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CHAPTER 3

ALUMINUM AND ITS ALLOYS

Seymour G. Epstein

J. G. Kaufman

Peter Pollak

The Aluminum Association, Inc.

Washington, D.C.

3.1 INTRODUCTION

3.9.1 Cutting Tools

3.9.2 Single-Point Tool Operations

3.9.3 Multipoint Tool Operations

3.2 PROPERTIESOFALUMINUM

3.10

CORROSION BEHAVIOR

3.10.1

General Corrosion

3.3 ALUMINUMALLOYS

3.10.2

Pitting Corrosion

3.10.3

Galvanic Corrosion

3.4 ALLOYDESIGNATIONSYSTEMS 46

3.11

FINISHING ALUMINUM

3.5 MECHANICAL PROPERTIES OF

ALUMINUM ALLOYS

3.11.1

Mechanical Finishes

3.11.2

Chemical Finishes

3.11.3

Electrochemical Finishes

3.6 WORKINGSTRESSES

3.11.4

Clear Anodizing

3.11.5

Color Anodizing

3.7 CHARACTERISTICS

3.11.6

Integral Color Anodizing

3.7.1 Resistance to General

Corrosion

3.11.7

Electroly tically Deposited

Coloring

5 1

3.7.2 Workability

3.11.8

Hard Anodizing

3.7.3 Weldability and Brazeability

3.11.9

Electroplating

3.11.10 Applied Coatings

3.8 TYPICAL APPLICATIONS

3.12

SUMMARY

3.9 MACHININGALUMINUM

3.1 INTRODUCTION

Aluminum is the most abundant metal and the third most abundant chemical element in the earth's

crust, comprising over 8% of its weight. Only oxygen and silicon are more prevalent. Yet, until about

150 years ago aluminum in its metallic form was unknown to man. The reason for this is that

aluminum, unlike iron or copper, does not exist as a metal in nature. Because of its chemical activity

and its affinity for oxygen, aluminum is always found combined with other elements, mainly as

aluminum oxide. As such it is found in nearly all clays and many minerals. Rubies and sapphires

are aluminum oxide colored by trace impurities, and corundum, also aluminum oxide, is the second

hardest naturally occurring substance on earth—only a diamond is harder.

It was not until 1886 that scientists learned how to economically extract aluminum from aluminum

oxide via electrolytic reduction. Yet in the more than 100 years since that time, aluminum has become

the second most widely used of the approximately 60 naturally occurring metals, behind only iron.

3.2 PROPERTIES OF ALUMINUM

Let us consider the properties of aluminum that lead to its wide use.

One property of aluminum that everyone is familiar with is its light weight or, technically, its low

specific gravity. The specific gravity of aluminum is only 2.7 times that of water, and roughly one-

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

ISBN 0-471-13007-9

third that of steel or copper. An easy number to remember is that 1 in. 3 of aluminum weighs 0.1 Ib;

1 ft 3 weighs 170 Ib compared to 62 Ib for water and 490 Ib for steel. The following are some other

properties of aluminum and its alloys that will be examined in more detail in later sections:

Formability. Aluminum can be formed by every process in use today and in more ways than

any other metal. Its relatively low melting point, 122O 0 F, while restricting high-temperature

applications to about 500-60O 0 F, does make it easy to cast, and there are over 1000 foundries

casting aluminum in this country.

Mechanical Properties. Through alloying, naturally soft aluminum can attain strengths twice

that of mild steel.

Strength-to-Weight Ratio. Some aluminum alloys are among the highest strength to weight

materials in use today, in a class with titanium and superalloy steels. This is why aluminum

alloys are the principal structural metal for commercial and military aircraft.

Cryogenic Properties. Unlike most steels, which tend to become brittle at cryogenic temper-

atures, aluminum alloys actually get tougher at low temperatures and hence enjoy many cry-

ogenic applications.

Corrosion Resistance. Aluminum possesses excellent resistance to corrosion by natural at-

mospheres and by many foods and chemicals.

High Electrical and Thermal Conductivity. On a volume basis the electrical conductivity of

pure aluminum is roughly 60% of the International Annealed Copper Standard, but pound for

pound aluminum is a better conductor of heat and electricity than copper and is surpassed

only by sodium, which is a difficult metal to use in everyday situations.

Reflectivity. Aluminum can accept surface treatment to become an excellent reflector and it

does not dull from normal oxidation.

Finishability. Aluminum can be finished in more ways than any other metal used today.

3.3 ALUMINUMALLOYS

While commercially pure aluminum (defined as at least 99% aluminum) does find application in

electrical conductors, chemical equipment, and sheet metal work, it is a relatively weak material, and

its use is restricted to applications where strength is not an important factor. Some strengthening of

the pure metal can be achieved through cold working, called strain hardening. However, much greater

strengthening is obtained through alloying with other metals, and the alloys themselves can be further

strengthened through strain hardening or heat treating. Other properties, such as castability and mach-

inability, are also improved by alloying. Thus, aluminum alloys are much more widely used than is

the pure metal, and in many cases, when aluminum is mentioned, the reference is actually to one of

the many commercial alloys of aluminum.

The principal alloying additions to aluminum are copper, manganese, silicon, magnesium, and

zinc; other elements are also added in smaller amounts for metallurgical purposes. Since there have

been literally hundreds of aluminum alloys developed for commercial use, the Aluminum Association

formulated and administers special alloy designation systems to distinguish and classify the alloys

in a meaningful manner.

3.4 ALLOY DESIGNATION SYSTEMS

Aluminum alloys are divided into two classes according to how they are produced: wrought and cast.

The wrought category is a broad one, since aluminum alloys may be shaped by virtually every known

process, including rolling, extruding, drawing, forging, and a number of other, more specialized

processes. Cast alloys are those that are poured molten into sand (sand casting) or high-strength steel

(permanent mold or die casting) molds, and are allowed to solidify to produce the desired shape.

The wrought and cast alloys are quite different in composition; wrought alloys must be ductile for

fabrication, while cast alloys must be fluid for castability.

In 1974, the Association published a designation system for wrought aluminum alloys that class-

ifies the alloys by major alloying additions. This system is now recognized worldwide under the

International Accord for Aluminum Alloy Designations, administered by the Aluminum Association,

and is published as American Standards Institute (ANSI) Standard H35.1. More recently, a similar

system for casting alloys was introduced.

Each wrought or cast aluminum alloy is designated by a number to distinguish it as a wrought

or cast alloy and to categorize the alloy. A wrought alloy is given a four-digit number. The first digit

classifies the alloy by alloy series, or principal alloying element. The second digit, if different than

O, denotes a modification in the basic alloy. The third and fourth digits form an arbitrary number

Table 3.1 Designation System for

Wrought Aluminum Alloys

Alloy

Series

Ixxx

2xxx

3xxx

4xxx

5xxx

6xxx

7xxx

8xxx

9xxx

Description or Major

Alloying Element

99.00% minimum aluminum

Copper

Manganese

Silicon

Magnesium

Magnesium and silicon

Zinc

Other element

Unused series

which identifies the specific alloy in the series.* A cast alloy is assigned a three-digit number followed

by a decimal. Here again the first digit signifies the alloy series or principal addition; the second and

third digits identify the specific alloy; the decimal indicates whether the alloy composition is for the

final casting (0.0) or for ingot (0.1 or 0.2). A capital letter prefix (A, B, C, etc.) indicates a modifi-

cation of the basic alloy.

The designation systems for wrought and cast aluminum alloys are shown in Tables 3.1 and 3.2,

respectively.

Specification of an aluminum alloy is not complete without designating the metallurgical condi-

tion, or temper, of the alloy. A temper designation system, unique for aluminum alloys, was developed

by the Aluminum Association and is used for all wrought and cast alloys. The temper designation

follows the alloy designation, the two being separated by a hyphen. Basic temper designations consist

of letters; subdivisions, where required, are indicated by one or more digits following the letter. The

basic tempers are:

F — As-Fabricated. Applies to the products of shaping processes in which no special control

over thermal conditions or strain hardening is employed. For wrought products, there are no

mechanical property limits.

O — Annealed. Applies to wrought products that are annealed to obtain the lowest strength

temper, and to cast products that are annealed to improve ductility and dimensional stability.

The O may be followed by a digit other than zero.

Table 3.2 Designation System for Cast

Aluminum Alloys

Alloy

Series

Ixx.x

2xx.x

3xx.x

4xx.x

5xx.x

6xx.x

7xx.x

Sxx.x

9xx.x

Description or Major Alloying Element

99.00% minimum aluminum

Copper

Silicon plus copper and /or magnesium

Silicon

Magnesium

Unused series

Zinc

Tin

Other element

*An exception is for the Ixxx series alloys, where the last two digits indicate the minimum aluminum

percentage. For example, alloy 1060 contains a minimum of 99.60% aluminum.

Table 3.3 Subdivisions of H Temper: Strain

Hardened

First digit indicates basic operations:

Hl—Strain hardened only

H2—Strain hardened and partially annealed

H3—Strain hardened and stabilized

HA —Strain hardened, lacquered, or painted

Second digit indicates degree of strain hardening:

HX2—Quarter hard

HX4—Half hard

HX8—Full hard

HX9—Extra hard

Third digit indicates variation of two-digit temper.

H — Strain-Hardened (Wrought Products Only). Applies to products that have their strength

increased by strain hardening, with or without supplementary thermal treatments to produce

some reduction in strength. The H is always followed by two or more digits. (See Table 3.3.)

W — Solution Heat Treated. An unstable temper applicable only to alloys that spontaneously

age at room temperature after solution heat treatment. This designation is specific only when

the period of natural aging is indicated; for example: W l /2 hr.

T — Thermally Treated to Produce Stable Tempers Other than F, O, or H. Applies to products

that are thermally treated, with or without supplementary strain hardening, to produce stable

tempers. The T is always followed by one or more digits. (See Table 3.4.)

3.5 MECHANICAL PROPERTIES OF ALUMINUM ALLOYS

Wrought aluminum alloys are generally thought of in two categories: nonheat-treatable and heat-

treatable. Nonheat-treatable alloys are those that derive their strength from the hardening effect of

elements such as manganese, iron, silicon, and magnesium, and are further strengthened by strain

hardening. They include the Ixxx, 3xxx, 4xxx, and 5xxx series alloys. Heat-treatable alloys are

Table 3.4 Subdivions of T Temper: Thermally Treated

First digit indicates specific sequence of treatments:

Tl—Cooled from an elevated-temperature shaping process and naturally aged to a substantially

stable condition

T2—Cooled from an elevated-temperature shaping process, cold worked, and naturally aged to a

substantially stable condition

T3—Solution heat-treated, cold worked, and naturally aged to a substantially stable condition

T4—Solution heat-treated and naturally aged to a substantially stable condition

T5—Cooled from an elevated-temperature shaping process and then artifically aged

T6—Solution heat-treated and then artifically aged

T7—Solution heat-treated and overaged/stabilized

T8—Solution heat-treated, cold worked, and then artificially aged

T9—Solution heat-treated, artificially aged, and then cold worked

TlO—Cooled from an elevated-temperature shaping process, cold worked, and then artificially

aged

Second digit indicates variation in basic treatment:

Examples:

T42 or T62—Heat treated to temper by user

Additional digits indicate stress relief:

Examples:

TX51 or TXX51—Stress relieved by stretching

TX52 or TXX52—Stress relieved by compressing

TX54 or TXX54—Stress relieved by combination of stretching and compressing

strengthened by a combination of solution heat treatment and natural or controlled aging for precip-

itation hardening, and include the 2xxx, some 4xxx, 6xxx, and 7xxx series alloys. Castings are not

normally strain hardened, but many are solution heat-treated and aged for added strength.

In Table 3.5 typical mechanical properties are shown for several representative nonheat-treatable

alloys in the annealed, half-hard and full-hard tempers; values for super purity aluminum (99.99%)

are included for comparison. Typical properties are usually higher than minimum, or guaranteed,

properties and are not meant for design purposes but are useful for comparisons. It should be noted

that pure aluminum can be substantially strain hardened, but a mere 1% alloying addition produces

a comparable tensile strength to that of fully hardened pure aluminum with much greater ductility

in the alloy. And the alloys can then be strain hardened to produce even greater strengths. Thus, the

alloying effect is compounded. Note also that, while strain hardening increases both tensile and yield

strengths, the effect is more pronounced for the yield strength so that it approaches the tensile strength

in the fully hardened temper. Ductility and workability are reduced as the material is strain hardened,

and most alloys have limited formability in the fully hardened tempers.

Table 3.6 lists typical mechanical properties and nominal compositions of some representative

heat-treatable aluminum alloys. One can readily see that the strengthening effect of the alloying

ingredients in these alloys is not reflected in the annealed condition to the same extent as in the

nonheat-treatable alloys, but the true value of the additions can be seen in the aged condition. Pres-

ently, heat-treatable alloys are available with tensile strengths approaching 100,000 psi.

Again, casting alloys cannot be work hardened and are either used in as-cast or heat-treated

conditions. Typical mechanical properties for commonly used casting alloys range from 20 to 50 ksi

for ultimate tensile strength, from 15 to 50 ksi tensile yield strength and up to 20% elongation. The

range of strengths available with wrought aluminum alloys is shown graphically in Fig. 3.1.

3.6 WORKINGSTRESSES

Aluminum is used in a wide variety of structural applications. These range from curtain walls on

buildings to tanks and piping for handling cryogenic liquids, and even bridges and major buildings

and roof structures. In establishing appropriate working stresses the factors of safety applied to the

ultimate strength and yield strength of the aluminum alloy vary with the specific application. For

building and similar type structures a factor of safety of 1.95 is applied to the tensile ultimate strength

Table 3.5 Typical Mechanical Properties of Representative Nonheat-Treatable Aluminum

Alloys (Not for Design Purposes)

Tensile

Yield

Nominal

Strength

Elongation

Hardness

Alloy

Composition

Temper

(ksi)

(% in 2 in)

(BHN)

1199

99.9+% Al

6.5

1.5

—

HIS

—

1100

99+% Al

H14

HIS

3003

1.2% Mn

H14

HIS

3004 1.2% Mn

1.0% Mg

H34

H38

5005

0.8% Mg

H14

HIS

5052

2.5% Mg

H34

H38

5456

5.1% Mg

0.8% Mn

H321, H116

B443.0

5.0% Si

514.0

4.0% Mg

"Sand cast.

^Permanent mold cast.

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