Production of aluminum products (all types of castings exclusive of ingots)
has increased over the past 30 years at a fairly steady rate.
Aluminum casting alloys must contain, in addition to strengthening elements,
sufficient amounts of eutectic-forming elements (usually silicon) in
order to have adequate fluidity to feed the shrinkage that occurs in
all but the simplest castings. Required amounts of eutectic formers
depend in part on casting process. Alloys for sand casting generally
are lower in eutectics than those for casting in metal molds, because
sand molds can tolerate a degree of hot shortness that would lead to
extensive cracking in non-yielding metal molds.
The range of cooling rates characteristic of the casting process being
used controls to some extent the distribution of alloying and impurity
elements. For example, the extremely high cooling rates inherent in die
casting result in fine dispersion of strengthening and eutectic-forming
constituents, and reasonably good castings can be obtained in spite of
impurity contents that would render sand or plaster-mold castings
unacceptable. However, with these minor exceptions, most aluminum
foundry alloys can be cast by all processes, and choice of casting
technique usually is controlled by factors other than alloy composition.
A large number of aluminum alloys has been developed for casting, but
most of them are varieties of six basic types: aluminum-copper,
aluminum-copper-silicon, aluminum-silicon, aluminum-magnesium,
aluminum-zinc-magnesium and aluminum-tin alloys.
Aluminum-copper alloys that contain 4 to 5% Cu, with the usual
impurities iron and silicon and sometimes with small amounts of magnesium,
are heat-treatable and can reach quite high strength and ductility,
especially if prepared from ingot containing less than 0.15% iron.
Manganese in small amounts also may be added, mainly to combine with the
iron and silicon and reduce their embrittling effect. However, these alloys
have poor castability and require very careful gating if sound castings
are to be obtained. Such alloys are used mainly in sand casting; when
they are cast in metal molds, silicon must be added to increase fluidity
and curtail hot shortness, and this addition of silicon substantially
AI-Cu alloys with somewhat higher copper contents (7 to 8%), formerly the
most commonly used aluminum casting alloys, have steadily been replaced
by AI-Cu-Si alloys and today are used to a very limited extent. The best
attribute of these higher-copper Al-Cu alloys is their insensitivity to
impurities, but they have very low strength and only fair castability.
Also in limited use are AI-Cu alloys that contain 9 to 11 % Cu, whose
high-temperature strength and wear resistance make them suitable for
automotive pistons and cylinder blocks. These alloys usually contain
manganese as an impurity because wrought metal scrap is used in
preparing them. The manganese has little effect.
Very good high-temperature strength is an attribute of alloys containing
copper, nickel and magnesium, sometimes with iron in place of part of
Aluminum-copper-silicon alloys. The most widely used aluminum
casting alloys are those that contain silicon together with copper.
The amounts of both additions vary widely, so that the copper predominates
in some alloys and the silicon in others. In these alloys, the copper
contributes to strength, and the silicon improves castability and reduces
hot shortness. Thus, the higher silicon alloys normally are used for more
complex castings and for permanent mold and die casting processes, which
cannot tolerate hot-short alloys.
Al-Cu-Si alloys with more than 3 to 4% Cu are heat treatable, but
usually heat treatment is used only with those alloys that also contain
magnesium, which enhances their response to heat treatment. Without
magnesium, response is too slow for heat treatment to be economical.
High-silicon alloys (> 10% Si) have low thermal expansion, which
makes them suitable for high-temperature operations. When silicon content
exceeds 12 to 13% (silicon contents as high as 22% are typical), primary
silicon crystals are present and, if properly distributed, cause excellent
wear resistance. Automotive engine blocks and pistons are major uses of
Aluminum-silicon alloys that do not contain copper additions are
used when good castability and good corrosion resistance are needed. If
high strength is also needed, magnesium additions make these alloys heat
Alloys with silicon contents as low as 2% have been used for casting, but
silicon content usually is between 5 and 13%. Strength and ductility of
these alloys, especially the ones with higher silicon, can be substantially
improved by "modification".
Modification of the hypoeutectic alloys is particularly advantageous in
sand castings, and can be effectively achieved through the addition of
a controlled amount of sodium or strontium, which refines the silicon
eutectic. Calcium and antimony additions are also used. Pseudomodification
of sand castings, in which the size of the eutectic but not the structure
is affected, may be achieved by solidification at high rates, such as
occurs when chills are used. With permanent mold castings, modification
of the eutectic also is advantageous, but the effect on properties is
not as dramatic as with sand castings.
Aluminum-magnesium alloys. High corrosion resistance, especially
to seawater and marine atmospheres, is the primary advantage of castings
made of Al-Mg alloys. Best corrosion resistance requires low impurity
content (both solid and gaseous), and thus alloys must be prepared
from high-quality metals and handled with great care in the foundry.
The relatively poor castability of Al-Mg alloys and the tendency of
the magnesium to oxidize increase handling difficulties and, therefore,
Aluminum-zinc-magnesium alloys have the ability to naturally age,
achieving full strength at room temperature 20 to 30 days after casting.
This strengthening process can be accelerated by furnace aging.
The high-temperature solution heat treatment and drastic quenching required
by other alloys (Al-Cu and AI-Si-Mg alloys, for example) is not necessary
for optimum properties in most Al-Zn-Mg alloy castings.
However, microsegregation of Mg-Zn phases can occur in these alloys, which
reverses the accepted rule that faster solidification results in higher
properties. When it is found in an Al-Zn-Mg alloy casting that the strength
of the thin or highly chilled sections are lower than the thick or slowly
cooled sections, the weaker sections can be strengthened to the required
level by solution heat treatment and quenching, followed by natural or
artificial (furnace) aging. Castability of Al-Zn-Mg alloys is poor, but
they have good general corrosion resistance despite some susceptibility
to stress corrosion.
Aluminum-tin alloys that contain about 6% Sn (and small amounts
of copper and nickel for strengthening) are used for cast bearings
because of the excellent lubricity imparted by tin. Bearing performance
of Al-Sn alloys is strongly affected by casting method. Fine interdendritic
distribution of tin, which is necessary for best bearing properties,
requires small interdendritic spacing, and small spacing is obtained
only with casting methods in which cooling is rapid.
Selection of Casting Alloy
The major factors that influence alloy selection for casting applications
include casting process to be used, casting design, required properties,
and economic (and availability) considerations.
Each casting process requires specific metal characteristics. For example,
die and permanent mold casting generally require alloys with good fluidity
and resistance to hot tearing, whereas these properties are less critical
in sand, plaster and investment casting, where molds and cores offer less
resistance to shrinkage. Discussions of required alloy characteristics,
and lists of alloys commonly used, are presented for the various casting
processes in the section that follows.
The application for which a casting is to be made affects alloy selection
by establishing requirements for strength and ductility, as well as special
service requirements such as pressure characteristics, corrosion resistance
and surface treatments.
Economic considerations also may be important in alloy selection. Total cost
of making a casting is affected by required heat treatment and by
weldability and machinability, in addition to ingot and melting costs.
Full development of the potential of any casting alloy depends in large
part on foundry technique. Foundry personnel should be consulted on alloy
selection; use of alloys with which such personnel are familiar often
results in better and more economical castings.
Selection of the proper alloy requires careful consideration of all the
factors discussed above, which are presented in the brief outline that
Alloy characteristics necessary for casting process selected:
- resistance to hot tearing
- solidification range
Casting design considerations:
- solidification range
- resistance to hot tearing
- die soldering (die casting)
- strength and ductility
- heat treatability
- pressure tightness characteristics
- corrosion resistance
- surface treatments
- dimensional stability
- thermal stability
- ingot and melting costs
- heat treatment
Aluminum is one of the few metals that can be cast by all of the processes
used in casting metals. These processes, in decreasing order of amount
of aluminum cast, are: die casting, permanent mold casting, sand casting
(green sand and dry sand), plaster casting and investment casting.