Note: Descriptions are shown in the official language in which they were submitted.
DUAL ALLOY _TURB I NE BLADE
TECHNICAL FIELD
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This inventiQn relates generally to gas turbine
power plants, and, more particularly, to turblne blades
used in high performance gas turbine engines.
BACKGROUN~ OF THE INVENTION
Gas turbine power plants are used as the primary
propulsive power source for aircraft, in the forms of
jet engines and turboprop engines, as auxiliary power
sources for driving air compressors, hydraulic pumps,
etc. on aircraft, and as stationary power supplies such
as backup electrical generators for hospitals and the
like. The same basic power generation principles apply
for all of these types of gas turbine power plants.
lS Compressed air i~ mixed with fuel and burned, and the
expanding hot combustion gases are directed against
stationary turhine vanes in the engine. The vanes turn
the ~high velocity gas flow partially sideways to impinge
upon turbine blades mounted on a turbine disk or wheel
that is free to rotate.
The force of the impinging gas causes the turbine
disk to sp~n at high speed. Jet propulsion engines use
this power to draw more air into the engine and then
high velocity combustion ga~ is passed out the aft end
of the gas turbine, creating forward thrust. Other
engines use this power to turn a propeller or an
electric generator.
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~; Thè turbine blades and vanes lie at the héart of
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the power plant, and ~t is well established that in most
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ca~ses, they are one of the limiting factors in achieving
improved power plant efficiency. In particular, because
they are subjected to high heat and stress loadings as
they are rotated and impacted by the hot gas, there is a
continuing effort to identify improvements to the
construction and/or design of turbine blades to achieve
ever higher performance.
Much research and engineering has been directed
to the problem of improved turbine blade materials. The
earliest turbine blades were made of simple cast alloys
having relatively low maximum operating temperatures.
The alloy materials have been significantly improved
over a period of years, resulting in various types of
nickel-based and cobalt-based superalloys that are in
use today.
As the alloy materials were improved, the
metallurgical microstructure of the turbine blades was
also improved. First, the polycrystalline grain
structures were modified by a wide variety of treatments
to optimize their performance. Directionally solidified
ôr orlented polycrystalline blades were then developed,
having elongated grains with deformation-resistant
orientations parallel to the radial axis of the blade in
order to best resist the centrifugal stresses. Each of
the~e advancements led to improved performance of the
blades. Polycrystalline and oriented polycrystalline
'-~ blade are widely used in most commercial and many
~ m,ilitary alrcraft engine~ today.
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-~ - It 'has been proposed to improve polycrystalline ,~
30 blades by including reinforcing ceramic fibers or the ~,
like in the structure but such approaches ;have not met
with success primarily because of the ~problems in
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adeguately bonding such differing materials so that
operating stresses are evenly distributed.
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More recently, single crystal turbine blades have
been introduced as a result of the development of
practical techniques to cast them in large qUantities.
These turbine blades have the advantage of eliminating
grain boundaries entirely, which are one of the
important causes of creep deformation and failure of the
airfoil. The elimination of grain boundaries allows the
chemical composition of the single crystal blade to be
ad~usted to achieve improved creep and high-cycle
fatigue performance at the highest engine operating
temperatures. Single crystal turbine blades are now
used in high performance military aireraft and may
eventually be intFoduced into commercial applications.
Whlle the single crystal turbine blades have
provided improved overall airfoil performance as
compared with polycrystalline blades, they still exhibit
problem areas. In many applications, the highly loaded
attachment area is subject to low cycle fatigue
failures. As a result, there is a continuing need to
provide yet further improvements to achieve higher
operati:ng temperatures and lengthened operating lives in
the blades used in high performance gas turbine
engine~.
It i~ therefore an object of the present
invention to provide a novel turbine blade, and method
of making samé, whlch has an increased operating life.
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Another obiect of the invention is to provide a
single~ crystal turbine blade having a reduced
susceptibility to fallure in its attachment area.
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A further ob~ect of the invention is to provide a
composite structure in at least a portion of the
attachment section of a single crystal turbine blade to
retard creep and/or crack growth in sald portion.
SUMMARY OF THE INVENTION
The present invention resides in an improved gas
turbine blade that utilizes a sinqle crystal alloy body
optimized for high temperature performance of the
airfoil section, with a reinforcing polycrystalline
alloy core within the interior of at least a portion of
the attachment or root section in order to form a
composite structure. The resulting turbine blade is
physically interchangeable with prior blades, but has
improved strength, stiffness and low cycle fatigue
resistance in the attachment section.
While turbine blade is a unitary structure, it
may be conveniently de~cribed ac having two sections: an
airfoil section and an attachment or root section. The
airfoil sectlon is elongated and curved slightly into a
shape suitable for reacting against the flow of the hot
combustion gas. The root section attaches the airfoil
section to the rotatable turbine disk or hubo The most
widely used attachment is a "firtree" shape, wherein the
attachment section of the blade has a serie~ of enlarged
ridge~ that fit into a conforming receptacle in the rim
of the turbine disk. The blade is held in place by the
` ~physical interlocking of the ridges and the receptacle,
yet is reiatively easy to insert and remove when
necessary.
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The airfoil section of the turbine blade is
subjected to a combination of stresses induced by
centrifugal forces and hot gas impingement. Centrifugal
forces induce slow creep deformation and, if rotational
speeds are high enough, failure by stress rupture. Hot
gas impingement combined with centrifugal loading can
lead to high-cycle (low-amplitude strain) fatigue. The
single crystal alloys have been .optimized to resist
these mechanisms of failure. However, it has been
observed that the attachment section is susceptible to
another, completely different failure mechanism: low
cycle (high amplitude strain) fatigue. Existing single
crystal turbine blades have their lives limited in many
cases, by this low cycle fatigue mode. Because the
tur~ine. blade single crystal alloy is optimized to
resist other failure mechanisms, low cycle fatique
failure of the attachment section becomes a more
prominent concern in high performance gas turbine
engines.
While the inventor does not wish to be held to
any particular theory, it ls believed that the source of
. the low cycle fatigue performance improvement arises
.- from the inherent differences between the lower modulus
- : single crystal and higher modulus polycrystalline
micro tructures. Low ..cycle fatigue occurs under
.. condltions .of high cyclic load and the .related large
plastic strai~ns.~. The absence of.grain boundaries in the
: slngle crystal~ materiaI has the effect of increasing the
.straln at any given stre~ and eliminating a major
;. .30 .. ..mlcrostructural restraint to the growth of micro cracks
~whlch are formed.during high plastic.strain. .The fine
. grained polycrystal}ine core material.is much stiffer
. and~stherefore ::attracts a larger :share of .the radial
. brood belng transferred through the blade. This reduces
~ 35 the ~critical stresses in the softer single crystal
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material of the attachment areas which increases the low
cycle fatigue life of the composite blade.
In accordance with the present invention, a
turbine blade comprises a low modulus, cast single
S crystal body having an airfoil section and an attachment
section, and a higher modulus structural core of a
polycrystalline alloy bonded within said attachment
section.
The turbine blade of the present invention has a
single crystal body having a composition, orientation,
and structure optimized to provide excellent creep and
high-cycle fatigue resistance in the airfoil section.
This blade is grown by existing single crystal growth
techniques, such as those reported in U.S. Patents Nos.
4,412,577 and 3,494,709, whose disclosures are
incorporated herein by reference. However, the blade is
grown with the attachment section containing a hollow
cavity. Alternately, a cavity may be later machined
into the blade.
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A core of a polycxystalline superalloy is applied
within the center of the attachment section. The
thickness, composition and microstructure of the core
are optimized to be resistant to low cycle, moderate
temperature fatigue damage and other failure mechanisms
that are predominant in the attachment section. The
entire attachment section is preferably not made of the
polycrystalline material. The lower-modulus
sin~le-cry~tal material receives the airfoil attachmen~
load from the stiffer, higher-modulus, polycrystalline
core. Notch-root stres~es are minimized in the single
crystal material by the support provided by the
high-modulu~ core. Reduced notch-sensitivity ~is also
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achieved by the use of the low-modulus single-crystal
material.
The polycrystalline core can be applied by any
number of techniques, but prefe~ably by plasma
spraying. The core material can then be metallurgically
refined .to improve the microstructure to be more
resistant to failure, for example by hot isostatic
pressing or heat treating.
In accordance with the processing aspect of the
present invention, a .process for preparing a turbine
blade generally comprises the steps of casting a single
crystal body having an airfoil section and an attachment
section, forming a cavity within the core of the
attachment section, reinforcing the core of the
attachment section by filIing the cavity with a
polycrystalline alloy, metallurgically refining the
polycrystalline core and, finally, machining the
attachment section into a desired final configuration
for attachment to a turbine disk. In a preferred
approach, a process for preparing a turbine blade
comprises the steps of casting a single crystal body
.having an.airfoil section and an attachment section,
plasma spraying a hlgh strength..polycrystalline alloy
: : .into ;a.core cavity formed within the control portion of
25~ the~ attachment section, and hot isostatic pressing the
body to~con-olldat- the polycrystalline alloy core.
: : In the most preferred approach, the single
c nstal portion of the ..blade is of SC180 composition
super~alloy (described in EPO Patent Appln. No. 246~082)
;30~ having a [OOl] crystallographic orientation parallel to
the blade's longitudinal.axis. The polycrystalline core
ls~preferably of U=270 superalloy since its composition
P~ is compatible to SCl80. The polycrystalline core is
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applied by vacuum plasma spray deposition and then
consolidated by hot isostatic pressing, so that the core
is dense and welI bonded to the sin~le crystal portion
of the attachment section.
It should be appreciated that the turbine blade
of the invention achieves improved performance and life
by incorporating the best features of two different
approaches, while minimizing the detractions of each.
Optimized airfoil section performance is attained by
using an optimized single crystal alloy, and optimized
attachment section performance is attained by using an
optimized polycrystalline alloy in the core to provide
additional strength. This composite structure behaves
in a complex fa~hion which is not entirely predictable
by only considering the individual properties of the
single crystal material or the polycrystalline
material. Initially the single crystal layer resists
the centrifugal stresses but after some small amount of
creep, the stresses are transferred into the stronger
polycrystalline core. Other features and advantages of
the present invention will be apparent from the
following more detailed description of a presently
preferred embodiment, taken in conjunction with the
accompanying drawlngs, which illustrate, by way of
example and not limitation, the principles of the
invention.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a conventlonal
single crystal turbine blade;
-FIG. 2 is a partial perspective view of a single
crystal turbine blade of the present invention; and
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FIG. 3 is an enlarged sectional view of the
attachment region o the blade shown ln FIG. 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
By way of background, FIG. 1 illustrates a prior
S single crystal turbine blade (10). The blade (10) has
an airfoil section (12), an attachment or root section
(14), and, usually, a platform or stabilizer (16)
between the two sections. The attachment section (14)
has the pattern of alternating ridges (17) and
depressions (18) ~hat form a "firtree" shape for
removable attachment to complementary grooves in a
turbine disk (not shown). The blade (10) is ~abricated
entirely of a piece of single crystal superalloy,
preferably with a [001] crystallographic direction
parallel to the blade's longitudinal axis.
As used herein, a single crystal article is one
in which substantially all of the article ha~ a single
crystallographic orientation through the load bearing
portions, without the presence of high angle grain
boundaries. A small amount of low angle grain
boundaries, such aq tilt or twist boundaries, are
permitted within such a single crystal article, but are
preferably not present. However, such low angle
boundaries are often present after solidification and
25 ~formation ~of the single crystal article, or after some
deformation of the article durlng creep or other light
-- deformation process. Other minor irregularities are
also permitted within the scope of the term "single
crystal". For example, small areas of high angle grain
boundaries may be formed in various portion~ of the
article, due to the inability of the single crystal to
grow perf~ctly near corners and the like. Such
deviations -Srom a perfect single crystal, which are
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found in normal commercial production operations are
within the scope of the term "single crystal" as used
herein.
FIG. 2 illustrates a dual alloy, dual structure
S turbine blade (20), which also has an airfoil section
(22), an attachment section (24), and a platform or
stabilizer (26). The attachment section (24) has a
firtree of the same outward configuration and dimensions
as the firtree of the prior blade (10). The physical
appearance and configuration of the blade (20) may be
identical with that of a prior blade (10), 90 that the
improved blade can directly replace the prior blade in
existing turbine wheels.
From the enlarged cross-sectional illustration of
lS FIG. 3, however, it is apparent that the structure of
the blade (20) differs from that of the blade (10). The
airfoil sections (12) and (22) are identical, but the
attachment sections (14) and (24) are not
metallurgically identical. ~he attachment section (24)
is formed with an~polycrystalline core (30) that extends
from the base of the blade up towards the platform (26)
beyond the firtree. The core (30) is preferably formed
of a size ~ust smaller than the entire attachment
sect$on (14) but large enough to provide re$nforcement
thereto. The core (30) preferably tapers sufficiently
~-~ to form a mechanical interlocking structure with the
outer layer;of single crystal material. Overlying the
~~core (30) $s at least a th$n layer of the single crystal
mater$al -(34). The layer (34) has its external
conf$gurat$on mach$ned with the same ridges (27) and
grooves (28) as the prlor art blade (10).
The polycrystail$ne metallic alloy core (30) must
be metallurg$cally bonded to the single crystal along
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the interfacial bond lines (36) without substantial
porosity and defects.
The single crystal material may be any acceptable
superalloy that can be prepared as a single crystal.
The preferred single crystal materials are those that
have compositions tailored to yield optimal high
temperature properties in the single crystal airfoil
section (22) but have a relatively low modulus in the
transverse [100] grain direction. The most preferred
single crystal material is an alloy known as SC180,
disclo~ed in European Patent Application No. 246,082.
In its most preferred form SC180 has a nominal
composition of about 10~ Co, 5% Cr, 1.7~ Mo, 53 W, 8.5%
Ta, 5.2% Al, 3% Re, 1.0% Ti, 0.1% Hf and the balance
nickel. Its modulus is relatively low at about 14.8 x
106 cm/cm. The crystalllne orientation of the single
crystal is preferably with the [001] direction parallel
to the blade's lonqitudinal axis. Other acceptable
single crystal materials are well known in the art.
See, for example, U.S. Patents Nos. 4,582,S48;
4,643,782; and 4,719,080.
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The polycrystalline material for u~e in the core
(30) may be any acceptable superalloy that can be
prepared with a fine grain. The preferred
polycrystalline- materials are tho~e that have
composition~, grain sizes, and processing optimized to
yield ~maxlmum performance as -an ~attachment- section
alloy.~ ~This criterion implieslan alloy having high
strength and excell~nt low cycle fatigue performance.
The most preferred polycrystalline material is U-720
which:haQ a nominal composition of about 14.5% Co, 18.0~
Cr,;3.0% Mo,-1.2% W, 2.5% Al, 5.0% Ti and minor amounts
of B, C, and Zr in a nickel matrix. This alloy has a
. ~ ~ relatively high modulus of about 28.2 x 106 cm/cm. In
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addition, the chemical composition is similar enough to
SCl80 to minimize phase instability near the interfacial
bond line (36). Other acceptable polycrystalline
superalloys include, but are not limited to well-known
wrought disk alloys such as those sold under the
trademarks or tradenames MAR M-247, Waspoloy, IN-100,
and Astroloy.
The turbine blade of the invention is fabricated
by first casting a single crystal piece having the shape
of the airfoil section (22~, platform (2~), and
preferably a channel or cavity for the tapered core (30~
in the attachment section (24). If the cavity is not
formed during the casting process, it may later be
electrochemically machined into the solid attachment
lS section (14). A more preferred process is to initially
cast a small undersized cavity in the blade and then
later machine the cavity to a desired final size and
shape to ensure greater uniformity in production blades.
Any fabrication technique which produces a
substantially slngle crystal article is operable in
con~unction with the present invention. The preferred
~technique, u~ed to prepare the single crystal articles
descri~ed herein, is the high thermal gradLent
solldlflcatlon method. : Molten metal of the desired
-25 composltion is placed into a heat resistant ceramic mold
: having essentially the desired shape of the final
: fabricated component. -~he mold and metal contained
thereln ~are~placed withln a furnace, induction heating
~coil,~ or other heating device to melt the metal, and the
: 30 mold -and molten metal are gradually cooled in a
- controiled temperature gradient. In this process, metal
ad~acent the-cooler end of the mold solldifies first,
and the lnterface -between the solidified and liquid
metal gradually moves through the metal as cooling
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continues. Such gradient solidification can be
accomplished by placing a chill block adjacent one end
of the mold and then turning off the heat source,
allowing the mold and molten metal to cool and solidify
in a temperature grad~ent. Alternatively, the mold and
molten metal can be gradually withdrawn from the heat
source.
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' It is known that certain preferred
crystallographic orientations such as 1001~ can be grown
to the exclusion of others during such a gradient
soIidification process, so that a single grain becomes
dominant throughout the article. Techniques have been
developed to promote the formation of the single crystal
orientation rapidly, so that substantially all of the
article has the same single crystal orientation. Such
techniques include seeding, described in U.S. Patent No.
4,412,577, whereby an oriented ~ingle crystal starting
material is positioned ad~acent the metal first
solidlfied, so that the metal initially develops that
orientation. Another' approach is a geometrical
selection proces~ such as described in U.S.'I''Patent No.
3,494,709.
`'As'lndicated, all' other techniques for forming a
single crystal are acceptable for-use in con~unction
~With the ~present invention. The floating'zone technique
mayS~bë~used~wherein a molten zone is passed through a
polycr'ystalline piece' of 'metal-'"to produce' a- movinq
solidificatlon 'front. Solid-state techniques are also
permltted wherein a solid piece of polycrystalline
30~ ~material i~ transformed to a single crystal in the solid
s~te.~ The-~-solid 'state' approach is not preferred
becau~ë it 'ls'typically slow andSproduces a relatively
'impêrfect`single crystal. ~' ~ '--' - '
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The polycrystalline core (30) is applied by any
technique that produces a sound microstructure that is
well bonded to the underlying single crystal substrate.
The preferred approach is vacuum plasma spray
deposition. The target to be coated, here the tapered
cavity of the blade (20), is placed into a vacuum
chamber which is evacuated to a relatively low
pressure. A plasma gun that melts metal fed thereto is
aimed at the target substrate, typically positioned
several centimeters from the plasma gun. Particles of
metal of the desired final composition are fed to the
plasma gun, which melts, ox at least softens, the
particles and propels them toward the target to impact
thexeupon. Different blends of particles can also be
used, but a single particulate feed material is
preferred for uniformity.
The plasma deposition process is continued for as
long as necessary to fill up the core caYity. By way of
example and not of limitation, a typical blade (20) may
be 5 to 10 centimeters long, and the depth of the core
(30) may be about 1.3 to 3.8 centimeters.
Such a blade was analyzed and calculated to have
about 10~ less stress in the attachment grooves (28)
which would increase~the low cycle fatigue life of ~he
attachment section by a factor of about 2. Of course
other blade designs will have to be analyzed to
determine the optimum proportions for the core and the
amount of increasod li~e provided thereby.
The as-deposited core may have a slight degree of
3~ porosity and possibly unmelted particles. To remove the
porosity and-irregularities, the blade (20) is placed
into a pressure chambex and hot i~ostatically pressed.
The hot isostatic pressing is conducted at an elevated
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pressure, typically 1034 to 1724 bars, and at anelevated temperature, typically 1080C to 1221C, for a
sufficient time, such as 4 hours. The exact temperature
and time may vary depending upon heat treatment
requirements for the single crystal and the core
materials. An acceptable and preferred hot isostatic
pressing treatment is 1221C and 1034 bars for 4 hou~s.
Upon completion of this treatment the porosity in the
core should be completely closed, with good bonding at
the bond line (36). After pressing, the composite blade
is preferably solution heat-treated and aged at about
649C to 1260C (more preferably 760C to 871C) to
optimize the polycrystalline microstructure. Care must
be taken to avoid incipient melting of the single
crystal material, and the appropriate combination of
pressing and heat treatment parameters will depend upon
the materials selected for the single crystal and
polycrystalline core in any particular case.
Any other acceptable procedure may also be used
to fill the single crystal cavity with the
polycrystalline material. Such other techniques
include, but are not limited to, vapor deposition,
plasma transfer arc, electrodeposition, deposition from
solution, and powder spraying.
As should now be appreciated, the turbine blades
of the invention provide an improved dual alloy
composlte structure and therefore improved performance
` compared to prior blades. Although a particular
embodiment of the invention has been described in detail
for purposes of illustration, various modifications may
be made without departing from the spirit and scope of
the invention. For example, some stationary vanes or
other components in a gas turbine engine may experience
attachm nt problems which could be solved by adding a
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reinforcing core of polycrystalline alloy. -Accordingly,
the invention is not to be limited except as by the
appended claims. t
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