4.5Cu-1.5wt%) Mg alloy
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Prediction of Freezing Range
To reduce tendency for solidification cracking, need to minimize absolute freezing range
Use thermodynamic model to predict freezing range for different alloy compositions
Effect of Mg content on freezing range of eutectic in Al-4.5Cu-x Mg alloy
0
5
10
15
20
25
30
35
40
45
50
0 0.5 1 1.5 2 2.5 3
wt.% Mg
T
(F
ree
zin
g R
an
ge
of
Eu
tec
tic
)
Binary Eutectic[ + ]
Ternary Eutectic[ + S]
Saddle Point[ + S]
Optimum composition range
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Value of Calculations
Thermodynamic calculations suggest modifications to current alloy compositions to improve weldability
Focus experimental investigation on promising compositions Save both development time and cost
New weld filler wires have been developed on the basis of these calculations and are now being tested
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Modelling Dispersoid Precipitation in 7xxx Aerospace Al Alloys
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Prediction of Microstructure Thermodynamic calculations give an indication of
likely phases but give no information about How phase is distributed
Particle size, spacing and location
How microstructure changes as function of time Transformation of metastable phases Evolution of volume fraction of phase and particle
size distribution
These factors depend on phase transformation kinetics and are critical in determining microstructure and hence properties
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Kinetic Modelling
Aim to predict key microstructural parameters as a function of alloy composition, temperature and time
Difficult problem for aerospace Al-alloys due to complex microstructures and processing routes Large number of possible phases evolving
simultaneously Metal subjected to thermal cycling and
complex deformation during processing
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7050 Plate
Components machined from 7050 alloy thick plate are widely used in load bearing applications e.g. wing spars
7050 composition specification
Focus on one alloy (7050) and product (thick hot rolled plate)
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Processing Sequence - 7050 Plate
CastDirect chill
Homogenize~475oC, 24h
Hot roll~350-450oC20+ passesreduction~70%
Solution treat475oC, 1hspray quenched
Age
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Cast SolutionizedHomogenized Rolled
RD
Microstructural ChangesT
em
pe
ratu
re
Aged
Time
50nm
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Dispersoids
Fine Al3Zr dispersoid particles precipitate during homogenization of 7050
Dispersoid particles are important for the control of grain structure during processing Act to “pin” grain boundaries
Al3Zr dispersoid particles in 7050 after homogenization
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Modelling Dispersoid Precipitation
Effectiveness of dispersoids depends on their size, spacing and distribution
Develop model for dispersoid precipitation and use to optimize homogenization treatment to give best dispersoid distribution
To model dispersoid precipitation must account for both non-uniform distribution of Zr due to microsegregation during casting and Al3Zr precipitation kinetics
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Start
Average zirconium
concentration (depends on
position in slab)
Microsegregation Model
(MTDATA Scheil Model)
Local zirconium concentration (as a function of position
within grain)
Homogenization temperature/time
profile
PrecipitationKinetics Model
Dispersoid size, number density, spacing and size
distribution
Schematic of Model
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Precipitation Kinetics
The precipitation of Al3Zr dispersoids is a diffusion controlled phase transformationClassically, precipitation of particles considered as 2-step process of nucleation and growth, followed by coarsening
Clusters of Al, Zr atoms form by random in matrix. Stable clusters become particle nuclei
Particles grow, controlled by diffusion of Zr
Small particles dissolve at the expense of large particles to reduce total interfacial area
Nucleation Coarsening
Time = t1 t2 t3
Nucleation+growth
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Kinetics Model
Time is divided into a large number of small steps
Growth, nucleation and coarsening allowed to occur concurrently governed by driving force and concentration gradients
At each step new particles nucleate and existing particles grow (or shrink) depending on local interfacial compositions
After each step, solute supersaturation in the matrix is recalculated and used for next step
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Nucleation
Nucleation rate (number of new particles formed/s) depends on Thermodynamic driving force for formation
of new phase Diffusion rate (temperature) Interfacial energy between nucleus and
matrix
Nucleation rate
Te
mp
era
ture
I/f e
nerg
yNucleation rate
Driving forceincreasing but diffusion ratedecreasing
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Growth
Growth rate for each particle depends on Concentration gradient ahead of particle
Equilibrium compositions from phase diagram Particle size
Diffusion rateZ
r co
ncen
trat
ion
distance
Concentration profiles
Small particle
Large particle
Zr in particle
Zr in matrix at interface(depends on particles size)
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Coarsening
Coarsening does not need to be modelled separately but arises naturally from growth model in later stages of precipitation
Early stages
All particles growing
c
Co
nce
ntr
atio
n Z
r
Late stages
Large particles growing, small particles shrinking
c
Co
nce
ntr
atio
n Z
r shrinkinggrowing
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Testing the Model
First test model against experiment for a single initial Zr concentration
Size
Num
ber
Evolution of size distribution with time
Comparison of model prediction and experiment at 500oC
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Effect of Zirconium Segregation
In practice, Zr concentration varies across a grain due to segregation during casting
Leads to non-uniform dispersoid precipitation during homogenization
Zr concentration after castingObserved dispersoid distribution after homogenization
Centre
Disperso
id free
zone
Edge Low Zr
High Zr
EDGE CENTRE
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Including Effect of Segregation
To model Al3Zr distribution across a grain Divide the distance from grain edge to
centre into large number of elements Model dispersoid evolution in each element Allow zirconium redistribution by diffusion
between elements
Edge Centre
Zr
conc
entr
atio
n
Zr removed into Al3Zr dispersoids
Zr diffusing into elementZr diffusing out of element
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Predicting Across a Grain
Volume Fraction Zr in solution Mean radius
Centre Edge Centre Edge Centre Edge
Centre
Edge Can the model reproduce the observed behaviour?
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Effect of Dispersoid Distribution Inhomogeneously distributed dispersoids
are not best for control of grain structure In regions where there are few
dispersoids, new grains can form (recrystallization) - this is undesirable
Structure after processingNew grains have formed and partially consumed original grains - this structure does not give best properties
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Optimizing Dispersoid Distribution
Use model to determine optimum homogenization conditions to promote dispersoid precipitation in low Zr regions
Aim is to reduce the formation of new (recrystallized) grains during processing
For best recrystallization resistance, want a large number of small dispersoid particles, as uniformly distributed as possible
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Model Predictions
Nucleation
Growth
Use model to investigate kinetics in detail
Temperature /oC Time /hT
em
per
atu
re /o
C
To promote dispersoid nucleation in low Zr regions need to hold at ~425oC
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Optimizing Homogenization
BUT Homogenization temperature for 7050 is restricted
Model suggests that best temperature for precipitating dispersoids in low Zr regions lies below this range
Hom
ogen
izat
ion
ran
ge
AA7050
Need to dissolve these phases during homogenization
Must avoid onset of melting
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Two Step Practice
Two step homogenization practice may be of benefit Step 1: Hold at a temperature to precipitate
optimum dispersoid distribution Step 2: Hold at final homogenization
temperature
Model used to determine best conditions for step 1 5h Hold time at 430oC
Test 2 step homogenization practice
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Effect on Dispersoids
Standard Homogenization Two step treatment
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Comparison of Recrystallization
Standard PracticeRecrystallized Fraction = 30.4%
Hold + Homogenize PracticeRecrystallized Fraction = 14.0%
Two step homogenization practice, developed entirely by computer modelling, is effective in significantly reducing the fraction of recrystallization
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Summary
Aerospace aluminium alloys are complex materials, developed over a long period of time by empirical experiment to meet industrial needsIn recent years, the understanding of the metallurgical processes governing the microstructure and properties of these alloys has greatly increased
This has led to the development of models that have practical application in the design of new alloys and processes
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Acknowledgements
For provision of data and examples of FE and thermodynamic modelling Dr Qiang Li, Birmingham University Dr Andy Norman, Manchester Materials
Science Centre
Luxfer and Alcoa for funding some of this research