Patent Application
20170284758
It has long been understood that weapon barrels have to withstand the pressure of the discharging ammunition and provide enough stiffness for sufficient accuracy. This need can be met simply by a high wall thickness of the barrel. With increased wall thickness, the maximum pressure load the barrel can bear is improved as well as its stiffness by the larger diameter and subsequent increased second moment of area. These advantages are offset by the barrel weight, which should be as low as possible to ensure swift weapon operation, especially in manually supported weapons like rifles and shotguns. Furthermore the barrel should allow repeated accuracy at consecutive shots, e.g. as found in automatic weapons. This is hindered by the heat up of the barrel which leads to thermal expansion and stress in the barrel and results in a loss of accuracy. Accordingly an ideal weapon barrel is stiff and light at the same time and heats up slowly and/or has great cooling efficiency by an advantageous surface-to-volume or more accurately an improved surface-to-heat capacity-ratio.
Metal-Metal-Matrix Composite Barrels are barrels comprised of an iron or nickel alloy core with at least one sleeve made from a metal-matrix material, applicable in rifles, shotguns, cannons, and mortars. Due to the properties of the metal-matrix materials (e.g. higher specific strength, higher specific heat capacity and lower density), the composite barrels can exhibit a higher rate of fire, reduced weight, improved accuracy, or a combination of any of these properties. Furthermore, the sleeve contributes to the pressure resistance of the barrel if the metal-matrix sleeve is properly fitted to the barrel core.
This application claims priority on Federal Republic of Germany patent application number DE 102014013663.9, submitted Sep. 16, 2014 at Deutsches Patent-und Markenamt DPMA (German Patent and Trademark Office). References/Patent Citations:
Two sheets of drawings: Vertical, tight cross hatchings symbolize homogeneous steel and nickel alloys, as found for example, in the barrel core of the composite barrels and the walls of conventional barrels.
45° cross hatching symbolize metal-matrix materials, which are found in the barrel jackets of the composite barrels.
a. Schematic of a conventional AR-15 style barrel
b. Schematic of a metal-metal-matrix composite barrel, in which the metal-matrix jacket only extends over the barrel and not the breach.
c. Schematic of a metal-metal-matrix composite barrel, in which the metal-matrix jacket extends over the whole barrel and the breach. This is the preferred technique since it allows the most weight savings.
1.1 Barrel+Breach Length, 1.2 Breach, 1.3 Barrel Length, 1.5 metal-matrix Jacket and 1.4 Steel/Nickel Alloy Barrel Core.
a. Schematic profile of a conventional barrel from steel or nickel alloys, without coatings. Only one material is used.
b. Schematic profile of a metal-metal-matrix composite barrel in an application of lowest weight. The diameter is smaller than that of the conventional barrel; accordingly, the area moment of inertia and the stiffness would be lower. In this case, the thickness of the metal-matrix material has to ensure sufficient pressure resistance.
c. Schematic profile of a metal-metal-matrix composite barrel with high diameter. Such an application does not necessarily provide weight savings. But due to the high diameter and increased heat capacity, it will provide slower heat up and increased stiffness/accuracy.
a. Schematic profile of a metal-metal-matrix composite barrel with six flutings for increased surface area and improve cooling efficiency.
b. Schematic profile of a metal-metal-matrix composite barrel with triangular cutouts with a side ratio of 1:3 to 1:4 for increased surface area and improve cooling efficiency.
c. Schematic profile of a metal-metal-matrix composite barrel with minimal metal-metal-matrix jacket, delivering sufficient pressure resistance and four cooling fins. The cooling fins provide increased surface area for more cooling efficiency and increased stiffness.
Currently, state of the art barrels are made from homogeneous metal alloys, especially steels (iron alloys) and nickel alloys (see
Accordingly, it is advantageous to increase the diameter of the barrel while minimally increasing the barrel's weight. It is most preferable that the barrel weight be decreased while its diameter is increased. In special cases, these characteristics are achieved by composite barrels which have layers of fiber composite wrapped around them. Examples of these barrels are rifle barrels with extremely thin wall thicknesses and fiber composite wraps as described in patent CA 2284893C and WO 2011146144 A2. Usually carbon fiber composites are used for this purpose with a resin matrix based on epoxy resins. Carbon fiber composites normally have a specific tensile strength higher than that of steels and nickel alloys. In short, a carbon fiber wrapped barrel can be lighter than a standard barrel made solely from a metal alloy. However the disadvantage of these systems is their low thermal stability, restricting their use in semi or fully automatic weapons to a few shots. In general, they can only be deployed in systems in which the temperature of the barrel doesn't rise above 100° C. for extended periods of times, especially not above 200° C. Above these temperatures the organic resin matrix will degrade permanently. The fiber composite wraps also act as an insulator due to their low thermal conductivity. The heat of the barrel is therefore slightly dissipated, greatly affecting the precision of the weapon by thermal barrel creep.
Another barrel type made from different materials are found in smooth bore cannons, as deployed in Abrams and Leopard II battle tanks. These barrels actually have an insulating outer layer for thermal management, which don't contributing to the barrels' mechanical integrity. This characteristic distinguishes these barrels from the previously described wrapped fiber composite barrels in which the fiber wrap contributes to the barrels' mechanical properties.
An alternative form of a multilayered composite barrel with improved stiffness is given in patent US 2011/0113667A1. The barrel's stiffness is provided with an outer metal sleeve. The void between the barrel and the sleeve is sealed with a light, hardening filler material. Unfortunately, the system described in patent US2011/0113667A1 has some significant flaws. The filler is a poor thermal conductor resulting in hot spots and barrels with thermal creep. For this reason, using this barrel system in semi and fully automatic weapons is impractical since the sustainable rate of fire would be reduced to prevent overheating and thermal creep. Also, the sleeve can separate from the filler and barrel core due to the higher thermal expansion coefficient of the sleeve material in comparison to both filler and barrel core, resulting in a loss of the system's mechanical integrity and accuracy.
The invention presented in this patent ‘Metal-Metal-Matrix Composite Barrels’ tackles these problems by using innovative metal-matrix materials which combine a linear thermal expansion coefficient similar to those found in iron and nickel alloys with a specific tensile strength greater than that commonly found in barrel steel and nickel alloys such as 316, 4140 and 4150 steels.
The metal-matrix materials used are based on light metals; accordingly, their density is significantly lower than the density of iron and nickel alloys. The consequence of this combination of material properties is that the greater specific tensile strength allows weight reduction while the lower density results in a higher diameter of the barrel. The increased diameter again leads to an increased stiffness of the barrel and improved accuracy (see
Consequently, a metal-metal-matrix composite barrel can have improved weight, accuracy/stiffness, and/or increased rate of sustainable fire. Although it is desirable to have all these characteristics improved, it can be advantageous for special applications to improve only one or two of the above mentioned properties significantly while sacrificing other properties. For example, it is possible to forgo weight savings when highest precision is desired by bringing the wall thickness of the barrel to the absolute maximum, gaining stiffness and heat capacity. Additionally, the metal-matrix materials presented in claims 2 to 6 allow continuous use of a barrel above 100° C., especially above 200° C. These advantages are due to the composite barrel being comprising materials with a metal-matrix and not organic fiber composites whose resin matrix would invariably degrade at these temperatures. Accordingly, they are advantageous over wrapped barrels and are truly suited for use as barrels in semi and fully automatic weapons or any barrel carrying high thermal load by for example either high rates of fire or a specific type ammunition.
Basically, every reinforced light metal, especially but not restricted to aluminum and magnesium alloys, is suitable for use as metal-matrix material in the presented metal-metal-matrix composite barrels. It is important in this application that they exhibit the properties given in patent claims 2 through 6. Especially suitable are metal-matrix materials, containing thermally highly conductive fillers like carbon nanotubes, boron nitride, diamond, or silicon carbide in an aluminum matrix. The degree of filler should be such that the thermal expansion coefficient is between 10 to 15 ppm/K and thermal conductivity higher than that of the barrel core, best 80 to 200 W/m·K. An example of such metal-matrix materials is the aluminum diamond composite described in the US patent ‘Aluminum Composite for Gun Barrels’ U.S. Pat. No. 6,482,248B1 by S. R. Holloway or the silicon carbide reinforced aluminum alloys produced by the Materion Cooperation (Mayfield Heights, Ohio, USA) under the brand name SupremEX, especially SupremEX AMC640XA. The properties of this material are given in Table 1 at the end of the patent description together with calculations for an AR-15 type barrel.
Realized are these improvements in a composite barrel featuring a barrel core made from steel or nickel alloys, like 316, 4140 or 4150 steel. The barrel core's lowest wall thickness is the minimum necessary to rifle the barrel and still guarantee sufficient wear resistance for commercial use but will normally higher and depends on the caliber and desired safety factor, e.g. for the tensile strength. The maximum diameter of the barrel core normally should not be larger than the diameter found in heavy barrel profiles. The barrel core is again sheathed with the barrel jacket made from metal-matrix material with the aforementioned properties (see claims 2 to 6). The metal-matrix material in this application has a thermal expansion coefficient similar to that of the barrel core, allowing thermal shrink fitting bonding while preventing separation of the two parts under thermal load.
Additionally, the similar thermal expansion coefficients prevent bending under thermal load, reducing thermally induced mechanical strain in the barrel. In the best case scenario, the thermal expansion coefficients of core and jacket are identical. Other bonding method to achieve a force bearing joint between core and jacket are for example hammer forging, direct extrusion of the jacket around the barrel core or welding.
If the thermal expansion coefficients of core and jacket differ less than 20%, they can be bonded directly, e.g. by shrink fitting or direct extrusion. This is the simplest embodiment of this invention. If the difference in their thermal expansion coefficients is higher, a second (inner) jacket has to be inserted between the barrel core and the outer jacket. The thermal expansion coefficient of the inner jacket has to be greater than that of the outer metal-matrix jacket and his wall thickness small. In this way, the expansion of the inner jacket during heating will be lessened by the outer jacket, ensuring a permanent force fit. During cooling the barrel core prevents the shrinkage of the inner jacket and a force baring joint is realized throughout the whole range temperature range. In this embodiment both the barrel core's and out jacket's wall have to be thick enough to withstand the pressure of the systems thermal expansion and shrink.
In both applications (with or without inner jacket), it has to be ensured the joint and especially the interference fit provide sufficient force fit in the temperature range between −40° C. and 150° C., ideally between −70° C. to 350° C. Additionally, it has to be ensured that the barrel core's wall is thick enough to withstand the forces occurring within this temperature range. The jacket or jackets should extend over the full length of the barrel, as shown in
Although the focus of this invention is to produce original barrels, it also allows the retrofit of existing barrels by sleeving them with a metal-matrix jacket. For this process, a steel barrel, which forms the new barrel core, has to be milled down evenly to provide a smooth surface. The metal-matrix jacket then can be attached by e.g. shrink. Alternatively, the metal-matrix jacket can also be produced from two or more sections placed around the barrel core. The sections then have to be welded or screwed together to stiffen and support the barrel core. The drawback of employing this ‘sectional’ technique (fastened with screws) is that the metal-matrix sleeve only provides minimal to no pressure resistance to the composite barrel. Only the thermal and stiffness improvements of the sleeve can take effect in this embodiment. Accordingly, the barrel core has to be much heavier compared to a solidly built metal-metal-matrix composite barrel to provide sufficient pressure resistance on its own. Also, when using a used barrel there is the danger of flaws and weaknesses in the barrel due to former abuse and use. In short, it is always best to build brand new composite barrels to ensure the best possible quality and to fine tune the material properties for optimal bonding.
If the materials and jacket diameters are properly chosen and assembled, it is possible to increase the stiffness and the heat capacity of the composite barrel by factors up to 12. Normally these properties will be improved by factors between 1.5 to 7 while the weight will be decreased or only slightly increased. The weight increase will be less than 25%. The exact improvements and values depend on the chosen wall thicknesses of both barrel core and jacket, the intended use of the metal-metal-matrix barrel, the caliber of the weapon and the properties of the barrel to which they are compared. For example, in AR-15 type barrels, the described barrel system can increase the accuracy by a factor of two while also increasing the sustainable rate of fire rate by a factor of two. A detailed example for an AR-15 is given at the end of the patent description.
It should be noted that the time of sustainable fire can be increased when the fire rate is not totally exploited and vice versa. For example, if the fire rate can be increased by a factor of 4 but the actual rate of fire is only increased by a factor of 2 then this fire rate can be sustained twice as long until the critical temperature is reached at which the barrel fails, since the heat energy necessary to make the barrel fail is proportional to the rate of fire and the time of fire and in general higher in the composite barrel. Accordingly, either rate of fire, time of fire, or both can be increased. Also both parameters depend on the mass and the surface-to-volume ratio of the barrel Therefore it is also possible to massively reduce the weight if rate of fire and time of fire stay the same.
The increased diameter of the composite barrel and the higher heat capacity of the metal-matrix materials result in a decrease of the surface-to-volume ratio and ratio of heat capacity to barrel surface area. These factors effectively decrease the capacity of a composite barrel to cool down at the same rate as a conventional steel barrel. This can be counteract by surface patterning the barrel, e.g. fluting and structuring. Ideally, these surface structures increase the barrel surface area by the same or greater amount by which the surface-to-volume ratio and surface-to-heat-capacity ratio have been decreased. An example of surface structure by six flutes is given in
In a further variation of the inventions embodiment the barrels can also be conically and decrease in diameter towards the muzzle to allow further weight reductions. In this instance, either both barrel core and jacket or one of the two pieces can taper. Furthermore, the profiles of both jacket and core can change along the barrel long axis, e.g. by different diameters. This change can help reduce higher harmonics in the barrel and allow barrel whip and vibrations to calm faster, meaning better accuracy and faster consecutive shots with the same precision. Again, these profile changes can occur independently or dependently within a barrel core and barrel jacket as long as the profiling doesn't interfere with the quality of the two parts connected. The realization of these variations will depend again on the intended use and the difficulty of manufacturing but are technically viable.
The so-produced barrels are especially useful for applications in semi and fully automatic weapons, such as rifles and cannons. In bolt actions rifle and shot guns, they have the advantage of providing higher accuracy. Moreover, in thermally, highly strained single fire weapons, such as mortars, these barrels allow higher sustained rates of fire and longer barrel life as well. Since the potential uses are extensive, they cannot be detailed in this document for every case—only their general design and properties. To clarify these possibilities, an example is given on the basis of an AR-15 platform in the next section.
The examples given here are for metal-metal-matrix composite barrels based on the AR-15 rifle system, caliber 5.56 Nato. An example of the metal-matrix materials are the SupremeEX AMC 640XA properties given in Table 1 together with the 4140 steel properties, the most widely used steel for AR-15 type barrels. The given thicknesses of the barrel core in Table 2 are sufficient to provide pressure resistance alone while the pressure resistance added by the metal-matrix jacket provides a safety factor. Depending on the thickness of the metal-matrix jacket, different properties of the barrel can be achieved. It is possible to save 35% of the weight if the stiffness of the barrel is only preserved, not improved. Alternatively, it is also possible to improve the accuracy of the composite barrel by a factor of 2.3 in comparison to heavy profile barrel profiles while having a weight savings of 18% (see Table 2).
| Number | Date | Country | Kind |
|---|---|---|---|
| 102014006081 | Apr 2014 | DE | national |
| 102014013663.9 | Sep 2014 | DE | national |
| Number | Date | Country | |
|---|---|---|---|
| 20170016691 A1 | Jan 2017 | US |
