Manufacture
Manufacture
Fig. 22-4 Automatic investment casting.
16. The high operating temperatures at which the turbine discs must operate necessitates the use of nickel base alloys. The compressor discs at the rear end of the compressor are produced from creep-resisting steels, or even nickel base alloys, because of the high temperatures to which they are subjected. The compressor discs at the front end of the compressor are produced from titanium. The higher strength of titanium at the moderate operating tem-peratures at the front end of the compressor, together with its lower weight provides a consider-able advantage over steel.
17.
Forging calls for a very close control of the temperature during the various operations. An exceptionally high standard of furnace control 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
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, 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.
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
Manufacture
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 configu-ration. 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. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：劳斯莱斯喷气引擎－中英(113)
