I know aluminium was extremely hard to refine until the 20th century, which is one of the reasons why it wasn't used much. But what are the drawbacks to using it as armor or weapons in this type of setting? Would it be too brittle, or tough to smith into shape?
Aluminium is too soft to build strong armor from it. On the Mohs scale aluminium just has a 2.75 while iron, for example has 4, copper has 3.
The Mohs scale is about how hard it is to scratch a given material and the higher the number the harder is the material. That means that while copper can scratch aluminium the opposite is not true.
Edit: While there are multiple properties to look at none of them alone shows what is good for armor and the kind of armor you want to craft.
Let's look at toughness:
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing
So assume you want to craft rigid armor. If it is as soft as aluminium but has a high toughness any meaningful blow will dent it without fracturing. No fracturing is good but a big dent in the breastplate means you can't breathe freely anymore. That has happened a lot with iron/steel armor in medieval times but it would be much worse with aluminium. Because of that soft but tough materials are just good for flexible armor that doesn't keep the dent, or at least not over a large area.
Back to aluminium. Aluminium armor could be good for presentation purposes like for ceremonial guards. The armor looks good, needs little maintenance and is light. If those kinds of armor ever see combat it would be short skirmishes not prolonged battles. So the problem described above would not matter as much as for armies.
There are other kinds of armor that might be enhanced by addition of aluminium like brigandine or jack of plates where metal plates are added to cloth. In those the malleability isn't as much of a problem and the lesser weight could be beneficial.
For a weapon aluminum is a poor choice since it can't hold an edge well and, due to the low density, can't concentrate force like iron (in a warhammer, for example). For PARTS of a weapon, it might be serviceable.
As for armor, depends on what you want it for. You can make chain mail from aluminum and it is comparatively light. This may work to resist slashing attacks. However it doesn't have the strength to stop piercing attacks, like the thrust of a dagger or an arrow head. The rings will spread and burst. So you can double or even triple up on the rings (or make them very thick) but then the armor starts to weigh as much as iron or steel. Same with a breastplate. Ok for turning a slash but unable to stop a thrust (either punch through or deform and crush the soft tissue underneath). This can be easily demonstrated with aluminum cookware versus stainless steel. The rigidity and strength of steel versus the same thickness of aluminum is readily apparent. And the aluminum cookware is significantly lighter.
One benefit will be a reduction in rust, as well as lighter weight for the same thickness of metal and the ability to take on a high polish. So aluminum would probably be great for a child's suit of armor or a parade suit, made for flash, not efficacy. Or if you are facing Magneto :)
Yes, when thick enough The Aluminum castings we use my factory are about about 8mm thick and quite tough. It's unlikely any sword and strong man will be able to do much other than bend it a bit. If these castings weren't so expensive, I'd take it out back and try to smash it for empiricism. I have broken (snapped) a 4mm aluminum cover by tightening bolts out of sequence, but there where factors such as torque and leverage at play which would not be involved in the physics of a sheer strike. In a related point, Plate Armor is designed to dent! chest plate fits over padding at a distance from the wearer's body. If the armor absorbs a strike, it has room to bend without hitting the occupant. A semi-soft metal may be a benefit to armor because of the ability to bend instead of break. Given sufficient thickness, Aluminum Armor is plausible.
One weakness of aluminum versus iron or steel is its lack of a real fatigue limit (aka Endurance Limit.) Fatigue is the accumulation and enlargement of small cracks and imperfections when the material is stressed repeatedly.
To cause fatigue in iron and steel you have exceed a certain threshold of stress. As long as the stress is below that point, the metal does not weaken, no matter how many times you repeat the stress.
The problem with aluminum is that it is subject to fatigue from much smaller stresses than steel. Even a small stress will cause some fatigue, and over time as the stress repeats, the fatigue increases and eventually the metal breaks.
Take a steel bar and an aluminum bar of equal strength, and repeatedly bend them both back and forth. The aluminum bar will weaken and break much sooner than the steel bar.
In the case of armor, repeated stresses both from impact and from normal stresses of marching, riding a horse, and especially the stress from repairs will cause fatigue, which could cause sudden, surprising failure.
Another problem with aluminum from a medieval perspective is that it's much more difficult to forge weld due to its lower melting point, more rapid oxidation, and the inability to use the color of the glow as a gauge of temperature. Aluminum is highly reactive with oxygen, and quickly forms a coating of oxide which will prevent a good weld.
One further small issue with aluminum is its lower mass. A more massive object has more momentum than a lighter one. It's simply harder to push around. Of course, lighter armor has plenty of benefits, easier to put on, less tiring, and so forth, so this is a minor disadvantage at worst.
Aluminium is a relatively soft, durable, lightweight, ductile, and malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness.
Emphasis mine. Medieval armor design was based on hardness, stiffness of steel. Weapon needs hard edge, too.
Aluminium has about one-third the density and stiffness of steel.
So to have the same stiffness, you basically need the same mass, and more thickness - only drawbacks.
Of course, modern aluminium alloys are better than that. At the same time, modern steel varieties are better, too.
In the case of the Bradley, the aluminium (presumably an alloy, most references to aluminium actually refer to Al-based alloys) is laminated. Wikipedia doesn't say what with, but in your case even a thin skin of steel over an aluminium plate could give a hard surface over a tough backing. This would be like ironclads, in which much of the strength came from wood, hence clad. Such an approach would be lighter than steel alone. Aluminium frames, even hollow tubes bent into shape could form a strong structure to hold its shape against impact and support a shell of steel over aluminium.
As Al alloys are quite easy to work, the steel skin might even be chainmail.
Inspired by your corundum answer: You could even replace the steel (if casting the aluminium) by a suitably hard stone embedded in the matrix. Of course individual chunks would get knocked out and broken, but so do ceramic trauma plates in modern armour (wikpiedia again) and they're widely used.
TL;DR: Aluminum armor is plausible in the form of corundum.
As many others have mentioned, pure aluminum foil/sheets is a poor (though light) armor choice. However, if someone really wants aluminum armor, we have another option. I understand it's not exactly what the question was answering, but let's explore it anyway for the benefit of others.
Crystallized aluminum oxide, better known as corundum, is a surprisingly feasible armor choice. It's extremely strong, the definition of a 9/10 on the Mohs scale. Normal steel has a hardness of 4-4.5 and hardened steel has a hardness of 7.5-8. What this means is that a steel blade, even a tempered one, wouldn't scratch the armor. In fact, the armor would be more likely to scratch it!
However, I understand hardness isn't the only consideration for armor material choice. We also must consider its weight. The specific gravity (A highly specific version of density) for corundum is around 4.1 The specific gravity of steel varies slightly, but is normally 7.7-7.8. This means that corundum armor would be nearly half as heavy as steel armor, weighing in at only around 26 kg!
We should also consider the availability of the material and manufacturing costs. This is where steel has a clear upper hand. Corundum is found only rarely (It's literally sapphires and rubies) and only in small sizes (rarely over 4-5 grams). This means that you couldn't have an armor made entirely of corundum (except if you allow magic, which I'm doubting based on the question), but you could have armor made entirely of small corundum gemstones inset in steel links.
1 I tried converting this to density and then multiplying by the volume of a suit of plate armor, but then I realized two things: 1. You can just compare the SGs, and 2: The volume of plate armor is really difficult to find.
The simplest problem is that it would have been cost a king's ransom to make. As you say it was hard to refine, and as a result of that, incredibly expensive.
emperor Napoleon III reserved a prized set of aluminum cutlery for special guests at banquets. (Less favored guests used gold knives and forks.)
This is quoted everywhere, unfortunately I can't find an original source, but this was already the 19thC. The earlier you go the less commonly available it is.
To disagree with the other stated ideas here, I think Aluminum could be a surprisingly good armor material.
Disclaimer: I don't have great sources on this and I am definitely not a materials engineer so please correct me if I'm wrong
From what I can tell aluminum is about a third as dense as steel, about a third as stiff as steel, and about half as hard as steel. That may sounds like a bad material, and it is if it is difficult to make in large quantities, but it also has some advantages.
- Because it is a third as heavy, you should in theory be able to make it about three times thicker than a steel alternative while being about as easy to wear and use. This more than makes up for the lack of hardness, and actually has a significant advantage just by being thicker. That advantage is that when slicing through it, you have to pass through three times as much material, which means three times as much friction and mass to displace, which means that you may actually have better piecing/slashing protection from aluminum armor than from steel armor by weight.
- Because aluminum is softer than steel, it is more likely to deform rather than totally break. This is particularly good when dealing with blunt impact because it absorbs a portion of the energy of the blow by bending, similar to crumple zones in a car
- Another advantage of being softer is that it could in theory be easier to shape for the smith, which would also potentially make it easier to repair when damaged. This may be contravened by needing to be thicker, but I don't actually know that much about smithing, so I'll leave that question to someone with expertise.
- If you need lighter armor at the expense of protection, aluminum would make for great chain mail. It would still do well enough for glancing/slashing blows, though it would do much worse than iron or steel at stopping a piecing weapon. The major concern (and I don't know how this would work in reality, but I would love to see a practical test), would be if the steel slashing weapon (lets say a knife) might just cut through the aluminum rings and get to you anyway.
Also, just to make this answer complete, I agree with the other answers about using aluminum as a weapon material. That would not work well because being soft makes it not hold an edge, and bending on impact is bad for a weapon for the same reason it is good for a set of armor, it spreads out the force of the impact. Additionally, if you are trying to cut someone else wearing iron or steel armor, your aluminum weapon won't be able to penetrate it at all.
Chris H already has pointed out that aluminium is used is some modern armour, so this is just about weapons.
If you look at a modern climbing axe, it has an aluminium shaft. If you look at a modern javelin, it (often) has an aluminium shaft. If you look at a modern trenching spade, it has (often) an aluminium shaft. Aluminium is much better than wood for pole arms which won't break, though you probably will want the tip or blade to be something which is harder.
The only true benefit for using aluminum is that it is light, but in the case of body armor, that's not necessarily too good of a thing. Of course, this isn't the main purpose of armor, but heavy armor prevents your opponent from knocking you back too much. Most importantly, though, is that aluminum is used because of how easy it is to mold and bend, the exact opposite of what you want for armor.