Aluminum Alloys
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Transcript of Aluminum Alloys
ALUMINUM ALLOYS:
MECHANICAL BEHAVIOUR
Dra. A. Salas Zamarripa
Mechanical Behavior
The principal microstructural features that control the mechanical
properties of aluminum alloys are as follows:
Coarse intermetallic compounds (often
called constituent particles).
Constituent particles serve no useful
function in high-strength wrought alloys and
they are tolerated in most commercial
compositions because their removal would
necessitate a significant cost increase.
Aligned stringers of coarse intermetallic
compounds in a rolled aluminium alloy ( 250 x).
Mechanical Behavior
Smaller submicron particles, or dispersoids (typically 0.05–0.5 m):
These are formed during homogenization of the ingots by solid state
precipitation of containing elements which have modest solubility and
which diffuse slowly in solid aluminium.
These particles resist either dissolution or coarsening.
They serve to retard recrystallization and grain growth during
processing and heat treatment of the alloys concerned.
They may also exert an important influence on certain mechanical
properties through their effects both on the response of some alloys to
ageing treatments, and on dislocation substructures formed as a result
of plastic deformation.
Mechanical behavior Schematic representation of the
substructure of a cold worked
alloy containing coarse and fine
intermetallic particles
Fine precipitates (up to 0.1 m) which
form during age-hardening and normally
have by far the largest effect on
strengthening of alloys that respond to
such treatments.
Mechanical behavior
Grain size and shape.:
The most significant microstructural feature that differentiates wrought
products such as sheet from plate, forgings and extrusions is the degree of
recrystallization.
Aluminium dynamically recovers during hot deformation producing a network
of subgrains and this characteristic is attributed to its relatively high stacking-
fault energy.
However, thick sections, which experience less deformation, usually do not
undergo bulk recrystallization during processing so that an elongated grain
structure is retained
Dislocation substructure, notably that caused by cold working of those
alloys which do not respond to age-hardening, and that developed due to
service stresses.
Mechanical behavior
Crystallographic textures that form as a result of working and annealing,
particularly in rolled products. They have a marked effect on formability
(Section 2.1.4) and lead to anisotropic mechanical properties.
Aerospace Aluminum
2XXX
Al–Cu Alloys.
Heat treatable
High strength, at room and elevated temperatures
Typical ultimate tensile strength range: 27–62
ksi
Usually joined mechanically but some
alloys are weldable
Not as corrosion resistant as other alloys.
7XXX
Al–Zn Alloys.
Heat treatable
Very high strength; special high
toughness versions
Typical ultimate tensile strength
range: 32–88 ksi
Mechanically joined
8XXX
Alloys with Al-Other Elements
Heat treatable
• High conductivity, strength, hardness
• Typical ultimate tensile strength
range: 17–60 ksi
• Common alloying elements include Fe,
Ni and Li
2xxx & 7xxx Series
The aluminum–copper (2XXX series) and aluminum–zinc (7XXX
series) alloys are the primary alloys used in airframe structural
applications.
The 2XXX alloys are used in damage tolerance applications,
such as the lower wing skins and fuselage structure of commercial
aircraft, while the 7XXX alloys are used where higher strength is
required, such as the upper wing skins.
The 2XXX alloys also have slightly higher temperature capability.
Reducing impurities, in particular iron and silicon, has resulted in
higher fracture toughness and better resistance to fatigue crack
initiation and crack growth.
Examples of these newer alloys are 2524-T3, 7150-T77 and 7055-
T77, which are used on the Boeing 777.
2xxx & 7xxx Series
The venerable alloy 2024-T3 has been one of the most widely used alloys in fuselage construction. While it only has a moderate yield strength, it has very good resistance to fatigue crack growth and good fracture toughness.
However, the newer alloy 2524-T3 has a 15–20% improvement in fracture toughness and twice the fatigue crack growth resistance of 2024-T3.
The 7XXX alloys have higher strengths than the 2XXX alloys and are used in sheet, plate, forgings and extrusions.
Like 2024-T3, 7075-T6 has been used for a great many years in airframe construction; however, stress corrosion cracking has been a recurring problem.
Newer alloys, such as 7055-T77, have higher strength and damage tolerance than 7050-T7451, while 7085-T7651 has higher thick section toughness.
Fracture Toughness vs. Yield Strength
Yield Strength vs. Year of Introduction
Aerospace Materials
Al-Li Alloys
Dra. Adriana Salas Zamarripa
Al-Li alloys
AL-LI 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 (for
example, liquid oxygen and hydrogen fuel tanks for aerospace
vehicles).
The major development work began in the 1970s, when
aluminum producers accelerated the development of Al-Li alloys
as replacements for conventional airframe alloys.
The goal was to introduce ingot Al-Li alloys that could be
fabricated on the existing equipment of aluminum producers and
then used by airframe manufacturers as direct replacements for
the conventional aluminum alloys (which typically have
constituted 70 to 80% of the weight of current aircraft).
Al-Li alloys
The development work led to the introduction of commercial alloys 8090, 2090, and 2091 in the mid- 1980s; Weldalite 049 and CP276 were introduced shortly thereafter.
These alloys are characterized by the following approximate nominal (wt%) compositions (balance aluminum):
Weldalite 049: 5.4 Cu, 1.3 Li, 0.4 Ag, 0.4 Mg, 0.14 Zr
Alloy 2090: 2.7 Cu, 2.2 Li, 0.12 Zr
Alloy 2091: 2.1 Cu, 2.0 Li, 0.10 Zr
Alloy 8090: 2.45 Li, 0.12 Zr, 1.3 Cu, 0.95 Mg
Alloy CP276: 2.7 Cu, 2.2 Li, 0.5 Mg, 0.12 Zr
Commercial Al-Li 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.
Al-Li alloys
The principal disadvantages of peak-strength Al-Li 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 microstructurally small.
Li and Be are the most effective metallic additions for lowering
density.
Li is the lightest metallic element, and each 1% of lithium (up to the
4.2% Li solubility limit) reduces alloy density by about 3% and
increases modulus by about 5%.
In addition, Li in small amounts allows the precipitation
strengthening of aluminum when a homogeneous distribution of
coherent, spherical δ' (Al3Li) precipitates is formed during heat
treatment.
Al-Li alloys
Like other age-hardened aluminum alloys, aluminum-lithium alloys
achieve precipitation strengthening by thermal aging after a
solution heat treatment.
The precipitate structure is sensitive to a number of processing
variables, including the quenching rate following the solution heat
treatment, the degree of cold deformation prior to aging, and the
aging temperature and time.
Aluminum-lithium-base alloys are microstructurally unique:
Once the major strengthening precipitate (δ') is homogeneously
precipitated, it remains coherent even after extensive aging.
Extensive aging at high temperatures (>190 °C, or 375 °F) can result in
the precipitation of grain-boundary precipitates with five-fold symmetry.
Al-Li alloys
Various modifications in alloy chemistry and fabrication techniques
have been used in an attempt to improve the ductility and
toughness of Al-Li alloys while maintaining a high strength.
Cu, Mg, and Zr solute additions have been shown to have
beneficial effects.
Mg and Cu improve the strength of Al-Li alloys through solid
solution and precipitate strengthening, and they can minimize the
formation of PFZs near grain boundaries.
Zr, which forms the cubic Al3Zr coherent dispersoid, stabilizes the
subgrain structure and suppresses recrystallization.
Al-Li-X alloys show 7 to 12% higher stiffness, generally superior
fatigue crack propagation resistance, and improved toughness at
cryogenic temperatures.
Al-Li Alloys On the negative side, however, they can suffer from poor
short-transverse properties, and they have been shown to
display significantly accelerated fatigue crack extension rates
when cracks are microstructurally small
In addition to precipitation hardening, aluminum-lithium alloys
derive part of their strength from a controlled grain
microstructure generated through hot and cold deformation.
Al-LI alloy with delta-phase particles
Weldalite 049
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.
Alloy 2090
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.
A variety of tempers are being developed to offer useful combinations of
strength, toughness, corrosion resistance, damage tolerance, and fabricability..
Data concerning strength and toughness may be incomplete for some forms.
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 2091
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.
In general, the behavior of 2091 is similar to that of other 2xxx and
7xxx alloys.
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
applications.
Alloy 2091
The exfoliation resistance of 2091 is generally comparable to that of
similar gages of 2024-T3.
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.
Alloy 8090
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 The alloy is available as sheet, plate, extrusions, and
forgings and it can also be used for welded applications.
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.
Al-Li Alloys applications
The alloy 2095 (Al-Li-Cu) has excellent weldability, superior to that
of the 2000 series alloys, including alloy 2219, and is a strong
contender as fuel tank material for NASA space shuttle because of
the materials excellent cryogenic properties.
The 2000 and 8000 series Al-Li alloys are available commercially
in a variety of forms and tempers which can be selected to meet
the specific design requirements of either high strength (e.g.,
2090-TSX, 8091-T8), medium strength combined with
corrosion resistance and damage tolerance (e.g., 8090- T8XXX,
2091-T8X), or high damage tolerance (e.g., 2091-T8XXX)
Al-Li Alloys applications
The commercial alloys normally have strongly developed
textures resulting in strong anisotropy of strength and fracture
properties. Strength and fracture are also strongly influenced
by grain size and structure.
Strength is derived from precipitation of ’ (Al3Li), T1 (Al2CuLi),
S’ (Al2CuMg), ’ (Al2Cu), and other phases.
Applications of Al-U alloys are not widespread to date.
Alloy 8090-T83 is used in limited quantities by Airbus Industries,
for the D-nose skins of the leading edge of the A330/ 340
aircraft wing.
Al-Li Alloys applications
Al-Li Alloys applications
Al-Li Alloys applications
Alloys 2090-T83 and 2090- T62 are used by McDonnell Douglas
for some flooring sections in the C-17 airlifter craft.
The new Boeing 777 aircraft makes only limited use of Al-U
alloys.
In contrast, Westland-Agusta, U.K. /Italy is unique in making
extensive use of 8090 forgings and sheets and 2090 and 2091
sheets for the EHlOl helicopter.
The alloys are also being tested for a variety of new
applications, including lower wing skins and fuselage
applications (panels and doors).