U.S. patent application number 11/480639 was filed with the patent office on 2008-05-29 for processing of rifled gun barrels from advanced materials.
Invention is credited to Animesh Bose, Robert J. Dowding, Jeffrey J. Swab.
Application Number | 20080120889 11/480639 |
Document ID | / |
Family ID | 39462250 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080120889 |
Kind Code |
A1 |
Bose; Animesh ; et
al. |
May 29, 2008 |
Processing of rifled gun barrels from advanced materials
Abstract
Gun barrels made from advanced materials have the potential to
provide a significant increase in barrel life as well as a
reduction in weight (for advanced ceramic materials) for small
caliber systems. The potential use of advanced materials as gun
barrels is severely limited due to the difficulty in introducing
the rifling pattern on the inner diameter. Most projectiles coming
out of the guns are spin stabilized (for aerodynamic flight
stability). This spin is imparted by a rifling pattern (lands and
grooves) in the inner surface of the gun barrel. The processing of
gun barrels made from advanced materials with internal rifling
pattern poses a tremendous processing challenge to the materials
community. The rifling lands and grooves and desired twist rate
coupled with the difficulty of machining some of the advanced
materials (ceramics, cermets, hardmetals, etc.) makes the economic
manufacturing of such gun barrels extremely difficult. Currently,
this form of rifling is achieved by machining in case of metallic
gun barrels. The limitation in producing the rifled pattern lies
with the conventional processing of complex shaped advanced
materials such as ceramics, cermets, or hardmetals. Shaping of
these typically requires careful diamond grinding. This grinding
process is not only very expensive but it also introduces flaws in
case of the brittle ceramics (microcracks). These flaws are
detrimental to the performance of these advanced materials as
rifled gun barrels. Thus, there is an opportunity and challenge to
the materials community to come up with a processing solution that
will allow advanced materials such as silicon nitride
(Si.sub.3N.sub.4), SiAlON, hardmetals, etc. to be used as gun
barrels that have the rifled pattern in the inner diameter. Herein
are provided methods and compositions useful to form the rifled gun
barrel tubes from advanced materials using little or no machining
of the internal rifled geometry.
Inventors: |
Bose; Animesh; (Fort Worth,
TX) ; Dowding; Robert J.; (Abingdon, MD) ;
Swab; Jeffrey J.; (Fallston, MD) |
Correspondence
Address: |
Animesh Bose
4413 Ledgeview Road
Fort Worth
TX
76109
US
|
Family ID: |
39462250 |
Appl. No.: |
11/480639 |
Filed: |
July 3, 2006 |
Current U.S.
Class: |
42/76.02 ; 419/5;
419/65; 419/66 |
Current CPC
Class: |
F41A 21/18 20130101;
B22F 2998/00 20130101; F41A 21/20 20130101; B22F 2998/00 20130101;
B22F 2998/10 20130101; B22F 3/225 20130101; B22F 2998/00 20130101;
B22F 2998/10 20130101; B22F 5/085 20130101; B22F 2998/10 20130101;
B22F 2998/10 20130101; B22F 3/15 20130101; B22F 3/225 20130101;
C22C 29/08 20130101; B22F 3/1025 20130101; B22F 2202/03 20130101;
B22F 3/1025 20130101; B22F 3/1035 20130101; B22F 3/10 20130101;
B22F 3/1025 20130101; B22F 3/1025 20130101; B22F 3/225 20130101;
B22F 3/225 20130101 |
Class at
Publication: |
42/76.02 ;
419/66; 419/65; 419/5 |
International
Class: |
F41A 21/04 20060101
F41A021/04; B22F 3/02 20060101 B22F003/02; B22F 5/12 20060101
B22F005/12 |
Goverment Interests
STATEMENT CONCERNING FEDERAL SUPPORT
[0002] This patent application is based to a large extent on the
research work carried out in connection with contract
W911QX-05-C-0029 with the US Army Research Laboratory. Accordingly,
the federal government retains certain rights in the invention
disclosed herein.
Claims
1. A powder injection molding (PIM) process for the manufacture of
a rifled gun barrel liner comprising an advanced material, said
liner having a multiplicity of lands and grooves having a uniform
twist, said process comprising feedstock formation to form a
feedstock; molding of the feedstock to form a molded feedstock,
wherein the molding imparts the multiplicity of lands and grooves;
debinding the molded feedstock to form a brown part; and
consolidation of the brown part to form the liner.
2. The process of claim 1, wherein the advanced material comprises
alumina, hardmetal, silicon nitride, SiAlON, a cermet, zirconia,
and/or a superalloy.
3. The process of claim 1, wherein the feedstock comprises alumina,
zirconia, SiAlON, silicon nitride, hardmetal, a cermet, a
superalloy, silicon and/or carbon.
4. The process of claim 1, wherein the lands and grooves which have
a sinusoidal pattern with a uniform and/or a square twist.
5. The process of claim 1, wherein the molding is effected through
use of a high-pressure molding machine, a medium-pressure molding
machine, and/or a low-pressure molding machine.
6. The process of claim 1, wherein the molding is effected through
use of a core rod.
7. The process of claim 1, wherein the feedstock formation
comprises mixing a powder of an advanced material or an advanced
material precursor with a composition comprising wax, polymer, oil,
water, and/or acid.
8. The process of claim 1, wherein the debinding comprises solvent
extraction, thermal extraction, wick debinding, catalytic
debinding, freeze-drying, and/or supercritical fluid
extraction.
9. The process of claim 1, wherein the consolidation comprises
pressureless sintering, pressureless liquid-phase sintering,
pressure-assisted sintering, and/or containerless hot isostatic
processing.
10. The process of claim 1, wherein the rifled gun barrel liner
extends throughout the entirety of the length of the breech portion
of, but extends less than the entirety of the length of, a gun
barrel to which the liner is fitted
11. The process of claim 1, wherein the rifled gun barrel liner
comprises a multiplicity of partial gun barrel liners, each of
which said partial gun barrel liners lines the entirety of the
inner diameter of a portion of, but less than the entirety of the
length of, a gun barrel to which the liner is fitted.
12. The process of claim 1, further comprising imparting a
compressive loading to the gun barrel liner by constraining the
outer diameter of the gun barrel by constraining the outer diameter
with a metal tube or a fiber reinforced epoxy winding.
13. A process for the manufacture of a rifled gun barrel liner
comprising an advanced material, said liner having a multiplicity
of lands and grooves having a uniform twist, said process
comprising feedstock formation; casting of a slurry or a gel, said
slurry or gel comprising an advanced material or an advanced
material precursor, wherein the casting imparts the multiplicity of
lands and grooves; drying/debinding; and consolidation.
Description
PRIORITY
[0001] Priority is claimed on the basis of Provisional Application
Number 60/697,183, Filed on Jul. 7, 2005.
FIELD OF THE INVENTION
[0003] The invention relates to the processing of rifled gun
barrels from advanced materials.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1: End view of a molded Si.sub.3N.sub.4 tube with
internal rifling pattern showing lands and grooves.
[0005] FIG. 2: View of a sintered Si.sub.3N.sub.4 rifling tube
sectioned in half to show the rifled geometry in the internal
diameter of the tube.
PROBLEM
[0006] With the emergence of new propellants, the bore surface
erosion in gun barrels has become a major problem that has to be
tackled in the very near future. This need is forcing the gun
barrel designers to try and develop various techniques to try and
improve the life of the current steel gun barrels as well as look
at new liner materials.
[0007] The other need of great importance to gun barrel materials
is to try and reduce the weight for improved mobility. These
requirements are challenging the designers and manufacturers of gun
barrel liners. The current state-of-the-art gun barrels use chrome
plating in their large caliber weapons to improve the bore life.
The problem is that the failure of the current coated gun barrels
frequently occurs after only about 250 rounds (compared to earlier
500 rounds). The failure occurs as the chromium coating is prone to
cracks which allow the hot propellant gases to reach the steel
substrate and eventually resulting in the coating layer spalling
off. Also, the chrome plating process itself has major
environmental concerns associated with the process itself and its
compliance cost is also very high. Thus, there is the need for a
change in the gun barrel material, and advanced materials such as
ceramics, cermets, or in some cases even hardmetals (though they
have higher densities) offer the best promise. However, the
machining of these materials to produce the complex shape of the
rifling pattern in the inner diameter of a tube is a major problem
that has not until now been solved.
SOLUTION
[0008] As the advanced materials are extremely difficult to shape
(through conventional machining or casting route), it was necessary
to come up with alternate processing techniques that can be used to
form these materials into near net shaped components. The process
of powder injection molding (PIM) is a relatively new technology
(its true commercialization started in mid to late 1980's) that is
used for the processing of complex shaped parts from advanced
materials (metals, ceramics, cermets, etc.). The processing of PIM
has not been exploited to manufacture rifled gun barrel tubes from
advanced materials. Other than PIM, processes that will produce net
shape or near net shape parts include gel casting of powdered
materials or slurry casting of advanced material on a shaped core
that can later be removed by a number of means such as machining,
leaching, or simple mechanical extraction, etc. All the above
processes use a shaped core rod to impart the internal geometry of
a rifled gun barrel tube. The invention provides a processing
method that can be used to form the net shaped or near net shaped
rifled gun barrel tubes.
BACKGROUND
[0009] Powder Injection molding (PIM) is a materials processing
technique that has the unique capability of producing complex
shapes from numerous high performance materials. This is a
relatively new technology that has come of age over the last 15
years. PIM uses the shaping advantages of traditional plastic
injection molding but expands the applications to numerous advanced
materials such as metals, alloys, ceramics, intermetallic
compounds, and composites. A number of these advanced materials
have dual use potential. Most of the development and applications
of PIM have been in the commercial sector. However, PIM is an ideal
dual use technology that has applications both in the military as
well as in the commercial arena.
[0010] Advanced materials, especially advanced ceramics such as
silicon nitride and SiAlON are extremely attractive due to their
unique property combination of high strength, oxidation resistance,
elevated temperature strength, and moderate toughness. They will
have numerous dual use applications if they can be manufactured
into complex shapes in a cost effective manner. Several cermets can
also provide extremely attractive properties that will be suitable
for rifled gun barrel applications. Another material known as
hardmetal (tungsten carbide), though having a higher density
compared to steel, could be useful as a short segment of a large
caliber gun barrel, especially in the breech area where the
pressure, temperature and erosion is maximum. The small addition in
weight may be worth it for large caliber systems if the erosion can
be significantly reduced. While for the low caliber gun barrel
tubes, the stress will be not on just a more high temperature
erosion resistant material but also on lowered weight. In the
latter case, the advanced ceramics will provide an ideal
solution.
[0011] It is, therefore, necessary to develop a PIM process that
will be able to incorporate the processing of fine silicon nitride
powder, fine alumina powder and a hardmetal (WC--Co), to form the
desired rifled shapes. Collectively, the use of these materials
shows the generality of the disclosed process. The ultimate goal is
to develop a PIM process that is capable of producing complex
shaped advanced material gun barrels with internal rifling pattern,
in an affordable manner.
[0012] Other alternate processes capable of forming the rifled gun
barrel tubes include gel casting and slurry casting. All these
processes involve the use of a core or mandrel that has the land
and groove and the rifled pattern already machined into it around
which the advanced material that will be used is molded or cast. In
all cases, the advanced material is in a powder form and is mixed
with other organic compounds and/or water to form suitable slurry
or a feedstock that can then be molded or poured around the
pre-shaped core rod which already has the desired lands and grooves
and the rifling pattern all formed into it.
PROCESS BACKGROUND
[0013] In general, the process of powder injection molding (PIM)
consists of four major steps: feedstock formation, molding,
debinding and consolidation. The process begins with the mixing of
a small amount of organic binder with a desired inorganic powder
(metal, alloy, intermetallic compound, ceramic, etc.). The mixture
is granulated (pelletized) to form a "Feedstock." The feedstock is
introduced into the hopper of a powder injection-molding machine,
where it is re-melted in the barrel and moves up to the front
section near a nozzle through which it is pushed into an oversized
die cavity. This molded "green" shape which is usually an oversized
replica of the final product is subjected to a step known as
debinding where the organic portion of the green part is removed.
The purpose of debinding is the gradual and complete removal of the
organic phase without leaving behind any residual contamination.
After the removal of the binder, the part is subjected to a thermal
treatment known as sintering which results in the consolidation or
densification of the part to the desired level. Post sinter heat
treatments and additional hot consolidation steps (Hot Isostatic
Pressing, HIP) can be used to attain the desired properties.
[0014] The PIM process is extremely suitable for moderate volume of
parts that are relatively small (golf ball being on the upper size
range of "conventional" PIM process). Thus, it provides the
designers and engineers with a powerful shaping technique that can
shape materials like plastics but does not confine them to the
plain engineering plastic materials (thermoplastic or thermosetting
type). It should be realized that the process has numerous variants
that is a reflection of the different combinations of powders,
organic binders, mixing and molding techniques, numerous debinding
routes (which are dependent on the initial binder choice), and
widely varied sintering practices (batch or continuous; vacuum or
atmosphere which can vary from air to hydrogen to dissociated
ammonia to inert gases). Some of the typical post sinter processes
includes coining (for metallic parts), HIP'ing, grinding,
polishing, etc.
[0015] The conventional PIM process is carried out in high pressure
plastic injection molding machines. As the size of the part
increases, the machine capacity typically has to be increased due
to the larger clamping force that is necessary. Thus, a different
molding approach can also be considered when trying to PIM process
moderately large parts using fine powders of advanced materials.
Some of these approaches include the use of a low or medium
pressure injection molding techniques.
[0016] At this point it would be worthwhile discussing some of the
characteristics of PIM Feedstock that forms the starting point of
the process. The ideal feedstock will have relatively high amount
of the inorganic powder loading (minimum 50 volume percent); can be
molded into relatively large components without molding defects;
the organic binder phase can be removed in an environmentally
friendly manner without debinding defects and distortion; and the
material can be sintered to the desired density without the loss of
shape (distortion). Unfortunately, PIM processing of fine materials
(cermets, ceramics, hardmetals, etc.) have contradictory property
requirements that complicate the process. For example, as the
relative solids loading of the powder is increased, the viscosity
of the feedstock increases rapidly and it becomes more difficult to
mold large parts free of defects. Alternately, if we go with lower
solids loading for the ease of mold filling in large parts,
subsequent processing will result in large shrinkage, which usually
results in problems with distortion and the part precision.
Similarly, for filling out a large size mold free of defects, it
would normally be necessary to use large injection molding
pressures. With the high pressures in the conventional PIM process,
there is the need for large clamping pressures to keep the molds
together so as to prevent parting line defects such as burrs, which
are difficult to remove. The other problem associated with
high-pressure injection molding is also the extensive wear and tear
on the tooling, which has to be made from the expensive high
quality steel. Also, the tooling in this case is extremely complex
and it will be cost prohibitive to build a new tooling only after
the fabrication of a relatively low number of parts due to
excessive tool wear. However, for making short sized tubes for use
in only the breech area of the gun barrel tubes (where the erosion
and temperature is at its peak), it may still be cost effective to
use conventional high pressure injection molding process. The
automation, experience, and the availability of these injection
molding machines are the major advantages of this process.
[0017] The other alternative is to make larger sized parts in a
cost effective manner. In this case it may be possible to use low
cost tooling made from aluminum or brass or some less expensive
steels. Such low cost tooling for larger parts is only possible
when we use a low pressure or medium pressure injection-molding
machine. It should be realized that the tooling cost difference
increases (between a tool steel mold and a mold made of aluminum,
bronze, or low cost steel) significantly as the part size becomes
larger. Thus, for larger parts, the tooling cost can be
significantly lower if the mold material can be made from aluminum,
bronze, or low cost steels instead of the relatively expensive tool
or die steels. In the overall economics of producing large size PIM
parts in small volume, the factors that start to play an important
role are the powder price, tool cost, and production cost. For
economic viability, it is vital to use relatively inexpensive
tooling, and have a lower operating cost. For making moderately
large sized ceramic parts in fairly moderate numbers (around 10,000
parts a year), it is best to make dies from conventional tool or
die steel materials and use low or medium pressure injection
molding machine. In this case, using a low pressure injection
molding will result in a tool life that will be around 70 to 100%
more due to the lowered wear on the die as the pressure used is
significantly lower.
[0018] The desired powder of the advanced material has to be mixed
together with the organic binder system. There are a host of
available organic binder systems to choose from. Some of the
organics that can be and have been used in powder injection molding
process are: a variety of different waxes such as paraffin wax,
microcrystalline wax, Carnauba wax, bees wax, etc.; variety of
different polymers such as polyethylene, polypropylene,
polystyrene, methyl ethyl ketone, butyl stearate, aniline,
polyethylene glycol, polyvinyl butyryl, dibutyl phthalate,
polymethyl methacrylate, ethylene vinyl acetate copolymer,
methacrylic acid ester copolymer, polyvinyl chloride, etc.; oils
and acids such as stearic acid, oleic acid, vegetable oil, palm
oil, fish oil, peanut oil, etc. Typically, the organic binder
system is a combination of waxes, polymers, oils and acids. The
viscosities of the typical binder systems can vary significantly
depending on the individual binder components and their
combinations. The temperature of the mixing and molding can be
quite high, which necessitates the use of high pressure injection
molding machines, or quite low which can allow the use of low
pressure injection molding machine.
[0019] The next important step in PIM is injection molding. In this
step, the desired feedstock is fed into the hopper of an
injection-molding machine, where a screw advances the feedstock to
the front of the barrel while also heating the feedstock to the
molding temperature. The molten feedstock is then pushed into an
oversized die cavity at high pressures. This type of injection
molding machine is used by more than 95% of the PIM industry. The
pressures used in these conventional PIM machines are high
(typically around 4000 to 5000 psi) and the tool material for the
dies are usually tool or die steels. The clamping force of a
conventional reciprocating screw type machine must exceed the peak
injection pressure times the projected cross sectional area of the
die cavity and the runner system. Thus, as the size of the part
increases, the clamping force of the machine also increases greatly
when the injection pressure is high. The fabrication of complex
dies from tool steels is quite expensive especially when the number
of parts is low (1000 parts/year). If a larger number of parts is
required, it makes sense to use a hardened steel die. For larger
parts, it may be reasonable to use a low pressure injection molding
machine as it would produce a larger number of parts compared to
high pressure molding machine due to the lower wear of the tools.
For large parts, dies made of softer metals would not work with the
conventional high-pressure injection molding machines. To combat
this, a low or medium pressure injection-molding machine may be
useful. In this type of machine the feedstock is fed into the die
cavity at low pressures, resulting in low tooling cost and tool
wear. Large size parts are possible with this type of low-pressure
machine. Gun barrel tubes with the rifling pattern have been
successfully molded using the low/medium pressure injection molding
machine. The tooling was made from conventional steel with a
pre-designed core rod that could be extracted. An alternate to the
elevated temperature injection molding process is a low temperature
injection molding where the feedstock is made from a water-based
binder system. The feedstock is molded at low pressures into the
die cavity where the feedstock is subjected to a low temperature
where the part freezes. The part is then extracted from the mold
and subjected to a process similar to freeze drying which removes a
significant part of the water which is a part of the feedstock.
[0020] After a PIM part has been molded and extracted, the parts
known as "green parts" are subjected to a step known as debinding.
A variety of different debinding schemes are available based on
some of the different binder combinations. Some of the debinding
techniques include solvent extraction, thermal debinding, catalytic
debinding, freeze drying, etc. The purpose here is to remove the
binder and yet retain the shape of the part itself. This process is
a very delicate step in the manufacturing cycle and needs to be
carefully controlled as numerous defects can result during this
step. Typically, a major part of the organic binder is removed
during this step, leaving behind a small amount of binder that
holds the shape of the part.
[0021] The final step in the PIM process is the consolidation step.
Typically, in this step, the debound part is heated to a
temperature where the parts undergo sintering. The sintering
process is responsible for the consolidation of the part, where due
to the temperature driven atomic motion, the pores are gradually
eliminated. The sintering process may be carried out in solid state
(where all material remains in the solid state throughout the
sintering cycle) or with the help of a liquid phase (known as
liquid phase sintering where a small volume of the material is in
the liquid state) that forms during the sintering cycle. The aim in
this case is the attainment of near full density of the advanced
material. It is conceivable that in some cases, complete
densification of the part may not be possible through pressureless
sintering techniques. In that case, pressureless sintering will be
used to densify the parts to a level where all the pores will be
closed pores. Once that is achieved, an additional step of
containerless hot isostatic pressing may be used to fully densify
the part. It should be noted that the final part is a miniature
version of the green part. Thus, it is important to know the exact
shrinkage of the part before the tooling is designed. Unless the
proper shrinkage is used to design the tooling, the part will not
be to final print. The process of PIM has good repeatability and if
the conditions of feedstock formation, molding, debinding and
sintering are kept unchanged, the parts are quite repeatable. This
can be used to a great advantage in the formation of the rifled gun
barrel tubes.
[0022] Other similar processing concepts can also be used to form
these rifled gun barrel tubes without the use of extensive
machining to form the intricate inner diameter lands and grooves
with the rifling pattern. Two of the techniques that can be used
are gel casting and slurry casting. In these processes, the powder
of the desired advanced material (ceramics, cermet, hardmetal,
etc.) is formed into slurry using water or solvents that have small
amounts of gel forming material. In this case, a machined core rod
is used as in case of PIM to form the internal diameter of the
rifled gun barrel tube. In this case, the slurry will be poured
into a mold that has the machined core rod. The material then can
gel and form a solid. The core rod can be made from a variety of
different materials such as machined ceramics, high temperature
wax, rigid plastics, gypsum, etc. The key purpose of the core rod
is to impart the desired shape to the internal diameter of the gun
barrel tube and then it should be easily extractable. The core rod
may be mechanically removed, may be thermally extracted, or may be
extracted chemically or through its dissolution in a solvent. The
gel cast or the slurry cast material will harden and take up the
shape of the mold and the core rod. Once the core rod is removed,
the process will include the removal of the gel or the small amount
of organic binder. The remaining step will be the same as the PIM
process where the part will be consolidated though the sintering
step, which is the high temperature exposure of the part.
EXAMPLE 1
[0023] This example describes the processing of a rifled alumina
gun barrel tube that was around 100 mm long having eight lands and
eight grooves. Over and above the lands and grooves in the ID, the
barrel had a 10:1 twist (1 complete twist in 250 mm) incorporated
into it.
[0024] The powder chosen for this investigation was an alumina
powder that had an average particle size of around 0.61 micrometer
with a D20 of 0.42 micrometer and a D90 of 1.39 micrometer. The BET
specific surface area of the powder was 5.1 m.sup.2/g. The powder
had a small amount of (515 ppm) MgO milled in as a sintering aid.
Other impurities in the powder are 37 ppm of K, 12 ppm of Na, and
19 ppm of Si. The powder had a density of 3.97 g/cc.
[0025] The powder was mixed with an organic binder using a double
planetary mixer. Initially the critical solids loading of the
powder was determined using a torque rheometer. It can be said that
this powder will have a critical solids loading of around 56 v/o.
Based on the loading curve and MPI's experience with the powder
injection molding process using the low/medium pressure molding,
the final loading of the material used was 53.4 volume percent. The
green density of the feedstock was measured by the pycnometer to be
around 2.38 g/cc.
[0026] Special tooling was designed for fabricating the injection
molded rifled gun barrel liners from alumina. The tooling consisted
of the mold cavity and a core rod. The molding conditions used for
molding the gun barrel tubes were determined and used to mold
several gun barrel tubes. The gate was removed from the main mold
body by cutting away the gate section. The as-molded samples of the
ceramic rifled gun barrel tube were debound using a thermal
debinding process. The debinding program was around 32 hours long.
After the parts had cooled to room temperature, they were weighed
to determine the amount of binder extracted. After an adequate
amount of binder is removed (typically greater than 50% of the
binder) the samples are taken to a different furnace for sintering.
The sintering was carried out at a temperature of around
1600.degree. C. with a hold of 2 hour at the peak temperature. The
sintering cycle consisted of a number of slow ramps in the early
stages to remove the remaining binder without creating debinding
defects in the part. After sintering, the rifled gun barrel tubes
were visually observed, followed by observations under a
stereo-microscope. The samples were dimensioned to determine the
shrinkage and the densities of the parts were also measured using
the water immersion technique. The densities of the parts were
determined to be around 3.7 g/cc which is almost close to full
density. Thus, the process of PIM is suitable for fabricating an
advanced ceramic gun barrel liner based on alumina that had 8
grooves and 8 lands which had a uniform twist of 1:10.
EXAMPLE 2
[0027] This example describes the processing of a rifled hardmetal
(WC--Co) gun barrel tube that was around 100 mm long with a 25 mm
inner diameter, a 33 mm outer diameter, having eight lands and
eight grooves. Over and above the lands and grooves in the ID, the
barrel had a 10:1 twist (1 complete twist in 250 mm) incorporated
into it.
[0028] The powder chosen for this investigation was a hardmetal
based on WC--Co. The desired WC--Co powder was mixed with an
organic binder using a double planetary mixer. The feedstock was
then molded into a special tooling that had an extractable core rod
that was machined to have the desired lands and grooves and the
uniform twist needed for the injection molded rifled gun barrel
liners to be fabricated from the hardmetal. The molding conditions
used for molding the gun barrel tubes were determined and used to
mold several hardmetal gun barrel tubes. The gate was removed from
the main mold body by cutting away the gate section. (As is known
to one skilled in the art, the feedstock during injection molding
flows through the runner and then through the gate into the actual
cavity of the mold.)
[0029] The as-molded WC--Co rifled gun barrel tubes were debound
using a thermal debinding process. After the parts had cooled to
room temperature, they were weighed to determine the amount of
binder extracted. After an adequate amount of binder was removed
(typically greater than 50% of the binder) the samples were taken
to a different furnace for sintering. The sintering was carried out
at a temperature of around 1450.degree. C. with a hold of 1 hour at
the peak temperature. The sintering cycle consisted of a number of
slow ramps in the early stages to remove the remaining binder
without creating debinding defects in the part. After sintering,
the rifled gun barrel tubes were visually observed, followed by
observations under a stereo-microscope. The samples were
dimensioned to determine the shrinkage and the densities of the
parts were also measured using the water immersion technique. The
densities of the parts were determined and found to be close to
full density. Thus, the process of PIM is suitable for fabricating
a hardmetal gun barrel liner based on alumina that had 8 grooves
and 8 lands which had a uniform twist of 1:10.
EXAMPLE 3
[0030] This example describes the processing of a rifled silicon
nitride (Si.sub.3N.sub.4) gun barrel tube that was greater than 100
mm long with around 25 mm inner diameter, a 33 mm outer diameter,
having eight lands and eight grooves. Over and above the lands and
grooves in the ID, the barrel had a 10:1 twist (1 complete twist in
250 mm) incorporated into it.
[0031] The powder chosen for this investigation was a silicon
nitride powder with some sintering additives consisting of alumina
and yttrium oxide. The combined powder had a measured density of
3.3 g/cc.
[0032] For preparing the actual feedstock, a loading lower than the
critical loading was needed. The critical solid loading was
determined to be around 62 volume percent. A final feedstock solid
loading of 56.7 volume percent was chosen. After weighing the
desired amount of organic binder, the binder constituents were
placed into the pot of a double planetary mixer and it was allowed
to heat until all the binder was melted. The powder was then added
(silicon nitride powder with the additives) to the melted organic
binder. The powder and the organic binder were then mixed for 12
hours. The binder and powder mix was allowed to cool and then it
was removed. This feedstock was introduced into the hopper of a
low/medium pressure injection molding machine. Several gun barrel
tubes were molded using a tooling that had an extractable core rod
that was machined to have the desired lands and grooves and the
uniform twist needed for the injection molded rifled gun barrel
liners to be fabricated from the silicon nitride. With respect to
the tooling, it should be noted that there is a core that has the
desired rifling patter built in and after the molding the core can
be removed by manual extraction or by a motor. The core has to turn
slowly and the lands and grooves have to follow the twist for
proper extraction, analogous to a bolt being removed from a
threaded hole. The molding conditions used for molding the gun
barrel tubes were determined and used to mold several silicon
nitride based gun barrel tubes.
[0033] The spiral samples were molded to provide an idea about the
molding parameters that can be used to provide good mold filling as
the mold to be filled is quite complex and moderately large in size
(relative to injection molded parts). The longer the spiral length
that is filled, the better is the mold filling. Spiral samples were
first molded using a constant pressure of 80 psi and varying the
molding temperature. The next set of spiral samples were molded
using a constant temperature but varying the molding pressure. The
results of the spiral tests provided an idea about the molding
parameters to be used for molding the gun barrel tubes. It was
observed that keeping the molding pressure constant, the length of
the molded spiral increases with increasing molding temperature.
This is to be expected as the viscosity of the feedstock is
expected to decrease with temperature, thus causing the material to
fill the die to greater lengths. It can also be seen that keeping
the molding temperature constant, the length of the molded spiral
increases with increasing molding pressure. This is also natural as
with increasing pressure, more feedstock tends to be pushed into
the die, thereby increasing the length of the mold fill.
[0034] Even though the spiral molding provided an idea about the
mold filling parameters to be used, multiple molding trials were
necessary to obtain the conditions necessary for proper molding of
the gun barrel tubes. Majority of the molded samples would crack
during the core rod extraction. After the samples were molded,
green weights and green dimensional measurements were obtained.
Using a stereomicroscope, each sample was inspected thoroughly for
any molding defects. Some samples showed signs of cracking along
the spiral edge where the groove meets the land. These samples were
reverted into the injection molding tank to be recycled. However, a
few good samples were eventually molded. One skilled in the art
can, without undue experimentation, likewise determine the proper
molding parameters needed in each instance to practice a process
according to the present invention.
[0035] The debinding of the rifled silicon nitride tubes was
carried out till the binder extracted was greater than 75%. This
precaution was taken since the silicon nitride powder was quite
fine and it was felt necessary that a major part of the binder be
removed before the part was subjected to pre-sintering and
sintering. Once this was achieved, the debound samples were taken
for pre-sintering and sintering.
[0036] The debound rifled tubes were first presintered. The
purposes of presintering the samples were a) Remove the binder
prior to sintering; and b) Increase strength for better handling.
The parts were presinterted in a Lindburg Box Furnace. The
presintering was carried out using a flowing nitrogen atmosphere of
5 cfm. The box furnace runs were carried out at 800.degree. C. It
was observed that all the rifled tubes that were presintered in the
larger box furnace had a slightly darker color. After presintering,
the samples still exhibited enough strength for handling which was
important as the sintering of these parts had to be carried out in
a different high temperature furnace. The Si.sub.3N.sub.4 samples
were sintered in a graphite furnace that had the capability to go
to 2000.degree. C. Two sintering temperatures were used. The first
sintering was carried out at 1800.degree. C. and the hold-time at
the peak temperature was around 120 minutes. A nitrogen pressure of
3 psig was maintained in the furnace throughout the run. The
sintered density of the samples was measured by water immersion
technique. The average of the densities of the parts was around 2.8
g/cc. This was lower than what was desired. Next the higher
sintering temperature of 1900.degree. C. was tried with a hold time
at peak temperature of 180 minutes. This resulted in rifled gun
barrels with sintered density of around 2.92 g/cc. Some of the
rifled gun barrels were then containerless hot isostatically
pressed at a temperature of 1750.degree. C., using a nitrogen gas
pressure of 15 ksi (kg per square inch) for 2 hours. The densities
of the rifled tubes were increased to 3.0 g/cc.
EXAMPLE 4
[0037] This example describes the processing of a rifled silicon
nitride (Si.sub.3N.sub.4) gun barrel tube that is greater than 100
mm long with around 25 mm inner diameter, a 33 mm outer diameter,
having eight lands and eight grooves. Over and above the lands and
grooves in the ID, the barrel has a 10:1 twist (1 complete twist in
250 mm) incorporated into it.
[0038] The powder chosen for this is a silicon nitride powder with
some sintering additives consisting of alumina and yttrium oxide.
The combined powder has a measured density of 3.3 g/cc. The powders
are mixed with a gel forming material and then poured into the
cavity of the die with the shaped core rod. Time is allowed for the
gel to dry and then the core rod is extracted. The part is then
heated to burn off the gel and then sintered to obtain high
sintered density of the part.
Silicon Nitride Rifling Tubes
[0039] The silicon nitride powder with alumina and yttrium oxide
was mixed in a double planetary mixer using an organic binder. The
mixed feedstock was granulated and then transferred to a molding
machine.
[0040] Various molding iterations were carried out. In some cases
going to high temperature resulted in sinkholes and lower
temperatures resulted in poor fill. The final parameters used on
the injection molding machine for molding the Si.sub.3N.sub.4
rifling tubes can be seen in Table SN1. These parameters were used
to successfully mold nine of the rifling tube samples. The molded
dimensions and weights are shown in Table SN2.
TABLE-US-00001 TABLE SN1 Molding parameters used for the tapered
Si.sub.3N.sub.4 rifling tubes Part Tank Pipe Orifice Fill Mold
Description Temp. Temp. Temp. Time Temp. Rifling Tube
Si.sub.3N.sub.4 182 177 177 12 Ambient Samples
TABLE-US-00002 TABLE SN2 Green weights and green measurements for
rifling tubes with a slight taper. Sample Weight Length O.D. I.D.
Number (g) (mm) (mm) (mm) 1-T 173.7384 125.45 42.05 30.33 2-T
172.8143 125.50 42.07 30.32 3-T 173.6280 125.45 42.07 30.31 4-T
171.3400 124.08 42.06 30.31 5-T 172.0314 125.25 42.02 30.32 6-T
173.2958 125.10 42.01 30.28 7-T 173.3372 125.48 42.02 30.32 8-T
172.4473 125.45 42.05 30.25 9-T 144.1747 104.85 42.07 30.32
[0041] The samples were debound using the same debinding cycle
twice. The detail of the preferred debinding cycle is shown in
Table SN3. The samples were allowed to cool to room temperature
before they were removed from the furnace.
TABLE-US-00003 TABLE SN3 Debinding cycle. Dewax Cycle .degree. C.
Ramp Time (hours) Ramp Rate (.degree./hr) Set Point .degree. C.
Soak Time (hours) 1.45 34 75.0 6 1.00 28 102.8 8 5.50 3 121.1 10
3.03 18 176.7 10 Dewax Cycle .degree. F. Ramp Time (hours) Ramp
Rate (.degree./hr) Set Point .degree. F. Soak Time (hours) 1.45 60
167.0 6 1.00 50 217.0 8 5.50 6 250.0 10 3.03 33 350.0 10
[0042] These samples were then weighed to determine the amount of
binder extracted. The binder extraction values can be seen in Table
SN4 for both dewax cycles.
TABLE-US-00004 TABLE SN4 Binder extracted from 1.sup.st and
2.sup.nd dewax cycles. Sample Binder Extracted Binder Extracted
Total binder Number From 1st Dewax From 2nd Dewax Extracted 1-T
53.62% 22.38% 76.00% 2-T 54.12% 21.03% 75.15% 3-T 53.36% 22.88%
76.25% 4-T 54.43% 23.10% 77.53% 5-T 55.97% 22.66% 78.63% 6-T 56.65%
21.29% 77.94% 7-T 57.06% 21.55% 78.61% 8-T 57.65% 19.71% 77.36% 9-T
51.63% 21.83% 73.46% Average 54.94% 21.83% 76.77%
[0043] These debound samples were presintered in flowing nitrogen
atmosphere (5 cfm) using a temperature of 900.degree. C. The
samples were then sintered using the following sintering cycle:
[0044] Room Temperature to 1000.degree. C. in 500 min (overnight
heating)
[0045] 1000.degree. C. to 1750.degree. C. at 10.degree. C./min
[0046] Hold at 1750.degree. C. for 15 min
[0047] 1750.degree. C. to 1900.degree. C. at 3.degree. C./min
[0048] Hold at 1900.degree. C. for 180 min
[0049] Cool from 1900.degree. C. to 1000.degree. C. in 200
minutes
[0050] Furnace Shut Off
[0051] The sintered sampled were subjected to containerless hot
isostatic pressing (HIP). The hot isostatic pressing was carried
out at a temperature of 1750.degree. C., using a gas pressure of 15
ksi for 2 hours. The resultant density was in the range of 3 g/cc.
The samples had 8 lands and 8 grooves and a uniform twist of 1:10.
The typical dimensions (nominal) of the rifled tubes are:
[0052] Outer diameter: 38 mm
[0053] Inner diameter: 27.5 mm
[0054] Length: 111 mm
Alumina Rifling Tubes
[0055] Many molding attempts were made. Table Al below shows the
molding conditions for a incomplete mold fill and a complete mold
fill.
TABLE-US-00005 TABLE A1 Molding conditions for ceramic rifling tube
Part Tank Pipe Orifice Description Temp Temp Temp Pressure Fill
Time Comments Ceramic Rifling Tube 165.degree. F. 155.degree. F.
150.degree. F. 65 psi 20 sec Incomplete Ceramic Rifling Tube
165.degree. F. 155.degree. F. 150.degree. F. 80 Psi 20 sec Good
[0056] Several rifled tubes were molded out of the alumina
feedstock. The as molded samples were characterized and then
debound. The wick debinding is carried out at 350 F. After cooling,
the samples were cleaned and the percent binder extracted is
calculated. The binder removal in all the samples was over 80%
which is considered adequate. Sintering was carried out at various
temperatures. The sintering is carried out at peak temperatures
that varied between 1550 to 1600 C.
[0057] The sintered densities of the samples were measured by water
displacement. The two different sintering cycles showed different
densities. The lower temperature (1550 C) sintered material showed
a density of 3.5 g/cc while the higher temperature (1600 C)
sintered sample showed a density of 3.7 g/cc.
OTHER EXAMPLES
Example OE1
[0058] Processing a rifled gun barrel tube using another ceramic
capable of withstanding high temperatures and with good erosion
resistance. The ceramic is AlON (aluminum oxynitride).
Example OE2
[0059] Processing of rifled gun barrel with a larger diameter to
fit the larger caliber guns such as 155 mm.
Example OE3
[0060] Processing of rifled gun barrel tubes with different land
and groove arrangements such as the lands and grooves being
sinusoidal in nature or like a saw-tooth in nature.
Example OE4
[0061] Very long tubes cannot be made as one piece by PIM. The way
to overcome this difficulty is to fabricate tubes with shorter
segments and join them together using a very high temperature
ceramic joining pastes (having lower melt temperatures than the
tube material). The small segments are designed in such a way that
a V-groove is created when the parts are butted up against each
other and the V-groove was filled with the ceramic joining paste.
This ensures that the tube is gas sealed.
WC--Co Rifling Tubes
[0062] Rifled gun barrel tubes fabricated by the process of powder
injection molding have been demonstrated with advanced ceramics.
This section shows the generic nature of the PIM processing in
fabricating the rifling tubes through the use of a hardmetal,
commonly known as tungsten carbide. Tungsten carbides are a special
class of ceramic-metal composite (sort of like a Cermet). There are
different types of WC--Co with widely varying properties depending
on the composition of the material. In this case, we have used
simply the cobalt content to vary the material.
[0063] Table WC1 shows the molding conditions for the WC--Co
rifling tubes with varying cobalt amounts. The WC-6Co alloy will be
hard and have low toughness, while the WC-12Co will be relatively
lower hardness (though it will have much higher hardness than most
steels), but have high toughness. The hardmetals are much more
dense compared to ceramics as well as compared to steels. So the
purpose of rifled gun barrels made of hardmetals would not be
useful for lightweight gun barrels that will be used by ground
troop (for which the ceramic gun barrels will be ideal). However,
the hardmetals are significantly tougher and impact resistant
compared to ceramic-based materials. An ideal application for
hardmetal gun barrels would be as a short segment, which can be
placed in the breech area of a large caliber gun (155 mm). At the
breech area, a slight pocket can be made by machining and then the
short segment of the hardmetal rifled gun barrel can be fitted in
place so as to match the rifling pattern with the rifling pattern
of the remaining gun barrel. It has to be remembered that
WC--Co-based hardmetals are much harder and erosion resistant as
well as higher temperature resistant compared to steel. Thus,
strategically placing a hardmetal segment at the breech area where
the erosion, pressure and temperature is at its peak will provide
major advantages. The slight additional weight will be negligible
compared to the gains this hardmetal section will provide. This
section shows the processing of two hardmetal compositions: WC-6Co
and WC-12Co. The molding conditions for the two materials have been
shown in Table WC1.
TABLE-US-00006 TABLE WC1 Molding parameters Part Tank Pipe Orifice
Pressure Fill Mold Description Temp Temp Temp Psi Time Temp 6%
Rifling 160.degree. F. 150.degree. F. 150.degree. F. 43 psi 15 sec
Room Tube Temp 12% Rifling 170.degree. F. 160.degree. F.
160.degree. F. 48 psi 15 sec Room Tube Temp
[0064] After the samples were molded they were weighed using an
analytical scale and then measured with Mitutoyo calibers. Table
WC2 shows the weights and measurements for each sample.
TABLE-US-00007 TABLE WC2 Green weights and measurements for molded
rifling tube samples Material Sample Weight O.D. Length Description
Number grams inches inches 6% WC--Co 2 614.3000 1.6470 4.9270 4
614.0000 1.6480 4.9270 6 613.9000 1.6480 4.9250 12% WC--Co 1
596.8000 1.6520 4.9330 2 260.9000 N/A 4.9340
[0065] When molding the 12% WC--Co, sample number 2 sample was
sectioned in half (that is why the weight is low).
[0066] The debinding cycle for the WC-6Co and WC-12Co rifling tubes
can be seen in Table WC3.
TABLE-US-00008 TABLE WC3 Debinding cycle for the WC-6Co and WC-12Co
rifling tubes Dewax Cycle .degree. C. Ramp Time (hours) Ramp Rate
(.degree./hr) Set Point .degree. C. Soak Time (hours) 1.45 34 75.0
6 1.00 28 102.8 8 5.50 3 121.1 10 3.03 18 176.7 10 Dewax Cycle
.degree. F. Ramp Time (hours) Ramp Rate (.degree./hr) Set Point
.degree. F. Soak Time (hours) 1.45 60 167.0 6 1.00 50 217.0 8 5.50
6 250.0 10 3.03 33 350.0 10
[0067] The binder extracted from the 6% WC--Co rifling tubes is
around 55%, while the binder removed from the WC-12Co rifled tube
was around 53%.
[0068] Sintering was carried out in a sinter-HIP furnace. The
sintering was carried out at a temperature in the range of
1375.degree. C. for times between 30 to 60 minutes. The samples
were almost fully dense.
ADDITIONAL EXAMPLES
[0069] A powder injection molding (PIM) process for the manufacture
of a rifled gun barrel liner comprising an advanced material, said
liner having a multiplicity of lands and grooves having a uniform
twist, said process comprising feedstock formation to form a
feedstock; molding of the feedstock to form a molded feedstock,
wherein the molding imparts the multiplicity of lands and grooves;
debinding the molded feedstock to form a brown part; and
consolidation of the brown part to form the liner.
Example 1
[0070] The process wherein the advanced material comprises of
alumina, hardmetal, silicon nitride, SiAlON, a cermet, zirconia,
and/or a superalloy.
Example 2
[0071] The process wherein the feedstock comprises alumina,
zirconia, SiAlON, silicon nitride, hardmetal, a cermet, a
superalloy, silicon and/or carbon.
Example 3
[0072] The process wherein the lands and grooves have a sinusoidal
pattern with a uniform twist.
Example 4
[0073] The process wherein the molding is effected through use of a
high-pressure molding machine, a medium-pressure molding machine,
and/or a low-pressure molding machine.
Example 5
[0074] The process wherein the molding is effected through use of a
core rod.
Example 6
[0075] The process wherein the feedstock formation comprises of
mixing a powder of an advanced material or an advanced material
precursor with a composition comprising wax, polymer, oil, water,
and/or acid.
Example 7
[0076] The process wherein the debinding comprises of solvent
extraction, thermal extraction, wick debinding, catalytic
debinding, freeze-drying, and/or supercritical fluid
extraction.
Example 8
[0077] The process wherein the consolidation comprises of
pressureless sintering, pressureless liquid-phase sintering,
pressure-assisted sintering, and/or containerless hot isostatic
processing.
Example 9
[0078] The process wherein the rifled gun barrel liner extends
throughout the entirety of the length of the breech portion of, but
extends less than the entirety of the length of, a gun barrel to
which the liner is fitted.
Example 10
[0079] The process wherein the rifled gun barrel liner comprises a
multiplicity of partial gun barrel liners, each of which said
partial gun barrel liners lines the entirety of the inner diameter
of a portion of, but less than the entirety of the length of, a gun
barrel to which the liner is fitted.
Example 11
[0080] The gun barrel liners fabricated can be put under
compressive loading by constraining the outer diameter of the gun
barrel with a metal tube or a fiber reinforced epoxy winding.
Example 12
[0081] A process for the manufacture of a rifled gun barrel liner
comprising an advanced material, said liner having a multiplicity
of lands and grooves having a uniform twist, said process
comprising feedstock formation; casting of a slurry or a gel, said
slurry or gel comprising an advanced material or an advanced
material precursor, wherein the casting imparts the multiplicity of
lands and grooves; drying/debinding; and consolidation.
[0082] The foregoing disclosure is intended to teach the disclosed
invention, not to enumerate each iteration of the infinite variety
of ways in which the disclosed process can be practiced. The
examples provided are accordingly illustrative. It will be
understood by those skilled in the art that the invention can be
practiced according to the disclosed process in ways equivalent to
those herein disclosed. It should accordingly be understood that
the rights of the inventors and any assignees of the invention will
be bounded only according to the full scope accorded at law and in
equity to the claims in any patent for which the present
application serves as a priority document. The following sample
claims represent an initial reckoning of aspects of such scope.
* * * * *