The Application of New Technology to Aluminum Armor Systems

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Historically, the superiority of weldable aluminum armor for protection against high explosive shell fragments was recognized about 60 years ago but it was not in use until the year 1960 when the first quantity of aluminum-armored vehicles appeared. This was the Ml13 and was constructed from the aluminium-4.5% magnesium alloy 5083; this vehicle was in production for the next 25 years.

Historically, the superiority of weldable aluminum armor for protection against high explosive shell fragments was recognized about 60 years ago but it was not in use until the year 1960 when the first quantity of aluminum-armored vehicles appeared. This was the M113 and was constructed from the aluminium-4.5% magnesium alloy 5083; this vehicle was in production for the next 25 years.

Although the resistance of this alloy to 0.3 inch armor-piercing (AP) attack is slightly less than that of steel, it is slightly better than steel for 14.5mm diameter AP and the vehicle is lighter than an equally protected steel version. Additional weight savings are gained from the use of aluminum because of the greater rigidity of the thicker, but lighter, plate -- nine times stiffer than steel -- with the consequent saving on stiffening structure.

A demand in the early 1960s for lighter, and therefore ballistically stronger aluminum armor led to the introduction of a heat treatable, weldable aluminium-4.5% zinc-2.5% magnesium alloy designated 7039. Alcan Co. developed this further into the slightly stronger and more corrosion-resistant alloy 7017. A comparison of this alloy with steel for 0.3 inch AP and 14.5 mm AP is a weight saving of about 20% is shown for 7017 when using the 14.5 mm round.

The intermediate strength heat-treated alloys 7020, favored in France and Germany, and 7018 developed by Alcan for use in parts of vehicles most vulnerable to blast attack, also are produced by Alcan. It can be seen that the heat treated 7000 series alloys offer appreciable strength increases over the earlier non-heat treated 5083 with the exception of alloy 7018 which has been designed to give similar properties.

Welding modifies the structure of the parent plate; by permanent softening of the heat affected zone and by dissolution of hardening precipitates in the heat-treated alloys. Some of the hardness is recovered in the latter by a process of natural age-hardening so that the welded joint yield strength of the 7018 is about 50% higher than that of the 5083 alloy. Testing of the relative ballistic performances of these alloys, based on attack by 0.3 inch AP rounds, show that 7017 is about 16% more resistant than 5083.

Selection of alloy by the vehicle designer requires consideration of the type of threat in each particular area of the vehicle but also must take into consideration other characteristics such as stress corrosion resistance. Like many other structural materials, the aluminum alloys have, to varying degrees, some susceptibility to stress corrosion. Generally this susceptibility increases as the alloy content is increased, but even in the area of decreasing resistance much can be achieved in processing to minimize the risk of attack. Alcan’s 7017 improvement program, has, over a number of years, increased the life in an accelerated stress corrosion test by a factor of about 40. This has been achieved by careful balancing of alloying elements and attention to processing parameters from melting and casting to final heat treatment.

However, the heat generated by welding does modify the metallurgical structure of the material in narrow zones adjacent to the weld and high residual stresses can be induced by incorrect assembly and welding techniques. Fabrication procedures must recognize these effects and be designed to minimize them. Advice on these aspects is always available from the material manufacturer.

Future developments will look at some areas of the work currently being undertaken with the projected targets of improving stress corrosion lives of vehicles and increasing ballistic resistance. Although inevitably interrelated, the various areas of development will be reviewed under the following categories:

  • Stress Corrosion Studies
  • Corrosion Protection
  • Alloy Refinement
  • New Alloys
  • Welding Techniques.

Stress Corrosion

Stress Corrosion was identified as a material characteristic that required careful consideration in designing armored vehicles and our work recognises the need to continually improve the resistance of welded structures to this form of attack. This is especially important in the new generation vehicles, which are required to have in-service lives of 30 years or more. Until relatively recently the mechanisms by which stress corrosion failures occurred were not fully understood. One of Alcan’s contributions in this area of science has been to conclusively demonstrate the fundamental role of hydrogen diffusion along grain boundaries. This process precedes any actual crack growth and the mechanism is one of intermittent diffusion and crack growth.

Studies of cracking susceptibility also have necessitated the devising of novel experimentation techniques to establish the actual condition obtaining at the crack front. The techniques include methods for extracting electrolyte from cracks in microlitre (μl) quantities and chemically analyzing them (this size sample is about the same as a bead of perspiration).

Another useful tool in these investigations is the slow strain rate stress corrosion test developed at Newcastle University. Research scientists have adopted and developed this test as a means of precisely assessing degrees of stress corrosion susceptibility. This is done by slowly straining duplicate specimens to failure; one in a corrosive environment and the other in dry air. A comparison of the plastic strain at failure in the two specimens can be used to evaluate the material. This procedure is particularly useful as a rapid screening test for candidate materials for improved performance applications. In addition to establishing the rate and manner of crack growth, research laboratories have devised techniques to simultaneously measure the crack growth of many test specimens loaded in series.

Corrosion Protection

With the knowledge of the mechanisms involved, engineers are now being able to develop protection systems for areas of welded structures where high residual stresses in corrosive environments are difficult to avoid.

The parent plate material, in the delivered condition, has been developed to a relatively stress corrosion resistant state but once it has been incorporated into a welded structure it may be left in a susceptible condition. This can result from residual stresses induced by adjacent welding and, in the parent metal/weld interface, by localized remelting or modification of the wrought alloy structure.

Exposed cut edge has successfully been protected by a process known as buttering in which the exposed end grain is coated with weld/filler metal applied with a welding gun. Such methods are not applicable at the weld/parent plate junction and a sprayed metal protection system has been developed for this area. The basis of this technique is successful use on 7004 alloy railcar hoppers in Canada used for carrying sea water washed bauxite. For armored vehicle use the basic alloy is being improved to optimize the electrochemical characteristics to particular alloy-weld conditions.

Alloy Refinement

In parallel with the other work on corrosion protection work continues with efforts to improve the intrinsic stress corrosion resistance of the parent plate-weld system by close attention to alloy composition. With the increased understanding derived from corrosion studies, it is possible to further increase the resistance of aluminum alloy systems.

New Alloys

New works obviously extends that carried out under the previous category and covers new heat treatable and non-heat treatable alloys. In the non-heat treated alloys research will involve evaluation of stronger, more corrosion resistant material. In the heat-treated alloys engineers will not be constrained by weldability requirements; applications are becoming increasingly apparent where weldability is not a primary requirement. This allows us to consider very high strength alloys and also high strength low density alloys. The very high strength material development is at the stage where it can produce plate possessing 50% more strength than that of the current best aluminum armor in use today.

The low density alloys are based on the aluminium-lithium system and offer up to 10% weight saving compared to conventional aluminum alloys but with comparable ballistic strength. Ballistic performances of these materials still have to be fully evaluated but for enhancing existing systems by increasing penetration resistance there is an important area of application that can be filled by one or both of these alloy concepts. Both of these approaches to material design have necessitated new designs of casting equipment to accommodate the more complex solidification characteristics of these alloys.

Welding Techniques

Because of its effect on strength (and ballistics) and stress corrosion cracking resistance it has being seen that welding is a most critical part of vehicle construction. The effects can be minimized by reducing heat input and attention to welding and assembly procedures to minimize residual stresses. Current trends are towards increased automated and robotized welding. These will demand new standards in arc-condition reproducibility through improvements in power supply and wire feed systems.

Further developments will include robotics development and weld/arc monitoring and control through direct control by expert systems.

Aluminum Armour Alloys

a) Typical Chemical Compositions (weight %)

Alloy Zn Mg Cu Mn Cr Zr Fe* Si* Al
5083 <0.05 4.8 <0.05 0.7 0.12 <0.05 0.3 0.1 Remainder
7017 5.0 2.3 0.1 0.3 0.17 0.13 0.2 0.1 Remainder
7018 5.0 1.1 0.1 0.3 0.17 0.13 0.2 0.1 Remainder
7020 4.5 1.2 0.1 0.3 0.17 0.13 0.2 0.1 Remainder
7039 4.3 2.5 0.1 0.2 0.15 <0.05 0.2 0.1 Remainder
(* impurity levels)

b) Typical Tensile Properties

Alloy 0.2% P.S.
(MPa)
U.T.S.
(MPa)
Elongation %
on 5D
Density
g/cm3
5083-Hl15 290 360 9 2.66
7017-T651 425 485 12 2.78
7018-T765l 300 360 13 2.79
7020-T6S1 360 400 12 2.78
7039-T65l 400 460 12 2.78

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