In high-purity form aluminum is soft and ductile.
Most commercial uses, however, require greater strength
than pure aluminum affords. This is achieved in aluminum first
by the addition of other elements to produce various alloys,
which singly or in combination impart strength to the metal.
Further strengthening is possible by means that classify the
alloys roughly into two categories, non-heat-treatable
and heat-treatable ...
The initial strength of alloys in this group depends upon the hardening
effect of elements such as manganese, silicon, iron and magnesium,
singly or in various combinations. The non-heat-treatable alloys are
usually designated, therefore, in the 1xxx, 3xxx, 4xxx, or 5xxx series.
Since these alloys are work-hard-enable, further strengthening is made
possible by various degrees of cold working. Alloys containing
appreciable amounts of magnesium when supplied in strain-hardened
tempers are usually given a final elevated temperature treatment
called stabilizing to ensure stability of properties.
The initial strength of alloys in this group is enhanced by the
addition of alloying elements such as copper, magnesium, zinc, and
silicon. Since these elements in various combinations show increasing
solid solubility in aluminum with increasing temperature, it is
possible to subject them to thermal treatments that will impart
These treatments include solution heat treatment, quenching and
precipitation or age, hardening. By the proper combination of
solution heat treatment, quenching, cold working and artificial
aging, the highest strengths are obtained.
All wrought aluminum alloys are available in annealed form.
In addition, it may be desirable to anneal an alloy from any
other initial temper, after working, or between successive
stages of working such as in deep drawing.
Effect of Alloying Elements
1xxx series - Aluminum of 99 percent or higher purity has many
applications, especially in the electrical and chemical fields.
Excellent corrosion resistance, high thermal and electrical
conductivity, low mechanical properties and excellent workability
characterize these compositions. Moderate increases in strength
may be obtained by strain-hardening. Iron and silicon are the
2xxx series - Copper is the principal alloying element in this group
often with magnesium as secondary addition. These alloys require
solution heat-treatment to obtain optimum properties. 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
strength is not so significant. The alloys in this series do not
have as good corrosion resistance as most other aluminum alloys,
and under certain conditions they may be subject to intergranular
3xxx series - 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 percent, can be
effectively added to aluminum, it is used as a major element in only
a few instances.
4xxx series - The major alloying element of this group is silicon,
which can be added in sufficient quantities (up to 12%) 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.
5xxx series - 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. Alloys in this series possess good welding
characteristics and good resistance to corrosion in marine atmosphere.
6xxx series - Alloys in this group contain silicon and magnesium in
approximate proportions to form magnesium silicone, thus making them
heat-treatable. Though less strong than most of the 2xxx or 7xxx
alloys, the magnesium-silicon alloys possess good formability and
corrosion resistance, with medium strength.
7xxx series – Zinc in amounts of 1 to 8% is the major alloying element
in this group, and when coupled with magnesium and copper (or without
copper) results in heat-treatable alloys of very high strength.
Usually other elements such as manganese and chromium are also added
in small quantities. The out-standing member of this group is 7075,
7050 and 7049, which is among the highest strength alloys available
and is used in air-frame structures and for highly stressed parts.