Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
This invention relates to the field of powder metal-
lurgy and specifically to a container for hot consolidating
powder of metallic and non-metallic composition and combinations
thereof and a method for using the same.
Hot consolidation of metallic, intermetallic, and
non-metallic powders and combinations thereof has become an
industry standard. The advantages of hot consolidation over
other techniques for consolidating powders are well known.
In some cases, hot consolidation is the only practical powder
metallurgical technique for consolidating certain high tem-
perature material. For example, hot consolidation is employed
extensively for high temperature - high stress materials,
such as nickel-base superalloys (e.g. IN-100).
Hot consolidation can be accomplished by filling
a container with a powder to be consolidated. The container
is usually evacuated prior to filling and then hermetically
sealed. Heat and pressure are applied to the filled and sealed
container. This can be accomplished by using an autoclave.
The gas pressure produced in the autoclave applies an equal
pressure over the surface of the container and causes the
container to shrink, or collapse, against the powder. As
the container shrinks, or collapses, the powder is densified.
In other words, at elevated temperatures, the container functions
as a pressure transmitting medium to subject the powder to
the pressure applied to the container. Simultaneously, the
heat causes the powder to fuse by sintering. This process
for densifying powdex is generally referred to as hot isostatic
pressing. In short, the combination of heat and pressure
causes consolidation of the powder into a substantially fully
densified and fused mass in which the individual powder particles
have lost their identity.
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P-3~9
After consolidation, the container is removed from
the densified powder compact. The compact is then further
processed through one or more steps, such as forging, machining,
and/or heat treating, to form a finished part.
An extremely critical element of the hot consolidation
process is the nature and characteristics of the container.
The material of which the container is made must be capable
of performing as a pressure transmitting medium at temperatures
high enough to cause sintering of the powder, that is, the
container must be flexible or deformable yet maintain structural
integrity at elevated temperatures. The container must be
non-reactive, or only slightly reactive, with respect to the
powder contained therein, or steps must be taken to shield
the container from the powder. Since the container must be
hermetically sealed, and in some cases vacuum evacuated, the
container must be capable of withstanding heating and pressing
without cracking. The type of container employed will also
determine, to a large extent, the degree of precision with
which the compact can be made. In other words, some types
of containers are only capable of producing simple billet
stock shapes and rough preforms which require extensive sub-
sequent forging and machining to produce a finished part.
Due to the high cost of raw material and the cost
of forging, recent efforts have been made to develop containers
capable of producing compacts of greater precision to thereby
reduce material and forming costs. Such high precision compacts
are generally referred to as "near-net shapes". Such precision
compacts would only require machining or, at most, a simple
forging operation, to produce a final shape, thus eliminating
extensive intermediate forging steps. This invention is directed
to a hot isostatic pressing container which meets the foregoing
requirements as well as demonstrating the capability of producing
10 ~ ~ ~ 3
P-3~9
near-net shapes.
PRIOR ART
The prior art includes many examples of containers
for hot consolidating powder. These containers are made of
various materials, such as metal, glass, and ceramics. The
earliest containers used for hot consolidating powder, and
the ones most commonly encountered in current industrial practice,
are made of metal. The particular type of m~tal employed
for the container is usually selected in view of the composition
of the powder to be consolidated. That is, the requisite
temperatures and pressures of consolidation and the reactivity
of the powder are taken into consideration when determining
the container material. Metal containers for hot consolidating
nickel-base superalloys are commonly made of stainless steel.
Other metals, however, are used for powders of different
composition.
Examples of typical metal containers are shown in
U.S. Patents 3,340,053, issued September 5, 1967, and 3,356,496,
issued December 5, 1967. It is noted that these metal containers
are relatively thin-walled and of simple shape. The reason
that thin-walled containers have been used is that an effort
was made to duplicate, as near as possible, the behavior of
a flexible rubber bag of the type which had been used to iso-
statically press powders at near room temperature. Of course,
rubber bags could not be used at the elevated temperatures
required for hot consolidation. The theory was, however,
that a thin-walled metal container would behave, at elevated
temperatures, much like a rubber bag at near room temperature.
It was learned that this was not the case. The walls of a
thin-walled metal container do not transmit pressure evenly
to the powder due to variations in the structural strength
of the container. Consequently, thin-walled metal containers
--3--
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tend to buckle or wrinkle in weaker sections. When simple
shapes, such as billet stock or forging preforms, are being
produced, surface defects caused by buckling and wrinkling
of the thin-walled metal container can sometimes be tolerated
since these defects can be removed by machining. It is very
difficult, if not impossible, however, to produce more compli-
cated precision shapes using thin-walled metal containers.
One of the greatest difficulties in producing precision shapes
using thin-walled metal containers is that the resulting com-
pact is greatly distorted due to non-uniform reduction in the
size of the container. In other words, the shape of the re-
sulting compact after compaction is far different from the
shape of the cavity initially defined by the thin-walled con-
tainer. Although such distortions, in most cases, can be accomo-
dated for by making a greatly over-sized compact, this is done
at the expense of excessive forging and/or machining and material
waste.
Attempts have heen made to solve the problems associ-
ated with thin-walled containers and to provide a container
capable of producing near net shapes. For example, in the U.K.
Patent 1,339,669, published July 2, 1975, a method of consoli-
dating metallic powder is disclosed in which a relatively thick-
walled container is formed by 30ining two mold halves which
are made of sintered metal powder and by encasing the mold
halves in an outer metal sheath. The mold havles are made of
sintered metal powder so that the porosity, or density, of the
walls of the mold halves are approximately e~ual to the tap
density of the powder contained in the cavity formed by the
mold halves. Upon the application of heat and pressure, it
is intended that the density of the container and the powder
contained therein both increase substantially simultaneously
to uniformly compact the powder without distortion. Another
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deviation from the traditional thin-walled metal container
is disclosed in U.S. Patent 3,230,286, issued January 18,
1966. The container disclosed in this patent is made of a
metal, such as cerium, bismuth, cesium, or alloys thereof,
5 which undergoes an abrupt densification, or reduction in volume, ;-
at a predetermined pressure. The abrupt densification, or
reduction in volume, is due to a rearrangement of the crystal
lattice structure of the material caused by the applied pressure.
The reduction in volume is relied upon to apply pressure to
the powder contained within the container.
In summary, in the development of containers and
methods for hot isostatic pressing powder, the first efforts
were to simulate a flexible rubber bag. Hence, thin-walled -~
metal containers were employed. As the art advanced, various
attempts were made using thicker walled containers; however,
in the case of metal containers, the containers were made
porous or an exotic alloy was employed which is capable of
an abrupt densification under the influence of extreme pressures.
These rather complicated measures were taken because it was
generally believed that a thick-walled container would not
effectively transmit pressure to the powder. When materials
other than metals were employed, such as glass or ceramics,
the container walls were also made relatively thin. If not
thin, then the material was in particulate form. This required
additional steps to contain the particulate material, such
as the use of the inner and outer containers shown in U.S.
Patent 3,700,435, issued October 24, 1972. Notwithstanding
all the development effort thus far expended, there is no
commercially acceptable container available which is capable
of producing precision compacts or near-net shapes.
SUMMARY OF THE INVENTION
This invention is based upon a recognition by the
10~06;~3
inventor that a superior container for hot consolidating powder
can be made from a substantially fully dense and incompressible
material if the material is capable of plastic flow at pressing ;:
temperatures and that, if the container walls are thick enough,
the container material will act like a fluid upon the application
of heat and pressure to apply hydrostatic pressure to the powder.
In other words, it is not necessary to use a porous material as
described in United Kingdom Patent 1,399,669 and United States
Patent 3,700,435 or a material which undergoes an abrupt densific-
ation as described in United States Patent 3,230,286. It has
been determined by the inventor that the container walls are
thick enough to function in the intended manner when the
exterior surface of the container walls does not closely follow
the contour of the container cavity. In other words, the exterior
surface of the walls of the container should not follow the con-
tour of the container cavity as, for example, do the walls of
the container described in United States Patent 3,841,870, issued
October 15, 1974. It is noted that the exterior surface of the
walls of the container described in this patent define a shape
which is substantially identical to the shape of the container
cavity. This is typical of what is referred to as a "thin-
walled" container.
Thus, this invention provides for a method for hot
consolidating powder of metallic and nonmetalic composition and
combinations thereof to form a densified compact comprising the
steps of: encapsulating a quantity of powder in a cavity in a
thick-walled container having walls entirely surrounding the
cavity and of sufficient thickness so as not to closely follow
the contour of the cavity and of a material which is substantially
fully dense and incompressible and capable of plastic flow at
lO90~iZ3
elevated temperatures, heating the container and powder to a
temperature at which the powder will densify and applying
external pressure to the entire exterior surface of the container
thereby causing plastic flow of the container walls to subject
the powder to a hydrostatic pressure which causes it to densify .
into the compact.
In a second embodiment, this invention provides for
a method for hot consolidating powder of metallic and non-
metallic composition and combinations thereof to form a densified
compact, said method comprising the steps of: providing a
container by forming a cavity having the general shape of the
compact to be produced in a mass of container material which is
substantially fully dense and incompressible and is capable of
plastic flow at elevated temperatures, the volume of said mass
being sufficiently large with respect to the volume of said cavity
to form walls entirely surrounding said cavity of sufficient
thickness so that the exterior surface of the walls do not
closely follow the contour of said cavity, filling said cavity
with a powder to be compacted, hermetically sealing said con-
tainer, heating said container and powder to a temperature atwhich said mass is capable of plastic flow and the powder is
susceptible to compaction and applying external pressure to the ~-
entire exterior surface of said container to apply a hydro-
static pressure to the powder in said cavity to densify said
powder into a compact, and cooling said container and compact
and removing said container from said compact.
In a third embodiment, this invention provides for a
method for hot consolidating powder of metallic and non-metallic
composition and combinations thereof to form a densified powder
compact comprising the steps of encapsulating a quantity of
-6a-
1090tiZ3
powder in a thick-walled metal container wherein the walls of
the container entirely surround the powder and are substantially
fully dense and incompressible and are capable of plastic flow
at predetermined temperatures and pressures, heating the con-
tainer and powder to a temperature at which the powder will
densify and applying pressure to the entire exterior surface -
of the heated container by pressing the container between the
dies of a press while restraining the container, the applied
pressure being of sufficient magnitude to cause plastic flow
at the container walls thereby subjecting the powder to a
hydrostatic pressure which causes it to densify.
In a fourth embodiment, this invention provides for a
method for hot consolidating powder to form a densified powder
compact comprising the steps of encapsulating a quantity of
powder in a thick-walled low carbon steel container wherein
the walls of the container entirely surround the powder and
are substantially fully dense and incompressible and are capable
of plastic flow at temperatures above 1000F. and a pressure
exceeding 5000 psi, preheating the container and powder to a
temperature above 1000 F, and applying a pressure above 5000 psi
to the entire exterior surface of the container by pressing the
container between the dies of a press while restraining the
container thereby causing plastic flow of the container walls
to subject the powder to a hydrostatic pressure which causes
it to densify.
The container of the instant invention constitutes
a radical departure from the generally accepted principles
relating to containers for hot consolidating powder. The
fact that the container is capable of applying hydrostatic
pressure to the powder facilitates uniform shrinkage, permits
-6b-
1090623
a closer prediction of final dimensions, and reduces distortion.
Hence, it is possible to produce near-net shapes. In order
to achieve these results, however, a container is used which
is made of a substantially fully dense and incompressible
material. The walls of the container surrounding the powder-
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10 ~ ~ ~ 3
P_319
receiving cavity are thicker than the prior art suggests would
be capable of transmitting pressure. Any heretofore known
containers having walls of any significant thickness have
been made of a compressible or particulate material. It has
been discovered by the inventor that the thickness of the
container walls does not hinder consolidation, but to the
contrary, is desirable and essential to produce a hydrostatic-
like pressure at the interface between the container material
and the powder in the cavity. In other words, a thick-walled
container of the type described herein produces better results
than thin-walled containers because of its ability to apply
hydrostatic pressure to the powder.
The container of the instant invention was specifically
designed for consolidating superalloy powder, such as, IN-
15 100, which is a well-known nickel-base alloy which includes --~
alloying elements of aluminum, titanium, tantalum, columbium,
molybdenum, tungsten, chromium and cobalt. IN-100, and other
superalloys, are employed in turbine engine components, for ~
example, because of their high strength characteristics at `
elevated temperatures. These high strength characteristics,
however, make these alloys difficult to work. Conventional
casting techniques cannot easily be used since the many alloying
elements produce segregation problems in the cast object.
Additionally, the inherent strength of these alloys at high
temperatures make forging difficult and expensive. Accordingly,
it has become necessary to employ powder metallurgy techniques
to produce superalloy parts having optimum physical charac-
teristics. Even present day powder metallurgy techniques
often require multiple forging and machining operations to
produce a final shape. Efforts have therefore been made to
produce precision powder metal compacts, or near-net shapes,
to reduce, or eliminate, forging and to reduce the amount
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P-319
of material which must be removed by machining to produce a
finished shape. The container constructed in accordance with
the instant invention offers this advantage.
Other advantages of the present invention will be
readily appreciated as the same becomes better understood by
reference to the following detailed description when considered
in connection with the accompanying drawings wherein:
FIGURE 1 is a cross-sectional elevational view of
a container for hot consolidating powder constructed in accor-
dance with the instant invention showing the container beforehot consolidation and, in phantom, after hot consolidation;
FIGURE 2 is a broken-away view of FIGURE 1 illustrat-
ing probable force distribution when pressure is applied to
the container;
FIGURE 3 is a cross-sectional elevational view of
another embodiment of a container for hot consolidating powder
constructed in accordance with the instant invention:
FIGURE 4 is a cross-sectional elevational view of
a densified compact subsequent to hot consolidation in the con-
tainer of FIGURE 3;
FIGURE S is a cross-sectional elevational view of
a finished part machined from the densified compact of FIGURE
4;
FIGURE 6 is a cross-sectional elevational view of
an embodiment of a container for hot consolidating powder de-
signed particularly for hot consolidation in a press; and
FIGURE 7 is a cross-sectional elevational view of
suitable upper and lower press dies for use with the container
of FIGURE 6.
Referring more particularly to the drawings, a con-
tainer for hot consolidating powder constructed in accordance
with the instant invention is generally shown at 10 in FIGURE 1.
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P-319
The container 10 includes an upper die section 12 and
a lower die section 14. In the embodiment shown, the upper -
and lower die sections 12 and 14 are made from standard billet
stock of low carbon steel, such as an SAE 1008 to 1015 steel.
S Low carbon steel is a particularly desirable material for the
container 10 since it is relatively inexpensive and easy to
machine. It is noted, however, that other metals can be employed
and, in fact, other materials, such as glass or ceramic, as
long as the materials behave in the manner set forth herein.
In order to make the container 10 shown in FIGURæ
1, two pieces of low carbon steel were machined using standard
..:; .
metal cutting techniques to form the upper and lower die sections --~ ~
12 and 14. When the die sections 12 and 14 are joined along ~ ~ -
their mating surfaces, the upper and lower die sections form -
a cavity 16 having a predetermined desired configuration. The
container 10, shown in FIGURE 1, is specially adapted to form
a type of turbine disc for a jet engine. For this particular -~
turbine disc, the cavity 16 is provided with a main section
18, which is generally disc-shaped, for foxming the body of
the turbine disc and a ring portion 20 which extends generally
laterally from each side of the disc-shaped main section 18.
The size and shape of the cavity is determined in
view of the final shape of the part to be produced. Since IN-
100 powder has a tap density which is less than its theoretical
density, typically 65% of theoretical density, the cavity is
made large enough to accomodate a reduction in size sufficient
to reach approximate theoretical density in the densified com- -
pact. Additionally, the container is designed so that the size
of the densified compact after consolidation is somewhat larger
than the final part. This extra material is removed by machin-
ing to form the final part.
Before the upper and lower die sections are assembled,
.
1090~;23
P-31~
a hole 22 is drilled in one of the die sections 12 and a fill
tube 24 is inserted. The fill tube 24 for the container 10
is a piece of cold-drawn seamless steel tubing. The fill tube
24 is attached to the upper die section 12 by welding. Care
is taken to insure that the welds do not leak since the fully
assembled container must be evacuated to a level of about 5 -
10 microns prior to filling.
After the fill tube 24 has been attached, the two
die sections 12 and 14 are placed in mating relationship and
welded together. In order to facilitate welding, the outer
edges of the die sections 12 and 14 are chamfered at approxi-
mately a 45 angle. When the two die sections 12 and 14 are
properly assembled, the chamfered edges form a welding trough
26 for receiving weld material 28. Again, care is taken during
welding to insure that a hermetic seal is produced to permit
evacuation.
It is noted that the starting pieces of billet stock,
out of which the upper and lower die sections 12 and 14 were
made, are of sufficient size so that, after machining, relatively
thick walls remain. That the container includes thick walls
is evidenced by the fact that the external shape of the container
10 has no relation to the complex shape of the cavity 16 therein.
A characteristic of the thick-walled containers tested is that
the volume of the cavity is not greater than the total volume -
of the container walls. As will be further described, the use
of thick walls reduces the distortion problems associated with
thin-walled containers and permits the production of near-net
shapes.
While low carbon steel was used to make the container
10, other materials can be employed. Suitable container material
is characterized by certain physical characteristics. Using
low carbon steel as an example, in billet stock form, the
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P-319
starting material for the container 10 described herein, the
material is substantially fully dense. In other words, ignoring
production defects, such as random porosity and the like, the
steel is as close to its theoretical density as can be obtained
by standard production methods. Low carbon steel is also sub-
stantially incompressible in that its volume cannot be signifi-
cantly reduced by the application of pressure. The container
material must also be gas impervious, as is low carbon steel,
to permit hermetic sealing of the container. These character-
istics and physical properties distinguish the container materialof the instant invention from many of the materials heretofore
employed. Further distinguishing characteristics are that the
container walls are substantially uniform in composition across
a cross-section from the exterior surface to the cavity and that
the container walls are of substantially uniform density.
In addition to the foregoing, the container material
must function as a pressure transmitting medium at the temperature
and pressure necessary to consolidate the powder. In order to
achieve this result, the container material must be capable of
plastic flow at suitable pressing temperatures. Specific pres-
sing temperatures are determined, in great part, by the compo-
si~ion of the particular type of powder being compacted. Once
the pressing temperature is determined, a suitable container
material can be selected which is capable of plastic flow at
such temperature. Most metals are capable of plastic flow even
at room temperature, therefore, consideration must also be given
to the amount of pressure required to cause plastic flow in the
container material at the suitable pressing temperature. As
the temperature increases the tensile strength of metal de-
creases so that lower pressures are required to cause signifi-
cant plastic flow. In other words, in order to consolidate any
given powder, the temperature, as well as the pressure, must
--11--
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P-319
be determined. Once these two parameters are determined then
a container material is selected which is plastic, i.e. has
a low enough tensile strength, at the particular temperature
so that it will deform plastically with relative ease at the
particular pressure employed. In the case of IN-100, pressing
temperatures of between 1850 and 2200F are common. As is
well-known, low carbon steel is capable of plastic flow under
stress, and this capability increases with increasing tempera-
ture. At temperatures of 1850 to 2200F, significant plastic
flow can be induced by the application of pressures of 10,000
to 15,000 psi. While these pressures are commonly used in prac-
tice, lower or higher pressures may be used. In all cases,
the extent of plastic flow depends upon the tensile strength
of the material at the pressing temperature.
Another significant consideration is that the struc-
tural integrity of the container must be maintained during hot
consolidation. Structural integrity of a metal container is
maintained as long as the temperature of consolidation does
not exceed the melting temperature of the container material.
More precisely, the temperature should not exceed the melting
temperature of any major solid phase of the container material.
If the melting point is exceeded, the container material will
lose its strength in shear. This would lead to the destruction
of the container. Since other potential container materials,
e.g., glass, consist of a super-cooled liquid, it cannot reach
the liquid state. A glass container formed in accordance
with the instant invention would retain sufficient strength
until its viscosity becomes so low that the glass is fluid.
Therefore, as a rule, the container material must retain suf-
ficient strength at pressing temperatures to maintain the struc-
tural integrity of the container.
lU90t;~3
P-319
Another physical property of the container material
which must be taken into consideration is the material's rate
of expansion and contraction with temperature. When complex
shapes are being produced, such as those which include undercuts
and the like, it is believed that the thermal expansivity of
the container must be reasonably close to that of the material
being consolidated. If the thermal characteristics of the two
materials are widely different, stresses will be built up in
the compact during cooling which could cause fracture. While
a critical difference has not been precisely determined, it is
at least known that the difference in thermal expansivity be-
tween an SAE 1010 steel and IN-100 is not deleterious. In order
to determine the best container material for consolidating other
types of powder it may be necessary to conduct preliminary tests
to insure that the thermal characteristics of the respective
materials are compatible.
IN-100 powder and other superall~ys are normally com-
pacted at temperatures of between 1850 and 2200F and pressures
from 10,000 to 15,000 psi. Such pressures can easily be attained
in commercially available autoclaves. At temperatures of between
1850 and 2200F and pressures of 10,000 to 15,000 psi, the
walls of a thick-walled low carbon steel container act very much
like a fluid. That is, the metal can flow under stress. Due
to the fluid-like behavior of the container walls at these tem-
peratures and pressures, a hydrostatic pressure is applied tothe powder contained in the cavity. As used herein, a hydro-
static pressure is one in which the direction of the force acting
on any surface of the powder is normal to the surface. A major
problem with thin-walled containers is that, while a hydrostatic
pressure may be applied to the external surface of the container,
the container is not capable of transmitting a hydrostatic pres-
sure to the powder. The application of a hydrostatic pressure
~0906Z3
P-319
will insure that near uniform shrinkage will occur.
It has been determined that the walls of a container
are thick enough to accomplish the intended result, i.e. a hydro-
static pressure, if the exterior surface of the walls do not
closely follow the contour of the cavity. This definition is,
at best, an approximation of what is referred to as a "thick-
walled" container. In terms of a desired result, a thick-walled
container is one which has walls that are thick enough to produce
a hydrostatic pressure on the powder upon the application of
heat and pressure. By way of an example, the greatest problem
associated with thin-walled containers arise when the part to
be produced includes an annular portion, such as the ring-shaped
extensions of the part shown in FIGURE 1. A typical thin-walled
container surrounds three sides of the extension, as viewed in
cross section, leaving the interior volume vacant. This arrange-
ment causes serious distortion problems during hot consolidation.
As a minimum, the thickness of the container in the region of
the annular portion must be sufficient to substantially fill
the interior volume. When this is accomplished it can no longer
be said that the exterior surface of the container follows the
contour of the cavity. The result is that the container walls
solidly support the sides of the ring portion so that practically
uniform and undistorted shrinkage will occur.
The container 10 was processed in the following manner.
Once the die sections 12 and 14 were welded together, a vacuum
pump was connected to the fill tube 24 and the cavity 16 was
evacuated. This procedure was followed in the case of IN-100
powder to prevent contamination by atmospheric gases which would
produce undesirable oxides and nitrides and to eliminate a poten-
tial source of porosity in the resulting compact. Additionally,a vacuum within the container increases the difference in pressure
between the external and internal surfaces to facilitate
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P-319
pressing. It is noted, however, that these precautions may not
be necessary for other types of powder. Once evacuated, the
container 10 was filled with atomized IN-100 powder. During
the filling stage it was necessary to fill all portions of the
cavity 16 and to achieve the highest tap density. This was
accomplished by rotating the container and by striking the sides
of the container with a mallet. It is noted that this procedure,
although highly successful in insuring complete filling and maxi-
mum tap density, is difficult to perform on a thin-walled metal
container without bending the walls and changing the shape of
the cavity. After the container 10 was filled, the fill tube
24 was hermetically sealed by pinching it closed and welding
it. The filled and sealed container 10 was then placed in an
argon gas autoclave. The autoclave was cycled to subject the
container 10 to a temperature of approximately 1950F and a pres-
sure ranging up to 10,000 to 15,000 psi over a period of approxi-
mately two hours.
The pressure in the autoclave produced an isostatic
pressure over the surface of the container. At the pressing
temperature of 1950F, the low carbon steel had softened to the
point where it could no longer support the pressure applied
(10,000 - 15,000 psi) and plastic flow occured. The applied
pressure provided the driving force for reducing the size of
the cavity. It is possible to reduce the size of the cavity
because the cavity is filled with a compressible material, i.e.,
the less than fully dense powder. The size of the cavity con-
tinues to shrink until the powder reaches approximately full
density. As the powder densifies it also fuses by sintering
so that the compact produced was a fully dense and solid mass.
After consolidation, the container 10 was removed from
the autoclave and cooled. The container was then removed from
the densified compact by pickling in a nitric acid solution.
~0906~,3
P-319
Since IN-100 is corrosion resistant, the nitric acid solution
preferentially attacked the low carbon steel container. The
container dissolved leaving the densified IN-100 compact. While
a nitric acid solution was employed, other types of solutions
can be used. Alternatively, the container could have been re-
moved by machining or a combination of rough machining followed
by pickling.
Before the container 10 was removed from the densified
compact, the external shape of the container was measured and
recorded. After the container 10 was removed, the dimensions
of the densified compact were measured. By comparing the size
and shape of the densified compact with the original cavity, ~-
the amount and manner of shrinkage could be determined. The
deminsions of the container after compaction and of the densified -
compact axe shown in phantom in FIGURE 1. Specifically, phantom
line 30 outlines the shape of the densified compact while phantom
line 32 outlines the exterior shape of the container 10.
It is noted that surprisingly uniform shrinkage oc-
curred. Moreover, wall thickness of the container increased. - -
The fact that areas such as 34 and 36 increased in thickness,
or size, during hot compaction indicates that the direction
of force applied to the powder was hydrostatic and unrelated
to the direction of the force acting upon the surface of the con- -
tainer. This is shown schematically in FIGURE 2 which illustrates
the probable directions of the forces acting upon the powder
and the container. The direction of the force acting upon the
surface of the container, which is indicated by arrows 38, is
perpendicular to the container surface. The direction of the
force's action upon the powder, which is indicated by arrows
40, is generally perpendicular to the surface of the cavity.
The direction of forces acting on the powder, however, is not
necessarily parallel to the direction of the force action on
-16-
l~gO~;~3
P-319
the container surface. This is characteristic of a hydrostatic
pressure. This indicates that the container walls actually act
like a fluid to apply a hydrostatic pressure to the powder.
The result is a more uniform reduction in the size of the cavity.
For a number of reasons low carbon steel appears to
be the most commercially attractive material from both economic
and processing standpoints for making containers to hot consoli-
date IN-100 and other superalloy powders. Low carbon steel is
relatively inexpensive (as compared to the cost per pound of
the powder to be consolidated) and is easy to obtain. Low carbon
steel is very machinable, it can be welded easily, and the fin-
ished container can withstand significant abuse. It is pointed
out, however, that thick-walled containers in accordance with
the instant invention may be made of other metals and other
materials. Glass and ceramics are examples of such materials.
The important result is that plastic flow coupled with suffi-
ciently thick container walls will produce a hydrostatic pressure
upon the powder.
It is also noted that the invention is not limited
to producing a cavity by machining. Other well-known metalworking
techniques, such as, casting or forging may be employed to produce
the container. For example, a cast container can be produced
by using an expendable core having the shape of the desired
cavity. After the metal is cast around the expandable core,
the core is removed, such as by leaching. A two-part container
could also be produced by a forging process. The only drawback
with forging is that undercuts could not be produced such as
are possible with machining or casting.
A unique method for assembling a container for pro-
ducing a part of extremely complex shape is shown in FIGURES
3 through 5. The desired part is shown generally at 42 in FIGURE
5. It is noted that this is a rather complex turbine disc which
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includes a number of undercuts. In order to produce a densified
compact which can be machined to produce the part 42 shown in
FIGURE 5, a cavity, generally indicated at 44, is formed in a
thick-walled container having the shape shown in FIGURE 3. It
should be apparent, that it would be difficult, if not possible,
to machine a cavity of such complex shape in a two-section con-
tainer, such as the one previously described and shown in FIGURE
1. In order to produce the part shown in FIGURE 5, the cavity ::
44 includes a generally disc-shaped portion 46 and a generally ~-
ring-shaped portion 48 extending substantially laterally from
the disc-shaped portion 46. In addition, the ring-shaped por-
tion 48 angles inwardly in such a manner that it would be diffi- .-
cult to machine. Hence, the container is made in three sections. : .
Specifically, the container includes a first main section 50,
15 a second main section 52, and an intermediate section 54. The ~; --
first main section 50 and the intermediate section 54 includes
surfaces 56 and 58 which generally define the disc-shaped portion
46 of the cavity 44. The second main section 52 and the inter-
mediate section 54 include surfaces 60 and 62 which define the
ring-shaped portion 48. These three sections are machined sep-
arately and then fitted together to form the complex cavity 46.
As in the first embodiment of the container, the first
and second main sections 50 and 52 include joinable mating sur-
faces. The outer edges of these surfaces are chamfered to form
a weld trough 64 for receiving weld material 66. A hole 68
is drilled in one of the main sections, in this case, the first
main section 50 for receiving a fill tube 70 which is attached
by welding. As shown in F~GURE 3, the intermediate section
54 is supported between the first and second main section 50 and
52 by cooperating the interfitting means which locate and support
the intermediate section 54. The cooperating interfitting means
includes an extension 72 of the cylindrical portion of the
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intermediate section 54 which fits into a cylndrical bore 74
in the second main section 52 and also an extension 76 of the
cylndrical portion of the first main section 50 which seats in
a cylindrical bore 78 in the intermediate section 54.
This container was processed in generally the same
manner as the first container. After hot colsolidation the den-
sified compact recovered had the shape shown generally at 80
in FIGURE 4. This densified compact was then machined to the
final shape shown in FIGURE 5. It is particularly pointed out
that the final part 42 was produced without a forging operation
and with minimal scrap.
The containers described above were subjected to heat
and pressure by using an argon gas autoclave. It is noted, how-
ever, that other means may be employed to apply heat and pressure.
One procedure which has been developed by the inventor herein
includes pressing the container between the dies of a press. -
In order to consolidate the powder using a press, a ~-
standard mechanical or hydraulic press is outfitted with upper
and lower dies similar to the upper and lower dies 82 and 84
shown in FIGURE 7. The lower die 84 includes a cavity for re-
ceiving a preheated, powder-filled container. The upper die
82 which is mounted on the ram of the press includes an exten-
sion 88 which enters the recess 86 to engage and apply pressure
to the container. Since the container material has been pre-
heated to a temperature at which plastic flow will occur rela-
tively easily and since the lower die 86 restrains the container,
the container material will act like a fluid to subject the
powder to a hydrostatic pressure. Since the powder in the con-
tainer is at less than full density, the pressure of the container
material will cause the powder to densify. Densification will
proceed until the powder achieves full density. At this point,
- the entire mass, that is, the container material and the powder,
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is at full density. The container is then removed from the
lower die 84 by a suitable stripping operation and the container
material is removed from the densified powdex compact.
It is noted that the side walls 90 of the recess 86
S in the lower die 84 are tapered and that the sides of the con- ~-
tainer 96 are provided with a corresponding draft angle to facili- ~ -
tate ejection of the container from the lower die 84 after
pressing. The upper die 82 is also tapered to correspond to
the taper of the lower die 84.
In the event that a mechanical press is employed,
damage to the press could be caused if the powder reaches full
density before the ram reaches the end of its downward stroke -
because the ram would be working against a fully dense and in-
compressible mass. This could cause breakage of the press crank `
15 or at least jamming of the press. Obviously, this problem is -
not presented in a hydraulic press since its stroke terminates
upon reaching a predetermined pressure.
In order to prevent damage to a mechanical press, the -
upper and lower dies 82 and 84 are designed to permit controlled
escape of container material from between the dies when the pres-
sure exceeds a desired maximum. In other words, a gap is pro-
vided between the sides 90 of the recess 86 and the lower die
84 and the sides 92 of the extension 88 of the upper die 82 to
permit formation of a flash under conditions of excessive pressure.
In order to insure that the pressure experienced by the container
is sufficiently high to achieve full densification of the powder,
it may be necessary to force the escaping container metal to - -
follow a tortuous path. For example, the sides 92 of the upper
die 82 may be continued to form a curved surface 94 which would
resist the flow of container metal by forcing it to reverse its
direction of flow thus extending the path of the material.
`~ The additional surface also increases the total frictional
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resistance experienced by the material. In any event, the upper
and lower dies are designed to relieve excess pressure by the
controlled escape of container material.
A container designed particularly for consolidating
S the powder using a press is shown generally at 96 in FIGURE 6.
The internal cavity of the container illustrates the rather com-
plicated shapes which can be produced by this method. It should
be apparent, therefore, that near net shapes can easily be pro-
duced. The container 96 includes an upper section 98 and a
lower section 100 which have been machined from a low carbon
steel. A core 102 is also machined from the same material and ~ -
fitted between the upper and lower sections 98 and 100. As with
the containers described above, the upper and lower sections
98 and 100 are welded together at their mating surfaces as in-
dicated by the weldment 104.
In order to fill the cavity defined by the walls of
the container 96 with powder, the upper section 98 is provided
with one or more passageways 104 which communicate with the
cavity. The passageway~ 104 extend through a conical-shaped
portion 106 formed in the upper section 98 of the container and -
merge in a single opening 108. A fill tube 110 is welded to -; ;
the upper section 98 of the container at the opening 108 for
conducting powder into the passageways 104. The fill tube 110
is also used to connect a vacuum pump to the container 96 for -~
evacuating the cavity prior to filling with powder.
After the container 96 has been evacuated and filled
with powder, the fill tube 110 is closed by crimping the end
of the fill tube as at 112.
When the powder is to be consolidated by pressing in
a press, it is necessary to protect the fill tube 110 from
damage. Since the contents of the container 96 are under a
vacuum, damage to the fill tube 110 could result in a leak which
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would cause contamination of the powder. In order to prevent
damage to the fill tube 110 a protective shield, generally in-
dicated at 112, is welded to the container and surrounds the
fill tube 110. The protective shield 112 comprises a sleeve
114 which is placed over the fill tube 110 and is welded to the
container 96. In order to provide additional support the vacant
space within the sleeve 114 may be filled with powder. A plug
116 is then welded across the entrance to the sleeve 114.
The upper die 82 includes a special configuration
which corresponds to the exterior shape of the upper portion
of the container 96. Specifically, it includes a tapered recess
118 which corresponds in size and shape to the conical portion -
106 of the container 96. An extension 120 of the tapered recess
is also formed to receive the protective shield 112 which is
located on top of the container 96. It is noted that the exten-
sion 120 is also tapered and that the protective shield 112 is
provided with a suitable draft angle for facilitating separation
of the upper die 82 from container 96.
It is not essential that the container 96 include a
domed portion 106. As an alternative the container 96 could
have the shape indicated by the dotted line 121. The container
shape shown, however, obviously re~uires less material than the
alternative and, for this reason, is more desirable.
A typical procedure for compacting powder using a press
includes the following steps. After the container 96 is fabri-
cated a vacuum pump is connected to the fill tube 110 and the
cavity of the container is pumped down to a level of about 10
microns. After evacuation the container is filled with powder
whi~ maintaining the cavity under vacuum. This can be accom-
plished by using a tee-type connection at the fill tube 110
wherein one branch of the tee is connected to the vacuum pump
while the other branch is connected to a supply of powder.
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After filling,the fill tube 110 is closed. This may be accom-
plished by crimping the fill tube 110 and welding the crimped
end.
As described above, the protective shield 112 is then
attached to the container 96 so that it surrounds the fill tube
110 .
The container 96 is then heated in a furnace to a tem-
perature at which the powder will densify. The container material
is selected so that at the appropriate densification temperature
the container material will be capable of plastic flow when sub-
jected to a pressure sufficient to cause densification of the
powder. It has been found that for most applications, the con-
tainer and powder are heated to a temperature of between l,700F
and 2,300F. The specific temperature is selected in view of
the alloy composition of the powder being compacted. Suitable
densification temperatures are well known for common alloys.
Within this range of temperatures a low carbon steel container
will maintain structural integrity, but is capable of plastic
flow at pressures exceeding about 5,000 psi. The heated container
is then transferred to a press for consolidation of the powder.
A test part was made from titanium powder using a con-
tainer having the configuration of container 96 by preheating
to a temperature of about 1,750F and applying a pressure of
about 15,000 psi by means of a standard mechanical press out-
fitted with tools similar to those shown in FIGURE 6. Afterheating in the furnace for a time sufficient to obtain a uniform
temperature throughout, the container was conveyed to a press
fitted with dies having the configuration of the upper and lower
dies 82 and 84. The press was then cycled through a single
stroke. As described above, because the container is restrained
by the lower die 82, the heated container material flows plas-
tically and subjects the powder to a hydrostatic pressure which
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causes it to densify. Thereafter, the container 9~ was ejected
from the lower die 82 and cooled. The container was then removed
from the densified powder compact.
Consolidating the powder using a press rather than
an autoclave is advantagous since cycle time at maximum tempera-
ture can be reduced significantly. The typical cycle time in
an autoclave can easily exceed four hours from loading to un-
loading while the cycle time for a press is measured in minutes.
Moreover, autoclaves which operate in the 15,000 psi range are
sophisticated pieces of equipment and are quite expensive. There-
fore, the use of mechanical and hydraulic presses significantly
simplify the consolidation process.
The invention has been described in an illustrative
manner, and it is to be understood that the terminology which has
been used is intended to be in the nature of words of description
rather than of limitation.
Obviously, many modifications and variations of the
present invention are possible in light of the above teachings.
It is therefore, to be understood that the invention may be prac-
ticed otherwise than as specifically described herein and yetremain within the scope of the appended claims.
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