Doncha just love it when suppliers refer to something as "alloy" One
thinks it must be aluminum, but as I understand things, many different
metals can be considered "alloys". I've used parts made from both "6"
series and "7" series aluminum with equal success. I have also seen
titanium used for valve spring caps. That said, here's the wordy answer
from http://www.materialsengineer.com/E-Aluminum.htm
1xxx Series. These grades of aluminum are characterized by excellent
corrosion resistance, high thermal and electrical conductivities, low
mechanical properties, and excellent workability. Moderate increases in
strength may be obtained by strain hardening. Iron and silicon are the
major impurities.
2xxx Series. These alloys require solution heat treatment to obtain
optimum properties; in the solution heat-treated condition, mechanical
properties are similar to, and sometimes exceed, those of low-carbon
steel. In some instances, precipitation heat treatment (aging) is
employed to further increase mechanical properties. This treatment
increases yield strength, with attendant loss in elongation; its effect
on tensile strength is not as great.
The alloys in the 2xxx series do not have as good corrosion resistance
as most other aluminum alloys, and under certain conditions they may be
subject to intergranular corrosion. Alloys in the 2xxx series are good
for parts requiring good strength at temperatures up to 150 0C (300 0F).
Except for alloy 2219, these alloys have limited weldability, but some
alloys in this series have superior machinability.
3xxx Series. These alloys generally are non-heat treatable but have
about 20% more strength than 1xxx series alloys. Because only a limited
percentage of manganese (up to about 1.5%) can be effectively added to
aluminum, manganese is used as major element in only a few alloys.
4xxx Series. The major alloying element in 4xxx series alloys is
silicon, which can be added in sufficient quantities (up to 12%) to
cause substantial lowering of the melting range. For this reason,
aluminum-silicon alloys are used in welding wire and as brazing alloys
for joining aluminum, where a lower melting range than that of the base
metal is required. The alloys containing appreciable amounts of silicon
become dark gray to charcoal when anodic oxide finishes are applied and
hence are in demand for architectural applications.
6xxx Series. Alloys in the 6xxx series contain silicon and magnesium
approximately in the proportions required for formation of magnesium
silicide (Mg2Si), thus making them heat treatable. Although not as
strong as most 2xxx and 7xxx alloys, 6xxx series alloys have good
formability, weldability, machinability, and relatively good corrosion
resistance, with medium strength. Alloys in this heat-treatable group
may be formed in the T4 temper (solution heat treated but not
precipitation heat treated) and strengthened after forming to full T6
properties by precipitation heat treatment.
7xxx Series. Zinc, in amounts of 1 to 8% is the major alloying element
in 7xxx series alloys, and when coupled with a smaller percentage of
magnesium results in heat-treatable alloys of moderate to very high
strength. Usually other elements, such as copper and chromium, are also
added in small quantities. 7xxx series alloys are used in airframe
structures, mobile equipment, and other highly stressed parts. Higher
strength 7xxx alloys exhibit reduced resistance to stress corrosion
cracking and are often utilized in a slightly overaged temper to provide
better combinations of strength, corrosion resistance, and fracture
toughness.
Hope this helped,
Hugh Barber
Stafford, VA, USA
'73 TR6
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