U.S. patent application number 09/270565 was filed with the patent office on 2001-05-24 for method of making a closed porosity surface coating on a low density preform.
This patent application is currently assigned to Steven A. Miller et al. Invention is credited to GAYDOS, MARK E., MILLER, STEVEN A., NACHTRAB, WILLIAM T..
Application Number | 20010001640 09/270565 |
Document ID | / |
Family ID | 23031828 |
Filed Date | 2001-05-24 |
United States Patent
Application |
20010001640 |
Kind Code |
A1 |
MILLER, STEVEN A. ; et
al. |
May 24, 2001 |
METHOD OF MAKING A CLOSED POROSITY SURFACE COATING ON A LOW DENSITY
PREFORM
Abstract
A method of making a low density part with a closed porosity
surface coating is formed by applying, to a porous powder preform,
a coating of powder finer than that of the preform which is
sinterable to a near full density below the melting temperature of
the powder; and heating the coated preform to sinter the coating to
form a near full density, gas impermeable, closed porosity surface
coating on the preform.
Inventors: |
MILLER, STEVEN A.; (CANTON,
MA) ; NACHTRAB, WILLIAM T.; (MAYNARD, MA) ;
GAYDOS, MARK E.; (NASHUA, NH) |
Correspondence
Address: |
IANDIORIO & TESKA
260 BEAR HILL ROAD
WALTHAM
MA
024511018
|
Assignee: |
Steven A. Miller et al
|
Family ID: |
23031828 |
Appl. No.: |
09/270565 |
Filed: |
March 16, 1999 |
Current U.S.
Class: |
419/9 |
Current CPC
Class: |
B22F 2998/00 20130101;
B22F 2998/10 20130101; B22F 3/1266 20130101; B22F 7/004 20130101;
B22F 2998/00 20130101; B22F 1/065 20220101; B22F 2998/10 20130101;
B22F 2201/20 20130101; B22F 3/1266 20130101; B22F 2998/00 20130101;
B22F 1/065 20220101 |
Class at
Publication: |
419/9 |
International
Class: |
B22F 007/00 |
Claims
What is claimed is:
1. A method of making a low density part with a closed porosity
surface coating comprising: applying, to a porous powder preform, a
coating of powder finer than that of the preform which is
sinterable to a near full density below the melting temperature of
the powder; and heating the coated preform to sinter the coating to
form a near full density, gas impermeable, closed porosity surface
coating on the preform.
2. The method of claim 1 further including applying an increased
pressure to the closed porosity coating for transmitting the force
of the pressure through the coating to consolidate the porous
powder preform to a predetermined density.
3. The method of claim 2 in which the increased pressure includes
gas pressure.
4. The method of claim 2 in which the preform is consolidated to
near full density.
5. The method of claim 1 in which the step of applying includes
adding the metal powder to a carrier and applying the carrier to
the preform.
6. The method of claim 5 in which the carrier includes a binder for
aiding the powder in adhering to the preform.
7. The method of claim 1 in which applying includes brushing the
powder onto the preform.
8. The method of claim 1 in which applying includes dipping the
preform into the powder.
9. The method of claim 1 in which applying includes spraying the
powder onto the preform.
10. The method of claim 9 in which the powder is melted and sprayed
on as molten droplets.
11. The method of claim 1 further including placing the powder
coated preform in a vacuum furnace and reducing the pressure to
remove any gas from the porous preform prior to heating.
12. The method of claim 1 in which the powder is spherical.
13. The method of claim 1 in which the coating of powder is the
same material as the preform.
14. A method of making a high density part from a low density part
comprising: applying, to a porous powder preform, a coating of
powder finer than that of the preform which is sinterable to a near
full density below the melting temperature of the powder coating;
heating the powder coated preform to sinter the coating to form a
near full density, gas impermeable, closed porosity surface coating
on the preform; and applying increased pressure to the closed
porosity coating for transmitting the force of the pressure through
the coating to consolidate the porous powder preform to a
predetermined density.
15. The method of claim 14 in which the increased pressure includes
gas pressure.
16. The method of claim 14 in which the predetermined density is
near full density.
17. The method of claim 14 in which the step of applying includes
adding the metal powder to a carrier and applying the carrier to
the preform.
18. The method of claim 17 in which the carrier includes a binder
for aiding the powder in adhering to the preform.
19. The method of claim 14 in which applying includes brushing the
powder onto the preform.
20. The method of claim 14 in which applying includes dipping the
preform into the powder.
21. The method of claim 14 in which applying includes spraying the
powder onto the preform.
22. The method of claim 21 in which the powder is melted and
sprayed on as molten droplets.
23. The method of claim 14 further including placing the coated
preform in a vacuum furnace and reducing the pressure to remove any
gas from the porous preform prior to heating.
24. The method of claim 14 in which the powder is spherical.
25. The method of claim 14 in which the coating of powder is the
same material as the preform.
26. A method of making a metallic part with a closed porosity
surface coating comprising: applying, to a metallic part, a coating
of powder sinterable to a near full density below the melting
temperature of the powder; and heating the coated part to sinter
the powder coating to form a near full density, gas impermeable,
closed porosity surface coating.
27. The method of claim 26 further including applying an increased
pressure to the closed porosity surface coating for transmitting
the force of the pressure through the coating to consolidate the
porous powder preform to a predetermined density.
28. The method of claim 27 in which the increased pressure includes
gas pressure.
29. The method of claim 27 in which the preform is consolidated to
near full density.
30. The method of claim 26 in which the step of applying includes
adding the metal powder to a carrier and applying the carrier to
the preform.
31. The method of claim 30 in which the carrier includes a binder
for aiding the powder in adhering to the preform.
32. The method of claim 26 in which applying includes brushing the
powder onto the preform.
33. The method of claim 26 in which applying includes dipping the
preform into the powder.
34. The method of claim 26 in which applying includes spraying the
powder onto the preform.
35. The method of claim 33 in which the powder is melted and
sprayed on as molten droplets.
36. The method of claim 26 further including placing the coated
preform in a vacuum furnace and reducing the pressure to remove any
gas from the porous preform prior to heating.
37. The method of claim 26 in which the powder is spherical.
38. The method of claim 26 in which the coating of powder is the
same material as the preform.
39. A low density powder metal near net shape part with a closed
porosity coating produced by: applying, to a porous powder preform,
a coating of powder finer than that of the preform which is
sinterable to a near full density below the melting temperature of
the powder; and heating the coated preform to sinter the coating to
form a near full density, gas impermeable, closed porosity surface
coating on the preform.
40. A near net shape part comprising: a porous metal powder
preform; and a fine metal powder coating disposed on said preform,
the metal powder coating sintered below the melting temperature of
the powder coating to form a full density, gas impermeable closed
porosity coating on the preform.
41. The near net shape part of claim 40 in which the metal preform
had a near full density.
42. A method of making a near net shape full density coating
comprising: applying a powder coating to a powder metal preform,
the powder coating sinterable to full density; and heating the
coated preform to sinter the coating to form a closed porosity, gas
impermeable near net shape coating.
43. The method of claim 42 in which the powder of the coating is
finer than that of the preform.
44. A method of making a low density part with a gas impermeable
coating comprising: applying, to a porous powder preform, a coating
of powder finer than that of the preform and having a lower melting
point than the preform; and heating the powder coated preform to
melt the coating such that the coating reacts with the preform to
form a near full density, gas impermeable, closed porosity surface
coating on the preform.
Description
FIELD OF INVENTION
[0001] This invention relates to manufacturing near net shape
powder metal parts and more particularly to a method of
manufacturing a near net shape powder metal part with a closed
porosity surface using a powder metal coating.
BACKGROUND OF INVENTION
[0002] Manufacturing of parts by consolidating metal powders offers
the potential of greatly reduced manufacturing costs when compared
to more traditional manufacturing such as cast or wrought parts. To
date, however, this potential has been realized only with part
shapes and powder that are amenable to die pressing or metal
injection molding followed by a sintering operation. To a large
extent, powder metallurgy techniques have not been amenable to
economically produce parts that are large, have complex shapes and
have a high density, e.g., near 100%. This is still true, despite
the fact that many processes have been developed over the last
twenty years directed to obtaining near 100% dense parts by
compacting metal powders. These processes include Hot Isostatic
Pressing (HIP), Rapid Omnidirectional Compaction (ROC), Pneumatic
Isostatic Forging (PIF), Rapid Isostatic Pressing (RIP) and the
Ceracon process.
[0003] Each of these processes rely on hydrostatic or near
hydrostatic compression forces being applied to the powder while
maintaining the powder at a high temperature. Due to the porosity
of the powder, it is necessary to place the mass in a sealed can in
order to transmit the compressive forces to the powder.
[0004] U.S. Pat. Nos. 3,700,435, 5,094,810 and 5,217,227 teach
forming an article by placing the powder in a mold, placing the
mold in a secondary media, such as sand, and then sealing the mold
and sand in a can and finally hot degassing the interior of the
can. The temperature is then elevated and pressure increased to
compact the sand and thus compact the metal powder. However, this
process is time consuming and expensive due to the many processing
steps required.
[0005] U.S. Pat. No. 3,982,934 teaches electroplating metal cans
over a disposable preform. The preform is removed and the can is
filled with metal powder and then sealed. Pressure is increased to
compact and consolidate the metal powder. However, this method
suffers from hot tearing of the can and subsequent loss of
compactability due to the loss of the can's integrity as well as
reaction between the powder and the metal can.
[0006] U.S. Pat. No. 3,622,313 teaches using a glass or vitreous
container instead of a can for applying rapid omnidirectional
compaction (ROC). However, the glass, like the can above, tends to
react with the powder. Moreover, the part must still be machined to
remove the glass.
[0007] Others have attempted to produce a can which is the same
shape as the finished part. See U.S. Pat. Nos. 5,561,834 and
4,487,096. A part is stamped or sintered to obtain the desired
shape. The part is thereafter oxidized to provide a gas impermeable
layer which allows the part to be compacted. The oxide layer must
then be machined off.
[0008] U.S. Pat. No. 5,640,667 teaches an exact shape can using
insitu lasering of the metal powder one layer at a time in layers
which are only thousandths of inches thick. Once the entire part is
lased, producing a shaped can, the part undergoes hot isostatic
pressing (HIP) to compact the metal powder. This method, however,
is very time consuming due to the many thin layers that are
required, e.g. it may take two days or more to produce one can. The
part is then compacted by HIP. This method is not an effective
method for mass manufacturing of powder metal near net shape
parts.
[0009] Finally, U.S. Pat. No. 5,816,090 teaches that the need for a
can may be circumvented by applying gas pressure to the preform
very rapidly so that the gas does not have time to penetrate the
preform thereby compacting the preform. However, this method
requires a preform with a density of near 90%. Typical sinter
densities, however, are approximately 80%, which is not sufficient
for this method to work.
[0010] Most of these processes start with loose powder that is
poured into a can which is evacuated and sealed and subsequently
compacted. The can is designed so that after compaction, the powder
in the can loosely approximates the final shape of the desired
part. When the part shape is very complex or large, however, can
fabrication is very expensive. To reduce the cost of the cans, the
quality of the shape is sacrificed by providing an excessively
large part envelope. This results in excessive and unnecessary use
of expensive powder to fill the can and unnecessary machining to
bring the part to its final geometry after the consolidation
process. Moreover, because the can is usually a different material
than the powder, and because it is a continuous solid rather than a
powder, it reacts differently than the powder to the applied
pressures. This fact severely complicates the designer's job in
predicting the can shape necessary for a particular part.
[0011] Given the problems of can design, the need to use oversized
cans, the cost in fabricating the cans, the sacrificed shape to
accommodate cost and the machining requirements to remove the can
as well as the need to machine the part to obtain the final shape,
powder metallurgy is not effectively being used to produce large,
complex, near net shape parts.
BRIEF SUMMARY OF THE INVENTION
[0012] It is therefore an object of this invention to provide a
method of making a metal powder near net shape part with a closed
porosity surface coating.
[0013] It is a further object of this invention to provide such a
method which does not require a can to consolidate the powder.
[0014] It is a further object of this invention to provide such a
method in which the closed porosity surface coating acts as a can
for consolidating the powder.
[0015] It is a further object of this invention to provide such a
method in which the coating is a near net shape.
[0016] It is a further object of this invention to provide such a
method in which the coating is integral with the near net shape
part.
[0017] It is a further object of this invention to provide such a
method which does not require machining the coating off of the near
net shape part.
[0018] It is a further object of this invention to provide such a
method in which the final part requires only finish machining.
[0019] It is a further object of this invention to provide such a
method which produces a near net shape powder metal part with near
full density.
[0020] It is a further object of this invention to provide such a
method which produces a gas free near net shape powder metal
part.
[0021] It is a further object of this invention to provide such a
method which produces a near net shape part with a closed porosity
surface coating of a material the same as or different than the
preform.
[0022] It is a further object of this invention to provide such a
method which is cost effective to implement.
[0023] The invention results from the realization that a truly near
net shape part may be accomplished by applying to a powder preform
a coating of fine powder, finer than that of the preform, and
heating the powder coated preform to sinter the coating to form a
near full density, gas impermeable, closed porosity surface coating
on the preform. The part may be utilized as is, or the part may be
subjected to increased pressure to further compact the preform to
achieve a near net shape part of predetermined density.
[0024] The invention features a method of making a low density part
with a closed porosity surface coating by applying, to a porous
powder preform, a coating of powder finer than that of the preform
which is sinterable to a near full density below the melting
temperature of the powder, and heating the coated preform to sinter
the coating to form a near full density, gas impermeable, closed
porosity surface coating on the preform.
[0025] In a preferred embodiment an increased pressure may be
applied to the closed porosity surface coating for transmitting the
force of the pressure through the coating to consolidate the porous
powder preform to a predetermined density. The increased pressure
may include gas pressure. The preform may be consolidated to near
full density. The step of applying may include adding the metal
powder to a carrier and applying the carrier to the preform. The
carrier may include a binder for aiding the powder in adhering to
the preform. Applying may include brushing the powder onto the
preform, dipping the preform into the powder or spraying the powder
onto the preform. Spraying may include melting the powder and
spraying the molten droplets onto the preform. The coated part may
be placed in a vacuum furnace and the pressure reduced to remove
any gas from the porous preform prior to heating. The powder may
spherical. The coating of powder may be the same material as the
preform.
[0026] The invention also features a method of making a high
density part from a low density part by applying, to a porous
powder preform, a coating of powder finer than that of the preform
which is sinterable to a near full density below the melting
temperature of the powder coating, heating the powder coated
preform to sinter the coating to form a near full density, gas
impermeable, closed porosity surface coating on the preform, and
increasing pressure to the closed porosity surface coating for
transmitting the force of the pressure through the coating to
consolidate the porous powder preform to a predetermined
density.
[0027] In a preferred embodiment the increased pressure may include
gas pressure. The preform may be consolidated to near full density.
The step of applying may include adding the metal powder to a
carrier and applying the carrier to the preform. The carrier may
include a binder for aiding the powder in adhering to the preform.
Applying may include brushing the powder onto the preform, dipping
the preform into the powder or spraying the powder onto the
preform. Spraying may include melting the powder and spraying the
molten droplets onto the preform. The coated part may be placed in
a vacuum furnace and the pressure reduced to remove any gas from
the porous preform prior to heating. The powder may be spherical.
The coating of powder may the same material as the preform.
[0028] The invention also features a method of making a metallic
part with a closed porosity surface coating by applying, to a
metallic part, a coating of powder sinterable to a near full
density below the melting temperature of the powder, and heating
the coated part to sinter the powder coating to form a near full
density, gas impermeable, closed porosity surface coating.
[0029] In a preferred embodiment an increased pressure may be
applied to the closed porosity surface coating for transmitting the
force of the pressure through the coating to consolidate the porous
powder preform to a predetermined density. The increased pressure
may include gas pressure. The preform may be consolidated to near
full density. The step of applying may include adding the metal
powder to a carrier and applying the carrier to the preform. The
carrier may include a binder for aiding the powder in adhering to
the preform. Applying may include brushing the powder onto the
preform, dipping the preform into the powder or spraying the powder
onto the preform. Spraying may include melting the powder and
spraying the molten droplets onto the preform. The coated part may
be placed in a vacuum furnace and the pressure reduced to remove
any gas from the porous preform prior to heating. The powder may
spherical. The coating of powder may the same material as the
preform.
[0030] The invention also features a low density powder metal near
net shape part with a closed porosity coating produced by applying,
to a porous powder preform, a coating of powder finer than that of
the preform which is sinterable to a near full density below the
melting temperature of the powder, and heating the coated preform
to sinter the coating to form a near full density, gas impermeable,
closed porosity surface coating on the preform.
[0031] The invention also features a near net shape part having a
porous metal powder preform, and a fine metal powder coating
disposed on said preform, the metal powder coating sintered below
the melting temperature of the powder coating to form a full
density, gas impermeable closed porosity coating on the
preform.
[0032] The invention also features a method of making a near net
shape full density coating by applying a powder coating to a powder
metal preform, the powder coating sinterable to full density, and
heating the coated preform to sinter the coating to form a closed
porosity, gas impermeable near net shape coating.
[0033] In a preferred embodiment the powder of the coating may be
finer than that of the preform.
[0034] The invention also features a method of making a low density
part with a gas impermeable coating by applying, to a porous powder
preform, a coating of powder finer than that of the preform and
having a lower melting point than the preform and heating the
powder coated preform to melt the coating such that the coating
reacts with the preform to form a near full density, gas
impermeable, closed porosity surface coating on the preform.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] Other objects, features and advantages will occur to those
skilled in the art from the following description of a preferred
embodiment and the accompanying drawings, in which:
[0036] FIG. 1 is a block diagram generally showing the method of
the present invention;
[0037] FIG. 2 is a more detailed block diagram of the method of
FIG. 1;
[0038] FIG. 3 is a block diagram, similar to FIG. 2, in which the
powder is applied to the inside of a mold surface prior to being
applied to the preform;
[0039] FIG. 4 is an example of a near net shape porous metal powder
preform prior to application of the fine metal powder coating;
[0040] FIG. 5 is a micrograph of the preform depicted in FIG. 4
demonstrating the porosity of the preform;
[0041] FIG. 6 shows the preform of FIG. 4 after a fine metal
powder, different than that of the preform, has been sintered to
form a fully dense, closed porosity surface coating according to
the method of the present invention;
[0042] FIG. 7 is a micrograph of the preform depicted in FIG. 6
demonstrating the fully dense, closed porosity surface coating;
[0043] FIG. 8 is micrograph, similar to FIG. 7, in which the fully
dense, closed porosity coating is the same material as the preform;
and
[0044] FIG. 9 is an example of the metal powder applied to a mold
surface according to the diagram of FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0045] As discussed in the Background of the Invention above,
powder metallurgy has not historically been economically feasible
for producing large, complex near net shape parts. Economic success
has been hindered due to the need for expensive prior art cans and
post machining operations of not only the final geometry of the
part, but also machining of the prior art can off the final
part.
[0046] The method of the present invention, however, solves these
problems by producing near net shape parts using porous, low
density powder metal preforms and forming on the preform a hermetic
or closed porosity surface coating or skin. Generally, a separate
can is required to translate gas pressure to the powder preform,
due to its porosity, in order to consolidate and compact the
preform to increase its density. Without a can, the gas pressure
passes right through the preform and no consolidation occurs. The
expense of the prior art can is reduced by lessening the detail of
the part thus requiring additional machining other than that
required to remove the can in order to obtain the final shape.
Where the material is, for example, titanium, the wasted material
in the form of machining scrap can be very expensive.
[0047] By forming a closed porosity surface coating on the preform,
no expensive can is needed. Instead, the surface acts as the can.
Moreover, this "can" becomes an integral part of the preform and
need not be machined off. Thus, a near net shape preform may be
used and a near net shape coating is produced, as the coating
exactly conforms to the shape of the preform, minimizing machining
costs and unnecessary waste of expensive material.
[0048] According to the method of the present invention, a metal
powder, step 10, FIG. 1, is applied to a low density preform, step
12, to coat the preform. The preform may be manufactured by cold
isostatic compaction, metal injection molding, or other techniques
well known in the art. However, the preform may also be
manufactured as taught in a co-pending application entitled METHOD
OF MAKING HIGH DENSITY SINTERED METAL ARTICLES, incorporated herein
by this reference. The coated preform is then heated to a
temperature below the melting point of the powder, typically
0.7-0.9 of the melting point, to sinter the powder to form a full
or near full density, closed porosity surface coating which is gas
impermeable. Small powder particles will sinter to a much higher
density than larger particles. Additionally, very fine particles
can be sintered to full density at temperatures which will not
sinter larger particles. Depending on the desired application, the
low density sintered part may be machine finished for use, or the
sintered preform may be consolidated, step 16, shown in phantom.
Increased pressure is applied to the gas impermeable, closed
porosity coating which transmits the force of the pressure, through
the coating, to consolidate and compact the preform to the desired
density, e.g. near full density. This compaction may be
accomplished by cold isostatic pressing, i.e., simply increasing
gas pressure applied to the surface. Thus, there is no need to heat
the part. However, other compaction techniques which require heat
may also be used, e.g., HIP, RIP, ROC, and Ceracon consolidation
methods.
[0049] Thus, the present invention enables powder metal near net
shape parts with near full density to be produced without the need
for expensive can production, wasted material or expensive
machining.
[0050] Fine metal powder 18, FIG. 2 may typically be in the range
of 1-44 microns. Nanopowders, powders less than 1 micron in
diameter, will fuse more rapidly and at lower temperatures than the
1-44 micron fine powders. In any case, the fine metal powder must
be at least smaller than the powder of the preform. While spherical
powders are preferred, this is not a limitation as non-spherical
powders will also work. In order to improve the application of the
powder to the preform, the powder may be combined with a carrier,
step 20, such as alcohol, water, or other volatile organic, however
this is not a necessary limitation of the invention, as the powder
may be applied directly to the preform. Whether or not a carrier is
used, the metal powder is applied to the preform, for example with
a brush, sprayer, or by dipping the preform in the powder or
carrier combination, dipping the part in a fluidized bed of powder
or even melting the powder and spraying onto the preform as molten
droplets, e.g. plasma spraying. If a carrier is used, the carrier
must be allowed to evaporate.
[0051] Once the metal powder is applied such that a visible and
continuous coating is obtained, the coated preform is heated below
the melting temperature of the powder, step 24, in a furnace.
[0052] Once the metal powder has sintered to full or near full
density to provide a closed porosity surface coating, the sintered,
coated preform is removed from the furnace.
[0053] In certain applications, the part's intended use may be for
high temperature applications. If there is gas within the preform,
the high temperatures will cause the gas to expand, affecting the
strength and integrity of the part, causing the part to eventually
fail. Thus, in high temperature applications the coated preform may
be placed in a vacuum furnace and the pressure reduced. Because the
coating is still a powder at this point, the furnace must be
evacuated sufficiently slow so that the powder will not be
dislodged from the preform by the escaping gas.
[0054] No matter which technique is used, however, the result is a
low density powder metal, near net shape part with a closed
porosity surface coating, step 26. Again, depending on the
application, the part produced may be machine finished and ready
for use.
[0055] However, where higher density is necessary the sinter coated
preform, step 24, may be consolidated by increasing the
environmental pressure such as by cold isostatic pressing or by
standard hot consolidation such as HIP, RIP, ROC and Ceracon
techniques, step 30. By using cold isostatic pressing, costs are
reduced since no heating is required. Because the surface coating
has a closed porosity, cold isostatic pressing is preferred because
the increased gas pressure does not pass through the coating, but
acts on the coating which in turn transmits the force of the gas
pressure to the powder preform to compact and consolidate the
powder of the preform to produce a powder metal, near net shape,
near full density part, step 32. However, this is not a necessary
limitation of the invention as the density required may be a
predetermined density somewhere between the preform density and
full density. Thus, the coating acts as a can which becomes
integral with the preform. In the case where the coating is to be a
material of a lower melting point than the preform, the preform can
even be dipped in a molten bath of the material. Pressure and the
vacuum infiltration techniques could be easily used to adjust the
depth of penetration of the coating.
[0056] Another feature of the present invention is that the fine
metal powder may be sintered in the same step as the preform yet
still eliminates the need for a prior art can. The fine metal
powder may be combined with another medium to form a slurry, step
34, FIG. 3, and poured into the mold, step 36, for example a
ceramic disposable shell mold or any standard mold. However, a
ceramic disposable shell mold is used to produce large, complex
parts. The slurry may be a combination of the metal powder and, for
example, a carrier for dispersing the powder and a wetting agent to
allow the slurry to adhere to the mold surface. While slurries can
be made of many different types of metal powder it is important
that the powder not rapidly settle out of the slurry, and that the
slurry fully wet the powder. To this end the smaller the powder
used the better. With the interior of the mold sufficiently coated,
the mold is filled with preform powder, step 38.
[0057] After coating the mold and filling with preform powder, the
mold is heated to sinter the coating and preform powder, step 40.
Because the powder coating consists of fine particles, the coating
will sinter to form a closed porosity surface first. As the
temperature increases, the preform powder will sinter. Once the
mold is removed, step 42, the low density near net shape part with
closed porosity coating may be machined for use, step 44, or the
part may undergo consolidation, step 46, to produce a near net
shape, near full, or predetermined density, step 46, as discussed
above.
[0058] Alternatively, once the slurry is poured into the mold, step
36, the coated mold may be heated, step 37, to sinter the powder
coating to produce a near net shape closed porosity coating inside
the mold. The mold with sintered coating may then be filled with
perform powder, step 39, and the mold heated a second time to
sinter the preform powder to the closed porosity coating, step 41.
After the second heating, the mold is removed, step 42, and the
part machined for use or consolidated as discussed above.
[0059] To demonstrate the feasibility of the present invention, a
1.5 kg porous powder preform 52, FIG. 4, was sintered using less
than 35 mesh, typically having a particle size less than 500
microns, Ti 6Al-4V powder (six percent aluminum and four percent
vanadium). The porosity of the powder preform is demonstrated by
its dull appearance and lack of metallic reflectivity. Preform 52,
FIG. 5, had a density of approximately 80%. As the micrograph
demonstrates, surface 54 of the preform 52 is very porous and no
consolidation could take place as the gas would easily flow through
preform 52.
[0060] Fine aluminum powder, 2-5 micron, was suspended in an
ethanol solution and painted onto preform 52 using a paint brush.
The ethanol carrier rapidly wicked into the interior of porous
preform 52 leaving the aluminum powder loosely caked to the
preform. The carrier was allowed to evaporate at room temperature
for twenty four hours.
[0061] Preform 52 was placed in a vacuum furnace and slowly
evacuated to 5.times.10.sup.-5 torr to avoid dislodging the powder
from the preform. The temperature was increased to 1200.degree.C.
at 10.degree.C./s and held thereafter for 120 minutes to sinter the
aluminum powder to the Ti-6Al-4V preform by transient liquid phase
sintering.
[0062] The closed porosity surface 56, FIG. 6, of sintered preform
58 is demonstrated by the metallic reflectivity due to the full
density fused coating.
[0063] The result is more clearly demonstrated by closed porosity
surface 56, FIG. 7. As can be seen from the micrograph, the
sintered preform 58 includes porous preform 52 with a closed
porosity, fully dense coating 56.
[0064] In a similar manner, a second less than 35 mesh, typically
having a particle size less than 500 microns, Ti 6Al-4V powder
preform was coated with less than 44 micron Ti-6Al-4V using ethyl
alcohol as a carrier. The coated preform was vacuum sintered at
1500.degree.C., below the melting point of Ti-6Al-4V, for 120
minutes. As above, the sintered preform emerged with a highly
reflective surface indicating the fully dense closed porosity
coating. The closed porosity surface of sintered coated preform
58', FIG. 8, is demonstrated by continuous surface 56' covering
porous preform 52'. Accordingly, one metal may be coated onto a
preform of another.
[0065] Thus, as demonstrated above, the method of the present
invention produces a closed porosity surface coating on a porous
powder preform. Accordingly, the coating need not be the same
material as the preform.
[0066] Further, the preform need not be a porous powder preform as
described above. The preform may be an already near fully dense
preform such as a cast steel sea water valve. The valve can then be
coated with titanium powder and sintered as above, to produce a
steel sea water valve with a thick titanium coating which is
impervious to sea water and thus corrosion resistant. The current
practice is to cast the titanium valve entirely from titanium at
considerable cost. Adhesion of the surface coat prior to sintering
can be improved by using small amounts of a residue binder in the
solvent such as agar, polysaccharides or any of the binders used in
conventional metal injection mold (MIM) processing. Thus the
invention may be applied to coat parts which are subject to
aggressive environments.
[0067] Another aspect of the invention, as discussed in FIG. 3, is
to coat the inside of a mold with the fine powder, then fill it
with coarse metal preform powder, e.g. 500 micron, and sinter.
[0068] A slurry was prepared by suspending 165 g of less than 325
mesh, or less than 44 micron Cp Ti (commercially pure titanium) in
a solution consisting of 200 ml of distilled water, 600 g of poly
vinyl alcohol, 2 ml Nalco 8815 wetting agent and 0.2 ml Nalco H10
antifoaming agent. The slurry was then poured into a conventional
porous, alumina shell crucible 60, FIG. 9. The crucible was mildly
shaken to distribute the slurry and coat the mold surface. The
excess slurry was drained from the mold leaving the dark titanium
powder coating 62. The crucible was dried at 100.degree.C., filled
with less than 35 mesh, or less than 500 micron, powder and
sintered for 120 minutes at 1500.degree.C. in a vacuum furnace. The
resulting near net shape may be finish machined or compacted as
above.
[0069] While only preforms of Ti-6Al-4V have been described, this
process can be used with any powder. Stainless steels, tool steels,
and super alloys are just a few examples that are sintered or hot
isostatically pressed to make parts. Although PREP powders and gas
atomized powders are preferred for the preform due to their
spherical shape, any type of powder such as water atomized or
chemically reduced powder will also work. Although spherical gas
atomized powder was employed for the coatings, non-spherical powder
such as is made by the hydride dehydride process or by comminution
will offer an advantage due to their blocky shape which will help
them interlock with the surface pores of the preform.
[0070] While the above work has used powder less than 44 microns in
diameter, this is not a necessary limitation of the invention as
there are many factors that could make smaller or larger sizes more
desirable. The required powder size will depend on the powder's
sintering kinetics for a specific alloy. Some alloys may require
finer powder, while other alloys may allow the use of coarser
powder. Finer powder will provide more rapid sintering and better
adhesion of the unsintered coat, while coarser powder will be less
expensive and result in less oxygen contamination.
[0071] Some typical powders and their respective sinter and melting
points are shown in the following table. However, this in no way
limits the type and sizes of metal powders which may be used, or
the sintering temperature for which the method of the present
invention will produce a gas impermeable, closed porosity coating.
Indeed, the method of the present invention may also be
accomplished with powders and preforms that produce a eutectic.
That is, a powder with a high melting point, such as iron
(1538.degree.C.), may be applied to a preform with a similarly high
melting point such as titanium (1668.degree.C). The two metals
together will actually melt below the melting point of either
metal, approximately 1085.degree.C., to form a eutectic.
1 Particle Size (range) Sinter (range) Melting Point Metal Powder
(.mu.) (.degree. C.) (.degree. C.) Al 2-5 1200 660 Ti <44
1120-1415 1600 1080 steel <37 1043-1415 1490 Stainless steel
<37 962-1306 1375 316 Cp Ti <44 1120-1520 1600 PWA 771 <44
931-1264 1330 F-75 <100 1015-1377 1450
[0072] Although specific features of the invention are shown in
some drawings and not in others, this is for convenience only as
each feature may be combined with any or all of the other features
in accordance with the invention.
[0073] Other embodiments will occur to those skilled in the art and
are within the following claims:
* * * * *