Sunday, July 8, 2012

Manufacture - Casting




CASTING


Fig. 22-3 Method of producing an engine component by sand casting.


Fig. 22-4 Automatic investment casting.
20. An increasing percentage of the gas turbine engine is produced from cast components using sand casting, ref. fig. 22-3, die casting and investment casting techniques; the latter becoming the foremost in use because of its capability to produce components with surfaces that require no further machining. It is essential that all castings are defect free by the disciplines of cleanliness during the casting process otherwise they could cause component failure.

21. All casting techniques depend upon care with methods of inspection such as correct chemica composition, test of mechanical properties, radiolog- ical and microscopic examination, tensile strength and creep tests. 22. The complexity of configurations together with accurate tolerances in size and surface finish is totally dependent upon close liaison with design, manufacturing, metallurgist, chemist, die maker, furnace operator and final casting.

23. In the pursuit of ever increasing performance, turbine blades are produced from high temperature nickel alloys that are cast by the investment casting or lost wax' technique. Directionally solidified and single crystal turbine blades are cast using this technique in order to extend their cyclic lives.


Fig. 22-5 Robot cut-off
24. Figure 22-4 illustrates automatic casting used in the production of equi-axed, directional solidified and single crystal turbine blades. The lost wax process is unparalleled in its ability to provide the highest standards of surface finish, repeatable accuracy and surface detail in a cast component. The increasing demands of the engine has manifested itself in the need to limit grain boundaries and provide complex internal passages. The moulds used for directional solidified and single crystal castings differ from con- ventional moulds in that they are open at both ends, the base of a mould forms a socketed bayonet fitting into which a chill plate is located during casting. Metal is introduced from the central sprue into the mould cavities via a ceramic filter. These and orientated seed crystals, if required, are assembled with the patterns prior to investment. Extensive automation is possible to ensure the wax patterns are coated with the shell material consistently by using robots. The final casting can also have their rises removed using elastic cut-off wheels driven from robot arms, ref. fig. 22-5.






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