U.S. patent application number 09/456972 was filed with the patent office on 2002-01-17 for die cast nickel base superalloy articles.
Invention is credited to GIUGNO, RALPH, GUSTAFSON, WALTER FREDERICK, MARCIN JR., JOHN JOSEPH, NORTON, DELWYN EARLE, SAMUELSON, JEFFERY WILLIAM, SCHIRRA, JOHN J..
Application Number | 20020005233 09/456972 |
Document ID | / |
Family ID | 26811198 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005233 |
Kind Code |
A1 |
SCHIRRA, JOHN J. ; et
al. |
January 17, 2002 |
DIE CAST NICKEL BASE SUPERALLOY ARTICLES
Abstract
A die cast article such is composed of nickel base superaloy IN
718 is disclosed. The microstructure is characterized by an absence
of flowlines and includes a fine average grain size, e.g., ASTM 3
or smaller. Exemplary articles include gas turbine engine
components, such as blades, vanes, cases and seals.
Inventors: |
SCHIRRA, JOHN J.;
(ELLINGTON, CT) ; GIUGNO, RALPH; (IVORYTON,
CT) ; GUSTAFSON, WALTER FREDERICK; (MANCHESTER,
CT) ; MARCIN JR., JOHN JOSEPH; (MARLBOROUGH, CT)
; SAMUELSON, JEFFERY WILLIAM; (JUPITER, FL) ;
NORTON, DELWYN EARLE; (MANCHESTER, CT) |
Correspondence
Address: |
F TYLER MORRISON
PRATT & WHITNEY
PATENT DEPARTMENT-MS132-13
400 MAIN STREET
EAST HARTFORD
CT
06108
|
Family ID: |
26811198 |
Appl. No.: |
09/456972 |
Filed: |
December 7, 1999 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60113755 |
Dec 23, 1998 |
|
|
|
Current U.S.
Class: |
148/428 ;
148/410; 420/448 |
Current CPC
Class: |
C22C 1/02 20130101; C22C
19/07 20130101; B22D 17/00 20130101; C22C 19/055 20130101; F01D
5/28 20130101; C22F 1/10 20130101; F05C 2201/0466 20130101; C22C
19/056 20130101 |
Class at
Publication: |
148/428 ;
148/410; 420/448 |
International
Class: |
C22C 019/05 |
Claims
What is claimed is:
1. A die cast article composed in weight percent of about 15-25 Cr,
2.5-3.5 Mo, about 5.0-5.75 (Cb +Ta), 0.5-1.25 Ti, 0.25-1.0 Al, up
to about 21 Fe, balance generally nickel.
2. The article of claim 1, wherein the article is characterized by
a microstructure having an absence of flowlines and having
strength, crack growth rates imd stress rupture resistance in
accordance with AMS 5663.
3. The article of claim 1, wherein the article comprises a gas
turbine engine component.
4. The article of claim 3, wherein the article is a compressor
component.
5. The article of claim 3, wherein the article is a turbine
component.
6. The article of claim 1, wherein the average grain size is
smaller than about ASTM 3.
7. The article of claim 1, wherein the article has ail ultimate
tensile strength at room temperature of at least 180 ksi and a 0.2%
yield strength of at least 145 ksi.
8. The article of claim 7, wherein the article has al ultimate
tensile strength at about 1200 F.of at least 150 ksi and a 0.2%
yield strength of at least 125 ksi.
9. The article of claim 1, wherein the article is characterized by
a microstructure having an absence of flowlines and having
strength, crack growth rates and stress rupture resistance in
accordance with AMS 5663.
10. The article of claim 1, wherein the article has ail ultimate
tensile strength at room temperature of at least 120 ksi and a 0.2%
yield strength of at least 105 ksi.
11. A die cast gas turbine engine component composed in weight
percent of about 15-25 Cr, 2.5-3.5 Mo, about 5.0-5.75 (Cb+Ta),
0.5-1.25 Ti, 0.25-1.0 Al, up to about 21 Fe, balance generally
nickel
12. The article of claim 11 characterized by a microstructure with
an absence of flowlines.
13. The article of claim 11, wherein the article has room
temperature and 1200 F. strength and stress rupture resistance in
accordance with AMS 5663.
14. The article of claim 11, wherein the article is a compressor
component.
15. The article of claim 11, wherein the article is a turbine
component.
16. The article of claim 20, wherein the average grain size is
smaller than about ASTM 3.
17. The article of claim 11, wherein the article has an ultimate
tensile strength at room temperature of at least 180 ksi and a 0.2%
yield strength of at least 145 ksi.
18. The article of claim 11, wherein the article has an ultimate
tensile strength at about 1200 F.of at least 150 ksi and a 0.2%
yield strength of at least 125 ksi.
19. The article of claim 11, wherein the article has room
temperature and 1200 F.strength and stress rupture resistance in
accordance with AMS 5383.
20. The article of claim 19, wherein the article has an ultimate
tensile strength at room temperature of at least 120 ksi and a 0.2%
yield strength of at least 105 ksi.
Description
[0001] This application claims the benefit of U.S. Provisional
application No. 60/113,755, filed on Dec. 23, 1998.
CROSS REFERENCE TO RELATED APPLICATIONS
[0002] Some of the material in the present application is also
disclosed in co-pending applications filed on even date herewith
and entitled "Method of Making Die Cast Articles of High Melting
Temperature or Reactive Materials", and "Apparatus for Die Casting
High Melting Temperature Materials", which are hereby expressly
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0003] The present invention relates generally to articles
fabricated from superalloy material, and relates more particularly
to articles fabricated from nickel base superalloys. Such alloys
typically have high melting temperatures, in excess of
2300-2500.degree. F.
[0004] Nickel base superalloys are employed in applications which
require high strength-weight ratios, corrosion resistance and use
up to relatively high temperatures, e.g. , up to and above about
2000.degree. F.
[0005] In gas turbine engines for example, these superalloys are
typically employed in the turbine section, and sometimes in the
latter stages of the compressor section of the engine, including
but not limited to airfoils such as blades and vanes, as well as
static and structural components such as intermediate and
compressor cases, compressor disks, turbine cases and turbine
disks. A typical nickel base superalloy utilized in gas turbine
engines is Inconel 718 (IN 718), in broad terms having general a
composition in weight percent, of about 0.01 -0.05 Carbon (C),
13-25 Chromium (Cr), 2.5-3.5 Molybdenum (Mo), 5.0-5.75 (Columbium
(Cb) [also referred to as Niobium (Nb)]+Tantalum (Ta)), 0.7-1.2
Titanium (Ti), 0.3-0.9 Aluminum (Al), up to about 21 Iron (Fe),
balance generally Ni.
[0006] In the gas turbine engine industry, forging is used to
produce parts having complex, three-dimensional shapes such as
blades and vanes. Nickel base superalloys have traditionally been
precision forged to produce parts having a fine average grain size
and a balance of high strength, low weight, and good high cycle
fatigue resistance. When properly produced, these parts do exhibit
a balance of high strength, low weight, and durability.
[0007] Briefly, in order to forge a part such as a blade of vane,
an ingot of material is first obtained having a composition
corresponding to the desired composition of the finished component.
The ingot is converted into billet form, typically cylindrical for
blades and vanes, and is then thermomechanically processed, such as
by heating and stamping several times between dies and/or hammers
which may be heated and are shaped progressively similar to the
desired shape, in order to plastically deform and flow the material
into the desired component shape. Each component is typically heat
treated to obtain desired properties, e.g.,
hardening/strengthening, stress relief, resistance to crack
nucleation and a particular level of HCF resistance, and is also
finished, e.g., machined, chem-milled and/or media finished, as
needed to provide the component with the precise shape, dimensions
or features.
[0008] The production of components by forging is an expensive,
time consuming process, and thus is typically warranted only for
components that require a particular balance of properties, e.g.,
high strength, low weight and durability, both at room temperature
and at elevated temperatures. With respect to obtaining material
for forging, certain materials require long lead times, sometimes
measured in months. Forging typically includes a series of
operation, each requiring separate dies and associated equipment.
The post-forging finishing operations, e.g., machining the root
portion of a blade and providing the appropriate surface finish,
comprise a significant portion of the overall cost of producing
forged parts, and include a significant portion of parts which must
be scrapped.
[0009] During component forging, much of the original material (up
to about 85%) is removed and does not form part of the finished
component, e.g., it is process waste. The complexity of the shape
of the component produced merely adds to the effort and expense
required to fabricate the component, which is an even greater
consideration for gas turbine engine components having particularly
complex shapes. Nickel base superalloys such as IN 718 also exhibit
significant springback, e.g., the material is resilient, and the
springback must be taken into account during forging, i.e. , the
parts must typically be "over forged". As noted above, finished
components may still require extensive post forging processing.
Moreover, as computer software is used to apply computational fluid
dynamics to analyze and generate more aerodynamically efficient
airfoil shapes, such airfoils and components have even more complex
three-dimensional shapes. It is correspondingly mnore difficult or
impossible to forge superalloys precisely into these advanced, more
complex shapes, e.g., due in part to the slightly resilient nature
many materials exhibit during forging, which adds further to the
cost of the components or renders the components so expensive that
it is not economically feasible to exploit certain advances in
engine technology, or to utilize particular alloys for some
components.
[0010] Forged components also often exhibit significant levels of
defects, including inclusions and carbides, which vary
significantly from component to component. Such components having
higher columbium contents, e.g. , IN 718, are also prone to
elemental segregation during forging. In addition, forged
components tend to be difficult to machine and inspect for such
defects. Moreover, precise reproducibility is also a
concern-forging does not result in components having dimensions
that are precisely the same from part to part. After inspection,
many parts must still be re-worked. As a general rule, forged parts
must be scrapped or significantly re-worked about 20 % of the time.
Moreover, newer, more advanced or more highly alloyed nickel base
superalloys (e.g., Waspaloy or IN 939) will be increasingly
difficult (if not impossible) and correspondingly more expensive to
forge. These concerns will only intensify as more complex
three-dimensional airfoil geometries are employed.
[0011] Casting has been extensively used to produce relatively
near-finished-shape articles.
[0012] Investment casting, in which molten metal is poured into a
ceramic shell having a cavity in the shape of the article to be
cast, can be used to produce such articles. However, investment
casting produces articles having extremely large grains (relative
to the small average grain size achievable by forging), and in some
cases the entire part comprises a single grain. In addition,
solidification rates may result in the presence of unacceptable
amounts of elemental segregation producing large scatter (variances
from part to part) in test results or in the presence of brittle
phases also resulting in reduced properties. Moreover, since an
individual mold is produced for each part, this process is
expensive. Reproducibility of very precise dimensions from part to
part is difficult to achieve. In addition, molten material is
melted, poured and/or solidified in air or other gas, results in
parts having undesirable properties such as inclusions and
porosity, particularly for materials containing reactive
elements.
[0013] Permanent mold casting, in which molten material is poured
into a multipart, reusable mold and flows into the mold under only
the force of gravity, has also been used to cast parts generally.
See, e.g., U.S. Pat. No. 5,505,246 to Colvin. However, permanent
mold casting has several drawbacks. For thin castings, such as
airfoils, the force of gravity may be insufficient to urge the
material into thinner sections, particularly so where high melting
temperature materials and low superheats are employed, and
accordingly the mold does not consistently fill and the parts must
be scrapped. Dimensional tolerances must be relatively large, and
require correspondingly more post casting work, and repeatably is
difficult to achieve. Permanent mold casting also results in
relatively poor surface finish, which also requires more post cast
work.
[0014] Die casting, in which molten metal in injected under
pressure into a re-usable die, has been used successfully in the
past to form such articles from materials having relatively
low-melting temperatures, e.g., Tm below about 2000 F.
[0015] One type of die casting machine is set forth in U.S. Pat.
No. 3,791,440. In that patent, the machine includes a fixed die
element 11 and a moveable die element 12. Briefly, metal which has
been melted is poured through a pour spout 22 and sprue 21, and
flowvs into an injection cylinder 30, which communicates wvith the
die cavity 15. Sufficient molten material is poured to fill the
injection cylinder 30 and a portion of the sprue 21, thus
displacing air from the injection cylinder. See, e.g., col. 6,
lines 7-17. An injection plunger 38 forces material from the
injection cylinder 30 into the die cavity 15. A sprue locking
cylinder and associated plunger 35 can seal the sprue 21, e.g.,
during injection. The injection cylinder 30 is embedded in one of
the die platens, thereby preventing distortion of the cylinder when
high melting temperature, molten material is poured into the
injection cylinder. The Cross-type machine does not utilize a
vacuum environment, but rather utilizes complete filling of the
cylinder to prevent injecting air into the die.
[0016] Such machines are expensive. Moreover, this type of machine
is not readily available, and is correspondingly expensive to
refurbish and repair, as needed. For example, it would be difficult
and expensive at best to attach a vacuum system to the machine,
since the sleeve is embedded in a platen and not readily
accessible. Moreover, it would be difficult at best to transfer
molten material from a melting unit to the pour spout 22, within a
vacuum environment controlling the temperature of the die would
also be difficult, not only due to the physical size of the
platen/embedded die combination, but also due to the thermal mass
of such a combination. The configuration of the machine would also
render release of the part difficult within a vacuum
environment.
[0017] Another type of die casting machine is the "cold chamber"
type. As set forth, for example, in U.S. Pat. Nos. 2,932,865,
3,106,002, 3,532,561 and 3,646,990, a conventional, cold chamber
die casting machine includes a shot sleeve mounted to one
(typicallyJ fixed) platen of a multiple part die, e.g., a two part
die including fixed and movable platens which cooperate to define a
die cavity. The shot sleeve can be oriented horizontally,
vertically or inclined between horizontal and vertical. The sleeve
communicates with a runner of the die, and includes an opening on
top of the sleeve through which molten metal is poured. A plunger
is positioned for movement in the sleeve, and forces molten metal
that is present in the sleeve into the die. In a "cold type"
machine, the shot sleeve is oriented horizontally and is unheated.
Casting typically occurs under atmospheric conditions, i.e., the
equipment is not located in a non-reactive environment such as a
vacuum chamber.
[0018] The drawbacks of such machines are discussed in U.S. Pat.
No. 3,646,990, particularly in connection with the inability to use
such machines to cast higher melting point materials (Tm above
about 2000 F.), such as nickel base, cobalt base and iron base
superalloys. In conventional cold chamber machines the shot sleeve
is not evacuated, and the plunger also forces air into the die
resulting in undesirable and impermissible porosity of die cast
articles. Accordingly, in order to avoid injecting bubbles with the
molten material the shot sleeve must be filled as completely as
possible, or is inclined such that any air in the molten material
migrates away from the die before injection.
[0019] Moreover, since the shot sleeve is unheated, a skin or "can"
of molten metal solidifies on the inside of the shot sleeve, and in
order to move the plunger through the sleeve to inject the molten
metal into the die, the plunger must scrape the skin off of the
sleeve and "crush the can". However, where the can forms a
structurally strong member, e.g., in the form of cylinder which is
supported by the sleeve, the plunger and/or associated structure
for moving the plunger can be damaged or destroyed. Where the
sleeve is thermally distorted and fails to match the plunger shape,
or the plunger is distorted and fails to match the sleeve shape,
the plunger can allow the passage, of metal between plunger and
sleeve ("blowback") and/or any entrapped gas between the plunger
and sleeve, all of which detrimentally affects the quality of the
resulting articles. See also U.S. Pat. No. 3,533,464 to Parlanti et
al.
[0020] Despite extensive efforts, the conventional "cold chamber"
die casting apparatus have not been used successfully to produce
articles composed of high melting temperature materials, such as
nickel base superalloys alloys. Past attempts to die cast high
melting temperature materials such as superalloys has resulted in
broken die casting machinery, as well as articles characterized by
inferior qualities such as impurities, excessive porosity and
segregation, and relatively poor strength and low and high cycle
fatigue properties.
[0021] It is an object of the present invention to provide die cast
articles composed of high melting temperature materials, such as
nickel base superalloys.
[0022] It is another object of the present invention to provide die
cast nickel base superalloy articles having properties comparable
to corresponding forged articles.
[0023] It is a more specific object of the present invention to
provide nickel base superalloy articles that have strength,
durability and fatigue resistance comparable to corresponding
forged superalloy articles.
[0024] It is still another object of the present invention to
provide such articles having complex, three dimensional shapes are
difficult if not impossible to forge.
[0025] Additional objects will become apparent to those skilled in
the art based upon the following disclosure and drawings.
SUMMARY OF THE INVENTION
[0026] According to one aspect of the invention, a die cast article
composed of nickel base superalloy such as IN 718 is disclosed. The
articles preferably at least meet the strength, low crack growth
rates and stress rupture resistance requirements of corresponding
forged articles, e.g., according to AMS 5663 or AMS 5383. The
article, for example includes a blade or vane for a gas turbine
engine. Each article has a microstructure similar to that of forged
material, and is characterized by a more uniform grains, and a fine
average grain size for a cast article, e.g., smaller than about
ASTM 3, more preferably ASTM 5 or smaller. The microstructure
preferably is further characterized by an absence of flowlines. In
the case of rotating components, such as gas turbine engine blades,
the preferred average grain size is smaller, e.g., preferably ASTM
5 or smaller, more preferably ASTM 6 or smaller.
[0027] The articles have both yield and ultimate tensile strengths
at both room and elevated temperatures that are comparable to
forged parts composed of the same material, and also have similar
high and low cycle fatigue properties.
[0028] An advantage of the present invention is that die casting
significantly reduces the time required to produce a part, from
ingot to finished part, as there is no need to prepare specially
tailored billets of material or ceramic investment shell, and
casting broadly is performed in a single step, as opposed to
multiple forging operations or shell preparations. In addition, die
casting enables the production of multiple parts in a single
casting. Die casting further enables production of parts having
more complex three dimensional shapes, thereby enabling production
of more aerodynamically efficient airfoils, and other components
relative to forging. The present invention will enable the
production of articles utilizing materials having shapes that are
difficult or impossible to forge into those shapes. Moreover, die
cast parts have greater reproducibility than forged or investment
cast articles, and can be produced nearer to their finished shape,
and with a superior surface finish, which minimizes post forming
finishing operations, all of which also reduces the cost of
producing such parts. Additional advantages will become apparent in
view of the following drawings and detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a view illustrating a die cast article composed of
IN 718 in accordance with the present invention.
[0030] FIG. 2 is a photomicrograph illustrating the microstructure
of a test bar composed of die cast IN 718 in accordance with the
present invention.
[0031] FIG. 3 is a photomicrograph illustrating the microstructure
of an airfoil composed of die cast IN 718 in accordance with the
present invention.
[0032] FIG. 4 is a photomicrograph of the airfoil of FIG. 4 after
hot isostatic pressing of the airfoil.
[0033] FIG. 5 is a photomicrograph illustrating the microstructure
of an airfoil composed of forged IN 718.
[0034] FIGS. 6 and 7 illustrate properties of a die cast IN 718
article in accordance with the present invention and corresponding
forged articles.
[0035] FIGS. 8 and 9 are schematic views of a die casting machine
used to produce articles composed of IN718.
[0036] FIG. 10 is a flow diagram illustrating a process of die
casting IN 718 in accordance with the present invention.
[0037] FIG. 11 is an exemplary heat treatment to reduce or
eliminate elemental segregation on a microscopic level.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Turning now to FIG. 1, a die cast nickel base superalloy
article in accordance with the present invention is indicated
generally by the reference numeral 10. In the illustrated
embodiment, the article includes a blade 10 composed of IN 718 and
which is used in a gas turbine engine. The article includes an
airfoil 12, a platform 14, and a root 16. The present invention is
broadly applicable to various applications, and is not intended to
be limited to any particular article or to use in gas turbine
engines. Preferably, the die cast components for use in a gas
turbine engine (as opposed to die cast components for other
applications) exhibit strengths, low crack growth rates and high
stress rupture resistance set forth in Aerospace Material
Specification AMS 5663 (Rev. J, publ. Sep. 1997) (for corresponding
forged components) or AMS 5383 (Rev. D, publ. Apr. 1993) (for
corresponding investment cast components-for lower strength
applications relative to AMS 5663) published by SAE Int'l of
Warrendale, PA., and incorporated by reference herein.
[0039] As noted above, a typical nickel base superalloy utilized in
gas turbine engines is Inconel 718 (IN 718), which generally
includes in weight percent about 19 Cr, 3.1 Mo, about 5.3 (Cb+Ta),
0.9 Ti, 0.6 Al, 19 Fe, balance. More broadly, IN 718 includes in
weight percent, about 0.01-0.05 Carbon (C), up to about 0.4
Manganese (Mn), up to about 0.2 Silicon (Si), 13-25 Chromium (Cr),
up to about 1.5 Cobalt (Co), 2.5-3.5 Molybdenum (Mo), 5.0-5.75
(Columbium (Cb)+Tantalum (Ta), 0.7-1.2 Titanium (Ti), 0.3-0.9
Aluminum (Al), up to about 21 Iron (Fe), balance essentially Ni.
Still more preferably, IN 718 has a composition of about 0.02-0.04
C, up to about 0.35 Mn, up to about 0.15 Si, 17-21 Cr, up to about
1 Co, 2.8-3.3Mo+W+Re, 5.15-5.5 Cb+Ta,0.75-1.15 Ti+V +Hf. 0.4-0.7
Al, up to about 19 Fe, balance essentially Ni. Other elements (also
by weight percent) may also include up to about 0.01 Sulfur (S), up
to about 0.015 Phosphorus (P), 0.002-0.006 Boron (B), up to about
0.10 Cu, up to about 0.0030 Magnesium (Mg), up to about 0.0005 Lead
(Pb), up to about 0.00003 Bismuth (Bi), up to about 0.0003 Selenium
(Se), up to about 0.0005 Silver (Ag), up to about 0.01 Oxygen (O)
and, up to about 0.01 Nitrogen (N).
[0040] Compositional modifications can be made to IN 718, e.g.,
increasing the Nb content of the material to be cast, as well as
other strengthening elements to improve strength and
capability.
[0041] We have produced die cast articles composed of nickel base
superalloys using a die casting machine of the type shown and
described, e.g., in U.S. Pat. Nos. 3,791,440 and 3,810,505 both to
Cross. We have also die cast such articles in "cold chamber type"
die casting machines, typically having an unheated shot sleeve and
as described above and in the '440 patent. We have subsequently
used and prefer to use the "cold chamber" machines in connection
with the present invention, at least since such machines are less
expensive, more readily available, may be refurbished as needed for
use in die casting such high melting temperature materials, and are
generally less expensive to repair if needed.
[0042] Briefly, in accordance with the present invention at least a
single charge of material is melted in a manner to minimize
contamination, either in connection with the melting apparatus or
from reaction of one or more elements of the material. Accordingly,
the alloy is heated and melted in a non-reactive, e.g., an inert or
preferably in a vacuum environment, preferably maintained at a
pressure of less than 100 .mu.m more preferably at less than 50
.mu.m. The alloy is also heated to a controlled, limited superheat,
e.g., typically within 100.degree. F.to 200.degree. F.above the
melting temperature of the alloy and more preferably within
50.degree. F. to 100.degree. F., and preferably using a
non-contaminating melting device. We prefer to use a ceramic free
melting system such as an inducto-skull melting unit. The material
is sufficiently superheated to ensure that it remains molten until
injected into the die, but not enough to prevent rapid
solidification of the molten material after injection. Molten alloy
is then transferred to a horizontal shot sleeve of the machine,
which is preferably located in a vacuum environment and the molten
material is injected under pressure into a reusable mold. The
process comprising pouring and injecting the molten material should
not exceed a few seconds, with injection occurring preferably in
less than one or two seconds, in a die casting machine having an
unheated shot sleeve.
[0043] It should be noted that the articles may be
thermomechanically processed after casting, if desired. In other
words, the articles may be forged after being die cast; e.g., the
die cast articles may serve as pre-forms for use in a forging
operation. We prefer that the die cast articles be cast to near net
shape, so as to minimize post-casting work and associated experise
performed on the articles.
[0044] In accordance with the present invention, articles prepared
in accordance with the present invention are characterized by a
microstructure having a fine, uniform average grain size,
particularly for cast articles, and an absence of flow lines. See,
FIGS. 2 and 3 illustrating the microstructure of a die cast IN 718
test bar and an airfoil, respectively, and FIG. 5 illustrating the
microstructure of a conventional, forged IN 718 airfoil. In FIG. 2,
the average grain size is roughly ASTM 6. In FIG. 5, the average
grain size is roughly ASTM 10.
[0045] The articles are characterized by a small average grain
size, e.g., for non-rotating gas turbine engine components such as
cases and seals, the average grain size is ASTM 3 or smaller, more
preferably ASTM 5 or smaller. In the case of rotating components,
such as gas turbine engine blades, the preferred average grain size
is smaller, e.g., preferably ASTM 5 of smaller, more preferably
ASTM 6 or smaller. The preferred average grain size and maximum
allowable grain size will depend upon the application of the part,
e.g., whether the article is intended for use in a gas turbine
engine versus other application, rotating vs. non-rotating parts,
operating in lower temperature versus higher temperature
environments. Such articles have properties comparable to, and
preferably at least equivalent to, corresponding articles composed
of forged material.
[0046] Surprisingly however, the die cast Inconel 718 article still
exhibited the presence of casting segregation in the as cast
condition, despite the relatively fine as, cast grain size and high
solidification rates. For critical applications (such as rotating
turbine engine hardware), the presence of casting segregation is
unacceptable. We have discovered that it is possible to thermally
homogenize the casting segregation while maintaimng the benefits
associated with the fine as cast grain size. This can be
accomplished through a stand alone heat treat cycle or a careful
selection of the HIP cycle temperature. See, co-pending application
entitled "Heat Treatment For Die Cast Superalloy Articles", filed
on even date herewith and expressly incorporated by reference
herein.
[0047] As noted above, the present invention enables the die
casting of articles that have not only good strength, but also have
other properties that are comparable to or better than
corresponding forged components, e.g., low crack growth rates and
high stress rupture resistance. Samples of die cast IN 718 in
accordance with the present invention were tested to determine
yield and ultimate tensile strengths, as well as ductility and
impact strength. With respect to tensile properties, samples of die
cast IN 718 articles were tested both at room temperature (about 70
.degree. F.) and elevated temperatures, e.g., about 1200.degree. F.
held for a period of time prior to testing. The samples were
subjected to strain rate of between 0.003-0.007 in./in./minute
through the yield strength, and then the rate was increased to
produce failure in about one minute later. As indicated by FIGS. 6
and 7, the die cast articles are characterized, at room temperature
and at elevated temperatures, by comparable 0.2% yield strengths,
ultimate tensile strengths, elongation at failure and impact
strengths.
[0048] More specifically, in the case of blades and vanes, e.g.,
rotating components, die cast parts require at least strength and
impact properties equivalent to those exhibited by corresponding
forged articles. Blades, vanes and rotating components composed of
IN 718 should have a 0.2% yield strength at room temperature of at
least 140 ksi and more preferably at least 150 ksi and most
preferably at least 160 ksi; and at yield strength at 1200.degree.
F. of at least 115 ksi and more preferably 125 ksi and most
preferably at least 135 ksi. Such articles have a ultimate tensile
strength at room temperature of at least 175 ksi and more
preferably at least 185 ksi and most preferably at least 195 ksi;
and an ultimate tensile strength at 1200.degree. F. of at least 140
ksi and more preferably 150 ksi and most preferably at least 160
ksi.
[0049] In addition, standard combination smooth and notched stress
rupture test specimens (comprising material produced in accordance
with the present invention), e.g., conforming to ASTM E292, were
tested. The specimens were maintained at about 1200.degree. F. and
loaded continuously, after generating an initial axial stress of
between about 105-110 ksi. In the case of material to be used for
blades and vanes, the specimens ruptured only after at least 23
hours. The values are comparable to those found in AMS 5663,
referenced above.
[0050] Similar standard combination smooth and notched stress
rupture test specimens (comprising material produced in accordance
with the present invention), e.g., conforming to ASTM E292, were
also tested at about 1300.degree. F. The specimens were loaded
continuously, after generating an initial axial stress of between
about 60-65 ksi. In the case of material to be used for blades and
vanes, the specimens ruptured only after at least 40 hours.
[0051] Creep properties were also evaluated, at about 1200.degree.
F. The specimens were maintained at about 1200.degree. F., and
loaded to produce an axial stress of at least about 80 ksi. The
time to 0.1% plastic deformation was measured, in the case of
material to be used for blades and vanes, should exceed about 15
hours. Again, the specific required values will differ depending
upon the particular use to which the articles are being put.
[0052] For non-rotating parts, such as cases, flanges and seals,
e.g., rings the above values are in excess of the values required.
More specifically, for non-rotating parts such as rings and seals
composed of IN 718 should have a 0.2% yield strength at room
temperature of at least 130 ksi and more preferably at least 140
ksi and most preferably at least 150 ksi; and at yield strength at
1200.degree. F. of at least 105 ksi and more preferably 115 ksi and
most preferably at least 125 ksi. Such articles have a ultimate
tensile strength at room temperature of at least 165 ksi and more
preferably at least 175 ksi and most preferably at least 185 ksi;
and an ultimate tensile strength at 1200.degree. F. of at least 125
ksi and more preferably 135 ksi and most preferably at least 145
ksi.
[0053] In addition, standard combination smooth and notched stress
rupture test specimens (comprising material produced in accordance
with the present invention), e.g., conforming to ASTM E292, were
tested. The specimens were maintained at about 1200.degree. F. and
loaded continuously, after generating an initial axial stress of
between about 105-110 ksi In the case of material to be used for
blades and vanes, the specimens ruptured only after at least 23
hours, and the elongation was at least about 6 %.
[0054] Similar standard combination smooth and notched stress
rupture test specimens (comprising material produced in accordance
with the present invention), e.g., conforming to ASTM E292, were
also tested at about 1300.degree. F. The specimens were loaded
continuously, after generating an initial axial stress of between
about 60-65 ksi. In the case of material to b used for blades and
vanes, the specimens ruptured only after at least 85 hours.
[0055] Creep properties were also evaluated, at about 1200.degree.
F. The specimens were maintained at about 1200.degree. F., and
loaded to produce an axial stress of at least albout 80 ksi. The
time to 0.1% plastic deformation was measured, in the case of
material to be used for blades and vanes, should exceed about 15
hours. Again, the specific required values will differ depending
upon the particular use to which the articles are being put.
[0056] AMS 5663 calls for the following properties:
1 Property Room Temp. 1200.degree. F. +/- 10 AMS 5663 calls for the
following properties: Tensile Strength, min. 180 ksi 140 ksi Yield
Strength, 0.2% offset, min. 150 ksi 125 ksi Elongation in 4D, min.
10% 10% Reduction in area, min. 12% 12% AMS 5383 calls for the
following properties: Tensile Strength, min. 120 ksi Yield
Strength, 0.2% offset, min. 105 ksi Elongation in 4D, min. 3%
Reduction in area, min. 8%
[0057] As noted in AMS 5663, the properties for forged material
differ depending upon whether the samples are tested longitudinally
or transversely, e.g., the properties are not isotropic and the
lower values are produced during transverse testing.
[0058] In addition, standard combination smooth and notched stress
rupture test specimens (comprising material produced in accordance
with the present invention), e.g., conforming to ASTM E292, are
tested. The specimens were maintained at 1200.degree. F. and loaded
continuously, after generating an initial axial stress of between
about 105-110 ksi. The specimens ruptured after at least 23 hours.
These values meet the requirements set forth in AMS 5663.
[0059] For lower strength articles, i.e., meeting the requirements
of AMS 5383 standard combination smooth and notched stress rupture
test specimens are tested. The specimens were maintained at
1300.degree. F. and loaded continuously, after generating an
initial axial stress of about 65 ksi. The specimens should rupture
only after at least 23 hours.
[0060] Turning to FIGS. 8, 9 and 10, such nickel base superalloys
such as IN 718 are preferably melted and cast in a non-reactive
environment, e.g., in the presence of an inert gas or more
preferably in a vacuum environment. The preferred manner of die
casting the articles is set forth in co-pending application
entitled "Method of Making die Cast Articles of High Melting
Temperature or Reactive Materials", and "Apparatus for Die Casting
High Melting Temperature Materials", filed on even date herewith
and which are each hereby incorporated explicitly herein by
reference. Preferably, a single charge or small batch (less than
about 10 pounds) of material is prepared (FIG. 10, step 44). The
charge is melted to ensure rapid melting without contaminating the
material. The molten material is then poured into a horizontal shot
sleeve of a cold chamnber-type die casting apparatus, which is also
preferably evacuated, so as to partially fill the sleeve. The
molten material is then injected into a die, which is preferably
unheated, where is solidifies to form the desired article.
[0061] Initially, material to be die cast must first be melted
(step 46-FIG. 10) in the apparatus 18 illustrated in FIGS. 8 and 9.
Where reactive materials, such as superalloys containing reactive
elements, are to be cast it is important to melt the materials in a
non-reactive environment, to prevent any reaction, contamination or
other condition which might detrimentally affect the quality of the
resulting articles. Since any gasses in the melting environment may
become entrapped in the molten material and result in excess
porosity in die cast articles, we prefer to melt the material in a
vacuum environment rather than in an inert environment, e.g.,
argon. More preferably the material is melted in a melt chamber 20
coupled to a vacuum source 22 in which the (chamber is maintained
at a pressure of less than 100 .mu.m, and preferably less than 50
.mu.m.
[0062] We prefer to melt nickel base superalloys such as IN 718 by
induction skull remelting or melting (ISR) 24, for example a
crucible manufactured by Consarc Corporation of Rancocas, N.J.
which is capable of rapidly, cleanly melting a single charge of
material to be cast, e.g., up to about 25 pounds of material. In
ISR, material is melted in a crucible defined a plurality of metal
(typically copper) fingers retained in position next to one
another. The crucible is surrounded by an induction coil coupled to
a power source 26. The fingers include passages for the circulation
of cooling water from and to a water source (not shown), to prevent
melting of the fingers. The field generated by the coil passes
through the crucible, and heats and melts material located in the
crucible. The field also serves to agitate or stir the molten
metal. A thin layer of the material freezes on the crucible wall
and forms the skull, thereby minimizing the ability of molten
material to atack the crucible. By properly selecting the crucible
and coil, and the power level and frequency applied to the coil, it
is possible to urge the molten material away from the crucible, in
effect levitating the molten material.
[0063] Since some amount of time will necessarily elapse between
material melting and injection of the molten material into the die,
the material is melted with a limited superheat-high enough to
ensure that the material remains at least substantially molten
until it is injected, but low enough to ensure that rapid
solidification occurs upon injection, e.g., so that small grains
can be formed. For superalloys, we prefer to limit the superheat to
within about 200 F. over the meltlng point, more preferably less
than 100 F., and most preferably less than 50 F.
[0064] While we prefer to melt single charges of the material using
an ISR unit, the material may be melted in other manners, such as
by vacuum induction melting (VIM), electron beam melting,
resistance melting or plasma arc. Moreover, we do not rule out
melting bulk material, e.g., several charges of material at once,
in a vacuum environment and then transferring single charges of
molten material into the shot sleeve for injection into the die.
However, since the material is melted in a vacuum, any equipment
used to transfer the molten material must typically be capable of
withstanding high temperatures and be positioned in the vacuum
chamber, and consequently the chamber must be relatively large. The
additional equipment adds cost, and the correspondingly large
vacuum chamber takes longer to evacuate thus adversely affecting
the cycle time.
[0065] In order to transfer molten material from the crucible to a
shot sleeve 30 of the apparatus (step 48 - FIG. 10), the crucible
is mounted for translation (arrow 32 in FIG. 9), and also for
pivotal movement (arrow 33 of FIG. 8) about a pouring axis (not
shown), and in turn is mounted to a motor (also not shown) for
rotating the crucible to pour molten material from the crucible
through a pour hole 32 of the shot sleeve 30, with or without a
pour cup or funnel coupled to the sleeve. Translation occurs
between the melt chamber 20 in which material is melted and a
position in a separate vacuum chamber 34 in which the shot sleeve
is located. The pour chamber 34 is also maintained as a
non-reactive environment, preferably a vacuum environment at a
pressure less than 100 .mu.m, and more preferably less than 50
.mu.m. The melt chamber 20 and pour chamber 34 are separated by a
gate valve or other suitable means (not shown) to minimize the loss
of vacuum in the event that one chamber is exposed to atmosphere,
e.g., to gain access to a component in the particular chamber.
[0066] As noted above, the molten material is transferred from the
crucible 24 into the shot sleeve 30 through a pour hole 34. The
shot sleeve 30 is coupled to a multipart, reusable die 36, which
defines a die cavity 38. A sufficient amount of molten material is
poured into the shot sleeve to fill the die cavity, which may
include one part or more than one part. We have successfully cast
as many as 12 parts in a single shot, e.g., using a 12 cavity
die.
[0067] The illustrated die 36 includes two parts, 36a, 36b, which
cooperate to define the die cavity 38, for example in the form of a
compressor airfoil for a gas turbine engine. The die 36 is also
coupled to the vacuum source, to enable evacuation of the die prior
to injection of the molten metal, and may be enclosed in a separate
vacuum chamber. One part of the two parts 36a, 36b of the die is
fixed, while the other part is movable relative to the one part,
for example by a hydraulic assembly (not shown). The dies
preferably include ejector pins (not shown) to facilitate ejecting
solidified material from the die.
[0068] The die may be composed of various materials, and should
have good thermal conductivity, and be relatively resistant to
erosion and chemical attack from injection of the molten material.
A comprehensive list of possible materials would be quite large,
and includes materials such as metals, ceramics, graphite and metal
matrix composites. For die materials, we have successfully employed
tool steels such as H13 and V57, molybdenum and tungsten based
materials such as TZM and Anviloy, copper based materials such as
copper beryllium alloy "Moldmax"-high hardness, cobalt based alloys
such as F75 and L605, nickel based alloys such as IN 100 and Rene
95, iron base superalloys and mild carbon steels such as 1018.
Selection of the die material is critical to producing articles
economically, and depends upon the complexity and quantity of the
article being cast, as well as on the current cost of the
component.
[0069] Each die material has attributes that makes it desirable for
different applications. For low cost die materials, mild carbon
steels and copper beryllium alloys are preferred due to their
relative ease of machining and fabricating the die. Refractory
metal such as turgsten and molybdenum based materials are preferred
for higher cost, higher volume applications due to their good
strength at higher temperatures. Cobalt based and nickel based
alloys and the more highly alloyed tool steels offer a compromise
between these two groups of materials. The use of coatings and
surface treatments may be employed to enhance apparatus performance
and the quality of resulting parts. The die may also be attached to
a source of coolant such as water or a source of heat such as oil
(not shown) to thermally manage the die temperature during
operation. In addition, a die lubricant may be applied to one or
more selected parts of the die and the die casting apparatus. Any
lubricant should generally improve the quality of resultant cast
articles, and more specifically shouldi be resistant to thermal
breakdown, so as not to contaminate the material being
injected.
[0070] Molten metal is then transferred from the crucible to the
shot sleeve. A sufficient amount of molten metal is poured into the
shot sleeve to fill the die cavity and associated runners, biscuit,
other cavities. Since IN 718 does not "can" to the extent that
titanium alloys do, it is possible to fill the shot sleeve.
However, we have produced good quality castings where the sleeve is
less than 50% filled, less than about 40% filled, and less than
about 30% filled.
[0071] An injection device, such as a plunger 40 cooperates with
the shot sleeve 30 and hydraulics or other suitable assembly (not
shown) drive the plunger in the direction of arrow 42, to move the
plunger between the position illustrated by the solid lines and the
position indicated by dashed lines, and thereby inject the molten
material from the sleeve 30 into the die cavity 38 (step 50-FIG.
15). In the position illustrated by solid lines, the plunger and
sleeve cooperate to define a volume that is substantially greater
than the amount of molten material that will be injected.
Preferably, the volume is at least twice the volume of material to
be injected, more preferably at least about three times.
Accordingly, the volume of molten material transferred from the
crucible to the sleeve. Where the sleeve is only partially filled,
any material or skin that solidifies on the sleeve forms only a
partial cylinder, e.g., an open arcuate surface, and is more easily
scraped or crushed during metal injection, and reincorporated into
the molten material. For injection, we have used plunger speeds of
between about 30 inches per second (ips) and 300 ips, and currently
prefer to use a plunger speed of between about 50-175 inches per
second (ips). The plunger is typically moved at a pressure of at
least 1200 psi, and more preferably at least 1500 psi. As the
plunger approaches the ends of its stroke when the die cavity is
filled, it begins to transfer pressure to the metal. The pressure
exerted on the metal is then intensified, preferably to at least
500 psi and more preferably to at least about 1500 psi, to (nsure
complete filling of the mold cavity. Intensification is also
performed to minimize porosity, and to reduce or eliminate any
material shrinkage during cooling. After a sufficient period of
time has elapsed to ensure solidification of the material in the
die, the ejector pins (not shown) are actuated to eject parts from
the die (step 52-FIG. 10).
[0072] As is known in the art, articles cast generally and die cast
in particular tend to include some porosity, generally up to a few
percent. Accordingly, and particularly where such articles are used
in more demanding applications, such as compressor airfoils for gas
turbine engines, there is a need to reduce and preferably eliminate
porosity and otherwise treated as needed (step 54-FIG. 10). The
parts are therefore hot isostatically pressed (HIP'd) as described
above to reduce and substantially eliminate any porosity in the
parts as cast. For nickel base superalloys such as IN 718, we
prefer to HIP at a temperature of between about 1800-2000 F., more
preferably between about 1800-1875 F., for a minimum of about 4
hours, and at a pressure of between about 15-25 ksi.
[0073] If desired, each article may then be heat treated. For
airfoils composed of die cast IN 718, the heat treatment includes
standard and commercially accepted treatment, such as is disclosed
in AMS 5663.
[0074] Actual heat treatment and HIP parameters may be varied
depending upon the desired properties and application for the
article and target cycle time for the process, however the
temperature, pressure and time used during HIP must be sufficient
to eliminate substantially all porosity, and homogenize any
residual casting segregation but not to allow significant grain
growth.
[0075] The parts are inspected (step 56-FIG. 10) using conventional
inspection techniques, e.g., by fluorescent penetrant inspection
(FPI), radiographic, visual, and after passing inspection may be
used or further treated/re-treated if necessary (step 58-FIG.
10).
[0076] As a result of our work with nickel base superalloys, we
believe that several conditions are important to produce good
quality castings. The melting, pouring and injection of material,
particularly for reactive materials, must be performed in a
non-reactive environment, and we prefer to perform these operations
in a vacuum environment maintained at a pressure preferably less
than 100 .mu.m and more preferably less than 50 .mu.m. The amount
of superheat should be sufficient to ensure that the material
remains substantially and completely molten from the time it is
poured until it is injected, but also to enable rapid cooling and
formation of small grains once injected, Due to the relatively low
superheat, molten metal transfer and injection must be rapid enough
to occur prior to metal solidification. The resulting
microstructure such as grain size appears to correspond to the
sectional thickness of the part being cast as well as the die
materials utilized and the superheat used, i.e., thinner sections
tend to 25 include smaller grains and thicker sections
(particularly internal portions of thicker sections) tend to
include larger grains. Higher thermal conductivity die materials
result in articles having smaller grains, as does use of lower
superheats. We believe that this results from relative cooling
rates of these sections. The rate at which the plunger is moved,
and correspondingly the rate at which material is injected into the
mold appears to affect the surface finish of the articles as cast,
although the design of 30 the gating as well as the die material
may also play a role in combination with the injection rate.
[0077] Careful control of the post cast thermal processing is
required to fully achieve the benefits offered by the relatively
fine as die cast microstructure.
[0078] Die casting provides other significant advantages over
forging. The time required to produce a part, from ingot to
finished part, is reduced significantly, since there is no need to
prepare specially tailored billets of material, and casting broadly
is performed in a single step, as opposed to multiple forging
operations. In die casting, multiple parts can be produced in a
single casting. Die casting enables production of parts having more
complex three dimensiorial shapes, thereby enabling new software
design technology to be applied to and exploited in areas such as
gas turbine engines and enabling production of more efficient
airfoils and other components. We believe that die casting will
enable the production of articles having complex shapes utilizing
materials that are difficult or impossible to forge into those
shapes. Moreover, die cast parts have greater reproducibility than
forged or investment cast articles, and can be produced nearer to
their finished shape, and with a superior surface finish, which
minimizes post forming finishing operations, all of which also
reduces the cost of producing such parts.
[0079] While the present invention has been described above in some
detail, numerous variations and substitutions may be made without
departing from the spirit of the invention or the scope of the
following claims. Accordingly, it is to be understood that the
invention has been described by way of illustration and not by way
of limitation.
* * * * *