Page 244 - The Jet Engine

Manufacture

sand casting, ref. fig. 22-3, die casting and nickel alloys that are cast by the investment casting
investment casting techniques; the latter becoming or lost wax' technique. Directionally solidified and
the foremost in use because of its capability to single crystal turbine blades are cast using this
produce components with surfaces that require no technique in order to extend their cyclic lives.
further machining. It is essential that all castings are
defect free by the disciplines of cleanliness during 24. Figure 22-4 illustrates automatic casting used in
the casting process otherwise they could cause the production of equi-axed, directional solidified and
component failure. single crystal turbine blades. The lost wax process is
unparalleled in its ability to provide the highest
21. All casting techniques depend upon care with standards of surface finish, repeatable accuracy and
methods of inspection such as correct chemical surface detail in a cast component. The increasing
composition, test of mechanical properties, radiolog- demands of the engine has manifested itself in the
ical and microscopic examination, tensile strength need to limit grain boundaries and provide complex
and creep tests. internal passages. The moulds used for directional
solidified and single crystal castings differ from con-
22. The complexity of configurations together with ventional moulds in that they are open at both ends,
accurate tolerances in size and surface finish is the base of a mould forms a socketed bayonet fitting
totally dependent upon close liaison with design, into which a chill plate is located during casting.
manufacturing, metallurgist, chemist, die maker, Metal is introduced from the central sprue into the
furnace operator and final casting.
mould cavities via a ceramic filter. These and
23. In the pursuit of ever increasing performance, orientated seed crystals, if required, are assembled
turbine blades are produced from high temperature 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.
FABRICATION
25. Major components of the gas turbine engine i.e.
bearing housings, combustion and turbine casings,
exhaust units, jet pipes, by-pass mixer units and low
pressure compressor casings can be produced as
fabricated assemblies using sheet materials such as
stainless steel titanium and varying types of nickel
alloys.

Fig. 22-5 Robot cut-off

234