[0001] The present invention relates generally to articles fabricated from superalloy material,
and relates more particularly to articles fabricated from nickel base superalloys
and to methods of heat treating such alloys. Such alloys typically have high melting
temperatures, in excess of 1260 - 1371 °C/2300 - 2500 °F. 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
1093 °C/2000 °F.
[0002] 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 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.
[0003] 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.
[0004] 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.
[0005] 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 operations, 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.
[0006] 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 more 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.
[0007] Forged components also often exhibit significant levels of defects, including inclusions
and carbides, which vary significantly from component to component. Components having
higher columbium contents, e.g., IN 718, are also prone to elemental segregation,
as well as the formation of phases such as Laves and other topologically close packed
phases. The presence and degree of these defects detrimentally affects the mechanical
properties, particularly at elevated temperatures. The extent of these defects typically
depends upon the material composition, and the length of time the component is exposed
to elevated temperature, e.g., during forging. Accordingly, the articles are heat
treated to reduce or remove the defects, e.g., homogenization heat treatment, which
occurs as a separate step in addition to any other treatment steps performed on the
articles. The heat treatment typically includes exposing the article to a relatively
high temperature, e.g., around 1093 °C/2000 °F for a period of up to several hours.
The temperature is high enough to reduce segregation, but not so high or for so long
as to enable significant grain growth to occur.
[0008] Casting has been extensively used to produce relatively near-finished-shape articles.
[0009] 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. The porosity must be eliminated, e.g., by heating the article and
subjecting the article to pressure. For IN 718, the articles are typically HIP'd at
a temperature of between 982 - 1024 °C/1800 - 1875 °F, at a pressure of between 105
- 154 MPa/15 - 22 ksi for several hours. Spallation of the ceramic shell also contributes
to the presence of inclusions and impurities in the cast articles.
[0010] 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.
[0011] 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 1093 °C/2000 °F.
[0012] 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, molten metal is poured through a pour spout 22 and sprue 21, and flows into
an injection cylinder 30 which communicates with 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.
[0013] 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.
[0014] 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 (typically
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.
[0015] 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 1093 °C/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.
[0016] 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.
[0017] 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.
[0018] 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 articles, for example
include 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.
[0019] According to another aspect of the invention, a method is disclosed for heat treating
a die cast superalloy article having porosity and elemental segregation as cast. The
method includes heating the article to a temperature of about 982-1121 °C/1800 - 2050
°F, preferably between about 982 - 1023 °C/1800 - 1875 °F for between about 1 - 24
hours, whereby the segregation is reduced. More preferably, the article is subjected
to a pressure of between about 105 - 154 MPa/15 - 25 ksi during the step of heating,
whereby porosity is substantially eliminated at the same time.
[0020] 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.
[0021] 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.
[0022] Certain preferred embodiments will now be described by way of example only and with
reference to the accompanying drawings, in which:
FIG. 1 is a view illustrating a die cast article composed of IN 718 in accordance
with a preferred embodiment of the present invention;
FIG. 2 is a photomicrograph illustrating the microstructure of a test bar composed
of die cast IN 718 in accordance with a preferred embodiment of the present invention;
FIG. 3 is a photomicrograph illustrating the microstructure of an airfoil composed
of die cast IN 718 in accordance with a preferred embodiment of the present invention;
FIG. 4 is a photomicrograph of the airfoil of FIG. 4 after hot isostatic pressing
of the airfoil;
FIG. 5 is a photomicrograph illustrating the microstructure of an airfoil composed
of forged IN 718;
FIGS. 6 and 7 illustrate properties of a die cast IN 718 article in accordance with
a preferred embodiment of the present invention and corresponding forged articles;
FIGS. 8 and 9 are schematic views of a die casting machine used to produce articles
composed of IN 718;
FIG. 10 is a flow diagram illustrating a process of die casting IN 718 in accordance
with a preferred embodiment of the present invention;
FIG. 11 is a graph illustrating the average grain size and percentage of segregation
in a die cast IN 718 article versus heat treatment temperature in accordance with
preferred embodiments of the present invention;
FIG. 12 is a photomicrograph illustrating the microstructure including elemental segregation
in a die cast IN 718 article; and
FIG. 13 is a photomicrograph illustrating the reduced segregation after being HIP'd
and heat treated in accordance with a further preferred embodiment of the present
invention.
[0023] Turning now to FIG. 1, a preferred 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.
[0024] As noted above, a typical nickel base superalloy utilized in gas turbine engines
is Inconel 718 (IN 718), which nominally 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 - 3.3 Mo + 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 and traces of other elements.
[0025] 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.
[0026] 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.
[0027] Briefly, in accordance with a preferred embodiment of 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 µm more preferably at less than 50 µm. The alloy is also heated to
a controlled, limited superheat, e.g., typically within 38 - 93°C/100°F to 200°F above
the melting temperature of the alloy and more preferably within 10 - 38°C/50°F to
100°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.
[0028] 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 expense performed on the articles.
[0029] In accordance with the present invention, articles prepared in accordance with the
preferred embodiments 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.
[0030] 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.
[0031] Examination of the as die cast articles such as die cast IN718 has, surprisingly,
also indicated the presence of at least some Laves phase and other TCP phases, as
well as elemental segregation. The presence of these defects is surprising, given
the relatively rapid rate (relative to investment casting) at which the molten material
cools upon being injected into the mold. As previously discussed, the presence of
these defects reduces mechanical properties of the articles. Depending upon the intended
use of the articles, these defects must be reduced or eliminated. Exemplary heat treatments
for reducing or eliminating these defects are discussed with respect to FIGS. 12 -
14.
[0032] 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 preferred embodiments
of 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 20 °C/ 70
°F) and elevated temperatures, e.g., about 650 °C/1200 °F held for a period of time
prior to testing. The samples were subjected to strain rate of between 0.076 - 0.178
mm./mm./minute or 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.
[0033] 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 1 MPa/140
ksi and more preferably at least 1.05 MPa/150 ksi and most preferably at least 1.12
MPa/160 ksi; and at yield strength at 650 °C/1200 °F of at least 805 kPa/115 ksi and
more preferably 875 kPa/125 ksi and most preferably at least 945 kPa/135 ksi. Such
articles have a ultimate tensile strength at room temperature of at least 1.23 MPa/175
ksi and more preferably at least 1.3MPa/185 ksi and most preferably at least 1.37
MPa/195 ksi; and an ultimate tensile strength at 650 °C/1200 °F of at least 1 MPa/a140
ksi and more preferably 1.05 MPa/150 ksi and most preferably at least 1.12MPa/160
ksi.
[0034] In addition, standard combination smooth and notched stress rupture test specimens
(comprising material produced in accordance with a preferred embodiment of the present
invention), e.g., conforming to ASTM E292, were tested. The specimens were maintained
at about 650°C/1200°F and loaded continuously, after generating an initial axial stress
of between about 735 - 770 MPa/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.
[0035] Similar standard combination smooth and notched stress rupture test specimens (comprising
material produced in accordance with a preferred embodiment of the present invention),
e.g., conforming to ASTM E292, were also tested at about 704 °C/1300 °F. The specimens
were loaded continuously, after generating an initial axial stress of between about
420 - 455 MPa/60 - 65 ksi. In the case of material to be used for blades and vanes,
the specimens ruptured only after at least 40 hours.
[0036] Creep properties were also evaluated, at about 650 °C/1200 °F. The specimens were
maintained at about 650° C/1200° F, and loaded to produce an axial stress of at least
about 560 MPa/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.
[0037] 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 910 MPa/130 ksi and more preferably at least 1 GPa/140 ksi and most preferably
at least 1.05 GPa/150 ksi; and at yield strength at 650 °C/1200 °F of at least 735
MPa/105 ksi and more preferably 805 MPa/115 ksi and most preferably at least 875 MPa/125
ksi. Such articles have a ultimate tensile strength at room temperature of at least
1.16 GPa/165 ksi and more preferably at least 1.23 GPa/175 ksi and most preferably
at least 1.3 GPa/185 ksi; and an ultimate tensile strength at 650°C/1200°F of at least
875 MPa/125 ksi and more preferably 945 MPa/135 ksi and most preferably at least 1.02
GPa/145 ksi.
[0038] In addition, standard combination smooth and notched stress rupture test specimens
(comprising material produced in accordance with a preferred embodiment of the present
invention), e.g., conforming to ASTM E292, were tested. The specimens were maintained
at about 650 °C/1200 °F and loaded continuously, after generating an initial axial
stress of between about 735 - 770 MPa/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 %.
[0039] Similar standard combination smooth and notched stress rupture test specimens (comprising
material produced in accordance with a preferred embodiment of the present invention),
e.g., conforming to ASTM E292, were also tested at about 704 °C/1300 °F. The specimens
were loaded continuously, after generating an initial axial stress of between about
420 - 455 MPa/60 - 65 ksi. In the case of material to be used for blades and vanes,
the specimens ruptured only after at least 85 hours.
[0040] Creep properties were also evaluated, at about 650 °C/1200 °F. The specimens were
maintained at about 650°C/1200°F, and loaded to produce an axial stress of at least
about 560MPa/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.
[0041] AMS 5663 calls for the following properties:
Property |
Room Temp. |
1200 °F +/- 10 (650 °C) |
Tensile Strength, min. |
1.26GPa/180 ksi |
IGPa/140 ksi |
Yield Strength, 0.2% offset, min. |
1.05GPa/150 ksi |
875MPa/125 ksi |
Elongation in 4D, min. |
10% |
10% |
Reduction in area, min. |
12% |
12% |
[0042] AMS 5383 calls for the following properties:
Property |
Room Temp. |
Tensile Strength, min. |
840MPa/120 ksi |
Yield Strength, 0.2% offset, min. |
735MPa/105 ksi |
Elongation in 4D, min. |
3% |
Reduction in area, min. |
8% |
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.
[0043] 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 650 °C/1200 °F and loaded
continuously, after generating an initial axial stress of between about 735 - 770
MPa/105 - 110 ksi. The specimens ruptured after at least 23 hours. These values meet
the requirements set forth in AMS 5663.
[0044] 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 704°C/1300 °F and loaded continuously, after generating an initial
axial stress of about 462MPa/65 ksi. The specimens should rupture only after at least
23 hours.
[0045] 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. A 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 4.5kg/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
chamber-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 it solidifies to form the desired article.
[0046] Initially, material to be die cast is 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 µm, and preferably less than 50 µm.
[0047] 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, NJ 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 attack 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.
[0048] 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 93 °C/200 °F over the melting point, more preferably less
than 38 °C/100 °F, and most preferably less than 10 °C/50 °F.
[0049] 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.
[0050] 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 µm, and more preferably less than 50 µm. 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] 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 tungsten 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 should be resistant to thermal breakdown,
so as not to contaminate the material being injected.
[0055] 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.
[0056] 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 0.77 m/s - 30 inches per second (ips) and 7.7 m/s -- 300 ips, and
currently prefer to use a plunger speed of between about 1.3 - 4.5 m/s -- 50 - 175
inches per second (ips). The plunger is typically moved at a pressure of at least
8.4 MPa/1200 psi, and more preferably at least 10.5 MPa/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 3.5 MPa/500 psi and more preferably to at least about 10.5 MPa/1500 psi,
to ensure 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).
[0057] 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 982 - 1093 °C/1800 - 2000 °F, more preferably between
about 982 - 1023 °C/1800 - 1875 °F, for a minimum of about 4 hours, and at a pressure
of between about 105 - 175 MPa/15 - 25 ksi.
[0058] 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.
[0059] 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.
[0060] 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).
[0061] Turning to the heat treatment, we have determined that the segregation and TCP phases
can be reduced or substantially eliminated at significantly reduced temperatures compared
to prior articles, and thus that it is possible also to address the presence of elemental
segregation within the same parameters as the HIP. The heat treatment includes heating
the material to a temperature of between about 982 - 1121 °C/1800 - 2050 °F, for a
period of between about 1 - 24 hours, and at a pressure of about 105 - 175 MPa./15
- 25 ksi where porosity is to be eliminated. The treatment preferably occurs in an
inert environment, such as argon. Actual parameters may be varied depending upon the
desired application for the article and target cycle time for the process, however
the temperature, pressure and time must be sufficient to eliminate substantially all
porosity and reduce segregation in the as cast articles (illustrated in FIG. 12),
but not to allow significant grain growth. FIG. 13 illustrates die cast IN 718 material
after being heated to a temperature of about 1010 °C/1850 ° F for 2 hours, without
the pressure, to illustrate the reduction in segregation. Application of an appropriate
HIP pressure during this time closes the existing porosity.
[0062] The temperature and time will affect the resulting grain size of the article. For
example and with reference to FIG. 11, an IN 718 article as die cast had an average
grain size of about ASTM 9, and a percent segregation of about 30% (see the picture
to the left of the curve). Samples were treated at temperatures between about 954
- 1121 °C/1750 - 2050 °F, and the treated articles had reduced segregation and average
grain sizes that increased with increasing temperatures. The increase in average grain
size is enhanced where longer times are used, particularly at higher temperatures.
The curve illustrated in FIG. 11 is for die cast IN 718, but other materials may exhibit
similar behavior. See, e.g., co-pending application entitled "Die Cast Superalloy
Articles".
[0063] 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 µm and more preferably less than 50 µm. 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 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 the gating
as well as the die material may also play a role in combination with the injection
rate. Careful control of the post cast thermal processing is required to fully achieve
the benefits offered by the relatively fine as die cast microstructure.
[0064] 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 dimensional 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.
[0065] The preferred heat treatment of the present invention provides advantages. The heat
treatment eliminates the adverse effect of casting, e.g., porosity, Laves segregation
and other unwanted TCP phases while retaining the fine grain size which provides superior
mechanical properties. Moreover, the treatment enables the elimination of all of the
above adverse effects in a single step, thereby enabling savings of cost, time and
handling.
[0066] Thus, in at least the illustrated embodiments, it can be seen that the present invention
provides die cast articles composed of high melting temperature materials, such as
nickel base superalloys having properties comparable to corresponding forged articles,
in particular having strength, durability and fatigue resistance comparable to corresponding
forged superalloy articles; furthermore also provides such articles having complex,
three dimensional shapes which are difficult if not impossible to forge; and provides
a method of reducing or eliminating any elemental segregation in die cast superalloy
articles, in particular it provides a heat treatment for reducing or eliminating elemental
segregation and TCP phases in die cast IN 718 articles, or viewed another way provides
a heat treatment which can also incorporate appropriate HIP parameters to reduce or
eliminate any residual porosity.
[0067] While the present invention has been described above in some detail, numerous variations
and substitutions may be made without departing from the scope of the invention as
defined by 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.
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 and stress rupture
resistance in accordance with AMS 5663.
3. The article of claim 1 or 2, wherein the article has an ultimate tensile strength
at about 650 °C/1200 °F of at least 1.05GPa/150 ksi and a 0.2% yield strength of at
least 875 MPa/125 ksi.
4. 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 5383.
5. The article of claim 4, wherein the article has an ultimate tensile strength at room
temperature of at least 840MPa/120 ksi and a 0.2% yield strength of at least 735 MPa/105
ksi.
6. The article of claim 1, wherein the article has an ultimate tensile strength at room
temperature of at least 1.26 GPa/180 ksi and a 0.2% yield strength of at least 1.02
GPa/145 ksi.
7. The article of any preceding claim, wherein the article comprises a gas turbine engine
component.
8. The article of any preceding claim, wherein the article is a compressor component
or a turbine component.
9. The article of any preceding claim, wherein the average grain size is smaller than
about ASTM 3.
10. A method of heat treating a cast superalloy article having porosity and elemental
segregation as cast comprising the step of heating the article to a temperature of
about 982 - 1121°C/1800 - 2050 °F for between about 1 - 24 hours, whereby the segregation
is reduced.
11. A method of making a cast superalloy article having porosity and elemental segregation
as cast comprising the steps of:
die casting the article, and
heating the article to a temperature of about 982 - 1121 °C/1800 - 2050 °F and at
a pressure above ambient pressure for between about 1 - 24 hours to reduce the segregation.
12. The method of claim 10 or 11, wherein the article is subjected to a pressure of between
about 105 - 175 MPa/15 - 25 ksi during the step of heating, whereby porosity is substantially
eliminated.
13. The method of claim 10, 11 or 12, wherein the step of heating comprises heating the
article to a temperature of between about 982 - 1023 °C/1800 - 1875 °F for at least
about 4 hours.
14. The method of any of claims 10 to 13, wherein the material has a composition 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.
15. The method of any of claims 10 to 14, wherein the resulting material exhibits elemental
segregation in an amount of between about 0 - 40 %.
16. The method of any of claims 10 to 15, wherein the resulting material exhibits a microstructure
having an average grain size of ASTM 3 or smaller.
17. A superalloy die cast article having substantially no porosity and reduced segregation
relative to the articles as die cast.
18. The die cast article of claim 17, wherein the article has a composition in weight
percent of about 0.02 - 0.04 C, 17 - 21 Cr, up to about 1 Co, 2.8 - 3.3 Mo + 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
generally Ni.
19. The die cast article of claim 17 or 18, wherein the resulting material exhibits elemental
segregation in an amount of between about 0 - 40 %.