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 weight of lithium.^[1]

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.
• 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 aluminum 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 aluminum 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 aluminum.^[5]

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, 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.^[7] In addition, Al–Li alloys are also used on both the Atlas V and Delta IV EELV rockets, and before its cancellation were to be used by NASA for Project Constellation, primarily, on its Ares I and Ares V rockets, as well as the Orion spacecraft.

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.

See also

• Aluminium alloy
• Magnesium-lithium alloys

References