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Aluminum and Aluminum Alloys Davis

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Aluminum and

Aluminum Alloys

Introduction and O verview

General Characteristics. The unique combinations of properties

provided by aluminum and its alloys make aluminum one of the most ver-

satile, economical, and attractive metallic materials for a broad range of

uses—from soft, highly ductile wrapping foil to the most demanding engi-

neering applications. Aluminum alloys are second only to steels in use as

structural metals.

Aluminum has a density of only 2 g/cm 3 , approximately one-third as

much as steel (7 g/cm 3 ). One cubic foot of steel weighs about 490 lb;

a cubic foot of aluminum, only about 170 lb. Such light weight, coupled

with the high strength of some aluminum alloys (exceeding that of struc-

tural steel), permits design and construction of strong, lightweight structures

that are particularly advantageous for anything that moves—space vehi-

cles and aircraft as well as all types of land- and water-borne vehicles.

Aluminum resists the kind of progressive oxidization that causes steel to

rust away. The exposed surface of aluminum combines with oxygen to form

an inert aluminum oxide film only a few ten-millionths of an inch thick,

which blocks further oxidation. And, unlike iron rust, the aluminum oxide

film does not flake off to expose a fresh surface to further oxidation. If the

protective layer of aluminum is scratched, it will instantly reseal itself.

The thin oxide layer itself clings tightly to the metal and is colorless and

transparent—invisible to the naked eye. The discoloration and flaking of

iron and steel rust do not occur on aluminum.

Appropriately alloyed and treated, aluminum can resist corrosion by

water, salt, and other environmental factors, and by a wide range of other

chemical and physical agents. The corrosion characteristics of aluminum

alloys are examined in the section “Effects of Alloying on Corrosion

Behavior” in this article.

Alloying: Understanding the Basics

J. Davis, p351-

DOI:10/autb2001p

Copyright © 2001 ASM International®

All rights reserved.

asminternational

352 / Light M etals and Alloys

Aluminum surfaces can be highly reflective. Radiant energy, visible

light, radiant heat, and electromagnetic waves are efficiently reflected,

while anodized and dark anodized surfaces can be reflective or absorbent.

The reflectance of polished aluminum, over a broad range of wave

lengths, leads to its selection for a variety of decorative and functional

uses.

Aluminum typically displays excellent electrical and thermal conduc-

tivity, but specific alloys have been developed with high degrees of elec-

trical resistivity. These alloys are useful, for example, in high-torque

electric motors. Aluminum is often selected for its electrical conductivity,

which is nearly twice that of copper on an equivalent weight basis. The

requirements of high conductivity and mechanical strength can be met by

use of long-line, high-voltage, aluminum steel-cored reinforced transmis-

sion cable. The thermal conductivity of aluminum alloys, about 50 to 60%

that of copper, is advantageous in heat exchangers, evaporators, electri-

cally heated appliances and utensils, and automotive cylinder heads and

radiators.

Aluminum is nonferromagnetic, a property of importance in the electri-

cal and electronics industries. It is nonpyrophoric, which is important in

applications involving inflammable or explosive-materials handling or

exposure. Aluminum is also non-toxic and is routinely used in containers

for food and beverages. It has an attractive appearance in its natural finish,

which can be soft and lustrous or bright and shiny. It can be virtually any

color or texture.

The ease with which aluminum may be fabricated into any form is one

of its most important assets. Often it can compete successfully with

cheaper materials having a lower degree of workability. The metal can be

cast by any method known to foundrymen. It can be rolled to any desired

thickness down to foil thinner than paper. Aluminum sheet can be

stamped, drawn, spun, or roll formed. The metal also may be hammered

or forged. Aluminum wire, drawn from rolled rod, may be stranded into

cable of any desired size and type. There is almost no limit to the differ-

ent profiles (shapes) in which the metal can be extruded.

Alloy Categories. It is convenient to divide aluminum alloys into two

major categories: wrought compositions and cast compositions. A further

differentiation for each category is based on the primary mechanism of

property development. Many alloys respond to thermal treatment based on

phase solubilities. These treatments include solution heat treatment,

quenching, and precipitation, or age, hardening. For either casting or

wrought alloys, such alloys are described as heat treatable. A large number

of other wrought compositions rely instead on work hardening through

mechanical reduction, usually in combination with various annealing pro-

cedures for property development. These alloys are referred to as work

hardening. Some casting alloys are essentially not heat treatable and are

1 xx: Controlled unalloyed (pure) compositions, especially for rotor

manufacture

2 xx: Alloys in which copper is the principal alloying element. Other

alloying elements may be specified.

3 xx: Alloys in which silicon is the principal alloying element. The

other alloying elements such as copper and magnesium are specified.

The 3xx series comprises nearly 90% of all shaped castings pro-

duced.

4 xx: Alloys in which silicon is the principal alloying element.
5 xx: Alloys in which magnesium is the principal alloying element.
6 xx: Unused
7 xx: Alloys in which zinc is the principal alloying element. Other

alloying elements such as copper and magnesium may be specified.

8 xx: Alloys in which tin is the principal alloying element.
9 xx: Unused

Heat-treatable casting alloys include the 2xx, 3 xx, and 7xx series.

Tables 4 and 5 list nominal compositions for representative wrought and

cast aluminum alloys. It should be noted that the alloy compositions list-

ed in these tables make up a rather small percentage of the total amount

of compositions developed. More than 500 alloy designations/composi-

tions have been registered by the Aluminum Association Inc. for alu-

minum alloys. Composition limits for these alloys can be found in the

Metals Handbook Desk Edition, 2nd ed., (see the article “Chemical

Compositions and International Designations on pages 426–436) and in

Registration Records on wrought alloys, castings, and ingots published by

the Aluminum Association.

354 / Light M etals and Alloys

Table 1 Classification of wrought aluminum alloys according to their strengthening mechanism Alloy system Aluminum series

Work-hardenable alloys

Pure Al 1 xxx Al-Mn 3 xxx Al-Si 4 xxx Al-Mg 5 xxx Al-Fe 8 xxx Al-Fe-Ni 8 xxx

Precipitation-hardenable alloys

Al-Cu 2 xxx Al-Cu-Mg 2 xxx Al-Cu-Li 2 xxx Al-Mg-Si 6 xxx Al-Zn 7 xxx Al-Zn-Mg 7 xxx Al-Zn-Mg-Cu 7 xxx Al-Li-Cu-Mg 8 xxx

Aluminum and Aluminum Alloys / 355

Table 2 Strength ranges of various wrought aluminum alloys

Aluminum Type of Tensile Association alloy Strengthening strength range series composition method MPa ksi

1 xxx Al Cold work 70–175 10– 2 xxx Al-Cu-Mg Heat treat 170–310 25– (1–2% Cu) 2 xxx Al-Cu-Mg-Si Heat treat 380–520 55– (3–6% Cu) 3 xxx Al-Mn-Mg Cold work 140–280 20– 4 xxx Al-Si Cold work 105–350 15– (some heat treat) 5 xxx Al-Mg Cold work 140–280 20– (1–2% Mg) 5 xxx Al-Mg-Mn Cold work 280–380 40– (3–6% Mg) 6 xxx Al-Mg-Si Heat treat 150–380 22– 7 xxx Al-Zn-Mg Heat treat 380–520 55– 7 xxx Al-Zn-Mg-Cu Heat treat 520–620 75– 8 xxx Al-Li-Cu-Mg Heat treat 280–560 40–

Table 3 Strength ranges of various cast aluminum alloys Tensile strength range

Alloy system (AA designation) MPa ksi

Heat treatable sand cast alloys (various tempers)

Al-Cu (201–206) 353–467 51– Al-Cu-Ni-Mg (242) 186–221 27– Al-Cu-Si (295) 110–221 16– Al-Si-Cu (319) 186–248 27– Al-Si-Cu-Mg (355, 5% Si, 1% Cu, 0% Mg) 159–269 23– Al-Si-Mg (356, 357) 159–345 23– Al-Si-Cu-Mg (390, 17% Si, 4% Cu, 0% Mg) 179–276 26– Al-Zn (712, 713) 241 35

Non-heat treatable die cast alloys

Al-Si (413, 443, F temper) 228–296 33– Al-Mg (513, 515, 518, F temper) 276–310 40–

Non-heat treatable permanent mold cast alloys

Al-Sn (850, 851, 852, T5 temper) 138–221 20–

Applications. Aluminum alloys are economical in many applications.

They are used in the automotive industry, aerospace industry, in construc-

tion of machines, appliances, and structures, as cooking utensils, as cov-

ers for housings for electronic equipment, as pressure vessels for

cryogenic applications, and in innumerable other areas. Tables 6 and 7 list

typical applications for some of the more commonly used wrought and

cast alloys, respectively.

W rought Alloy Classes

As described in the “Introduction and Overview” to this article, alu-

minum alloys are commonly grouped into an alloy designation series. The

general characteristics of the wrought alloy groups are described below.

Strength ranges, nominal compositions, and applications for wrought alu-

minum alloys are listed in Tables 2, 4, and 6, respectively.

1 xxx Series. Aluminum of 99% or higher purity has many applica-

tions, especially in the electrical and chemical fields. These grades of alu-

minum are characterized by excellent corrosion resistance, high thermal

Aluminum and Aluminum Alloys / 357

Table 5 D esignations and nominal compositions of common aluminum alloys used for casting

Composition, %

AA number Product(a) Cu Mg Mn Si Others

201 S 4 0 0 ... 0 Ag, 0 Ti 206 S or P 4 0 0 0(b) 0 Ti, 0 Fe(b) A206 S or P 4 0 0 0(b) 0 Ti, 0 Fe(b) 208 S 4 ... ... 3. ... 242 S or P 4 1 ... ... 2 Ni 295 S 4 ... ... 0. ... 96 P 4 ... ... 2. ... 308 S or P 4 ... ... 5. ... 319 S or P 3 ... ... 6. ... 336 P 1 1 ... 12 2 Ni 354 P 1 0 ... 9. ... 355 S or P 1 0 0(b) 5 0 Fe(b), 0(b) C355 S or P 1 0 0(b) 5 0 Fe(b), 0(b) 356 S or P 0(b) 0 0(b) 7 0 Fe(b), 0 Zn(b) A356 S or P 0(b) 0 0(b) 7 0 Fe(b), 0 Zn(b) 357 S or P ... 0 ... 7. ... A357 S or P ... 0 ... 7 0 Ti, 0 Be 359 S or P ... 0 ... 9. ... 360 D ... 0 ... 9 2 Fe(b) A360 D ... 0 ... 9 1 Fe(b) \380 D 3 ... ... 8 2 Fe(b) A380 D 3 ... ... 8 1 Fe(b) 383 D 2 ... ... 10. ... 384 D 3 ... ... 11 3 Zn(b) A384 D 3 ... ... 11 1 Zn(b) 390 D 4 0 ... 17 1 Zn(b) A390 S or P 4 0 ... 17 0 Zn(b) 413 D ... ... ... 12 2 Fe(b) A413 D ... ... ... 12 1 Fe(b) 4430 S 0(b) ... ... 5. ... A443 S 0(b) ... ... 5. ... B443 S or P 0(b) ... ... 5. ... C443 D 0(b) ... ... 5 2 Fe(b) 514 S ... 4. ... ... ... 518 D ... 8. ... ... ... 520 S ... 10. ... ... ... 535 S ... 6 0 ... 0 Ti A535 S ... 7 0. ... ... B535 S ... 7 ... ... 0 Ti 712 S or P ... 0 ... ... 5 Zn, 0 Cr, 0 Ti 713 S or P 0 0 ... ... 7 Zn, 0 Cu 771 S ... 0 ... ... 7 Zn, 0 Cr, 0 Ti 850 S or P 1 ... ... ... 6 Sn, 1 Ni

(a) S, sand casting; P, permanent mold casting; D, die casting. (b) Maximum

358 / Light M etals and Alloys

1100 Commercially pure aluminum highly resistant to chemical attack and weathering. Low cost, ductile for deep drawing, and easy to weld. Used for high-purity applications such as chemical processing equipment. Also for nameplates, fan blades, flue lining, sheet metal work, spun holloware, and fin stock 1350 Electrical conductors 2011 Screw machine products. Appliance parts and trim, ordnance, auto- motive, electronic, fasteners, hardware, machine parts 2014 Truck frames, aircraft structures, automotive, cylinders and pis- tons, machine parts, structurals 2017 Screw machine products, fittings, fasteners, machine parts 2024 For high-strength structural applica- tions. Excellent machinability in the T-tempers. Fair workability and fair corrosion resistance. Alclad 2024 combines the high strength of 2024 with the corro- sion resistance of the commer- cially pure cladding. Used for truck wheels, many structural aircraft applications, gears for machinery, screw machine products, automotive parts, cylinders and pistons, fasteners, machine parts, ordnance, recre- ation equipment, screws and rivets 2219 Structural uses at high temperature (to 315 °C, or 600 °F). High- strength weldments 3003 Most popular general-purpose alloy. Stronger than 1100 with same good formability and weldabilty. For general use including sheet metal work, stampings, fuel tanks, chemical equipment, con- tainers, cabinets, freezer liners, cooking utensils, pressure ves- sels, builder’s hardware, storage tanks, agricultural applications, appliance parts and trim, archi- tectural applications, electronics, fin stock, fan equipment, name plates, recreation vehicles, trucks and trailers. Used in drawing and spinning. 3004 Sheet metal work, storage tanks, agri cultural applications, building products, containers, electronics, furniture, kitchen equipment, recreation vehicles, trucks and trailers

3105 Residential siding, mobile homes, rain-carrying goods, sheet metal work, appliance parts and trim, automotive parts, building prod- ucts, electronics, fin stock, furniture, hospital and medical equipment, kitchen equipment, recreation vehicles, trucks and trailers 5005 Specified for applications requiring anodizing; anodized coating is cleaner and lighter in color than 3003. Uses include appliances, utensils, architectural, applica- tions requiring good electrical conductivity, automotive parts, containers, general sheet metal, hardware, hospital and medical equipment, kitchen equipment, name plates, and marine appli- cations 5052 Stronger than 3003 yet readily formable in the intermediate tempers. Good weldability and resistance to corrosion. Uses include pressure vessels, fan blades, tanks, electronic panels, electronic chassis, medium- strength sheet metal parts, hydraulic tube, appliances, agri- cultural applications, architect- ural uses, automotive parts, building products, chemical equipment, containers, cooking utensils, fasteners, hardware, highway signs, hospital and medical equipment, kitchen equipment, marine applications, railroad cars, recreation vehicles, trucks and trailers 5056 Cable sheathing, rivets for magne- sium, screen wire, zippers, auto- motive applications, fence wire, fasteners 5083 For all types of welded assemblies, marine components, and tanks requiring high weld efficiency and maximum joint strength. Used in pressure vessels up to 65 °C (150 °F) and in many cryogenic applications, bridges, freight cars, marine components, TV towers, drilling rigs, trans- portation equipment, missile com- ponents, and dump truck bodies. Good corrosion resistance 5086 Used in generally the same types of applications as 5083, particularly where resistance to either stress corrosion or atmospheric corro- sion is important

5454 For all types of welded assemblies, tanks, pressure vessels. ASME code approved to 205 °C (400 °F). Also used in trucking for hot asphalt road tankers and dump bodies; also, for hydrogen peroxide and chemical storage vessels

5456 For all types of welded assemblies, storage tanks, pressure vessels, and marine components. Used where best weld efficiency and joint strength are required. Restricted to temperatures below 65 °C (150 °F) 5657 For anodized auto and appliance trim and name plates 6061 Good formability, weldability, corro- sion resistance, and strength in the T-tempers. Good general- purpose alloy used for a broad range of structural applications and welded assemblies including truck components, railroad cars, pipelines, marine applications, furniture, agricultural applica- tions, aircrafts, architectural applications, automotive parts, building products, chemical equipment, dump bodies, electri- cal and electronic applications, fasteners, fence wire, fan blades, general sheet metal, highway signs, hospital and medical equipment, kitchen equipment, machine parts, ordnance, recre- ation equipment, recreation vehicles, and storage tanks 6063 Used in pipe railing, furniture, archi- tectural extrusions, appliance parts and trim, automotive parts, building products, electrical and electronic parts, highway signs, hospital and medical equipment, kitchen equipment, marine appli- cations, machine parts, pipe, rail- road cars, recreation equipment, recreation vehicles, trucks and trailers 7050 High-strength alloy in aircraft and other structures. Also used in ordnance and recreation equip ment 7075 For aircraft and other applications requiring highest strengths. Alclad 7075 combines the strength advantages of 7075 with the corrosion-resisting properties of commercially pure aluminum- clad surface. Also used in machine parts and ordnance

Table 6 Selected applications for wrought aluminum alloys

Alloy Description and selected applications Alloy Description and selected applications Alloy Description and selected applications

360 / Light M etals and Alloys

make truck and aircraft wheels, truck suspension parts, aircraft fuselage

and wing skins, structural parts, and those parts requiring good strength at

temperatures up to 150 °C (300 °F). Figure 1 shows the relationships

between some of the more commonly used alloys in the 2xxx series.

3 xxx Series. Manganese is the major alloying element of 3xxx series

alloys. These alloys generally are non-heat-treatable but have about 20%

more strength than 1xxx series alloys. Because only a limited percentage

of manganese (up to about 1%) can be effectively added to aluminum,

manganese is used as a major element in only a few alloys. However, one

of these, the popular 3003 alloy, is widely used as a general-purpose alloy

for moderate-strength applications requiring good workability.

4 xxx Series. The major alloying element in 4xxx series alloys is silicon,

which can be added in sufficient quantities (up to 12%) to cause substan-

tial lowering of the melting range without producing brittleness. For this

reason, aluminum-silicon alloys are used in welding wire and as brazing

2011

2117 2017

2048 2024

2025

2124

2018

2218

2618

2020

2090

2519

2219

2014

2036

2319

2419

Extrusions, bar, tubing; screw machine stock; good strength/machinability

Duralumin Rivet wire, softer

Rivets Fasteners

Forging version of 2024

Plate product Better toughness (lower impurities) Good SCC resistance

Add Ni for improved high-tempertature strength

Add Li for high modulus, low density

Good weldability

Higher strength (≅ 7039)

Greater SCC resistance than 5083 Used as armor plate

High-strength plate/sheet up to 1 in.; good SCC resistance

Commercial version Good formability, moderate strength, spot weldable

Highest strength Poor SCC resistance Good properties (thick sections) Good elevated-temperature properties

Weld wire for 2219 (better strength than 4043, possible better ductility)

Good weldability Good elevated-temperature strength (up to 316 °C, or 600 °F) High fracture toughness

Good weldability Higher toughness

Lower strength, very high toughness

Fig. 1 Relationships among commonly used alloys in the 2xxx series (Al-Cu)

Aluminum and Aluminum Alloys / 361

alloys for joining aluminum, where a lower melting range than that of the

base metal is required. Most alloys in this series are non-heat treatable, but

when used in welding heat-treatable alloys, they pick up some of the

alloying constituents of the latter and so respond to heat treatment to a

limited extent. The alloys containing appreciable amounts of silicon

become dark gray to charcoal when anodic oxide finishes are applied and

hence are in demand for architectural applications. Alloy 4032 has a low

coefficient of thermal expansion and high wear resistance; thus it is well

suited to production of forged engine pistons.

5 xxx Series. The major alloying element in 5xxx series alloys is mag-

nesium. When it is used as a major alloying element or with manganese,

the result is a moderate-to-high-strength workhardenable alloy.

Magnesium is considerably more effective than manganese as a hardener,

about 0% Mg being equal to 1% Mn, and it can be added in consid-

erably higher quantities. Alloys in this series possess good welding char-

acteristics and good resistance to corrosion in marine atmospheres.

However, certain limitations should be placed on the amount of cold work

and the safe operating temperatures permissible for the higher-magnesium

alloys (over ~3% for operating temperatures above ~65 °C, or 150 °F)

to avoid susceptibility to stress-corrosion cracking. Figure 2 shows the

relationships between some of the more commonly used alloys in the 5xxx

series.

5005

5050

5252

5457

5405

5052

5154

5652

5254

5086

5356

5083

5456

5454

5654

5356

5183

5556

5554

5182 5056

Mg,% Mg,% Weldable / strength (TS/YS) Mg,%

Weld wires

Increase strength, Mg content

(28/13)

(35/17)

(38/17)

(40/18)

(42/21)

(45/23)

(36/17)

2.

3.

4.

5.

4.

5.

2.

0.

2.

1.

2.

Mg,% Nonweldable / strength (TS/YS)Mg,%

4 (42/22) 5.

3.

Beauty

Anodize Auto trim Super clean Anodize Auto trim Formable Eyeglass frames

Auto panels

Container ends

Fig. 2 Relationships among commonly used alloys in the 5xxx series (Al-Mg). Tensile

strength (TS) and yield strength (YS) are in ksi units.

Aluminum and Aluminum Alloys / 363

toughness. Figure 4 shows the relationships between some of the more

commonly used alloys in the 7xxx series.

8 xxx series alloys constitute a wide range of chemical compositions.

For example, improved elevated-temperature performance is achieved

through the use of dispersion-strengthened Al-Fe-Ce alloys (e., 8019) or

Al-Fe-V-Si alloys (e., 8009) made by powder metallurgy processing.

Lower density and higher stiffness can be achieved in lithium-containing

alloys (e., 8090). The latter alloy, which is precipitation hardenable, has

replaced medium-to-high strength 2xxx and 7xxx alloys in some air-

craft/aerospace applications (e., helicopter components).

7001 7008

7178

7076

7079

7049

7115

7108

7005

7150

7050*

7010*

T76 temper

T73 temper

7475

7149

Rod, extrusions Ultrahigh strength (98TS/91YS) Cladding

Forgings and extrusions

More Zn, less Cr

Forgings and extrusions High SCC resistance Tougher than 7075

Forgings, plate, extrusions Highest SCC resistance High strength

7010 and 7050 use Zr rather than Cr for deeper hardening

Moderate strength Weldable Eliminate Cu Reduce Zn, Cr, Mn Add Ti, Zr

Clad alloy like 7008 without Cr+ 0% more of others

Higher strength than 7050 in thicker sections (84TS/78YS)

Forgings, plate extrusions High strength and toughness High resistance to SCC and exfoliation

High strength High SCC resistance Good toughness in thick sections

Better SCC resistance Excellent exfoliation resistance Moderate strength

Best SCC resistance Lowest strength Excellent exfoliation resistance

7075

very similar

7072

Forging Excellent forgeability and elongation

High strength Highest toughness (Si instead of Cu, reduce Mg+Cr, slightly increase Zn)

Highest strength (88TS/78YS) Exfoliation problem (use T76 to avoid)

Basic high-strength alloy Poor corrosion resistance Poor SCC resistance Poor exfoliation resistance

Higher strength than 7075 Excellent exfoliation/SCC resistance High toughness Better fatigue resistance

Fig. 4 Relationships among commonly used alloys in the 7xxx series (Al-Zn-Cu-Mg-Cr).

Tensile strength (TS) and yield strength (YS) are in ksi units

364 / Light M etals and Alloys

Cast Alloy Classes

Aluminum casting alloys are based on the same alloy systems as those

of wrought aluminum alloys, are strengthened by the same mechanisms

(with the general exception of strain hardening), and are similarly classi-

fied into non-heat-treatable and heat treatable types. The major difference

is that the casting alloys used in the greatest volumes contain alloying

additions of silicon far in excess of the amounts in most wrought alloys.

Silicon is the alloying element that literally makes the commercial viabil-

ity of the high-volume aluminum casting industry possible. Silicon con-

tents from ~4% to the eutectic level of ~12% reduce scrap losses, permit

production of much more intricate designs with greater variations in section

thickness, and yield castings with higher surface and internal quality. These

benefits derive from the effects of silicon in increasing fluidity, reducing

cracking, and improving feeding to minimize shrinkage porosity.

Figure 5 shows the complete phase diagram of the binary aluminum-

silicon system. This is a simple eutectic system with limited terminal solu-

bility and is the basis for the 4xx alloys. Metallographic structures of the

pure components and of several intermediate compositions show typical

morphologies. The intermediate compositions are mixtures of aluminum

Fig. 5 Aluminum-silicon phase diagram and cast microstructures of pure components and of alloys of various compo-

sitions. Alloys with less than 12% Si are referred to as hypoeutectic, those with close to 12% Si as eutectic, and

those with over 12% Si as hypereutectic

366 / Light M etals and Alloys

these phases. The alloys containing both copper and magnesium have

higher strengths at elevated temperatures.

Higher-silicon-content alloys are preferred for casting by the permanent

mold and die casting processes. The thermal expansion coefficient

decreases with increasing silicon and nickel contents. A low expansion

coefficient is beneficial for engine applications such as pistons and cylin-

der blocks. When the silicon content exceeds 12%, as in alloys 390.

through 393, primary silicon crystals are present and, if fine and well

distributed, enhance wear resistance.

4 xx Series. Alloys of the 4xx group, based on the binary aluminum-

silicon system and containing from 5 to 12% Si, find many applications

where combinations of moderate strength and high ductility and impact

resistance are required. Bridge railing support castings are a representative

example.

5 xx Series. The aluminum-magnesium alloys in the 5xx group are

essentially single phase binary alloys with moderate-to-high strength

and toughness properties. High corrosion resistance, especially to sea-

water and marine atmospheres, is the primary advantage of castings

made of Al-Mg alloys. Best corrosion resistance requires low impurity

content (both solid and gaseous), and thus alloys must be prepared from

high-quality metals and handled with great care in the foundry. These

alloys are suitable for welded assemblies and are often used in architec-

tural and other decorative or building needs. Aluminum-magnesium

alloys also have good machinability and an attractive appearance when

anodized.

7 xx Series. The 7xx aluminum-zinc-magnesium alloys are notable

for their combinations of good finishing characteristics, good general cor-

rosion resistance, and the capability of developing high strength through

natural aging without heat treatment.

8 xx Series. Alloys of the 8xx group contain ~6% Sn and small

amounts of copper and nickel for strengthening. These alloys were devel-

oped for bearing applications (tin imparts lubricity), for example, con-

necting rods and crankcase bearings for diesel engines.

Alloying and Second-Phase Constituents

The predominant reason for alloying is to increase strength, hardness,

and resistance to wear, creep, stress relaxation or fatigue. Effects on these

properties are specific to the different alloying elements and combinations

of them, and are related to their alloy phase diagrams and to the

Aluminum and Aluminum Alloys / 367

microstructures and substructures that they form as a result of solidifica-

tion, thermomechanical history, heat treatment and/or cold working.

The tensile yield strength of super-purity aluminum in its annealed

(softest) state is approximately 10 MPa (1 ksi), whereas those of some

heat treated commercial high-strength alloys exceed 550 MPa (80 ksi).

When the magnitude of this difference (an increase of over 5000%) is con-

sidered, this practical, everyday accomplishment, which is just one aspect

of the physical metallurgy of aluminum, is truly remarkable. Higher

strengths, up to a yield strength of 690 MPa (100 ksi) and over, may be

readily produced, but the fracture toughness of such alloys does not meet

levels considered essential for aircraft or other critical-structure applica-

tions.

The elements that are most commonly present in commercial alloys to

provide increased strength—particularly when coupled with strain hard-

ening by cold working or with heat treatment, or both—are copper, mag-

nesium, manganese, silicon, and zinc (Fig. 6). These elements all have

significant solid solubility in aluminum, and in all cases the solubility

increases with increasing temperature (see Fig. 7).

For those elements that form solid solutions, the strengthening effect

when the element is in solution tends to increase with increasing differ-

ence in the atomic radii of the solvent (Al) and solute (alloying element)

atoms. This factor is evident in data obtained from super-purity binary

solid-solution alloys in the annealed state, presented in Table 8, but it is

evident that other effects are involved, chief among which is an electronic

bonding factor. The effects of multiple solutes in solid solution are some-

what less than additive and are nearly the same when one solute has a

larger and the other a smaller atomic radius than that of aluminum as

Fig. 6 The principal aluminum alloys

Aluminum and Aluminum Alloys / 369

precipitation in the solid state during postsolidification heating also

increases strength and hardness. The rates of increase per unit weight of

alloying element added are frequently similar to but usually lower than

those resulting form solid solution. This “second-phase” hardening occurs

even though the constituent particles are of sizes readily resolved by opti-

cal microscopy. These irregularly shaped particles form during solidifica-

tion and occur mostly along grain boundaries and between dendrite arms.

Manganese and chromium are included in the group of elements that

form predominantly second phase constituents, because in commercial

alloys they have very low equilibrium solid solubilities. In the case of

many compositions containing manganese, this is because iron and silicon

are also present and form the quaternary phase Al 12 (Fe, Mn) 3 Si. In alloys

containing copper and manganese, the ternary phase Al 20 Cu 2 Mn 3 is

formed. Most of the alloys in which chromium is present also contain

magnesium, so that during solid-state heating they form Al 12 Mg 2 Cr,

which also has very low equilibrium solid solubility. The concentrations

of manganese and/or chromium held in solid solution in as-cast ingot that

has been rapidly solidified and cooled from the molten state greatly

exceed the equilibrium solubility. The solid solution is thus supersaturat-

ed and metastable. Ingot preheating for wrought commercial alloys con-

taining these elements is designed to cause solid-state precipitation of the

complex phase containing one or the other of these elements that is appro-

priate to the alloy composition. This precipitation does not cause apprecia-

ble hardening, nor is it intended that it should. Its purpose is to produce

finely divided and dispersed particles that retard or inhibit recrystallization

Fig. 8 Correlation between tensile yield, strength elongation, and magne-

sium content for some commercial aluminum alloys

370 / Light M etals and Alloys

and grain growth in the alloy during subsequent heatings. The precipitate

particles of Al 12 (Fe,Mn) 3 Si, Al 20 Cu 2 Mn 3 , or Al 12 Mg 2 Cr are incoherent

with the matrix, and concurrent with their precipitation the original solid

solution becomes less concentrated. These conditions do not provide

appreciable precipitation hardening. Changes in electrical conductivity

constitute an effective measure of the completeness of these precipitation

reactions that occur in preheating.

For alloys that are composed of both solid-solution and second-phase

constituents and/or dispersoid precipitates, all of these components of

microstructure contribute to strength, in a roughly additive manner. This

is shown in Fig. 9 for Al-Mg-Mn alloys in the annealed condition.

Effects of Specific Alloying Elements and Impurities

The mechanical, physical, and chemical properties of aluminum alloys

depend upon composition and microstructure. The addition of selected

elements to pure aluminum greatly enhances its properties and usefulness.

Because of this, most applications for aluminum utilize alloys having one

or more elemental additions. The major alloying additions used with alu-

minum are copper, manganese, silicon, magnesium, and zinc. The total

amount of these elements can constitute up to 10% of the alloy composi-

tion (all percentages given in weight percent unless otherwise noted).

Impurity elements are also present, but their total percentage is usually

less than 0% in aluminum alloys.

Fig. 9 Tensile properties in Al-Mg-Mn alloys in the form of annealed (O tem-

per) plate 13 mm (0 in.) thick

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