US3539308A - Composite aluminum armor plate - Google Patents

Description

United States Patent U.S. Cl. 29-197.5 2 Claims ABSTRACT OF THE DISCLOSURE A composite armor plate structure consisting of a wrought 7039 aluminum alloy member having welded thereto a cast aluminum alloy member produced from an alloy consisting essentially of 3.8-4.3 weight percent zinc, 3.3-3.8 Weight percent magnesium, 0.03-0.15 weight percent titanium, and the balance aluminum.

This invention relates to a new aluminum alloy, and more particularly to an aluminum cast structural alloy of superior quality.

A recent development in wrought aluminum vehicular armor has been the introduction of a weldable heat treatable alloy, commonly identified as Wrought 7039, Aluminum Alloy Armor Plate, Heat Treatable, Weldable, described in Military Specification MIL-A-46063B(MR), Feb. 15, 1965. The general composition of 7039 aluminum alloys as set forth in the Alloy Digest published in April 1966 by Engineering Alloy Digest Inc., Upper Montclair, N.J., is 2.3-3.3% magnesium, 3.54.5% zinc, 0.10-0.40% manganese, 0.l5-0.25% chromium, 0.30% silicon (max), 0.10% copper (max), 0.40% iron (max), 0.10% titanium (max.), remainder aluminum. Wrought 7039 alloy possesses a high yield strength and produces an improvement of about 15% in resistance to armor piercing projectiles over previous aluminum alloys. Although this new wrought alloy has been extremely eifective when employed as the body structure for armored vehicles, such as tanks, trucks, and the like, further problems were created due to a lack of compatible cast alloys. The available cast alloys do not have adequate combined yield strength-elongation properties to provide desirable armor piercing protection, nor are they compatible for welding to wrought 7039 alloy or resistant to galvanic corrosion when Welded thereto. Alloys which can be cast and are compatible with wrought 7039 are of vital importance since these cast alloys are often employed as housings and hatch covers on tanks fabricated of wrought 7039.

There is no current commercial cast aluminum alloy which provides the combination of 50,000 p.s.i. with 3% or better elongation for a 1-inch gauge length. An example of the deficiency in these desired properties can be seen in the Military Specification MIL-A-2ll80C, Aluminum Alloy Castings, High Strength, dated Jan. 15, 1964. In this report the highest minimum yield strength specified is 40,000 p.s.i. There are alloys, such as Ternalloy 8, a trademark product of Apex Smelting Co., and comprising an Al-base alloy, which has the necessary level of 45,000 to 55,000 p.s.i. However, the elongation at these yield strengths is 1% or less. Since such excessive brittleness is known to degrade the ballistic protection of the alloy, these alloys will not give the same protection provided by wrought 7039 armor.

It is, therefore, an object of our invention to produce an aluminum-base alloy which is capable of being cast.

Another object of our invention is to provide a cast alloy which is compatible with wrought 7039 alloy.

More specifically, it is the object of our invention to provide a cast alloy having at least 50,000 p.s.i. yield strength with 3% or better elongation for a 1 inch gauge length.

Further objects of the invention will become apparent as the invention is more fully disclosed.

The alloy of this invention consists essentially of by weight 3.8 to 4.3% zinc, from 3.3 to 3.8% magnesium, 0.03 to 0.15% titanium, and the balance aluminum. Such an alloy will attain yield strengths between 45,000 and 51,000 p.s.i. with 3% or better elongation for a 1 inch gauge length, and provide armor piercing ballistic protection better than 95% of the specified minimum required for wrought 7039 armor as specified in MIL-A- 46063B(MR) aforementioned. The closeness of the cast alloy composition to wrought 7039 enables both alloys to remain compatible for protection against galvanic corrosion.

A preferred composition within the ranges defined above consists essentially by weight of 4% zinc, 3.5% magnesium, 0.05% titanium and the balance aluminum. The minute traces of titanium are present for grain refining purposes. This preferred composition exhibited physical properties of about 53,500 p.s.i. yield strength while reaching an elongation of about 3.9% for a 1-inch gauge length.

In general, the alloy of this invention is prepared by melting the following materials: Aluminum as 99.99% virgin ingot, zinc as 99.99% virgin ingot, magnesium as an aluminum-10% magnesium alloy, titanium as an aluminum5% titanium alloy. The specific quantities of each metal are dependent upon the total weight of the alloy desired. Melting is performed in an induction furnace at approximately 1300-1350 F. using a claygraphite crucible with the aluminum being melted first. The zinc and magnesium are then added separately to the aluminum melt in either order by submerging the zinc or magnesium metals below the surface of the aluminum melt to minimize their melting time. As soon as the metals are homogeneously melted, degassing is carried out at about 1300 F. by stirring in about 7 grams of commercial hexachlorethane powder for every pound of melt and holding the melt for 10 minutes at this temperature to allow the chlorine and hydrogen gases to evolve and the dross to rise to the surface. After determining that the apparent gas content was below 0.05 cc./ 100 grams of metal (reduced pressure test), the titanium addition was made. The melted metal was then adjusted to about 1350 F. While still in the induction furnace, the dross skimmed off, and the melted metal poured into molds. Preferably, either sand or investment molds are used in forming the final product. A room temperature aluminum chill was placed at the end of the molds to promote suificiently high undirectional thermal gradients such that gross unsoundness and hot tears were avoided. The alloy castings were then heat treated in air circulation furnaces by solutionizing at about 830 F. for about 16 hours, and aging at about 260 F. about 20-30 hours for the investment castings and about 8-12 hours for the sand castings. The aging time within the above-mentioned ranges will increase upon an increase in the aluminum content.

In order to determine the spalling characteristics of our new alloy, a single caliber 0.50 fragment simulator round was directed into the central region of three alloy plates 1% inches thick. The velocity used was about of the minimum Protection Ballistic Limit (extraploated to about l At-inch thickness) for weight 7039 aluminum alloy armor plate. The resulting complete penetrations were compatible with the results of extensive prior ballistic testing, which established the fragment simulator resistance of castings at 70% to of specification minimums for comparable wrought material. The plates did not spall even when attacked in regions of 4 /2 elongation. Wrought 7039 plates with approximately 57,000 p.s.i. yield strength and 13% elongation for a 1-inch gauge length do exhibit considerable spalling. There was star-cracking and petalling which is usual for castings, but no pieces left the plate or plug.

Cast aluminum alloys are usually not used for armored vehicle components having ballistic requirements because of their low yield strengths and inferior spalling resistance Which causes undesirable behind-the-plate damage. This spalling characteristic necessarily degrades the ballistic protection level.

Tests were also run to determine the weldability of our new alloy to wrought 7039 alloy. As stated previously, the cast alloy is used for small parts which are secured to the Wrought aluminum alloy substrate. It is essential for successful deployment of the new alloy that it be capable of being Welded to the Wrought alloy. Cast plates of the new alloy were butt welded (double V joint) to l A-inch thick plate of wrought 7039. The plates were Welded along the chill end. The alloys were successfully Welded and the weld joint tensile properties for the new alloy welded to Wrought were comparable to those for Wrought 7039 Welded to itself.

Thus the cast alloy of the present invention possesses the unique and valuable combination of high strength, ductility, and weldability. These properties make it useful in the applicatiton of armor piercing protection on tanks, and other such vehicles.

I claim:

1. A composite armor plate structure consisting of a wrought 7039 aluminum alloy member having Welded thereto a cast aluminum alloy member having improved resistance to spalling, said cast aluminum alloy member being formed of an alloy consisting essentially of about 3.8-4.3 weight percent zinc, about 3.3-3.8 Weight percent magnesium, about 0.03'0.15 Weight percent titanium, and the remainder aluminum, and being characterized by a yield strength of about 50,000 p.s.i. and a retained elongation of at least 3% for a 1-inch gauge length.

2. A composite armor plate structure as set forth in claim 1 wherein the cast aluminum alloy member consists essentially of about 4 weight percent zinc, 3.5 weight percent magnesium, 0.05 weight percent titanium and the remainder aluminum, said alloy member being characterized by a yield strength of about 53,500 p.s.i. with a 3.9% elongation for a 1-inch gauge length.

References Cited UNITED STATES PATENTS 3,133,839 5/1964 Thomas 75146 3,171,760 3/1965 Vernam et al. 75-146 3,290,187 12/1966 Paul 148-159 OTHER REFERENCES Alloy Digest (A1 154) April 1966, published by Engineering Alloys Digest Inc., Upper Montclair, NJ.

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 75-146