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-H115
|
290
|
360
|
9
|
2.66
|
|
7017-T651
|
425
|
485
|
12
|
2.78
|
|
7018-T7651
|
300
|
360
|
13
|
2.79
|
|
7020-T651
|
360
|
400
|
12
|
2.78
|
|
7039-T651
|
400
|
460
|
12
|
2.78
|
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