Casting of alloys
For alloys having copper as main element, see Casting of copper.
For alloys having magnesium or zinc as main element, see Casting of other metals
Casting of superalloys
The superalloys are typically Ni, Ni-Fe, and Co based alloys with Cr, Ti, W, Al additions. They were originally used for high temperature applications (over 810 ºC) or in severe corrosive media. Superalloys can be distinguished from high alloyed steels. Since iron is not the major compound (as defined in [201, CEN, 2000]), they are considered non-ferrous materials. The casting of superalloys may occur in certain investment casting foundries, as well as partly in foundries which specialise in high alloyed steel qualities. The nickel base alloys are produced from a group of alloys which have chemical compositions generally over 50 % nickel and less than 10 % iron. They are mainly strengthened by intermetallic precipitation in an austenitic matrix. The cobalt base alloys have a high Co content (40 to 70 %), high Cr (over 20 %), high W (7 to 15 %) and they are strengthened by a combination of carbides and solid solution hardeners.
Some superalloys, particularly Ni-Fe and Co based alloys, are directly melted in electric furnaces by classical methods usually applicable to stainless steels. However for Ni and special Ni-Fe superalloys, vacuum induction melting is required in order to reduce the content of interstitial gases (O, H, N) to a very low level. This enables foundries to achieve high and controlled contents of oxidisable elements such as Ti or Al.
The control of interstitial gases and oxidisable elements is very important for the product’s mechanical properties, the corrosion resistance and its reliability. In general, superalloys are cast into complex final shapes where machining is not possible. Therefore, they are mainly produced by investment casting (i.e. using a ceramic mould). This casting process produces a product of very precise dimensions with a very smooth surface. Additional processes, such as HIP (hot isostatic pressing), can be used to eliminate the internal porosity that can appear in large castings. In aircraft gas turbine manifolds, directional casting technology is commonly applied. This technology eliminates the grain boundaries and greatly increases the strength of the material.
Initially superalloys were developed for high temperature applications. However, their field of application continues to expand and now covers areas such as cryogenic temperature appliances and orthopaedic and dental prostheses. In general, superalloys are mainly used in aircraft and industrial gas turbines, in nuclear reactors, in aircraft and spacecraft structures, in petrochemical production and in medical applications. [202, TWG, 2002]
Source: European Commission, Reference Document on Best Available Techniques in the Smitheries and Foundries Industry, May 2005