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).