Functionally and Geometrically Ordered Ti Armor
Design and Processing of Ti Composite Armors
1: Ti Armor
Titanium has long been recognized as a superior material for many combat systems and components due to an excellent combination of properties. It has a high strength-to-weight ratio, excellent ballistic mass efficiency, and is corrosion resistant. However, the high cost and limited availability of titanium has restricted its use to only the most critical applications, such as in high performance aircraft. To address these limitations, DoD, DARPA, and titanium powder producers recently undertook programs to develop low-cost titanium and titanium alloy powders having powder metallurgy (PM) grade quality. Various studies have shown that these powders can be consolidated in the solid state to form fully dense metals that meet military and industry specifications. In case studies to investigate the production of titanium components for military ground vehicles using PM processing, high quality parts were fabricated with an estimated cost savings of at least 50% when compared to the conventional melt-processing methods that are currently used for titanium production.
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Mechanical properties of P/M Ti meet the requirements of ASTM B-348-09 and MIL-T-9047G specifications.
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Typical Ti-6Al-4V α/β microstructure seen with consolidated Armstrong powder.
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Front and back of V50 test plate made by solid state vacuum hot pressing of Armstrong Ti-6Al-4V powder. Ballistic performance of P/M Ti-6Al-4V plate was shown to be equivalent to standard wrought armor plate vs. 30 cal APM2 threat.
Good ballistic impact resistance and a high strength-to-weight ratio make titanium invaluable to DoD at a time when combat systems are required to outperform previous generations of equipment, especially in terms of mobility, rapid deployment, and protection against improvised explosive devices, as well as conventional munitions. Land combat systems that have been designed and built with extensive use of titanium have typically achieved 40% overall weight reductions. In addition to improved mobility, titanium offers a significant advantage in lowering fuel consumption. The latest generation of fighter aircraft use a high percentage of titanium to lower weight and increase fuel efficiency: 40% on the F-22 Raptor and 20% on the F-35 Joint Strike Fighter. Even new commercial airliners, such as the Boeing 787, are making extensive use of titanium in combination with carbon fiber composites to achieve a 20% improvement in fuel economy.
The Ti-6Al-4V alloy provides superior ballistic protection when compared on a weight basis to conventional rolled homogeneous armor (RHA) steel, but it is far less efficient on both a weight and volume basis than state-of-the-art ceramic armor. Armor materials can be compared by their mass efficiency rating, Em. Em is the ratio of the weight of RHA steel to the weight of the alternate armor material that is needed to stop identical threats. The Em of Ti-6Al-4V is about 1.5, while the Em of boron carbide (B4C) ceramic is 3. This means that if it takes 3 lbs. of RHA steel to stop a given threat, then the same protective capability can be achieved with 2 lbs. of Ti-6Al-4V or with 1 lb. of B4C armor. But, the ceramics are brittle and can be costly, making them less desirable for combined structural and protective applications.
Titanium and other metal armors achieve defeat of hardened (armor piercing) ballistic threats primarily through energy absorption by plastic deformation and shear of the armor, and by friction between the ballistic projectile and the armor plate.
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In contrast, the ceramic armor materials achieve defeat by deformation and fracture of the projectile and by the work of fracture in cases where the ceramic plate is broken.
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By combining a ceramic material with titanium to form a composite structure, it should be possible to combine the different defeat mechanisms and produce an enhanced armor material. By blunting and/or fracturing the projectile upon initial impact with the composite, much more energy will be expended in the subsequent process of penetration through the remaining armor thickness. This enhanced performance can be used to either increase the protection level or reduce the amount of armor needed when compared to the baseline configuration. However, combining Ti metal with ceramic powders, inserts, or plates is difficult, if not impossible, when using conventional melt processing. This is primarily due to chemical reactivity at molten metal temperatures (>1700 C), differences in specific gravity which result in phase separation, and differences in the thermal expansion coefficient which can result in interfacial stresses that are large enough to distort or shear the materials during cooling. Powder metallurgy processing in the solid state can minimize or eliminate these problems.