Original Cobras, and most aluminum cars of that era, were made from 1100 alloy aluminum. 1100 is pure aluminum. Original Cobras are extremely fragile cars. You can dent 1100 aluminum by simply pushing your finger into it
It is fun to work with...but, it is so weak many people use alloy 3003 here in the US today. In Poland we use an old Soviet alloy (which has no equivalent in the US) to make our bodies. It is alloyed with iron. It is tricky to weld, but it really polishes up quite nicely--much better than 1100 or even 3003. Original Cobras were made from 0.050" thick aluminum sheet. In Poland we use 1.5 mm thick (0.059") aluminum. Most coach builders in the US (and England) use 0.063" (1/16") aluminum to build bodies today as it is far more durable.
Originally, Cobras were made by a combination of techniques. Deep shapes (say the area around the headlight) were pounded out on a sand bag with a rawhide or wooden mallet. Those rough shapes were then rolled smooth with a "wheeling machine" (AKA "English wheel" here in the US). It takes extremely skilled craftsmen to use a wheeling machine. The shapes were constantly fit back and forth against a buck (a model of the car body--typically a plywood "egg crate" design of some sort). All fitment was done by eye. As you can imagine, this left for quite a bit of "interpretation" by the craftsman. His interpretation may, or may not, have varied depending upon how close he was to quitting time. On CSX3104 the left fender is 1 inch narrower than the right fender and the fender wasn't wrecked. The left fender also had 1/2" of bondo in it on top under the original paint. (The fender really was that far out--but, that was "original.") Thinner aluminum was originally used because it was much easier (cheaper) to form by hand and it, of course, cost less than thicker aluminum.
We do not use a wheeling machine very much. We really only use it for prototype parts. It is exceptionally difficult to make two parts match up perfectly that are wheeled. I am sure many of you have seen beautifully wheeled parts. They are all nice and shiney. But, I am sure you have also noticed the welds are invariably filed smooth and left in a brushed state so the car looks like a mirror with stripes running all over it. Those transition areas are rarely smooth. In fact, I have never seen one smooth. Our panels don't look as nice as a wheeled panel, but our overall car is smoother. That is why we are able to polish the entire car to a mirror finish.
In Italy, they rarely used a wheeling machine. Parts were just beaten out and then welded together. After a bit of planishing (straightening with a hammer and dolly) the car was slathered in body filler and sanded smooth. If you have ever seen an old Ferrari you would be amazed at the amount of filler they used. There is no doubt the body shapes were stunningly beautiful nonetheless.
Originally the aluminum panels were gas welded together. Gas welding is very fast. Probably twice as fast as TIG welding. We also gas weld our body panels together in Poland. Repair work is generally done with a TIG. Incidentally, silicon bronze welds very easily with a TIG.
In terms of material science...
Young's modulus, or stiffness, is about 10,000,000 psi for aluminum. For comparison, it is 30,000,000 psi for steel. In other words, steel is 3 times stiffer than aluminum. Bronze is about 1/2 as stiff as steel in case you wanted to know.
That said...be careful!!! Hardness and stiffness does NOT always equate to strength.
Aluminum (the soft body forming alloys we use, like 3003-0) has a yield strength of 6,000 psi and an ultimate tensile of 16,000 psi.
Mild steel has a yield strength of 50,000 psi and an ultimate tensile of about 60,000 psi.
655 Bronze has a yield strength of 55,000 psi and an ultimate tensile of 85,000 psi.
As you can see, the question is more complex than it first may appear. Material science is fascinating.
For body parts we make here in the US, we use 3003 aluminum in the H0 and H14 state. 3003 means the aluminum was alloyed with manganese to make it a bit stronger than pure aluminum. H0 means its hardness is 0, or fully annealed. H14 is a little different. Each number, the 1 and the 4 mean something different. The 1 means "strain hardened" (by the rolling process) and the 4 means "1/2 hard." In other words, H14 was rolled until it was 1/2 hard. H0 is really only good for a few parts, or parts with a LOT of shape as it is so soft there is not much strength left in the part when you are finished. You have to move H0 a long way to get the hardness back up to something durable. Sometimes, we start with 3003 H0 and form the part 1/2 way. Then we anneal and form it the rest of the way. 1100 also comes in H0 and H14.
We also use 5052 H32, 6061 T-6, and 7075 T-6 to make parts. The "T" designates the aluminum was solution heat treated and aged to make it harder and stronger.
For superplastic forming we use (generally, but not always) alloy 5083 to form the part. 5083 works really well because it doesn't have to be heat treated and it has a very high strength when it comes out of the press.
For comparison, bronze is much, much harder than aluminum. Remember, bronze was used to make weapons for thousands of years after it replaced copper. Swords were invented in the bronze age (and not in the copper age) because bronze was finally strong enough to make a long blade. Copper wasn't strong enough to make swords (they did make copper axes, however, as evidenced by the "Ice Man.") We advanced from the copper age to the bronze age when we figured out how to alloy copper with tin and make bronze. (Now, there are many different kinds of bronze and I am just giving a very brief over view here of the history involved.)
The bottom line is, "old world metal forming" is a dying art. It is very hard to get a perfect part by hand. There really isn't anything that can compare to a super plastic formed part. SPF parts are simply stunning. We certainly still use the old methods in Poland, but there are modern ways to get much stronger and repeatable parts--like superplastic forming. We will make our new Coupe with super plastic forming.
David
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Aluminum Grade used by KMS
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