U.S. patent application number 10/791079 was filed with the patent office on 2004-09-23 for metal powder composition for laser sintering.
This patent application is currently assigned to 3D Systems, Inc.. Invention is credited to Geving, Brad, Newell, Kenneth J., Schmidt, Kris Alan.
Application Number | 20040182202 10/791079 |
Document ID | / |
Family ID | 32988347 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040182202 |
Kind Code |
A1 |
Geving, Brad ; et
al. |
September 23, 2004 |
Metal powder composition for laser sintering
Abstract
A powder blend for use in laser sintering and a method for
forming tough, strong, wear-resistant, corrosion-resistant
infiltrated metal products are provided. The powder blend comprises
a steel alloy, a polymeric binder and a high melting temperature
fine particulate which are blended together, then applied layer by
layer to a working surface in a laser sintering system, exposed a
layer at a time to fuse together the powder until a green part of
high strength is formed, and then the green part is infiltrated
with a metal infiltrant in a non-reducing gas atmosphere at an
effective temperature for an effective period of time. The
preferred steel is a mild steel alloy.
Inventors: |
Geving, Brad; (Newhall,
CA) ; Schmidt, Kris Alan; (Granada Hills, CA)
; Newell, Kenneth J.; (Valencia, CA) |
Correspondence
Address: |
Ralph D'Alessandro
3D Systems, Inc.
26081 Avenue Hall
Valencia
CA
91355
US
|
Assignee: |
3D Systems, Inc.
|
Family ID: |
32988347 |
Appl. No.: |
10/791079 |
Filed: |
March 2, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10791079 |
Mar 2, 2004 |
|
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10394312 |
Mar 19, 2003 |
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Current U.S.
Class: |
75/252 |
Current CPC
Class: |
Y02P 10/25 20151101;
B22F 2003/248 20130101; B22F 2998/10 20130101; C22C 33/02 20130101;
B22F 2999/00 20130101; B33Y 10/00 20141201; B33Y 40/00 20141201;
B33Y 70/00 20141201; B22F 2998/10 20130101; B22F 10/20 20210101;
B22F 3/26 20130101; B22F 3/24 20130101; B22F 2999/00 20130101; B22F
3/24 20130101; B22F 2202/03 20130101; B22F 2998/10 20130101; B22F
10/20 20210101; B22F 3/26 20130101; B22F 3/24 20130101 |
Class at
Publication: |
075/252 |
International
Class: |
C22C 029/08 |
Claims
What is claimed:
1. A powder blend for use in a laser sintering process comprising a
steel alloy selected from the group consisting of a mild steel
alloy, a carbon steel and a stainless steel, a polymeric binder and
a high melting temperature fine particulate.
2. The powder blend according to claim 1 wherein the steel alloy
ranges in size from less than about 90 microns to about 4
microns.
3. The powder blend according to claim 2 wherein the steel alloy
ranges in size from less than about 75 microns to about 8
microns.
4. The powder blend according to claim 2 wherein the steel alloy is
less than about 45 microns.
5. The powder blend according to claim 1 wherein the steel alloy is
spherical.
6. The powder blend according to claim 2 wherein the high melting
temperature fine particulate has a particle size less than about 10
microns.
7. The powder blend according to claim 6 wherein the high melting
temperature fine particulate has a particle size less than about 2
microns.
8. The powder blend according to claim 7 wherein the high melting
temperature fine particulate comprises greater than about 5 weight
percent and less than about 15 weight percent of the powder
blend.
9. The powder blend according to claim 8 wherein the high melting
temperature fine particulate comprises about 8 weight percent of
the powder blend.
10. The powder blend according to claim 1 wherein the polymeric
binder is a thermoplastic or a thermoset.
11. The powder blend according to claim 1 wherein the polymeric
binder is selected from the group consisting of polyethylene,
polypropylene, polyacetal, polymethacrylate, polyvinylacetate,
nylon, wax, phenolic and combinations thereof.
12. The powder blend according to claim 11 wherein the polymeric
binder is nylon.
13. The powder blend according to claim 12 wherein the nylon is one
selected from the group consisting of polymers and co-polymers of
nylon 6, nylon 9, nylon 10, nylon 11, and nylon 12.
14. The powder blend according to claim 1 further comprising a flow
agent.
15. The powder blend according to claim 14 wherein the flow agent
is fumed silica.
16. A method of forming a tough, strong, wear-resistant,
corrosion-resistant, infiltrated metal product comprising the steps
of: a. mixing together a powder blend comprising a steel alloy
selected from the group consisting of a mild steel alloy, a carbon
steel and a stainless steel, a polymeric binder and a high melting
temperature fine particulate; b. applying a layer of the powder
blend to a working surface in a laser sintering system; c. exposing
the layer of the powder blend to heat energy to fuse together the
steel alloy and high melting temperature fine particulate by the
melting and subsequent rehardening of the binder material; d.
applying a new layer of powder blend and exposing the new layer of
powder blend in sequential fashion repeatedly until a
three-dimensional green metal part is formed; and e. infiltrating a
green metal part with metal infiltrant in a gas atmosphere at an
effective temperature for an effective time period.
17. The method according to claim 16 wherein using a powder blend
comprising about 88.75 to about 92.75 weight percent mild steel
alloy; about 6 to about 9 percent tungsten carbide, and about 1.25
to about 2.25 weight percent polymer binder.
18. The method according to claim 16 further comprising using
copper and/or copper containing alloys as a metal infiltrant.
19. The method according to claim 18 further comprising using a gas
selected from the group consisting of nitrogen, argon, or a
nitrogen argon blend as the gas atmosphere during infiltration.
20. The method according to claim 19 using nitrogen as the gas
atmosphere during infiltration.
21. The method according to claim 19 further comprising performing
the infiltrating using an infiltration cycle having a peak
temperature of about 1070.degree. C.
22. The method according to claim 16 further comprising exposing
the infiltrated green metal part to a heat treatment cycle.
23. The method according to claims 20 further comprising using a
fine grit alumina packing medium as a support material to encase
the green metal part during infiltration.
24. The method according to claim 17 further comprising using a
powder blend having about 8 weight percent high melting temperature
fine particulate, about 1.6 to about 2.1 weight percent nylon
binder, and the remainder a mild steel alloy.
25. The method according to claim 24 further comprising
deagglomerating the mild steel alloy to a range in size from less
than about 90 microns to about 4 microns.
26. The method according to claim 24 further comprising
deagglomerating the mild steel alloy to a range in size from less
than about 75 microns to about 8 microns.
27. The method according to claim 25 further comprising
deagglomerating the high melting temperature fine particulate.
28. The method according to claim 27 further comprising using a
high melting temperature fine particulate having a particle size
less than about 10 microns.
29. The method according to claim 22 further comprising performing
the heat treatment cycle having a peak temperature of about
840.degree. C. for at least one hour, quenching the infiltrated
part with room temperature nitrogen to reduce the part temperature
over an effective time, sub-cooling the part to about -79.degree.
C. over at least a 90 minute period, return the part to room
temperature, and temper the part for about 3 hours at about
163.degree. C.
30. The method according to claim 16 further comprising prior to
infiltrating, absorbing nitrogen into a mild steel alloy in the
green metal part.
31. The method according to claim 30 further comprising prior to
infiltrating, maintaining the green metal part at a temperature in
excess of about 850.degree. C. and less than about 900.degree. C.
for about 4 to about 6 hours in a nitrogen atmosphere.
32. The powder blend according to claim 9 wherein the high melting
temperature fine particulate is tungsten carbide.
33. The method according to claim 28 further comprising using
tungsten carbide as the high melting temperature fine particulate.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to the field of solid
freeform fabrication ("SFF") of parts and more specifically to the
powder blend for use in the selective laser sintering process
utilizing a steel alloy and the method of forming three-dimensional
parts employing that powder blend.
[0003] 2. Description of the Relevant Art
[0004] SFF generally refers to the manufacture of articles in a
layer-wise or additive fashion directly from computer-aided-design
(CAD) databases in an automated fashion, as opposed to conventional
machining of prototype articles from engineering drawings in
subtractive processes. SFF has, in recent years, made substantial
improvements in providing high strength, high density parts for use
in the design and pilot production of many useful articles. As a
result, the time required to produce prototype parts from
engineering designs has reduced from several weeks, using
conventional machinery and subtractive processes, to a matter of
hours.
[0005] One example of an SFF technology is the selective laser
sintering process practiced by systems available from 3D Systems,
Inc. of Valencia, Calif. According to this technology, articles are
produced in layer-wise fashion from a laser-fusible powder that is
dispensed one layer at a time. The powder is fused, or sintered, by
the application of laser energy that is directed to those portions
of the powder corresponding to a cross-section of the article.
After the fusing of powder in each layer, an additional layer of
powder is then dispensed, and the process repeated, with fused
portions of later layers fusing to fused portions of previous
layers (as appropriate for the article), until the article is
complete. Detailed description of the selective laser sintering
technology may be found in U.S. Pat. Nos. 4,863,538 and 5,017,753,
both assigned to Board of Regents, The University of Texas System,
and in U.S. Pat. No. 4,247,508, to Housholder. The selective laser
sintering technology has enabled the direct manufacture of
three-dimensional articles of high resolution and dimensional
accuracy from a variety of materials including nylons,
polystyrenes, and composite materials such as polymer coated metals
and ceramics. Examples of composite powder materials are described
in U.S. Pat. Nos. 4,944,817; 5,076,869; and 5,296,062, all assigned
to Board of Regents, The University of Texas System, and
incorporated herein by reference in pertinent part.
[0006] A related SFF technology, referred to as 3-Dimensional (3D)
Printing, is described in U.S. Pat. Nos. 5,340,656 and 5,387,380.
From a computer (CAD) model of the desired part, a slicing
algorithm draws detailed information for every layer. Each layer
begins with a thin distribution of powder spread over the surface
of a powder bed. Using a technology similar to ink-jet printing, a
binder material selectively joins particles where the object is to
be formed. A piston that supports the powder bed and the
part-in-progress lowers so that the next powder layer can be spread
and selectively joined. This layer-by-layer process repeats until
the part is completed. Following a heat treatment, unbound powder
is removed, leaving the fabricated part.
[0007] As SFF technology has evolved, it has increasingly been used
not only to make prototype parts but also to make final useful
parts as well as tools or molds that can be used to make multiple
parts. It is becoming more common to fabricate such parts, tools,
or molds with an "indirect" process that uses a powder of metal
and/or ceramic particles either coated by or blended with a
polymer. The powder is used in the selective laser sintering
process to fabricate a "green" article that binds the particles to
one another. The green article is then heated to a temperature
above the decomposition temperature of the polymer, which both
drives off the polymer and also binds the metal and/or ceramic
substrate particles to one another to form an intermediate porous
article. The porous article can then be infiltrated with another
material, such as a lower melting temperature metal to give a fully
dense article with desirable properties. The green article can also
be fabricated with 3D printing.
[0008] Some examples of the use of these approaches for functional
applications are described, for example, in U.S. Pat. Nos.
5,433,280, 5,544,550, and 5,839,329 to Smith et al. These describe
the use of selective laser sintering a tungsten carbide-polymer
composite powder to generate a "green" drill bit which is then
infiltrated in a furnace cycle with a copper alloy to generate a
fully functional drill bit for down hole oil exploration. U.S. Pat.
No. 4,554,218 describes the use of a powder mixture having a first
metal and a second metal, such as A6 tool steel, and a fugitive
binder that is placed in a mold, cured to a green part and then
infiltrated with a third metal, preferably a copper or
copper-containing alloy, to form an infiltrated, molded metal
composite article. Another commercial application of these indirect
approaches is a product called ProMetal by ExtrudeHone. Utilizing
the 3D Printing technology described above, ProMetal builds metal
components by selectively binding metal powder layer by layer. The
finished structural skeleton is then sintered and infiltrated with
bronze to produce a finished part that is 60% steel and 40% bronze
and is used for injection molding tools or final metal parts.
Another commercial example is 3D Systems' ST-100 system, which uses
selective laser sintering of a steel polymer composite powder to
generate a green article. The green article or part is subsequently
put through a furnace cycle that removes the polymer binder and
infiltrates the metal skeleton with bronze to create a functional
fully dense article that can also be used for injection mold tools
or final parts.
[0009] As is well known in the art, the structural strength of the
green article is an important factor in its utility, since weak
green articles cannot be safely handled during subsequent
operations. Another important factor in the quality of a prototype
article is its dimensional accuracy relative to the design
dimensions. However, these factors of part strength and dimensional
accuracy are generally opposed to one another, since the
densification of the powder that occurs in the sintering of the
post-process anneal also causes shrinkage of the article. The
polymer content of a metal and/or ceramic composite powder
described above could be increased in order to provide higher green
part strength, but the shrinkage of the part in post-process anneal
would increase accordingly. As a result, compromises between
article strength and dimensional stability must be made in the
design of a composite powder system using a polymeric binder.
[0010] Some drawbacks of conventional composite powders
incorporating thermoplastic polymer binders have been observed. In
the post-process anneal of green articles using such binders, creep
deformation has been observed as the article is heated to a
temperature above the glass transition temperature of the polymer
binder, but below the decomposition temperature at which the binder
is released. The viscosity of the polymer decreases to such an
extent that the metal or ceramic substrate particles slide past one
another under the force of gravity. Not only do the dimensions of
the article change as a result of this creep deformation, but also
this dimensional change is not uniform. Taller features deform by a
larger extent than do shorter features. This non-uniformity in
deformation precludes the use of a constant shrinkage correction
factor in the selective laser sintering fabrication of the green
part, further exacerbating the difficulty of achieving
dimensionally accurate articles of high density and strength.
[0011] Creep deformation has been observed to deform not only the
height but also the shape of vertical features, such as sidewalls.
For example, vertical walls of mold cavities formed by selective
laser sintering of polymer-coated metal powders, and having a
thickness of 0.75 inches and a height of 1.5 inches, have been
observed to bow outwardly as a result of creep deformation. The
dimensional accuracy of the infiltrated final part is, of course,
severely compromised by such deformation.
[0012] To address this tendency of creep deformation, another prior
art technique was developed that combined the use of a
thermoplastic binder with a thermoset binder. This is described in
U.S. Pat. No. 5,749,041. In this approach a "green" part is formed
by the selective laser sintering of a metal-polymer composite
powder, in which the polymer binder is a thermoplastic polymer.
Following its fabrication, the green article is infiltrated with a
thermosetting material prior to heating the part. The thermosetting
material may be an aqueous emulsion of a cross-linkable polymer
with a cross-linking agent, or may instead be an aqueous emulsion
of only the cross-linking agent. In the first case, the
cross-linking agent reacts with the cross-linkable polymer in the
infiltrant to form a rigid skeleton for the green article; in the
second case, the cross-linking agent reacts with the polymer binder
of the green article to form the rigid skeleton. Following the
formation of the rigid skeleton, the article may be heated to
decompose the polymer and sinter the metal substrate particles,
followed by infiltration with a metal for added strength. This
prior art approach provided a solution to the creep deformation
problem, but added significant time to the post processing of the
part to dry out the article after the aqueous infiltration
step.
[0013] Another approach used commercially to avoid the
aforementioned drying step incorporates both a thermoplastic and
thermoset binder in the formulation of the metal-polymer composite
article. In one successful version a phenolic type thermoset is
combined with a wax binder to give a system that provides adequate
initial green strength and a more rigid skeleton for the green
article. The green strength of this system though, while improved,
still suffers from unacceptable failure rates due to breakage of
green parts in handling. Thus the search for stronger green part
systems has continued.
[0014] The trend has been to use more polymer binder materials as
one approach to achieve higher green strength parts. However, as
the amount and complexity of binders used in these metal and/or
ceramic polymer composite powders has increased, it has been
increasingly difficult to removing all of the polymer system
binders during the decomposition and burn-out phase. The
decomposition of the polymer into smaller fragments should be
complete enough to ensure that the bulk of the hydrocarbon
fragments can escape the article skeleton before the infiltrating
metal (copper or bronze, for example) enters the skeleton. If all
of the hydrocarbon fragments do not escape, the interconnectivity
of pores in the resulting metal part is decreased and outgassing is
hampered as the interpassages become blocked by trapped hydrocarbon
fragments leading to a phenomena of blistering on the surface and
potential delamination of the final article. In some systems the
presence of too much residual carbon can also impede the
infiltration process. The presence of a reducing atmosphere, such
as hydrogen or forming gas helps the polymer degradation greatly,
but is a more expensive alternate than a non-reducing gaseous
atmosphere.
[0015] Accordingly, there is a need to provide an improved powder
blend for use in conjunction with laser sintering and an
infiltration process to achieve finished articles possessing high
strength, enhanced toughness (resistance to crack growth),
increased surface hardness, increased corrosion resistance or
stainless property and improved surface finish with less distortion
and shrinkage during heat treatment and infiltration.
BRIEF SUMMARY OF THE INVENTION
[0016] It is an aspect of the present invention to provide a method
of fabricating high density and high strength articles and tooling
via SFF techniques from powder blend and a powder blend comprising
at least a steel alloy selected from the group consisting of mild
steel, carbon steel and stainless steel, a polymeric binder and a
high melting temperature fine particulate metallic, intermetallic
or ceramic with improved initial green strengths.
[0017] It is another aspect of the present invention that a metal
powder blend is employed which provides improved part building in a
laser sintering process.
[0018] It is a still another aspect of the present invention to
provide such a powder blend comprising at least a steel alloy
selected from the group consisting of mild steel, carbon steel and
stainless steel, a polymeric binder and a high melting temperature
fine particulate metallic, intermetallic or ceramic, and a method
using such a powder blend in a laser sintering SFF technique that
improves dimensional accuracy.
[0019] It is a further aspect of the invention to provide such a
method for use in a laser sintering SFF technique while avoiding
blistering phenomena even in nitrogen atmospheres.
[0020] It is a feature of the present invention that the base alloy
is mild steel that yields low shrinkage during infiltration with a
preferred metal infiltrant.
[0021] It is another feature of the present invention that a
preferred infiltrant is copper and/or a copper containing
alloy.
[0022] It is still another feature of the present invention that
the material composition is blended for an extended period of time
to break up agglomerations of a high melting temperature fine
particulate metallic, intermetallic or ceramic material.
[0023] It is a further feature of the present invention that the
infiltration occurs in a gas atmosphere.
[0024] It is yet another feature of the present invention that the
polymeric binder is a co-polymer of nylon 6 and nylon 12.
[0025] It is still another feature of the present invention that
the metal powder composition is able to employ a lower percentage
of binder material to increase the green strength of the part
formed from the laser sintering process.
[0026] It is an advantage of the present invention that the powder
blend and method using such powder blend produce high quality,
fine-featured green parts with minimal curl.
[0027] It is an another advantage of the present invention that the
powder blend and method using such powder blend produce parts after
infiltration with excellent surface finish and high hardness.
[0028] It is a further advantage of the present invention that the
powder blend and method using such powder blend produce parts after
infiltration having a desirable balance of toughness, strength and
wear resistance.
[0029] It is yet another advantage of the present invention that
the powder blend and method using such powder blend produce parts
with excellent thermal conductivity after infiltration with a
copper containing alloy.
[0030] It is a further advantage of the present invention that the
metal powder composition flows without caking as it is distributed
across the part bed of the laser sintering system.
[0031] It is yet a further advantage of the present invention that
the laser sintered part obtained using the metal powder composition
of the present invention has a higher green strength than prior
metal compositions despite not employing an increased binder
composition.
[0032] It is still a further advantage of the present invention
that the laser sintered part obtained using the metal powder
composition of the present invention is corrosion resistant or
possesses stainless property.
[0033] The invention may be incorporated into a method of
fabricating an article, such as a prototype part or a tooling for
injection molding, using selective laser sintering. According to
the present invention, the selective laser sintering of a powder
blend comprising at least a steel alloy selected from the group
consisting of mild steel, carbon steel and stainless steel, a
polymeric binder, and a high melting temperature fine particulate
metallic, intermetallic or ceramic forms a "green" part. After
removal of unfused material from the green part, it is placed in an
oven or furnace preferably in a non-reducing atmosphere such as,
for example, nitrogen, argon or a nitrogen-argon blend for
subsequent heat treatment to decompose and drive off the binder and
sinter the metal substrate particles prior to infiltration by a
metal with a lower melting point. During the critical step of
infiltrating the green part, the green part is packed in a fine
grit alumina in a non-reducing gas atmosphere that creates the
conditions necessary to provide sufficient surface area to absorb
the outgassing of the binder material and prevent redeposit on the
part. A preferred steel alloy is a mild steel alloy and a preferred
high melting temperature fine particulate is tungsten carbide.
[0034] These and other aspects, features and advantages are
obtained in the present invention wherein the powder blend for use
in a laser sintering process comprises a steel alloy selected from
the group consisting of mild steel, carbon steel and stainless
steel, a polymeric binder, and a high melting temperature fine
particulate metallic, intermetallic or ceramic and the powder blend
is employed in a method to form a tough, strong and wear resistant
product comprising the steps of:
[0035] a. mixing together a powder blend comprising a steel alloy
selected from the group consisting of mild steel, carbon steel and
stainless steel, a high melting temperature fine particulate
metallic, intermetallic or ceramic and a polymeric binder;
[0036] b. applying a layer of the powder blend to a working surface
in a laser sintering system;
[0037] c. exposing the layer of the powder blend to heat energy to
fuse together the mild steel alloy and tungsten carbide by the
melting and subsequent rehardening of the binder material;
[0038] d. applying a new layer of powder blend and exposing the new
layer of powder blend in sequential fashion repeatedly until a
three-dimensional green metal part is formed; and
[0039] e. infiltrating a green metal part with metal infiltrant in
a non-reducing gas atmosphere at an effective temperature for an
effective time period in an oven.
BRIEF DESCRIPTION OF THE DRAWING
[0040] These and other aspects, features and advantages of the
present invention will become apparent upon consideration of the
following detailed disclosure of the invention, especially when it
is taken in conjunction with the accompanying drawing, wherein:
[0041] FIG. 1 is a flow diagram illustrating a method of
fabricating a three-dimensional object according to an embodiment
of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0042] According to the present invention, three-dimensional
objects or articles of complex shapes may be made with high
dimensional accuracy and good part strength both in their green
state and also as finished articles. It is to be understood that,
while the present invention is particularly useful in the
fabrication of prototype injection molds and tooling, the present
invention may also be used to advantage in the fabrication of
prototype parts, such as used in the modeling of mechanical
systems. Indeed, it is contemplated that the selective laser
sintering process and the method of the present invention may be
used to manufacture end use articles and parts therefor,
particularly in custom or limited runs, as economics permit. As
such, the use of the term "article" in this description will be
employed to refer either to a part (prototype or end-use), or to
tooling for injection molding, thus encompassing various eventual
uses of the article.
[0043] Referring now to FIG. 1, a method of fabricating a
three-dimensional object or article according to a first embodiment
of the invention will now be described in detail. The method begins
with process 10, which is the selective laser sintering of a
blended metal powder to form a "green" article. The term "green"
refers to the intermediate state of the article, prior to its
densification as will be described hereafter.
[0044] The powder blend used in process 10 according to this
embodiment of the invention is a metal powder blended with or
coated by a polymeric binder system and also includes a high
melting temperature fine particulate metallic, intermetallic or
ceramic. The metal is a steel alloy and can be a stainless, carbon
or a low alloy or mild steel. Carbon steels are steels containing
less than about 2% by weight total alloying elements and low alloy
or mild steels are steels containing an alloy content from about
2.07% to about 10% by weight total alloy content. Stainless steels
contain at least 11% by weight chromium. The metal preferably is a
mild steel alloy. Alternate suitable metals can include, for
example, 17-4 PH (precipitated hardened) steel or 316 stainless
steel. The polymeric binder system may use thermoplastics,
thermosets, or a combination thereof. The binder system preferably
is a thermoplastic, such as a copolymer of nylon. Optionally a flow
agent may be used in the blend. Where one is used, fumed silica can
be employed such as Cabosil 720 available from Cabot
Corporation.
[0045] The alloy has a particle size ranging from less than about
90 .mu.m (microns) to about 4 .mu.m, more preferably from less than
about 75 .mu.m to about 8 .mu.m, and most preferably is less than
or equal to about 45 .mu.m. The steel alloy is deagglomerated by an
appropriate deagglomerating technique, including but not limited to
blending, mixing and attriting, prior to blending to obtain the
desired particle size range. The individual particles of steel
alloy preferably are spherical. Use of spherical particles appears
to obviate or at least lessen the need for separate flow agents
since the powder blend does not cake during its distribution or
spreading across the powder bed in the laser sintering system. A
preferred suitable mild steel alloy is A6 steel powder available
from the Stellite Division of Cabot Corporation or from UltraFine
Powder Technologies. A6 steel powder is an air-hardening tool and
die steel with non-deforming properties with especially attractive
properties for many die and mold applications.
[0046] The polymeric binder system is selected from the group
consisting of polyethylene, polypropylene, polyacetal,
polymethacrylate, polyvinylacetate; co-polymers of polyethylene,
polypropylene, polyacetal, polymethacrylate, polyvinylacetate;
nylon, wax, phenolic and combinations thereof. More preferably the
binder system utilizes polymers and co-polymers of nylon, such as
ones selected from the group consisting of co-polymers of nylon 6,
nylon 9, nylon 10, nylon 11, and nylon 12. Most preferable are
co-polymers of nylon 6 and nylon 12, such as nylon 6, 12. Nylon
homopolymers may also be appropriate, such as nylon 6 or nylon 12.
The polymeric binder must melt and freeze or recrystallize between
about 75.degree. C. and about 200.degree.0 C. and more preferably
between about 100.degree. C. and about 150.degree. C. to obtain
optimal processing. It is theorized that a co-polymer having a
lower melt viscosity facilitates optimal processing.
[0047] The powder blend also includes a high melting temperature
fine particulate metallic, intermetallic or ceramic which has a
particle size less than about 10 microns and more preferably less
than about 2 microns. The high melting temperature fine particulate
metallic, intermetallic or ceramic comprises greater than about 5
percent and less than about 15 percent by weight of the powder
blend, more preferably between about 6 to about 9 percent by weight
of the powder blend and most preferably comprises about 8 percent
by weight of the powder blend. Suitable high melting temperature
fine particulate metallic, intermetallic or ceramic materials
include tungsten, tantalum, hafnium, rhenium, molybdenum, titanium
aluminides, silicon carbide, tungsten carbide, boron carbide,
alumina and diamond. A high melting temperature is deemed within
the context of the present invention to be a temperature less than
about 1500.degree. C. and as a general guide should be about 1.5
times the infiltration temperature required to have the metal
infiltrant effectively infiltrate the green part formed using the
powder blend. The high melting temperature fine particulate
metallic, intermetallic or ceramic is deagglomerated during
blending to obtain the desired particle size range. The presence of
the high melting temperature fine metallic, intermetallic or
ceramic particles in the powder blend minimizes distortion in the
finished infiltrated articles by controlling the formation of
diffusional necking or surface diffusional neck growth in the solid
phase between the particles of the steel alloy. The high melting
temperature fine particulate limits the occurrence of solid phase
diffusional sintering among the steel alloy particles. The high
melting temperature fine particulate metallic, intermetallic or
ceramic also contribute to the increased green strength of the
preformed articles. A preferred high melting temperature fine
particulate is tungsten carbide.
[0048] Using particle sizes for the steel alloy and the high
melting temperature fine particulate as described prevents gouging
or streaking of the powder bed in the laser sintering system that
will occur with large sized particles during the application of the
powder blend to the working surface of the powder bed.
[0049] Selective laser sintering process 10 is preferably performed
in a modern selective laser sintering apparatus, such as the
VANGUARD.TM. system available from 3D Systems, Inc. of Valencia,
Calif. As described in the above-referenced patents, process 10
fabricates the green article in a layer wise fashion, by first
preheating the powder bed and then dispensing a thin layer of the
powder over a target surface, preferably in a controlled
environment. Laser energy is then applied to selected locations of
the powder layer to fuse, or sinter, the powder in the exposed
areas. According to the present invention, the powder is a blend or
composite powder of metal particles, polymeric binder particles and
high melting temperature fine metallic, intermetallic or ceramic
particles. The powder particles are fused to one another by the
melting and cooling of the polymeric binder, rather than by
sintering of the metal substrate particles, which would require
very high laser power or very slow processing. The selected
locations of the powder layer correspond to those portions of the
layer in which the article is to be formed, as defined by a
computer-aided-design (CAD) data base representation of the
article. After the selective fusing of a layer, a subsequent layer
is dispensed over the previously processed layer, and the selective
fusing is repeated in the new layer at locations of the layer
corresponding to the CAD "slice" of the article to be formed
therein. Those portions of a layer that overlie fused portions of
the powder in the prior layer are bonded to the fused portions in
the prior layer, such that a solid three-dimensional object or
article results. The unfused powder in each layer serves as a
support medium for subsequent layers, enabling the formation of
overhanging elements in the article. As a result of process 10, the
green article is formed to the desired size and shape.
[0050] It is contemplated that the particular settings and
operating parameters of the selective laser sintering system used
in process 10 may be readily selected by one of ordinary skill in
the art. These parameters include such items as the laser power,
laser scan rate, ambient chamber temperature, layer thickness and
the like. Typically, the values of these operating parameters are
optimized for a given commercially-available powder, such as the
composite powder described above, according to documentation
provided by the system manufacturer.
[0051] Other thermal-based additive processes may alternatively be
used to form the green article. For example, it is contemplated
that process 10 may be performed by the layer wise masked flood
exposure of the composite powder to light, so that the portions of
the powder to be fused are exposed to the light and the unfused
portions are masked therefrom.
[0052] Upon completion of process 10, process 12 is then performed
to remove the unfused or unsintered powder from around the article
in the conventional manner. Such removal is commonly referred to as
"rough break-out", and generally involves the mechanical removal of
the unfused powder to yield the green article. Further surface
finishing of the green article may be performed at this time, if
desired.
[0053] Upon completion of process 12, process 14 is then performed.
In process 14 the green article is placed in a non-reducing gas,
preferably nitrogen, atmosphere in an oven or furnace, usually
packed in inert powder packing made up of fine alumina or silica
powders to provide support during the subsequent heating steps. The
fine grit alumina is -240 mesh so that it provides the needed
surface area to absorb outgassing during the oven cycle and prevent
any redeposit of outgassed carbon on the surface of the article.
The absorption of the outgassing prevents surface blistering of the
final infiltrated article due to residual carbon material being
forced to the surface during final infiltration. A lower melting
infiltrant material, such as copper or a copper-containing alloy
such as bronze, is placed in the oven or furnace in contact with
the green article.
[0054] During process 14 the temperature of the objects or articles
in the oven or furnace is raised about 90.degree. C. per hour to a
temperature of about 550.degree. C. This first temperature is high
enough to begin to decompose the polymeric binder present.
[0055] After process 14, process 16 is performed; the temperature
of the oven or furnace is raised to increase the temperature of the
article further to begin a preliminary sintering of the composite
articles to form a rigid skeleton. The temperature of the object is
raised from about 550.degree. C. to about 850.degree. C. over about
a three hour period. The articles are maintained at this
temperature for about four hours to permit absorption of nitrogen
gas and vapor phase sintering to occur. The high melting
temperature fine particulate, such as tungsten carbide, positioned
at the iron particle interface facilitates vapor phase diffusional
neck growth and reduces solid phase diffusional neck growth of the
steel alloy particles, such as mild steel. These now stronger
articles are often referred to as brown parts or brown
articles.
[0056] After process 16, the temperature of the oven or furnace is
raised to a peak temperature of about 1070.degree. C. over about a
two and one half period and is held at that temperature for about
3-4 hours to perform process 18. Now the presence of the high
melting temperature fine metallic, intermetallic or ceramic
particles, such as tungsten carbide, positioned along interstitial
spaces of the articles reduces liquid phase sintering shrinkage and
distortion of the steel alloy particles. Process 18 causes the
infiltrant that was placed in the oven or furnace in contact with
each article to melt and spontaneously infiltrate the brown
article, resulting in a fully dense article. The infiltration
process dissolves iron from a tab that is in contact with each
article producing a ternary alloy of copper, tin and iron. The
solubility of the iron into the copper can be as high as about 6 to
about 7 percent by weight of the copper in the final article. The
entire oven cycle process of binder removal, sintering,
infiltration and cool down to room temperature can span from about
20 to as long as about 30 hours, dependent upon the volume of the
part or article being formed.
[0057] An additional heat treatment process 20 can then be
performed to further harden the articles. After the articles from
process 18 have been cooled to room temperature, they are placed in
a preheated oven in an air atmosphere and over about a 15 minute
period brought from room temperature to a temperature of about
700.degree. C. Thereafter the temperature is gradually raised from
700.degree. C. to 840.degree. C. over a two hour period. The
articles are then left at this temperature for one hour. The
atmosphere in the oven or furnace is then quenched with a room
temperature (about 25.degree. C.) gas, preferably a non-reducing
gas such as nitrogen, argon or a nitrogen argon blend to bring the
temperature of the articles down to about 93.degree. C. over about
a 15 minute period. The articles are then cooled to about
38.degree. C. over a 15 to 20 minute period and then sub-cooled for
about 90 minutes in the non-reducing gas to about -79.degree. C.
The articles are returned to room temperature and then are tempered
for about 3 hours at about 163.degree. C. Alternatively the
quenching gas could be hydrogen or a hydrogen blend, such as argon
hydrogen or nitrogen hydrogen. However, the use of a hydrogen or
hydrogen blend gas causes the scale that is formed on the surface
of the article from carbon residue stemming from deposits of the
binder material to be reduced away. The presence of the carbon
residue scale prevents the copper in the copper or
copper-containing infiltrant from wicking to the surface of the
article.
[0058] An additional nitrogen absorption cycle 19 can also be
performed before the infiltration process 18 is performed by
holding the articles above about 900.degree. C. for about 4-6
hours. This process converts a mild steel alloy into a
corrosion-resistant steel by the absorption of the nitrogen during
the heat up stage when nitrogen is used as the non-reducing
atmosphere. While not being bound by theory, it is theorized that
the process involves the solid state formation of a higher nitrogen
stainless steel because the nitrogen is absorbed into the ferritic
body-center-cubic alloy so that at about 910.degree. C. the
ferritic iron begins a slow phase transformation to an austenitic
face-center-cubic. It is believed this slow and brief
transformation opens the surface and grain boundary crystal lattice
for the inward diffusion of the nitrogen, forming an in situ A6
stainless steel. It is theorized that upon slow cooling, the
austenitic face-center-cubic structure may transform back to the
ferritic body-center-cubic structure with what appears to be a
supersaturated level of nitrogen.
[0059] A preferred example of a composite powder to be used in the
selective laser sintering process that is useful in connection with
this embodiment of the invention has a substrate of a mild steel
powder, such as spherical particles of -325 mesh, 45 micron, A6
tool steel powder from Ultrafine Powder Technologies of Woonsocket,
R.I., a polymeric binder system made up of about 1.25% to about
2.25% and more preferably about 1.6% to about 2.1% by weight of
3501UD nylon 6,12 co-polymer available commercially from Atofina
Chemicals, Inc. of Philadelphia, Pa.; and about 8% by weight of
less than 2 microns tungsten carbide from Teledyne Inc. of
Huntsville, Ala. The polymer binder is preferably blended with the
A6 mild steel alloy and tungsten carbide powder substrate
particles. The resultant infiltrated articles have a smoother
surface finish than previous powder blends and methods, achieving a
surface finish of about 3.5 .mu.mRa without the presence of bronze
bleed through to the surface in parts that can be machined like
higher grade steels. The final infiltrated articles are heat
treatable and ASTM 638 tensile samples yielded air hardening to a
Rockwell hardness (R.sub.c) of 15-30, possess a tensile yield
strength of 60 Ksi (413 Mpa), a tensile ultimate strength of 105
Ksi (735 Mpa), a modulus of 20.5 Msi (180 Gpa), a maximum
percentage of elongation of about 4.0, a thermal conductivity of
about 38-40 W/m-C at 200.degree. C., linear shrinkage of <2% and
an accuracy of .+-.0.005 inch per inch. Accuracy is defined as
dimensional variation from part to part of the same feature or
variation from the desired dimension.
[0060] It should be recognized that other waxes, polyamides, and
phenolics could be combined into workable systems for the purposes
of this invention. In addition, other thermoplastics could be
substituted for the polyamide and other thermosets for the
phenolic.
[0061] While the invention has been described above with reference
to specific embodiments thereof, it is apparent that many changes,
modifications and variations can be made without departing from the
inventive concept disclosed herein. Accordingly, it is intended to
embrace all such changes, modifications and variations that fall
within the spirit and broad scope of the appended claims. All
patent applications, patents and other publications cited herein
are incorporated by reference in their entirety.
* * * * *