Aluminum Armor

Because of their light weight, aluminum alloys have found wide use in military applications, including military vehicles such as personnel carriers. The light weight of aluminum allows for improved performance and ease of transporting equipment, including air transport of military vehicles. In some vehicles, it is advisable to provide shielding or protection against assault, such as by providing armor plate to protect the occupants of the vehicle. Aluminum has enjoyed substantial use as such armor plate, military specifications pertaining to certain aluminum alloys for armor plate applications applying thereto. Basically, the requirements for aluminum alloy armor plate are resistance to projectiles, good corrosion resistance, and, in some applications, good weldability. Ballistics tests are often conducted with armor-piercing projectiles such as 0.30 caliber and with fragment-simulating projectiles such as the common 20 millimeter projectile. Obviously, aluminum alloys which satisfy all the requirements for armor plate are desirable, and these desires have been met to varying degrees.

Traditionally, the search for better combat vehicle armor has emphasized stronger and tougher materials to resist more durable projectiles moving at higher velocities. Conventional approaches have led to progressively thicker armor, harder armor, and, inevitably, to heavier armor, and a corresponding sacrifice in vehicle mobility.

In extensive studies of armor, a surprising material feature was noticed which has given new impetus to materials armor design. Specifically, it involved the anomalous differences in ballistic behavior between aluminum armor and steel armor at low and high obliquities. People with a great deal of armor experience began to ask: why does a material having one-third the strength of armor steel sometimes show better results against projectiles than steel does? Assuming no change in projectile form, the most apparent answer points to the thickness or path presented by the armor to the projectile. The greater the length of this path the greater the work resistance or consumption of projectile energy. The lower the density of the material, the greater the "work path" for a given weight of armor.

If this feature were combined with high strength, even greater ballistic capability could be provided in a chosen material configuration. The effectiveness of this lower density core is further improved if the shape or form of the projectile is altered. This is accomplished by the hard steel front plate which can crack or fracture the projectile. The medium strength steel back plate adds shock toughness to the total material system and facilitates easy fabrication since a weldable steel is present on both faces of the armor package. Thus, the invention presents an armor concept which attempts to accentuate the ballistic advantage of increased penetration resistance available in a low density-high strength material through a composite material arrangement.

The "meat of the sandwich" is the key to more effective armor plate. The outer lamination, which is directly exposed to ballistic attack, is a substantially conventional high-alloy, high-hardness armor steel plate which confronts the projectile with high reflective shock tensile forces in a relatively short travel path, and thus severely stresses the projectile, so much so that the structural integrity of the projectile is impaired by the formation of cracks. When the projectile reaches the low-density core, the long path of travel through the core brings about more complete breakup of the cracked projectile and substantially reduces its velocity because of the effort by the projectile to displace the great bulk of the low-density metal. Thereafter, if the projectile or its pieces can penetrate the entire thickness of the cor, the velocity thereof is so greatly reduced that the inner plate cannot be penetrated.

Armor plate of aluminum alloys has become established for specialized purposes where not only ballistic resistance, but also lightweight are important considerations. This is notably true in the case of armored military personnel carriers which operate on the ground but must be transportable by air. U.S. military specifications have been developed for such alloys, dealing with ballistic performance in terms of the speeds of two different kinds of projectiles fired at specified obliquities to the target. One of these is an armor piercing projectile (e.g., .30 caliber) designated "AP", characterized by a pointed leading end. The other is a fragment simulating projectile (e.g., 20 mm) designated "FS", characterized by a blunt leading end. The latter projectile tends to create flying fragments from the inner side of the armor plate, even when the projectile fails to penetrate the plate, so that speeds less than penetration speeds have to be considered for purposes of FS tests.

Experience shows that an armor alloy better than another for one kind of these projectiles may be worse for the other kind of projectile. Weldability (joining characteristics and joint performance) and corrosion resistance, which are also important considerations, may also vary for different alloys. Consequently, the general objective is to develop armor plate alloys having improved performance in dealing with both kinds of projectiles, while also achieving good weldability and corrosion resistance.

The aluminum armor alloys which have become most widely accepted are 5083 meeting the requirements of U.S. Military Specification MIL-A46027F (MR), and 7039 meeting the requirements of U.S. Military Specification MIL-A46063E. Alloy 5456 is listed in the former specification, but apparently has had little, if any, acceptance for armor plate purposes. These and all other four digit alloy designations herein are in accordance with alloy numbers and corresponding definitions registered by The Aluminum Association, Washington, DC. As shown in these military specifications, armor plate of alloy 7039 is considerably superior to armor plate of alloy 5083 for AP ballistic performance, but less so in FS ballistic performance. In fact, below 1.235 inch gauge, 7039 armor plate is rated below 5083 armor plate in FS ballistic performance, according to the military specifications. In any case, the generally favorable ballistic performance of 7039 armor plate is seriously offset by the fact that it is more susceptible to stress corrosion than 5083 armor plate, especially when welded into an armored structure. It is also less readily weldable than 5083 armor plate, and is more dense than 5083 armor plate, due to the relatively high magnesium and low zinc content of 5083.

Substantial increases in magnesium offer benefits of substantially improved strength and ballistics performance along with slightly reduced weight since magnesium is lighter than aluminum. It is recognized, however, that increasing the level of magnesium introduces problems in stress corrosion cracking where significant amounts of cold work are imparted to the sheet or plate product, for instance, amounts of cold work in excess of 10% or 15%. The prior art recognized the corrosion problem in aluminum-magnesium alloys which receive substantial cold work, especially as the magnesium content is increased, and that by eliminating the cold rolling and employing instead warm rolling, the stability of the product was substantially improved such that resistance to stress corrosion cracking was generally acceptable, but resistance to exfoliation was not improved. A preferred minimum for magnesium is about 6.3% with a more preferred minimum being about 6.6%. A preferred maximum for magnesium is about 7.8%, more preferably about 7.4%.

Developed by Aleris of Germany, the aluminum alloy 5059 (AA5059) offers greater ballistic and blast protection for armored hull-type vehicles. Because it can be readily welded and offers superior corrosion resistance, the alloy was an excellent candidate material for a wide range of applications. The Army used AA5059 in the RG-33 Mine Resistant Ambush Protected (MRAP) vehicle that was deployed in Iraq, and the alloy is being considered for other programs as well. In addition to protecting our warfighters, the lighter weight of AA5059 means greater fuel economy and enhanced system performance, and the decreased rate of corrosion lowers overall life- cycle costs. More than 2,000 RG-33 MRAPs were fielded.