US3649227A

US3649227A - Aluminum composite

Info

Publication number: US3649227A
Application number: US5860A
Authority: US
Inventors: Maurice C Fetzer; John D Sprowl; William R Mohondro
Original assignee: Kaiser Aluminum and Chemical Corp
Current assignee: Kaiser Aluminum and Chemical Corp
Priority date: 1970-01-26
Filing date: 1970-01-26
Publication date: 1972-03-14
Anticipated expiration: 1989-03-14
Also published as: DE2105851A1; DE2105851C2; GB1291965A

Abstract

An aluminum composite having improved penetration and spall resistances comprising a high-strength aluminum frontal member having a tensile strength between 65,000 and 90,000 p.s.i. and an aluminum backing member having a tensile strength between about 45,000 and 70,000 p.s.i. The backing member must have a thickness between about 4 and 20 percent of the total composite thickness and have a tensile strength of at least 2,000 p.s.i., preferably 7,000 p.s.i., less than the frontal member.

Description

XR 3,649,227 SR Fetzer et al.

[54] ALUMINUM COMPOSITE [72] Inventors: Maurice C. Fetzer, Walnut Creek; John D.

Sprowl, Pleasanton; William R. Mohondro, Dublin, all of Calif.

[73] Assignee: Kaiser Aluminum & Chemical Corporation, Oakland, Calif.

[22] Filed: Jan. 26, 1970 [21] App1.No.: 5,860

[52] US. Cl ..29/l97.5, 29/1835, 148/34 [51] Int. Cl ..B32b 15/00 [58] Field ofSearch.... ..29/197.5

[56] References Cited UNITED STATES PATENTS 2,821,014 4/1935 Miller ..29/197.5

[451 Mar. 14, 1972 Brown ..29/197.5

Gauthier ..29/197.5

Primary Examiner-,Hyland Bizot Attorney-James E. Toomey, Paul E. Calrow, Harold L. Jenkins and Edward .I. Lynch [57] ABSTRACT 6 Claims, 1 Drawing Figure -BAC/(//V6 MEMBER 74/00/555 0/ form 7 //(K/1/55-" ALUMINUM COMPOSITE BACKGROUND OF THE INVENTION The demand for high spall and penetration resistance lightweight materials has increased considerably in the last few years, particularly for armor plate applications. To a certain extent, the 7039 alloy (Aluminum Association designation) has enjoyed much popularity due to the low spalling and high-penetration resistance to projectiles characteristic of this alloy. In addition, the alloy has high strength and corrosion resistance as well as being easily welded by conventional techniques.

With aluminum materials, the resistance to armor piercing (AP) projectiles is directly related to the tensile strength and hardness of the material. However, as the strength and hardness increase, the tendency to spall from projectile impact, such as fragment simulator (FS) projectiles, increases. With aluminum materials having tensile strengths above about 65,000 to 70,000 p.s.i., the spalling from F8 projectiles is so great that the materials are more or less useless as armor plate. H. P. George et al., in US. Pat. No. 3,042,555 attempted to overcome this problem by overaging the rear face of 7075 and 7178 alloy plates. However, the process described is very difficult to control, i.e., it is difficult to obtain consistent properties, and, although the product exhibits improved spalling resistance, the improved properties are obtained at the expense of a reduced AP resistance.

For aluminous materials, the 7039 alloy provides the best balance of penetration and spalling resistance. However, for many applications the 7039 alloy is not completely satisfactory due to the relatively low-ballistic resistance to armor piercing (AP) projectiles striking the plate at obliquities greater than 30, i.e., a projectile path at an angle greater than 30 from a line normal to the surface. At angles greater than 30 high-hardness steel armor has superior ballistic resistance on an equal weight basis.

SUMMARY OF THE INVENTION This invention relates to an all-aluminum composite materi- 21 having improved penetration resistance and resistance to spalling. The present invention provides a composite material having improved resistance to penetration by armor piercing (AP) projectiles while maintaining, and in many instances increasing, the resistance to impact by other projectiles such as a fragment simulator (FS) projectiles.

Generally, the composite of the present invention comprises a frontal member of a high strength aluminum alloy having a tensile strength between about 65,000 and 90,000 p.s.i. and a tough aluminum backing member having a tensile strength between about 45,000 and 70,000 p.s.i. The thickness of the backing member should be between 4 and percent of the total composite thickness. Within the context of this invention, the total thickness of the composite ranges from onefourth to about 6 inches thick. The tensile strength of the frontal plate should exceed the tensile strength of the backing member or cladding by at least 2,000 p.s.i., preferably greater than 7,000 p.s.i. The various members must exhibit the above strength values in the composite material.

A particularly advantageous aluminum alloy for the frontal plate consists essentially of 4 to 7 percent zinc, 1.5 to 5 percent magnesium, up to 3.0 percent copper, and the balance aluminum and inconsequential amounts of other elements, i.e., amounts which do not detrimentally affect the penetration and spalling resistance in a substantial manner.

DETAILED DESCRIPTION OF THE INVENTION percent, preferably between about 5 and 12 percent of the thickness of the composite. An aluminum alloy is herein considered to be an aluminum base alloy having more than 80 percent aluminum.

Many aluminum alloys, such as 7075T6, 7079-T6, 7178T6, 2024-T3 and the like, have been found to have the desired characteristics for the frontal member within the broadest scope of the present invention. The primary requirement for the frontal plate composition is that it must be capable of developing between about 65,000 and 90,000 p.s.i. tensile strength upon appropriate heat treatments after bonding with the backing member. The higher strength alloys, such as 7178T6, have a greater tendency to shatter when struck by higher caliber projectiles such as 0.50 cal and 20 mm. projectiles; and this characteristic severely limits the multihit capabilities of this material. However, such high-strength alloys employed in the composite of the present invention do exhibit superior single hit properties. The preferred alloy composition of the frontal member consists essentially of about 4 to 7 percent zinc, 1.5 to 5 percent magnesium, up to 3.0 percent copper, and the balance aluminum and inconsequential amounts of other elements which do not substantially affect the strength and ballistic properties of the alloy. Generally, a high magnesium to zinc ratio provides a member which is more easily welded by conventional GMA techniques.

The backing member or cladding comprises a relatively medium strength, tough aluminum alloy having a tensile strength between about 45,000 and 70,000 p.s.i. The backing member tensile strength must be at least 2,000, preferably 7,000 p.s.i., less than the tensile strength of the frontal plate. For substantially improved penetration resistance and spalling resistance, the backing member must be between about 4 and 20 percent, preferably between about 5 and 12 percent, of the total composite thickness. Any heat treatable aluminum alloy can be employed if the backing member exhibits the required strength in the final composite form. The preferred alloy for the backing member consists essentially of 4 to 6 percent zinc, 0.5 to 3.5 percent, preferably 0.5 to 1.5 percent, magnesium and the balance aluminum and inconsequential amounts of other elements. To facilitate the roll bonding of the composite, the weight ratio of the magnesium content of the backing member to themagnesium content of the frontal member should be less than about 0.7. Other suitable alloys include 7039 and 6053.

In the alloys-described above for the frontal and backing members the silicon should not exceed 0.3 percent, iron 0.4 percent, manganese 0.5 percent, titanium 0.10 percent, chromium and/0r zirconium 0.25 percent. Although the above limitations are primarily given as maximum amounts of impurities, positive additions of these elements are included within the scope of the invention. For example, chromium and zirconium may be added or controlled in the composition between 0.10 and 0.20 percent to improve stress corrosion resistance and inhibit recrystallization; copper may be added or controlled between 0.10 and 0.20 percent to improve stress corrosion resistance in the T6 temper; manganese may be added or controlled between 0.1 and 0.5 percent to improve strength and stress corrosion resistance; and titanium may be added or controlled between 0.01 and 0.03 percent for grain refining purposes. Generally, for most commercial applications the copper content will be less than 0.2 percent for welding purposes although it is recognized that alloys having between 2 and 3 percent copper can be welded by conventional techniques.

All compositions given herein are on a weight percent basis and all strength properties are in the long-transverse direction. The expression long-transverse direction" refers to the direction normal to the direction of metal working or flow and in the longer dimension of a cross section taken in a plane normal to the direction of metal flow.

It has been found that the composite material of the present invention exhibits substantially improved penetration resistance to AP projectiles with substantially no reduction in spalling resistance in comparison to 7039-T64. In most of the composites tested, a slight increase in spalling resistance was noted in comparison with 7039-T64 on an equal weight basis. The improvement in spalling resistance would be considered substantial if compared with materials having equivalent AP resistance such as high-hardness homogeneous steel plate and high-strength aluminum alloy plate such as 7075-T6 and 7178T6.

A critical feature of the instant discovery is the relationship of the thickness of the backing member with respect to the total thickness of the composite to obtain an improved spalling resistance, and attendant with the improved spalling resistance, a substantially improved resistance to penetration. It has been found that a backing member thickness between about 4 and 12 percent provides for improved resistance to AP projectiles. Greater or lesser amounts than this generally provides for no substantial improvement over the frontal plate alone. A maximum resistance to AP projectiles is reached with a backing member thickness of about 5 to 6 percent, which is particularly noticeable when the projectiles strike at high obliquities, e.g., 45. However, the resistance to F8 projectiles is more or less a function of the backing member thickness up to a thickness of over 20 percent of the total composite thickness. The greater the thickness of the backing member, the greater is the resistance to ES projectiles. A backing member thickness between about 4 percent and 20 percent, preferably between about 5 percent and 12 percent, provides the best balance of AP and FS penetration resistance. The relationship of backing member thickness to ballistic properties for the preferred alloys of the present invention is indicated inthe drawing. The nominal composition for the frontal member was 6.6 percent zinc and 2.5 percent magnesium and for the backing member was 5.5 percent zinc and 0.75 magnesium. The present invention uniquely allows the ballistic properties of the composite to be specifically designed to satisfy the requirements of a wide range of applications.

The several components of the present invention are joined to form the composite through an intimate metal-to-metal bond, which can be formed by several methods, such as roll bonding, explosive bonding and the like. It is believed that the composite structure acts as an energy absorber and disperser, in that, as the projectile penetrates the frontal plate, the plug displaced thereby forces the backing member to be stretched and torn away from the frontal plate.

Conventional roll-bonding techniques are preferred which include heating the members to a temperature between about 650 to 900 F., passing the members through rolls several times with slight thickness reduction to seal the backing member to the core, then subsequently further hot-rolling the members to form the desired composite. The thickness reduction of the composite in the hot-rolling step should normally exceed 50 percent, preferably about 80 percent.

With aluminum alloys containing more than about 1.5 percent magnesium, a sound bond is difficult to obtain by conventional roll-bonding procedures; and it has been found that a more complete bond can be made by precladding these alloys with small amounts (e.g., one-half to 2 percent of the total thickness) of relatively magnesium-free aluminum alloy, i.e., not more than 0.1 percent magnesium, then roll bonding the preclad members with the precladded faces in contact. Suitable precladding alloys include 3003, 1 100, 7072 and 1230.

Due to the fact that the elements are bonded to form the composite, the alloy selection is restricted because both plates must usually respond to the same thermal treatment to reach the required strength properties. The preferred alloy compositions of the present invention, set forth above, are particularly compatible in this regard. Many artificial aging treatments can be employed with the preferred alloy composition of the present invention. For suitable ballistic properties, the solution heat treated composite may be heated to between 220 and 250 F. and maintained at that temperature between about 24 and 48 hours, depending on thickness of the composite and aging temperature. Other aging practices can of 5 course be employed. It has been found that overaging, by either single-step or multiple-step aging practices, increases the spalling resistance of the exposed face of the frontal plate, particularly at projectile paths greater than 30 from a line normal to the surface.

Several examples of the present invention are given in Table l. A comparison of ballistic properties of the examples with 7039-T64 is given in Table 11. The specimens of the examples were prepared by first precladding the frontal member and backing member on one face with 1,230 alloy in thickness between 1 and 2 percent, heating the members to a roll-bonding temperature between 650 and 850 F. and then roll bonding the members (with the 1230 alloy surfaces in contact) to effect thickness reductions between 60 and 80 percent. The specimens of Examples 1 through 9 were solution heat treated by heating at 850 F. and quenching in cold water, and subsequently aged to the T-6 temper. The artificial aging practice consisted of naturally aging for 5 days, heating at a rate of 35 F./hr. to the aging temperature of 240 F. and artificially aging at that temperature for 48 hours. The specimen for Example 10 was given conventional thermal treatment to obtain the T-6 temper. The tensile properties (long transverse direction) of the members of the samples are shown in Table l.

The prepared samples were then subjected to ballistic tests with Cal. 0.30 AP projectiles at 0 obliquity, Cal. 0.50 AP projectiles at 45 and 0 and mm. AP projectiles at 30 and 20 mm. FS as shown in Table 11. The tests comprised determining the lowest velocity for complete penetration by the projectile and the maximum velocity for partial penetration by the projectile. The ballistic limit is the average of these two velocities. The ballistic limits were compared with known ballistic limit values for 7039-1'64 to determine the thickness of this alloy necessary to give the same protection, i.e., same ballistic limit as the composite material, and then the merit rating was determined according to the formula given in Table ll.

As is evident from the merit ratings shown in Table II, considerable increases in AP resistance can be obtained with the composite material of the present invention. More importantly, no substantial reduction in PS resistance was found and in many cases, as in Examples 4 and 5, an increase in PS resistance was noted. Although the improvement in PS resistance may seem inconsequential, the improvement is considerable when compared with other materials having equivalent AP resistance. Moreover, the invention provides for up to a percent decrease in weight requirements over 7039-T64 for equivalent protection. The AP resistance of the present invention (on an equivalent weight basis) is superior to 300 Brinell hardness steel and equivalent or superior to 500 Brinell hardness steel. Moreover, the spall resistance of the composite of the present invention is greatly superior to that of homogeneous, high hardness steel plate.

Although the invention is described herein primarily as lightweight armor plate with improved ballistic properties, the invention has applications in other fields. However, this invention is directed to a relatively thick composite material, i.e., one-fourth to 6 inches thick, and is not concerned with cladding to improve corrosion resistance, although such improvement may be obtained depending upon the selection of the alloy composition.

What is claimed is:

Tablel Frontal Backing membser mer i iber Example Frontal member composition Backing member composition psi 1 6.6% Zn, 2.5% Mg, bal. aluminum... 5.5% Zn, 1.0% Mg, bal. aluminum... 88,000 57,000 2-.. do 0 86,000 57,000 3 do 5.5% Zn, 0.75% Mg bal. a1uminum 83,000 54, 000 4 do d0 0... 83 000 54,000

Table l Continued Frontal Backing member member 'I.S., I.S., Example Frontal member composition Backing member composition p.s.i. p.s.i.

7 ..d -.dO 81. 000 54,000 4.5% Zn, 4.2% Mg, bal. aluminum..- 5.0% Zn, 0.5% Mg, bal. aluminum". 81,000 44,000 do .d0 80,000 58,000 10 7075 3.75% Zn, 2.0% Mg, bal. aluminu m 83,000 58,00

TABLE 11.-BALLISTIC PROPERTIES Nominal Backing composplate Merit rating* ite thick, thick, percent Cal. 30 Cal. 50 Cal. 50 mm. 20 mm. in. of total AP/lO" API45 AP/0 AP/30 FS Weight of 7039-164 (per unit area) required to protect *Merit rating thickness being between about 4 and 20 percent of said total composite thickness and said backing member having a tensile strength at least 2,000 p.s.i. less than the frontal plate tensile strength, said tough aluminum alloy consisting essentially of about 4.0 to 6.0% zinc, from 0.5 to 3.5% magnesium, up to 3.0% copper and the balance aluminum and inconsequential X Weight of composite (per unit area) providing equivalent protection amounts of other elements.

2. The composite of claim 1 wherein the tough aluminum alloy contains 0.5 to L5 percent magnesium.

3. The composite of claim 1 wherein the copper content of the frontal plate and backing plate is less than 0.2 percent.

4. The composite of claim 1 wherein the backing member thickness is between 5 and 12 percent of the total composite thickness.

5. The composite of claim 1 wherein the backing member thickness is between 5 and 12 percent of the total composite thickness.

6. The composite of claim 1 wherein the yield strength of the backing member is at least 7,000 p.s.i. less than the frontal plate tensile strength.

Claims

2. The composite of claim 1 wherein the tough aluminum alloy contains 0.5 to 1.5 percent magnesium.
3. The composite of claim 1 wherein the copper content of the frontal plate and backing plate is less than 0.2 percent.
4. The composite of claim 1 wherein the backing member thickness is between 5 and 12 percent of the total composite thickness.
5. The composite of claim 1 wherein the backing member thickness is between 5 and 12 percent of the total composite thickness.
6. The composite of claim 1 wherein the yield strength of the backing member is at least 7,000 p.s.i. less than the frontal plate tensile strength.