Aluminium–lithium alloy

Aluminium–lithium alloys (Al–Li) are a series of alloys of aluminium and lithium, often also including copper and zirconium. Since lithium is the least dense elemental metal, these alloys are significantly less dense than aluminium. Commercial Al–Li alloys contain up to 2.45% by mass of lithium.[1]

Crystal structure

Alloying with lithium reduces structural mass by three effects:

Displacement 
A lithium atom is lighter than an aluminium atom; each lithium atom then displaces one aluminium atom from the crystal lattice while maintaining the lattice structure. Every 1% by mass of lithium added to aluminium reduces the density of the resulting alloy by 3% and increases the stiffness by 5%.[1] This effect works up to the solubility limit of lithium in aluminium, which is 4.2%.
Strain hardening
Introducing another type of atom into the crystal strains the lattice, which helps block dislocations. The resulting material is thus stronger, which allows less of it to be used.
Precipitation hardening
When properly aged, lithium forms a metastable Al3Li phase (δ') with a coherent crystal structure.[2] These precipitates strengthen the metal by impeding dislocation motion during deformation. The precipitates are not stable, however, and care must be taken to prevent overaging with the formation of the stable AlLi (β) phase.[3] This also produces precipitate free zones (PFZs) typically at grain boundaries and can reduce the corrosion resistance of the alloy.[4]

The crystal structure for Al3Li and Al–Li, while based on the FCC crystal system, are very different. Al3Li shows almost the same-size lattice structure as pure aluminium, except that lithium atoms are present in the corners of the unit cell. The Al3Li structure is known as the AuCu3, L12, or Pm3m[5] and has a lattice parameter of 4.01 Å.[3] The Al–Li structure is known as the NaTl, B32, or Fd3m[6] structure, which is made of both lithium and aluminium assuming diamond structures and has a lattice parameter of 6.37 Å. The interatomic spacing for Al–Li (3.19 Å) is smaller than either pure lithium or aluminium.[7]

Usage

Al–Li alloys are primarily of interest to the aerospace industry due to the weight advantage they provide. They are currently used in a few commercial jetliner airframes, the fuel and oxidizer tanks in the SpaceX Falcon 9 launch vehicle, Formula One brake calipers, and the AgustaWestland EH101 helicopter.[8]

The third and final version of the US Space Shuttle's external tank was principally made of Al–Li 2195 alloy.[9] In addition, Al–Li alloys are also used in the Centaur Forward Adapter in the Atlas V rocket,[10] in the Orion Spacecraft, and were to be used in the planned Ares I and Ares V rockets (part of the cancelled Constellation program).

Al–Li alloys are generally joined by friction stir welding. Some Al–Li alloys, such as Weldalite 049, can be welded conventionally; however, this property comes at the price of density; Weldalite 049 has about the same density as 2024 aluminium and 5% higher elastic modulus.

Although aluminum–lithium alloys are generally superior to aluminum–copper or aluminum–zinc alloys in ultimate strength-to-weight ratio, their poor fatigue strength under compression remains a problem, which is only partially solved as of 2016.[11][12] Also, high costs (around 3 times or more conventional aluminum alloys), poor corrosion resistance and strong anisotropy of mechanical properties of rolled aluminum–lithium products has resulted in the paucity of the applications.

On narrow-body airliners, Arconic (formerly Alcoa) claims up to 10% weight reduction compared to composites, leading to up to 20% better fuel efficiency, at a lower cost than titanium or composites. Al–Li is used on the Airbus A380 and A350, Boeing 787 and Bombardier CSeries airliners, and on Gulfstream G650 and Bombardier Global 7000/8000 business jets.[13]

List of aluminium-lithium alloys

  • 1429 aluminum alloy[14]
  • 2055 aluminium alloy[15]
  • 2090 aluminium alloy
  • 2091 aluminium alloy
  • 2099 aluminium alloy (2nd generation Al-Li alloy)[16]
  • 2195 aluminium alloy, used in the last revision of the Space Shuttle External Tank
  • 8090 aluminium alloy
  • Weldalite 049

Production sites

Key world producers of aluminium–lithium alloy products are Alcoa, Constellium and Kamensk-Uralsky Metallurgical Works.

  • Arconic Technical Center (Pennsylvania)
  • Arconic Lafayette (Indiana); capacity 20,000 metric tons of aluminum–lithium and capable of casting round and rectangular ingot for rolled, extruded and forged applications
  • Arconic Kitts Green (United Kingdom)
  • Rio Tinto Alcan Dubuc Plant (Canada); capacity 30,000 metric tons
  • Constellium Issoire (Puy-de-Dôme); capacity 14,000 metric tons/annum
  • Kamensk-Uralsky Metallurgical Works (KUMZ)
  • Aleris (Koblenz, Germany)
  • FMC
  • Southwest Aluminium

See also

References

  1. 1 2 Joshi, Amit. "The new generation Aluminium Lithium Alloys" (PDF). Indian Institute of Technology, Bombay. Metal Web News. Archived from the original (PDF) on 28 September 2007. Retrieved 2008-03-03.
  2. ↑ E. Starke, T. Sanders Jr, and I.G. Palmer, "New Approaches to Alloy Development in the Al–Li System" Journal of Metals, vol. 33, Aug. 1981, pp. 24–33.
  3. 1 2 K. Mahalingam, B. Gu, G. Liedl, and T. Sanders Jr, "Coarsening of [delta]'(Al3Li) Precipitates in Binary Al–Li Alloys", Acta Metallurgica, vol. 35, Feb. 1987, pp. 483–498.
  4. ↑ S. Jha, T. Sanders Jr, and M. Dayanada, "Grain Boundary Precipitate Free Zones in Al–Li Alloys", Acta Metallurgica, vol. 35, 1987, pp. 473–482.
  5. ↑ http://cst-www.nrl.navy.mil/lattice/struk/l1_2.html Archived 6 April 2010 at the Wayback Machine..
  6. ↑ http://cst-www.nrl.navy.mil/lattice/struk/b32.html Archived 12 June 2011 at the Wayback Machine..
  7. ↑ K. Kishio and J. Brittain, "Defect structure of [beta]-LiAl", Journal of Physics and Chemistry of Solids, vol. 40, 1979, pp. 933–940.
  8. ↑ Queen's University Faculty of Applied Science, Aluminium-Lithium Alloys Archived 28 February 2007 at the Wayback Machine..
  9. ↑ NASA, Super Lightweight External Tank.
  10. ↑ "Atlas V Launch Services User's Guide" (PDF). March 2010.
  11. ↑ Effect of Mg and Zn Elements on the Mechanical Properties and Precipitates in 2099 Alloy.
  12. ↑ MEE433B Aluminum-Lithium Alloys.
  13. ↑ Kerry Lynch (August 8, 2017). "FAA Issues Special Conditions for Global 7000 Alloy". Aviation International News.
  14. ↑ Development of Aluminum-Lithium alloys processed by the Rheo container process
  15. ↑ https://www.arconic.com/adip/catalog/AFE2055-factsheet.pdf
  16. ↑ Aluminum-lithium alloy 2099-T86
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