Aluminum-Lithium Alloys: Part One

요약:

Aluminum alloys containing lithium are under much scrutiny to perform with expectations that they will improve weight and stiffness critical application even against other aluminum alloys and even carbon-fiber composites.
Specific benefits of Al-Li alloys are a reduced density and an increased modulus of elasticity with an increase in the specific modulus of 26% compared to alloy 7075-T651.

In recent years, the family of lithium containing aluminum alloys has received much attention because it offers the promise of substantial specific strength and specific stiffness advantages over other commercial 2XXX- and 7XXX-series aluminum alloys and carbon-fiber composites. This advantage makes them an ideal candidate for weight-critical and stiffness-critical structures for military, space and commercial applications.

Most high-strength aluminum alloys used in aircraft structures are mechanically fastened, which has the drawbacks of slow assembly and limitations in joining thin sections. Fusion welding of lightweight aluminum-lithium alloys has been evaluated by several investigators and has resulted in various degrees of success. However, there exists a critical need to develop new joining methods in order to extend the range of applications for these alloys and to improve the overall performance, durability, damage tolerance and life of safety-critical components and structures. Furthermore, new joining methods could facilitate use of these alloys in marine hardware, lightweight pressure vessels and lightweight armored vehicles.

Magnesium and lithium are the only two elemental additions which, when added to aluminum, have the potential ability to decrease its density. Beryllium also decreases the density of aluminum but it is extremely toxic and is a health hazard. Lithium is one of the few elements with substantial solubility in aluminum (4.2 wt % at 600°C/1112°F in a binary aluminum lithium alloy).

The potential for aluminum alloy density reduction through lithium additions is evident by comparing its atomic weight (6.94) with that of aluminum (26.98). Lithium additions to aluminum also cause a significant increase in elastic modulus. Each 1% lithium addition of aluminum, up to 4 wt-% lithium, decreases the density by 3%, and increases the elastic modulus by 6%. The specific modulus (modulus of elasticity by weight) of an alloy with 2.8 wt-% lithium is 21% higher than that of aluminum alloy 2024-T351 and 26% higher than that of aluminum alloy 7075-T651.

The higher specific modulus reduces the rate of fatigue crack growth. The decrease in density is far more effective in reducing structural weight than improved strength, modulus, toughness or fatigue resistance. For example, in an aluminum alloy having 3 wt-% lithium, structural weight savings of 10% on affected structures could be realized by direct substitution, and over 16% by design modification based on improved mechanical properties.

Properties

Li is the lightest metallic element and each 1% of Li reduces an alloy density by about 3% and increases modulus by about 6%

  • 7-10% Lower density.
  • 10-15% Higher Modulus.
  • Excellent fatigue and cryogenic toughness properties.
  • Higher stiffness.
  • Superior fatigue crack growth resistance.
  • Reduced ductility.
  • Low fracture toughness.
  • Ease of fabrication using conventional equipment and methods.
  • Some operations that generate molten metal, such as flame cutting and welding, are hazardous because of the possibility of explosion if moisture is present.
  • These alloys are typically solution-heat-treated and aged in conventional furnace equipment.

Disadvantages

Tendency to oxidize at elevated temperatures, leading to decreases in

  • Micro-hardness
  • electrical resistivity
  • precipitate density (material softens)

Applications

1) Aircraft parts such as leading and trailing edges, access covers, seat tracks and wing skins.
2) Military Applications: Certain types of military aircrafts parts like main wing box, center fuselage, control surfaces are made by Al-Li alloys. Al-Li alloys are used as a substitute for conventional Al alloys in helicopters, rockets and satellite systems.
3) Space Applications: Of all the benefits offered, by the use of Al-Li alloys, weight savings is the most critical in space applications. Al-Li alloy is a candidate material for the cryogenic tankage of booster systems. These alloys are used in cryogenic applications for example, liquid oxygen and hydrogen fuel tanks for aerospace vehicles.



Table 1: Physical properties of Al-Li alloys

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