U.S. patent application number 10/453542 was filed with the patent office on 2004-12-09 for method of liquid phase sintering a two-phase alloy.
This patent application is currently assigned to LOCKHEED MARTIN CORPORATION. Invention is credited to Spencer, William R..
Application Number | 20040247479 10/453542 |
Document ID | / |
Family ID | 33489565 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040247479 |
Kind Code |
A1 |
Spencer, William R. |
December 9, 2004 |
Method of liquid phase sintering a two-phase alloy
Abstract
Liquid phase sintering method for a two-phase alloy includes
forming a green body billet of a two-phase alloy, solid state
sintering the green body billet, surrounding the solid state
sintered billet with a refractory barrier medium within a
refractory container to form a charge, optionally flowing wet
hydrogen through at least a portion of the charge, equilibrating a
charge temperature below a solidus temperature of the two-phase
alloy, changing the charge temperature to a liquid phase sintering
temperature of the two-phase alloy, maintaining the liquid phase
sintering temperature for a period of time of .ltoreq.four hours,
reducing the charge temperature to less than the solidus
temperature of the two-phase alloy, and optionally holding the
charge stationary as the charge temperature passes through the
solidus temperature. Optionally, the charge can be rotated about an
axis of symmetry during liquid phase sintering and a portion of the
charge can be zone heated.
Inventors: |
Spencer, William R.;
(Longwood, FL) |
Correspondence
Address: |
BURNS, DOANE, SWECKER & MATHIS, L.L.P.
P.O. Box 1404
Alexandria
VA
22313-1404
US
|
Assignee: |
LOCKHEED MARTIN CORPORATION
Bethesda
MD
|
Family ID: |
33489565 |
Appl. No.: |
10/453542 |
Filed: |
June 4, 2003 |
Current U.S.
Class: |
419/47 |
Current CPC
Class: |
B22F 3/1035 20130101;
B22F 2003/1042 20130101; B22F 2998/00 20130101; B22F 3/1017
20130101; B22F 2998/00 20130101; B22F 2998/10 20130101; B22F
2999/00 20130101; B22F 2998/10 20130101; B22F 2999/00 20130101;
B22F 3/1017 20130101; B22F 3/1216 20130101; B22F 2201/013 20130101;
B22F 3/105 20130101; B22F 3/1007 20130101; B22F 3/1035
20130101 |
Class at
Publication: |
419/047 |
International
Class: |
B22F 003/10 |
Goverment Interests
[0001] At least some aspects of this invention were made with
Government support under contract no. F08630-96-C-0042 DMCPW. The
Government may have certain rights in this invention.
Claims
What is claimed is:
1. A method to liquid phase sinter a two-phase alloy, the method
comprising: forming a green body billet of a two-phase alloy; solid
state sintering the green body billet; forming a charge by
surrounding the solid state sintered billet by a refractory barrier
medium within a refractory container, wherein the refractory
barrier medium prevents contact between the solid state sintered
billet and the refractory container; equilibrating a temperature of
the charge below a solidus temperature of the two-phase alloy;
changing the temperature of the charge to a liquid phase sintering
temperature of the two-phase alloy; maintaining the liquid phase
sintering temperature for a period of time of less than or equal to
four hours; and reducing the temperature of the charge to less than
the solidus temperature of the two-phase alloy.
2. The method of claim 1, wherein solid state sintering the green
body billet results in at least 80% theoretical density.
3. The method of claim 1, wherein the refractory barrier medium is
a ceramic liner, a ceramic sand, or an open cell ceramic foam.
4. The method of claim 3, wherein the ceramic sand is
Al.sub.2O.sub.3, ZrO.sub.2, or MgO.
5. The method of claim 3, wherein the ceramic sand has a grain size
of -325 to 80 mesh
6. The method of claim 1, wherein the refractory container is
formed of a metallic material.
7. The method of claim 6, wherein the metallic material is a
Mo-based alloy or a W-based alloy.
8. The method of claim 1, wherein the refractory barrier medium is
permeable to a wet hydrogen atmosphere and the method comprises
flowing wet hydrogen through at least a portion of the charge.
9. The method of claim 8, wherein the wet hydrogen atmosphere
contacts at least a portion of the two-phase alloy.
10. The method of claim 8, wherein the wet hydrogen has a pressure
of 3 to 4 psi.
11. The method of claim 1, wherein equilibrating a temperature of
the charge below a solidus temperature of the two-phase alloy is
equilibrating at less than 20.degree. C. below the solidus
temperature.
12. The method of claim 1, wherein changing the temperature of the
charge to a liquid phase sintering temperature of the two-phase
alloy is changing the temperature at a rate of from 40.degree.
C./hr to 400.degree. C./hr.
13. The method of claim 1, wherein the period of time for
maintaining the liquid phase sintering temperature is from 0.3
hours to 1.5 hours.
14. The method of claim 1, wherein reducing the temperature of the
charge to less than the solidus temperature of the two-phase alloy
is reducing the temperature at a rate of from 20.degree. C./hr to
100.degree. C./hr.
15. The method of claim 1, wherein the two-phase alloy is a
tungsten heavy alloy.
16. The method of claim 15, wherein the tungsten heavy alloy
includes less than or equal to 93 wt. % tungsten.
17. The method of claim 15, wherein the tungsten heavy alloy
includes less than or equal to 93 wt. % tungsten and the balance at
least one secondary element selected from the group consisting of
Ni, Fe and Co.
18. The method of claim 15, wherein the solidus temperature is
1475.+-.20.degree. C.
19. The method of claim 15, wherein the liquid phase sintering
temperature is 1535.+-.20.degree. C.
20. The method of claim 1, wherein the charge has an axis of
symmetry and the method comprises rotating the charge about the
axis of symmetry during at least a portion of the method during
which the temperature of the charge is above the solidus
temperature.
21. The method of claim 20, wherein a rotation rate of the rotating
charge is from 1 to several cycles per minute.
22. The method of claim 1, wherein the charge has a cylindrical
shape with an axis in a height dimension and the method comprises
rotating the charge about the axis of symmetry during at least a
portion of the method during which the temperature of the charge is
above the solidus temperature.
23. The method of claim 22, wherein a rotation rate of the rotating
charge is from 1 to several cycles per minute.
24. The method of claim 1, comprising holding the charge stationary
as the temperature passes through the solidus temperature during
the step of reducing the temperature of the charge to less than the
solidus temperature.
25. The method of claim 1, wherein the charge is placed in a
partial vacuum or an atmospheric furnace.
26. The method of claim 1, wherein the temperature of the charge is
equilibrated, changed, or maintained by radiative heating,
resistive heating, or electromagnetic heating.
27. The method of claim 26, wherein electromagnetic heating
includes RF heating or MW heating.
28. The method of claim 1, comprising zone heating a portion of the
charge to liquid phase sinter the two-phase alloy.
29. The method of claim 28, wherein zone heating comprises heating
the portion of the charge to the liquid phase sintering temperature
to form a heating zone and traversing the heating zone from a first
end of the charge to a second end of the charge by relative motion
between the charge and a heating element.
30. The method of claim 29, wherein the relative motion is
step-wise or continuous.
31. The method of claim 29, wherein the relative motion is at a
rate of 1 to 5 cm per hour.
32. The method of claim 28, wherein zone heating occurs during the
step of changing the temperature of the charge to a liquid phase
sintering temperature of the two-phase alloy.
33. The method of claim 28, wherein the charge has an axis of
symmetry and the method comprises rotating the charge about the
axis of symmetry during at least a portion of the method during
which the temperature of the charge is above the solidus
temperature.
34. The method of claim 33, wherein a rotation rate of the rotating
charge is from 1 to several cycles per minute.
35. The method of claim 28, wherein the charge has a cylindrical
shape with an axis in a height dimension and the method comprises
rotating the charge about the axis of symmetry during at least a
portion of the method during which the temperature of the charge is
above the solidus temperature.
36. The method of claim 35, wherein a rotation rate of the rotating
charge is from 1 to several cycles per minute.
Description
BACKGROUND
[0002] The present invention relates to liquid phase sintering of a
two-phase metal alloy. More particularly, the present invention
relates to a method to liquid phase sinter a tungsten heavy
alloy.
[0003] Large size and/or geometrically complex two-phase alloy
materials, such as tungsten heavy alloy (WHA), are difficult to
produce as a single piece.
[0004] Liquid phase sintering (LPS) can be used to produce WHA
parts. However, LPS can be limited by, for example, maximum furnace
size, severe slumping of parts, liquid matrix runout of WHA
material, and substantial compositional variation due to alloying
elements, such as tungsten in WHA, settling under gravity. Further,
long process times at temperatures where liquid matrix is present
can exacerbate the limitations. For example, LPS processes can
include up to 14 to 20 hours at greater than 1475.degree. C.,
resulting in settling of the tungsten grains toward the bottom of
the part and/or the formation of a portion of the part that is
matrix rich.
[0005] Current methods of producing large pieces include liquid
phase sintering in a pusher furnace. A pusher furnace LPS process
can include pushing a WHA billet at a given rate through a hot zone
of a long furnace with an essentially fixed temperature profile
along the length. However, part size in a typical pusher furnace
LPS consolidation process can be limited by the furnace opening,
which is approximately 20 inches wide and 4 inches high. Further,
tungsten particle settling can occur in a WHA billet higher than
four inches when processed in a pusher furnace due to gravity
during the time when the matrix of the material is liquid. This can
be especially pronounced in thicker WHA parts, which can show
compositional variations of up to 10 weight percent (wt. %) or more
in the part.
SUMMARY
[0006] An exemplary liquid phase sintering method for a two-phase
alloy comprises forming a green body billet of a two-phase alloy,
solid state sintering (SSS) the green body billet, forming a charge
by surrounding the solid state sintered billet by a refractory
barrier medium within a refractory container, wherein the
refractory barrier medium prevents contact between the solid state
sintered billet and the refractory container, equilibrating a
temperature of the charge below a solidus temperature of the
two-phase alloy, changing the temperature of the charge to a liquid
phase sintering temperature of the two-phase alloy, maintaining the
liquid phase sintering temperature for a period of time of less
than or equal to four hours, and reducing the temperature of the
charge to less than the solidus temperature of the two-phase
alloy.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0007] The following detailed description makes reference to the
accompanying drawings in which like numerals designate like
elements and in which:
[0008] FIG. 1 schematically illustrates an exemplary liquid phase
sintering method.
[0009] FIG. 2 shows a cross section of an exemplary charge used in
the liquid phase sintering method of FIG. 1.
[0010] FIG. 3 shows an exemplary temperature versus time plot for a
charge indicating the temperature profile in the charge during the
liquid phase sintering method of FIG. 1.
[0011] FIG. 4 shows a cross section of an exemplary rotating
apparatus used in the liquid phase sintering method of FIG. 1.
[0012] FIG. 5 shows an exemplary embodiment of a charge and a
heating element for a liquid phase sintering method with zone
heating.
[0013] FIGS. 6a and 6b show micrographs of a tungsten heavy alloy
after (a) solid state sintering and (b) rapid liquid phase
sintering according to the exemplary method, respectively.
DETAILED DESCRIPTION
[0014] FIG. 1 schematically illustrates an exemplary liquid phase
sintering method for a two-phase alloy. The method 100 comprises
forming a green body billet of a two-phase alloy 102, solid state
sintering (SSS) the green body billet 104, forming a charge 106 by
surrounding the solid state sintered billet by a refractory barrier
medium within a refractory container, wherein the refractory
barrier medium prevents contact between the solid state sintered
billet and the refractory container, optionally flowing wet
hydrogen through at least a portion of the charge 108,
equilibrating a temperature of the charge below a solidus
temperature of the two-phase alloy 110, changing the temperature of
the charge to a liquid phase sintering temperature of the two-phase
alloy 112, maintaining the liquid phase sintering temperature for a
period of time of less than or equal to four hours 114, optionally
rotating the charge about an axis of symmetry 116, optionally zone
heating 118 a portion of the charge to liquid phase sinter the
two-phase alloy, reducing the temperature of the charge to less
than the solidus temperature of the two-phase alloy 120, and
optionally holding the charge stationary as the temperature passes
through the solidus temperature 122.
[0015] Forming the green body billet can be by any suitable method.
For example, a green body billet can be cold pressed to about
50-60%, or higher, theoretical density. The shape of the green body
billet can be any suitable shape such as a solid shape, a hollow
shape, or a shape containing both solid and hollow sections. For
example, the shape of the green body billet can be a solid
geometric form, both regular and irregular, or the shape can have
one or more hollows open to an outer surface of the green body
billet.
[0016] Solid state sintering of the green body billet can occur at
any suitable temperature for the materials used. For example, WHA
sintering can occur in a multistep process with a final step at
approximately 1400.degree. C. for several hours, e.g., 8 hours.
Theoretical densities of the solid state sintered billet can be
from 50 to 95% theoretical density or higher, preferably greater
than 80% theoretical density and most preferably greater than 90%
theoretical density.
[0017] FIG. 2 shows an exemplary embodiment of a charge 200. The
charge 200 includes a refractory container 202 in which the solid
state sintered (SSS) billet 204 of a two-phase alloy is placed. The
SSS billet 204 is surrounded by a refractory barrier medium 206,
such that the SSS billet 204 does not touch the refractory
container 202.
[0018] The refractory container 202 can take any suitable form and
can be made of any suitable material. For example, the refractory
container 202 can be formed of a metallic materials. Exemplary
metallic materials include molybdenum (Mo) based alloys and
tungsten (W) based alloys. Ceramic materials can also be used for
the refractory container.
[0019] The refractory barrier medium 206 can function to provide a
barrier between the refractory container 202 and the SSS billet
204, such that the SSS billet 204 does not contact the refractory
container 202. In addition, the refractory barrier medium 206 can
be permeable to hydrogen, especially wet hydrogen, which can reduce
oxides. Further, the refractory barrier medium 206 can function to
constrain and/or mold the SSS billet when the two-phase alloy is
above the solidus temperature. For example, the restraining
function of the refractory medium can assist in maintaining the
shape of the two-phase alloy during the liquid phase sintering
portion of the method. If freestanding when heated above the
solidus temperature, the two-phase alloy can undergo severe
slumping and ejection of liquid matrix from the bulk. In an
exemplary embodiment, the refractory barrier medium can be a
ceramic liner, a ceramic sand, an open cell ceramic foam, or any
suitable form that can meet one or more of the functions of the
barrier medium.
[0020] For example, the ceramic of the refractory barrier medium
can be Al.sub.2O.sub.3, ZrO.sub.2, MgO, or any other suitable oxide
or combinations thereof. In a preferred embodiment, the refractory
barrier medium is a ceramic sand formed of Al.sub.2O.sub.3.
Al.sub.2O.sub.3 sand can be easier to remove from the charge at the
end of the liquid phase sintering method, and also allows wet
hydrogen atmosphere to permeate through and contact at least a
portion of the SSS billet, preferably the entire surface of the SSS
billet. Additionally, Al.sub.2O.sub.3 sand has a suitable grain
size such that seepage of liquid matrix during the liquid phase
sintering process does not seep into the sand. For example, a
preferred Al.sub.2O.sub.3 sand has a grain size of between -325 and
80 mesh. However, a uniform grain size is not required and a
suitable distribution of grain sizes may be used.
[0021] The charge can be configured to allow wet hydrogen to
contact at least a portion of the SSS billet. The contact can be
for any suitable time, such as, for example, up to 12 hours or
longer, depending on the size of the SSS billet, the type and/or
permeability to wet hydrogen of the refractory barrier medium,
and/or the manner in which the wet hydrogen is supplied to the
charge.
[0022] For example, the refractory barrier material can be
permeable to a wet hydrogen atmosphere. The refractory barrier
medium can either be diffusively permeable, i.e., a blanket of wet
hydrogen atmosphere in contact with the refractory barrier medium
will defuse through and to at least a portion of the SSS billet,
and/or the wet hydrogen atmosphere can be forced to flow through
the refractory barrier medium to contact the SSS billet, such as
wet hydrogen having a pressure of 3 to 4 psi.
[0023] The wet hydrogen atmosphere can be supplied to the charge by
any suitable means. For example, the refractory container can have
one or more inlets, connections, or other suitable openings or
fixtures to port weight hydrogen atmosphere diffusively and/or at a
specified pressure into the interior of the refractory container.
FIG. 2 shows the charge 200 with a hydrogen inlet 208. Optionally,
the hydrogen inlet 208 can be located on the closure 210.
[0024] In the exemplary embodiment shown in FIG. 2, the refractory
barrier medium 206 completely fills the interior space of the
refractory container 202 and completely surrounds the SSS billet
204. Further, in embodiments in which the shape has one or more
hollow sections, the hollow sections contain the refractory barrier
medium to assist in obtaining a uniform temperature profile across
a cross section of the SSS billet during the liquid sintering
process, e.g., the refractory barrier medium can be packed around
the SSS billet and into any hollows, such as troughs, holes,
cut-outs, and so forth, so that the refractory barrier medium
contacts all exterior surfaces of the SSS billet and/or is tightly
packed about the SSS billet. Optionally, the refractory barrier
medium can surround only a portion of the SSS billet that is to be
liquid phase sintered and/or the refractory barrier medium can also
only partially fill the refractory container. However, partially
surrounding the SSS billet or partially filling the refractory
container can result in voids which can allow seepage of matrix
and/or slumping of the two-phase alloy during liquid phase
sintering. The restraining function of the refractory medium
assists in maintaining the shape of the two-phase alloy during the
process. For example, if freestanding when heated above the solidus
temperature, the two-phase alloy can undergo severe slumping and
ejection of liquid matrix from the bulk. The refractory medium,
such as an aluminum oxide sand, can serve to protect the container
as well as allowing hydrogen gas to contact the surfaces of the SSS
billet and prevent the SSS billet contacting the refractory
container because liquid billet material can alloy with select
materials, such as molybdenum alloys of the refractory
container.
[0025] The charge 200 can be an open vessel, i.e., open at one end,
or can be closed. As shown in the exemplary embodiment of FIG. 2,
the charge 200 has a closure 210 at one end. The closure 210 can
include threads 212 for cooperating with threads 214 on the
refractory container 202 to form a closed refractory container.
Further, the closure 210 at an outer surface 216 can have a
connection 218, such as a socket, threaded connection, bolt, and so
forth, for connecting to a mechanical device (not shown), such as a
motor, for moving or imparting motion, rotation, or other motive
force to the charge 200.
[0026] FIG. 3 shows an exemplary temperature versus time plot for a
charge indicating the temperature profile in the charge during the
liquid phase sintering method of FIG. 1. Temperature in the charge
at a starting time is initially at a starting temperature (point
A), which is changed at a reasonable rate (.DELTA..sub.1) to a
first temperature T.sub.1 (point B). A reasonable rate can be any
suitable rate that does not unnecessarily prolong the process, for
example, the heating rate can be 50 to 60.degree. C. per hour to a
temperature of 750.degree. C. and 30 to 40.degree. C. thereafter.
Temperature T.sub.1 can be any suitable temperature below the
solidus temperature (T.sub.solidus) of the two-phase alloy. For
example, T.sub.1 can be 20-40.degree. C. below the solidus
temperature. The temperature of the charge is equilibrated at
T.sub.1 for a period of time, the equilibration period (t.sub.eq).
For example, the equilibration period can be approximately 6 to 8
hours, depending on the size of the charge and the furnace. At the
end of the equilibration period (point C), the temperature of the
charge is increased to a liquid phase sintering temperature
(T.sub.LPS) at a suitable rate (.DELTA..sub.2). The rate of change
(.DELTA..sub.2) from the end of the equilibration period to
T.sub.LPS can be, for example, 40-400.degree. C. per hour or can
occur over approximately 0.1-2 hours.
[0027] Liquid phase sintering (starting at point D) continues for a
liquid phase sintering period (t.sub.LPS) of less than or equal to
four hours. The liquid phase sintering period can vary depending on
the medium of the two-phase alloy and on the size of the SSS billet
and/or the charge. For example, a larger SSS billet or charge can
require additional time at the liquid phase sintering temperature
to liquid phase sinter the two-phase alloy. Preferably, the liquid
phase sintering period is from 0.3-1.5 hours.
[0028] At the end of the liquid phase sintering period (point E),
the temperature of the charge is reduced to less than the solidus
temperature of the two-phase alloy. A suitable rate of change
(.DELTA..sub.3) for the reduction of temperature is 20-100.degree.
C. per hour or can occur over approximately 0.2-4 hours. If the
rate of change (.DELTA..sub.3) is too fast, the two-phase alloy can
have increased porosity. However, if the rate of change
(.DELTA..sub.3) is too slow, the two-phase alloy can have increased
settling of tungsten particles within liquid metal matrix. Further,
the rate of change (.DELTA..sub.3) to a temperature below the
solidus temperature of the two-phase alloy can occur by suitable
cooling methods including ambient cooling and/or forced
cooling.
[0029] At the end of the process (point F), the two-phase alloy can
be removed from the charge for subsequent processing and/or
use.
[0030] Tungsten heavy alloy (WHA) is a two-phase alloy or
metal-matrix composite consisting of almost pure tungsten (W)
grains surrounded by a matrix that consists of an alloy of tungsten
with secondary elements, e.g., nickel (Ni), iron (Fe), and/or
cobalt (Co). WHA can vary in composition from at least 80-90 wt. %
W to about 95 wt. % W and the balance Ni, Fe and/or Co. In an
exemplary embodiment, the two-phase alloy is a tungsten heavy alloy
including .ltoreq.93 wt. % W. Further, the WHA can optionally
include a balance of at least one secondary element selected from
the group consisting of Ni, Fe, and Co. An exemplary WHA comprises
90 wt. % W, 8 wt. % Ni, and 2 wt. % Co.
[0031] An exemplary WHA, such as tungsten heavy alloy formed of 93
wt. % W and the balance Ni, Fe, and Co that has been solid state
sintered to about 95% theoretical density, e.g., greater than 90%,
can have a solidus temperature of 1475.degree. C..+-.20.degree. C.
and a liquid phase sintering temperature of 1535.degree.
C..+-.20.degree. C. The solidus temperature is the temperature at
which the components of the matrix, such as nickel, begin to melt.
For a tungsten heavy alloy solid state sintered to 90% theoretical
density, the solidus temperature is 1455.degree. C..+-.20.degree.
C.
[0032] As depicted in FIG. 2, the charge 200 has a cylindrical
shape with an axis X-X' in the height dimension. However, the
charge can have any form with an axis of symmetry about which the
charge can be rotated during an optional rotation step of the
method. For example, when the charge is cylindrical shaped, the
charge can be rotated about the axis of symmetry X-X' by a suitable
rotating apparatus. Alternatively, the charge can be rotated about
the axis of symmetry Y-Y' by a suitable rotating apparatus. Other
suitable forms for the charge include a sphere, a cone, a box, or
other suitable form that has an axis of symmetry about which
rotation can occur in a suitable rotating apparatus.
[0033] FIG. 4 shows an exemplary rotating apparatus 400 for use in
the method of FIG. 1. As shown, the rotating apparatus 400 places
the charge 402 (shown in cross section corresponding to section A-A
in FIG. 2) in contact with rotating bars 404 seated in notches 406
of support blocks 408. A motor or other means of imparting motive
force (not shown) can be attached to the charge 402 by way of the
connection on the closure (shown in FIG. 2 as connection 218). For
example, a bar can connect the motor and the charge via the socket
in the closure. In other respects, the charge can include a SSS
billet 410, a refractory barrier medium 412, a refractory container
414, and a hydrogen connection (not shown).
[0034] Rotation (.omega.) of the charge 402 can occur in any
direction around any axis of symmetry, such as clockwise or counter
clockwise around axis X-X'. Rotation can be from one to several
cycles per minute to limit centrifugal forces acting on the
particles and to limit the settling due to gravity. The exemplary
method of FIG. 1 can optionally include rotating the charge during
at least a portion of the method during which the temperature of
the charge is above the solidus temperature, e.g., during the
portion of the temperature-time profile represented in FIG. 3
between points D and E. Such rotation can limit settling of the
tungsten which is surround by liquid matrix alloy and also can
assist in compositional uniformity. However, the charge can also be
maintained in a fixed position during the period of time the charge
is above solidus temperature.
[0035] Further, when the charge has been rotated during at least a
portion of the method during which the temperature of the charge is
above the solidus temperature, the charge can be held stationary as
the temperature passes through the solidus temperature. For
example, during the reduction of temperature from the liquid phase
temperature to the end of the process, e.g., during the portion of
the temperature-time profile represented in FIG. 3 between points E
and F, the charge can be held stationary during the time when the
temperature passes through the solidus temperature (T.sub.solidus).
The period of time during which the charge is held stationary can
be any suitable time, for example, the charge can be held
stationary within a temperature range of .+-.5.degree. preferably
.+-.2.degree., about the solidus temperature.
[0036] The temperature of the charge during any point of the
exemplary process, can be equilibrated, changed, or maintained by
suitable methods such as radiative heating, resistive heating, or
electromagnetic heating. Exemplary electromagnetic heating methods
include radio frequency (RF) heating or microwave (MW) heating.
[0037] The charge can be heated in any suitable environment. For
example, the charge or the charge in the rotating apparatus can be
placed within a furnace to achieve the desired temperature profile
during the method. Suitable furnaces include partial vacuum
furnaces and atmospheric furnaces.
[0038] An exemplary method of liquid phase sintering can optionally
include zone heating a charge or a portion of a charge to liquid
phase sinter the two-phase alloy. Zone heating can include heating
the portion of the charge to the liquid phase sintering temperature
to form a heating zone and traversing the heating zone from a first
end of the charge to a second end of the charge by relative motion
between the charge and a heating element. The temperature profile
produced in the portion of a charge is sufficient to liquid phase
sinter a two-phase alloy. For example, the temperature profile
presented and described with reference to FIG. 3 can be applied to
a charge or portions of a charge with a WHA SSS billet and the
heating element can traverse the geometry of the charge. Zone
heating can occur both alternative to and in combination with
rotating the charge about an axis of symmetry.
[0039] FIG. 5 shows an exemplary embodiment of a charge and a
heating element for a liquid phase sintering method with zone
heating. As shown in FIG. 5, a zone heating apparatus 500 includes
a heating element 502 positioned about a charge 504. Examples of
heating elements include an inductively, resistively, radiatively
or electromagnetically heated ring, jacket, coupling, sleeve, or
other heating element that can be placed around or approximate to a
portion of the outer surface of the charge and that can produce a
suitably constrained heating zone projected toward the charge to
achieve, within a two-phase alloy located in a charge, at least the
liquid phase sintering temperature of the two-phase alloy.
Preferably, the heating element is heated by an electric resistance
furnace or an induction furnace. In the exemplary embodiment, the
charge 504 is cylindrical and the heating element 502 is an
inductive ring about the circumference of the cylindrical charge
504.
[0040] In the exemplary embodiment depicted in FIG. 5, the charge
504 includes a 90% dense SSS billet 506 constrained vertically
within a refractory container 508 and surround by a refractory
medium 510. The charge 504 is heated by the heating element 502,
which is depicted as an inductive coil, so as to melt a heating
zone 512 at a first end 514 of the charge 502. As shown, the
heating zone 512 is disc-like and is approximately 10 centimeters
or less in height. The heating zone 512 is then moved up the charge
504 toward a second end 516 by movement of the charge 504 and/or
the heating element 502.
[0041] Relative motion can occur between the charge 504 and the
heating element 502 such that a temperature profile in the charge
504 is controlled and the solidifying front of the liquid phase
sintered material is moved uniformly toward a free surface of the
two-phase alloy, e.g., toward an end of the two-phase alloy.
Movement of the heating zone 512 can be coordinated with achieving
a desired peak temperature within the charge and relative motion
can occur either step wise or continuously. Once the heating zone
512 at any one portion of the charge completes the liquid phase
sintering time period, the heating element is moved relative to the
charge by either moving the heating element, the charge, or both.
For example, the heating zone can be moved from a first end 514 of
the charge 504 to a second end 516 of the charge 504 by any
suitable means, such as by a mechanical arm, a conveyor system, a
stepper motor, and so forth. Further, the charge can be stationary
or can also be moved through the heating zone by a suitable
elevating or conveying system.
[0042] The rate of movement of the heating zone, and thus of the
temperature profile, can depend upon the size of the part and the
heating system. Traverse rates in the range of about 1-5
centimeters per hour can be used to achieve melting and
solidification gradients in the heating zone to achieve the desired
compositional and mechanical results. Solidification gradients in
the range of 50-200.degree. per hour are preferred in order to
avoid generation of porosity defects in the material.
[0043] The moving heating zone can eliminate both defects caused by
conventional methods, e.g., leakage and settling. For example,
movement of the temperature profile toward a free surface can avoid
shrinkage defects within the two-phase alloy, e.g., an ingot of
tungsten heavy alloy, formed by the liquid phase sintering process.
Further, the size of the heating zone, e.g., cylindrical with less
than 10 centimeters in height depending on the thickness of the
charge and/or solid state sintered billet in the transverse
direction, and the rapid movement of the solidifying front, e.g., 1
to 5 centimeters per hour, can result in insufficient time for any
significant tungsten green settling. Also, the directional
solidification of the moving zone can sweep shrinkage or evolved
gas porosity up the ingot to the free surface of the top, thereby
reducing porosity to less than or equal to 5%, preferably less than
or equal to 2%.
[0044] FIGS. 6a and 6b show micrographs of a tungsten heavy alloy
after (a) solid state sintering and (b) rapid liquid phase
sintering according to the exemplary method, respectively. The
photomicrograph in FIG. 6a shows porosity distributed throughout
the image. This porosity is approximately 5%. Further, the tungsten
phase (the light shaded phase) is contiguous and the matrix
material (the dark gray phase) is not contiguous. The contiguous
tungsten phase, which has low ductility, can negatively impact
crack propagation within the material.
[0045] The photomicrograph in FIG. 6b shows that the tungsten phase
has ripened into substantially spherical phase regions. Further,
the matrix material, e.g., nickel, iron and/or cobalt, has an
increased contiguous character, e.g., a larger proportion of the
matrix material is contiguous than in the solid state sintered
sample of FIG. 6a, and the porosity is not evident. Accordingly,
the increase in proportion matrix material that is contiguous
improves the ductility of the liquid phase sintered two-phase
alloy. As shown, the tungsten phase is approximately 50 microns in
diameter. Also, FIG. 6b shows that at least a portion of the
tungsten phase is not contiguous or is completely surround by
matrix phase, e.g., does not contact a neighboring tungsten
phase.
[0046] While the present invention has been described by reference
to the above-mentioned embodiments, certain modifications and
variations will be evident to those of ordinary skill in the art.
Therefore, the present invention is to be limited only by the scope
and spirit of the appended claims.
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