aluminum - nist

CHAPTER 26

Aluminum

ALUMINUM has many outstanding attri-butes that lead to a wide range of applications,including:

� Good corrosion and oxidation resistance� High electrical and thermal conductivities� Low density� High reflectivity� High ductility and reasonably high strength� Relatively low cost

Aluminum is a consumer metal of greatimportance. Aluminum and its alloys are usedfor foil, beverage cans, cooking and food-processing utensils, architectural and electricalapplications, and structures for boats, aircraft,and other transportation vehicles. As a result of anaturally occurring tenacious surface oxide film(Al2O3), a great number of aluminum alloyshave exceptional corrosion resistance in manyatmospheric and chemical environments. Itscorrosion and oxidation resistance is especiallyimportant in architectural and transportationapplications. On an equal weight and cost basis,aluminum is a better electrical conductor thancopper. Its high thermal conductivity leads toapplications such as radiators and cookingutensils. Its low density is important for handtools and all forms of transportation, especiallyaircraft. Wrought aluminum alloys display agood combination of strength and ductility.Aluminum alloys are among the easiest of allmetals to form and machine. The precipitation-hardening alloys can be formed in a relativelysoft state and then heat treated to much higherstrength levels after forming operations arecomplete. In addition, aluminum and its alloysare not toxic and are among the easiest to recycleof any of the structural materials.

26.1 Aluminum Metallurgy

Aluminum is a lightweight metal with a den-sity of 2.70 g/cm3 (0.1 lb/in.3) and a moderately

low melting point of 655 �C (1215 �F). Since ithas a face-centered cubic crystalline structure,the formability of aluminum and aluminumalloys is good. The good formability is furtheraided by its rather low work-hardening rate.Aluminum alloys are classified as either wroughtor cast alloys. Some of the wrought alloys arehardened by work hardening, while othersare precipitation hardenable. Likewise, some ofthe cast alloys can be hardened by precipitationhardening, while others cannot. Some of theimportant properties of each of the wrought alloyseries are given in Table 26.1.

Microstructural control is extremely impor-tant in the production and processing of alumi-num alloys. Important microstructural featuresinclude constituent particles and dispersoids.Constituent particles are coarse intermetalliccompounds that form by eutectic decompositionduring ingot solidification. Some are soluble,while others are virtually insoluble. The insol-uble compounds usually contain the impurityelements iron or silicon and form compoundssuch as Al6(Fe,Mn), Al2Fe, Al7FeCu2, andaAl(Fe,Mn,Si). The soluble compounds areequilibrium intermetallic compounds of one ofthe major alloying elements, such as CuAl2 orSiMg2. One of the major reasons for ingothomogenization before hot working is to dis-solve these soluble compounds. During hotworking, the large insoluble compounds arebroken up and aligned as stringers in the work-ing direction. Dispersoids are smaller submicronparticles (typically 0.05 to 0.5 mm) that formduring ingot homogenization by solid-stateprecipitation from elements that have onlylimited solubility and that diffuse slowly. Oncethey form, they resist dissolution and/or coar-sening. They usually consist of one of the tran-sition elements; examples are Al20Cu2Mn3,Al12CrMg2, and Al3Zr. Dispersoids are useful inretarding recrystallization and grain growth.Other microstructural features that can affectproperties include oxide inclusions, porosity,

Name ///sr-nova/Dclabs_wip/Metallurgy/5224_487-508.pdf/Chap_26/ 3/6/2008 10:19AM Plate # 0 pg 487

Elements of Metallurgy and Engineering Alloys F.C. Campbell, editor, p 487-508 DOI: 10.1361/emea2008p487

Copyright © 2008 ASM International® All rights reserved. www.asminternational.org

grain size and shape, and crystallographic tex-tures that can lead to anistropic properties.

Strengthening of non-heat-treatable alloys is aresult of a combination of solid-solutionstrengthening, second-phase constituents, dis-persoid precipitates, and work hardening. Thealloys normally hardened by work or strainhardening include the commercially pure alu-minums (1xxx), the aluminum-manganese alloys(3xxx), some of the aluminum-silicon alloys(4xxx), and the aluminum-magnesium alloys(5xxx). These can be work hardened to variousstrength levels with a concurrent reduction inductility. Since these alloys will undergorecovery at moderate temperatures, they areused mainly for lower-temperature applications.

The highest strength levels are attained bythe precipitation-hardenable alloys, whichinclude the aluminum-copper alloys (2xxx),the aluminum-magnesium+silicon alloys(6xxx), the aluminum-zinc alloys (7xxx), and thealuminum-lithium alloys of the 8xxx series. Forthe cast alloys, this includes the aluminum-copper alloys (2xx.x), some of the aluminum-silicon+copper and/or magnesium alloys(3xx.x), and the aluminum-zinc alloys (7xx.x).One rather disappointing property of high-strength aluminum alloys is their fatigue per-formance. Increases in static tensile propertieshave not been accompanied by proportionateimprovements in fatigue properties (Fig. 26.1).The precipitation-hardened alloys exhibit onlyminimal fatigue improvement because of twofactors: (1) cracks initiating at precipitate-freezones adjacent to grain boundaries, and (2) there-solution of precipitate particles when they arecut by dislocations. The cut precipitate particlesbecome smaller than the critical size for ther-modynamic stability and redissolve.

26.2 Aluminum Alloy Designation

A four-digit numerical designation system,developed by the Aluminum Association, isused to designate wrought aluminum and alu-minum alloys. As shown in Table 26.2, the firstdigit defines the major alloying element of theseries. The 1xxx series is handled a little differ-ently than the 2xxx through 8xxx series. In the1xxx series of commercially pure aluminums,the last two of the four digits in the designationindicate the minimum aluminum percentage.These digits are the same as the two digits to the

Table 26.1 Major attributes of wroughtaluminum alloys

1xxx: Pure Al. The major characteristics of the 1xxx series are:

� Strain hardenable� Exceptionally high formability, corrosion resistance, and

electrical conductivity� Typical ultimate tensile strength range: 69–186 MPa

(10–27 ksi)� Readily joined by welding, brazing, soldering

2xxx: Al-Cu Alloys. The major characteristics of the 2xxxseries are:

� Heat treatable� High strength at room and elevated temperatures� Typical ultimate tensile strength range: 186–428 MPa

(27–62 ksi)� Usually joined mechanically, but some alloys are weldable� Not as corrosion resistant as other alloys

3xxx: Al-Mn Alloys. The major characteristics of the 3xxxseries are:

� High formability and corrosion resistance; medium strength� Typical ultimate tensile strength range: 110–283 MPa

(16–41 ksi)� Readily joined by all commercial procedures� Hardened by strain hardening

4xxx: Al-Si Alloys. The major characteristics of the 4xxxseries are:

� Some heat treatable� Good flow characteristics; medium strength� Typical ultimate tensile strength range: 172–379 MPa

(25–55 ksi)� Easily joined, especially by brazing and soldering

5xxx: Al-Mg Alloys. The major characteristics of the 5xxxseries are:

� Strain hardenable� Excellent corrosion resistance, toughness, weldability;

moderate strength� Building and construction, automotive, cryogenic, marine

applications� Typical ultimate tensile strength range: 124–352 MPa

(18–58 ksi)

6xxx: Al-Mg-Si Alloys. The major characteristics of the6xxx series are:

� Heat treatable� High corrosion resistance, excellent extrudability;

moderate strength� Typical ultimate tensile strength range: 124–400 MPa

(18–58 ksi)� Readily welded by gas metal arc welding and gas tungsten

arc welding methods� Outstanding extrudability

7xxx: Al-Zn Alloys. The major characteristics of the 7xxx series are:

� Heat treatable� Very high strength; special high-toughness versions� Typical ultimate tensile strength range: 221–607 MPa

(32–88 ksi)� Mechanically joined

8xxx: Alloys with Al/other elements (not covered by other series).The major characteristics of the 8xxx series are:

� Heat treatable� High conductivity, strength, hardness� Typical ultimate tensile strength range: 117–414 MPa

(17–60 ksi)� Common alloying elements include Fe, Ni, and Li

Source: Ref 1

488 / Elements of Metallurgy and Engineering Alloys


right of the decimal point in the minimum alu-minum percentage when expressed to the near-est 0.01%. When the second digit is a numberother than zero, it indicates that special controlhas been used to control one or more of thenaturally occurring impurities.

In the 2xxx through 8xxx alloy series, thesecond digit in the designation indicates thealloy modification. When the second digit iszero, it indicates the original alloy. The numbers1 through 9, assigned consecutively, indicatemodifications of the original alloy. Explicit ruleshave been established for determining whether a

proposed composition is merely a modificationof a previously registered alloy or if it is anentirely new alloy. The last two of the four digitsin the 2xxx through 8xxx series have no specialsignificance but serve only to identify the dif-ferent alloys in the series.

For cast alloys (Table 26.3), a four digitnumerical designation incorporating a decimalpoint is used. The first digit indicates the majoralloying element, while the second and thirddigits identify the specific alloy. For the 1xx.xseries, the second two digits indicate purity. Thelast digit, which is separated from the others by a

0 14 28 42 56 70 84 98

Aged aluminum alloys

Non-heat-treatable aluminum alloys

Magnesium alloys

Steels

7

14

21

28

35

42

50

100

150

200

250

300

End

uran

ce li

mit

5×10

8 cy

cles

, ksi

End

uran

ce li

mit

5×10

8 cy

cles

, MP

a

Tensile strength, ksi

0 100 200 300 400 500 600 700

Tensile strength, MPa

Slope 0.3Slope 0

.5

Fig. 26.1 Fatigue strength comparison for aluminum. Source: Ref 2

Table 26.2 Designations for aluminum wroughtalloys

Series Aluminum content or main alloying element

1xxx 99.00% minimum2xxx Copper3xxx Manganese4xxx Silicon5xxx Magnesium6xxx Magnesium and silicon7xxx Zinc8xxx Others9xxx Unused

Table 26.3 Designations for aluminum castingalloys

Series Aluminum content or main alloying element

1xx.0 99.00% minimum2xx.0 Copper3xx.0 Silicon with copper and/or magnesium4xx.0 Silicon5xx.0 Magnesium6xx.0 Unused7xx.0 Zinc8xx.0 Tin9xx.0 Other

Chapter 26: Aluminum / 489


decimal point, indicates the product form,whether casting or ingot; the zero after a periodidentifies the alloy as a cast product. If the periodis followed by the number 1, it indicates an ingotcomposition that would be supplied to a castinghouse. A modification of an original alloy, or ofthe impurity limits for unalloyed aluminum, isindicated by a serial letter preceding thenumerical designation. The serial letters areassigned in alphabetical sequence starting with“A” but omitting “I,” “O,” “Q,” and “X,” the“X” being reserved for experimental alloys. Forexample, the designation A357.0 indicates ahigher purity level than the original alloy 357.0.

The temper designations for aluminumalloys are shown in Table 26.4. The basic temperdesignations are as follows.

As-fabricated (F) applies to products inwhich there is no special control over the ther-mal conditions or work hardening (if applied),and there are no mechanical property limits.

Annealed (O) applies to wrought and castproducts that are annealed to produce the loweststrength condition. Cast products are oftenannealed to improve ductility and dimensionalstability.

Work hardened (H) applies only to wroughtproducts that have been strengthened by work orstrain hardening. A subsequent thermal treat-ment is sometimes used to produce somereduction in strength. The work-hardened con-dition H is always followed by one digit and

sometimes two. The first digit signifies the spe-cific work-hardening process, and the seconddigit gives the amount of residual hardening.

Solution heat treated (W) applies to pro-ducts that are solution heat treated. The Wcondition is unstable since alloys will slowly ageat room temperature. Wrought heat treatablealloys are often formed in the W condition sincetheir formability is almost as good as theannealed condition. In this case, they are refri-gerated after solution heat treating but beforeforming to retard natural aging. The refrigera-tion temperature must be in the range of �45 to�75 �C (�50 to �100 �F).

Solution heated treated and aged(T) applies to products that have been solutionheat treated and either aged at room temperature(naturally aged) or aged at elevated temperature(artificially aged). The specific aging treatmentis designated with a “T” followed by a number(1 through 10) for the specific aging treatment.Wrought products are also stress relieved aftersolution treating but before aging, designated asTx5x. This is conducted by stretching in tension(Tx51), compression (Tx52), or a combinationof tension and compression (Tx54). The smallamount of stress relief (1 to 5%) reduces war-page during machining and improves the fatigueand stress-corrosion resistance. If the product isan extrusion, a third digit may be used. Thenumber “1” for extruded products indicates theproduct was straightened by stretching, while

Table 26.4 Temper designations for aluminum alloys

Suffix letter “F,” “O,” “H,”“T,” or “W” indicates basictreatment condition

First suffix digit indicatessecondary treatment usedto influence properties

Second suffix digitfor condition H onlyindicates residual

hardening

F—As-fabricatedO—Annealed-wrought products onlyH—Cold worked, strain hardened

1—Cold worked only2—Cold worked and partially annealed3—Cold worked and stabilized

2—1/4 hard4—1/2 hard6—3/4 hard8—Hard9—Extra hard

W—Solution heat treatedT—Heat treated, stable

T1—Cooled from an elevated-temperature shaping operation+natural agedT2—Cooled from an elevated-temperature shaping operation+cold worked+natural agedT3—Solution treated+cold worked+natural agedT4—Solution treated+natural agedT5—Cooled from an elevated-temperature shaping operation+artificial agedT6—Solution treated+artificial agedT7—Solution treated+overagedT8—Solution treated+cold worked+artificial agedT9—Solution treated+artificial aged+cold workedT10—Cooled from an elevated-temperature shaping operation+cold worked+artificial aged

Source: Ref 3



the number “0” indicates that it was notmechanically straightened.

26.3 Aluminum Alloys

The principal alloying elements in wroughtaluminum alloys include copper, manganese,magnesium, silicon, and zinc. Alloys containingcopper, magnesium+silicon, and zinc are pre-cipitation hardenable to fairly high strengthlevels, while those containing manganese ormagnesium are hardened primarily by coldworking.

26.3.1 Wrought Non-Heat-Treatable Alloys

The wrought non-heat-treatable alloysinclude the commercially pure aluminum alloys(1xxx), the aluminum-manganese alloys (3xxx),the aluminum-silicon alloys (4xxx), and thealuminum-magnesium alloys (5xxx). Thesealloys cannot be hardened by heat treatment andare therefore hardened by a combination ofsolid-solution strengthening (Fig. 26.2) and coldworking (Fig. 26.3). The chemical compositionsof a number of wrought non-heat-treatablealloys are shown in Table 26.5, and representa-tive mechanical properties are given inTable 26.6.

Commercially Pure Aluminum Alloys(1xxx). The 1xxx alloys normally have tensilestrengths in the range of 69 to 186 MPa (10 to27 ksi). The 1xxx series of aluminum alloysinclude both the superpurity grades (99.99%)and the commercially pure grades containing upto 1 wt% impurities or minor additions. The lasttwo digits of the alloy number denote the twodigits to the right of the decimal point of the

percentage of the material that is aluminum. Forexample, 1060 denotes an alloy that is 99.60%Al. The more prevalent commercially puregrades (99.0 wt% minimum aluminum) areavailable in most product forms and are used forapplications such as electrical conductors, che-mical processing equipment, aluminum foil,cooking utensils, and architectural products.Since these alloys are essentially free of alloyingadditions, they exhibit excellent corrosionresistance to atmospheric conditions. The most

0 0.5 1.0 1.5 2.0 2.5 3.0

Mg

Cu

Zn

Mn

Si

Solute (wt%)

1.5

2.9

4.3

5.8

7.3

Yie

ld S

tren

gth

(ksi

)

10

20

30

40

50

Yie

ld S

tren

gth

(MP

a)

Annealed High Purity Aluminum

Fig. 26.2 Solid-solution strengthening of aluminum.Source: Ref 4

300

200

100

300

MP

aM

Pa

200

100

Fig. 26.3 Work-hardening curves for wrought non-heat-treatable aluminum alloys. Source: Ref 5

Table 26.5 Compositions of select wrought non-heat-treatable aluminum alloys

Alloy

Alloying element content, wt%

Cu Mn Mg Cr

3003 0.12 1.2 . . . . . .3004 . . . 1.2 1.0 . . .3005 . . . 1.2 0.4 . . .3105 . . . 0.6 0.5 . . .5005 . . . . . . 0.8 . . .5050 . . . . . . 1.4 . . .5052 . . . . . . 2.5 0.255252 . . . . . . 2.5 . . .5154 . . . . . . 3.5 0.255454 . . . 0.8 2.7 0.125056 . . . 0.12 5.0 0.125456 . . . 0.8 5.1 0.125182 . . . 0.35 4.5 . . .5083 . . . 0.7 4.4 0.155086 . . . 0.45 4.0 0.15

All contain iron and silicon as impurities. Source: Ref 6



popular 1xxx alloy is alloy 1100; it has atensile strength of 90 MPa (13 ksi), which canbe increased to 165 MPa (24 ksi) by workhardening. The 1xxx series are also used forelectrical applications, primarily alloy 1350,which has relatively tight controls on impuritiesthat would adversely affect electrical con-ductivity.

Aluminum-Manganese Alloys (3xxx). The3xxx alloys are often used where higher strengthlevels are required along with good ductility andexcellent corrosion resistance. The aluminum-manganese alloys contain up to 1.25 wt% Mn;higher amounts are avoided because the pre-sence of iron impurities can result in the

formation of large primary particles of Al6Mn,which causes embrittlement. Additions of mag-nesium provide improved solid-solution hard-ening, as in the alloy 3004, which is used forbeverage cans, the highest single usage of anyaluminum alloys, accounting for approximately1/4 of the total usage of aluminum. Their mod-erate strength (ultimate tensile strengths of 110to 297 MPa, or 16 to 43 ksi) often eliminatestheir consideration for structural applications.These alloys are welded with 1xxx-, 4xxx-, and5xxx-series filler alloys, dependening on thespecific chemistry, specific application, andservice requirements.

Aluminum-Silicon Alloys (4xxx). The 4xxxseries of alloys is not as widely used as the 3xxxand 5xxx alloys. Ultimate tensile strengths rangefrom 172 to 379 MPa (25 to 55 ksi). Becauseof the relatively high silicon content, the 4xxxseries has excellent flow characteristics, makingthem the alloys of choice for two major appli-cations. Alloy 4032 is used for forged pistons;the high silicon content contributes to completefilling of complex dies and provides wearresistance in service. The 4xxx alloys are alsoused for weld and braze filler metals, where thesilicon content promotes molten metal flow tofill grooves and joints during welding andbrazing. Although aluminum-silicon alloys willnot respond to heat treatment, some of the 4xxxalloys also contain magnesium or copper, whichallows them to be hardened by precipitation heattreating.

Aluminum-Magnesium Alloys (5xxx). The5xxx alloys have the highest strengths of thenon-heat-treatable alloys, with tensile strengthsranging from 124 to 434 MPa (18 to 63 ksi).They develop moderate strengths when workhardened; have excellent corrosion resistance,even in saltwater; and have very high tough-ness, even at cryogenic temperatures to nearabsolute zero. They are readily weldable by avariety of techniques, at thicknesses up to20 cm (8 in.). Since aluminum and magnesiumform solid solutions over a wide range ofcompositions, alloys containing magnesium inamounts from 0.8 to approximately 5 wt%are widely used. The 5xxx-series alloys haverelatively high ductility, usually in excess of25%.

The 5xxx alloys, although still having verygood overall corrosion resistance, can be subjectto intergranular and stress-corrosion crackingattack. In alloys with more than 3 to 4 wt% Mg,there is a tendency for the b phase (Mg5Al8)

Table 26.6 Mechanical properties of selectwrought non-heat-treatable aluminum alloys

Alloy Temper

Tensile strength Yield strength Elongationin 50 mm(2 in.), %MPa ksi MPa ksi

3003 O 100 16 40 6 30H14 125 18 115 17 9H18 165 24 150 22 5

3004 O 180 26 70 10 20H34 240 35 200 29 9H38 285 41 250 36 5H19 295 43 285 41 2

3005 O 130 19 55 8 25H14 180 26 165 24 7H18 240 35 225 33 4

3105 O 115 17 55 8 24H25 180 26 160 23 8H18 215 31 195 28 3

5005 O 125 18 40 6 25H34 160 23 140 20 8H38 200 29 185 27 5

5050 O 145 21 55 8 24H34 190 28 165 24 8H38 220 32 200 29 6

5052 O 195 28 90 13 25H34 260 38 265 31 10H38 290 42 255 37 7

5252 O 180 26 85 12 23H25 235 34 170 25 11H28 285 41 240 35 5

5154 O 240 35 115 17 27H34 290 42 230 33 13H38 330 48 270 39 10H12 240 35 115 17 25

5454 O 250 36 115 17 22H34 305 44 240 35 10H111 250 36 125 18 18H112 250 36 125 18 18

5056 O 290 42 150 22 35H18 435 63 405 59 10H38 415 60 345 50 15

5456 O 310 45 160 23 24H112 310 45 165 24 22H116 350 51 255 37 16

5182 O 275 40 130 19 21

Source: Ref 6



to precipitate at the grain boundaries, makingthe alloy susceptible to grain-boundary attack.The precipitation of b occurs slowly at roomtemperature but can accelerate at elevatedtemperatures or under highly work-hardenedconditions. A second problem that can beencountered with the 5xxx alloys is one of agesoftening at room temperature; that is, over aperiod of time, there is some localized recoverywithin the work-hardened grains. To avoid thiseffect, a series of H3 tempers is used in which thealloy is work hardened to a slightly greater leveland then subjected to a stabilization agingtreatment at 120 to 150� C (250 to 300 �F). Thistreatment also helps reduce the tendency forb precipitation.

The 5xxx alloys are used extensively in thetransportation industries for boat and ship hulls;dump truck bodies; large tanks for carryinggasoline, milk, and grain; and pressure vessels,especially where cryogenic storage is required.The weldability of these alloys is excellent, andthey have excellent corrosion resistance.

26.3.2 Wrought Heat Treatable Alloys

The wrought heat treatable alloys includethe aluminum-copper (2xxx) series, the alu-minum-magnesium-silicon series (6xxx), thealuminum-zinc (7xxx) series, and the aluminum-lithium alloys of the 8xxx series. These alloys

are strengthened by precipitation hardening,which is covered in more detail in Chapter 9,“Precipitation Hardening,” in this book. Theimportance of precipitation hardening of alu-minum alloys can be appreciated by examiningthe data presented in Fig. 26.4 for naturallyaged 2024 and artificially aged 7075. Note thedramatic increase in strength of both due toprecipitation hardening, with only moderatereductions in elongation. The chemical compo-sitions of a number of the wrought heat treatablealuminum alloys are given in Table 26.7, and themechanical properties of a number of alloys areshown in Table 26.8.

Aluminum-Copper Alloys (2xxx). Thehigh-strength 2xxx and 7xxx alloys arecompetitive on a strength-to-weight ratio withthe higher-strength but heavier titanium andsteel alloys and thus have traditionally been thedominant structural material in both commercialand military aircraft. In addition, aluminumalloys are not embrittled at low temperatures andbecome even stronger as the temperature isdecreased, without significant ductility losses,making them ideal for cryogenic fuel tanks forrockets and launch vehicles.

The wrought heat treatable 2xxx alloys gen-erally contain magnesium in addition to copperas an alloying element. Other significant alloy-ing additions include titanium to refine the grain

10 20 30 40 50 60 70 80

O

T3 T4

YS

O

T3 T4

UTS

5 10 15 20 25 30

O T3 T4

Elongation

10 20 30 40 50 60 70 80

O

T6

YS

O

UTS

5 10 15 20 25 30

O

T6

Elongation

T6

Elo

ngat

ion

(%)

Elo

ngat

ion

(%)

7075

Str

engt

h (k

si)

Str

engt

h (k

si)

100

200

300

400

500

100

200

300

400

500

Str

engt

h (M

Pa)

S

tren

gth

(MP

a)

2024

Fig. 26.4 Effect of heat treatment on 2024 and 7075. YS,yield strength; UTS, ultimate tensile strength.

Source: Ref 7

Table 26.7 Compositions of select wrought heattreatable aluminum alloys

Alloy

Alloying element content, wt%

Fe Si Cu Mn Mg Cr Zn Zr

2008 0.40(a) 0.65 0.9 0.3(a) 0.4 . . . . . . . . .2219(b) 0.30(a) 0.20(a) 6.3 0.3 . . . . . . . . . 0.182519(b) 0.39(a)(c) 0.30(a)(c) 5.8 0.3 0.25 . . . . . . 0.182014 0.7(a) 0.8 4.4 0.8 0.5 . . . . . . . . .2024 0.50(a) 0.50(a) 4.4 0.6 1.5 . . . . . . . . .2124 0.30(a) 0.20(a) 4.4 0.6 1.5 . . . . . . . . .2224 0.15(a) 0.12(a) 4.4 0.6 1.5 . . . . . . . . .2324 0.12(a) 0.10(a) 4.4 0.6 1.5 . . . . . . . . .2524 0.12(a) 0.06(a) 4.25 0.6 1.4 . . . . . . . . .2036 0.50(a) 0.50(a) 2.6 0.25 0.45 . . . . . . . . .6009 0.50(a) 0.8 0.4 0.5 0.6 . . . . . . . . .6061 0.7(a) 0.6 0.3 . . . 1.0 0.2 . . . . . .6063 0.50(a) 0.4 . . . . . . 0.7 . . . . . . . . .6111 0.4(a) 0.9 0.7 0.3 0.8 . . . . . . . . .7005 0.40(a) 0.35(a) . . . 0.45 1.4 0.13 4.5 0.147049 0.35(a) 0.25(a) 1.6 . . . 2.4 0.16 7.7 . . .7050 0.15(a) 0.12(a) 2.3 . . . 2.2 . . . 6.2 0.127150 0.15(a) 0.10(a) 2.2 . . . 2.4 . . . 5.4 0.127055 0.15(a) 0.10(a) 2.3 . . . 2.1 . . . 8.0 0.127075 0.50(a) 0.40(a) 1.6 . . . 2.5 0.25 5.6 . . .7475 0.12(a) 0.10(a) 1.6 . . . 2.2 0.20 5.7 . . .

(a) Maximum allowable amount.

(b) 2219 and 2519 also contain 0.10% V and 0.06% Ti.

(c) 0.40% max Fe plus Si. Source: Ref 6



structure during ingot casting, and transitionelement additions (manganese, chromium, and/or zirconium) that form dispersoid particles(Al20Cu2Mn3, Al18Mg3Cr2, and Al3Zr), whichhelp control the wrought grain structure. Ironand silicon are considered impurities and areheld to an absolute minimum, because they formintermetallic compounds (Al7Cu2Fe and Mg2Si)that are detrimental to both fatigue and fracturetoughness.

Due to their superior damage tolerance andgood resistance to fatigue crack growth, the 2xxxalloys are used for aircraft fuselage skins forlower wing skins on commercial aircraft. The7xxx alloys are used for upper wing skins, wherestrength is the primary design driver. The alloy2024-T3 is normally selected for tension-tensionapplications because it has superior fatigueperformance in the 105 cycle range as comparedto the 7xxx alloys.

Alloy 2024 has been the most widely used ofthe 2xxx series, although there are now newer

alloys with better performance. Alloy 2024 isnormally used in the solution-treated, cold-worked, and then naturally aged condition (T3temper). Cold working is achieved at the mill byroller or stretcher rolling, which helps to pro-duce flatness along with 1 to 4% strains. It has amoderate yield strength (448 MPa, or 65 ksi)but good resistance to fatigue crack growth andfairly high fracture toughness. Another commonheat treatment for 2024 is the T8 temper(solution treated, cold worked, and artificiallyaged). Like the T3 temper, cold working priorto aging helps in nucleating fine precipitatesand reduces the number and size of grain-boundary precipitates. In addition, the T8 tem-per reduces the susceptibility to stress-corrosioncracking.

One of the developments that led to improvedproperties in the high-strength 2xxx and 7xxxaluminum alloys is impurity control, specificallythe reduction in the impurities iron and silicon.While 2024 has a combined iron and silicon

Table 26.8 Mechanical properties of select wrought heat treatable aluminum alloys

Alloy Temper Product form

Tensile strength Yield strength Elongationin 50 mm(2 in.), %MPa ksi MPa ksi

2008 T4 Sheet 250 36 125 18 28T6 Sheet 300 44 240 35 13

2014 T6, T651 Plate, forging 485 70 415 60 132024 T3, T351 Sheet, plate 450 65 310 45 18

T361 Sheet, plate 495 72 395 57 13T81, T851 Sheet, plate 485 70 450 65 6T861 Sheet, plate 515 75 490 71 6

2224 T3511 Extrusion 530 77 400 58 162324 T39 Plate 505 73 415 60 122524 T3, T351 Sheet, plate 450 65 310 45 212036 T4 Sheet 340 49 195 28 242219 T81, T851 Sheet, plate 455 66 350 51 10

T87 Sheet, plate 475 69 395 57 102519 T87 Plate 490 71 430 62 106009 T4 Sheet 220 32 125 18 25

T62 Sheet 300 44 260 38 11

6111 T4 Sheet 285 41 165 24 25T6 Sheet 350 51 310 45 10

6061 T6, T6511 Sheet, plate, extrusion, forging 310 45 275 40 12T9 Extruded rod 405 59 395 57 12

6063 T5 Extrusion 185 27 145 21 12T6 Extrusion 240 35 215 31 12

7005 T5 Extrusion 350 51 290 42 137049 T73 Forging 540 78 475 69 107050 T74, T745X Plate, forging, extrusion 520 74 450 65 137150 T651, T6151 Plate 600 87 560 81 11

T77511 Extrusion 650 94 615 89 127055 T7751 Plate 640 93 615 89 10

T77511 Extrusion 670 97 655 95 117075 T6, T651 Sheet, plate 570 83 505 73 11

T73, T735X Plate, forging 505 73 435 63 137475 T7351 Plate 505 73 435 63 15

T7651 Plate 455 66 390 57 15

Source: Ref 6



impurity level of 0.50 wt%, the newer alloy2224 contains a maximum iron and silicon levelof only 0.22 wt%. This lower fraction of im-purities produces a better combination ofstrength and fracture toughness (Fig. 26.5).Other improvements for the plate materialsinclude increasing the amount of cold work bystretching after quenching and the developmentof improved aging procedures.

The high-strength 2xxx alloys, which usuallycontain approximately 4 wt% Cu, are the leastcorrosion resistant of the aluminum alloys.Therefore, sheet products are usually clad onboth surfaces with a thin layer of an aluminumalloy containing 1 wt% Zn. Since the clad isanodic to the underlying core alloy, it pre-ferentially corrodes, leaving the core protected.The clad, which amounts to 1.5 to 10% of thethickness, is applied during hot rolling. Sincethe clad is weaker than the core alloy, there isa slight sacrifice in mechanical properties, es-pecially fatigue cracking resistance.

Aluminum-Magnesium-Silicon Alloys (6xxx).The combination of magnesium (0.6 to1.2 wt%) and silicon (0.4 to 1.3 wt%) in alu-minum forms the basis of the 6xxx precipitation-hardenable alloys. During precipitationhardening, the intermetallic compound Mg2Siprovides the strengthening. Manganese orchromium is added to most 6xxx alloys forincreased strength and grain size control. Copperalso increases the strength of these alloys, but if

present in amounts over 0.5 wt%, it reduces thecorrosion resistance. These alloys are widelyused throughout the welding fabrication indus-try, are used predominantly in the form ofextrusions, and are incorporated in many struc-tural components.

The 6xxx alloys are heat treatable to moder-ately high strength levels, have better corrosionresistance than the 2xxx and 7xxx alloys, areweldable, and offer superior extrudability. Witha yield strength comparable to that of mildsteel, 6061 is one of the most widely used of allaluminum alloys. The highest strengths areobtained when artificial aging is started im-mediately after quenching. Losses of 21 to28 MPa (3 to 4 ksi) in strength occur if thesealloys are room-temperature aged for 1 to 7 days.Alloy 6063 is widely used for general-purposestructural extrusions because its chemistryallows it to be quenched directly from theextrusion press. Alloy 6061 is used where higherstrength is required, and 6071 where the higheststrength is required.

The 6xxx alloys can be welded, while most ofthe 2xxx and 7xxx alloys have very limitedweldability. However, these alloys are solidifi-cation crack sensitive and should not be arcwelded without filler material. The addition ofadequate amounts of filler material during arcwelding processes is essential to prevent basemetal dilution, thereby preventing the hotcracking problem. They are welded with both

300 400 500 600 700

220

165

110

55

2024-T351

2224-T3511

7055-T7751

7150-T651

2324-T39

2324-T39 Type II

Upper wing skin(High Strength)

Lower wing skin(Durability)

Fuselage(Durability)

7075-T651

7178-T651

200

150

40 60 80 100

Typical yield strength (MPa)

100

50

Typical yield strength (ksi)

Fra

ctur

e to

ughn

ess

Kap

p (k

si/in

.1/2

)

Fra

ctur

e to

ughn

ess

Kap

p (M

Pa/

m1/

2 )

Fig. 26.5 Fracture toughness versus yield strength for high-strength aluminum alloys. Source: Ref 8



4xxx and 5xxx filler materials, depending on theapplication and service requirements.

Although the 6xxx alloys have not tradition-ally been able to compete with the 2xxx and 7xxxalloys in applications requiring high strength, arelatively new alloy (6013-T6) has 12% higherstrength than clad 2024-T3, with comparablefracture toughness and resistance to fatiguecrack growth rate, and it does not have to be cladfor corrosion protection.

Aluminum-Zinc Alloys (7xxx). The wroughtheat treatable 7xxx alloys are even moreresponsive to precipitation hardening than the2xxx alloys and can obtain higher strengthlevels, approaching tensile strengths of690 MPa (100 ksi). These alloys are based onthe Al-Zn-Mg(-Cu) system. The 7xxx alloys canbe naturally aged but are not because they are notstable if aged at room temperature; that is, theirstrength will gradually increase with increasingtime and can continue to do so for years.Therefore, all 7xxx alloys are artificially aged toproduce a stable alloy.

Although the Al-Zn-Mg alloys cannot attainas high a strength level as those containingcopper, they have the advantage of being weld-able. In addition, the heat provided by thewelding process can serve as the solution heattreatment, and they will age at room temperatureto tensile strengths of approximately 310 MPa(45 ksi). The yield strengths may be as much astwice that of the commonly welded alloys of the5xxx and 6xxx alloys. To reduce the chance ofstress-corrosion cracking, these alloys are airquenched from the solution heat treating tem-perature and then overaged. Air quenchingreduces residual stresses and reduces the elec-trode potential in the microstructure. The agingtreatment is often a duplex aging treatment of theT73 type. The commonly welded alloys in thisseries, such as 7005, are predominantly weldedwith the 5xxx-series filler alloys.

The Al-Zn-Mg-Cu alloys attain the higheststrength levels when precipitation hardened.Since these alloys contain up to 2 wt% Cu,

they are the least corrosion resistant of theseries. However, copper additions reduce thetendency for stress-corrosion cracking becausethey allow precipitation hardening at highertemperatures. As a class, these alloys arenot weldable and are therefore joined withmechanical fasteners. The best known of thesealloys is alloy 7075.

Some of the newer alloys have been producedto optimize their fracture toughness and resis-tance to corrosion, primarily stress-corrosioncracking and exfoliation corrosion. This has beenaccomplished through a combination of com-positional control and processing, primarilythrough the development of new overaging heattreatments. As for some of the newer 2xxx alloys,reduced iron and silicon impurity levels are usedto maximize fracture toughness. As an example,the older 7075 alloy contains a total iron andsilicon content of 0.90 wt%, while it has beenreduced to a maximum of only 0.22 wt% in 7475.The improvement in fracture toughness withreduced impurity levels is shown in Table 26.9,where the newer alloys 7149 and 7249 havehigher fracture toughness values that the original7049 composition.

One of the problems with 7075 and similaralloys when they are heat treated to the peak-aged T6 temper has been stress-corrosioncracking (SCC). Thick plate, forging, andextrusions of these alloys are particularly vul-nerable when stressed in the through-the-thickness (short-transverse) direction. Inresponse to these in-service failures, a numberof overaged T7 tempers have been developed.Although there is some sacrifice in strengthproperties, they have dramatically reduced theoccurrence of SCC failures. The T73 temperreduces the yield strength of 7075 by 15% butincreases the SCC threshold stress by a factorof 6. Additional overaged tempers (T74, T75,and T77) have been developed that providetrade-offs in strength and SCC and exfoliationcorrosion resistance. The T77 temper, devel-oped by Alcoa, is of particular interest because

Table 26.9 Effect of impurity content on high-strength aluminum extrusions

Alloy and temper

Composition, wt %Fracture toughness

yield strengthFracture toughness

ultimate tensile strengthElongation,

%

KIc

Si max Fe max Mn max MPa ksi MPa ksi MPa �m1/2

ksi � in:1/2

7049-T73511 0.25 0.35 0.35 503 73 552 80 11.6 26 247149-T73511 0.15 0.20 0.20 517 75 565 82 13.3 33 307249-T73511 0.20 0.12 0.12 531 77 579 84 13.3 37 34

Source: Ref 9



it maintains strength levels close to the T6temper. This temper is a variation of a treatmentcalled retrogression and reaging and producesthe best combination of mechanical propertiesand corrosion resistance. Although the specificheat treatment parameters depend on thealloy composition, the part is first heat treated tothe T6 temper. It is then reheated to approxi-mately 205 �C (400 �F) for 1 h, water quen-ched, and reaged again for 24 h at 120 �C(250 �F).

Other Aluminum Alloys (8xxx). The 8xxxseries is reserved for those alloys with lesscommonly used alloying elements, such asiron, nickel, and lithium. A series of electricalconductor alloys contains iron and nickelfor strength, with only a minimal loss inconductivity. A number of aluminum-ironalloys have been studied for high-temperatureapplications. In addition, the 8xxx series,along with the 2xxx series, contains some ofthe high-strength aluminum-lithium alloysthat have been evaluated for aerospace applica-tions.

Aluminum-lithium alloys, part of both the2xxx and 8xxx alloy series, are attractive foraerospace applications, owing to their reduceddensities and higher elastic moduli. Lithium isan even lighter element than aluminum, andeach 1 wt% of lithium alloyed with aluminumreduces the density by 3%. In addition, lithiumadditions increase the elastic modulus of alu-minum alloys, with each 1 wt% Li producing anincrease in the modulus by approximately 6%.However, lithium is a very active metal, makingmelting and casting difficult and expensive.Aluminum-lithium alloys cost approximatelythree times as much as conventional competitivehigh-strength aluminum alloys.

Aluminum-lithium alloys were initially pro-duced as early as the 1950s and have progressedthrough several generations of improvements.The so-called second-generation alloys thatwere produced in the 1980s were designed asdrop-in replacements for existing high-strengthalloys. These alloys were classified as highstrength (2090, 8091), medium strength (8090),and damage tolerant (2091, 8090). These alloyscontain 1.9 to 2.7 wt% Li, which results inapproximately a 10% lower density and 25%higher specific stiffness than equivalent 2xxxand 7xxx alloys. One of the major initial appli-cations was for structural members on theC-17 transport aircraft. However, due to prop-erty and manufacturing problems, they were

removed and replaced with conventionalhigh-strength aluminum alloys. Technical pro-blems included excessive anisotropy in themechanical properties, lower-than-desired frac-ture toughness and ductility, hole cracking anddelamination during drilling, and low SCCthresholds. The anisotropy experienced by thesealloys is a result of the strong crystallographictextures that develop during processing, with thefracture toughness problem being one of pri-marily low strength in the short-transversedirection.

A third generation of alloys has been devel-oped with lower lithium contents. One successstory is alloy 2195, which has a lower coppercontent and has replaced 2219 for the cryogenicfuel tank on the space shuttle, where it provides ahigher strength, higher modulus, and lowerdensity than 2219.

26.4 Melting and Primary Fabrication

To produce pure aluminum, alumina (Al2O3)is first extracted from the mineral bauxite, whichcontains approximately 50% Al2O3. In theBayer process, a sodium hydroxide solution isused to precipitate aluminum hydroxide, whichis then calcined to form alumina. Alumina isthen converted to pure aluminum by electro-lysis, using the Hall-Heroult process illustratedin Fig. 26.6. The cell is lined with carboncathodes, and consumable electrodes aregradually fed into the top of the cell. The elec-trolyte is cryolite (Na3AlF6) with 8 to 10 wt%Al2O3 dissolved in it. The cell operates attemperatures in the range of 955 to 1010 �C(1750 to 1850 �F), with a power rating of 10to 12 kW �h/kg aluminum. Pure aluminum(99 wt%) is reduced at the cathode and forms amolten pool in the bottom of the cell, which isdrained from the bottom and cast into aluminumingots. Since impurities such as iron and siliconare reduced along with the aluminum, the rawmaterials used in the Bayer process are carefullycontrolled to minimize these metal oxides. Sincethe production of aluminum takes a lot of elec-trical energy, and recycling aluminum takesmuch less energy, a large portion of general-purpose aluminum currently is made fromrecycled material.

During casting of aluminum alloy ingots,oxides and gases must be controlled. Aluminumoxidizes rapidly in the liquid state and revertsback to alumina. Once the oxide becomes



entrapped in the liquid metal, it is difficult toremove and remains dispersed in the liquidmetal. The main source of oxygen is moisturefrom the furnace charge. Oxides are removed byfluxing with gases, solids, or molten salts. Var-ious filtration methods are also used to removeoxides during pouring. Hydrogen is the only gaswith appreciable solubility in molten aluminumand can cause blisters in heat treated partsand porosity in aluminum castings. Again,moisture from the furnace charge is the mainsource of hydrogen. Hydrogen can also beintroduced from moisture in the air and productsof combustion. Hydrogen is removed by bub-bling chlorine, nitrogen, or argon gas throughthe melt.

The semicontinuous direct-chill process is themostly widely used process for casting com-mercial ingots that will receive further proces-sing, such as rolling, extrusion, or forging. Itproduces fine-grained ingots at high productionrates. As shown in Fig. 26.7, the molten alumi-num is poured into a shallow, water-cooledmold of the required shape. When the metalbegins to freeze, the false bottom in the mold islowered at a controlled rate, and water is sprayedon the freshly solidified metal as it exits themold. A water box or spray rings are placedaround the ingot to rapidly cool the ingot. Metalswith low melting points, such as magnesium,copper, and zinc, are added directly to the mol-ten charge, while high-melting-point elements(e.g., titanium, chromium, zirconium, andmanganese) are added as master alloys. Inocu-lants, such as titanium and titanium-boron, are

added to reduce hot cracking and refine grainsize.

One of the main advantages of direct-chillcontinuous casting is that it helps to eliminatethe segregation that occurs in high-strengthalloys produced by the older tilt-casting proce-dures. These alloys, when produced by tiltcasting, are highly segregated because of thebroad solidification temperature ranges and theshape of the freezing front. Direct-chill castingeliminates most of this type of segregationbecause the liquid metal freezing front is almosthorizontal, and the liquid metal freezes fromthe bottom to the top of the ingot. During direct-chill casting, the pouring temperature is main-tained at only approximately 28 �C (50 �F)above the liquidus temperature; this helpsreduce oxide formation and hydrogen pickupand produces a fine-grained structure. It can alsoproduce fairly large ingots at slow speeds, anecessary requirement for the high-strengthalloys to prevent cracking. Typical castingspeeds are in the range of 2.5 to 13 cm/min (1 to5 in./min).

26.4.1 Rolling Plate and Sheet

Rolled aluminum is the most common of thewrought aluminum product forms. Sheet isdefined as rolled aluminum in the range of 0.15to 6.3 mm (0.006 to 0.250 in.) thick. If thethickness is greater than 6.3 mm (0.250 in.),then it is called plate. Foil refers to aluminumproduct that is less than 0.15 mm (0.006 in.)thick. Aluminum foil, sheet, and plate are

Molten Aluminum

Fig. 26.6 Electrolytic cell used to produce aluminum. Courtesy of Alcoa, Inc.



produced from aluminum ingots using the fol-lowing steps:

1. Scalping of the ingot2. Preheating and homogenizing the ingot3. Reheating the ingot, if required, to the hot

rolling temperature4. Hot rolling to form a slab5. Intermediate annealing6. Cold rolling along with intermediate anneals

to form foil and sheet product forms

Scalping of Ingots. To prevent surfacedefects on the cast ingot from being rolled intothe surface, approximately 6.3 to 9.5 mm (0.250to 0.375 in.) are removed from the surfaces to berolled. Some alloys, such as 1100, 3003, and3015, have fairly smooth as-cast surfaces and donot require scalping. Other, more highly alloyedingots, such as magnesium-containing alloys

and high-strength aircraft alloys, are alwaysscalped.

Preheating or homogenizing puts into solidsolution all the constituents that are soluble andreduces the coring that occurs during the castingprocess. It relieves stresses in the ingot andmakes the cast structure more uniform and morereadily hot worked. Soaking temperatures andtimes depend on the specific alloy. For example,1100 aluminum is soaked for approximately 1 hat 455 to 510 �C (850 to 950 �F), while 7075requires up to 24 h at 455 to 470 �C (850 to875 �F). Ingots that are going to be clad duringhot rolling are scalped after preheating to avoidthe heavy oxidation that occurs during the longpreheating cycle. This allows the cladding toform a better bond during hot rolling.

Hot rolling conducted above the recrystalliz-ation temperature produces a fine grain structure

Control Valve

Molten Metal Transfer Trough

Mold

Baffle

Water Box

Water Quench Inlet

Liquidus Surface Water

Spray

Ingot

Fig. 26.7 Semi-continuous direct-chill casting. Source: Ref 10



and a minimum of grain directionality. The upperlimit is set by the lowest-melting-point eutecticpresent in the alloy, while the lower temperatureis the temperature at which the metal is hotenough to be sufficiently reduced with each passthrough the mill without cracking. The ingot isremoved from the soaking pit and initially putthrough a series of four-high rolling mills tobreak down the ingot structure. Breakdowntemperatures are in the range of 400 to 540 �C(750 to 1000 �F), with continuous rolling tem-peratures of approximately 290 to 455 �C (550 to850 �F). Since the work rapidly lengthens inthe direction of rolling, it is necessary to removethe slab from the mill, turn it around, and then

cross roll it to produce wide sheet or plate. Thegrain structure becomes elongated in the rollingdirection, as shown in Fig. 26.8. This results inanistropic mechanical properties in which theproperties are lowest in the through-the-thickness (short-transverse) direction. Rollinginto thinner plate and sheet is then conducted onfive-stand four-high mills, with successivereductions at each mill. Aluminum sheet stock,which can exit the last mill at speeds approaching485 km/h (300 mph), is coiled into large coilsprior to cold rolling.

Intermediate Annealing. Since hot rollingproduces some cold work, coiled aluminumsheet stock is given an intermediate anneal prior

Short Transverse

Longitudinal

Longitudinal

Long Transverse

Transverse

Rolling Direction

Direction of rolling

Fig. 26.8 Grain directionality due to rolling. Original magnification at 40 · . Source: Ref 11



to cold rolling. Since the amount of cold workintroduced during hot rolling is sufficient tocause recrystallization, annealing not only low-ers the strength and increases ductility, it alsoimparts a fine grain structure.

Cold Rolling. Sheet and foil are coldrolled after hot rolling to produce a muchbetter surface finish and control the strengthand ductility through work hardening. Again,four-high mills are used along with lubricantsand, if a bright surface finish is required, polishedrolls. Depending on the amount of reduction andthe final strength required, intermediate annealsare conducted during the cold rolling process.Normally, reductions in the range of 45 to 85%are taken between anneals.

26.4.2 Extrusion

Both cold and hot extrusion methods are usedto produce extruded aluminum shapes. Cold orimpact extrusions are made by a single sharpblow of a punch into a die cavity that contains ablank or slug of the correct size and shape.Almost all aluminum alloys can be formed byimpact extrusion. The slugs are annealed andthen generally impact extruded at room tem-perature, although the temperature may rise to asmuch as 230 �C (450 �F) due to the extensiveplastic deformation. The slugs are also lubri-cated prior to extrusion to prevent excessivegalling and die wear.

Direct hot extrusion is used to make structuralshapes. In the direct extrusion process, thecylindrical ingot is preheated and then extrudedin the temperature range of 340 to 510 �C (650 to950 �F), depending on the specific alloy. Thepreheated ingot is placed in a hydraulic press andsqueezed at high pressure through a steel die toproduce the desired shape. During extrusion, themetal flows most rapidly at the center of theingot. Since oxide and surface defects are left inthe last 10 to 15% of the ingot, this part of theingot (called the butt) is discarded. The 6xxx-series alloys, because of their easy extrudability,are the most popular alloys for producing shapes.The 2xxx- and 7xxx-series alloys are used inapplications requiring higher strength; however,these alloys are more difficult to extrude.

26.5 Casting

Aluminum castings can offer significant costsavings by reducing the number of components

and the associated assembly cost. Three types ofcasting processes are used extensively for alu-minum alloys: sand casting for small numbers oflarge pieces, permanent mold casting for smalland medium part sizes, and die castings for smallparts where a large quantity can justify the costof the die-casting tooling.

26.5.1 Aluminum Casting Alloys

The major attributes of the aluminum castingseries are shown in Table 26.10. Although allof the aluminum casting alloys are covered inthis section, it should be emphasized that the

Table 26.10 Major attributes of cast aluminumalloys

2xx.x: Al-Cu Alloys. The major characteristics of the 2xx.xseries are:

� Heat treatable; sand and permanent mold castings� High strength at room and elevated temperatures; some

high-toughness alloys� Approximate ultimate tensile strength range: 131–448 MPa

(19–65 ksi)

3xx.x: Al-Si+Cu or Mg Alloys. The major characteristics ofthe 3xx.x series are:

� Heat treatable; sand, permanent mold, and die castings� Excellent fluidity; high-strength/some high-toughness alloys� Approximate ultimate tensile strength range: 131–276 MPa

(19–40 ksi)� Readily welded

4xx.x: Al-Si Alloys. The major characteristics of the 4xx.xseries are:

� Non-heat-treatable; sand, permanent mold, and die castings� Excellent fluidity; good for intricate castings� Approximate ultimate tensile strength range: 117–172 MPa

(17–25 ksi)

5xx.x: Al-Mg Alloys. The major characteristics of the 5xx.xseries are:

� Non-heat-treatable; sand, permanent mold, and die� Tougher to cast; provides good finishing characteristics� Excellent corrosion resistance, machinability,

surface appearance� Approximate ultimate tensile strength range: 117–172 MPa

(17–25 ksi)

7xx.x: Al-Zn Alloys. The major characteristics of the 7xx.xseries are:

� Heat treatable; sand and permanent mold cast (harder to cast)� Excellent machinability and appearance� Approximate ultimate tensile strength range: 207–379 MPa

(30–55 ksi)

8xx.x: Al-Sn Alloys. The major characteristics of the 8xx.xseries are:

� Heat treatable; sand and permanent mold castings(harder to cast)

� Excellent machinability� Bearings and bushings of all types� Approximate ultimate tensile strength range: 103–207 MPa

(15–30 ksi)

Source: Ref 1



aluminum-silicon alloys with magnesium and/orcopper, the 3xx.x alloys, are by far the mostwidely used of the aluminum casting alloys.The silicon addition greatly increases liquidmetal fluidity and produces superior castings.The order of the alloy series in order ofdecreasing castability is 3xx.x, 4xx.x, 5xx.x,2xx.x, and 7xx.x.

Commercially pure aluminum alloys(1xx.x) are used only for applications wherehigh electrical conductivity is needed andstrength is not very important.

Aluminum-Copper Alloys (2xx.x). Theheat treatable aluminum-copper alloys thatcontain 4 to 5 wt% Cu include the highest-strength aluminum castings available and areoften used for premium-quality aerospace pro-ducts. The ductility can also be quite good, ifprepared from ingot containing less than0.15 wt% Fe. When these alloys are cast inpermanent molds, special gating and riseringtechniques are required to relieve shrinkagestresses. However, when done correctly, thealuminum-copper alloys are capable of produ-cing high-strength and ductile castings. Alumi-num-copper alloys have somewhat marginalcastability and are susceptible to SCC when heattreated to high strength levels. Manganese canbe added to combine with iron and silicon andreduce the embrittling effect of insoluble phases.However, this reduces the castability. Manga-nese-containing alloys are mainly sand cast;when they are cast in permanent molds, siliconmust be added to increase fluidity and reduce hotshortness, but silicon additions reduce ductility.Aluminum-copper alloys retain reasonablestrength at elevated temperatures (up to 315 �C,or 600 �F). The high-temperature strength isa result of copper, nickel, and magnesiumadditions.

Aluminum-Silicon1Copper and/or Mag-nesium Alloys (3xx.x). The 3xx.x alloys are theworkhorses of the aluminum casting industry,accounting for more than 95% of all die castingsand 80% of all sand and permanent mold cast-ings produced. Silicon is by far the mostimportant alloying element in aluminum castingalloys. Silicon greatly improves the fluidity ofmolten aluminum, especially when the amountapproaches the eutectic composition. Siliconincreases fluidity, reduces cracking, and im-proves feeding to minimize shrinkage porosity.

The most widely used aluminum castingalloys are those containing between 9.0 and13.0 wt% Si. These alloys are of approximately

eutectic composition (Fig. 26.9), which makesthem suitable as die-casting alloys, since theirfreezing range is small. However, they form arather coarse eutectic structure (Fig. 26.9a) thatis refined by a process known as modification,where small amounts of sodium (~0.01% byweight) are added to the melt just before casting.The sodium delays the precipitation of siliconwhen the normal eutectic temperature is reachedand also causes a shift of the eutectic com-position toward the right in the phasediagram. Therefore, as much as 14 wt% Si maybe present in a modified alloy without any pri-mary silicon crystals forming in the structure(Fig. 26.9b). It is thought that sodium collects inthe liquid at its interface with the newly formedsilicon crystals, inhibiting and delaying theirgrowth. Thus, undercooling occurs and newsilicon nuclei are formed in large numbers,resulting in a relatively fine-grained eutecticstructure. Modification raises the tensile strengthand the elongation in the manner shown inFig. 26.10. The relatively high ductility of thiscast eutectic alloy is due to the solid-solutionphase in the eutectic constituting nearly 90% ofthe total structure. Therefore, the solid-solutionphase is continuous in the microstructure andacts as a cushion against much of the brittlenessarising from the hard silicon phase. Morerecently, modifying with strontium is replacingsodium, because there is less loss duringmelting due to oxidation or evaporation; over-modification (too much addition) is not a pro-blem because it forms the innocuous compoundSrAl3Si3, and strontium suppresses the forma-tion of large primary silicon particles in hyper-eutectic alloys.

Aluminum-Silicon alloys (4xx.x) are usedwhen good castability and good corrosionresistance are requirements. Alloys of the 4xx.xgroup, based on the binary aluminum-siliconsystem and containing from 5 to 12 wt% Si, areused in applications where combinations ofmoderate strength and high ductility and impactresistance are required.

Aluminum-Magnesium Alloys (5xx.x). Thealuminum-magnesium casting alloys are single-phase binary alloys with moderate-to-highstrength and toughness. The aluminum-magnesium alloys have excellent corrosionresistance. High corrosion resistance, especiallyto seawater and marine atmospheres, is theprimary advantage of castings made of thesealloys. The best corrosion resistance requireslow impurity content (both solid and gases), and



thus, alloys must be prepared from high-qualitymetals and handled with great care at thefoundry. The aluminum-magnesium alloyslack fluidity, are prone to hot tearing, and havea greater tendency to oxidize when molten.The 5xx.x alloys are weldable, have good

machinability, and have an attractive appear-ance when anodized.

Aluminum-Zinc Alloys (7xx.x). The alu-minum-zinc casting alloys do not have the goodfluidity or shrinkage-feeding characteristics ofthe silicon-containing alloys, and hot crackingcan be a problem in large, complex shapes.However, the aluminum-zinc alloys are usedwhere the castings are going to be brazed,because they have the highest melting points ofall of the aluminum casting alloys. The 7xx.xalloys have moderate-to-good tensile propertiesin the as-cast condition. Annealing can be usedto increase dimensional stability. They are alsocapable of self-aging at room temperature aftercasting, reaching quite high strengths after 20 to30 days. Therefore, they are used in applicationsrequiring moderate-to-high strength levels,where a full solution heating and quenchingoperation could cause severe warpage orcracking. They have good machinability andresistance to general corrosion but can besomewhat susceptible to SCC. They should notbe used at elevated temperatures due to rapidoveraging.

Modified

% Elongation

Unmodified

Modified TensileStrength

Unmodified

Silicon (wt%)

10

5 10 15

15

20

Elo

ngat

ion

(%)20

30

Ten

sile

str

engt

h (k

si)

70

140

210

Ten

sile

str

engt

h (M

Pa)

0

Fig. 26.10 Effects of silicon content and modification onaluminum casting alloy

α+β

α

β+Lα+L

L

0 2 4 6 8 10 12 14 16 18750

930

1110

1220

400

500

600

700

1.65% 11.6%14.0% Eutectic Point

Displacementdue to Modification

577 °C(1071 °F)

Silicon (wt%)[Al]

Tem

pera

ture

(°F

)

(a) (b)

564 °C(1047 °F)

Fig. 26.9 Modification of aluminum-silicon casting alloys. (a) Unmodified. (b) Modified. Original magnification: 100 · .Source: Ref 12



Aluminum-Tin alloys (8xx.x) are used forcast bearings and bushings where good com-pression strength is required. Heat treatment tothe T5 condition improves their compressionstrength. Aluminum-tin alloys with approxi-mately 6 wt% Sn, along with copper and nickeladditions for strengthening, have excellentlubricity due to the tin content. These alloys canbe cast by sand or permanent mold casting, butthey are susceptible to hot cracking and there-fore have rather poor castability. The bestbearing properties are obtained when the castinghas a small interdendritic spacing, produced byrapid cooling rates.

26.5.2 Aluminum Casting Control

Molten aluminum alloys are extremely reac-tive and readily combine with other metals, gas,and sometimes with refractories. Molten alu-minum dissolves iron from crucibles; therefore,aluminum is usually melted and handled inrefractory-lined containers. For convenience inmaking up the charge, and to minimize thechance of error, most foundries use standardprealloyed ingot for melting rather than doingtheir own alloying. Most alloying elements usedin aluminum castings, such as copper, silicon,manganese, zinc, nickel, chromium, and tita-nium, are not readily lost by oxidation, eva-poration, or precipitation. Alloying elementsthat melt at temperatures higher than the meltingtemperature of aluminum, such as chromium,silicon, manganese, and nickel, are added to themolten metal as alloy-rich ingots or masteralloys. Some elements, such as magnesium,sodium, and calcium, which are removed fromthe molten bath by oxidation and evaporation,are added in elemental form to the molten bath,as required, to compensate for loss.

Because of its reactivity, molten aluminumis easily contaminated. The principal con-taminants are iron, oxides, and inclusions. Whenthe iron content exceeds 0.9 wt%, an undesir-able acicular grain structure develops in thethicker sections of the casting. When the ironcontent exceeds 1.2 wt% in the higher-siliconalloys, sludging is likely to occur, particularlyif the temperature drops below 650 �C(1200 �F). To prevent sludging, the quantitywt%Fe+2(wt%Mn)+3(wt%Cr) should notexceed 1.9 wt%. When this quantity exceeds1.9 wt%, the castings are likely to containhard spots that impair machining and mayinitiate stress cracks in service. Iron also causes

excessive shrinkage in aluminum castings andbecomes more severe as the iron contentincreases beyond 1 wt%.

Oxides must be removed from the melt; ifthey remain in the molten metal, the castingswill contain harmful inclusions. Magnesium is astrong oxide former, making magnesium-containing alloys difficult to control whenmelting and casting. Oxides of aluminum andmagnesium form quickly on the surface of themolten bath, developing a thin, tenacious skinthat prevents further oxidation as long as thesurface is not disturbed. Molten aluminum alsoreacts with moisture to form aluminum oxide,releasing hydrogen. Oxidation is also caused byexcessive stirring, overheating of the moltenmetal, pouring from too great a height, splashingof the metal, or disturbing the metal surface withthe ladle before dipping. Oxides that form on thesurface of the molten bath can be removed bysurface-cleaning fluxes. These fluxes usuallycontain low-melting-point ingredients that reactexothermically on the surface of the bath. Theoxides separate from the metal to form a dry,powdery, floating dross that can be skimmed.Some denser oxides sink to the bottom and areremoved by gaseous fluxing or through a drainhole located in the bottom of the furnace.

Hydrogen is the only gas that dissolves to anyextent in molten aluminum alloys and, if notremoved, will result in porosity in the castings.As shown in Fig. 26.11, the solubility ofhydrogen is significantly higher in the liquidthan in the solid state. During cooling and solid-ification, the solubility decreases, and hydrogenis precipitated as porosity. Hydrogen is intro-duced in the molten aluminum by moisture anddirt in the charge and by the products of com-bustion. Degassing fluxes to remove hydrogenare used after the surface of the bath has beenfluxed to remove oxides. The degassing fluxesalso help to lift fine oxides and particles to thetop of the bath. Removal of hydrogen bydegassing is a mechanical action; hydrogen gasdoes not combine with the fluxing gases.Degassing fluxes include chlorine gas, nitrogen-chlorine mixtures, and hexachloroethane.

The grain size of aluminum alloy castings canbe as small as 0.13 mm (0.005 in.) in diameter toas large as 13 mm (0.5 in.) in diameter. Fine-grained castings are desired for several reasons.First, while any porosity is undesirable, coarseporosity is the most undesirable. Since thecoarseness of porosity is proportional to grainsize, porosity in fine-grained castings is finer and



less harmful in fine-grained castings. Second,shrinkage and hot cracking are usually as-sociated with coarse-grained structures. A finergrain size minimizes shrinkage, resulting insounder castings. Third, the mechanical pro-perties, such as tensile strength and ductility, arebetter for fine-grained castings than those ofcoarse-grained castings.

The grain size of aluminum alloy castings isinfluenced by pouring temperature, solidifica-tion rate, and the presence or absence of grainrefiners. For all aluminum alloys, the grain sizeincreases as the pouring temperature isincreased. This is the main reason that aluminumalloy castings should be poured at the lowesttemperature that will produce a sound casting.Rapid solidification rates produce finer grainsizes; therefore, castings done in steel dies willhave finer grain sizes due to the faster solidifi-cation rates than those produced by sand casting.Grain-refining elements, such as titanium,boron, and zirconium, are helpful in producingfiner-grained castings.

26.6 Heat Treating

The most common heat treatments of alumi-num alloys are precipitation hardening and

annealing. The details of the precipitation-hardening process have previously been coveredin Chapter 9, “Precipitation Hardening,” in thisbook. Annealing can be used for both the non-heat-treatable and the precipitation-hardenablegrades of wrought and cast alloys. In this chap-ter, only the specifics of annealing of aluminumalloys are covered.

26.6.1 Annealing

Full annealing (O temper) produces the low-est strength and highest ductility. Cold-workedproducts will normally undergo recrystalliza-tion, while hot-worked products may remainunrecrystallized, depending on the amount ofcold work introduced during hot deformation.The recrystallization temperature is not a fixedvalue; it depends on alloy composition, amountof cold work, rate of heating, and time at tem-perature.

For both the non-heat-treatable and heat trea-table alloys, reduction or elimination of thestrengthening effects of cold working is accom-plished by heating at temperatures in the range of260 to 440 �C (500 to 825 �F), depending on thespecific alloy. A 1 h soak at a temperature of345+8 �C (650+15 �F) is a satisfactoryannealing treatment for the 1xxx- and 5xxx-seriesalloys. Longer times and higher temperatures arenecessary for the 3xxx alloys. High heating ratesto the annealing temperature are desirable to givefiner grain structure. The time at temperaturedepends on the type of anneal, the thickness ofthe material, the method of furnace loading, andthe temperature. The time at temperature for afull anneal is usually 1 h. For heat treatablealloys, the cooling rate should be slow enough sothat precipitation reactions do not occur. Acooling rate not exceeding 25 �C/h (50 �F/h) isusually sufficient.

Annealing to remove the effects of cold workis conducted at approximately 345 �C (650 �F).If it is necessary to remove the hardening effectsof heat treatment or of cooling from hot workingtemperatures, a treatment designed to produce acoarse, widely spaced precipitate is used, con-sisting of soaking at 415 to 440 �C (775 to825 �F), followed by slow cooling (25 �C/h, or50 �F/h max) to approximately 260 �C(500 �F). The high diffusion rates during soak-ing and slow cooling permit maximum coales-cence of precipitate particles that result inminimum hardness. In the 7xxx alloys, partialprecipitation occurs, and a second treatment of

10002.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0500 550 600 650 700

Temperature, °C

Solid

Liquid

Hyd

roge

n so

lubi

lity,

mL/

100

g

750 800 850 900

1200

Temperature, °F

1400 1600

1 Atm Hydrogen Pressure

Fig. 26.11 Solubility of hydrogen in aluminum. Source:Ref 13



soaking at 230+5 �C (450+10 �F) for 2 h isrequired.

In annealing, it is important to ensure that theproper temperature is reached in all portions ofthe load; therefore, it is common to specify asoaking period of at least 1 h. The maximumannealing temperature is only moderately cri-tical; however, temperatures exceeding 415 �C(775 �F) can result in oxidation and graingrowth. Relatively slow cooling, in still air or inthe furnace, is recommended for all alloys tominimize distortion.

Stress-relief anneals are used to reducestresses without causing recrystallization. Tem-peratures in the range of 220 �C (425 �F) willproduce a reasonable degree of stress relief. The5xxx series of alloys tend to soften at room tem-perature after cold working. They are normallygiven stabilization anneals at the mill by heatingto 120 to 150 �C (250 to 300 �F) to ensure thestability of mechanical properties after shipment.

26.7 Fabrication

Aluminum alloys are forged using hammers,mechanical presses, and hydraulic presses. For-ging is conducted in the range of 360 to 470 �C(680 to 880 �F), depending on the specificalloy. Somewhat surprisingly, aluminum alloysgenerally require greater working forces for anequal amount of deformation than low-carbonsteels, due to the difference in flow stress levels attheir optimal hot working temperatures. There-fore, equipment used for forging aluminum mustsupply higher forces than that used for the low-carbon steels. For larger and more complex parts,hydraulic presses are preferred.

As a result of their face-centered cubic crys-talline structure and their relatively low rates ofwork hardening, aluminum alloys are readilyformable at room temperature. The choice oftemper for forming depends on the severityof the forming operation and the alloy beingformed. Aluminum alloys can be readily formedat room temperature in either the O or W temper.For alloys formed in the W (solution heattreated) temper, it is normal practice to refrig-erate the solution heat treated material to preventnatural aging before forming.

Aluminum alloys are extremely easy tomachine. Cutting speeds as high as 5 surface m/s(1000 surface ft/min) are common. The imple-mentation of high-speed machining during the1990s allowed even higher metal removal rates;

three times greater metal removal rates aretypical. In addition, since the cuts are light, mostof the heat is removed with the chips. Thisallows extremely thin walls and webs to bemachined without distortion. The significance isthat parts that once had to be assembled frommany formed pieces can now be machined froma single block of aluminum, resulting in aweight-competitive assembly at a large costsavings. A comparison of a sheet metal built-upassembly and a high speed machine integralassembly is shown in Fig. 26.12.

As a metal class, aluminum alloys arerather difficult to weld but can be welded bygas metal arc welding, gas tungsten arcwelding, and resistance welding. The 2xxx andcopper-containing 7xxx are either very difficultto weld or unweldable by conventional arcwelding methods. However, a relatively newprocess, called friction stir welding, illustratedin Fig. 26.13, is capable of welding even themost difficult of the aluminum alloys. In thisprocess, the weld joint never becomes a trueliquid; it is a solid-state process.

26.8 Corrosion

The corrosion and oxidation resistance ofaluminum is due to a very adherent oxide film(Al2O3) that immediately forms on exposure toair. Aluminum is corrosion resistant in neutralsolutions but is attacked by both basic and acidicsolutions. Most, but not all, aluminum alloys areless corrosion resistant than pure aluminum.General corrosion resistance of aluminum alloysis usually an inverse function of the amount ofcopper used in the alloy. Thus, the 2xxx-seriesalloys are the least corrosion-resistant alloys,since copper is their primary alloying elementand all have appreciable (approximately 4 wt%)levels of copper.

Some 7xxx series alloys contain approxi-mately 2 wt% Cu in combination with magne-sium and zinc to develop strength. Such alloysare the strongest but least corrosion resistant oftheir series. Low-copper aluminum-zinc alloys,such as 7005, are also available and havebecome more popular recently. However, cop-per does have a beneficial effect on the SCCresistance of the 7xxx alloys by allowing them tobe precipitated at higher temperatures withoutloss of strength in the T73 temper.

Among the 6xxx-series alloys, higher coppercontent (1 wt% in 6066) generally decreases



corrosion resistance, but most 6xxx-series alloyscontain little copper. Some other alloyingelements also decrease corrosion resistance.Lead (added to 2011 and 6262 for machin-ability), nickel (added to 2018, 2218, and 2618for elevated-temperature service), and tin (usedin 8xx.x castings) tend to decrease the corrosionresistance but not enough to matter in mostapplications. Many of the 5xxx-series alloys havecorrosion resistance as good as commerciallypure aluminum, are more resistant to saltwater,and thus are useful in marine applications.

Single-phase alloys tend to be more corrosionresistant than the two-phase precipitation-hardened alloys. With multiple phases, galvaniccells can arise between the phases. Therefore,the 3xxx and 5xxx alloys are more corrosion

resistant than the 2xxx and 7xxx alloys. In the2xxx alloys, precipitation of CuAl2 at the grainboundaries can cause a depletion of copperadjacent to the boundaries, making these regionsanodic relative to the centers of the grains,resulting in rapid intergranular corrosion. Of theprecipitation-hardenable alloys, the 6xxx alloyshave the best corrosion resistance but are not asstrong as the heat treated 2xxx and 7xxx alloys.

The naturally forming alumina (Al2O3) coat-ing is thin (0.005 to 0.015 mm, or 0.0002 to0.0006 in. thick) and a poor base for paint. Twotypes of coatings, chemical conversion coatingsand anodizing, are used to form a more uniformand thicker oxide for enhanced corrosion pro-tection. Chemical conversion coatings produce aporous and absorptive oxide (0.05 to 0.076 mm,or 0.002 to 0.003 in. thick) that is very uniformand morphologically tailored to bond well withpaint primers. The oxides are chromate- orphosphate-based, which further aids in corrosionprotection.

To further enhance corrosion resistance, fin-ished parts are frequently anodized before beingplaced in service to increase the thickness of theAl2O3 layer on the surface. Anodizing is anelectrolytic process that produces thicker (0.05to 0.13 mm, or 0.002 to 0.005 in.) and moredurable oxides than those produced by conver-sion coatings; therefore, it provides better cor-rosion resistance. Both sulfuric and chromic acid

Fig. 26.13 Friction stir welding process. Source: Ref 14

Sheet metalassembly

High speedmachined

Was

Number of Pieces...........................………44

Number of Tools.............................……....53

Design and Fabrication hr (Tools)...........965

Fabrication hr...........................…...........13.0

Assembly Manhours.....................….….....50

Weight (lb).....................….....................9.58

Now

Number of Pieces..................….…............6

Number of Tools..................……...............5

Design and Fabrication hr (Tools)...........30

Fabrication hr..............…........................8.6

Assembly Manhours…............................5.3

Weight (lb)....................….....................8.56

0.025 in. 0.025 in.

Note: Minimum gage thickness for conventional machining is ~0.060 in. Fastener

Integralstiffener

Fig. 26.12 Comparison between assembled and high-speed machined assembly



baths are used along with an electrical current todeposit a porous oxide layer on the surfaces. Thecomponent is the anode in an electrolytic cell,while the acid bath serves as the cathode. Anod-izing consists of degreasing, chemical cleaning,anodizing, and then sealing the anodized coatingin a slightly acidified hot water bath. Dependingon the temperature and time of the anodizingoperation, the oxide layer can range from 5 to13 mm (0.2 to 0.5 mil) to provide enhancedcorrosion protection to the underlying metal.After anodizing, the pores in the oxide film aresealed by placing the part in slightly acidified 80to 90 �C (180 to 200 �F) water. This sealingtreatment converts the aluminum oxide to alu-minum monohydrate, which expands to fill thepores.

REFERENCES

1. J.G. Kaufman, Aluminum Alloys, Hand-book of Materials Selection, John Wiley &Sons, Inc., 2002, p 89–134

2. P.C. Varley, The Technology of Aluminumand Its Alloys, Newnes-Butterworths, Lon-don, 1970

3. J.R. Kissell, Aluminum Alloys, Handbookof Materials for Product Design, McGraw-Hill, 2001, p 2.1–2.178

4. R.E. Sanders et al., Proceedings of theInternational Conference on AluminumAlloys—Physical and Mechanical Proper-ties (Charlottesville, VA), EngineeringMaterials Advisory Services, Warley, U.K.,1941, 1986

5. W.A. Anderson, Aluminum, Vol 1, Amer-ican Society for Metals, 1967

6. R.N. Caron and J.T. Staley, Aluminum andAluminum Alloys: Effects of Composition,Processing, and Structure on Properties ofNonferrous Alloys, Materials Selection andDesign, Vol 20, ASM Handbook, ASMInternational, 1997

7. F.C. Campbell, Manufacturing Technologyfor Aerospace Structural Materials, Else-vier Scientific, 2006

8. B. Smith, The Boeing 777, Adv. Mater.Process., Sept 2003, p 41–44

9. J.-P. Immarigeon et al., Lightweight Mate-rials for Aircraft Applications, Mater.Charact., Vol 35, 1995, p 41–67

10. Aluminum Casting Principles, Lesson 5,Aluminum and Its Alloys, ASM Course,American Society for Metals, 1979

11. K.R. Van Horn, Ed., Aluminum, Vol 1,Properties, Physical Metallurgy and PhaseDiagrams, American Society for Metals,1967

12. M. Warmuzek, Metallographic Techniquesfor Aluminum and Its Alloys, Metallo-graphy and Microstructures, Vol 9, ASMHandbook, ASM International, 2004

13. E.L. Rooy, Aluminum and AluminumAlloys, Casting, Vol 15, ASM Handbook,ASM International, 1988

14. R.S. Mishra and M.W. Mahoney, FrictionStir Welding and Processing, ASM Inter-national, 2007

SELECTED REFERENCES

� Alloy and Temper Designation Systems forAluminum, Metals Handbook Desk Edi-tion, 2nd ed., ASM International, 1998

� Aluminum Foundry Products, MetalsHandbook Desk Edition, 2nd ed., ASMInternational, 1998

� Aluminum Wrought Products, MetalsHandbook Desk Edition, 2nd ed., ASMInternational, 1998

� J.W. Bray, Aluminum Mill and EngineeredWrought Products, Properties and Selec-tion: Nonferrous Alloys and Special-Purpose,Materials,Vol 2,ASMHandbook,ASM International, 1990

� R.F. Gaul, Hot and Cold Working Alumi-num Alloys, Lesson 7, Aluminum and ItsAlloys, ASM Course, American Society forMetals, 1979

� Heat Treating of Aluminum Alloys, HeatTreating, Vol 4, ASM Handbook, ASMInternational, 1991

� I.J. Polmear, Light Alloys—Metallurgy ofthe Light Metals, 3rd ed., ButterworthHeinemann, 1995

� W.F. Smith, Precipitation Hardening ofAluminum Alloys, Lesson 9, Aluminumand Its Alloys, ASM Course, AmericanSociety for Metals, 1979

� E.A. Starke and J.T. Staley, Application ofModern Aluminum Alloys to Aircraft,Prog. Aerosp. Sci.,Vol 32, 1996, p 131–172

� W.M. Thomas and E.D. Nicholas, FrictionStir Welding for the Transportation Indus-tries, Mater. Des., Vol 18 (No. 4/6), 1997,p 269–273

� J.C. Williams and E.A. Starke, Progress inStructural Materials for Aerospace Systems,Acta Mater., Vol 51, 2003, p 5775–5799



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