Advanced aluminum armor alloys


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Article published in the December 2016 issue of Light Metal Age
By Michael Niedzinski from Constellium

Published in: Engineering
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Advanced aluminum armor alloys

  1. 1. LIGHT METAL AGE, DECEMBER 201634 A luminum armor solutions have been in exis- tence for the past 60 years, almost since the start of the Aluminum Association circa 1954. These plate products were developed for armored per- sonnel carriers, either wheeled or tracked. Armored vehi- cles serve an important function as reconnaissance, ambu- lance, scout, or infantry delivery vehicles. One of the first armored vehicles, which used aluminum armor, was the M113 armored personnel vehicle (Figure 1). The M113 introduced new aluminum armor that made the vehicle much lighter than earlier vehicles; it was thick enough to protect the crew and passengers against small arms fire, but light enough that the vehicle was air transportable. The plate products used were developed during the Vietnam War era. Since then, many enhancements and improvements have been made and thousands of these vehicles are in service. Aluminum armor was developed and selected based on its lower density. Density of alumi- num alloys is about 2.7 g/cm3 (0.1 lb/in3 ) and can vary ±7% from the nominal based on the level of alloying ele- ments. Depending on the level of alloying elements, den- sity of steel can vary between 7.75 and 8.05 g/cm3 (0.280 and 0.291 lb/in3 ). Lower density translates into lighter vehicles, thus improved mobility. Higher mobility results in higher speed and better acceleration and decelera- tion, which results in higher responsiveness on the battle- field and more maneuverability in urban warfare. Lighter weight also allows easier negotiation of steep slopes. All of these attributes result in lowering the vulnerability of the vehicle. Additionally, most of these vehicles are transferred to the war zone via air transport. More lightweight vehicles can be deployed using fewer aircraft. Lighter weight also reduces the need for refueling, providing a longer range. Based on their ballistic characteristics, alloys 5083-H131 and 7039-T64 have been the preferred alloys for armored vehicles. Alloy 5083 in H131 temper is a high strength temper developed through a high level of cold work (cold roll/stretch). Based on the high level of cold work, form- ability is reduced but the material is very weldable and ex- hibits good corrosion resistance. Conversely, alloy 7039 has superior protection against armor piercing and fragment Advanced Aluminum Armor Alloys By Michael Niedzinski, Constellium threats, but it has substandard corrosion (especially stress corrosion) resistance. Alloy 7039 is more difficult to fusion weld and thus the only way to join individual components is by using mechanical fasteners. Other alloys have been employed (Table I), however their ballistic performance was not as effective as the two aforementioned products. However, lower density does not necessarily mean light- er weight armor. The concept of areal density was devel- oped to quantify the ability of armor to defeat particular or combinations of threats. Since armor is used to protect a particular area, its practical weight is best described by its areal density. Areal density is described as the ratio of weight of the armor system to area being protected. In the U.S., pounds per square foot are the typical units. Thus areal density is a physical characteristic of the armor and does not indicate if that armor is effective. The effective- ness of two armor systems can only be assessed by com- paring their performance against the same threat. Based on multiple tests, a lower areal density was observed for aluminum armor when compared to steel, for the threats mentioned hereafter. Thus, for a given protection level, lower weight armor can be used, resulting in a lighter vehi- cle. Primary threats for the occupants of these vehicles are Armor Piercing (AP) rounds. Another threat was shrapnel from artillery rounds, such as Caliber .30 AP M2, Caliber .50 AP M2, Caliber .50 FSP, and 20 mm FSP. The Army Research Laboratory developed a series of tests to help evaluate and properly size armor plates used on the sides and front of the vehicles. Standardized AP rounds and projectiles, which simulate shrapnel, or frag- ment simulator projectiles (FSPs) are fired at a test plate from a specific distance to obtain the ballistic protection limit for a given thickness of aluminum or steel armor (Figure 2). Ballistic protection limit is a number based on multiple shots, which provide a balance between com- plete and partial penetration. Tables that list ballistic pro- tection limit or V50 (limit protection curve correspond- ing to a complete perforation by a given projectile with a 50% probability) for each combination of threat and ar- mor thickness are provided in MIL-DTL standards, which are public documents. Therefore, designers of armored vehicles have a ready reference to select a particular aluminum armor type and thickness to defeat a specific combination of threats. Over the past 15 years, new types of warfare have emerged. Soldiers are increasingly engaged in urban war- fare with multiple threats. AP rounds are fired at close distance and rocket propelled grenades are a weapon of choice. Beyond this, a new threat has emerged af- fecting armored columns on the roadways and on city streets. The emergence of improvised explosive devices (IEDs), which detonate under a moving vehicle, forced the Army to seek new solutions that would improve the Figure 1. Army (Bradley) M113 armored personnel carrier. Alloy Temper Available  Gauge  Range 2024 T351 0.25"  -­‐  4.0" 5083 H131 0.25"  -­‐  3.0" 6061 T651 0.25"  -­‐  9.0" 7039 T64 0.25"  -­‐  3.0" 7075 T651 0.25"  -­‐  4.0" Table I. Conventional armor alloys. Reprinted for Constellium, © 2016 Light Metal Age
  2. 2. LIGHT METAL AGE, DECEMBER 2016 35 performance of aluminum armor alloys by adding blast protection while enhancing AP and FSP protection. Im- proved occupant safety and crew survivability from blast, fragmenting, and armor piercing threats for combat and tactical wheeled vehicles became a primary objective, as shown in a survey of major threats affecting combatants in current warfare (Figure 3). Aluminum producers have responded by developing more advanced armor solutions. Constellium, which is a global producer for a broad scope of markets and appli- cations, including the aerospace, defense, and transporta- tion industries, has provided a number of new aluminum armor solutions under the KEIKOR™ brand (named after the armor of Samurai warriors, which was able to defeat many contemporary threats). Two alloys, which draw their heritage from aerospace applications, emerged as the preferred choice based on an excellent combination of strength, blast/ballistic protection, corrosion resistance, and high formability in intermediate tempers. These ar- mor plate products are KEIKOR 2139™ and KEIKOR 7056™. This article will highlight alloy 2139, which has been standardized in MIL-DTL 32341A. Constellium Plate Alloy for Armor Keikor 2139 was developed for armor applications in the 0.500-4.000 inch thickness range. Originally, this alloy was developed for aircraft wing applications under AMS 4468, but its high strength and tough- ness balance prompted examination as an armor al- loy. Standardized ballistic testing, described earlier, was used to develop a ballistic table for the four types of threats in the AP and FSP categories. Alloy 2139 is a high strength/toughness copper-magnesium- manganese-silver alloy with superior corrosion resis- tance and ability to resist softening at elevated tem- peratures. Corrosion resistance, specifically stress corrosion cracking (SCC), is a very important aspect because these vehicles are used in various environ- ments, including marine. The early precursors in the 2xxx armor plate family, specifically armor 2519-T87, has improved ballistic performance when compared to legacy armor but has very poor SCC resistance due to its high copper level. Thus, 2519 is not suitable for multiuse environments. Chemical analysis and mini- mum tensile properties of 2139 are shown in Tables II and III. Keikor 2139 is produced in two discrete tempers, an AP/FSP resistant temper (T8) and a blast resistant temper (T84). Alloy 2139 T8 (AP/FSP resistant) is pro- duced using a proprietary level of cold work and fol- lowed by artificial aging treatment to attain balanced properties. It is typically used for the applique or side and top armor. Ballistic testing indicated superior per- formance when compared to the two legacy armor so- lutions, i.e., 5083-H131 and 7039-T64. A relative com- parison of the V50 velocity for two threats—20 mm FSP and .50 caliber AP—is shown in Figure 4. This chart also includes Constellium’s Keikor 7056 produced in two discrete tempers, AP resistant and blast resilient (which are not discussed in this article but may be cov- ered in the future). The chart indicates that, for 1.28 inch thick armor plate, V50 for the 20 mm FSP threat is about 2,550 and 2,650 ft/sec for 5083-H131 and 7039-T64, respectively. Alloy 2139-T9 exhibits V50 of about 3,050 ft/sec. In practical terms it means that, for Figure 3. Survey of the major threats facing contemporary soldiers. ELEMENTS SYMBOL 2139  ALLOY  2 Silicon Si 0.10 Iron Fe 0.15 Copper Cu 4.5  -­‐  5.5 Manganese MN 0.20  -­‐  0.60 Magnesium Mg 0.20  -­‐  0.80 Chromium CR 0.005 Zinc Zn 0.25 Titanium Ti 0.15 Vanadium V 0.05 Zirconium Zr N/A Lithium Li N/A Silver Ag 0.15  -­‐  0.60 Other,  max.  Each -­‐-­‐-­‐ 0.05 Other,  max  Total  3/ -­‐-­‐-­‐ 0.15 Aluminum Al Remainder Table II. Chemical analysis of alloy 2139 (Aluminum Association reg- istered limits). Thickness,   inches Class I Class II Class I Class  II Class I Class II 0.500  to   3.000,  incl. 67 71  3/ 64 63  3/ 9 9  3/ 3.001  to   4.000,  incl. 67 N/A 64 N/A 9 N/A Tensile  Strength,  ksi Yield  Strength,  0,2%   Offest,  ksi Elongation  percent Table III. Minimum mechanical properties of alloy 2139 (MIL-DTL 32341A) Figure 2. Examples of armor test projectiles: 20 mm fragment simula- tor (a), .50 caliber AP round (b), and .30 caliber M2AP round (c). (a) (b) (c)
  3. 3. LIGHT METAL AGE, DECEMBER 201636 a given armor thickness, alloy 2139 offers 400-500 ft/ sec extra margin of safety. Similarly, in the .50 Cal AP area alloy 5083-H131 and 7039-T64 offer V50 of 1,780 and 1,910 ft/sec, respectively. Alloy 2139 exhibits V50 of 2,110 ft/sec. Margin of safety ranges from 200 to 330 ft/sec. Thus, manufacturers of armored vehicles can either provide extra survivability or consider a down gauge of the design yielding equivalent protection with lighter, more maneuverable construction. Keikor 2139’s minimum ballistic properties compare favorably with the legacy armor offering. More signifi- cant are the blast characteristics of the second temper, 2139-T84. Due to its higher strength and toughness, 2139-T84 experiences lower plastic deformation under a standardized blast test when compared to legacy 5083 armor. In a standardized test, a sample plate is exposed to a blast of equivalent weight of TNT with constant standoff distance. The amount of buckling of perma- nent plastic deformation is measured after the blast. Figure 5 indicates that, under standardized test condi- tions, 2139-T84 exhibits plastic deformation about 50% lower than equivalent thickness of 5083-H131 armor. Occupants of the vehicle experience significantly lower impulse/impact during the explosive event. This reduc- es the potential for injuries. T84 (blast resistant) temper is generally used for the underbelly of the armored vehicle and is obtained by the end user through aging Keikor T34 at an elevated temperature using a thermal treatment recipe supplied by Constellium. Plate for the complex, heavily formed bottom of the vehicle is supplied in T34 or an interme- diate temper produced by solution heat treat (SHT) and proprietary cold work of the plate. This results in a product with high formability and low flow stresses needed for the bending and forming processes. Form- ability of alloy 2139 is illustrated in Figure 6. Constel- lium is in partnerships with metal machine forming facilities to optimize the fabrication needed to pro- duce the required shapes. Older constructions utilized welded structures or bolted constructions. Welds and bolt holes introduce weak areas or stress raisers during a blast event, which may cause perforation of the un- derside of the vehicle with resultant intrusion of lethal fragments into the crew compartment. Conversely, a monolithic formed underside provides a much stron- ger structure, which may plastically deform but will survive a blast event without fracturing. This also mini- mizes and limits welding activity to the less vulnerable sections of the vehicle. Constellium developed welding techniques using commonly available filler wires in the 2xxx and 4xxx weld wire family. Weldability of alloy 2139 provides an extra manufacturing flexibility not readily attainable with 7xxx series alloys. Several vehicles have been designed and fielded by taking advantage of the ballistic and blast superiority of 2139. Specifics of the design of these vehicles are clas- sified for security reasons. One example of a vehicle, which was built using Keikor 2139 was the Concept for Advanced Military Explosion-Mitigating Land (CAMEL) program vehicle. The vehicle (Figure 7) was built by De- troit, MI, based Pratt & Miller and was successfully tested under various threat scenarios at the Army’s Tank Auto- motive Research Development and Engineering Center (TARDEC). This work was done in support of the Com- bat Vehicle Prototype (CVP) platform. In September 2016, it was announced that Constellium will supply Keikor 2139 for TARDEC’s entire CVP inte- grated hull capsule. Constellium’s manufacturing plant in Ravenswood, WV, will supply the 2139 plate. Erik Pol- sen, TARDEC’s chief engineer for CVP survivability, said Constellium’s 2139 aluminum alloy “met the necessary performance properties and characterized manufactur- ing process for the performance and packaging require- ments that we were looking for in our hull structure.” Figure 5. Plastic deformation (buckling) during blast testing for Keikor 2139-T8 versus legacy alloys. 0 10 20 30 40 50 60 70 80 90 Deflection Permanentdeflection(mm) 5083-H131 7020-T6 2139-T8 0 10 20 30 40 50 60 70 80 90 Deflection Permanentdeflection(mm) 5083-H131 7020-T6 2139-T8 - 55% 5083-H131 7020-T6 2139-T8 Figure 6. Formability of 2139-T84 plate. Figure 7. CAMEL vehicle. Figure 4. Comparison of Keikor 2139 with multiple legacy alloys in regards to V50 velocity for two threats—20 mm FSP and .50 Cal AP.