Aluminium Castings

Aluminium castings

Using an aluminium casting to replace an iron casting will result in reducing component weight by half. That is why auto producers are searching the areas where they may replace iron parts with aluminium parts. Most auto manufacturers prefer alloys of aluminium specially the A 300 series which have a mix of silicon + a minor amount of magnesium.

The silicon provides fluidity while casting, which allows thinner parts to be cast. The magnesium gives strength (via heat treatment). An important feature of aluminium and its alloys is that in comparison with ferrous alloys which show a unlimited fatigue strength, the fatigue strength of aluminium alloys goes down with increase in number of stress cycles this needs to be taken in account while designing its parts.

For getting high quality castings a variety of processes are followed. While casting permanent moulds precautions are taken to ensure that it gets required cooling and vacuum pressure is also applied to ensure correct flow and lessen porosity. Aluminium is one of the few metals which may be cast by all processes of casting.

Die casting:
Aluminium alloys are more utilized in die casting than alloys of any metal. In USA alone 2.5 billion dollars worth aluminium alloy die castings are manufactured every year. Die casting is suitable for production of smaller parts. But when expensive manufacturing equipment is used the castings weigh 5kg-50kg. Cost effectiveness can be obtained only by large scale production. With die casting it is possible to have close tolerance and fine finishing surface. Die castings are manufactured by injecting melted metal into metal moulds under considerable pressure. Quick injecting and quick solidification under high pressure make a fine grain surface, which leads to having best wear and fatigue properties. Aluminum die castings are generally not given heat treatment, so they cannot be welded.

Permanent die casting:
It is also known as gravity die casting and is suitable for large scale production. Their maximum weight is 10kg. But where heavy cost machinery and tools are used the weight of casting may go up to 50kg. Permanent mold casting have low pouring rate but metal molds solidify quickly, mechanical properties of permanent mould castings may be further bettered by heat treatment.

Sand casting:
Sand casting is casting of a mold with sand. In normal sand casting, the mould is made around a design by forcing sand mixed along with a bonding agent on the design. Then the design is taken out which leaves a cavity for casting. Melted metal is poured into the mould and after its solidification the mould is broken to remove casting. The most normal bonding agent is a mix of clay and water. Good metal handling practice is needed for getting good castings.

Investment castings:
It normally uses plaster moulds and throwaway designs of wax or other burnable materials. Plaster slurry is “invested” all round designs for many castings the designs are melted out as the plaster is baked. By these castings you can make precision parts, the aluminium parts can have walls as thin 0.40 to 0.75 mm. It is also used to make large number of parts of fine shapes which need no further machining. These parts have low mechanical properties.