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
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 pronounced strengthening.
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 major impurities.
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
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.
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