Aluminium–lithium alloys (Al–Li alloys) are a set of alloysofaluminium and lithium, often also including copper and zirconium. Since lithium is the least denseelementalmetal, these alloys are significantly less dense than aluminium. Commercial Al–Li alloys contain up to 2.45% lithium by mass.[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 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%.
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]
When properly aged, lithium forms a metastableAl3Li 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]
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 aluminium–lithium alloys are generally superior to aluminium–copper or aluminium–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 aluminium alloys), poor corrosion resistance, and strong anisotropy of mechanical properties of rolled aluminium–lithium products has resulted in a paucity of applications.
Al-Li alloy powder is used in the production of lightweight sporting goods, including bicycles, tennis rackets, golf clubs, and baseball bats. Its high strength combined with reduced weight significantly enhances performance, speed, and maneuverability.[18][19] It is also used in the automobile industry as body panels, chassis parts, and suspension components.[20]
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;[21] 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).[22]
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 aluminium–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]
^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. 33 (8) (published August 1981): 24–33. doi:10.1007/BF03339468. ISSN1047-4838. OCLC663900840.
^ abMahalingam, K.; Gu, B. P.; Liedl, G. L.; Sanders, T. H. (February 1987). "Coarsening of [delta]'(Al3Li) Precipitates in Binary Al–Li Alloys". Acta Metallurgica. 35 (2): 483–498. doi:10.1016/0001-6160(87)90254-9. ISSN0001-6160. OCLC1460926.
^"NASA Facts: Super Lightweight External Tank"(PDF) (Press release). Huntsville, Alabama: National Aeronautics and Space Administration (NASA) Marshall Space Flight Center. April 2005. Archived(PDF) from the original on 4 January 2006.
^"Atlas V". Archived from the original on 30 October 2008. Retrieved 7 March 2019.
