Patent Application
20140020590
The present invention relates to a method of producing a sintered fragmentation casing for explosives-charged warheads by applying powder metal technology. The present invention also includes various configurations of fragmentation casings produced in accordance with the said technology. A special feature of the fragmentation casings as claimed in the present invention is that their powder metal technology produced supporting main section or moulded part contains a large quantity of fragment bodies embedded at predetermined locations and distributed differently, and produced in a harder and heavier material than that used for the main mass of the moulded part. In this context the said fragment bodies are preferably comprised of heavy metal balls.
By powder metal technology is meant here that the single-piece supporting main section or moulded part is completely or partially formed by a suitable powder metal that is compressed until it assumes the desired form and is sintered together to form a homogenous metal.
Two different methods of producing homogenous metal bodies using powder metallurgy technology are well known. One of the said methods is designated in everyday language as HIP-ing or hot isostatic pressing which means that the basic powder material being used is isostatic compressed at the same time as it is sintered to form a homogenous metal. The other method is designated SIP-ing which means that the powder material is first cold isostatic compressed until the desired density is achieved, after which the compressed powder granules are sintered in a separate process until a homogenous metal is formed.
Both of these general methods can be utilised within the basic concept of the present invention.
By the designation heavy metal is meant here primarily high density Wolfram alloys. Depleted uranium has also been used in similar circumstances but it is still regarded with doubts regarding its effect on health during handling prior to use as well as any radioactive fallout after use.
When combating airborne targets such as aircraft and various types of missiles using barrel-fired projectiles or own missiles, as a rule it cannot be counted on that a direct hit on the target will be achieved and instead a near-miss must suffice and that the explosive charge-loaded warhead can be detonated as close to the target as possible. For this to be enabled the said warhead must be provided with a proximity fuze or equivalent that controls its detonation until the optimal point in time for combating the target with pressure and fragments. In most cases the greatest effect in the target from the said type of near-miss is achieved when the explosive charge is enclosed in a fragmentation jacket comprising a large number of pre-formed fragment bodies. Heavy metal bodies are now assumed to be the best technical and most economic fragment bodies as they have a high level of density and when they are enclosed in a fragmentation jacket they also create large quantities of fragments. The said heavy metal balls that are projected at high velocity by the detonation of the explosive generate good penetration even in semi-hard targets and in addition their size and consequently their dispersion pattern are predetermined. On the other hand it is more difficult to determine exactly how an originally homogenous fragmentation jacket for an explosive charged projectile will disintegrate when subjected to the detonation of an explosive charge and consequently the fragment dispersion pattern thus formed will be difficult to determine and partially at random. Therefore the intention was to provide air defence explosive-charged projectiles with a fragmentation jacket containing a large quantity of heavy metal balls that when the explosive is detonated it will eject a swarm of the said heavy metal balls in the direction of the target. However, to produce such a fragmentation jacket is not the easiest of tasks because the object is to have the greatest possible number of heavy metal balls penetrate the target and therefore the form of the fragmentation jacket is a critical factor in this context. Even in relatively simple forms this type of fragmentation jacket is relatively problematic to manufacture using the technologies currently available.
In this context U.S. Pat. No. 3,815,504 describes a method of producing fragmentation jackets for use in artillery shells where heavy metal balls are filled in between an inner and an outer tubular casings until the space between them is completely filled after which the inner tubular casing is subjected to high inner pressure either via a slightly conical “dolly” device or an inner detonation which secures the heavy metal balls by means of deformation of the inner tubular casing.
The said method of producing fragmentation jackets however, has the disadvantage of leaving a gap between the heavy metal balls which at an early stage of the detonation phase of the explosive contained in the complete shell causes pressure leakage between the said heavy metal balls thus exerting a lower velocity on them than would have been the case had they been completely encased by a moulded part.
U.S. Pat. No. 4,503, 776 further describes a fragmentation jacket comprising projectile-formed fragment bodies that are provided with a rear free opening that is used partly to fix the said fragment bodies in position in a fixture while the said fragment bodies are moulded in a base material, and partly for filling with incendiary material or equivalent after the moulding process is completed and the fixture has been removed. The moulding material used is cast iron and the said fixture can be of a ceramic material that can be either left in place or be removed when the moulding base material has set. The most immediate problem with this method would appear primarily to be the risk of porosity in the moulded material.
Finally, U.S. Pat. No. 4,129,061 describes a prefragmented shell having an outer casing produced using powder metallurgy technology. In this variant a compact layer of heavy metal balls is arranged around a single-piece body and thereafter the said compact layer of heavy metal balls is covered with powder metal that is then compacted and sintered together after which the centre body bored out to receive the explosive charge and the sintered powder jacket is finish-machined to the intended shape of the shell. However, the said patent does not disclose how the heavy metal balls are retained in their positions until the powder metal is introduced and compacted to form a single unit. Moreover, the said method requires considerable subsequent work and creates the risk of irregular powder density in critical areas.
Several years ago we made several attempts to produce shells provided with a prefragmented casing by applying powder metallurgy technology but the results were not completely satisfactory. Even though current conventional powder metallurgy technology is used to produce a large variety of different products there is a particular problem involved when producing prefragmented casings, namely the said casings shall contain such a large quantity of separately produced heavy metal balls from the very start. That is to say it is the material between the heavy metal balls that holds them together and gives the prefragmented casing its outer form that is to be created by powder metallurgy technology and inside the said single-unit casing or moulded part the heavy metal balls shall be embedded.
This is to say that a prefragmented shell casing containing embedded heavy metal balls comprises two different materials of which the heavy metal balls are already produced completely prior to formation of the single-unit casing and compaction by the powder metal that is then sintered together to form a single homogenous unit. The greatest difficulties with manufacturing prefragmented shell casings by applying powder metallurgy technology is that the materials to be included will have completely coefficients of expansion while the sintering phase involves the entire pre-formed body must be heated to the sintering temperature of the powder component. In previous attempts to produce prefragmented casings by applying powder metallurgy technology the frequency of shrinkage cracks in the casings was so high that as far as we are aware they never appeared on the market.
Previously tested techniques in this field are described in Swedish Patent SE 450294 (=U.S. Pat. No. 4,644,867) represented in the form of powder metallurgy technology produced prefragmented shells the casings of which were produced by means of completely pre-formed heavy metal balls embedded in powder metal that are then subjected to high temperature and high pressure from all directions to form a tightly compacted casing. Even if this patent, which is our own, does not state clearly how we were able to retain the heavy metal balls in their correct positions in the metal powder jacket, at that period of time we utilised a technique where we first attached the pre-formed heavy metal balls to a single-piece prefragmented casing which we then surrounded with powder steel which was then compacted under high pressure and sintered together to form a single uniform material. The problem using this technique was that the heavy metal balls formed a single inter-connected layer having completely different shrinkage properties than the surrounding powder metal technology produced material. Consequently, the frequency of cracks in the powder metal technology produced fragmentation jacket was too high for the production method to be utilised for mass production.
Unless we are very much mistaken the inventors responsible for U.S. Pat. No. 4,129,061 must have experienced a similar problem only more extensive as in the sintering phase their product contained compacted powder metal, a fragmentation jacket comprised of tightly packed heavy metal balls and an inner “dolly” that had a very large volume compared with the rest of the material.
The present invention now relates to an improved powder metallurgic method of producing fragmentation jackets or parts thereof containing large quantities of heavy metal balls distributed in the jacket in accordance with a predetermined pattern and intended for use in explosives-charged warheads. The present invention also includes a prefragmented casing produced in accordance with the said method.
A particular advantage gained from utilisation of the method as described in the present invention is that it enables production of fragmentation jackets having varied fragment dimensions contained within different sections of a more or less cylindrical prefragmented casing. This is to say that the said method would for example enable a rotating projectile moving along trajectory in the direction of the target and provided with a fragmentation jacket to detonate the built-in explosive warhead charge at a specific rotational position where the fragment bodies best suited for the target type in question are expelled towards the target. The advantage with this type of projectile containing differently dimensioned fragment bodies contained within different sections of its own exterior periphery is therefore that first when very near the target it can be decided which fragmentation bodies would have the best effect in the target. In this context the specific need to always be aware of the immediate roll position of the projectile does not present any difficulty for current sensor technology.
A further method for utilisation of this type of fragmentation casing having various sections containing differently dimensioned or differing in some other way fragment bodies is application in fin-stabilised, roll-stable, flying projectiles where the type of fragmentation casing to be used can be selected for use against an expected target.
A further characteristic feature of the present invention is that it presents a production method that makes possible the manufacture of fragmentation jackets in which the heavy metal balls are located completely free from contact with each other embedded in a powder metal technology produced main section or moulded part which in turn constitutes the exterior form of the fragmentation jacket and enables it to be further machined. As the heavy metal balls utilised as fragment bodies are located free from contact with each other in the powder metal technology produced moulded part, the said moulded part material can move during the sintering and cooling of the metal independently of the material in the heavy metal balls thus preventing the formation of cracks due to shrinkage in the homogenous metal after sintering of the powder metal.
When applying the production method characteristic for the present invention the desired location of the heavy metal balls completely free from contact with each other embedded in the yet to be sintered powder metal casing is defined first with the aid of a fixture after which the said heavy metal balls are surrounded with a suitable powder metal, preferably a powder steel that is then compacted and sintered to form a homogenous single-piece moulded part which if necessary can then be conventionally machined to the desired form and measurement accuracy. By utilising the method as defined in the present invention the occurrence of heat cracks in the material created during and in conjunction with sintering the powder metal and subsequent setting is avoided.
Production-wise, the method as claimed in the present invention can be divided in to three stages the first of which involves defining the desired location of the heavy metal balls relative to each other with the use of a fixture. The said fixture can be formed in several different ways but one variant is that its base is provided with the same number of guide cavities as there are fragment bodies or parts thereof as there will be when complete. The said guide cavities or guide means shall in this way define the locations of the fragment bodies relative to each other even though they shall only contact a small part of each fragment body when they are placed in their intended location in the supporting main section or moulded part. In this way a large part of the exterior surface of each fragment body remains free from contact and preferably more than half of its volume is left free to be surrounded by the metal powder used to produce the said moulded part during its subsequent production stage. In other words the said metal powder shall be added in sufficient quantity as to completely fill the space between the fixture and the heavy metal balls as well as between and over the said heavy metal balls to a predetermined level over the base of the fixture after which this initial layer of powder metal is compacted to form a single homogenous unit after which the fixture is then removed. While in the fixture the powder metal can be compacted mechanically, isostatically or semi-isostatically and in this way a further development of the present invention can be utilised to advantage by using a relatively thick rubber mat as a pressure equaliser in order to ensure a uniform distribution of compression on the entire volume of the powder metal.
Any tendency of the fragment bodies to leave their intended locations in the fixture prematurely can be prevented by application of an adhesive that does not have a greater adhesion strength than would prevent subsequent removal of the fixture. Similarly, adhesion of the powder metal to the fragment bodies can be improved with a for this purpose suitable substance. The entire process is based it being possible to remove the fixture without disturbing the fragment bodies from their intended locations in the compressed metal powder mass.
After this first powder metal compression stage there is now access to a single-piece moulded part in which the locations of the heavy metal balls are clearly fixed and where the said heavy metal balls are completely free from contact with each other inside the powder metal moulded part and where only that part of each heavy metal ball that has been in direct contact with the fixture protrudes from the first compressed powder metal moulded part. The next stage in the process as claimed in the present invention is to remove the said fixture and then to cover those parts of the heavy metal balls that previously were in direct contact with the fixture with a predetermined depth layer of powder metal mass after which it is also as previously described isostatically, semi-isostatically or mechanically compressed in a similar way to form a single-piece moulded part.
During this second powder metal compression it is advantageous to utilise a fixed surface to provide resistance to the previously compressed powder metal body. The second addition of powder metal/powder metal compression stage cal also be performed on individual or several previously moulded units produce during the first powder metal compression stage If during the second stage several units produced in accordance with the first stage as defined in this present invention are to be progressed then it is this second compressed powder metal mass that will unite the various units up to and including the sintering stage. The result from the said second powder metal compression will be a semi-fabricated powder metal containing embedded fragment bodies in the form of heavy metal bodies at predetermined locations The said semi-fabricated powder metal that can withstand a certain amount of handling is then subjected to sintering which converts the extremely compressed powder metal to a homogenous moulded metal part within which the fragment bodies in the form of heavy metal balls lie embedded at their predetermined distance from each other. After completion of the sintering process the prefragmented casing as defined in the present invention is machined to the desired inner and outer forms by applying normal conventional metal machining methods. The of powder metal used in the method as defined in the present invention is not dealt with in detail as the choice of the said powder metal is based on conventional powder metallurgical knowledge. With regard to the method of compacting the powder metal used in both the powder metal compression stages as defined in the present invention it is the case that as previously stated it can be performed isostatically, semi-isostatically and/or more or less mechanically and as regards the final sintering it can be performed as a separate final process stage or be included in a hot isostatic combined compression and sintering stage.
Within the framework of the above basic principles the method described in the present invention can be varied in its practical implementation in that the simplest variant is the one that involves producing fragmentation casing or parts thereof while utilising a more or less flat or slightly domed geometry and can be produced by the vertical compression of primarily horizontal layers of powder metal.
A characteristic feature of the present invention is that its basic principle enables several variants of prefragmented casing to be produced. For example it is possible to directly fabricate prefragmented tubular casing but this requires either a considerably more complex fixture or some other aid that can hold the heavy metal balls in position until the powder metal has attained a sufficiently high degree of compaction.
As a simple guideline it can be said that it is difficult hold the heavy metal balls in position using only gravitation and simple guide cavities during the first addition of powder metal/powder metal compression stage if the surface of the moulded part is inclined more than 30° relative to the horizontal surface.
As a further aid for holding the heavy metal balls in position on a surface with an excessive inclination for example is an adhesive that has sufficient adhesion properties to hold the heavy metal balls in position during the first stage while allowing the fixture to be removed prior to stage two. A further variant would be to fix the heavy metal balls in their original locations in the fixture using a hot adhesive that loses its adhesion after limited heating.
In accordance with a further variant of the present invention tubular or spherical prefragmented casing or in some other complex form can however be produced via several bulged, convex or concave or some other integral form horizontally placed parts to be united in conjunction with the second addition of powder metal/powder metal compression stage, whereas until now fabrication of the parts was performed individually. The limiting factor for the simplest variant of the present invention is when the heavy metal balls will no longer remain in their intended locations on the more or less horizontally arranged fixture. As already pointed out more than half of each heavy metal ball must protrude from the fixture so that the powder metal affixes their location in all directions thus defining their completely free from contact relative to each other location in the compacted powder material. For this first variant the fixture in general can take the form of a relatively simple hole-patterned disc or a disc with a number of guide cavities for the heavy metal balls where a ball is placed in each guide cavity.
In a further variant of the present invention the fixture is formed in such a way that the heavy metal balls can be held in their guide holes or cavities by suction. The suction force that holds the heavy metal balls in place in accordance with this variant of the present invention must be sufficient to overcome the force of gravity. When producing e.g., heavy metal ball tubular fragmentation bodies we start with a fixture cylinder having the desired form and provided with a large number of holes drilled through the wall of the said cylinder where each hole is in contact with a suction source which draws a heavy metal ball to itself and to each of the openings of the said holes which means that the said cylinder can be arranged vertically and on the side to which the heavy metal balls have been suctioned against be surrounded by or internally supplemented with an opposing wall preferably made from a relatively stiff but flexible material such as stiff rubber matting after which the space between this said new wall and the fixture is filled with powder metal around and over the heavy metal balls after which the said powder metal is compressed so that the said balls are embedded in the layer of powder metal. In this context compression of the powder metal is preferably performed semi-isostatically after which the said flexible material wall via which the said compression of the powder metal was performed in this first stage is removed and replaced with a fixed retainer, while the fixture is replaced with the same type of stiff but flexible material as was used in the first production stage and at a distance from the powder surface that was compressed also during the said first stage, that provides sufficient space for the remaining required quantity of powder metal that is added and then compressed so that even the remaining parts of the fragment bodies are covered completely after which the powder metal body so created is ready for sintering and possible subsequent final machining by conventional means.
The method described above can also be modified so that the first stage is performed primarily horizontally after which similar horizontally produced stage one modules or powder fragment body parts are combined to form a single-piece unit surrounded by powder that is compressed to form a second layer of powder the holds the resulting body together until the sintering process is completed.
The present invention is defined in the subsequent Patent Claims, and shall now be described in further detail with reference to the appended figures.
In these figures:
In order to produce the sector of prefragmented shell casing 1 shown in
After the so-produced powder body 9 is removed from the fixture 1 and possibly been machined the said powder body is turned over and given a second layer of powder 12 that will enclose those parts of the balls 5 that were located down in the guide cavities 4 in the fixture 2 during the initial production phase. The application and forming of this second layer of powder 12 is performed in a second fixture 13 to which the originally produced powder body has now been transferred. The said second fixture has in this case an oppositely bulged base 14 that is to say concave as the fixture 2 base 3 was convex during stage one. The second layer of powder is compacted in the fixture 13 using the compaction “dolly” 15 with the rubber matting 16 for pressure compensation. The final stage in the production of the intended powder metal casing as shown in
The cross-section shown in
The equipment required to produce this variant of the present invention will be rather more complicated and consequently in order not to make the Figure confusing the details have only been drawn within one of three identical sector elements in each Figure.
In practice it is also opportune to prepare the production of one sector at a time each of which is represented by a fixture 18 of the type indicated in
As soon as the powder layer is self-supporting the isostatic pressure P2, the tubular unit 23 and the “dolly” or holding device are removed, after which the now created powder body can be sintered to the desired material strength level. In this way a tubular casing is created and in which heavy metal balls 4 are sealed freely suspended relative to each other and the said tubular casing can then be machined by conventional means to the desired form and dimensions.
In the case of the variant shown in
As also shown in the said FIG. 1arge quantities of powder metal are required outside the concave sections of casing 37 and 38 but parts of the said sections can be replaced with inserts in which case the said inserts should be provided with a pressure-equalising, plastic deformation intermediate wall located facing in towards the sections of the powder casing.
In accordance with the general method as now described in the context of
After completion of the sintering operation the completed fragmentation casing can be shaped to the intended form and dimensions by means of pressing or some other conventional metal forming process. The exterior of the fragmentation casing can for example be pressed to its specified final dimensions in a calibration device.
A further variant of the present invention is based on producing several fragmentation casing sections as described above but are joined together while still in the powder stage and in a third compaction stage are pressed to become a single unit after which the powder metal is sintered to become a homogenous metal.
This said variant facilitates production of fragmentation casings containing several layers of fragmentation bodies.
With the fragmentation charge 45 mounted in a roll-stabilised projectile or in a rolling projectile where there is constant monitoring of the roll movement consequently the desired type of fragments with which to combat the target can be selected using fragmentation charges formed in this way.
