Developments in Aluminium Lithium Alloys

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Developments in Aluminium - Lithium Alloys

• T.R. Ramachandran
• Nonferrous Materials Technology Development
  Centre
• Hyderabad 500 058

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INTRODUCTION

• Addition of Li to Al improves elastic modulus (
  6 per wt.) and reduces density ( 3/wt.)
• Al-Li alloys are precipitation hardenable, d
  (Al3Li) being the hardening phase. The
  equilibrium phase is d (AlLi)
• Alloys with high specific strength and modulus
  can be developed.

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• Al-Li alloys have good fatigue and cryogenic
  toughness
• properties.
• Main disadvantages of peak-aged alloys are
  reduced
• ductility and fracture toughness in the short
  transverse
• direction, anisotropy of in-plane properties,
  need for
• cold work to attain peak properties and
  accelerated
• fatigue crack extension rates when the cracks are
  
• microstructurally small.

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Historical Perspective

• Scleron (Al-Zn-Cu-Li) introduced by the Germans
• in 1920s.
• Alloy 2020 (Al-Cu-Li-Cd) used in USA in the RA5C
• Vigilante aircraft in the late 1950s.
• Alloy 01420 (Al-5.3Mg-2Li-0.3Mn) introduced in
  the
• Soviet Union in mid 1960s - limited acceptance
• Extensive RD efforts in 1970s and 1980s led to
  better
• understanding of causes of poor ductility and
• fracture toughness development of 2090, 8090.
• Weldalite series introduced early 1990

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Causes for poor toughness

• Binary Al-Li alloys have poor toughness
  associated with
• Easy shear of d phase during deformation and
• consequent planar slip
• Localization of slip in the soft d-PFZ near GBs
• Precipitation of d and Fe-Si bearing compounds on
  GBs
• Hydrogen embrittlement due to enhanced hydrogen
• Solubility in Al-Li alloys

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DF electron micrograph of d bulls eye
structure
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d particles in Al-Li-Mg-Zr alloy aged at 190?C
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Dislocation loop free zone (LFZ) near GB and
dislocation in quenched Al
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PFZ in Al-4Zn-3Mg aged 24 h at 150?Ceffect of
0.3Ag
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Effect of alloying elements on hydrogen solubility
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Causes for poor toughness (contd.)

• GB embrittlement due to segregation of Na and K
  
• Unrecrystallized structure in most Al-Li alloys
  becomes highly textured in thin products
  resulting low angle boundaries allow cracks to
  propagate easily.
• Formation of low melting eutectic constituents
  with certain combination of alkali metals at
  grain boundaries can cause embrittlement.

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• Problems of poor toughness are overcome by
  additions
• of Cu and Mg.
• Small amounts of Zr (0.1) are added to retard
  recrystallization.
• Additional precipitating phases such as T1
  (Al2CuLi), S(Al2CuMg) and ? (CuAl2) resist shear
  by dislocations and contribute to homogenization
  of slip.

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Al-Li Alloys for Commercial Use
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Properties of Al-Li alloys
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Replacement of Existing Alloys by Al-Li Alloys
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Replacement of Existing Alloys by Al-Li Alloys
(contd.)
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Replacement of Existing Alloys by Al-Li Alloys
(contd.)
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Replacement of Existing Alloys by Al-Li Alloys
(contd.)
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Heat Treatment Tempers
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Heat Treatment Tempers (contd.)
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Phases in Al-Li Alloys
Insolubles Al12(Fe,Mn)3Si, Al6(Cu,Fe,Mn),
Al8Fe2Si Al5FeSi, Al20Cu2Mn3, Al23CuFe4 Al7Cu2Fe
Dispersoids Al3Zr
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Phases in Al-Li Alloys (contd.)
Precipitates Al3Li(d), AlLi(d), Al2CuLi(T),
Al6CuLi3(T2), Al15Cu8Li2(Tß), T,
Al2CuMg(S), Al2MgLi, AlLiSi, CuAl2(?),
CuAl2(?)
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Alloy 2090

• (2.7 Cu, 2.2 Li, 0.4 Ag, 0.12
  Zr)
• 8 lower density and 10 higher elastic modulus
  than 7075-T6.
• In-plane anisotropy of tensile properties
  considerably higher than in conventional
  aluminium alloys
• Elevated temperature exposure for T8 tempers
  shows good stability within 10 of the original
  properties
• Excellent fatigue growth behaviour
• Need for cold work for achievement of optimum
  properties
• Shape dependent behaviour for extrusions with
  very high strengths.

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Alloy 2091

• (2.1 Cu, 2.0 Li, 0.10 Zr)
• 8 lower density and 1 higher elastic modulus
  than 2024-T3 damage tolerant alloy.
• Depends less on cold work to attain optimum
  properties than does 2024.
• Exfoliation resistance of 2091 is generally
  comparable to that of similar gages of 2024-T3.
• Fatigue properties comparable to 2024.

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Alloy 8090

• (1.3 Cu, 2.45 Li, 0.95 Mg, 0.12 Zr)
• 10 lower density and 11 higher elastic modulus
  than 2024 and 2014 medium strength damage
  tolerant alloy.
• Changes in strength and toughness at cryogenic
  temperatures are more pronounced than in
  conventional aluminium alloys 8090 has
  substantially higher strength and toughness at
  these temperatures.
• Significant improvements in short transverse
  ductility in improved quality alloys

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WELDALITE SERIES ALLOYS
Weldlalite series have a broad range of
composition Element I series II
series Cu 5.0-7.0 3.5-5.0 Li 0.1-2.5 0.8-1
.8 Mg 0.05-4.0 0.25-1.0 GRF 0.01-1.5 0.01-
1.5 (US Patent 5,259,897, Nov 9, 1993 Martin
Marietta Corporation Reynolds manufactures
2094 2095.)
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WELDALITE 049

• (5.4 Cu, 1.3 Li, 0.4 Ag, 0.4 Mg, 0.14 Zr)
• Good ageing response in T3 and T4 tempers
• natural ageing response stronger than any other
• known aluminium alloy
• Tensile strengths of 700 MPa obtained in T6 T8
• tempers
• Alloy has good weldability

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Ageing response of Weldalite 049 at 170?C
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Temperature dependence of yield strength of 2219
and Weldalite
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Precipitation Sequence

• A variety of phases precipitate in Al-Li alloys
  depending on solution treatment and ageing
  temperatures.
• Cu Li ratio influences precipitation sequence.
• Cu/Li gt 4, SSSS ? GP zones ? ? ? ? ? ?
• Cu/Li (2.5 to 4.0), SSSS ? GP zones d ? ?
  d ? d T1 T1
• Cu/Li (1.0 to 2.5), SSSS ? GP zones d ? ?
  d ? d T1 T1
• Cu/Li lt1, SSSS ? d T1 ? T1

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Microstructural Features
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Effect of deformation on S precipitation in
Al-Cu-Mg alloy (a) no deformation 12h,
190?C (b) Stretch 6, same ageing.
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BF image T2(eq.) phase ppn on GB with SAD.
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Microstructure of peak aged 2090 (a) BF, (b)
SAD, (c) DF d, (d) DF T1 ppt. (e) DF ?.
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• 8090 alloy
• 2 stretch
• BF
• SAD
• DF S
• Weak
• Beam, S,
• T1.

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POINTS OF CONCERN

• Safety High explosion potential when molten
  metal is in contact with water problems of DC
  casting water as coolant is to be replaced.
• Industrial hygiene Oxidation in air leads to the
  formation of lithium aluminate which hydrolyses
  to LiOH the fumes of hydroxide cause irritation
  to the skin and nose.
• LiOH emissions cause opaqueness of stack
  emissions and down-wind problems result is
  corrosion and irritation during breathing.

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POINTS OF CONCERN (contd.)

• Dross resulting from remelting of Al-Li scrap can
  not be treated by conventional methods as Li
  leaching into ground water system is a major
  concern.
• Adverse effect on refractories by molten Al-Li
  alloys
• Lithium depletion in surface layers during heat
  treatment
• Explosion potential in salt bath

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Pechiney Aerospace) Al-Li Alloys
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Pechiney Al-Li (Aerospace) alloys - typical
property levels
Other Main Features
TYS L typical (Mpa / ksi)
Th(mm/in)
Alloy
3 to 5x life improvement over 2124-T8 Excellent
Corrosion resistance
420 Mpa 61 ksi
152 mm 6 in
2297-T851
High Strength (40 to 50 above 2024-T62) 3 to 5x
life improvement over 2024-T62 High toughness (80
to 100 above 2024-T62)
530 Mpa 76.9 ksi
6.35 mm 0.25 in
2098-T8
High Strength High Toughness at cryogenic T
580 Mpa 84.1 ksi
38.1 mm 1.5 in
2195-T8
(Post forming TYS) High forming rates
480 Mpa 69.6 ksi
2 5 mm 0.08 0.2in
2195 (SPF)
Very low density (2.63 g/cm3) High Tougness
530 Mpa 76.9 ksi
Extrusions
2196-T8
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Space Applications
The most critical factor is weight
savings 2090-T81 is useful for cryogenic tankage
of booster Systems 2195 used for fabrication of
super light weight tanks for the space shuttle
this tank is 3500 kg lighter than the current
external tank. Anisotropy of 2090, 8090 a
limitation for thicker sections.
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