Lightweight cartridge case of improved aluminum alloy material which eliminates catastrophic failures

Abstract

A high strength aluminum alloy cartridge case wherein said alloy includes a ispersoid of finely divided aluminum minum oxide (Al.sub.2 0.sub.3) which coats the aluminum alloy particles to markedly improve the burnthru resistance of the case, said Al.sub.2 0.sub.3 comprising about 6 to 13% by weight of the cartridge case, and preferably about 10 to 12% thereof.

Government Interests

The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalty thereon.

Description

This invention relates to ammunition and more particularly concerns an improved aluminum alloy cartridge case having excellent burnthru resistance.

Presently in the United States and in many foreign countries, brass is widely used for cartridge cases for military small arms ammunition. This metal, however, contains about 70% copper, a material which is now strategic, and especially so in times of war. The United States obtains over 1/2 of its copper needs from foreign sources, principally Chile and the Congo, a logistically dangerous situation.

Steel cartridge cases, whose cost approximates that of brass cartridge cases have been used as a substitute therefor in certain applications. The use of steel substantially overcomes the problem of the use of strategic copper, but does not lessen the weight of the cartridge case which aluminum does so well with the attendant advantages of increased firepower for a given weight of ammunition carried by an Infantryman. Further, the complex heat treatments required by steel do not lend themselves to high production processes, so vital in times of war. Also steel cartridge cases cause excessive wear on gun components.

When a brass or steel cartridge case fails during firing, no gun malfunction occurs. There may be a small flow of propellant gases at low velocity thru the failure, i.e., a crack or split in the case sidewall, but no catastrophic failure will occur. A castastrophic failure may be defined as a high velocity flow of high temperature propellant gases passing thru the crack or split and carrying particles and melted globules of aluminum therewith. The melted aluminum particles or globules react exothermally with the propellant gases producing a large plume of flame in the exit gases which result in permanent or serious malfunction of the weapon and/or injury to the operator.

It is therefore an object of this invention to provide a high strength aluminum alloy for small arms cartridge cases having excellent burnthru resistance.

Another object of the invention is to provide such a small arms cartridge case having about 1/3 the weight of a corresponding conventional brass case.

Still another object of the invention is to provide such a lightweight case which utilizes none or only a minor proportion of strategic copper.

A still further object of the invention is to provide such a cartridge case which will not undergo any catastrophic failure.

These and other objects of the invention will become apparent from the following description.

Briefly, I have discovered that conventional high strength aluminum alloys can be rendered resistant to burnthru by providing an Al.sub.2 O.sub.3 coating around each aluminum alloy particle and/or an Al.sub.2 O.sub.3 dispersoid throughout the aluminum alloy matrix.

More specifically, I have discovered that about 6 to 13% by weight of Al.sub.2 O.sub.3 when incorporated into high strength aluminum alloys and thus resulting in a material which by subsequent compacting and/or sintering, hot and cold working will yield a lightweight high strength product admirably suited for small arms cartridge cases.

My Al.sub.2 O.sub.3 dispersoid does not react with the aluminum alloy matrix and possesses a much higher melting point (2015.degree.C) than aluminum (660.2.degree.C). The dispersoid increases the high temperature strength of the cartridge case. For example, the Al.sub.2 O.sub.3 dispersion-strengthened pure aluminum has a tensile strength of approximately 14,000 psi at 500.degree.C and maintains this strength at this temperature for a period exceeding 700 hours, as compared to 3000 psi for the standard high strength aluminum alloys, measured after the same number of hours at the same temperature. My dispersoid may comprise from about 6 to 13% by weight of the final alloy composition, 10 to 12% being preferred. Proportions of my dispersoid up to about 15% however may be effective in preventing burnthru. Below about 6% of the oxide, effective burnthru resistance could not readily be provided to the case material, whereas above about 15% of the oxide, a product insufficiently ductile resulted.

In the actual production of my powder for use in my lightweight aluminum cartridge case, any of the high strength aluminum alloy powders, such for example, as the 2000, 6000, or 7000 series, when processed to obtain proper coating of the alloy particles with Al.sub.2 O.sub.3 and dispersion of Al.sub.2 O.sub.3 throughout the matrix, have been found suitable and satisfactory for the purpose of this invention.

A method for obtaining this coating of Al.sub.2 O.sub.3 and its dispersion may readily be obtained by ball milling the aluminum alloy powders until the desired Al.sub.2 O.sub.3 content is achieved. The ball milling continuously exposes new aluminum alloy surfaces which immediately oxidize in the ball mill with a coating of Al.sub.2 O.sub.3. In order to reduce ball milling time, the aluminum alloy powder may be pre-oxidized by conventional methods well known in the art, such as anodic chemical processes wherein the powder becomes the anode in an electrolytic bath of chromic, sulphuric or oxalic acid. The preferred amount of aluminum oxide, by weight, is 10 to 12 percent of the weight of the cartridge case.

In the actual preparation of my powdered metal aluminum alloy sheet used in the fabrication of my lightweight cartridge cases, the process steps are:

1. The aluminum alloy powder of about 1 to 20 microns in diameter contains an oxide or hydrate film of about 0.01 micron thickness around each particle. This powder is ball milled using strainless steel balls (or corundum balls if freedom from iron is required). Ball milling is continued until the desired Al.sub.2 O.sub.3 content is obtained. The extent of powder grinding is based on the desired oxide content, which is proportional to the generated surface area. The preferred oxide content for my applications is about 10 to 12 weight %. A lubricant such as isopropyl alcohol should be used in the grinding operation. The resulting powder should then be vacuum dried at about 300.degree.F to complete alcohol removal.

2. The dried powder is then cold pressed isostatically and sintered at approximately 540.degree.C in any suitable neutral atmosphere, such as argon, to produce a billet.

3. The billet is then hot-pressed at about 700.degree.F at about 90,000 psi into a rod.

4. The rod may next be hot extruded at a reduction ratio in a range of about 10 to 1, to 30 to 1, into a rod of smaller diameter or configuration to partially break down the oxide coating and provide aluminum alloy to aluminum alloy bonding of the particles to increase the mechanical strength of the alloy and to also provide very uniform dispersion of Al.sub.2 O.sub.3.

5. The extruded rod is now hot and cold rolled to obtain the desired thickness of sheet for subsequent production processes of conventional stamping, heat treating, and drawing operations.

In the following Examples, the above described process steps were followed, the extrusion reduction ratio being about 25 to 1, the values being obtained at room temperature:

EXAMPLE I

7075 aluminum alloy atomized powder having an average particle size of about 1 to 10 microns diameter, was ball milled with stainless steel balls until 11% by weight of Al.sub.2 O.sub.3 resulted:

Tensile Strength : 70,000 psi (min.) Yield Strength : 60,000 psi (min.) Elongation in 2" : At least 10%

EXAMPLE II

6061 aluminum alloy atomized powder having an average particle size of about 1 to 10 microns diameter, was ball milled with stainless steel balls, until about 10% by weight of Al.sub.2 O.sub.3 was obtained:

Tensile Strength : 43,000 psi Yield Strength : 38,000 psi Elongation in 2" : At least 10%

EXAMPLE III

Same Conditions as in EXAMPLE II, except 2024 aluminum alloy atomized powder used in lieu of 6061:

Tensile Strength : 60,000 psi Yield Strength : 45,000 psi Elongation in 2" : At least 15%

Upon metallographic examination of each Al.sub.2 O.sub.3 dispersion-strengthened aluminum alloy, a continuous web-like netting of Al.sub.2 O.sub.3 was observed to permeate the entire alloy which is believed to impart the desirable high temperature properties to the standard high strength aluminum alloys making it readily usable for applications exceeding 200.degree.C and providing the excellent burnthru resistance needed for cartridge case applications.

Test samples of my improved aluminum alloys which comprise about 10 to 12% by weight Al.sub.2 O.sub.3 and containing a predrilled hole of 0.0135" diameter to simulate a failed cartridge case were experimentally fired in a firing chamber in which actual small arms cartridge propellants generated temperatures and pressures approximating those actually obtained in small arms firing chambers. Absolutely no evidence of burnthru occured up to a pressure of 46,300 psi. Spectrographic readings of the plume showed no AlO present which would be an indication of the rapid oxidation or burning of aluminum in the propellant gas stream. When similar experiments were repeated with standard 6061T6 aluminum alloy, severe erosion of the hole in the test sample occured with a resulting plume of flame which would cause a catastrophic failure if this alloy were used in small arms cartridge cases.

Summarizing, it is apparent from the foregoing description that I have provided an aluminum alloy cartridge case which is strengthened by a dispersoid of Al.sub.2 O.sub.3, which not only increases the high temperature properties of the resulting alloy but provides excellent burnthru resistance to the case to eliminate catastrophic failures thereof.

Claims

I claim:

1. A lightweight cartridge case of a high strength aluminum alloy, said alloy including Al.sub.2 O.sub.3 dispersoid in an aluminum alloy matrix uniformly distributed throughout said alloy matrix resulting in a substantially continuous network of Al.sub.2 O.sub.3 throughout said alloy.

2. The cartridge case as described in claim 1 wherein said Al.sub.2 O.sub.3 is present therein in proportions ranging between about 6 to 13% by weight.

3. The cartridge case as described in claim 2 further characterized by said Al.sub.2 O.sub.3 being present in proportions ranging between about 10 to 12% by weight.