Aluminium Alloys

EDIT: Disclaimer, this post probably is inaccurate. The characteristics given (especially heat treatment etc) probably contain mistakes. End EDIT.

or aluminum, as they say in the US.

It was a bit weird experience researching this on the internet a while back, so I thought maybe a short recap is useful.

Basically, clean aluminium is quite weak. If you add some other substances like copper, magnesium, silicon, zinc or manganese you get an alloy that has different properties. The alloys are numbered to make it easier to know what you’re working with:

  • 1000 series. Clean Al, so it’s cheap, weak and quite corrosion resistant. 1050 is very common.
  • 2000 series. Strong, but corrodes. Heat treated. 2014 and 2024 are the most usual ones, not weldable. Expensive.
  • 3000 series. Easily formable.
  • 4000 series. Used in welding sticks.
  • 5000 series. No need for heat treatment. Good corrosion resistance. Easy welding. Some are marine aluminium.
  • 6000 series. Intermediate all round heat treated. Common.
  • 7000 series. Strong, but corrodes. Heat treated. 7075 is an example. Not weldable. Expensive.
  • There are also hardening treatments that are important. -O is soft and -T6 is hard. Some materials need to be formed annealed and heat treated after for strength.
  • Different temperature profiles for hardening and tempering produce different strengths etc, for example air hardening after welding etc.
  • Easily corroded alloys are often coated with pure Al to help (Alclad). Anodizing helps too (you can do it at home for small parts).
  • There are big differences in formability, weldability  and machinability.

The 2000 and 7000 series are most interesting to me as strong sheets with no need of welding. Some forming is possible with heat treated sheets.

2090 is a new lithium aluminium alloy that is a few % lighter than 2024 but is a few % stronger and stiffer. SpaceX uses an Al-Li alloy, don’t know if it’s that. Since they use friction stir welding, traditional weldability is probably not important to them. I don’t know about machinability or heat treatments.

Here are some examples of a few specific alloys from the excellent matweb.com:

Density for all of these is around 2700 kg/m³, and modulus of elasticity around 69-72 GPa, except 2090 for which they are 2600 kg/m³ and 76 GPa.

It’s noteworthy that 1050 is about 3-5 times as strong as birch plywood, but 4 times as dense as well. Steels like 4021 or 4310 are perhaps 11-12 times as strong as plywood but 11-12 as dense too, meaning that at first brush, structures for the same purpose weigh the same no matter which one of these three materials are used.

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