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 weight 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 weight 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.^{[citation needed]}
• 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 lithium atoms are present in the corners of the unit cell. The Al₃Li structure is known as the AuCu₃, L1₂, or Pm3m and has a lattice parameter of 4.01 Å.^[3] The Al–Li structure is known as the NaTl, B32, or Fd3m 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 AlLi (3.19 Å) is smaller than either pure lithium or aluminium.^[5]

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.^[6]

The third and final version of the US Space Shuttle's external tank was principally made of Al-Li 2195 alloy.^[7] In addition, Al–Li alloys are also used in the Centaur Forward Adapter in the Atlas V rocket,^[8] 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.^{[citation needed]}

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.^[9][10] 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 single-aisle airliners, Arconic (ex Alcoa) claims up to 10% weight reduction compared to composites leading to up to 20% better fuel economy, at a lower cost than titanium or composites. Al-Li is used on on the Airbus A380 and A350, Boeing 787 and Bombardier CSeries airliners, and on Gulfstream G650 and Bombardier Global 7000/8000 business jets.^[11]

List of Aluminium-lithium alloys

• 1429 aluminum alloy^[12]
• 2090 aluminium alloy
• 2091 aluminium alloy
• 2099 aluminium alloy (2nd generation Al-Li alloy)^[13]
• 2195 aluminium alloy
• 8090 aluminium alloy
• Weldalite 049

Production sites

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

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

See also

• Aluminium alloy
• Magnesium-lithium alloys