U.S. patent application number 12/409928 was filed with the patent office on 2009-10-08 for joining of difficult-to-weld materials and sintering of powders using a low-temperature vaporization material.
This patent application is currently assigned to Energy & Environmental Research Center Foundation. Invention is credited to Carsten Heide, John Hurley.
Application Number | 20090252637 12/409928 |
Document ID | / |
Family ID | 41133453 |
Filed Date | 2009-10-08 |
United States Patent
Application |
20090252637 |
Kind Code |
A1 |
Hurley; John ; et
al. |
October 8, 2009 |
JOINING OF DIFFICULT-TO-WELD MATERIALS AND SINTERING OF POWDERS
USING A LOW-TEMPERATURE VAPORIZATION MATERIAL
Abstract
The present invention discloses a process for sintering
particles using a sintering aid. The sintering aid can be brought
into contact with a plurality of particles to be sintered such that
a mixture of the particles and the sintering aid is provided. The
mixture of particles and the sintering aid is heated and at least
part of the sintering aid is vaporized. Sintering of the particles
to form a sintered component followed by cooling of the sintered
component can complete the process, or in the alternative, a
subsequent heating step or steps can be included whereby additional
vaporization of the sintering aid can occur.
Inventors: |
Hurley; John; (Grand Forks,
ND) ; Heide; Carsten; (Grand Forks, ND) |
Correspondence
Address: |
GIFFORD, KRASS, SPRINKLE,ANDERSON & CITKOWSKI, P.C
PO BOX 7021
TROY
MI
48007-7021
US
|
Assignee: |
Energy & Environmental Research
Center Foundation
Grand Forks
ND
|
Family ID: |
41133453 |
Appl. No.: |
12/409928 |
Filed: |
March 24, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12327385 |
Dec 3, 2008 |
|
|
|
12409928 |
|
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|
|
60991966 |
Dec 3, 2007 |
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Current U.S.
Class: |
419/32 ; 419/35;
419/36 |
Current CPC
Class: |
B22F 1/025 20130101;
B22F 3/1003 20130101; B22F 7/06 20130101 |
Class at
Publication: |
419/32 ; 419/36;
419/35 |
International
Class: |
B22F 1/02 20060101
B22F001/02; B22F 1/00 20060101 B22F001/00 |
Goverment Interests
STATEMENT REGARDING GOVERNMENT INTERESTS
[0002] This invention was made with government support under
Cooperative Agreement No. DE-FC26-05NT42465 awarded by the U.S.
Department of Energy. The government has certain rights in the
invention.
Claims
1. A process for sintering particles, the process comprising:
providing a plurality of particles; providing a sintering aid;
bringing the sintering aid into contact with the plurality of
particles and making a mixture of particles and sintering aid;
heating the mixture of particles and sintering aid whereby
vaporizing at least part of the sintering aid and sintering of the
particles to form a sintered component occurs; and cooling of the
sintered component.
2. The process of claim 1, wherein the sintering aid is selected
from the group consisting of a low melting point metal and a low
melting point alloy.
3. The process of claim 2, wherein the sintering aid contains
zinc.
4. The process of claim 3, wherein the sintering aid is a zinc
alloy.
5. The process of claim 1, wherein the plurality of particles is
metallic.
6. The process of claim 5, wherein the plurality of particles is
metallic alloy particles.
7. The process of claim 1, wherein bringing the sintering aid into
contact with the plurality of particles is coating the plurality of
particles with the sintering aid.
8. The process of claim 1, wherein bringing the sintering aid into
contact with the plurality of particles is mechanically mixing the
plurality of particles with a plurality of sintering aid
particles.
9. The process of claim 1, wherein bringing the sintering aid into
contact with the plurality of particles is passing a vapor of the
sintering aid past the plurality of particles whereby the vapor
comes into contact with a surface of at least part of the plurality
of particles.
10. The process of claim 1, wherein bringing the sintering aid into
contact with the plurality of particles is passing a liquid of the
sintering aid past the plurality of particles whereby the liquid
comes into contact with a surface of at least part of the plurality
of particles.
11. The process of claim 1, wherein the mixture of particles and
sintering aid are heated while under an external pressure.
12. The process of claim 11, wherein the heating continues and the
external pressure is removed after the forming of sintered
component occurs.
13. The process of claim 12, wherein at least part of the sintering
aid is vaporized after the external pressure is removed.
14. A process for sintering particles, the process comprising:
providing a plurality of metallic particles; providing a sintering
aid in the form of a low melting material; bringing the sintering
aid into contact with the plurality of particles and making a
mixture of particles and sintering aid; heating the mixture of
particles and sintering aid; vaporizing at least part of the
sintering aid; sintering the particles to form a sintered
component; and cooling of the sintered component.
15. The process of claim 14, wherein the sintering aid contains
zinc.
16. The process of claim 14, wherein the sintering aid is in the
form of particles containing zinc.
17. The process of claim 14, wherein bringing the sintering aid
into contact with the plurality of particles is mechanically mixing
the plurality of particles with the sintering aid particles
containing zinc.
18. The process of claim 14, wherein bringing the sintering aid
into contact with the plurality of particles is coating the
plurality of particles with the sintering aid.
19. The process of claim 14, wherein bringing the sintering aid
into contact with the plurality of particles is passing a vapor of
the sintering aid past the plurality of particles whereby the vapor
comes into contact with a surface of at least part of the plurality
of particles.
20. The process of claim 19, further including heating the sintered
component in a vacuum after it has cooled.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation-in-part of U.S. patent application
Ser. No. 12/327,385 filed Dec. 3, 2008, which claims the benefit
under 35 U.S.C. .sctn. 119(e) of U.S. Provisional Patent
Application No. 60/991,966 entitled "Joining of Difficult-to-Weld
Materials," filed Dec. 3, 2007, the disclosure of which is hereby
incorporated herein by reference.
FIELD OF THE INVENTION
[0003] The present invention relates to a process for joining
materials and, in particular, to a process for joining
difficult-to-weld materials. In addition, the present invention
relates to a process for sintering powders using a sintering
aid.
BACKGROUND OF THE INVENTION
[0004] The manufacture of electrical power plants, petrochemical
refineries, and other industrial facilities requires joining of
various components. Joining of such components can be performed
using welding, adhesives, threaded joints, flanges that can be
bolted together, and the like. In many instances, the welding of
components provides a sound engineering and economical method for
joining said components, and in fact, the ability of a material to
be welded can have a great impact on the material's commercial
viability.
[0005] With demands for increasing the efficiency of electrical
power plants, gas turbine engines, and the like, the need for the
use of materials that can withstand ever-increasing high
temperatures continues. For example, dispersion-strengthened alloys
are known to exhibit excellent high-temperature properties and have
shown potential for use in many high-temperature applications.
Likewise, nickel-based alloys strengthened by internal
precipitants, such as gamma prime, are currently used in the hot
sections of gas turbines. However, alloys such as these can present
problems with respect to traditional fusion welding techniques
since the melting of the base material results in the destruction
of the microstructure which provides the excellent high-temperature
properties.
[0006] Heretofore, joining techniques for such alloys have included
diffusion bonding, friction welding, and other solid-state welding
processes. Diffusion bonding is a process wherein two nominally
flat interfaces are joined at an elevated temperature using an
applied pressure upon the interfaces to be joined. The diffusion
bonding process affords the joining of dissimilar materials and/or
similar materials wherein the melting of the base material has
detrimental effects. However, the presence of oxide layers at the
joining surfaces can affect the quality of the joint, thereby
making sound, reproducible joints difficult to obtain.
[0007] A modified form of diffusion bonding is known as transient
liquid phase (TLP) diffusion bonding wherein liquid-state diffusion
bonding relies on the formation of a liquid phase provided by a
bonding film that is inserted between the interfaces to be joined
during an isothermal bonding cycle. The liquid phase subsequently
diffuses into the base material and eventually solidifies as a
consequence of continued diffusion into the bulk material at the
isothermal temperature. The liquid phase enhances dissolution
and/or disruption of any oxide layer that may be present on the
interfaces to be joined and, thereby, promotes intimate contact
between said interfaces, possibly dissolving a portion of the metal
present at the joint faces into the joint. As such, the presence of
the bonding film and thus the liquid phase reduces pressure and
time that may be required for diffusion bonding. However, methods
to TLP diffusion bond dispersion-strengthened high-temperature
alloys and gamma prime nickel-based alloys have met with limited
success. Therefore, a process for bonding of such alloys and/or a
bond foil having a composition that affords improved bond joints is
desirable. In addition, a clamping device that affords the
application of applied stress to the components to be joined is
desirable.
[0008] The sintering of powders also includes many aspects that are
present during the diffusion bonding of materials. For example,
sintering involves heating of powders below their melting point
until the particles adhere to each other. In addition, an oxide
layer can be present on the surface of the particles and cause
difficulties in proper adherence therebetween.
[0009] Various components can be advantageously made using a
sintering process. For example, sintered bronze can be used as a
bearing material, since porosity within the sintered component
allows for lubricants to flow therethrough and remain captured
therein. As such, a self-lubricating structure can be provided. In
addition, materials having a high melting point such as tungsten
can be sintered into a suitable shape when alternative
manufacturing techniques are not available. In some instances, very
low porosity can be obtained, and additional heating and/or
pressurizing steps can be included in order to obtain a more dense
structure. As with diffusion bonding, a process that affords for
improved sintering of powders is desirable.
SUMMARY OF THE INVENTION
[0010] The present invention discloses a process for joining
materials. The process can include providing a first component with
a first joint face and a second component with a second joint face.
The first joint face and the second joint face can be prepared for
bonding, and a bonding layer can be provided. The first component,
second component, and bonding layer can be assembled such that the
first joint face is oppositely disposed from the second joint face
with the bonding layer located at least partially therebetween. In
addition, a force can be applied to the assembly of the first
component, second component, and bonding layer such that the first
joint face is compressed against the second joint face with the
bonding layer therebetween. In some instances, the bonding layer
can be a bonding foil, and the bonding foil may or may not be a
zinc foil.
[0011] A thermal treatment can be applied to the first joint face
and the second joint face with the bonding layer therebetween,
thereby affording for at least part of the bonding layer material
to melt, the first joint face coming into intimate contact with the
second joint face and forming a bond interface, and the first
component being bonded to the second component across the bond
interface, with at least part of the bonding layer vaporizing
during the process. In addition, an atmosphere surrounding the
first joint face and the second joint face with the bonding layer
therebetween can be controlled before, during, and/or after the
thermal treatment. In some instances, the thermal treatment can be
a multiple-step thermal treatment to the first joint face and the
second joint face with the bonding layer therebetween.
[0012] In addition to disclosing a process for joining materials, a
process for joining particles is also disclosed. In particular, a
process for sintering particles is provided where a plurality of
particles are provided along with a sintering aid. The sintering
aid is brought into contact with the plurality of particles such
that a mixture of the particles and the sintering aid is provided.
The mixture of particles and the sintering aid is heated, and at
least part of the sintering aid is vaporized. Sintering of the
particles to form a sintered component followed by cooling of the
sintered component can complete the process, or in the alternative,
a subsequent heating step or steps can be included whereby
additional vaporization of the sintering aid can occur.
[0013] The sintering aid can be a low melting point metal and/or a
low melting point alloy. In addition, the sintering aid can be
brought into contact with the plurality of particles by mixing the
plurality of particles to be sintered with a plurality of sintering
aid particles, coating the plurality of particles with the
sintering aid, passing a vapor of the sintering aid past the
plurality of particles, and/or passing a liquid of the sintering
aid past the plurality of particles. It is appreciated that with
the sintering aid in contact with the plurality of particles,
heating of the mixture of particles and sintering aid before,
during, and/or after the sintering process can at least partially
melt the sintering aid and disrupt any oxide film that is present
on the surface of the plurality of particles. Furthermore, melting
of the sintering aid can enhance wetting between surfaces of
adjacent particles, thereby increasing sintering kinetics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a schematic diagram illustrating a process
according to an embodiment of the present invention;
[0015] FIG. 2 is a side view of a clamping device that can be used
with an embodiment of the present invention;
[0016] FIG. 3 is a schematic diagram illustrating sintering of a
plurality of particles;
[0017] FIG. 4 is a schematic diagram illustrating the presence of
an oxide film on surfaces of adjacent particles;
[0018] FIG. 5 is a schematic diagram illustrating the oxide film
shown in FIG. 4 having been at least partially disrupted;
[0019] FIG. 6A is a schematic diagram illustrating particles of a
sintering aid mixed with particles to be sintered;
[0020] FIG. 6B is a schematic diagram illustrating particles to be
sintered having been coated with a sintering aid;
[0021] FIG. 6C is a schematic diagram illustrating vapor of a
sintering aid passing through a plurality of particles;
[0022] FIG. 7 is a schematic diagram illustrating a process for
sintering particles using a sintering aid;
[0023] FIG. 8A is a schematic diagram illustrating percent porosity
as a function of time during a sintering process with and without
the use of a sintering aid; and
[0024] FIG. 8B is a schematic diagram illustrating sintering
kinetics during a sintering process with and without the use of a
sintering aid.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0025] The present invention discloses a process for joining
materials using diffusion bonding, TLP diffusion bonding, and
modifications thereof. As such, the process has utility as a
process for joining materials and, in particular, for joining
difficult-to-weld materials.
[0026] The process includes providing components to be joined, for
example, a first component having a first joint face and a second
component having a second joint face. The first joint face and/or
the second joint face can be prepared for bonding to each other. In
some instances, the first joint face and/or the second joint face
are machined. Optionally, the first joint face and/or the second
joint face can be polished or otherwise finished in addition to, or
in place of, the machining.
[0027] A bonding layer can be provided. In some instances, the
bonding layer is a bonding foil. The bonding foil can be a metallic
foil such as a zinc foil; the term "zinc foil" for the purposes of
the present invention includes foil made from high-purity zinc,
commercial pure zinc, zinc alloys, and the like. For example, the
zinc foil can be made from zinc alloyed with aluminum, copper,
lead, magnesium, nickel, iron, and/or tin. It is appreciated that
the bonding layer can be a paste that is applied to the first joint
face and/or second joint face or a coating that has been applied to
one of the joint faces. The coating can be applied by any method
known to those skilled in the art, illustratively including
sputtering, chemical vapor deposition, physical vapor deposition,
and the like.
[0028] The first component, second component, and bonding layer can
be assembled such that the first joint face is oppositely disposed
from the second joint face and at least part of the bonding layer
is located therebetween. A force can be applied to the assembly of
the first component and the second component with the bonding layer
therebetween and afford for the first joint face to be compressed
against the second joint face and the bonding layer.
[0029] The first joint face and the second joint face with the
bonding layer located therebetween can be subjected to a thermal
treatment, the thermal treatment affording for at least part of the
bonding layer material to melt and the first joint face bonding to
the second joint face, with at least part of the bonding layer
vaporizing, possibly after diffusing through the structure being
joined. In some instances, the thermal treatment can be a
multiple-step thermal treatment, or in the alternative, a
single-step thermal treatment where the temperature of the joint
region is continuously increased to a final temperature. In the
instance of a multiple-step thermal treatment, the thermal
treatment can include a first step that includes heating the first
joint face and the second joint face with the bonding layer
therebetween to a first temperature, followed by holding at the
first temperature for a predetermined amount of time, and a second
step that includes heating to a second higher temperature followed
by holding at the second temperature for a predetermined amount of
time.
[0030] The first temperature may or may not be higher than the
melting point or solidus temperature of the bonding layer, and the
second temperature may or may not be higher than a
recrystallization temperature of the first component and/or the
second component. In this manner, the first temperature may result
in the melting of the bonding layer, and the second temperature may
result in grain growth across a bond interface between the first
and second components. It is appreciated that melting of the
bonding layer can afford for wetting of the first and/or second
joint face and/or disrupting of any surface oxide on the first
joint face, second joint face, and/or bonding layer, possibly
dissolving a portion of the metal present at the joint faces into
the joint.
[0031] It is appreciated that grain growth across the bond
interface can result in improved bond joint quality and strength.
It is further appreciated that cold-working of the first and/or
second component proximate the first joint face and/or second joint
face, respectively, can enhance grain growth across the bond
interface. In the alternative, the bonding of components having
different compositions can afford for one or more concentration
gradients across the bond interface, the concentration gradient(s)
enhancing grain growth across the bond interface and cold-working
of the first and/or second component not being required.
[0032] In some instances, an atmosphere surrounding the first joint
face and the second joint face with the bonding layer therebetween
can be controlled. The atmosphere can be controlled by purging with
an inert gas and/or by pulling or drawing a vacuum on a chamber in
which the first joint face and the second joint face with the
bonding layer therebetween is contained within. The inert gas can
include a reducing gas such as hydrogen, for example argon with 5
volume percent hydrogen. It is appreciated that terms such as
"draw," "drawing," "pull," "pulled," and "pulling" are terms of art
when used in the context of a vacuum and refer to the removal of
atoms and/or molecules from an enclosed container, i.e., a chamber,
and the establishment of a pressure that is less than atmospheric
pressure therewithin.
[0033] Control of the atmosphere surrounding the first joint face
and the second joint face with the bonding layer therebetween can
be combined with the multiple-step thermal treatment. For example
and for illustrative purposes only, a chamber containing the first
joint face and the second joint face with the bonding layer
therebetween can be purged with an inert gas before and/or during
the first step, followed by establishing a vacuum before and/or
during the second step.
[0034] During at least part of the thermal treatment, with or
without the atmosphere control, contact between the interfaces to
be bonded is sufficient such that diffusion takes place
therebetween, and a sound metallurgical bond is provided. As stated
above, the thermal treatment can include a step that affords grain
growth across the bond interface, the bond interface being defined
herein as an interface between two components to be joined, across
which diffusion occurs to form a bonded joint. In this manner, a
process wherein joints having acceptable room-temperature and/or
high-temperature properties is provided.
[0035] Components that can be joined using the process disclosed
herein range from typical metals and alloys used for fabricating
structures to difficult-to-weld metals and alloys. For example and
for illustrative purposes only, materials such as the commercial
alloys MA956, PM2000, CM247LC, APMT, and the like can be joined to
themselves and/or to other materials. It is appreciated that the
MA956 alloy is an oxide dispersion-strengthened (ODS) alloy having
a nominal chemical composition of
Fe-20Cr-4.5Al-0.5Ti-0.5Y.sub.2O.sub.3 (wt %); the PM2000 alloy is
also an ODS alloy having a nominal chemical composition of
Fe-20Cr-5.5Al-0.5Ti-0.5Y.sub.2O.sub.3 (wt %); the CM247LC alloy is
a gamma prime-strengthened alloy having a nominal composition of
Ni-8.1Cr-9.2Co-0.5Mo-9.5W-3.2Ta-0.7Ti-5.6Al-0.01Zr-0.01B-0.07C-1.4Hf
(wt %); and the APMT alloy is a dispersion-strengthened powder
metallurgy alloy having a nominal composition of Fe-21Cr-3Mo-5Al
(wt %). It is further appreciated that these alloys, and other
alloys joined by the process disclosed herein, can have other
incidental impurities and additional alloying elements.
[0036] The process can also include the use of a fixture device for
holding the components to be joined in an appropriate orientation
with a desired stress applied thereon.
[0037] Similar to the use of a bonding layer to enhance the joining
of two components, a sintering aid can be used to enhance the
joining of adjacent particles during a sintering process. A process
for sintering particles can include providing a plurality of
particles and providing a sintering aid. The sintering aid can be
brought into contact with the plurality of particles and thereby
result in a mixture of the particles and the sintering aid. Heat
can be applied to the mixture of particles and sintering aid,
thereby affording for vaporization of at least part of the
sintering aid.
[0038] The sintering aid can be selected from a low melting point
metal, a low melting point alloy, and the like. In some instances,
the sintering aid contains zinc and/or a zinc alloy. The mixture of
the plurality of particles to be sintered and the sintering aid can
be in the form of the particles to be sintered mechanically mixed
with a plurality of sintering aid particles. In addition, the
plurality of particles can be at least partially coated with the
sintering aid, and/or a vapor and/or a liquid of the sintering aid
can be afforded to flow past or through the plurality of
particles.
[0039] In some instances, the mixture of particles to be sintered
and the sintering aid can be heated after a sintered component has
been made and cooled, thereby affording for additional vaporization
of the sintering aid from the sintered component. The heating of
the sintered component can occur in a controlled environment, e.g.,
a vacuum, an inert gas atmosphere, etc., which may or may not
enhance the vaporization of the sintering aid.
[0040] In FIG. 1, a process for joining difficult-to-weld materials
is illustrated generally at reference number 5. The process 5
includes providing components to be joined at step 10, wherein the
components can be made from any alloy or combination of alloys,
metals, etc., illustratively including oxide or other ceramic
dispersion-strengthened alloys, directionally solidified alloys,
internal precipitate-strengthened alloys, solid
solution-strengthened alloys, castings, and the like.
[0041] Included in the process 5 is a bonding layer at step 20. The
bonding layer can be a zinc foil or, in the alternative, made from
a material not containing zinc so long as the material has a
tendency to vaporize during thermal treatment of a joint region as
taught below. For example, zinc has a vapor pressure of 0.13
kilopascal (kPa) (1 torr) at 487.degree. C., 101.3 kPa (760 torr)
at 907.degree. C. and 10,132 kPa (7600 torr) at 1180.degree. C. In
addition, zinc has a melting point of 420.degree. C., which is less
than, or about the same as, other iron-zinc or nickel-zinc alloy or
intermetallic melting temperatures. As such, a zinc foil will melt
before or at the same temperature as other possible zinc-containing
compounds in an iron-based or nickel-based component, and
vaporization of at least part of the zinc foil will reduce or
eliminate it from the joined structure. Vaporization may occur from
near the joint itself, or from the surface of the joined structure
after the zinc has diffused through the structure.
[0042] It is appreciated that other low boiling point elements can
be used for the bonding layer. For example and for illustrative
purposes only, foils can be made primarily from elements such as
arsenic (T.sub.b=610.degree. C.), cadmium (T.sub.b=765.degree. C.),
cesium (T.sub.b=690.degree. C.), magnesium (T.sub.b=1110.degree.
C.), mercury (T.sub.b=357.degree. C.), phosphorus
(T.sub.b=283.degree. C.), polonium (T.sub.b=960.degree. C.),
potassium (T.sub.b=770.degree. C.), rubidium (T.sub.b=700.degree.
C.), selenium (T.sub.b=685.degree. C.), sodium (T.sub.b=890.degree.
C.), sulfur (T.sub.b=445.degree. C.), and/or tellurium
(T.sub.b=962.degree. C.) where T.sub.b is the boiling point of the
given element at atmospheric pressure. It is appreciated that some
of these materials are considered poisonous, fire hazardous, and/or
radioactive and thus may limit their use as a bonding layer but
have in common with zinc a relatively low vaporization
temperature.
[0043] After the components and the bonding layer have been
provided, the components and said bonding layer are assembled at
step 30. It is appreciated that the components to be joined can
have joint faces that have been properly prepared, for example, by
machining and/or polishing or other surface preparation, and the
bonding layer can be dimensioned to fit between the joint faces.
The bonding layer can have a thickness between 1 .mu.m and 1 mm,
inclusive. In some instances, the bonding layer has a thickness
between 5 .mu.m and 200 .mu.m and in other instances can be between
20 .mu.m and 50 .mu.m. Assembly of the components with the bonding
layer at step 30 includes bringing the joint faces to be bonded
into intimate contact with the bonding layer, the joint faces being
oppositely disposed from each other with the bonding layer
therebetween. In addition, pressure or an applied stress can be
applied to the components such that the interfaces to be bonded and
the bonding layer are under compression.
[0044] The pressure can assist in the breaking up or disruption of
any oxide scale that is present on the first joint face, second
joint face, and/or bonding layer and possibly dissolving a portion
of the metal present at the joint faces into the joint. It is
appreciated that the pressure can be applied with a fully
articulated press or assembly device that affords for the first
joint face and the second joint face to be easily aligned with each
other and thus provide for intimate contact therebetween once the
bonding layer has melted and diffused into the components and/or
been vaporized away from the joint region.
[0045] An assembly device shown generally at reference numeral 80
in FIG. 2 can be included to assist in the assembly of the
components to be joined. As shown in FIG. 2, the assembly device 80
can include a body 100 having a top portion 110 and a bottom
portion 120. Within the body 100 can also be at least one cavity
130 that affords for the placement of a first component 210 to be
joined to a second component 220. A bonding foil 230, for example,
a zinc foil, can be placed between the first component 210 and the
second component 220 as illustrated in the figure. The first
component 210 can have a first joint face 212 and the second
component 220 can have a second joint face 222. As stated above,
the first joint face 212 and/or the second joint face 222 can be
prepared for bonding to the oppositely disposed joint face.
[0046] Proximate to the top portion 110 is a pressure application
member 112. In some instances, the pressure application member 112
can be a threaded bolt, screw, and the like. The pressure
application member 112 can have a pressure end 114 that can be
moved in a back and forth direction 1. In addition to the body 100
and the pressure application member 112, a hemispherically shaped
cap 140 can be placed between the pressure end 114 of the pressure
application member 112 and the first component 210 to be joined.
Likewise, a second hemispherical cap 150 can be placed between the
member 100 and the second component 220 to be joined. It is
appreciated from FIG. 2 that the cap 140 and the cap 150 are placed
at distal ends or locations from first joint face 212 and second
joint face 222, respectively. It is also appreciated that the
pressure end 114 of the pressure application device 112 has a shape
that is complementary with the hemispherical cap 140 as illustrated
in FIG. 2. The body 100 can also have a machined region 122 that is
complementary to the spherical portion of the hemispherical cap
150.
[0047] The hemispherical caps 140 and 150 can be made from any
material known to those skilled in the art, illustratively
including high-temperature alloys, alumina, silica, and the like.
In some instances, the hemispherical cap 140 and 150 has a
hemisphere diameter that is generally equivalent to the diameter of
a rod, tube, and the like that is to be joined; however, this is
not required. If the components to be joined have a cross-sectional
polygon shape such as a square, rectangle, and the like, the cap
140 and cap 150 can be manufactured such that one end is
complementary to the pressure end 114 and/or machined region 122
and the other end is complementary to the components to be
joined.
[0048] The arcuate surface of the hemispherical cap 140 and/or 150
affords for the components to fully articulate, or move,
independently from the body 100 and the pressure applied by the
pressure application device 112. This articulation or movement can
be critical since the joint diffusion zone that results in the bond
can be relatively thin and the interfaces to be joined are
preferably in intimate contact along the complete surfaces of the
joint. Without the articulation, the joints can become cocked,
misaligned, etc., with force on one portion being greater than
another portion and the interfaces to be joined not being parallel
with each other.
[0049] It is appreciated that the member 100 and the pressure
application device 112 can be made from any material known to those
skilled in the art for use at generally high temperatures, such as
molybdenum, niobium, other metals having high-temperature strength,
high-temperature nickel-based alloys, high-temperature iron-based
alloys, high-temperature cobalt-based alloys, ceramics, metal
matrix composites, and the like.
[0050] Returning to FIG. 1, after the components to be joined have
been assembled at step 30, the atmosphere surrounding the first
joint face 212 and the second joint face 222 with the bonding foil
230 therebetween, i.e., the joint region, can be controlled at step
40. In some instances, the assembly of the components to be joined
is placed within an enclosed chamber such that the chamber can be
evacuated and/or purged with an inert gas. In other instances, the
region wherein the joint is to occur is enclosed without the entire
assembly being placed in a chamber. The control of the atmosphere
surrounding the joint region can be critical and, in some
instances, is provided by a high vacuum. The atmosphere can also be
controlled by purging the joint region with an inert and/or
reducing gas, or inert gas mixture, illustratively including argon,
nitrogen, mixtures of those gases with hydrogen, and the like. It
is appreciated that the atmosphere can be controlled by a
combination of vacuum and gas purging. A vacuum of less than
10.sup.-4 kPa (10.sup.-3 millibar) can assist in the vaporization
of the bonding foil material away from the joint region and/or aid
in decomposing of any oxide scale that is present.
[0051] An oxygen getter can be placed proximate to the joint region
and/or within an enclosed chamber that contains the joint region
such that excess oxygen within the atmosphere is reduced. Any
oxygen getter known to those skilled in the art can be used,
illustratively including an oxygen getter made from zirconium,
aluminum, tantalum, titanium, and the like. In some instances, the
oxygen getter is in the form of a sponge or some other
high-surface-area structure. In addition, the joint region can be
wrapped with oxygen getter foils such as aluminum, zirconium,
tantalum, titanium, and the like.
[0052] A thermal treatment of the joint region can be provided at
step 50. The thermal treatment can result in the heating of the
joint region, and the heating can be provided by thermal
resistance, thermal resistance furnaces, induction heating, radiant
heating, and the like. The thermal treatment can include a series
of time-temperature steps, such as a ramp up to a first
temperature, holding the first temperature for a predetermined
amount of time, ramp up or down to a second temperature, holding at
the second temperature for a predetermined amount of time, ramp up
or down to a third temperature, holding at the third temperature at
a predetermined amount of time, and so on.
[0053] For example and for illustrative purposes only, a first
component 210 made from the CM247LC alloy can be joined to a second
component 220 made from the APMT alloy using a zinc foil. A chamber
surrounding the first joint face 212 and the second joint face 222
with the zinc foil 230 therebetween can be purged with an argon+5%
hydrogen gas and held at a pressure of between 10 to 304 kPa (0.1
to 3 atmospheres) while the joint is heated to 700.degree. C. and
held for 1 hour. This initial step can result in the melting of the
zinc foil and disruption or dissolving of oxide surfaces at the
first joint face 212 and/or second joint face 222, possibly
dissolving a portion of the metal present at the joint faces into
the joint. Thereafter, a high vacuum, for example a vacuum of
10.sup.-7 kPa (10.sup.-6 millibar), can be pulled around the joint
region and the temperature increased to 1214.degree. C. and held
for 24 hours. During this second thermal step, grain growth and
interdiffusion across the joint interface can be promoted and at
least part of the zinc from the zinc foil vaporized, possibly after
diffusing through the structures being joined to the surface of the
structures.
[0054] It is appreciated that the second thermal processing step
can include holding the joint region at the second temperature for
a shorter amount of time, for example, 1 hour and thereafter
reducing the vacuum and providing an Ar+5% H.sub.2 gas. Such an
alternative thermal treatment can reduce vaporization losses from
assembly devices, furnace tubes, clamps, joint rods, and the like.
It is appreciated that with any of these thermal treatment steps,
the atmosphere can be further controlled by the introduction of
oxygen getter materials therein.
[0055] Additional thermal treatment steps can be included, such as
additional heating steps and subsequent cooling steps. Heat
treatment, stress relief, and/or aging thermal treatment steps can
be included along with the joining steps and still fall within the
scope of the invention.
[0056] In this manner, a first component can be joined to a second
component using a bonding layer that melts at a temperature that is
lower than the melting temperature of the first component and the
second component. In addition, at least part of the melted bond
layer can vaporize after it wets and dissolves at least a portion
of a first joint face and/or a second joint face, possibly after
diffusing through the structures being joined.
[0057] In FIG. 3, a schematic diagram illustrating sintering of a
plurality of particles is shown. A plurality of particles to be
sintered 300 can be accumulated within a preform (not shown) with a
plurality of voids 310 being present between the particles 300.
Upon the application of heat and/or pressure, the porosity 310
between the particles 300 can be reduced and regions of contact
between the particles resulting in diffusion therebetween and their
adherence to each other. In some instances, generally all of the
porosity 310 can be removed with subsequent grain growth and/or
recrystallization occurring and affording for enlarged grains
320.
[0058] In some instances, an oxide film 302 can be present on at
least part of a surface of the particles 300 as illustrated in FIG.
4. It is appreciated that the oxide film 302 can act as a diffusion
barrier between adjacent particles 300 and thereby retard the
sintering process. In contrast, the use of a sintering aid as
disclosed herein can result in the oxide film 302 being broken up
into discrete oxide regions 302 as illustrated in FIG. 5. It is
appreciated that the breaking up of the oxide 302 is a result of
the sintering aid being present within the mixture of the plurality
of particles 300 to be sintered.
[0059] For example and for illustrative purposes only, FIG. 6A
illustrates a plurality of sintering aid particles 330 having been
admixed with the plurality of particles 300 before the sintering
process is initiated. In the alternative, a coating of the
sintering aid 330 can be present on at least a portion of the
surface of the particles 300 and/or the oxide film 302 as
illustrated in FIG. 6C. In another alternative, a vapor and/or
liquid of the sintering aid 330 can be allowed to flow past or
through a plurality of particles 300 and/or the porosity 310 as
illustrated in FIG. 6C. In this manner, the sintering aid 330 is
brought into contact with the plurality of particles 300 and
heating of the sintering aid 330 can result in at least partially
melting thereof and afford for disruption of the oxide 302 and/or
wetting of adjacent surfaces of particles 300. In addition, at
least part of the oxide 302 and/or particle 300 can dissolve into
the melted sintering aid 330.
[0060] Referring to FIG. 7, a method of sintering powders with the
use of the sintering aid is shown generally at reference numeral
40. The process 40 can include providing powders to be sintered at
step 400 and a sintering aid at step 410. The powders to be
sintered and the sintering aid are mixed at step 420. For the
purposes of the present invention, the mixing of the powders and
the sintering aid can include a mechanical mixture of the powders
and particles of the sintering aid, coating of the powders with the
sintering aid, flowing of a vapor and/or a liquid of the sintering
aid past or through the powders, and the like. At step 430 the
powders are sintered by heating and/or applying pressure thereto,
with vaporization of at least part of the sintering aid optionally
occurring at step 432. It is appreciated that the sintering aid can
vaporize while it is initially on the surface of a particle and/or
after having diffused into the particle and reached a surface
thereof. In some instances, the powders with the sintering aid can
be contained within a preform that can be used to produce a desired
component.
[0061] After sintering of the powders at step 430, cooling of a
sintered component made from the sintered powders occurs at step
440. In some instances, an optional heating of the sintered
component can occur at step 450 and thereby allow for additional
vaporization of the sintering aid to occur. It is appreciated that
one or more heating steps can occur with or without a controlled
environment in order to maximize the properties of the sintered
component and/or remove additional sintering aid by vaporization.
In addition, it is further appreciated that the vaporization of the
sintering aid can occur after the component is sintered. For
example and for illustrative purposes only, the powders can be
sintered while under external pressure in order to increase the
kinetics of the sintering process, but which reduces the
vaporization of the sintering aid. Thereafter, the sintered
component can be heated without the presence of the external
pressure, or possibly in vacuum, and thereby afford for
vaporization of at least part of the sintering aid. The heating of
the sintered component can also include an additional heating step
after the sintered component has been cooled, or in the
alternative, a continuous heating of the sintered component with
the removal of the external pressure. For the purposes of the
present invention, the term "external pressure" is defined as
pressure greater than atmospheric pressure.
[0062] With reference to FIGS. 8A and 8B, the result of using the
sintering aid is shown. In particular, FIG. 8A illustrates the use
of the sintering aid affording increased kinetics with respect to
the reduction of percent porosity as a function of time during the
sintering process. Likewise, FIG. 8B illustrates an increase in the
average grain size and/or percent recrystallization as a function
of sintering time. As stated above, the sintering aid enhances the
sintering process by disruption of an oxide film that is present on
the surface of the particles to be sintered and/or enhancing
wettability between adjacent particles. In this manner, diffusion
between adjacent particles can occur at an earlier stage of the
sintering process and/or be enhanced by the presence of the
sintering aid.
[0063] Similar to the materials used for the bonding layer
described above, it is appreciated that other low boiling point
elements can be used as the sintering aid. For example and for
illustrative purposes only, the sintering aid can be made primarily
from elements such as arsenic, cadmium, cesium, magnesium, mercury,
phosphorus, polonium, potassium, rubidium, selenium, sodium,
sulfur, tellurium, and/or zinc. As stated above, it is appreciated
that some of these materials are considered poisonous, fire
hazardous, and/or radioactive and thus may limit their use as a
sintering aid. Zinc is a preferred sintering aid as in the case of
the material used for the bonding layer described above because of
a relatively low vaporization temperature, its ease of handling,
and commercial availability.
[0064] The foregoing drawings, discussion, and description are
illustrative of specific embodiments of the present invention, but
they are not meant to be limitations upon the practice thereof.
Numerous modifications and variations of the invention will be
readily apparent to those of skill in the art in view of the
teaching presented herein. It is the following claims, including
all equivalents, which define the scope of the invention.
* * * * *