ALUMINUM AND ITS ALLOYS
Aluminum owes most of its applications to its low density [about 0.1
lb/in3 (0.16 kg/m3)] and to the relatively high strength of its alloys.
Other uses depend upon its comparatively good corrosion resistance,
good working properties, high electrical and thermal conductivity, reflectivity,
and toughness at low temperatures. Designs utilizing aluminum
should take into account its relatively low modulus of elasticity
(10 3 103 ksi) (69 3 103 MPa) and high coefficient of thermal expansion
[13 3 1026/°F (2.3 3 1025/°C)]. Commercially pure aluminum
contains a minimum of 99 percent aluminum and is a soft and ductile
metal used for many applications where high strength is not necessary.
Aluminum alloys, on the other hand, possess better casting and machining
characteristics and better mechanical properties than the pure metal
and, therefore, are used more extensively.
Aluminum alloys are divided into two general classes: wrought alloys
and cast alloys, each with its own alloy designation system as specified
in ANSI H35.1, Alloy and Temper Designation Systems for Aluminum,
maintained by the Aluminum Association.
Aluminum Alloy Designation System Wrought aluminum and aluminum
alloys are designated by a four-digit number. The first digit
indicates the alloy group according to the main alloying element as
follows:
Aluminum, 99.00 percent or more 1xxx
Copper 2xxx
Manganese 3xxx
Silicon 4xxx
Magnesium 5xxx
Magnesium and silicon 6xxx
Zinc 7xxx
Other element 8xxx
Unused series 9xxx
The last two digits identify the aluminum alloy or indicate the aluminum
purity in the case of the 1xxx series. The second digit indicates modifications of the original alloy or impurity limits. Experimental
alloys are indicated by the prefix X.
Cast aluminum and aluminum alloys are also designated by a fourdigit
number, but with a decimal point between the last two digits. The
first digit indicates the alloy group according to the main alloying element
as follows:
Aluminum, 99.00 percent or more 1xx.x
Copper 2xx.x
Silicon, with added copper and/or magnesium 3xx.x
Silicon 4xx.x
Magnesium 5xx.x
Zinc 7xx.x
Tin 8xx.x
Other element 9xx.x
Unused series 6xx.x
The second two digits identify the aluminum alloy or indicate the
aluminum purity in the case of the 1xx.x series. The last digit indicates
whether the product form is a casting (designated by 0) or ingot (designated
by 1 or 2). A modification of the original alloy or impurity limits
is indicated by a serial letter prefix before the numerical designation.
The serial letters are assigned in alphabetical order but omitting I, O, Q,
and X. The prefix X is used for experimental alloys.
The temper designation system applies to all aluminum and aluminum
alloys, wrought and cast, except ingot. The temper designation
follows the alloy designation, separated by a hyphen. The basic tempers
are designated by letters as follows:
F as fabricated (no control over thermal conditions or strain hardening)
O annealed (the lowest-strength temper of an alloy)
H strain-hardened (applies to wrought products only)
T thermally treated
The H and T tempers are always followed by one or more numbers,
say, T6 or H14, indicating specific sequences of treatments. The properties
of alloys of H and T tempers are discussed further below.
Wrought Aluminum Alloys The alloys listed in Table 6.4.2 are divided
into two classes: those which may be strengthened by strain hardening
only (non-heat-treatable, designated by the H temper) and those
which may be strengthened by thermal treatment (heat-treatable, designated
by the T temper).
The heat-treatable alloys (see Table 6.4.3) first undergo solution heat
treatment at elevated temperature to put the soluble alloying elements
into solid solution. For example, alloy 6061 is heated to 990°F (530°C).
This is followed by rapidly dropping the temperature, usually by
quenching in water. At this point, the alloy is very workable, but if it is
held at room temperature or above, strength gradually increases and
ductility decreases as precipitation of constituents of the supersaturated
solution begins. This gradual change in properties is called natural
aging. Alloys that have received solution heat treatment only are designated
T4 temper and may be more readily cold-worked in this condition.
Some materials that are to receive severe forming operations, such as
cold-driven rivets, are held in this workable condition by storing at
freezing temperatures.
By applying a second heat treatment, precipitation heat treatment or
artificial aging, at slightly elevated temperatures for a controlled period,
further strengthening is achieved and properties are stabilized. For example,
6061 sheet is artificially aged when held at 320°F (160°C) for
18 h. Material so treated is designated T6 temper. Artificial aging
changes the characteristic shape of the stress-strain curve above yielding,
thus affecting the tangent modulus of elasticity. For this reason,
inelastic buckling strengths are determined differently for artificially
aged alloys and for alloys that have not received such treatment.
Non-heat-treatable alloys cannot be strengthened by heating, but may
be given a heat treatment to stabilize mechanical properties. Strengthening
is achieved by cold-working, also called strain hardening.
All wrought alloys may be annealed by heating to a prescribed temperature.
For example, 6061 is annealed when held at 775°F (415°C) for
approximately 2 to 3 h. Annealing reduces the alloy to its lowest strength
but increases ductility. Strengths are degraded whenever strain-hard-ened or heat-treated material is reheated. The loss in strength is proportional
to the time at elevated temperature. For example, 6061-T6 heated
to 400°F (200°C) for no longer than 30 min or to 300°F (150°C) for 100
to 200 h suffers no appreciable loss in strength. Table 6.4.4 shows the
effect of elevated temperatures on the strength of aluminum alloys.
Some common wrought alloys (by major alloying element) and applications
are as follows:
1xxx Series: Aluminum of 99 percent purity is used in the electrical
and chemical industries for its high conductivity, corrosion resistance,and workability. It is available in extruded or rolled forms and is hardened
by cold working but not by heat treatment.
2xxx Series: These alloys are heat-treatable and may attain strengths
comparable to those of steel alloys. They are less corrosion-resistant
than other aluminum alloys and thus are often clad with pure aluminum
or an alloy of the 6xxx series (see alclad aluminum, below). Alloys 2014
and 2024 are popular, 2024 being perhaps the most widely used aircraft
alloy. Many of the alloys in this group, including 2014, are not usually
welded.
3xxx Series: Alloys in this series are non-heat-treatable. Alloys 3003,
3004, and 3105 are popular general-purpose alloys with moderate
strengths and good workability, and they are often used for sheet metal
work.
4xxx Series: Silicon added to alloys in this group lowers the melting
point, making these alloys suitable for use as weld filler wire (such as
4043) and brazing alloys.
5xxx Series: These alloys attain moderate-to-high strengths by strain
hardening. They usually have the highest welded strengths among aluminum
alloys and good corrosion resistance. Alloys 5083, 5086, 5154,
5454, and 5456 are used in welded structures, including pressure vessels.
Alloy 5052 is a popular sheet metal alloy.
6xxx Series: Although these alloys usually are not as strong as those
in the other heat-treatable series, 2xxx and 7xxx, they offer a good
combination of strength and corrosion resistance. Alloys 6061 and 6063
are used widely in construction, with alloy 6061 providing better
strength at a slightly greater cost.
7xxx Series: Heat-treating alloys in this group produces some of the
highest-strength alloys, frequently used in aircraft, such as 7050, 7075,
7178, and 7475. And 7178-T651 plate has a minimum ultimate tensile
strength of 84 ksi (580 MPa). Corrosion resistance is fair. Many alloys
in this group (such as 7050, 7075, and 7178) are not arc-welded.
Aluminum-lithium alloys have been produced with higher strength
and modulus of elasticity than those of any of the alloys previously
available, but they are not as ductile.
Wrought alloys are available in a number of product forms. Extrusions,
produced by pushing the heated metal through a die opening, are
among the most useful. A great variety of custom shapes, as well as
standard shapes such as I beams, angles, channels, pipe, rectangular
tube, and many others, are extruded. Extrusion cross-sectional sizes
may be as large as those fitting within a 31-in (790-mm) circle, but more
commonly are limited to about a 12-in (305-mm) circle. Alloys extruded
include 1100, 1350, 2014, 2024, 3003, 5083, 5086, 5454, 5456,
6005, 6061, 6063, 6101, 6105, 6351, 7005, 7075, and 7178. The most
common are 6061 and 6063. Rod, bar, and wire are also produced rolled
or cold-finished as well as extruded. Tubes may be drawn or extruded.
Flat rolled products include foil, sheet, and plate. Foil is defined as
rolled product less than 0.006 in (0.15 mm) thick. Sheet thickness is less
than 0.25 in (6.4 mm) but not less than 0.006 in; aluminum sheet gauge
thicknesses are different from those used for steel, and decimal thicknesses
are preferred when ordering. Sheet is available flat and coiled.
Plate thickness is 0.25 in and greater, and it ranges up to about 6 in
(150 mm). Minimum bend radii for sheet and plate depend on alloy and
temper, but are generally greater than bend radii for mild carbon steel.
Commercial roofing and siding sheet is available in a number of profiles,
including corrugated, ribbed, and V-beam.
Aluminum forgings are produced by open-die and closed-die methods.
Minimum mechanical strengths are not published for open-die
forgings, so structural applications usually require closed-die forgings.
Like castings, forgings may be produced in complex shapes, but have
more uniform properties and better ductility than castings and are used
for products such as wheels and aircraft frames. For some forging
alloys, minimum mechanical properties are slightly lower in directions
other than parallel to the grain flow. Alloy 6061-T6 is popular for forgings.
Minimum mechanical properties (typically tensile ultimate and yield
strengths and elongation) are specified for most wrought alloys and
tempers by the Aluminum Association in ‘‘Aluminum Standards and
Data.’’ These minimum properties are also listed in ASTM specifications, but are grouped by ASTM by product (such as sheet and plate)
rather than by alloy. The minimum properties are established at levels at
which 99 percent of the material is expected to conform at a confidence
level of 0.95. The strengths of aluminum members subjected to axial
force, bending, and shear under static and fatigue loads, listed in the
Aluminum Association ‘‘Specifications for Aluminum Structures,’’ are
calculated using these minimum properties. Aluminum and some of its
alloys are also used in structural applications such as storage tanks at
temperatures up to 400°F (200°C), but strengths are reduced due to
creep and the annealing effect of heat.
Cast Aluminum Alloys These are used for parts of complex shapes
by sand casting, permanent mold casting, and die casting. The compositions
and minimum mechanical properties of some cast aluminum
alloys are given in Tables 6.4.5a to 6.4.5d. Castings generally exhibit
more variation in strength and less ductility than wrought products.
Tolerances, draft requirements, heat treatments, and quality standards
are given in the Aluminum Association ‘‘Standards for Aluminum Sand
and Permanent Mold Castings.’’ Tolerances and the level of quality and
frequency of inspection must be specified by the user if desired.
Sand castings (see ASTM B26) produce larger parts—up to 7,000 lb
(3,200 kg)—in relatively small quantities at slow solidification rates.
The sand mold is used only once. Tolerances and minimum thicknesses
for sand castings are greater than those for other casting types.
Permanent mold castings (see ASTM B108) are produced by pouring
the molten metal into a reusable mold, sometimes with a vacuum to
assist the flow. While more expensive than sand castings, permanent
mold castings can be used for parts with wall thicknesses as thin as
about 0.09 in (2.3 mm).
In die casting (see ASTM B85), aluminum is injected into a reusable
steel mold, or die, at high velocity; fast solidification rates are achieved.
The lowest-cost general-purpose casting alloy is 356-T6, while
A356-T6 is common in aerospace applications. Alloy A444-T4 provides
excellent ductility, exceeding that of many wrought alloys. These
three rank among the most weldable of the casting alloys. The cast
alloys utilizing copper (2xx.x) generally offer the highest strengths at
elevated temperatures. In addition to end use, alloy selection should
take into account fluidity, resistance to hot cracking, and pressure tightness.
Machining (see Sec. 13) Many aluminum alloys are easily machined
without special technique at cutting speeds generally much
higher than those for other metals. Pure aluminum and alloys of aluminum-
manganese (3xxx) and aluminum-magnesium (5xxx) are harder to
machine than alloys of aluminum-copper (2xxx) and aluminum-zinc
(7xxx). The most machinable wrought alloy is 2011 in the T3, T4, T6,
and T8 tempers, producing small broken chips and excellent finish; they
are used where physical properties are subordinate to high machinability,
such as for screw machine products. Castings are also machined;
aluminum-copper (such as 201, 204, and 222), aluminum-magnesium
(5xx.x), aluminum-zinc (7xx.x), and aluminum-tin (8xx.x) alloys are
among the best choices.
Joining Mechanical fasteners (including rivets, bolts, and screws)
are the most common methods of joining, because the application of
heat during welding decreases the strength of aluminum alloys. Aluminum
bolts (usually 2024-T4, 6061-T6, or 7075-T73) are available in
diameters from 1⁄4 in (6.4 mm) to 1 in (25 mm), with properties conforming
to ASTM F468. Mechanical properties are given in Table 6.4.6.
Aluminum nuts (usually 2024-T4, 6061-T6, and 6262-T9) are also
available. Galvanized steel and 300-series stainless steel bolts are also
used to join aluminum. Hole size usually exceeds bolt diameter by 1⁄16 in
(1.6 mm) or less. Rivets are used to resist shear loads only; they do not
develop sufficient clamping force between the parts joined and thus
cannot reliably resist tensile loads. In general, rivets of composition
similar to the base metal are used. Table 6.4.7 lists common aluminum
rivet alloys and their minimum ultimate shear strengths. Hole diameter
for cold-driven rivets may be no larger than 4 percent greater than the
nominal rivet diameter; hole diameter for hot-driven rivets may be no
larger than 7 percent greater than the nominal rivet diameter. Screws of
2024-T4, 7075-T73 aluminum, or 300-series stainless steels are often
used to fasten aluminum sheet. Holes for fasteners may be punched,
drilled, or reamed, but punching is not used if the metal thickness is
greater than the diameter of the hole. Applications of adhesive joining
are increasing.
Welding (see Sec. 13) Most wrought aluminum alloys are weldable
by experienced operators using either the fusion or resistance method.
Fusion welding is typically by gas tungsten arc welding (GTAW), commonly
called TIG (for tungsten inert gas) welding, or gas metal arc
welding (GMAW), referred to as MIG (for metal inert gas) welding. TIG
welding is usually used to join parts from about 1⁄32 to 1⁄8 in 0.8 to
3.2 mm) thick; MIG welding is usually used to weld thicker parts. The
American Welding Society Standard D1.2, Structural Welding Code—
Aluminum, provides specifications for structural applications of aluminum
fusion-welding methods. Filler rod alloys must be chosen carefully
for strength, corrosion resistance, and compatibility with the parent
alloys to be welded. Cast alloys may also be welded, but are more
susceptible to cracking than wrought alloy weldments. All aluminum
alloys suffer a reduction in strength in the heat-affected weld zone,
although this reduction is less in some of the aluminum-magnesium
(5xxx series) alloys. Postweld heat treatment may be used to counter the
reduction in strength caused by welding, but extreme care must be taken
to avoid embrittling or warping the weldment. Resistance welding includes
spot welding, often used for lap joints, and seam welding. Methods
of nondestructive testing of aluminum welds include dye-penetrant
methods to detect flaws accessible to the surface and ultrasonic and
radiographic inspection.
Brazing is also used to join aluminum alloys with relatively high
melting points, such as 1050, 1100, 3003, and 6063, using aluminumsilicon
alloys such as 4047 and 4145. Aluminum may also be soldered,
but corrosion resistance of soldered joints is inferior to that of welded,
brazed, or mechanically fastened joints.
Corrosion Resistance Although aluminum is chemically active,
the presence of a rapidly forming and firmly adherent self-healing oxide
surface coating inhibits corrosive action except under conditions that
tend to remove this surface film. Concentrated nitric and acetic acids are
handled in aluminum not only because of its resistance to attack but also
because any resulting corrosion products are colorless. For the same
reason, aluminum is employed in the preparation and storage of foods
and beverages. Hydrochloric acid and most alkalies dissolve the protective
surface film and result in fairly rapid attack. Moderately alkaline
soaps and the like can be used with aluminum if a small amount of
sodium silicate is added. Aluminum is very resistant to sulfur and most
of its gaseous compounds.
Galvanic corrosion may occur when aluminum is electrically connected
by an electrolyte to another metal. Aluminum is more anodic
than most metals and will be sacrificed for the benefit of the other metal,
which is thereby cathodically protected from attack. Consequently, aluminum
is usually isolated from other metals such as steel (but not stainless
steel) where moisture is present.
Another form of corrosion is exfoliation, a delamination or peeling of
layers of metal in planes approximately parallel to the metal surface,
caused by the formation of corrosion product. Alloys with more than 3
percent magnesium (such as 5083, 5086, 5154, and 5456) and held at
temperatures above 150°F (65°C) for extended periods are susceptible
to this form of attack.
Care should be taken to store aluminum in a manner to avoid trapping
water between adjacent flat surfaces, which causes water stains. These
stains, which vary in color from dark gray to white, do not compromise
strength but are difficult to remove and may be cosmetically unacceptable.
Ordinary atmospheric corrosion is resisted by aluminum and most of
its alloys, and they may be used outdoors without any protective coating.
(An exception is 2014-T6, which is usually painted when exposed
to the elements.) The pure metal is most resistant to attack, and additions
of alloying elements usually decrease corrosion resistance, particularly
after heat treatment. Under severe conditions of exposure such as
may prevail in marine environments or where the metal is continually in
contact with wood or other absorbent material in the presence of moisture,
a protective coat of paint will provide added protection.
Alclad Aluminum The corrosion resistance of aluminum alloys maybe augmented by coating the material with a surface layer of high-purity
aluminum or, in some cases, a more corrosion-resistant alloy of aluminum.
Such products are referred to as alclad. This cladding becomes an
integral part of the material, is metallurgically bonded, and provides
cathodic protection in a manner similar to zinc galvanizing on steel.
Because the cladding usually has lower strength than the base metal,
alclad products have slightly lower strengths than uncoated material.
Cladding thickness varies from 1.5 to 10 percent. Products available
clad are 3003 tube, 5056 wire, and 2014, 2024, 2219, 3003, 3004, 6061,
7075, 7178, and 7475 sheet and plate.
Anodizing The corrosion resistance of any of the alloys may also be
improved by anodizing, done by making the parts to be treated the
anode in an electrolytic bath such as sulfuric acid. This process produces
a tough, adherent coating of aluminum oxide, usually 0.4 mil
(0.01 mm) thick or greater. Any welding should be performed before
anodizing, and filler alloys should be chosen judiciously for good ano-dized color match with the base alloy. The film is colorless on pure
aluminum and tends to be gray or colored on alloys containing silicon,
copper, or other constituents. To provide a consistent color appearance
after anodizing, AQ (anodizing quality) grade may be specified in certain
alloys. Where appearance is the overriding concern, 5005 sheet and
6063 extrusions are preferred for anodizing. If a colored finish is desired,
the electrolytically oxidized article may be treated with a dye
solution. Care must be taken in selecting the dye when the part will be
exposed to the weather, for not all have proved to be colorfast. Two-step
electrolytic coloring, produced by first clear anodizing and then electrolytically
depositing another metal oxide, can produce shades of bronze,
burgundy, and blue.
Painting When one is painting or lacquering aluminum, it is important
that the surface be properly prepared prior to the application of
paint. A thin anodic film makes an excellent paint base. Alternately, the
aluminum surface may be chemically treated with a dilute phosphoric
acid solution. Abrasion blasting may be used on parts thicker than 1⁄8 in
(3.2 mm). Zinc chromate is frequently used as a primer, especially for
corrosive environments. Most paints are baked on. That affects the
strength of the metal, for baking tends to anneal it, and must be taken
into account where strength is a factor. Sometimes the paint baking
process is used as the artificial aging heat treatment. Aluminum sheet is
available with factory-baked paint finish; minimum mechanical properties
must be obtained from the supplier. High-, medium-, and low-gloss
paint finishes are available and are determined in accordance with
ASTM D523.

Additional comments :