Besides aluminum forging or aluminum machining, lots of aluminum parts are created by casting process. In China, the main aluminum casting methods are: gravity die casting, pressure die casting and sand casting.
The selection of a suitable aluminum casting method depend on several factors, such as part structure, aluminum grade, cost, quality, etc. CFS Foundry is the professional aluminum casting company supplying cast aluminum parts in any casting method you want.
Gravity Die Casting
Gravity die casting, also known as permanent mold casting, is suited to production in different volumes. Aluminum castings made from gravity die casting process could be larger than die casting process. Maximum weight of gravity die cast aluminum parts can reach 10 kg, but much larger castings sometimes sand casting is considered for its less production cost.
As this casting process utilizes gravity to pour metals, the pouring rate is relatively low, but the solidification is rapid. Gravity die casting provides excellent mechanical properties of aluminum castings. Castings are generally sound, provided that the alloys used exhibit good fluidity and resistance to hot tearing.
Generally, T6 heat treatment can help to improve the mechanical properties of aluminum castings after gravity die casting process. 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.
Most common aluminum alloys by gravity die casting in China are: A356 alloy, A380 alloy, etc. It is worthy to mention that A356 aluminum alloy is most commonly used, as T6 heat treatment is always operated after heat treatment to improve its strength for applications in marine, pumps, etc.
Pressure Die Casting
Pressure die casting is another aluminum casting method that differs from gravity die casting. This type of casting method is especially suited to producte relatively small aluminum parts in large volumes. Pressure die casting injects 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.
Pressure die casting can guarantee close tolerances and good surface finishes of aluminum castings. It can produce aluminum castings with small or uniform wall thickness.
Aluminum castings by pressure die casting process are usually are not welded or heat treated, because of entrapped gases. Special techniques and care in production are required for pressure-tight parts.
Typical pressure die cast aluminum alloys include: A360, A380, etc. Approximately 85% of aluminum pressure die castings are produced in aluminum-silicon-copper alloys such as A380. Aluminum castings in A380 alloy can achieve good strength and corrosion resistance, together with the high fluidity and freedom from hot shortness that are required for ease of casting. Aluminum castings in pressure die casting process are mostly applied in automotive, truck and other markets.
Sand Casting
Sand casting, which in a general sense involves the forming of a casting mold with sand.
This process starts with a resin pattern, a copy of the product, which is packed in sand to produce a hardened mould. Other sand shapes, called cores, are placed in the mould to produce internal cavities. Molten metal is hand poured into the mould through a running system and allowed to cool. Once out of the mould box, the casting’s surface is shot blasted and finished.
Sand casting is quick and relatively cost-effective and can be used to make aluminum castings in different sizes and with complex geometric shapes. But both the surface roughness and dimensional accuracy can not compare with gravity die casting or pressure die casting methods.
Although initial investment is relatively low, unit costs can be considerably higher than gravity die casting process. The same can be said of investment casting (lost wax), which is only economical in very low quantity, prototype-natured work.
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