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Effect of Alloying Elements
Series |
Main Alloy |
Effect of Alloying Elements |
| 1000 |
None
(99% Alum.) |
Unalloyed
aluminum is highly corrosion resistant, low strength, workable, conductive.
Non-heat-treatable. |
| 2000 |
Copper |
Gives
strength, hardness, machinability. Heat-treatable. |
| 3000 |
Manganese |
Adds
moderate strength, good workability. Non-heat-treatable. |
| 5000 |
Magnesium |
Moderate
to high strength. Corrosion resistant. Non-heat-treatable. |
| 6000 |
Magnesium
and Silicon |
Increases
strength, formability, corrosion resistance. Heat-treatable. |
| 7000 |
Zinc |
For
greatest strength. Heat-treatable. |
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Aluminum of 99% or higher purity has many applications,
especially in the electrical and chemical fields. these alloys are
characterized by excellent corrosion resistance, high thermal and
electrical conductivity, low mechanical properties and excellent
workability. Moderate increases in strength may be obtained by strain-hardening.
Iron and silicon are the major impurities.
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Copper
is the principal alloying element in this group. These alloys require
solution heat-treatment to obtain optimum properties; in the heat-treated
condition mechanical properties are similar to, and sometimes exceed,
those of mild steel. In some instances artificial aging is employed
to further increase the mechanical properties. This treatment materially
increases yield strength, with attendant loss in elongation; its
effect on tensile (ultimate) strength is not as great. The alloys
in the 2000 series do not have as good corrosion resistance as most
other aluminum alloys and under certain conditions they may be subject
to intergranular corrosion. Therefore, these alloys in the form
of heat are usually clad with a high purity alloy or a magnesium-silicon
alloy of the 6000 series which provides galvanic protection to the
core material and thus greatly increases resistance to corrosion.
Alloy 2024 is perhaps the best known and most widely used aircraft
alloy.
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Manganese is the major alloying element of alloys
in this group, which are generally non-heat-treatable. Because only
a limited percentage of manganese, up to about 1.5%, can be effectively
added to aluminum, it is used as a major element in only a few instances.
One of these, however, is the popular 3003, which is widely used
as a general-purpose alloy for moderate-strength applications requiring
good workability.
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The major alloying element of this group is silicon,
which can be added in sufficient quantities to cause substantial
lowering of the melting point without producing brittleness in the
resulting alloys. For these reasons aluminum-silicon alloys are
used in welding wire and as brazing alloys where a lower melting
point than that of the parent metal is required. Most alloys in
this series are non-heat-treatable, but when used in welding heat-treatable
alloys they pick up some of the alloying constituents of the latter
and so respond to heat-treatment to a limited extent. The alloys
containing appreciable amounts of silicon become dark gray when
anodic oxide finishes are applied, and hence are in demand for architectural
applications.
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Magnesium is one of the most effective and widely
used alloying elements for aluminum. When it is used as the major
alloying element or with manganese, the result is a moderate to
high strength non-heat-treatable alloy. Magnesium is considerably
more effective than manganese as a hardener, about 0.8% magnesium
being equal to 1.25% manganese, and it can be added in considerably
higher quantities. Alloys in this series possess good welding characteristics
and good resistance to corrosion in marine atmosphere. However,
certain limitations should be placed on the amount of cold work
and the safe operating temperatures permissible for the higher magnesium
content alloys (over about 3½% for operating temperatures
above about 150oF) to avoid susceptibility to stress corrosion.
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Alloys in this group contain silicon and magnesium
in approximate proportions to form magnesium silicide, thus making
them heat-treatable. Major alloy in this series is 6061, one of
the most versatile of the heat-treatable alloys. Though less strong
than most of 2000 or 7000 alloys, the magnesium-silicon (or magnesium-silicide)
alloys possess good formability, and corrosion resistance, with
medium strength. Alloys in this heat treatable group may be formed
in the T4 temper (solution heat-treated but not artificially aged)
and then reach full T6 properties by artificial aging.
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Zinc is the major alloying element in this group,
and when coupled with a small percentage of magnesium results in
heat-treatable alloys of very high strength. Usually other elements
such as copper and chromium are also added in small quantities.
The outstanding member of this group is 7075, which is among the
highest strength alloys available and is used in air-frame structures
and for highly stressed parts.
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