Aluminium alloy castings

Aluminium is one of the few metals that can be cast by all of the processes used in casting metals. These processes are: die casting, permanent mould gravity casting, sand casting, plaster casting, investment casting, and continuous casting.

There are many factors that affect selection of a casting process for producing specific aluminium alloy parts. The most important factors for all casting processes are feasibility and cost factors and quality.

In terms of feasibility, many aluminium 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 moulds 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 gravity castings.

Small castings usually are made with metal moulds 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 centre. 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 castings, which are more sound in spite of slower cooling.

Aluminium alloy die Castings

Aluminium alloy die castings are more extensively used than alloys of any other base metal. In the United States alone, about 2.5 billion dollars worth of aluminium 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. Aluminium 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.

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 aluminium alloy die castings is dependent on casting size.
Die castings are made by injection of molten metal into metal moulds 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.

Aluminium alloy die castings usually are not heat treated but occasionally are given dimensional and metallurgical stabilization treatments.

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 applications.

Approximately 85% of aluminium alloy die castings are produced in aluminium-silicon-copper alloys. 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. It is difficult to cast, which is reflected in higher cost per casting.

Gravity die castings

Gravity die casting, like die casting, is suited to high-volume production. Typically larger than die castings, maximum weight of gravity die 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 the casting.
Permanent mold castings are gravity-fed and pouring rate is relatively low, but the metal mould produces rapid solidification. Permanent mould castings exhibit excellent mechanical properties. Castings are generally sound, provided that the alloys used exhibit good fluidity and resistance to hot tearing.

Mechanical properties of permanent mould 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 aluminium permanent mould casting alloys are used (Alloy 366.0) for engine pistons, (Alloys 355.0, C355.0, A357.0) for gears, impellers, compressors, and aircraft and missile components requiring high strength.
Alloys 356.0, A356.0 are used for Machine tool parts, pump parts, hardware and valve bodies.

Aerospace Sand castings

Sand casting, which in a general sense involves the forming of a casting mould with sand, includes conventional sand casting and evaporative pattern (lost-foam) casting.
In conventional sand castings, the mould 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 mould, and after it has solidified the mould is broken to remove the casting. In making moulds 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 castings

Evaporative pattern casting (EPC) is a sand casting process that uses an unbounded sand mould with an expendable polystyrene pattern placed inside of the mould. 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 mould of unbounded sand.

Shell Mould Castings

In shell mould castings, 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 mould castings surpass ordinary sand castings in surface finish and dimensional accuracy and cool at slightly higher rates; however, equipment and production are more expensive.

Plaster Castings

In this method, either a permeable (aerated) or impermeable plaster is used for the mould. 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 moulds 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 moulds 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 moulding is slower than sand moulding, cost of operation is high. Aluminium alloys commonly used for plaster casting are 295.0, 355.0, C355.0, 356.0 and A356.0.

Investment castings

Investment castings of aluminium most commonly employs plaster moulds 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 baked.
Investment casting produces precision parts; aluminium alloy castings by this method can have walls as thin as 0.40 to 0.75 mm. However, investment casting 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 especially suited to aerospace castings. 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 practical.

Aluminium alloys commonly used for investment castings are 208.0, 295.0, 308.0, 355.0, 356.0, 443,0, 514.0, 535.0 and 712.0.

Centrifugal Castings

Centrifuging is another method of forcing metal into a mould. Steel baked sand, plaster, cast iron, or graphite moulds and cores are used for aluminum alloy castings. Metal dies or moulds provide rapid chilling, resulting in a level of soundness and mechanical properties comparable or superior to that of gravity-poured permanent mould castings.