Fe-Sn-Cu-Pb sintered composite metal article and process
United States Patent 3900317
Composite metal articles of high density are made from powders of the following metals: about 25 to about 90 wt. % iron, up to about 5 wt. % of tin, copper or mixtures thereof, and the balance substantially all lead. Preferably zinc in amounts up to about 1.5% is also present. Composite bird shot, for instance, is prepared by agglomeration techniques, sintered and partially alloyed at below about 1,200°C and if desired mechanically compacted to further increase the density. A lead-rich outer layer is advantageous.
US Patent References:
Method of making projectiles
Werme - December 1943 - 2336143
Plastic coated shotgun pellets
Irons - January 1968 - 3363561
WATER DISINTEGRABLE LEAD SHOT
Foerster - August 1969 - 3463637
FLOWABLE METAL POWDERS
Harrington et al. - December 1969 - 3481714
METHOD OF PREPARING NUCLEAR FUEL ELEMENTS INCORPORATING FISSION PRODUCT RETAINING FUEL PARTICLES
Redding - January 1970 - 3492379
Method of making projectiles
Werme - December 1943 - 2336143
Plastic coated shotgun pellets
Irons - January 1968 - 3363561
WATER DISINTEGRABLE LEAD SHOT
Foerster - August 1969 - 3463637
FLOWABLE METAL POWDERS
Harrington et al. - December 1969 - 3481714
METHOD OF PREPARING NUCLEAR FUEL ELEMENTS INCORPORATING FISSION PRODUCT RETAINING FUEL PARTICLES
Redding - January 1970 - 3492379
Inventors:
Meadus, Frederick W. (Ottawa, CA)
Sparks, Bryan D. (Ottawa, CA)
Puddington, Ira E. (Ottawa, CA)
Sparks, Bryan D. (Ottawa, CA)
Puddington, Ira E. (Ottawa, CA)
Application Number:
05/338563
Publication Date:
08/19/1975
Filing Date:
03/06/1973
View Patent Images:
Export Citation:
Assignee:
Canadian Patents and Development Limited (Ottawa, CA)
Primary Class:
Other Classes:
419/35, 102/459, 86/57, 419/46, 428/567, 428/548
International Classes:
B22F9/04; C22C1/04; F42B7/04; B22F9/02; F42B7/00; B22F7/00; C22C1/04
Field of Search:
75/28R,211,212,200,.5R 29/182,182.2,1.23 102/42R,92.2 264/.5
Primary Examiner:
Padgett, Benjamin R.
Assistant Examiner:
Schafer R. E.
Attorney, Agent or Firm:
Thomson, Alan A.
Claims:
We claim
1. A method of preparing a composite iron-lead article from metal powders comprising iron particles in about 25 to about 90% wt.; a metal selected from the group consisting of tin, copper or mixtures thereof in up to about 5%; and the balance substantially all lead powder, including:
2. The method of claim 1 wherein zinc is present in amounts up to about 1.5% wt.
3. The method of claim 1 wherein the article is a pellet and the pellets are formed by steps including:
4. The method of claim 3 wherein the first liquid is a hydrocarbon and the second liquid is aqueous.
5. The method of claim 4 wherein a surfactant is present in the second liquid to aid wetting the metal powders.
6. The method of claim 3 wherein the compaction is carried out by a step selected from (i) rolling between hard surfaces, and (ii) ricocheting against hard surfaces.
7. The method of claim 3 wherein before separating the balls, a further charge of metal powder more enriched in Pb is dispersed in the system and further agglomeration carried out to form a layer of the additional more Pb-rich charge on the surface of the balls.
8. The method of claim 4 wherein surface residual hydrocarbon liquid is allowed to remain on the pellets into the sintering stage to aid sintering.
1. A method of preparing a composite iron-lead article from metal powders comprising iron particles in about 25 to about 90% wt.; a metal selected from the group consisting of tin, copper or mixtures thereof in up to about 5%; and the balance substantially all lead powder, including:
2. The method of claim 1 wherein zinc is present in amounts up to about 1.5% wt.
3. The method of claim 1 wherein the article is a pellet and the pellets are formed by steps including:
4. The method of claim 3 wherein the first liquid is a hydrocarbon and the second liquid is aqueous.
5. The method of claim 4 wherein a surfactant is present in the second liquid to aid wetting the metal powders.
6. The method of claim 3 wherein the compaction is carried out by a step selected from (i) rolling between hard surfaces, and (ii) ricocheting against hard surfaces.
7. The method of claim 3 wherein before separating the balls, a further charge of metal powder more enriched in Pb is dispersed in the system and further agglomeration carried out to form a layer of the additional more Pb-rich charge on the surface of the balls.
8. The method of claim 4 wherein surface residual hydrocarbon liquid is allowed to remain on the pellets into the sintering stage to aid sintering.
Description:
This invention is directed to composite metal articles and a method of preparing these articles. Of particular interest is bird shot of reduced toxicity compared to the commonly used lead shot.
A serious problem of lead poisoning has been found to exist with respect to waterfowl and gamebirds due to the ingestion of spent shot during feeding. The shot is retained in the gizzard and is ground and eroded by the gizzard motion and acid content, releasing soluble lead into the system. Even one pellet has been found sufficient to cause death, usually within 20 days. The lead shot is very stable to corrosion and weathering under normal marsh and soil conditions and where gunning has occurred repeatedly, the density of spent shot and the resulting toxicity hazard can become very high.
Efforts are being made to develop a non-toxic or less-toxic birdshot and a number of metals, alloys and coated pellets have been tried. Iron shot has received the main emphasis since the toxicity due to iron is low (about 12 percent compared to greater than 90% mortality for lead). However iron causes undue wear on gun barrels and chokes and ballistic behaviour beyond about 40 yards becomes poor. Lead alloys have been tried but toxicity remains unacceptably high. Copper, zinc and some other metals have been tested but while toxicity is decreased they are otherwise inferior to or less suitable then lead. Plastic-coated lead shot has had a high mortality close to that of lead itself.
Thus there is a need to develop a shot which will have acceptably low toxicity, acceptable gun barrel and ballistic performance and yet be of reasonable cost and easily made.
In accordance with the present invention we have found that high density pellets of reduced toxicity compared to lead, can be prepared from certain metal powders. Density is sufficiently high to give reasonable ballistic performance and gun barrel and choke wear is within acceptable limits. We have been able to demonstrate that the addition of iron to lead shot reduces the toxicity. Addition of as little as about 25% wt. iron produces a shot that is no longer unacceptably toxic. Iron and lead have no mutual solubility so that alloying is very difficult. Alloying metal additives are costly, but a partial alloying with small amounts of ternary or quaternary additions has been found very satisfactory.
An intimate dispersion of powders is prepared, shaped, sintered-and-partially-alloyed between the iron particles, and preferably mechanically compacted to a higher density.
In the attached drawings:
FIG. 1 is a photomicrograph of a cross-section of a typical article taken at 1000 X, and
FIG. 2 is a photomicrograph taken using an electron probe adapted to give a profile analysis of the iron component of the same cross-section as in FIG. 1, at 1000 X.
Composite metal articles are provided which contain from about 25 to about 90 wt. % iron, up to about 5 wt. % tin or copper or mixtures thereof, and the balance substantially all lead. Zinc is preferably added in amounts up to about 1.5 wt. % to obtain additional iron-Pb alloy compatibility. The tin and/or copper should be present in sufficient amounts to give adequate wetting of the iron particles. There is no critical lower limit - usually at least 0.5% is beneficial. The cost and other factors would indicate decreased benefit above 5%.
The cross-section is not uniform as can be seen in the accompanying photomicrographs (using a pellet as in Example 2(a) below). The photomicrograph shown as FIG. 1 was obtained from an electron scanning microscope using a conventional configuration. The pellets were set in epoxy resin, carefully sectioned and polished. The overall structure appears largely as Fe particles shown as dark grey in FIG. 1 --imbedded in a matrix alloy of Pb, Sn and Zn-shown as white. FIG. 2 was obtained using the electron probe attachment to give a profile analysis of the Fe of the same cross-section. In FIG. 2 the iron appears as the light-colored areas. Similar photomicrographs were taken with the probe for Pb, Sn and Zn separately. The Pb, Sn and Zn were seen to lie between the iron particles and served to fill the voids and cement the iron particles together. If one superimposed these photomicrographs (as negatives) on top of each other one sees that the Fe is not alloyed except possibly at the Fe surfaces with the Pb, Sn and Zn. There are also indications that the Sn is preferentially adsorbed at the Fe interface.
In addition to pellets and metal shot, other shaped articles of the sintered composite powders can be made e.g. by powder metallurgy techniques. For instance the articles may be bearings, magnets, blanks for preparing porous Pb alloy objects, etc.
Metal shot has usually been prepared by some manner of disruption of metal into particles followed by cooling and shaping the particles in a moving fluid or in a disc mill etc. Shot towers of considerable size have been required. Pellets have been prepared from fine metal powders by mixing with a volatile binder and tumbling in the presence of a small amount of moisture to give balls which are then heated to remove the binder and to sinter the metal powder. In this latter case, the binder is an added cost and the density and sphericity are difficult to control and optimize.
To ensure a uniform final sintered product, it is desirable to pre-mix the metal powders thoroughly. A mild grinding of the mixed powder was found to give satisfactory results. The non-iron metals may be pre-alloyed, ground, and mixed with iron powders. In preparing the composite pellets, we prefer to carry out an agglomeration from a liquid dispersion of the metal powders. The metal powder may have been wet ground in the same liquid to a suitable particle size (the particle size is not critical and preferably contains a spectrum of sizes in the range of about 20 to 200 micrometers, when pellets of the order of 1 -6 mm. diameter are being produced). The continuous phase liquid is preferably an inert organic liquid such as a low viscosity hydrocarbon. Other organic liquids which may also be used, include halogenated hydrocarbons, (carbon tetrachloride, trichlorethylene, etc.) petroleum ethers, and other organic liquids which are water-immiscible.
When the continuous phase liquid is organic, an aqueous briding liquid is used and is usually applied as a spray to the surface of the agitated or rotating slurry. It may be necessary to add a small amount (up to 0.1%) of a wetting agent, e.g. tannic acid or cationic urea-formaldehyde resin dispersions, to the aqueous bridging liquid to compensate for different surface characteristics of the metal powders. This addition of wetting agent would only be needed during nuclei formation. The amount of aqueous bridging liquid applied is of the order of 5 to 40% by wt. of the solids. This amount has been found most suitable for producing agglomerates of the size of about 1-6 mm. diameter. No binder is necessary.
A large dish rotating about an axis at an angle to the vertical has been found very suitable for the agglomeration process (see C. E. Capes et al U.S. Pat. No. 3,471,267 October 7/69 and U.S. Pat. No. 3,665,066 issued 23 May 1972). A rotating or reciprocating drum could also be used. In all cases agglomeration is preferably initiated by the formation of small nuclei. These nuclei may be formed in the dish itself by the controlled addition of bridging liquid to small amounts of prepared powder dispersion. However, it is ofter more convenient to prepare the nuclei by some other technique. For example, vigorous agitation of a slurry (about 10 to 50% solids) plus bridging liquid in a high speed mixer, a stirred tank or a tank fitted with a sump pump and recirculating limb, has given satisfactory nuclei. Alternatively solid particle nuclei (non-agglomerated) of up to 75% of final pellet diameter may be used. The composition of the nuclei is not critical but may comprise a variety of materials compatible with the outside layers (e.g. particles of iron, copper, nickel, ceramics, cerments, etc. and mixtures thereof).
After nucleation, it has been found advantageous to increase the nuclei size by depositing the metal powder on the nuclei in layers, by alternately adding the powder and bridging liquid spray to the system undergoing agitation. This layering can be repeated in a number of stages, and has been found to give a better control of pellet size and sphericity. With proper agitation and uniform feeding, less off-sized material and recycling has been found necessary. The wet agglomerates are sufficiently strong to be separated by screening, decanting etc. and if necessary segregated within selected size limits. Off-sized agglomerates can be recycled.
A preferred feature of the invention is to use different metal powders in different stages of the layering operation. Thus for metal shot it is advantageous to have a higher concentration of lead (and tin or copper) in the outer layer to minimize wear in the gun barrel and choke. The harder iron material is concentrated in the interior of the pellet.
For ballistic performance as high a density as possible is desired and sintering is necessary to achieve high density and strength. The alloying additions and the sintering temperature affect the sintered density. Sintering temperatures over about 1200°C are deemed too costly for this type of product. With control of the alloying additions and with preferred auxiliary steps, quite high densities have been achieved at sintering temperatures below 1200°C.
One auxiliary step which has been found to increase the density is to impact, repeatedly hammer or roll the sintered pellets with hard surfaces. One convenient way of accomplishing this re-compression is to agitate pellets in a closed partiallyfilled container in a 3-dimensional path to effect repeated ricocheting impacts with the container walls. Another preferred technique is to roll the pellets between hard surfaces e.g. between rotating grooved plates being urged together.
The presence of some residual carbon on the pellets during sintering has been found to give small increases in density. The carbon was conveniently deposited from excess organic liquid left on the pellets from the agglomeration stage.
For overall performance, cost, and toxicity, presently preferred compositions for bird shot are:
iron in about 25 to about 50% wt.;
copper in about 0.5 to about 5% wt.;
zinc in about 0.5 to about 1% wt.;
and the balance substantially all lead.
The following examples are illustrative.
EXAMPLE 1
Finely divided metal powders of from about 50 to 150 micrometers diameter were mixed in a high speed blender to give a 20% wt. solids slurry in an aliphatic hydrocarbon solvent (Varsol-trademark). The metals were present in the following proportions: 55% wt. Pb, 40% Fe, 4% Sn and 1% Zn. Water containing 0.1% Wt. of tannic acid, was metered to the surface of the slurry while stirred in the blender, in an amount of about 10% wt. of the solids. Agglomerates of about 0.5 mm diameter quickly formed, and were used as nuclei.
Slurry containing a lesser concentration of nuclei in the solvent was placed in a rotating dish. A small amount of water was sprayed onto the surface of the rotating slurry and further mixed metal powder was then added. (The powder was preferably pre-wetted with solvent to minimize dusting and aid incorporation). Water was again sprayed onto the surface to give a total of about 8% by wt. based on the further powder added. A layer of powder was formed on the nuclei. This alternate powder addition and water spray was repeated a number of times to build up successive layers on the nuclei until the balls reached the size of about 3 mm. diameter. The balls at this stage should be about 14% larger than the final desired shot size.
The balls were separated from the solvent by screening and air dried slowly. They were sintered by heating slowly in an inert atmosphere up to about 940°C over 1.5 hours and held at this temperature for about 15 min. The balls were packed in graphite powder which maintained a reducing atmosphere around the charge. (Hydrogen atmospheres have also given good sintering). The sintered balls were cooled in a reducing or inert atmosphere. A sintered density of 7.5 g/cc was obtained, and a diameter of about 2.6 mm.
The lead content of each powder addition during the multi-layering operation can be increased to give lead-rich surface layers.
EXAMPLE 2
Balls were made as outlined in Example 1 containing
a. 40 % wt. Pb, 55% Fe, 4% Sn and 1% Zn and
b. 30 % wt. Pb, 65% Fe, 4% Sn and 1% Zn
Both groups were packed in graphite powder and sintered at about 1000°C for 20 min. (a), and 1 hr. (b), followed by quick cooling in an inert atmosphere. Good balls having a density of 8.4 (a), and 7.2 (b) g/cc were obtained. One of the (a) pellets was used to obtain the photomicrographs FIGS. 1 and 2.
Large samples of (a) and (b) pellets were tested for toxicity to birds, and for shotgun wear (300 rounds) and were found acceptable on both counts.
EXAMPLE 3
The effect of temperature on sintered density was examined for several compositions of Pb/Fe/Sn/Zn. Balls were prepared as in Example 1 having the following compositions:
a. 55% wt. Pb, 40% Fe, 4% Sn and 1% Zn.
b. 30% wt. Pb, 65% Fe, 4% Sn and 1% Zn.
c. 28% wt. Pb, 65% Fe, 4% Sn and 3% Zn.
Samples from each series were then sintered at various temperatures for 1 hour. The pellets were packed in graphite powder during sintering and cooled in an inert atmosphere.
For 3 (a), densities of 7.5-7.6 g/cc were achieved at temperatures within about 930°-1000°C. From over about 1000°C to about 1140°C, the sintered densities were lower than this with a distinct minimum at about 1050°C. Beyond 1140°C the densities increased above 7.6, with a density of 8.5 being reached at 1180°C.
With 3 (b), a distinct maximum in density occurred at about 1040°-1050°C (about 7.1 g/cc). At 1000°C and 1080°C the sintered density had dropped to about 6.6 g/cc.
With 3 (c), a maximum occurred within about 1050 to 1090°C of about 7.2 g/cc. Below and above this temperature range, the density fell off sharply. Above about 1230°C the density was increasing above 7.2.
EXAMPLE 4
Composite pellets of 55% wt. Pb, 40% Fe, 4% Cu and 1% Zn were prepared as in Example 1. Pellets were sintered for 1 hour at various temperatures (by direct introduction into the hot furnace) and the densities measured. Some sintered pellets were then compacted in a container in a high speed rotary shaker. The sintered densities increased only slightly (from 7.4 to 7.6 g/cc) as the temperature increased from about 950° to 1150°C and there was no distinct maximum. This was in contrast to the behaviour of the Sn-containing pellets of Examples 1-3. The compacted sintered pellets had densities about 0.3-0.4 g/cc higher than the uncompacted ones. The highest density achieved was 8.2 g/cc at 1100°C where some residual carbon from the organic liquid remained with the pellets into the sintering operation.
When the pellets were heated slowly to sintering temperature (over a period of 1.5 hours), a greater increase in density was observed with increasing sintering temperature than before. Densities as high as 8.5 - 8.8 g/cc were obtained at 1140°C using the additional compaction described.
EXAMPLE 5
Composite pellets of 30% Pb, 65% Fe, 4% Cu and 1% Zn were prepared as before and sintered for 1 hour at various temperatures. The samples were introduced into the hot furnace. The density increased approximately linearly from about 5.8 g/cc at 1000°C to about 6.8 at 1200°C. with compaction after sintering, the linear increase was from about 6.6 at 1000°C to about 7.8 at 1200°C.
The pellets of Ex. 4 and 5 also gave acceptable toxicity and gun barrel wear.
A serious problem of lead poisoning has been found to exist with respect to waterfowl and gamebirds due to the ingestion of spent shot during feeding. The shot is retained in the gizzard and is ground and eroded by the gizzard motion and acid content, releasing soluble lead into the system. Even one pellet has been found sufficient to cause death, usually within 20 days. The lead shot is very stable to corrosion and weathering under normal marsh and soil conditions and where gunning has occurred repeatedly, the density of spent shot and the resulting toxicity hazard can become very high.
Efforts are being made to develop a non-toxic or less-toxic birdshot and a number of metals, alloys and coated pellets have been tried. Iron shot has received the main emphasis since the toxicity due to iron is low (about 12 percent compared to greater than 90% mortality for lead). However iron causes undue wear on gun barrels and chokes and ballistic behaviour beyond about 40 yards becomes poor. Lead alloys have been tried but toxicity remains unacceptably high. Copper, zinc and some other metals have been tested but while toxicity is decreased they are otherwise inferior to or less suitable then lead. Plastic-coated lead shot has had a high mortality close to that of lead itself.
Thus there is a need to develop a shot which will have acceptably low toxicity, acceptable gun barrel and ballistic performance and yet be of reasonable cost and easily made.
In accordance with the present invention we have found that high density pellets of reduced toxicity compared to lead, can be prepared from certain metal powders. Density is sufficiently high to give reasonable ballistic performance and gun barrel and choke wear is within acceptable limits. We have been able to demonstrate that the addition of iron to lead shot reduces the toxicity. Addition of as little as about 25% wt. iron produces a shot that is no longer unacceptably toxic. Iron and lead have no mutual solubility so that alloying is very difficult. Alloying metal additives are costly, but a partial alloying with small amounts of ternary or quaternary additions has been found very satisfactory.
An intimate dispersion of powders is prepared, shaped, sintered-and-partially-alloyed between the iron particles, and preferably mechanically compacted to a higher density.
In the attached drawings:
FIG. 1 is a photomicrograph of a cross-section of a typical article taken at 1000 X, and
FIG. 2 is a photomicrograph taken using an electron probe adapted to give a profile analysis of the iron component of the same cross-section as in FIG. 1, at 1000 X.
Composite metal articles are provided which contain from about 25 to about 90 wt. % iron, up to about 5 wt. % tin or copper or mixtures thereof, and the balance substantially all lead. Zinc is preferably added in amounts up to about 1.5 wt. % to obtain additional iron-Pb alloy compatibility. The tin and/or copper should be present in sufficient amounts to give adequate wetting of the iron particles. There is no critical lower limit - usually at least 0.5% is beneficial. The cost and other factors would indicate decreased benefit above 5%.
The cross-section is not uniform as can be seen in the accompanying photomicrographs (using a pellet as in Example 2(a) below). The photomicrograph shown as FIG. 1 was obtained from an electron scanning microscope using a conventional configuration. The pellets were set in epoxy resin, carefully sectioned and polished. The overall structure appears largely as Fe particles shown as dark grey in FIG. 1 --imbedded in a matrix alloy of Pb, Sn and Zn-shown as white. FIG. 2 was obtained using the electron probe attachment to give a profile analysis of the Fe of the same cross-section. In FIG. 2 the iron appears as the light-colored areas. Similar photomicrographs were taken with the probe for Pb, Sn and Zn separately. The Pb, Sn and Zn were seen to lie between the iron particles and served to fill the voids and cement the iron particles together. If one superimposed these photomicrographs (as negatives) on top of each other one sees that the Fe is not alloyed except possibly at the Fe surfaces with the Pb, Sn and Zn. There are also indications that the Sn is preferentially adsorbed at the Fe interface.
In addition to pellets and metal shot, other shaped articles of the sintered composite powders can be made e.g. by powder metallurgy techniques. For instance the articles may be bearings, magnets, blanks for preparing porous Pb alloy objects, etc.
Metal shot has usually been prepared by some manner of disruption of metal into particles followed by cooling and shaping the particles in a moving fluid or in a disc mill etc. Shot towers of considerable size have been required. Pellets have been prepared from fine metal powders by mixing with a volatile binder and tumbling in the presence of a small amount of moisture to give balls which are then heated to remove the binder and to sinter the metal powder. In this latter case, the binder is an added cost and the density and sphericity are difficult to control and optimize.
To ensure a uniform final sintered product, it is desirable to pre-mix the metal powders thoroughly. A mild grinding of the mixed powder was found to give satisfactory results. The non-iron metals may be pre-alloyed, ground, and mixed with iron powders. In preparing the composite pellets, we prefer to carry out an agglomeration from a liquid dispersion of the metal powders. The metal powder may have been wet ground in the same liquid to a suitable particle size (the particle size is not critical and preferably contains a spectrum of sizes in the range of about 20 to 200 micrometers, when pellets of the order of 1 -6 mm. diameter are being produced). The continuous phase liquid is preferably an inert organic liquid such as a low viscosity hydrocarbon. Other organic liquids which may also be used, include halogenated hydrocarbons, (carbon tetrachloride, trichlorethylene, etc.) petroleum ethers, and other organic liquids which are water-immiscible.
When the continuous phase liquid is organic, an aqueous briding liquid is used and is usually applied as a spray to the surface of the agitated or rotating slurry. It may be necessary to add a small amount (up to 0.1%) of a wetting agent, e.g. tannic acid or cationic urea-formaldehyde resin dispersions, to the aqueous bridging liquid to compensate for different surface characteristics of the metal powders. This addition of wetting agent would only be needed during nuclei formation. The amount of aqueous bridging liquid applied is of the order of 5 to 40% by wt. of the solids. This amount has been found most suitable for producing agglomerates of the size of about 1-6 mm. diameter. No binder is necessary.
A large dish rotating about an axis at an angle to the vertical has been found very suitable for the agglomeration process (see C. E. Capes et al U.S. Pat. No. 3,471,267 October 7/69 and U.S. Pat. No. 3,665,066 issued 23 May 1972). A rotating or reciprocating drum could also be used. In all cases agglomeration is preferably initiated by the formation of small nuclei. These nuclei may be formed in the dish itself by the controlled addition of bridging liquid to small amounts of prepared powder dispersion. However, it is ofter more convenient to prepare the nuclei by some other technique. For example, vigorous agitation of a slurry (about 10 to 50% solids) plus bridging liquid in a high speed mixer, a stirred tank or a tank fitted with a sump pump and recirculating limb, has given satisfactory nuclei. Alternatively solid particle nuclei (non-agglomerated) of up to 75% of final pellet diameter may be used. The composition of the nuclei is not critical but may comprise a variety of materials compatible with the outside layers (e.g. particles of iron, copper, nickel, ceramics, cerments, etc. and mixtures thereof).
After nucleation, it has been found advantageous to increase the nuclei size by depositing the metal powder on the nuclei in layers, by alternately adding the powder and bridging liquid spray to the system undergoing agitation. This layering can be repeated in a number of stages, and has been found to give a better control of pellet size and sphericity. With proper agitation and uniform feeding, less off-sized material and recycling has been found necessary. The wet agglomerates are sufficiently strong to be separated by screening, decanting etc. and if necessary segregated within selected size limits. Off-sized agglomerates can be recycled.
A preferred feature of the invention is to use different metal powders in different stages of the layering operation. Thus for metal shot it is advantageous to have a higher concentration of lead (and tin or copper) in the outer layer to minimize wear in the gun barrel and choke. The harder iron material is concentrated in the interior of the pellet.
For ballistic performance as high a density as possible is desired and sintering is necessary to achieve high density and strength. The alloying additions and the sintering temperature affect the sintered density. Sintering temperatures over about 1200°C are deemed too costly for this type of product. With control of the alloying additions and with preferred auxiliary steps, quite high densities have been achieved at sintering temperatures below 1200°C.
One auxiliary step which has been found to increase the density is to impact, repeatedly hammer or roll the sintered pellets with hard surfaces. One convenient way of accomplishing this re-compression is to agitate pellets in a closed partiallyfilled container in a 3-dimensional path to effect repeated ricocheting impacts with the container walls. Another preferred technique is to roll the pellets between hard surfaces e.g. between rotating grooved plates being urged together.
The presence of some residual carbon on the pellets during sintering has been found to give small increases in density. The carbon was conveniently deposited from excess organic liquid left on the pellets from the agglomeration stage.
For overall performance, cost, and toxicity, presently preferred compositions for bird shot are:
iron in about 25 to about 50% wt.;
copper in about 0.5 to about 5% wt.;
zinc in about 0.5 to about 1% wt.;
and the balance substantially all lead.
The following examples are illustrative.
EXAMPLE 1
Finely divided metal powders of from about 50 to 150 micrometers diameter were mixed in a high speed blender to give a 20% wt. solids slurry in an aliphatic hydrocarbon solvent (Varsol-trademark). The metals were present in the following proportions: 55% wt. Pb, 40% Fe, 4% Sn and 1% Zn. Water containing 0.1% Wt. of tannic acid, was metered to the surface of the slurry while stirred in the blender, in an amount of about 10% wt. of the solids. Agglomerates of about 0.5 mm diameter quickly formed, and were used as nuclei.
Slurry containing a lesser concentration of nuclei in the solvent was placed in a rotating dish. A small amount of water was sprayed onto the surface of the rotating slurry and further mixed metal powder was then added. (The powder was preferably pre-wetted with solvent to minimize dusting and aid incorporation). Water was again sprayed onto the surface to give a total of about 8% by wt. based on the further powder added. A layer of powder was formed on the nuclei. This alternate powder addition and water spray was repeated a number of times to build up successive layers on the nuclei until the balls reached the size of about 3 mm. diameter. The balls at this stage should be about 14% larger than the final desired shot size.
The balls were separated from the solvent by screening and air dried slowly. They were sintered by heating slowly in an inert atmosphere up to about 940°C over 1.5 hours and held at this temperature for about 15 min. The balls were packed in graphite powder which maintained a reducing atmosphere around the charge. (Hydrogen atmospheres have also given good sintering). The sintered balls were cooled in a reducing or inert atmosphere. A sintered density of 7.5 g/cc was obtained, and a diameter of about 2.6 mm.
The lead content of each powder addition during the multi-layering operation can be increased to give lead-rich surface layers.
EXAMPLE 2
Balls were made as outlined in Example 1 containing
a. 40 % wt. Pb, 55% Fe, 4% Sn and 1% Zn and
b. 30 % wt. Pb, 65% Fe, 4% Sn and 1% Zn
Both groups were packed in graphite powder and sintered at about 1000°C for 20 min. (a), and 1 hr. (b), followed by quick cooling in an inert atmosphere. Good balls having a density of 8.4 (a), and 7.2 (b) g/cc were obtained. One of the (a) pellets was used to obtain the photomicrographs FIGS. 1 and 2.
Large samples of (a) and (b) pellets were tested for toxicity to birds, and for shotgun wear (300 rounds) and were found acceptable on both counts.
EXAMPLE 3
The effect of temperature on sintered density was examined for several compositions of Pb/Fe/Sn/Zn. Balls were prepared as in Example 1 having the following compositions:
a. 55% wt. Pb, 40% Fe, 4% Sn and 1% Zn.
b. 30% wt. Pb, 65% Fe, 4% Sn and 1% Zn.
c. 28% wt. Pb, 65% Fe, 4% Sn and 3% Zn.
Samples from each series were then sintered at various temperatures for 1 hour. The pellets were packed in graphite powder during sintering and cooled in an inert atmosphere.
For 3 (a), densities of 7.5-7.6 g/cc were achieved at temperatures within about 930°-1000°C. From over about 1000°C to about 1140°C, the sintered densities were lower than this with a distinct minimum at about 1050°C. Beyond 1140°C the densities increased above 7.6, with a density of 8.5 being reached at 1180°C.
With 3 (b), a distinct maximum in density occurred at about 1040°-1050°C (about 7.1 g/cc). At 1000°C and 1080°C the sintered density had dropped to about 6.6 g/cc.
With 3 (c), a maximum occurred within about 1050 to 1090°C of about 7.2 g/cc. Below and above this temperature range, the density fell off sharply. Above about 1230°C the density was increasing above 7.2.
EXAMPLE 4
Composite pellets of 55% wt. Pb, 40% Fe, 4% Cu and 1% Zn were prepared as in Example 1. Pellets were sintered for 1 hour at various temperatures (by direct introduction into the hot furnace) and the densities measured. Some sintered pellets were then compacted in a container in a high speed rotary shaker. The sintered densities increased only slightly (from 7.4 to 7.6 g/cc) as the temperature increased from about 950° to 1150°C and there was no distinct maximum. This was in contrast to the behaviour of the Sn-containing pellets of Examples 1-3. The compacted sintered pellets had densities about 0.3-0.4 g/cc higher than the uncompacted ones. The highest density achieved was 8.2 g/cc at 1100°C where some residual carbon from the organic liquid remained with the pellets into the sintering operation.
When the pellets were heated slowly to sintering temperature (over a period of 1.5 hours), a greater increase in density was observed with increasing sintering temperature than before. Densities as high as 8.5 - 8.8 g/cc were obtained at 1140°C using the additional compaction described.
EXAMPLE 5
Composite pellets of 30% Pb, 65% Fe, 4% Cu and 1% Zn were prepared as before and sintered for 1 hour at various temperatures. The samples were introduced into the hot furnace. The density increased approximately linearly from about 5.8 g/cc at 1000°C to about 6.8 at 1200°C. with compaction after sintering, the linear increase was from about 6.6 at 1000°C to about 7.8 at 1200°C.
The pellets of Ex. 4 and 5 also gave acceptable toxicity and gun barrel wear.