Age-hardening wrought Al-alloys

Note: only one typical science note is linked from this example of a CES EduPack Level 2 datasheet. Additional eco properties and durability properties are also available at level 2, but not listed in this view—see also this ABS datasheet

Description

THE MATERIAL
The high-strength aluminum alloys rely on age-hardening: a sequence of heat treatment steps that causes the precipitation of a nano-scale dispersion of intermetallics that impede dislocation motion and impart strength. This can be as high as 700 MPa giving them a strength-to-weight ratio exceeding even that of the strongest steels. This record describes for the series of wrought Al alloys that rely on age-hardening requiring a solution heat treatment followed by quenching and ageing. This is recorded by adding TX to the series number, where X is a number between 0 and 8 that records the state of heat treatment. They are listed below using the IADS designations (see Technical notes for details).2000 series: Al with 2 to 6% Cu -- the oldest and most widely used aerospace series.6000 series: Al with up to 1.2% Mg and 1.3% Si -- medium strength extrusions and forgings.7000 series: Al with up to 8% Zn and 3% Mg -- the Hercules of aluminum alloys, used for high strength aircraft structures, forgings and sheet. Certain special alloys also contain silver. So this record, like that for the non-age hardening alloys, is broad, encompassing all of these.
COMPOSITION
2000 series: Al + 2 to 6% Cu + Fe, Mn, Zn and sometimes Zr
6000 series: Al + up to 1.2%Mg + 0.25% Zn + Si, Fe and Mn
7000 series: Al + 4 to 9 % Zn + 1 to 3% Mg + Si, Fe, Cu and occasionally Zr and Ag

The 2000 and 7000 series age-hardening aluminum alloys are the backbone of the aerospace industry. The 6000 series has lower strength but is more easily extruded: it is used for marine and ground transport systems.
GENERAL PROPERTIES
Density
2500
-
2900
kg/m^3
Price
*1.46
-
1.6
GBP/kg

MECHANICAL PROPERTIES

Young’s modulus

68

82

GPa

Shear modulus

25

28

GPa

Bulk modulus

64

70

GPa

Poisson’s ratio

0.32

0.36

Yield strength (elastic limit)

95

610

MPa

Tensile strength

180

620

MPa

Compressive strength

95

610

MPa

Elongation

1

20

%

Hardness—Vickers

60

160

HV

Fatigue strength at 10^7 cycles

57

210

MPa

Fracture toughness

21

35

MPa.m^1/2

Mechanical loss coefficient (tan delta)

1e-4

1e-3

 
THERMAL PROPERTIES

Melting point

495

640

°C

Maximum service temperature

120

200

°C

Minimum service temperature

-273.2

°C

Thermal conductor or insulator?
Good conductor
 

Thermal conductivity

118

174

W/m.K

Thermal expansion coefficent

22

24

µstrain/°C

Specific heat capacity

890

1020

J/kg.K

 
ELECTRICAL PROPERTIES

Electrical conductor or insulator?

Good conductor

Electrical resistivity

3.8

6

µohm.cm

 
OPTICAL PROPERTIES

Transparency

Opaque

 
Processability

Castability

4

5

 

Formability

3

4

 

Machinability
4
-
5
 
Weldability
3
-
4
 
Solder/brazability
2
-
3
 
 
Eco properties

Embodied energy, primary production

200

218

MJ/kg

CO2 footprint, primary production

11.3

12.8

kg/kg

Recycle
True
     
 

Supporting information

Design guidelines

The age-hardening alloys have exceptional strength at low weight, but the origin of the strength -- age hardening -- imposes certain design constraints. At its simplest, age-hardening involves a three step heat treatment.

Step 1: the wrought alloy, as sheet, extrusion or forging, is solution heat treated -- held for about 2 hours at around 550 C (it depends on the alloys) to make the alloying elements (Cu, Zn, Mg, Si etc) dissolve.

Step 2: the material is quenched from the solution-treatment temperature, typically by dunking or spraying it with cold water. This traps the alloying elements in solution. Quenching is a savage treatment that can cause distortion and create internal stresses that may require correction, usually by rolling.

Step 3: the material is aged, meaning that it is heated to between 120 and 190 C for about 8 hours during which the alloying elements condense into nano-scale dispersions of intermetallics (CuAl, CuAl2, Mg2Si and the like). It is this dispersion that gives the strength.

The result is a material that, for its weight, has remarkably high strength and corrosion resistance. But if it is heated above the solution treatment temperature -- by welding, for example -- the strength is lost. This means that assembly requires fasteners such as rivets, usual in airframe construction, or adhesives. Some 6000 series alloys can be welded, but they are of medium rather than high strength.

 

Technical notes

Until 1970, designations of wrought aluminum alloys were a mess; in many countries, they were simply numbered in the order of their development. The International Alloy Designation System (IADS), now widely accepted, gives each wrought alloy a 4-digit number. The first digit indicates the major alloying element or elements. Thus the series 1xxx describe unalloyed aluminum; the 2xxx series contain copper as the major alloying element, and so forth. The third and fourth digits are significant in the 1xxx series but not in the others; in 1xxx series they describe the minimum purity of the aluminum; thus 1145 has a minimum purity of 99.45%; 1200 has a minimum purity of 99.00%. In all other series, the third and fourth digits are simply serial numbers; thus 5082 and 5083 are two distinct aluminum-magnesium alloys. The second digit has a curious function: it indicates a close relationship: thus 5352 is closely related to 5052 and 5252; and 7075 and 7475 differ only slightly in composition. To these serial numbers are added a suffix indicating the state of hardening or heat treatment. The suffix F means 'as fabricated'. Suffix O means 'annealed wrought products'. The suffix H means that the material is 'cold worked'. The suffix T means that it has been 'heat treated'. More information on designations and equivalent grades can be found in the Users section of the Granta Design website, www.grantadesign.com

 
Phase diagram of Al
 

Phase diagram description

The 2000 series of wrought aluminium alloys are based on aluminium (Al) with 2.5-7% copper (Cu). This is the relevant part of the phase diagram.

 

Typical uses

2000 and 7000 series: aerospace structures, ultralight land-based transport systems.6000 series: cladding and roofing; medium strength extrusions, forgings and welded structures for automotive and general engineering.

 
Links

Reference

ProcessUniverse

Producers

No warranty is given for the accuracy of this data. Values marked * are estimates