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equipment, careful maintenance and cleanliness of
the forging hammers, presses and dies, is essential.
18. Annular combustion rings can be cold forged to
exacting tolerances and surfaces which alleviates
the need for further machining before welding
together to produce the combustion casing.
19. H.P. compressor casings of the gas turbine
engine are forged as rings or half rings which, when
assembled together, form the rigid structure of the
engine. They are produced in various materials, i.e.,
stainless steel, titanium and nickel alloys.
CASTING
20. An increasing percentage of the gas turbine
engine is produced from cast components using
Manufacture
233
Fig. 22-4 Automatic investment casting.
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 chemical
composition, test of mechanical properties, radiological
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.
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 conventional
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.
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.
Manufacture
234
Fig. 22-5 Robot cut-off
26. Other fabrication techniques for the
manufacture of the low pressure compressor wide
chord fan blade comprise rolled titanium side panels
assembled in dies, hot twisted in a furnace and finally
hot creep formed to achieve the necessary configuration.
Chemical milling is used to recess the centre
of each panel which sandwiches a honeycomb core,
both panels and the honeycomb are finally joined
together using automated furnaces where an
activated diffusion bonding process takes place, ref.
fig. 22-6.
WELDING
27. Welding processes are used extensively in the
fabrication of gas turbine engine components i.e.,
resistance welding by spot and seam, tungsten inert
gas and electron beam are amongst the most widely
used today. Care has to be taken to limit the
distortion and shrinkage associated with these
techniques.
Tungsten inert gas (T.I.G.) welding
28. The most common form of tungsten inert gas
welding, fig, 22-7, in use is the direct current straight
polarity i.e., electrode negative pole. This is widely
used and the most economical method of producing
high quality welds for the range of high strength/high
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