# Aircraft aluminum grade 7075 used for body armor in medieval setting

I've read a few discussion on the use of aluminum in medieval armor. There are a lot of people saying that aluminum is a bad choice for armor because it's too soft and brittle. There are many different alloys of aluminum available. Each one has properties specific to the task for which it is designed.

The premise of my novel is a 737 is transported into another universe with 12th century technology. The 7075 alloy on the wings and body of an aircraft are strong, nearly the same as hot rolled steel. The material can be annealed at 775 degrees and is then workable with a hammer. The wings are about a quarter inch thick. I'm thinking it can be fairly easily reworked into exceptional armor for the period.

• It better than nothjng, that for sure, and it has low melting point and can be cast, which is an advantage as well. Not sure about specific properties of 7075 however. Thickness on a low side if u wish to use it directly, probably okay against slashing, but not necessarly arrows. Hardness also important, it allows cone shapes to work as deflectors, without a projectile or weapon to bite into surface and render shape advantage less useful. Interesting question, not that simple to see a winner. Jun 6, 2021 at 10:49
• Annealing on this site suggests that annealing 7075 (7xxx types generally) is anything but easy. It is described as "extremely tricky and best avoided" and " prone to stress-cracking". 7075 itself is described as "This is fussy stuff to work with". Hard to see this been done successfully by a 12th century blacksmith, especially as it would be completely new to them. Jun 6, 2021 at 12:13
• Can i tempt you to send an SR-71 instead? The 0.5mm-1mm thick, corrugated Titanium skin will be cuttable (not workable!) using blackmith's tools. It will be bendable along the corrugations, making cylindrical segments reasonably easily. And a full suit of 1mm titanium should be about as sword and arrow resistant as a normal full plate steel suit, but will mass 1/8th as much. Jun 6, 2021 at 15:44
• With any metal armor in the 12th century, you're already leagues ahead. Your average conscript or even professional soldier probably only had cloth or leather armor while the wealthier knights could afford a hauberk and a metal bucket helm. Classical plate armor of any sort won't be around for another 100 to 200 years. Also, keep in mind that metal was expensive for a long period of history and metallurgy was rather bad back then. Jun 6, 2021 at 21:11

The main issue with aluminum is that you can't work it to display the dual properties of quenched iron-based alloys: though surface and elastic core.

That aside, in medieval time your only source of heat is burning carbon based substances. This means that carbon will end up in your final product. For iron-based alloys that's no big deal, carbon is an intended element of the alloy, but for aluminum it's not something you want

Carbon may occur infrequently as an impurity in aluminum in the form of oxycarbides and carbides, of which the most common is $$Al_4C_3$$, but carbide formation with other impurities such as titanium is possible. $$Al_4C_3$$ decomposes in the presence of water and water vapor, and this may lead to surface pitting.

Unless you have the proper metallurgic techniques to process it, aluminum cannot be used.

Also don't forget that for an armor you want a material which can plastically absorb a good amount of energy. Aluminum is pretty scarce under that point of view: while steel has a broad plastic area, aluminum goes from plasticization to rupture very soon.

As you can see from the reference stress-strain curves below

the amount of energy absorbed by aluminum before rupture, which is represented by the area under the curve, is smaller than the one for steel, and the elastic limit is reached earlier.