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
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
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
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.
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.
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
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.
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.
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
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.
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
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 125oC) exposure
are relatively stable in that changes in properties during
the lifetime of a component are acceptable for most commercial
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
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.
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
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 190oC. The level of the
plateau toughness is affected by impurity content.
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