Technical Field
[0001] This invention relates to aluminum alloys; particularly, to aluminum casting alloys;
and more particularly, to aluminum casting alloys used for sand and permanent mold
casting processes.
Background of the Invention
[0002] Components formed from casting of known aluminum alloys may not be sufficiently resistant
to corrosion for certain automotive applications. Aluminum automotive components that
are exposed to temperature extremes, water, snow, ice and humidity, as well as corrosion
inducing materials such as salt, and dirt and road grime that can retain moisture
and salt, eventually tend to exhibit significant corrosion.
[0003] United States Patent
US 6,733,726 B2 provides for an aluminum alloy that is suitable for die-casting components for automotive
applications. The aluminum die casting alloy is characterized by a very low copper
content, a manganese content that is sufficient to enhance the stability of the alloy
and have a positive influence on the strength properties, and a silicon content that
is sufficient to impart excellent fluidity, hot tear resistance and feeding characteristics
for good die-castability. The aluminum die casting alloy contains about 4.5 to about
12 percent silicon by weight, at least 87 percent aluminum by weight, about 0.25 percent
to about 0.5 percent manganese by weight, and a maximum of 0.08 percent copper by
weight. The resulting aluminum alloy has improved corrosion resistance and excellent
strength characteristics; however, the alloy is formulated mainly for die-casting
processes in which the alloy is subjected to high gauge pressure for introducing molten
alloy into the die-cast dies.
[0004] Sand and permanent mold casting may be a more cost efficient alternative to die-casting
for forming certain components. As compared to die-casting, both sand and permanent
mold casting processes require very low to no gauge pressure in introducing the molten
aluminum alloy into the mold. Sand casting is a process in which molten metal is poured
into a mold formed of sand under gravity or low pressure or in vacuum and held until
the alloy is cooled and solidified. Permanent mold casting is similar to sand casting
except that the mold is typically formed of a metal that has a higher melting point
than the alloy that is poured into the mold. Components formed from sand and permanent
mold casting can be heat treated to obtain the desired mechanical properties. Components
formed from aluminum alloys designed for die casting cannot be heat treated, and therefore
requires the alloy formulation be tailored to provide the desired mechanical properties
in an as-cast component.
[0005] It is desirable to have an aluminum casting alloy best suited for sand and permanent
mold casting processes, in which the alloy is highly resistant to corrosion yet exhibits
adequate strength similar to that of the aluminum alloy for die-casting as disclosed
in
US 6,733,726 B2. It is also desirable to have an aluminum casting alloy that is amenable to heat
treatment. It is still further desirable to have an aluminum casting alloy that has
a reduced natural affinity for the aluminium to attack and dissolve the tooling steel,
a condition referred to as soldering. It is an object of the present invention to
provide an aluminum alloy with the above mentioned advantages.
Summary of the Invention
[0006] This invention is directed to aluminum alloys having improved corrosion resistance
and excellent strength characteristics for sand and permanent mold casting processes.
[0007] An aluminum-based alloy in accordance with the present invention comprises at least
about 87 percent aluminum by weight; from about 4.0 percent to about 8.5 percent silicon
by weight; from about 0.25 percent to about 0.5 percent manganese by weight; a maximum
of about 0.08 percent copper by weight; and from about 0.2 percent to about 0.8 percent
iron by weight.
[0008] Relative to known aluminum alloys, the aluminum alloys of this invention are characterized
by a very low copper content, a manganese content that is sufficient to input excellent
strength properties, a silicon content that is suitable for fluid flow of molten alloy
into a mold under normal gravity, and an iron content sufficient to minimize soldering
of metallic molds. The lower silicon content provides for improved mechanical properties,
while the lower iron content provides for increased strength and better creep characteristics
at moderately elevated temperatures and improved ductility.
[0009] These and other features, advantages, and objects of the present invention will be
further understood and appreciated by those skilled in the art by reference to the
following specification and claims.
Description of the Preferred Embodiments
[0010] In accordance with the principles of this invention, an aluminum casting alloy having
improved corrosion resistance and excellent sand and permanent casting properties
includes a relatively low copper content that is effective to achieve enhanced corrosion
resistance, in conjunction with a relatively lower silicon content for improved mechanical
properties, and a lower iron content to increase tensile strength and ductility as
well as to reduce shrinkage and soldering.
[0011] The aluminum alloys of this invention typically have a silicon content of from about
4.0 percent by weight to about 8.5 percent by weight; preferably between about 4.0
to about 7.0 percent by weight.
[0012] Iron is preferably added to the aluminum alloys of this invention to decrease the
tendency for mold sticking or soldering during casting. A suitable amount of iron
is from about 0.2 percent to about 0.8 percent by weight.
[0013] Conventional aluminum casting alloys typically contain relatively high amounts of
copper in order to improve the machinability, strength, and hardness of the casting.
However, copper reduces resistance to general corrosion, and therefore, is present
in the aluminum alloys of this invention in relatively low amounts, if at all. In
order to achieve excellent corrosion resistance, the aluminum alloys of this invention
typically contain 0.08 percent copper by weight or less, and more preferably 0.05
percent or less.
[0014] Most aluminum casting alloys have an aluminum content of about 86 percent by weight
or less. For example, the most commonly used aluminum die casting alloy (alloy 380.0)
contains from about 79 to about 83 percent aluminum by weight. The conventional corrosion
resistant aluminum die casting alloys, alloys 360.0 and 413.0, contain from about
85 to about 86.5 percent aluminum by weight and from about 82 percent to about 84
percent aluminum by weight, respectively. In contrast, the aluminum alloys of this
invention for sand and permanent mold casting have a relatively high (more than 87
percent by weight) aluminum content, and as a result, exhibit a thermal conductivity
that is about 20 percent greater than that of alloy 380.0.
[0015] Manganese is present in an amount from about 0.25 to about 0.5 percent by weight
to enhance strength, and more preferably from about 0.3 to about 0.5, with about 0.40
percent manganese being most preferred. These levels of manganese have been found
to compensate, at least in part, for the relatively low levels of copper, to enhance
strength properties without significantly adversely affecting corrosion resistance.
[0016] Magnesium, nickel, zinc and tin may be present in the alloy in relatively minor amounts,
preferably about 1.5 percent or less, more preferably about 1 percent or less, and
even more preferably about 0.6 percent or less.
[0017] Other elements are not desirable, and are preferably present in an amount of less
than 0.5 percent by weight, and more preferably less than 0.03 percent by weight.
[0018] The aluminum alloys of this invention use lower silicon and higher iron and manganese
contents, as compared to known aluminum alloys, to increase strength and control the
grain structure. An advantage of having higher manganese content is the increase in
the sensitivity of heat treating in obtaining the desired properties. Another advantage
of having higher manganese content is that the manganese combines with magnesium to
insure a greater degree of stability to the alloy. Also, the higher iron aids in reducing
the natural affinity for aluminium to attack and dissolve tooling steel, a condition
referred to as soldering.
[0019] In comparison to
US 6,733,726 B2, the alloy of the present invention has a lower range limit for the silicon content,
and a lower range for the iron content. Lower iron content has been found to reduce
shrinkage and increases tensile strength and ductility. Lower silicon provides improved
mechanical and elongation properties.
1. An aluminum-based alloy comprising:
at least about 87 percent aluminum by weight;
from about 4.0 percent to about 8.5 percent silicon by weight;
from about 0.25 percent to about 0.5 percent manganese by weight;
a maximum of about 0.08 percent copper by weight; and
from about 0.2 percent to about 0.8 percent iron by weight.
2. The alloy of claim 2, wherein the silicon is present in an amount of from about 4.0
to 7.0 percent by weight.
3. The alloy of claim 1, having a maximum of 0.05 percent copper by weight.
4. The alloy of claim 1, wherein the aluminum is present in the alloy in an amount of
at least 88 percent by weight.
5. The alloy of claim 1, wherein the aluminum is present in the alloy in an amount of
at least 89 percent by weight.
6. The alloy of claim 1, wherein the manganese is present in an amount of from about
0.3 percent to about 0.5 percent by weight.
7. An article of manufacture made from an aluminum alloy comprising:
at least about 87 percent aluminum by weight;
from about 4.0 percent to about 8.5 percent silicon by weight;
from about 0.25 percent to about 0.5 percent manganese by weight;
a maximum of about 0.08 percent copper by weight; and
from about 0.2 percent to about 0.8 percent iron by weight.