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Source: Key to Metals

Metals Knowledge:Aluminum - Lithium Alloys

Published: 09 Oct 2009 01:17:40 PST

Aluminum-lithium alloys have been developed primarily to reduce the weight of aircraft and aerospace structures. More recently, they have been investigated for use in cryogenic applications.

The major development work began in the 1970-1980, when aluminum producers accelerated the development of aluminum-lithium alloys as replacements for conventional airframe alloys. The lower-density aluminum-lithium alloys were expected to reduce the weight and improve the performance of aircraft.

Commercial aluminum-lithium alloys are targeted as advanced materials for aerospace technology primarily because of their low density, high specific modulus, and excellent fatigue and cryogenic toughness properties. The superior fatigue crack propagation resistance of aluminum-lithium alloys, in comparison with that of traditional 2xxx and 7xxx alloys, is primarily due to high levels of crack tip shielding, meandering crack paths, and the resultant roughness-induced crack closure. However, the fact that these alloys derive their superior properties from the above mechanisms has certain implications with respect to small crack and variable-amplitude behavior.

The principal disadvantages of peak-strength aluminum-lithium alloys are reduced ductility and fracture toughness in the short transverse direction, anisotropy of in-plane properties, the need for cold work to attain peak properties, and accelerated fatigue crack extension rates when cracks are micro structurally small.

Commercial Aluminum-Lithium Alloys

Development of commercially available aluminum-lithium-base alloys was started by adding lithium to aluminum-copper, aluminum-magnesium, and aluminum-copper-magnesium alloys. These alloys were chosen to superimpose the precipitation-hardening characteristics of aluminum-copper-, aluminum-copper-magnesium-, and aluminum-magnesium-base precipitates to the hardening of lithium-containing precipitates. Proceeding in this manner, alloys 2020 (Al-Cu-Li-Cd), 01429 (Al-Mg-Li), 2090 (Al-Cu-Li), and 2091 and 8090 (Al-Cu-Mg-Li) evolved. Besides these registered alloys, other commercial aluminum-lithium alloys include Weldalite 049 and CP276.

Weldalite 049

Chemical composition: Cu - 5.4, Li - 1.3, Ag - 0.4, Mg - 0.4, Zr - 0.14.

Weldalite 049 shows high strength in variety of products and tempers. Its natural aging response is extremely strong with cold work (temper T3), and even stronger without cold work (T4); in fact, it has a stronger natural aging response than that of any other known aluminum alloy. Weldalite 049 undergoes reversion during the early stages of artificial aging and its ductility increases significantly up to 24%. Tensile strengths of 700 MPa have been attained in both T6 and 18 tempers produced in the laboratory.

Weldalite 049 has very good weldability. For example, it displays no discernable hot cracking in highly restrained weldment made by gas tungsten arc, gas metal arc and variable polarity plasma arc (VPPA) welding. Extremely high weldment strengths have been reported using conventional 2319 filler, and even higher weldment strengths have been obtained with the use of proprietary Weldalite filler.

Alloy 2090

Chemical composition: Cu - 2.7, Li - 2.2, Ag - 0.4, Zr - 0.12.

Alloy 2090 was developed to be a high-strength alloy with 8% lower density and 10% higher elastic modulus than 7075-T6, a major high-strength alloy used in current aircraft structures. Alloy 2090 was registered with the Aluminum Association in 1984. A variety of tempers are being developed to offer useful combinations of strength, toughness, corrosion resistance, damage tolerance, and fabricability.

Because alloy 2090 and its tempers are relatively new and in different phases of registration and characterization, data concerning strength and toughness may be incomplete for some forms.

In general, the engineering characteristics of aluminum-lithium alloys are similar to those of the current 2xxx and 7xxx high-strength alloys used by the aerospace industry. However, some material features of the 2090 products vary somewhat from those of the conventional aluminum alloys and should be considered during the design and material design phase.

These distinct characteristics of 2090 include:

  • An in-plane anisotropy of tensile properties that is higher than in conventional alloys.
  • An elevated temperature exposure for the peak-aged tempers (T86, T81 and T83) that shows good stability within 10% of original properties.
  • Excellent fatigue crack growth behavior.
  • The need for cold work to achieve optimum properties. In this characteristic, 2090 is similar to 2219 and 2024.
  • Shape-dependent behavior for extrusions with very high strengths.

Alloy 2090 sheet and plate, and 2090-T86 extrusions have demonstrated excellent resistance to exfoliation corrosion in extensive seacoast exposure tests. The resistance of these alloys and tempers is superior to that of 7075-T6, which, in some product forms, can suffer very severe exfoliation during a two-year seacoast exposure.

The stress-corrosion cracking (SCC) resistance of 2090 is strongly influenced by artificial aging. Tempers that are under-aged, such as T84, may be more susceptible to SCC than the near-peak-aged T83, T81, and T86 tempers.

Alloy 2091

Chemical composition: Cu - 2.1, Li - 2.0, Zr - 0.10.

Alloy 2091 was developed to be a damage-tolerant alloy with 8% lower density and 1% higher modulus than 2024-T3, a major high-toughness damage-tolerant alloy currently used for most aircraft structures. Alloy 2091 is also suitable for use in secondary structures where high strength is not critical.

Alloy 2091 has been registered with the Aluminum Association. A variety of tempers are being developed to offer useful combinations of strength, corrosion resistance, damage tolerance, and fabricability. The microstructure of 2091 varies according to product thickness and producer; in general, gages above 3.5 mm have an unrecrystallized microstructure, and lighter gages feature an elongated recrystallized grain structure.

In general, the behavior of 2091 is similar to that of other 2xxx and 7xxx alloys. Material characteristics that have been cause for concern in other aluminum-lithium alloys are of less concern in 2091. Alloy 2091 depends less on cold work to attain its properties than does 2024. The properties of 2091 after elevated-temperature (up to 125℃) exposure are relatively stable in that changes in properties during the lifetime of a component are acceptable for most commercial applications.

The exfoliation resistance of 2091-T84, like that of 2024, varies depending on the microstructure of the product and its quench rate. The more unrecrystallized the structure, the more even the exfoliation attack. However, the exfoliation resistance of 2091 is generally comparable to that of similar gages of 2024-T3.

The microstructural relationship for stress-corrosion cracking in sheet products is the converse of that for exfoliation. As the microstructure becomes more fibrous, the SCC threshold increases. For thicker unrecrystallized structures and thinner elongated recrystallized structures, it is possible to attain an SCC threshold of 240 MPa, which is quite good compared to that of 2024-T3. For thinner products, the threshold varies by gage and producer; it may be as low as 50 to 60% of the yield strength or as high as 75% of the yield strength.

Although fatigue testing on 2091 has been done by a number of labs, producers, and users, the results have been difficult to interpret. The results for 2091 have been superior to those for 2024, roughly equivalent to those for 2024, or inferior to those for 2024. In general, the consensus is that under controlled and similar circumstances, the fatigue properties of 2091-T84 are sufficient to allow it to be used as a substitute for 2024.

Alloy 8090

Chemical composition: Li - 2.45, Zr - 0.12, Cu - 1.3, Mg - 0.95.

Alloy 8090 was developed to be a damage-tolerant medium-strength alloy with about 10% lower density and 11% higher modulus than 2024 and 2014, two commonly used aluminum alloys. Its use is aimed at applications where damage tolerance and the lowest possible density are critical. The alloy is available as sheet, plate, extrusions, and forgings and it can also be used for welded applications.

The chemical composition of 8090 has been registered with the Aluminum Association. A variety of tempers have been developed that offer useful combinations of strength, corrosion resistance, damage tolerance, and fabricability.

Because alloy 8090 and its tempers and product forms are relatively new and unregistered, property data are incomplete. The medium-strength products of alloy 8090 are aged to near-peak strength and show small changes in properties after elevated-temperature exposure. The very underaged (damage-tolerant) products will undergo additional aging upon exposure to elevated temperatures.

Changes in strength and toughness at cryogenic temperatures are more pronounced in 8090 than in conventional aluminum alloys: 8090 has a substantially higher strength and toughness at cryogenic temperatures.

The improving quality of commercially available aluminum-lithium alloys such as 8090 has resulted in significant improvements in short-transverse ductility and, consequently, short-transverse tensile strength. Research on the short-transverse fracture toughness of 8090 has shown that the property reaches a minimum plateau at an aging temperature of 190℃. The level of the plateau toughness is affected by impurity content.

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