The different types of Aerospace Castings
Cast irons are generally formed either in discrete parts using moulds or by continuous casting of a generic shape of constant cross-section. Mould casting has been successfully employed for centuries with little change in a process that involves the pouring of molten steel into a fixed shaped cavity to produce aerospace technical castings. There are six commonly used methods:
– as implied by the name, this process involves the use of sand as a means of handling the molten iron as it cools into a desired form. A wooden pattern is used to first define the shape in the sand, then it is removed and the cavity filled with molten iron. This is an inexpensive method for limited volume applications.
Permanent mould castings
– similar to a sand casting but done using permanent mould made from a water-cooled steel mould. This is a costly method that is best suited for high volume applications.
– a process similar to plastic injection moulding involving the pressurized injection of molten metal into a mould. This is expensive, requiring a large number of parts to amortize the high tooling cost.
– this process involves first making a casing or shell of the pattern (or actual part), splitting it to remove it, reassembling the pieces and finally placing shell in sand. The sand supports the shell while the molten metal is poured into it. After cooling, the part is removed by breaking the shell to expose the completed part. This process is labour intensive but is a good one for intricate parts that are produced in low volumes.
– this casting process produces similar results as shell casting but is aimed at higher volumes. A mould is built to form a wax pattern which is then removed, coated with a ceramic material, heated to remove the wax, and then placed in sand. The molten metal is then poured inside, allowed to cool, and then the ceramic shell is removed. This is the method most frequently used to produce aerospace aluminium castings.
– this process involves producing a part by using a rotating drum with the mould being the inner diameter of the drum. It is rotated while molten metal is poured inside, forming the desired part which is removed when cooled by splitting the mould. This is most commonly used to form pipes.
Continuous casting is a relatively newer process which was fully developed after the World War II. As the name states, the process involves continuously pouring molten metal from the bottom of the crucible on to a water-cooled mould, forming a skin that allows it to be further handled down the line. The process inherently produces high quality castings, because the material is drawn from the bottom of the crucible, away from the slag and other impurities that float on the surface. It is also subjected to differential cooling results which produces varied cross-sectional material properties, with the outer region generally being made up of a finer graphite structure than the core to give a combined surface hardness and overall toughness. Two forms of graphite are typically produced, flakes (present in gray iron) and nodules (present in ductile iron). The solubility limit at which these form involve many factors that are not easy or economical to control, requiring additions, known as inoculants to be added which force the graphite out of solution and make it possible to control the size and shape of the graphite particles.
The casting process requires careful consideration of three parameters of the cast part that will often determine the method of casting selected. These are part size, required tolerances and surface finish. Generally, the larger the part, the more expensive the tooling and handling equipment. Those processes which require hard tooling such as die castings or investment castings, are not used to produce large parts, while sand castings have no such limitation. The tradeoff here is the quality of the part and the surface finish. Larger parts can have internal cavities caused by shrinkage while sand castings do not have the improved surface finish afforded by hard tooled processes.