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I love the super-rigidity found in aluminum squares and straight-edge rules. I've purchased cold rolled steel flat bars that flex far easier. In fact, when applying force against the thin edge of these aluminum squares and rules, I cannot get them to flex at all.
How do they get the aluminum to be this rigid and strong? What kind of aluminum is this? It would be great for a project I am currently working on.
Aluminum is made of aluminum - processed from rocks. This is actually a great "How It's Made" Episode. There is very little that really changes with most aluminum that you buy other than size and shape. I am sure there are slight differences in strength due to tempering processes and coating but this is usually minimal. If some aluminum is a lot "softer" than others it is probably because it is not pure aluminum.
Steel is an alloy. There are literally thousands of different steel mixtures. Depending on how they want the steel to turn out they add (or take out) different minerals - including aluminum sometimes. Steel is basically a mostly iron alloy and a lot of its properties are subject to the amount of carbon. I think if the carbon amount hits 2-3% steel becomes very rigid but also is very brittle. A ruler would not have a high carbon % in the alloy because it would need to be very flexible (relatively).
Steel for the most part is "stronger" than aluminum but it depends on the alloy composition and the tempering process (which has a stronger affect than it does on aluminum). The strongest steel is much stronger than strongest aluminum.
When you buy 'aluminum' extrusions or find a tool made of 'aluminum' at the hardware store, you can be virtually certain that it is not pure aluminum but is instead an aluminum alloy. Pure aluminum is not very strong by itself. However, there are a number of different aluminum alloys for different applications, many of which are quite strong. Some of these can be further modified in useful ways using heat treatments. You can read about some of the common aluminum alloys here Online Metals Aluminum Guide (no affiliation other than past customer). The 6000 series alloys are commonly used for extrusions and other structural components where strength is important but cost is also a factor.
If you are looking for a similar material it may be useful to find a specialty metal supplier near you as they would have a much wider selection of shapes and sizes than the big box store, and you can probably get some advice on what alloy is appropriate for your project. Also they will cut it to your specifications for a small charge. I have had good luck doing this even when buying small quantities.
