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 casting, are: die casting,
permanent mold casting, sand casting (green sand and dry sand),
plaster casting, investment casting, and continuous casting.
Other processes such as lost foam, squeeze casting, and hot
isostatic pressing are also mentioned.
There are many factors that affect selection of a casting
process for producing a specific aluminum alloy part. The
most important factors for all casting processes are:
- Feasibility and cost factors
- Quality factors.
In terms of feasibility, many aluminum alloy castings can be
produced by any of the available methods. For a considerable
number of castings, however, dimensions or design features
automatically determine the best casting method. Because
metal molds weigh from 10 to 100 times as much as the
castings they are used in producing, most very large cast
products are made as sand castings rather than as die or
permanent mold castings. Small castings usually are made with
metal molds to ensure dimensional accuracy.
Quality factors are also important in the selection of a
casting process. When applied to castings, the term quality
refers to both degree of soundness (freedom from porosity,
cracking, and surface imperfections) and levels of mechanical
properties (strength and ductility).
However, it should be kept in mind that in die casting,
although cooling rates are very high, air tends to be trapped
in the casting, which gives rise to appreciable amounts of
porosity at the center. Extensive research has been conducted
to find ways of reducing such porosity; however, it is
difficult if not impossible to eliminate completely, and die
castings often are lower in strength than low-pressure or
gravity-fed permanent mold castings, which are more sound in
spite of slower cooling.
Alloys of aluminum are used in die casting more extensively
than alloys of any other base metal. In the United States
alone, about 2.5 billion dollars worth of aluminum alloy die
castings is produced each year. The die casting process
consumes almost twice as much tonnage of aluminum alloys as
all other casting processes combined.
Die casting is especially suited to production of large
quantities of relatively small parts. Aluminum die castings
weighing up to about 5 kg are common, but castings weighing
as much as 50 kg are produced when the high tooling and
casting-machine costs are justified.
Typical applications of die cast aluminum alloys include:
- Alloy 380.0 - Lawnmower housings, gear
- Alloy A380.0 - Streetlamps housings, typewriter frames,
- Alloy 360.0 - Frying skillets, cover plates, instrument
cases, parts requiring corrosion resistance.
- Alloy 413.0 - Outboard motor parts such as pistons,
connecting rods, and housings
- Alloy 518.1 - Escalator parts, conveyor components,
aircraft and marine hardware and lit tings.
With die casting, it is possible to maintain close tolerances
and produce good surface finishes. Die castings are best
designed with uniform wall thickness: minimum practical wall
thickness for aluminum alloy die castings is dependent on
Die castings are made by injection of molten metal into metal
molds under substantial pressure. Rapid injection and rapid
solidification under high pressure combine to produce a dense,
fine-grain surface structure, which results in excellent wear
and fatigue properties. Air entrapment and shrinkage, however,
may result in porosity, and machine cuts should be limited to
1.0 mm to avoid exposing it.
Aluminum alloy die castings usually are not heat treated but
occasionally are given dimensional and metallurgical
Die castings are not easily welded or heat treated because of
entrapped gases. Special techniques and care in production
are required for pressure-tight parts. The selection of an
alloy with a narrow freezing range also is helpful. The use
of vacuum for cavity venting is practiced in some die casting
foundries for production of parts for some special
Approximately 85% of aluminum alloy die castings are produced
in aluminum-silicon-copper alloys (alloy 380.0 and its several
modifications). This family of alloys provides a good
combination of cost, strength, and corrosion resistance,
together with the high fluidity and freedom from hot shortness
that are required for ease of casting. Where better corrosion
resistance is required, alloys lower in copper, such as 360.0
and 413.0 must be used.
Alloy 518.0 is occasionally specified when highest corrosion
resistance is required. This alloy, however, has low fluidity
and some tendency to hot shortness. It is difficult to cast,
which is reflected in higher cost per casting.
Permanent mold casting
Permanent mold (gravity die) casting, like die casting, is
suited to high-volume production. Permanent mold castings
typically are larger than die castings. Maximum weight of
permanent mold castings usually is about 10 kg, but much
larger castings sometimes are made when costs of tooling and
casting equipment are justified by the quality required for
Permanent mold castings are gravity-fed and pouring rate is
relatively low, but the metal mold produces rapid
solidification. Permanent mold castings exhibit excellent
mechanical properties. Castings are generally sound, provided
that the alloys used exhibit good fluidity and resistance to
Mechanical properties of permanent mold castings can be further
improved by heat treatment. If maximum properties are required,
the heat treatment consists of a solution treatment at high
temperature followed by a quench and then natural or artificial
aging. For small castings in which the cooling rate in the
mold is very rapid or for less critical parts, the solution
treatment and quench may be eliminated and the fast cooling
in the mold relied on to retain in solution the compounds
that will produce age hardening.
Some common aluminum permanent mold casting alloys, and
typical products cast from them, are presented below.
- Alloy 366.0 - Automotive pistons
- Alloys 355.0, C355.0, A357.0 - Timing gears, impellers,
compressors, and aircraft and missile components requiring
- Alloys 356.0, A356.0 - Machine tool parts, aircraft
wheels, pump parts, marine hardware, valve bodies
- Other aluminum alloys commonly used for permanent mold
castings include 296.0, 319.0, and 333.0.
Sand casting, which in a general sense involves the forming
of a casting mold with sand, includes conventional sand
casting and evaporative pattern (lost-foam) casting.
In conventional sand casting, the mold is formed around a
pattern by ramming sand, mixed with the proper bonding agent,
onto the pattern. Then the pattern is removed, leaving a
cavity in the shape of the casting to be made. If the casting
is to have internal cavities or undercuts, sand cores are
used to make them. Molten metal is poured into the mold, and
after it has solidified the mold is broken to remove the
casting. In making molds and cores, various agents can be
used for bonding the sand. The agent most often used is a
mixture of clay and water.
Casting quality is determined to a large extent by foundry
technique. Proper metal-handling practice is necessary for
obtaining sound castings. Complex castings with varying wall
thickness will be sound only if proper techniques are used.
Evaporative (lost-foam) pattern casting
Evaporative pattern casting (EPC) is a sand casting process
that uses an unbounded sand mold with an expendable
polystyrene pattern placed inside of the mold. This process
is somewhat similar to investment casting in that an
expendable material can be used to form relatively intricate
patterns in a surrounding mold material. Unlike investment
casting, however, evaporative pattern casting (EPC) involves
a polystyrene foam pattern that vaporizes during the pouring
of molten metal into a surrounding mold of unbounded sand.
Shell Mold Casting
In shell mold casting, the molten metal is poured into a shell
of resin-bonded sand only 10 to 20 mm thick - much thinner
than the massive molds commonly used in sand foundries. Shell
mold castings surpass ordinary sand castings in surface
finish and dimensional accuracy and cool at slightly higher
rates; however, equipment and production are more expensive.
In this method, either a permeable (aerated) or impermeable
plaster is used for the mold. The plaster in slurry form is
poured around a pattern, the pattern is removed and the
plaster mold is baked before the casting is poured. The high
insulating value of the plaster allows castings with thin
wads to be poured.
Minimum wall thickness of aluminum plaster castings typically
is 1.5 mm. Plaster molds have high reproducibility, permitting
castings to be made with fine details and close tolerances.
Mechanical properties and casting quality depend on alloy
composition and foundry technique. Slow cooling due to the
highly insulating nature of plaster molds tends to magnify
solidification-related problems, and thus solidification must
be controlled carefully to obtain good mechanical properties.
Cost of basic equipment for plaster casting is low; however,
because plaster molding is slower than sand molding, cost of
operation is high. Aluminum alloys commonly used for plaster
casting are 295.0, 355.0, C355.0, 356.0 and A356.0.
Investment casting of aluminum most commonly employs plaster
molds and expendable patterns of wax or other fusible materials.
Plaster slurry is "invested" around patterns for several
castings, and the patterns are melted out as the plaster is
Investment casting produces precision parts; aluminum castings
can have walls as thin as 0.40 to 0.75 mm. However, investment
molding is often used to produce large quantities of
intricately shaped parts requiring no further machining so
internal porosity seldom is a problem. Because of porosity
and slow solidification, mechanical properties are low.
Investment castings usually are small, and it is especially
suited to production of jewelry and parts for precision
instruments. Recent strong interest by the aerospace industry
in the investment casting process has resulted in limited use
of improved technology to produce premium quality castings.
Combining this accurate dimensional control with the high and
carefully controlled mechanical properties can, at times,
justify casting costs and prices normally not considered
Aluminum alloys commonly used for investment castings are
208.0, 295.0, 308.0, 355.0, 356.0, 443,0, 514.0, 535.0 and
Centrifuging is another method of forcing metal into a mold.
Steel baked sand, plaster, cast iron, or graphite molds and
cores are used for centrifugal casting of aluminum. Metal
dies or molds provide rapid chilling, resulting in a level of
soundness and mechanical properties comparable or superior to
that of gravity-poured permanent mold castings.
Wheels, wheel hubs, and papermaking or printing rolls are
examples of aluminum parts produced by centrifugal casting.
Aluminum alloys suitable for permanent mold, sand, or plaster
casting can be cast centrifugally.
Long shapes of simple cross section (such as round, square,
and hexagonal rods) can be produced by continuous casting,
which is done in a short, bottomless, water-cooled metal
The casting is continuously withdrawn from the bottom of the
mold; because the mold is water cooled, cooling rate is very
high. As a result of continuous feeding, castings generally
are free of porosity. In most instances, however, the same
product can be made by extrusion at approximately the same
cost and with better properties, and thus use of continuous
casting is limited. The largest application of continuous
casting is production of ingot for rolling, extrusion, or
Many of the molding methods described above can be combined
to obtain greater flexibility in casting. Thus, dry sand
cores often are used in green sand molds, and metal chills
can be used in sand molds to accelerate local cooling.
Hot isostatic pressing
Hot isostatic pressing of aluminum castings reduces porosity
and can thus decrease the scatter in mechanical properties.
The method also makes possible the salvaging of castings that
have been scrapped for reasons of internal porosity, thereby
achieving improved foundry recovery. This advantage is of
more significant importance in the manufacture of castings
subject to radiographic inspection when required levels of
soundness are not achieved in the casting process. The
development of hot isostatic pressing is pertinent to the
broad range of premium castings, but is especially relevant
for the more difficult-to-cast aluminum-copper series.
Hybrid Permanent Mold Processes
Although die casting, centrifugal casting, and gravity die
casting constitute, on a volume basis, the major permanent
mold processes, there are also some hybrid processes that use
permanent molds. This includes squeeze casting and semisolid