INVESTMENT CASTING
Turbine blades have a complex geometry and contain many areas of
double curvature. Therefore the blades have to be precisely manufactured by the
precision casting process of investment casting, also known as the ‘lost wax
process’. Ceramic cores for the cooling channels are positioned within a master
mould pattern. Wax is then injected into the mould cavity to produce a preform
of the turbine blade. Pinning wires are then pressed through the wax to butt
against the ceramic cores within the preform. Next, the preform is coated with
multiple layers of ceramic, ultimately forming a thick casing around the
preform with the pinning wires embedded in it. The assembly is heated to melt
out the wax and then tired to strengthen the ceramic. The result is a ceramic
shell mould containing a complex ceramic core pattern which is held in position
by pinning wires anchored in the ceramic shell.
Finally, the mould assembly is preheated prior to casting the turbine
blade.
Turbine blade manufacture crucially requires the nucleation and
growth of precisely controlled microstructures. The grain structure within the
turbine-blade super alloy material is frequently described by terms such as
‘equi-axed’, ‘directionally solidified’ and ‘single crystal’. These
characterise the grain boundary length and thus ultimately the performance of
the turbine blade.
TURBINE BLADES MACHINING
The turbine components manufacturing technologies comprise grinding,
broaching and milling with the grinding wheels, grinding with the rasping
belts, deburring with the abrasive brushes and eccentric expertise in the
turbine blades and blisks machining. (Abrasive Water Jet Machining –AWJM,
Abrasive Flow Machining –AFM, Electro-Chemical Machining – ECM,
Electro-Discharge Machining – EDM).
High-speed cutting (HSC) permits stress-free sculptured machining of
the high performance alloys and composites being used by aircraft and
power-generation manufacturers. The multi-axes CNC machining centres guarantee high
level of shape repetition accuracy in series production.
The modern 5-axes machining centres facilitate machining of the
turbine and compressor blade leading sharp edges, trailing edges and tips in
one set-up procedure. The centres are equipped with CNC control system with
software for NC simulation of the blade aerofoil and the blade root machining method,
enabling the machinist to ensure milling operations and tools on the computer
screen and optimize the NC program. However finishes grinding of the blade
aerofoil surface have been impracticable on those machine tools. These blades
require general hand-polishing to breed the requisite shape and surface
roughness.
The machine tool manufacturers developed the other type of the
machining centres to avoid those disadvantages and to reduce the number of the
necessary machine tools by introducing new techniques n milling and grinding.
Nowadays in the different technologies of the turbine blade and vanes
removal machining the machining centres (MakinoA99-5XR-CD, Bridgeport FGC1000)
is applied by the producers. These centres were espoused by transformation to crushing
and milling the blades with VIPER method (an acronym of Rolls-Royce: Vitreous
Improved Performance Extreme Removal), however their main intention were
machining with the cutting tools of distinct edges geometry. The innovation integrated
lodgings and adjustments to the conditions of grinding: the core spindle, the splatter
guards of roller rail linear movements, a cooling medium discharge and
filtration unit, an sufficient sort of coolant assortment, equipping the device
tool with grinding wheel dressing attachment and with numerically controlled
system of the nozzles to convey the coolant unswervingly into the machining region,
to work out a appropriate software for CNC control system.
MANUFACTURING
SINGLE CRYSTAL TURBINE BLADE
Different methods of manufacturing are in practice to create single
crystal turbine blades. Theses manufacturing methods mainly use the idea of
directional solidification, or self-directed solidification. A common method
used is the Bridgman method for growing single crystals which uses casting
furnace for crystal growth. This process involves a mould that must be firstly
made of the blade itself. Molten wax is infused into a clanging mould of the required
turbine blade and left to set and eventually it takes the form of the turbine
blade after undergoing the proper process. The wax model is then used to make a
ceramic cast that is used for manufacturing of the single crystal turbine
blades. To add strength to the cast, it is heated at high temperature which
increases its final strength. Once the mould is ready for use, the wax is
melted out from the inside of the cast. The mould is now filled with the molten
form of the nickel based super alloy. The molten super alloy contained within
the mould is placed in some type casting furnace, often a vacuum induction
melting furnace, which uses Bridgman techniques.
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