U.S. patent application number 11/330325 was filed with the patent office on 2006-06-08 for composite material containing tungsten and bronze.
This patent application is currently assigned to International Non-Toxic Composites. Invention is credited to Kenneth H. Elliott.
Application Number | 20060118211 11/330325 |
Document ID | / |
Family ID | 23284923 |
Filed Date | 2006-06-08 |
United States Patent
Application |
20060118211 |
Kind Code |
A1 |
Elliott; Kenneth H. |
June 8, 2006 |
Composite material containing tungsten and bronze
Abstract
High-density composite materials comprising tungsten and bronze
are useful as lead replacements in the production of ammunition,
weights and other high density articles. The composition of the
composite, articles manufactured using the composite, and a process
for making the composite are disclosed.
Inventors: |
Elliott; Kenneth H.;
(Ontario, CA) |
Correspondence
Address: |
STERNE, KESSLER, GOLDSTEIN & FOX PLLC
1100 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
International Non-Toxic
Composites
Baltimore
CA
|
Family ID: |
23284923 |
Appl. No.: |
11/330325 |
Filed: |
January 12, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10270526 |
Oct 16, 2002 |
|
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11330325 |
Jan 12, 2006 |
|
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60329340 |
Oct 16, 2001 |
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Current U.S.
Class: |
148/423 |
Current CPC
Class: |
B22F 3/1025 20130101;
B22F 1/0096 20130101; B22F 3/10 20130101; B22F 3/26 20130101; B22F
1/0059 20130101; B22F 3/1025 20130101; B22F 1/0059 20130101; B22F
3/26 20130101; C22C 1/045 20130101; B22F 1/0003 20130101; C22C
1/0425 20130101; B22F 2998/10 20130101; C22C 1/045 20130101; B22F
1/0003 20130101; B22F 1/0003 20130101; B22F 2998/00 20130101; B22F
2998/10 20130101; C22C 1/0425 20130101; B22F 2998/00 20130101; F42B
7/046 20130101; B22F 2998/10 20130101; F42B 12/74 20130101; A01K
95/005 20130101; B22F 2998/10 20130101; B22F 2998/10 20130101 |
Class at
Publication: |
148/423 |
International
Class: |
C22C 27/04 20060101
C22C027/04 |
Claims
1-37. (canceled)
38. A projectile comprising a composite consisting essentially of a
suspension of tungsten and pre-alloyed copper/tin bronze.
39. A projectile according to claim 38, which is a bullet.
40. A projectile according to claim 38, which is a bullet core.
41. A projectile according to claim 38, which is shot.
42. A projectile according to claim 38, wherein 40-85% by weight of
said composite is tungsten.
43. A projectile according to claim 42, wherein 50-55% by weight of
said composite is tungsten.
44. A projectile according to claim 43, wherein 52% by weight of
said composite is tungsten.
45. A projectile according to claim 38, wherein 80-95% by weight of
said bronze is copper.
46. A projectile according to claim 42, wherein 80-95% by weight of
said bronze is copper.
47. A projectile according to claim 38, wherein said bronze has a
copper:tin ratio of about 9:1.
48. A projectile according to claim 38, further comprising a
surfactant or a wetting agent.
49. A projectile according to claim 38, wherein said composite
consists of tungsten, copper/tin bronze and iron.
50. A projectile according to claim 49, wherein 0.5-5% by weight of
said composite is iron.
51. A projectile according to claim 50, wherein about 0.8% by
weight of said composite is iron.
52. A projectile according to claim 49, wherein 40-85% by weight of
said composite is tungsten.
53. A projectile according to claim 52, wherein 50-55% by weight of
said composite is tungsten.
54. A projectile according to claim 53, wherein 52% by weight of
said composite is tungsten.
55. A projectile according to claim 49, wherein 80-95% by weight of
said bronze is copper.
56. A projectile according to claim 52, wherein 80-95% by weight of
said bronze is copper.
57. A projectile according to claim 49, wherein said bronze has a
copper:tin ratio of about 9:1.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application 60/329,340 filed Oct. 16, 2001.
FIELD OF THE INVENTION
[0002] This invention relates to composite materials, particularly
to composite materials that can be used as lead replacements.
BACKGROUND OF THE INVENTION
[0003] Lead has been used in a variety of industrial applications
for many thousands of years. In the last hundred years, the toxic
effects of lead have become apparent. In an effort to reduce
reliance on lead, there has recently been extensive research into
materials that could be used to replace lead.
[0004] In this regard, much effort has been focussed on producing
metal composites that mimic the properties of lead. Since the
density of lead is the most obvious characteristic to mimic, most
efforts have concentrated on finding composites that have the same
or similar density as lead. However, other important properties of
lead have been largely ignored and, as a result, no completely
satisfactory lead replacement has yet been found.
[0005] In addition to being non-toxic and to having a similar
density to lead, a successful composite should have reasonable
softness coupled with structural rigidity. Ideally the composite is
substantially homogeneous and relatively cheap to manufacture in
large quantities.
[0006] U.S. Pat. No. 5,279,787 discloses high density projectiles
formed by mixing a high density with a lower density metal. This
patent does not disclose a composite made from tungsten and
bronze.
[0007] U.S. Pat. No. 5,760,331 discloses projectiles comprising a
metal having a higher density than lead and a metal having a lower
density than lead. This patent does not disclose a composite
comprising tungsten and bronze.
[0008] U.S. Pat. No. 5,894,644 discloses lead-free projectiles
formed by liquid metal infiltration. In one embodiment,
ferrotungsten is infiltrated by molten copper, tin or brass. Such
composites do not have sufficient homogeneity to possess desirable
processing characteristics and properties.
[0009] U.S. Pat. No. 5,950,064 discloses lead-free shot comprising
a mixture of three metal components. This patent does not disclose
a composite formed by mixing tungsten with bronze.
[0010] There still remains a need for a composite materials having
a suitably high density, suitable processing characteristics and
suitable properties for a variety of applications.
SUMMARY OF THE INVENTION
[0011] There is provided a composite comprising tungsten and
bronze.
[0012] There is also provided a composite consisting essentially of
tungsten, bronze, and iron.
[0013] There is also provided a process for producing a composite,
the process comprising: blending powdered tungsten, powdered
bronze, and an organic binder, thereby forming a homogeneous
mixture; compounding the mixture at elevated temperature; and,
cooling the mixture to form a composite having consistent
characteristics throughout the composite.
[0014] There is also provided a process for producing an article
comprising: providing a mold having an open ended cavity; placing a
quantity of a homogeneous mixture of powders comprising tungsten
and bronze in the cavity; placing a quantity of a powdered
infiltrant on the mixture of powders in the cavity; sintering the
mixture of tungsten and bronze powders at a first temperature
followed by melting the infiltrant at a second temperature; and,
cooling the mold and the articles formed therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention will now be described by way of non-limiting
example with reference to the following drawings, wherein:
[0016] FIG. 1A is an electron micrograph at 2000.times.
magnification showing the microstructure of the fracture surface of
a composite of the present invention;
[0017] FIG. 1B is an electron micrograph at 4000.times.
magnification showing the microstructure of the fracture surface of
a composite of the present invention;
[0018] FIG. 2 is an optical micrograph of a composite of the
present invention showing tungsten particles dispersed in a bronze
matrix;
[0019] FIG. 3A is a photograph of a bullet comprising a
tungsten-bronze composite of the present invention;
[0020] FIG. 3B is a photograph of shot comprising a tungsten-bronze
composite of the present invention;
[0021] FIG. 3C is a photograph of a wheel weight comprising a
tungsten-bronze composite of the present invention;
[0022] FIG. 4 is an optical micrograph at 500.times. magnification
of a composite of the present invention made using separate
tungsten, copper, and tin powders;
[0023] FIG. 5 is an optical micrograph at 1000.times. magnification
of a composite of the present invention made using tungsten and
bronze powders;
[0024] FIG. 6 is a plot of sintered density versus iron content for
a composite of the present invention;
[0025] FIG. 7 is a schematic of a process for manufacturing a
composite of the present invention;
[0026] FIG. 8 is an optical micrograph at 200.times. magnification
of a composite made in a mold having an open cavity without using
an infiltrant;
[0027] FIG. 9 is an optical micrograph at 500.times. magnification
of a composite made in a mold having an open cavity using an
infiltrant;
[0028] FIG. 10 is an electron micrograph of a composite shot of the
present invention mechanically plated with tin.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Tungsten is generally used in the form of tungsten powder of
polygonal shape and may be milled to the desired shape and mean
particle size. The mean particle size is preferably about 0.5-50
.mu.m, more preferably about 1-20 .mu.m.
[0030] Bronze is typically an alloy of copper and tin. The ratio of
copper to tin may vary depending on the particular alloy and the
desired proportions of copper to tin in the composite. Most
industrially useful compositions have a tin content of under 25% by
weight. Many other additions at various levels are commonly used to
alter the properties of bronzes. These may include but are not
limited to metals and non-metals such as zinc, iron, manganese,
magnesium, aluminium, phosphorus, silicon, lithium compounds, etc.
Preferably, bronze having a Cu:Sn ratio of about 9:1 is used, this
includes bronze having a Cu:Sn ratio of 89:11. Bronze is preferably
used in the form of a powder and may be milled to the desired shape
and mean particle size. The mean particle size is preferably under
100 .mu.m, more preferably under 50 .mu.m.
[0031] Generally, the density of the composite can be adjusted at
will by varying the ratio of tungsten (density=19.3 g/cc) and
bronze (density=8.9 g/cc for a 90:10 alloy of Cu:Sn). A partial
list is provided in Table 1.
[0032] It was found that a composite comprising 40-85% tungsten by
weight of the composite, the balance being bronze comprising 80-95%
copper and 5-20% tin by weight of the bronze, was effective at
producing a composite suitable for use as a lead replacement.
Preferably, the composite comprises 50 to 55% tungsten, even more
preferably 52% tungsten by weight of the composite and the bronze
comprises copper and tin in a ratio of about 9:1 by weight of the
bronze.
[0033] Tungsten particles offer resistance to densification during
compaction as well as sintering. These issues may place an upper
limit on the useful fraction of tungsten. The latter issue can also
be partially offset by using finer tungsten grains.
[0034] The use of bronze in the formation of tungsten composites
offers significant advantages over composite materials that have
previously been described in the art, including the three component
systems described in U.S. Pat. No. 5,950,064. Surprisingly, it has
been found that suspensions of tungsten in bronze are more
homogeneous and denser than suspensions of tungsten in other
materials, particularly other metallic materials. The more even
distribution of tungsten in the bronze matrix leads to superior and
more consistent composite properties, such as higher impact
strength and greater density. The use of tungsten in bronze also
permits the use of a wider range of processing characteristics than
the use of a three component system, such as those described in
U.S. Pat. No. 5,950,064. When processing a three component system
involving separate powders of tungsten, copper and tin, molten tin
will dissolve into the copper matrix leaving non-removable voids or
porosity, permitting aggregation of the tungsten particles that
were next to the tin particles. As a result, the composite formed
from the three component system is less homogeneous and of lower
density than one formed from tungsten and bronze.
[0035] Other processing aids may be used during the production of
tungsten/bronze composites, such as lubricants (for example,
organic polymers, waxes, molybdenum disulphide, calcium difluoride,
ethylene-bis-stearamide, lithium stearate, lithium carbonate,
copper stearate, copper oleate, copper amines, and graphite),
surfactants (for example, stearic acid) mould releasing agents (for
example, zinc stearate) and wetting agents (for example, aluminum
and basic polymers such as polyvinyl pyrrolidone). TABLE-US-00001
TABLE 1 Density of Composites Having Various Proportions of
Tungsten and Bronze Tungsten Mixture Fractional Fractional loading
density weight of weight of (vol %) (g/cc) W powder bronze 20.000
10.980 0.352 0.648 20.500 11.032 0.359 0.641 21.000 11.084 0.366
0.634 21.500 11.136 0.373 0.627 22.000 11.188 0.380 0.620 22.500
11.240 0.386 0.614 23.000 11.292 0.393 0.607 23.500 11.344 0.400
0.600 24.000 11.396 0.406 0.594 24.500 11.448 0.413 0.587 25.000
11.500 0.420 0.580 25.500 11.552 0.426 0.574 26.000 11.604 0.432
0.568 26.500 11.656 0.439 0.561 27.000 11.708 0.445 0.555 27.500
11.760 0.451 0.549 28.000 11.812 0.458 0.542 28.500 11.864 0.464
0.536 29.000 11.916 0.470 0.530 29.500 11.968 0.476 0.524 30.000
12.020 0.482 0.518 30.500 12.072 0.488 0.512 31.000 12.124 0.493
0.507 31.500 12.176 0.499 0.501 32.000 12.228 0.505 0.495 32.500
12.280 0.511 0.489 33.000 12.332 0.516 0.484 33.500 12.384 0.522
0.478 34.000 12.436 0.528 0.472 34.500 12.488 0.533 0.467 35.000
12.540 0.539 0.461 35.500 12.592 0.544 0.456 36.000 12.644 0.550
0.450 36.500 12.696 0.555 0.445 37.000 12.748 0.560 0.440 37.500
12.800 0.565 0.435 38.000 12.852 0.571 0.429 38.500 12.904 0.576
0.424 39.000 12.956 0.581 0.419 39.500 13.008 0.586 0.414 40.000
13.060 0.591 0.409 40.500 12.112 0.596 0.404 41.000 13.164 0.601
0.399 41.500 13.216 0.606 0.394 42.000 13.268 0.611 0.389 42.500
13.320 0.616 0.384 43.000 13.372 0.621 0.379 43.500 13.424 0.625
0.375 44.000 13.476 0.630 0.370 44.500 13.528 0.635 0.365 45.000
13.580 0.640 0.360
[0036] The final composite may consist essentially of tungsten and
bronze. However, as indicated previously, the composite may include
other materials to alter properties, for example iron. In addition,
as one skilled in the art will appreciate, incidental impurities,
for example carbon, may be present that do not unduly affect the
properties of the composite.
[0037] Iron may be added to the composite to increase the
densification of the composite during sintering. Iron is readily
dissolved in bronze, and tungsten dissolves more readily in iron
than in bronze. The addition of iron to the composite has the
effect of aiding the dissolution of tungsten into the bronze,
improving the overall densification of the composite during
sintering. Also, any carbon present in the composite does not
readily dissolve in bronze, but does dissolve in iron. Iron
therefore helps to dissolve and disperse carbon throughout the
composite, minimizing the likelihood of carbon filled voids forming
during sintering that would reduce the density of the composite.
The effect of iron on composite density is illustrated in FIG. 6.
Iron can be added in selected amounts to tailor the composite
density over a narrow range to fit the requirements of a given
application.
[0038] It was found that the addition of iron to the composite,
preferably in the range of 0.5-5%, more preferably 0.8% by weight
of the composite, was useful in affecting the composite density and
especially useful in tailoring the composite density for a desired
application.
[0039] An example of a composite according to the present invention
consists essentially of tungsten, bronze and iron, preferably 52%
tungsten, 47.2% bronze, and 0.8% iron by weight of the composite,
the bronze consisting essentially of copper and tin in a ratio of
9:1 by weight of the bronze.
[0040] The composites of this invention can be used in a variety of
articles such as projectiles or ammunition (for example, bullets,
bullet cores and shot), weights (for example, wheel weights),
radiation shielding and high-density gyroscopic ballasts, among
others. Articles manufactured using the composite of the present
invention enjoy a significant price advantage, typically 33-50%,
over comparable articles manufactured using alternative
commercially available lead replacements. Also, ammunition
manufactured using the composite of the present invention exhibits
ballistic performance at least equal to or better than that of
ammunition manufactured using lead.
[0041] Numerous powder metallurgy forming techniques known in the
art can be used to create composites according to the present
invention and to mold the composites into articles. A number of
processes are generally disclosed in Manufacturing with Materials,
eds. Lyndon Edwards and Mark Endean, 1990, Butterworth-Heinemann,
Oxford, UK; and, Process Selection: From Design to Manufacture, K.
G. Swift and J. D. Booker, 1997, Arnold Publishers, London, UK, the
disclosures of which are hereby incorporated by reference.
[0042] An example of a process for manufacturing an article, for
example a bullet core, using a composite of the present invention
involves mixing tungsten and bronze powders together with any other
materials, for example iron, that may be present to alter the
properties of the composite. A mold, for example made of a
machinable ceramic material such as alumina, having a plurality of
open ended cavities in the shape of the article to be manufactured
is oriented with the open ends of the cavities facing up. The
mixture of powders is then placed in the cavities and the mold is
tapped to promote settling of the powders. A ram may optionally be
used to compress the powders. An infiltrant, for example copper
powder, is then placed on top of the mixed powders, generally
filling the remainder of the cavity. The mold and powders are then
sintered at a first temperature, for example 800-1000.degree. C.,
for a sufficient time to promote densification of the tungsten and
bronze, for example 1 to 3 hours. The mold and powders are then
raised to a second temperature that is higher than the first
temperature and sufficient to melt the infiltrant, for example
1000-1100.degree. C. The infiltrant fills the voids in the sintered
composite, typically increasing the final density to within 1-3% of
the theoretical mixture density. Copper is particularly desirable
for use as an infiltrant, since it raises the melting point of the
bronze as it is introduced, thus avoiding slumping, and since it
alloys with the bronze, thereby maintaining the corrosion
resistance of the composite. Upon cooling, a dense final article is
created with minimal slumping that is malleable and resists
cracking sufficiently to allow cold shaping using, for example,
swaging dies or other sizing processes.
[0043] In another type of manufacturing process, the composite of
the present invention may be formulated using an organic binder,
generally a thermoplastic binder, in sufficient quantity to allow
use of fluid processing techniques to manufacture articles using
the composite. A wax or blend of waxes is the preferred binder. The
preferred binder comprises a low molecular weight wax or wax blend
that preferably melts at a temperature from about room temperature
to about 120.degree. C., more preferably from about 50-90.degree.
C., yet more preferably from about 55-65.degree. C. The wax may be,
for example, paraffin wax, microcrystalline wax, polybutene wax,
polyethylene wax, Carnauba wax, among others, or a blend of two or
more thereof. The binder preferably has a thermal de-binding
temperature that allows it to be completely removed from the
composite material prior to sintering. The binder preferably has a
pyrolysis temperature less than 375.degree. C., even more
preferably about 350.degree. C., and preferably leaves little or no
ash residue upon pyrolysis. Additionally, the binder should have a
viscosity which changes gradually with temperature. A single
melting point wax undergoes an abrupt change in viscosity when
heated. Sudden shifts in viscosity can cause the metal powder to
fall out of suspension in the binder, creating zones of non-flowing
materials that may cause damage to equipment. To widen the useful
temperature range of the binder and prevent sudden shifts in
viscosity, a blend of low molecular weight waxes of various melting
points may be used. Optionally, a surfactant may be added to
promote adhesion of the powder to the binder and keeping the
powders in suspension. The surfactant used is preferably pyrolysed
at about the same temperature as the waxes and is preferably
removed completely during de-binding without the formation of ash
that may inhibit sintering of the composite.
[0044] An example of a binder according to the present invention
includes a blend of paraffin waxes having melting points between 50
and 73.degree. C. By adjusting the relative amounts of the waxes,
the softening range and melting point of the binder may be tailored
to the composite and the molding equipment being used.
[0045] Formulations of the composite that include an organic binder
are generally made using a compounder. Tungsten and bronze powders
are dry blended along with the organic binder and any other
additional components, for example iron, that may be added to alter
the properties of the composite. The result is preferably a
homogeneous mixture. The mixture is then introduced into a
compounder and compounded at elevated temperature. The temperature
of the compounder is preferably less than the melting point of the
binder, but high enough to allow the binder to soften, thereby
allowing the binder and powders to be mixed, for example
55-65.degree. C. The compounder typically has a heated bore with a
screw or twin screws and a series of paddles or cams for slicing
and shearing the mixture during compounding. This type of
compounder permits good control over particle distribution and
loading resulting in high volume throughput and good mixture
consistency and homogeneity. The compounder typically produces a
pelletized mixture that may be cooled for later use in the molding
of articles using fluid processing techniques.
[0046] Examples of processes for making articles that use organic
binders and fluid processing techniques include Powder Injection
Molding (PIM), tape casting, and polymer-assisted extrusion. These
techniques all involve an organic binder that contributes fluidity
to the composite thus permitting the forming of molded shapes.
[0047] In recent years, Powder Injection Molding (PIM) has emerged
as a method for fabricating precision parts in the aerospace,
automotive, microelectronics and biomedical industries. The
important benefits afforded by PIM include near net-shape
production of articles having complex geometries in the context of
low cost and rapid fabrication at high production volumes.
[0048] The overall PIM process consists of several stages. Metal
powders and organic materials that include waxes, polymers and
surfactants are compounded as previously described to form a
homogeneous mixture that is referred to as the feedstock. The
feedstock may, for example, be pelletized. Ideally, the feedstock
is a precisely engineered system. The constituents of the feedstock
are selected and their relative amounts are controlled in order to
optimize their performance during the various stages of the
process. The feedstock is used to mould parts in an injection
moulding machine, in a manner similar to the forming of
conventional thermoplastics.
[0049] The injection molding machine has a feed hopper which
supplies feedstock to an elongated processing barrel. The
processing barrel may be jacketed and is heated to the desired
molding temperature. The molding temperature is preferably below
the melting point of the binder but high enough to soften the
binder, for example 55-65.degree. C. The barrel typically contains
an elongated screw concentrically aligned with the barrel. The
barrel is generally tapered and as the screw is rotated, the
softened material is advanced through the barrel under an
increasing pressure. A mold having an internal cavity corresponding
in shape to the article being manufactured is provided at the
outlet of the barrel and receives an injection of the heated
pressurized material. The material is cooled in the mold under
pre-determined pressure and temperature conditions to plasticize
the binder and the formed article is removed from the mold for
further processing. Injection molding is particularly useful for
manufacturing wheel weights and bullets.
[0050] Additional shape forming methods that make use of fluid
processing techniques will be described below.
[0051] Extrusion and injection molding are typically done at
elevated temperatures. Extrusion is generally a melt-processing
technique that involves mixing the metal constituents and the
organic binder at an elevated temperature followed by extruding the
molten mixture through an open die into the form of wires, sheets
or other simple shapes. Tape casting usually involves mixing metal
constituents with a solution of organic binder and extruding the
mixture at room temperature into sheets. These techniques are
fairly slow for the commercial production of shot but may be most
applicable to the manufacture of articles like wheel weights and
bullets.
[0052] Compaction is another technique wherein composite
ingredients including organic binder are pressed to form a compact.
The compact may then be sintered at an elevated temperature.
Compaction techniques of this nature are typically not viable for
the volume production of articles such as shot.
[0053] In yet another technique, particularly adapted to producing
shot, the ingredients of the composite including organic binder are
mixed together and the binder is melted and dripped into small
spheres.
[0054] Heading or roll-forming techniques, either cold or warm, are
more rapid than casting, moulding, pre-forming or dripping
techniques and are ideally suited to the manufacture of ammunition,
such as shot, since high throughput is required to make the process
more economical. Generally, tungsten and bronze are mixed to form a
suspension and extruded to form a wire, strip or sheet. The wire,
strip or sheet may then be processed into the desired article. For
the production of shot, the wire, strip or sheet is stamped or
rolled out to give substantially or essentially spherical composite
particles. Press rolls may also be used to press the extruded
composite into a desired thickness before the spherical composite
particles are formed. The spherical composite particles may then be
finished to produce shot.
[0055] In such heading or roll-forming processes, tungsten and
bronze may be pre-mixed to form a pre-mixture and charged to an
extruder; or, they may be pre-mixed then compounded and pelletized,
and charged to an extruder. Pre-mixing is generally done at ambient
(room) temperature. Bronze, together with any other additives that
may be used, are typically mixed first to form a mixture which is
then mixed with tungsten to form the pre-mixture. Compounding and
pelletization is typically done at an elevated temperature. The
extruded composite, in the form of a wire, strip or sheet, may then
be stamped progressively using a series or an array of punches to
form regular indentations until the spherical composite particles
are finally stamped out. Alternatively, spinning rolls with a
dimpled texture may be used to form spherical composite
particles.
[0056] In another aspect of the invention that may be used to form
a variety of articles, special processing steps and binder
selection allow the cooled and solidified article to have a high
powder content (beyond the limit of random order) such that when
reheated the object will not lose its shape. Use of such a binder
dramatically improves the processibility of the composite
permitting the formation of a pourable mixture that can be easily
formed into the desired shape. The filled moulds may be vibrated
lightly to create a more ordered packing arrangement of the powder
particles. Successful reproductions may be formed with highly
repeatable accuracy and powder loading.
[0057] Following the shape forming stage as described in any of the
foregoing methods, removal of organic constituents may be achieved
by pyrolysis prior to sinter densification of the article. The
process of removal of the binders is referred to as debinding in
general, and the pyrolysis method of binder elimination is termed
as thermal debinding. The thermal debinding operation involves
heating the shaped article in a furnace to a temperature that
rapidly transforms the binder by pyrolysis into gaseous products
that are swept away by a flowing protective atmosphere. As the
article is heated, the binder melts. A wicking powder, for example
a powder comprising alumina, may be used to create a capillary
force gradient that draws the binder out of the part. Since melting
of the binder occurs from the outside in, the entire article is not
liquefied at one moment. As the liquid front moves from outside to
centre, it is immediately drawn out by the wicking powder. Much is
known about the removal of such binders in this manner, and the
calculations are well published to determine the basis of
operation.
[0058] Solvent de-binding using a liquid organic solvent, for
example heptane, heated to a temperature below its boiling point
but greater than the melting point of the organic binder, for
example 70.degree. C., may optionally be used prior to thermal
de-binding. When solvent de-binding is used as a pre-treatment, a
portion of the liquid binder is removed by the solvent and a
wicking powder is generally not needed during thermal
de-binding.
[0059] Once the de-binding is complete the furnace is heated to a
temperature adequate for the degree of sinter bonding required for
the application. Typically, the temperature may be from about
600-1100.degree. C. For composites according to the present
invention, the sintering temperature is preferably 800-1100.degree.
C., more preferably 1000-1100.degree. C. Sintering is generally
done under a reducing atmosphere to prevent oxidation of the metal
components. A protective gas, for example, pure hydrogen gas, a 10%
hydrogen/90% nitrogen gas mix or cracked ammonia gas, may be used
to provide a reducing atmosphere. The gas usually flows at from 5
to 10 times the volume of the furnace per hour to remove
impurities. Batch or continuous furnaces may be used for thermal
de-binding and sintering. In a batch furnace, the desired
temperature profile versus time is typically programmed into the
furnace. After completion of the program, the parts are left in the
furnace under a controlled atmosphere for cooling. In a continuous
furnace, the molded articles are introduced to the furnace on a
moving belt conveyor and a large flow of protective gas is used to
maintain the controlled atmosphere in the furnace. The furnace is
programmed with zones of varying size and temperature to produce
the desired temperature profile as the articles move through the
furnace. Cooling of the articles usually occurs outside of the
furnace under a controlled atmosphere. Either type of furnace may
be used to manufacture articles according to the present
invention.
[0060] To alter the surface properties of articles manufactured
using a composite according to the present invention, the articles
may be mechanically plated with another metal. The plating metal
may be, for example, tin, zinc, chromium, molybdenum, or mixtures
thereof, including alloys. The plating may be useful in imparting
corrosion resistance, hardness, or lubrication to the article. The
metals may be mechanically plated on to the articles by, for
example, introducing the articles and the powdered plating metals
into a ball mill and tumbling the articles and powders in the ball
mill. Altering the surface properties in this manner in no way
changes the structure or composition of the composite according to
the present invention.
EXAMPLES
Example 1
[0061] A powdered bronze alloy with a 90:10 ratio of Cu:Sn was
mixed with tungsten powder in a 1:1 blend by weight. The mixture
was compacted in the shape of rectangular bars with 0.5%
ethylene-bis-stearamide lubricant at 50 psi compaction pressure.
The bars were sintered at 1100.degree. C. to produce sintered bars
consisting essentially of tungsten particles dispersed in a bronze
matrix. The bars had a sintered density of 12.3 g/cc and a
transverse rupture strength of 600 MPa. The resulting component had
high impact toughness hitherto unseen in tin-tungsten
composites.
[0062] The microstructure of the fracture surface showed ductile
fracture with relatively high wettability of the tungsten grains
(FIGS. 1A and 1B). Optical micrographs of the bulk of the composite
confirmed the presence of wetted tungsten particles dispersed in a
bronze matrix (FIG. 2).
Example 2
[0063] A mixture comprising 60% by weight of tungsten powder and
40% by weight of bronze powder is mixed with a blend of waxes
comprising 20% by weight paraffin wax, 40% by weight
microcrystalline wax and 40% by weight Carnauba wax at 190.degree.
F. (about 88.degree. C.) under 28 inches of vacuum for 30 minutes,
such that the wax blend comprises 55 vol % of the metal/wax
mixture. The metal/wax mixture is then brought back to atmospheric
pressure and poured into a preheated rubber mould (about 82.degree.
C.). The filled mould is vibrated and returned to 26 inches of
vacuum for one minute in a heated oven (about 82.degree. C.), with
vibration continuing for 5 minutes. The filled mould is then
removed from the oven and allowed to cool until below about
27.degree. C.
[0064] Debinding is then done at about 300.degree. C. for 1 hour,
then at 450.degree. C. for 1 hour and finally at 550.degree. C. for
1 hour under an atmosphere of hydrogen gas. Sintering is then done
at a temperature of 850.degree. C. for 1 hour under hydrogen
gas.
[0065] Parts made using this process have high impact strength
together with excellent ductility and energy absorption capability.
Repeatedly hammering a tungsten-bronze pellet made using this
process results in almost total flattening of the pellet without
breaking. The flattened pellet is very hot to touch.
Example 3
[0066] To determine the effect of varying the relative chemical
composition of the material, several different compositions were
tested and the average density of the composite material was
measured. The results are presented in Table 2. TABLE-US-00002
TABLE 2 Material composition and average composite density
Experimental Density Series Material Composition (g/cm.sup.3) 1
50W--50Cu 10.8 50W--7.5Sn--42.5Cu 10.58 (6.4 g/cm.sup.3 tap density
W) 50W--7.5Sn--42.5Cu 10.7 10 g/cm.sup.3 tap density W)
55W--4.5Sn--40.5Cu 10.3 50W--5Sn--45Cu 10.6 45W--5.5Sn--49.5Cu
10.65 40W--6Sn--54Cu 10.45 2 54W--1Sn-45Bronze(90Cu10Sn) 11.2
50W--5Sn-45Bronze(90Cu10Sn) 10.6 50W-50Bronze(80Cu20Sn) 10.9
82W-18Bronze(90Cu10Sn) 9.3 72W-28Bronze(90Cu10Sn) 10.3
62W-38Bronze(90Cu10Sn) 11.0 52W-48Bronze(90Cu10Sn) 11.2 3
52W-47.5Bronze(90Cu10Sn)0.5Fe 10.6 52W-47.2Bronze(90Cu10Sn)0.8Fe
11.9 52W-47Bronze(90Cu10Sn)1Fe 10.7 52W-46.5Bronze(90Cu10Sn)1.5Fe
10.6 52W-45.5Bronze(90Cu10Sn)2.5Fe 9.8 52W-43Bronze(90Cu10Sn)5Fe
9.8
[0067] The densities shown in Table 1 are the average for a number
of samples tested. Sintering was for one hour in a 100% hydrogen
atmosphere with a temperature between 1080 and 1100.degree. C.,
depending on the composition being made. In the first series of
experiments, the composite material was made by mixing tungsten,
copper, and tin powders. The numeral preceding the chemical symbol
of each constituent indicates the weight percentage of that
constituent in the composite. In the first experimental series, a
variety of compositions were tested, producing a variety of
composite densities. None of the composite densities proved
acceptable, however, due primarily to the presence of voids in the
particles formed by the dissolution of tin into the copper along
the copper grain boundaries. The appearance of voids is illustrated
in FIG. 4, which shows a three component composite material made
during the first series of experiments in cross section at 500
times optical magnification, with the voids clearly visible as dark
black spots. To prevent void formation, the next series of
experiments was conducted using bronze powder. The ratio of copper
to tin in the bronze powder is indicated in brackets. Using bronze
rather than separate copper and tin powder proved effective at
preventing void formation in the composite, as indicated in FIG. 5,
which shows a two component composite material made during the
second series of experiments in cross section at 1000 times optical
magnification. In contrast to FIG. 4, no voids are visible when a
bronze powder and a tungsten powder are used to form the composite
material. In the third series of experiments, a small amount of
iron was added to the composite. The number preceding the chemical
symbol for iron indicates the weight percentage of iron in the
composite. The iron improved the sintering of the materials and had
a noticeable effect on density. As illustrated in FIG. 6, the
maximum density for the compositions tested was at 0.8% iron by
weight, with too much or too little iron having a detrimental
effect on composite density. The composite consisting essentially
of 52W47.2Bronze(90Cu10Sn) 0.8Fe had a density of 11.9
g/cm.sup.3.
Example 4
[0068] To arrive at the desired binder formulation, experiments
were conducted with a variety of wax blends and surfactants. Low
molecular weight paraffin waxes were selected from Table 3 and
blended in a variety of combinations. TABLE-US-00003 TABLE 3
Paraffin waxes for blending experiments (Source: Strahl &
Pitsch, West Babylon, NY) Melting Penetration Point Open ASTM S
& P Cap. Tube D-1 Acid Saponification Number USP Class II
321100/77/5 Value Value Color 206 122-127.degree. F. 18-43 Nil Nil
White 192 124-130.degree. F. 9-15 Nil Nil White 227B
128-135.degree. F. 11-16 Nil Nil White 1275 127-135.degree. F.
11-16 Nil Nil White 173 138-144.degree. F. 10-16 Nil Nil White 673
141-146.degree. F. 10-15 Nil Nil White 434 150-156.degree. F. 10-16
Nil Nil White 674 156-163.degree. F. 10-15 Nil Nil White Note: EPA
- Toxic Substances Control Act - Chemical Substance Inventory,
Substance Name Index - PARAFFIN WAX/CAS NUMBER 8002-74-2, Cosmetic,
Toiletry & Fragrance Association - Cosmetic Ingredient
Dictionary, CAS NUMBER 800-74-2
[0069] All of the paraffin waxes in the above table have a melting
point between 122.degree. F. (50.degree. C.) and 163.degree. F.
(73.degree. C.). Combining waxes of various melting points has the
effect of increasing the softening of the wax over a range of
temperatures. This is in contrast to the sharp transition from
solid to liquid over a relatively narrow temperature range for a
single wax, as listed above. By blending the binders, a more
gradual shift from solid to liquid is created, which provides a
range of operating temperatures that are compatible with commercial
compounding and molding equipment. Additionally, stearic acid
(Fisher Scientific: Atlanta, Ga.) was selected as a surfactant for
use in helping to keep metal powder suspended within the binder,
due to its comparable melting point of 54.degree. C. and since it
burns cleanly without leaving an ash residue.
[0070] By blending various waxes and stearic acid in a number of
combinations and qualitatively observing the softening and melting
points, the blend of materials shown in Table 4 was selected.
TABLE-US-00004 TABLE 4 Blend of materials in preferred binder
formulation Organic Material Melting Point (.degree. C.) Weight
Percentage (%) S&P 206 50-53 31.3 S&P 1275 53-57 34.9
S&P 674 69-73 31.3 Stearic Acid 54 2.5
[0071] For a binder formulated using the blend of materials in
Table 4, the physical properties listed in Table 5 were observed by
measuring the temperature of the binder upon qualitative
observation of the indicated parameter. TABLE-US-00005 TABLE 5
Physical properties of binder formulated as per Table 4. Binder
Parameter Temperature (.degree. C.) Softening of binder 45-55
Melting of binder 55-65 Flash Point (in air) 230
Example 5
[0072] A process for manufacturing the composite of the present
invention is shown schematically in FIG. 7. The tungsten powder 2
and the bronze powder 3 were dry blended together with a small
amount of iron and an organic binder 4. The dry blender used was a
double cone 500 lb capacity heavy duty tumbler with an open volume
of approximately 12 liters, rotating at 30 rpm for approximately 15
minutes. The composition of the blended powders is provided in
Table 6.
[0073] The total experimental batch size was 1000 cm.sup.3 and the
blended batch had an overall density of approximately 8.05
g/cm.sup.3. It is important to maximize the solid volume fraction
prior to compounding to ensure good pellet formation.
TABLE-US-00006 TABLE 6 Composition of blended powders Weight
Percentage Density Mass Powders (%) (g/cm.sup.3) (g) Bronze
(90Cu10Sn) 47.2 8.9 3641.87 Tungsten (W) 52 19.3 4012.23 Iron (Fe)
0.8 7.8 61.73 Total Metal Powders 100 12.35 7715.83 S&P 206
31.3 0.88 104.36 S&P 1275 34.9 0.89 116.37 S&P 674 31.3
0.90 104.36 Stearic Acid 2.5 0.86 8.34 Total Organic Binders 100
0.89 333.43 Total Batch Weight 8049.26
[0074] The powders were compounded into pellets using a compounder
5. The compounder 5 was a 2'' diameter twin-screw compounder with a
single feed port 6 for receiving the mixed powders (Readco: York,
Pa.). The feed port 6 was electrically heated to 65.degree. C. and
included a feed tube that was air cooled to approximately
23.degree. C. The compounder was oil jacketed and had a homogeneous
temperature throughout of 65.degree. C. A self-pelletizing two hole
die plate was installed at the outlet of the coupounder. The die
plate was not heated, but reached a temperature of 65.degree. C.
The powders were fed through the coupounder at a rate of 100 kg/hr
with a screw rotation rate of 150 rpm, consuming 1.2 HP of
electrical power. The pelletized material 7 was collected, tumbled
for 5 minutes, and compounded a second time to ensure homogeneity
of the compounded material.
[0075] The pellets 7 were then fed to an injection molding machine
8. The injection molding machine 8 was a 55 ton type 270V 500-150
(Arburg: Lossburg, Germany) with a PVC processing barrel and screw
of a type suitable for PIM applications. The mold 9 was a long
sprue type with four insert cavities. Two molds 9 were used, one
designed to produce tensile bars and the other designed to produce
9 mm bullets. The mold was jacketed and cooled to 10.degree. C. The
parameters used in the operation of the molding machine 8 are
provided in Table 7. TABLE-US-00007 TABLE 7 Molding parameters Mold
parameter Value Mold temperature 10.degree. C. Material charge 25
cm.sup.3 Injection time 0.4 s Injection speed 75 cm.sup.3/s
Injection pressure 500 bar Holding pressure 500 bar, 0.5 s 200 bar,
2 s Cooling time 12 s Pressure during plasticizing 200 bar
[0076] The resulting molded articles were then subjected to
de-binding. In this series of experiments, the optional solvent
de-binding system was not used and the articles were placed in
alumina powder for wicking of the wax. The articles were placed in
a furnace 10 with a 100% hydrogen atmosphere flowing at 10 times
the furnace volume per hour. The furnace temperature was ramped at
5.degree. C. per minute until it reached a temperature of
350.degree. C. The temperature was then held at that level until
all of the binder was removed, as measured by a drop in the furnace
exhaust flame temperature after about 2 to 2.5 hours.
[0077] Without removing the articles from the furnace 10, the parts
were sintered by ramping the temperature at 5.degree. C. per minute
until the temperature reached 1080.degree. C., where it was held
for 1 hour. The articles were then allowed to cool in the furnace
10 in the inert gas atmosphere before being removed from the
furnace.
[0078] The physical and mechanical properties of the cooled
composite articles were measured and are reported in Table 8.
TABLE-US-00008 TABLE 8 Physical and mechanical properties of
52W-47.2Bronze(90Cu10Sn)0.8Fe Property Value Average density 11.9
g/cm.sup.3 Porosity 4% Shrinkage 12.72% Brinnell Hardness 87-90
Tensile Strength 87,000 lbs/in.sup.2
[0079] The density of the material was determined using Archimedes
principle in oil. The finished parts had an average density of 11.9
g/cm.sup.3, which is 96% of the theoretical density for the
combination of metal powders of 12.35 g/cm.sup.3, calculated in
Table 6. This indicated that the material produced using the above
process had a porosity of about 4%. During de-binding and
sintering, the average dimension was reduced by 12.72% due to the
removal of the organic binder and densification of the metal
particles during sintering. Molds to produce the articles made from
this composite should be scaled up by a factor of 1.14 to account
for this shrinkage in the final parts. This combination of physical
properties is particularly desirable for composite materials that
can be used as lead replacements.
Example 6
[0080] To manufacture an article, particularly a 5.56 mm bullet
core for a full metal jacket bullet, in an alternative process, a
mold was made from a machinable ceramic comprising alumina. A
plurality of cavities corresponding in shape to a bullet core were
machined into the ceramic and the mold was oriented so that the
open ends of the cavities faced up. A mixture of powders consisting
essentially of 52% tungsten and 48% bronze by weight of the
mixture, the bronze consisting essentially of 90% copper and 10%
tin by weight of the bronze, was prepared and homogeneously mixed
using a dry blender. No organic binder was present in the mixture
of powders. The mixture was poured into the cavities and the mold
was tapped lightly to settle the powders in the cavities. The
powder was then compressed using a ram of the same diameter as the
cavity to create a specific empty volume above the level of the
mixed powders. An infiltrant powder comprising copper was then
added to the empty volume of all but three of the cavities and the
mold was tapped to settle the infiltrant powder. The infiltrant
powder was added in an amount of about 11% of the weight of the
mixed tungsten and bronze powders.
[0081] The filled mold was placed in a furnace with a pure hydrogen
atmosphere flowing at 5 times the furnace volume per hour. The
temperature of the furnace was increased at a rate of 10.degree. C.
per minute until the temperature reached a first temperature of
840.degree. C., where it was held for one hour to cause sintering
and densification of the mixed tungsten and bronze powders. The
first temperature was below the melting point of copper,
1083.degree. C. The furnace temperature was then elevated at a rate
of 10.degree. C. per minute until the temperature reached a second
temperature of 1100.degree. C., where it was held for one hour to
cause infiltration of the melted copper into the sintered tungsten
bronze matrix. The mold and bullet cores were then allowed to cool
in the furnace under a protective atmosphere of hydrogen.
[0082] Analysis of the bullet cores without infiltrant showed that
approximately 86% of the theoretical density was achieved during
sintering, leaving a porosity of 14%. An optical micrograph of a
cross section of one of these cores is provided in FIG. 8, showing
dark black voids that were left after sintering. The cores with
infiltrant had a density of approximately 97% of the theoretical
density, indicated that the molten copper had infiltrated the
pores. An optical micrograph of a copper infiltrated core is
provided in FIG. 9, showing an absence of voids and tungsten grains
well distributed in a bronze matrix with no individual copper
grains present.
[0083] The final density and weight of the bullet cores were
tailored to a military specification for this size of bullet core
using the composite and process of the present invention. The
bullet cores met the specification with an average density of 11.4
g/cm.sup.3 and an average weight of 62 gr+/-0.5 gr (4.018 g+/-0.032
g). The bullet cores had minimal slumping and distortion and were
malleable enough to allow shaping using swaging dies to bring the
bullet cores to exact final dimensions.
Example 7
[0084] To reduce the potential for corrosion, the composite
material was mechanically plated with tin. Approximately 100 g of
substantially spherical shot having an average diameter of 3.2 mm
was placed in a ball mill with 20 g of tin powder having a mean
diameter of 6 .mu.m and 10 hardened steel ball bearings with a
diameter of 20 mm. The ball mill was operated at 270 rpm for 1
hour. The resulting shot was analyzed using a scanning electron
microscope (SEM), as shown in FIG. 10. The surface of the shot
appeared smooth and shiny, indicating that some of the tin had
mechanically plated on to the surface of the shot. A cross section
of the shot analyzed using elemental mapping showed that a
continuous layer of tin approximately 1 .mu.m thick had been
mechanically plated on to the shot.
[0085] Other advantages which are inherent to the structure are
obvious to one skilled in the art. It is apparent to one skilled in
the art that many variations on the present invention can be made
without departing from the scope or spirit of the invention claimed
herein.
[0086] It will be understood that certain features and
sub-combinations are of utility and may be employed without
reference to other features and sub-combinations. This is
contemplated by and is within the scope of the claims.
[0087] Since many possible embodiments may be made of the invention
without departing from the scope thereof, it is to be understood
that all matter herein set forth or shown in the accompanying
figures is to be interpreted as illustrative and not in a limiting
sense.
[0088] Having described the invention, what is claimed is:
* * * * *