anothe artical how this helped in making SCB for engines.
New Page 4
Directionally Solidified &
Single Crystal Turbine Blades
& Vanes
A
state-of-the-art gas turbine
aerofoil component sometimes
works in an environment of
temperature beyond even the
melting point of the alloy used
for manufacturing it. These
superalloy components possess a
complex aerofoil geometry and
are investment cast to equiaxed
component or columnar grained
or single crystal components by
directional solidification to
increase the performance. The
technologies for both equiaxed
and columnar grains or single
crystal have been indigenously
developed and demonstrated by
DRDO for a variety of
components. In the directional
solidification, a bottom open
ceramic shell mould is placed on
a water cooled copper plate
mounted on a ram shaft and is
heated by an in situ mould
heater in a vacuum casting
furnace to a temperature
beyond the melting point of the
alloy to be cast. When the
melted alloy is poured into the
mould the alloy at the copper
plate gets chilled and a
columnar grained structure
develops in the direction of heat
extraction. The solidification
proceeds into the mould cavity
when the mould is lowered out
of the mould heater. To produce
a single crystal structure, some
constriction ahead of initial
columnar grain structure that
allows growth of only one of the
grains of the columnar structure,
usually a helix with about one
turn, is used.
Leachable fused silica cores are
used for casting aerofoils to very
tight tolerances with complex
internal cooling passages
characterised by thin walls of the
order of 0.5 mm. The shell and
core system is designed so that
no distortion occurs at the
temperature of directional
solidification of about 1500 oC.
