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Aluminium–lithium alloys (Al–Li alloys) are a set 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.[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 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.

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.[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

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. 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.[8] Aluminum–lithium alloys were first used in the wings and horizontal stabilizer of the North American A-5 Vigilante military aircraft. Other Al–Li alloys have been employed in the lower wing skins of the Airbus A380, the inner wing structure of the Airbus A350, the fuselage of the Bombardier CSeries[9] (where the alloys make up 24% of the fuselage),[10] the cargo floor of the Boeing 777X,[11] and the fan blades of the Al–Li alloys are primarily of interest to the aerospace industry due to the weight advantage they provide. 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.[8] Aluminum–lithium alloys were first used in the wings and horizontal stabilizer of the North American A-5 Vigilante military aircraft. Other Al–Li alloys have been employed in the lower wing skins of the Airbus A380, the inner wing structure of the Airbus A350, the fuselage of the Bombardier CSeries[9] (where the alloys make up 24% of the fuselage),[10] the cargo floor of the Boeing 777X,[11] and the fan blades of the Pratt & Whitney PurePower geared turbofan aircraft engine.[12] They are also used in the fuel and oxidizer tanks in the SpaceX Falcon 9 launch vehicle, Formula One brake calipers, and the AgustaWestland EH101 helicopter.[13]

The third and final version of the US Space Shuttle's external tank was principally made of Al–Li 2195

The third and final version of the US Space Shuttle's external tank was principally made of Al–Li 2195 alloy.[14] In addition, Al–Li alloys are also used in the Centaur Forward Adapter in the Atlas V rocket,[15] 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] Al–Li is also produced in rolls as wide as 220 inches (18 feet; 5.6 metres), which can reduce the number of joins.[16]

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.[17][13] Also, high costs (around 3 times or more than for conventional aluminum alloys), poor corrosion resistance, and strong anisotropy of mechanical properties of rolled aluminum–lithium products has resulted in a paucity of applications.

Aside from its formal four-digit designation derived from its element composition, an aluminium–lithium alloy is also associated with particular generations, based primarily on when it was first produced, but secondarily on its lithium content. The first generation lasted from the initial background research in the early 20th century to their first aircraft application in the middle 20th century. Consisting of alloys that were meant to replace the popular 2024 and 7075 alloys directly, the second generation of Al–Li had high lithium content of at least 2%; this characteristic produced a large reduction in density but resulted in some negative effects, particularly in fracture toughness. The third generation is the current generation of Al–Li product that is available, and it has gained wide acceptance by aircraft manufacturers, unlike the previous two generations. This generation has reduced lithium content to 0.75–1.8% to mitigate those negative characteristics while retaining some of the density reduction;[18] third-generation Al–Li densities range from 2.63 to 2.72 grams per cubic centimetre (0.095 to 0.098 pounds per cubic inch).[19]

First-generation alloys (1920s-1960s)[18]

Other alloys

  • 1424 aluminium alloy[30]
  • 1429 aluminium alloy[31]
  • 1441K aluminium alloy[30]
  • 1445 aluminium alloy[30]
  • V-1461 aluminium alloy

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

    • Arconic Technical Center (Upper Burrell, Pennsylvania, USA)[9]
    • Arconic Lafayette (Indiana, USA); annual capacity of 20,000 metric tons (22,000 short tons; 20,000,000 kg; 44,000,000 lb) of aluminum–lithium[9] 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 t (33,000 short tons; 30,000,000 kg; 66,000,000 lb)
    • Constellium Issoire (Puy-de-Dôme), France; annual capacity of 14,000 t (15,000 short tons; 14,000,000 kg; 31,000,000 lb)[9]
    • Kamensk-Uralsky Metallurgical Works (KUMZ)
    • Aleris (Koblenz, Germany)
    • FMC Corporation
    • Southwest Aluminium (PRC)

    See also

    References

    1. ^ a b 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 3 March 2008.
    2. ^ Starke, E. A.; Sanders, T. H.; Palmer, I. G. (20 December 2013). "New Approaches to Alloy Development in the Al–Li System". JOM: The Journal of the Minerals, Metals & Materials Society (published August 1981). 33 (8): 24–33. doi:10.1007/BF03339468. ISSN 1047-4838. OCLC 663900840.