Bimetallic armor: aluminum and steel in one bottle
There are no questions today about how to combine aluminum alloys and steel in the structure of protection of light combat vehicles, since a kind of gold standard in this matter has been developed for a long time. It implies the use of aluminum as the basis of armor, and steel plates - in the form of complementary screens (as in the BMP-3 and Bradley), which allows for the required bulletproof and projectile-proof resistance within the strict restrictions on the weight of the vehicle.
However, in the old days, about fifty years ago, other options for combining these materials were actively developed. One of them was to create bimetallic armor or, in other words, to turn sheets of steel and aluminum alloy into a single plate by welding. We will talk about the advantages of such armor parts in this article.
A little about the interest in bimetallic armor
Perhaps we should start with the main thing - why was it necessary to go through all this fuss with welding steel and aluminum sheets. To do this, let's remember why aluminum began to be used in the production of light military equipment.
The reason is simple: aluminum alloys have a much lower density than any type of steel. Yes, because of this, as well as low hardness values, they in any case lose in terms of resistance to destructive means, since where ten millimeters of steel armor will suffice, twenty (or even more) millimeters of aluminum will be needed. But the main thing is the gain in weight.
With the same requirements for protection against bullets or small-caliber gun shells, aluminum armor, although thicker than its steel counterpart, will be much lighter. Therefore, it is often given priority - especially in those situations when a combat vehicle must have not only good air transportability (so that several units fit into an airplane and there is a reserve for other cargo), but also airdroppability.
However, there is a fly in the ointment: it is practically impossible to radically improve the characteristics of aluminum alloys in terms of their resistance to destructive agents. This is not steel, after all, with the hardness, impact toughness and other parameters of which you can "play" in a fairly wide range using various technologies.
BMD-1 - carrier of armor made of aluminum alloy ABT-101
160–170 HB (Brinell hardness) is the limit beyond which brittle destruction begins in the form of breaches, other substandard damage and low survivability of the armor "aluminum". And in principle, nothing could be done about it before - if we take the USSR as an example, they even did some "downgrade", abandoning the harder alloy ABT-101 (used, for example, in the BMD-1) to the less hard and plastic ABT-102 (in the BMP-3, etc.) in favor of improving the survivability and resistance of the armor to shelling in exchange for increasing its thickness.
So the idea of creating bimetallic plates from aluminum and steel sheets, providing much higher characteristics compared to homogeneous aluminum armor, was treated with some enthusiasm. And one example of this enthusiasm was far from in vain is the research conducted in the second half of the 70s in the USSR.
Manufacturing technology
During these works, the researchers, of course, had to suffer greatly with the development of the technology for manufacturing bimetallic armor, since the usual method of diffusion welding and rolling of steel and aluminum sheets did not give any positive results due to the formation of an intermetallic layer between the sheets being joined, which became a source of brittle destruction during shelling.
To avoid these consequences, Soviet engineers used explosive welding of steel and aluminum sheets. And to get rid of the negative impact of the intermetallic layer, they used sublayers (spacers between the welded sheets) of copper and pure aluminum, which, in general, gave a fairly good and relatively durable connection.
The process of making “bimetal” itself looked like this.
A BT-70 steel sheet of the required dimensions, completely cleaned of any contamination, was taken and placed on a rigid base of steel plates. A copper plate about 5 mm thick was installed on top, with a gap of 10-0,5 millimeters, glued with tar to the duralumin sheet. And then an explosive mixture of ammonite and ammonium nitrate was applied to the duralumin sheet in an even layer of 15-20 mm, detonated using a detonation cord.
After the copper and steel had been welded together by the energy of the explosion, the duralumin sheet was removed. In its place, again with a gap, a plate of pure aluminum 0,8–1 mm thick was installed over the copper layer, also glued to the duralumin sheet. Then the explosive was applied again and detonated.
Then a sheet of aluminum alloy D20 was welded to the plated steel blank in a similar way - a kind of "sandwich". It seems to be crudely made, but it works. Which is what the tests showed.
Test
These armored parts from BT-70/D20 were tested, as they say, to the fullest extent: they were fired at with armor-piercing bullets of 7,62 mm and 12,7 mm caliber, as well as 23 mm armor-piercing incendiary shells BZT. The results of the shelling were compared with the resistance of homogeneous plates made of such materials as aluminum alloy ABT-102, titanium alloy VT-6 and steel BT-70Sh.
What came out of this can be seen in the images below.
This image shows the resistance (A PKP – maximum angle of non-penetration) of bimetallic armor against 7,62 mm armor-piercing bullets in comparison with homogeneous plates made of other materials. 1 - bimetallic armor, 2 - aluminum alloy ABT-102, 3 - steel armor BT-70Sh, 4 - titanium armor VT-6.
This shows the durability of bimetallic armor (shaded area) compared to homogeneous steel and aluminum armor of similar mass when fired at by 12,7 mm armor-piercing bullets from a distance of 100 meters. 1 - bimetallic armor, 2 - aluminum alloy ABT-102, 3 - steel armor BT-70Sh. The thickness of aluminum and bimetallic armor here is reduced to the thickness of equilibrium steel.
Projectile resistance of bimetallic armor (V pkp — maximum velocity of non-penetration), obtained by explosion welding, and folded aluminum and steel plates when fired at by 23-mm armor-piercing incendiary shells at an angle of 90 degrees. 1 — steel armor BT-70Sh, 2 — aluminum armor ABT-102, circles with a cross inside — bimetallic armor, welded by explosion.
Thus, if we evaluate the level of armor protection of bimetallic sheets from BT-70/D20 by the maximum speed of non-penetration and maximum angles of non-penetration, we can say the following. In comparison with the homogeneous steel armor of BT-70Sh and aluminum armor alloys, "bimetal" clearly wins in terms of durability, especially when fired at small angles or even at normal.
Against 7,62 mm armor-piercing bullets, the gain in this case is about 10%; 7,62 mm armor-piercing bullets - 25%; 23 mm armor-piercing incendiary shells BZT - 15%. So, for example, for protecting the frontal parts of the hull and turret of combat vehicles, where there is not much room for play with the angles of inclination, "bimetal" was very well suited, surpassing other materials in durability. However, ideally, this armor could be useful not only in the forehead - in other projections too.
It is also important to understand that these figures are far from being the standard. Using other types of armor steel and aluminum alloys, you can achieve an even greater increase in durability. And this is without taking into account that explosion welding does not provide a 100% weld - with another, more advanced technology, the quality of the armor could be even higher.
Conclusions
Bimetallic armor is a good thing, of course, but why then was it not used in the production of military equipment? There is no point in looking for insidious saboteurs. There is only one reason: high costs with an uncertain result.
Yes, in the future, if work on “bimetal” was closely pursued, a good alternative to homogeneous aluminum armor could be obtained, but to produce it, many different methods would have to be tried, and it is not a fact that they will be cheap and relatively simple, because the price and technological advancement determine the mass production.
It is enough to recall the tension with which the development of layered aluminum armor (LAA), which was planned to be installed on promising combat vehicles in the Russian army, was carried out - a working technology had already been invented and successfully tested, but it was still expensive to make, although easier than pairing aluminum with steel.
Source:
I.D. Zakharchenko, M.I. Maresev, N.P. Neverova-Skobeleva, et al. Combined armor made of steel and aluminum alloy for light VGM/ I.D. Zakharchenko, M.I. Maresev, N.P. Neverova-Skobeleva, et al. // Issues of defense technology. – 1979. – No. 86.
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