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Pure aluminum is soft. Almost everything useful about it comes from what gets mixed in and how it’s heat-treated afterwards. In the first episode of Grain Boundary, Sam and Dan walk through the aluminum alloy families from the 1000 to the 8000 series — what each one is good at, where you’ll find it, and how to choose one for a project.
In this episode
- 01:07 How alloy numbers and tempers (O, H, T6) work
- 02:12 1000 series: pure aluminum for wires and foil
- 02:52 2000 series: copper alloys for aircraft, and why planes are riveted
- 04:07 3000 series: manganese, cans and cookware
- 05:01 4000 series: silicon, welding wire and castings
- 05:33 5000 series: magnesium for boats and cryogenic tanks
- 06:11 6000 series: 6061 and 6063, the all-rounders
- 07:15 7000 series: zinc, the strongest aluminum alloys
- 08:17 How to pick an aluminum for your project
- 09:38 Myth: aluminum doesn’t corrode
Quick reference
| Series | Main addition | Example | Hardness (Brinell) | Typical uses |
|---|---|---|---|---|
| 1000 | None (99%+ pure) | 1100-O | Low 20s | Wire, bus bars, chemical tanks, foil |
| 2000 | Copper | 2024-T3 | ~120 | Aircraft fuselage and wing structures |
| 3000 | Manganese | 3003 | ~40 | Cookware, roofing, radiators, can bodies |
| 4000 | Silicon | 4043 | — | Welding wire, brazing |
| 5000 | Magnesium | 5052 / 5083 | ~60 (5052) | Boat hulls, trailers, fuel and LNG tanks, can lids |
| 6000 | Magnesium + silicon | 6061-T6 / 6063 | ~95 (6061-T6) | Bike frames, machine parts, extrusions, heat sinks |
| 7000 | Zinc | 7075-T6 | ~150 | Wing spars, carabiners, high-end bike and race parts |
| 8000 | Other (iron, lithium…) | — | — | Foil and packaging |
For comparison, mild steel is around 120 Brinell. Hardness depends heavily on temper: 6061 is about 30 Brinell in the soft O temper and about 95 in T6.
Transcript
00:00 Sam: Hey, welcome back to Grain Boundary, the show where we look at everyday stuff and ask, okay, but what is this actually made of? I’m Sam.
00:08 Dan: And I’m Dan. And today’s material, honestly, you’ve probably touched it like a dozen times already this morning.
00:14 Sam: Your phone.
00:16 Dan: Your phone, your laptop, the soda can, maybe your bike. The plane you flew on last month.
00:21 Sam: Aluminum!
00:23 Dan: Or aluminium, if you ask my British cousins.
00:26 Sam: We’re not doing that debate today.
00:29 Dan: Fair. Fair.
00:31 Sam: So, here’s what got me into this topic. When people say “aluminum,” they picture one material, right? One shiny, light metal. But there are literally hundreds of aluminum alloys, and they behave completely differently.
00:43 Dan: Yeah. Some of them are so soft you can dent them with your thumbnail. And some are harder than regular structural steel.
00:49 Sam: Which, when I first read that, I didn’t believe it.
00:52 Dan: It’s true though. And that’s kind of the whole story. Pure aluminum is actually pretty weak. It’s light, it doesn’t rust, it conducts heat and electricity really well, but it’s soft. Almost everything useful about aluminum comes from what you mix into it, and how you treat it afterwards.
01:07 Sam: Okay, so give people a map. How do engineers keep all these straight?
01:12 Dan: So, for wrought aluminum, which just means metal that’s rolled, or extruded, or forged into shape, there’s a four-digit numbering system. And the first digit tells you the main thing that’s been added. So you’ve got the one-thousand series, the two-thousand series, all the way up to eight thousand.
01:27 Sam: And then there’s that extra bit at the end. Like, the famous one, sixty sixty-one, T6.
01:34 Dan: Right, that’s the temper. It tells you how the metal was processed. “O” means annealed, so, fully softened. “H” means strain-hardened, basically it got tougher from being rolled or bent while cold. And “T” means heat-treated. T6 is heated, quenched, and then aged to squeeze out the most strength.
01:51 Sam: Got it. And one more quick thing. When we talk about hardness, we’ll mostly use the Brinell scale. You basically press a hard ball into the metal and measure the dent. Bigger number, harder metal.
02:02 Dan: And just so you have something to compare to, regular mild steel is around one hundred twenty Brinell.
02:07 Sam: So, keep one twenty in your head.
02:09 Dan: One twenty. Got it.
02:12 Sam: Alright, let’s start at the bottom. The one-thousand series.
02:15 Dan: This is basically pure aluminum. Ninety-nine percent or more. Alloys like eleven hundred. And when it’s fully soft, it’s somewhere in the low twenties on the Brinell scale.
02:25 Sam: So, what, a fifth as hard as mild steel?
02:28 Dan: Roughly, yeah. But that’s not really a flaw. This series has the best corrosion resistance and the best conductivity of the whole family. So you’ll see it in electrical cables, bus bars, chemical tanks, reflectors… and a lot of foil.
02:41 Sam: Like kitchen foil?
02:43 Dan: Kitchen foil’s usually either this series or the eight-thousand series, which we’ll get to. Either way, you want something that’ll roll super, super thin.
02:52 Sam: Okay. Two-thousand series. That’s copper.
02:55 Dan: Now we’re getting serious. Copper lets you heat-treat the alloy, and the strength just jumps. The classic one is twenty twenty-four. In the T3 temper, it’s around one-twenty Brinell.
03:06 Sam: Wait, so that’s already mild steel territory.
03:09 Dan: Yep. At about a third of the weight.
03:12 Sam: That’s wild.
03:14 Dan: And that’s why twenty twenty-four became the backbone of airplanes. Fuselage skins, wing structures. It’s also really good in fatigue. Meaning, it handles being flexed back and forth over and over, which is… exactly what a wing does all day.
03:28 Sam: Okay, so what’s the catch?
03:30 Dan: Two big ones. First, corrosion. Copper and aluminum don’t really get along, chemically. So aircraft sheet usually has a thin layer of pure aluminum bonded on each side. That’s called Alclad. And second, it doesn’t weld well.
03:44 Sam: Oh. Is that why planes are covered in rivets?
03:48 Dan: That’s a big part of it, yeah.
03:50 Sam: Huh. I always wondered.
03:52 Dan: There’s a cool modern twist too. Aluminum-lithium alloys. Lithium is the lightest metal there is, so a little bit of it makes the alloy lighter and stiffer. One of them, twenty-one ninety-five, was used in the Space Shuttle’s external tank, and in SpaceX’s Falcon 9.
04:07 Sam: Rocket aluminum. Love it. Okay, three-thousand series. Manganese.
04:13 Dan: This is the everyday workhorse. Manganese gives you a little more strength, but you keep the great formability and corrosion resistance. Thirty oh-three is the classic, around forty Brinell.
04:24 Sam: And where do I run into it?
04:26 Dan: Pots and pans, roofing, siding, car radiators. And the big one: the body of your soda can.
04:33 Sam: Oh, nice.
04:35 Dan: And here’s my favorite fact of the whole episode. The can isn’t one alloy.
04:39 Sam: What?
04:40 Dan: Nope! The body is a three-thousand series alloy, because it has to be stretched into this deep, thin cup. But the lid is a five-thousand series alloy, because it needs to be stiffer, to handle the pressure and the pull tab. So every soda can is actually two different aluminums.
04:56 Sam: Okay, that’s a great bar fact.
04:59 Dan: Use it wisely.
05:01 Sam: Four-thousand series. Silicon.
05:04 Dan: So silicon lowers the melting point and helps molten metal flow nicely. So you mostly see this series as welding wire and brazing material. Any aluminum welder knows forty forty-three.
05:15 Sam: And silicon is huge in cast aluminum too, right?
05:18 Dan: Oh, massive. Cast alloys have their own numbering, but the big names are A356, which is used for car wheels and engine parts, and A380, which is the most common die-casting alloy. So, power tool housings, transmission cases, that kind of thing.
05:33 Sam: Okay, five-thousand series. Magnesium.
05:37 Dan: I call this one the sailor’s aluminum. Magnesium gives you good strength without heat treatment, and amazing resistance to salt water. Fifty fifty-two is around sixty Brinell. And fifty eighty-three is a bit stronger, and it’s used for boat hulls and offshore structures.
05:52 Sam: And they weld?
05:54 Dan: Beautifully. Which is a big deal. You’ll also find this series in truck trailers, fuel tanks, and tanks for liquefied natural gas, because it actually gets tougher when it’s super cold.
06:04 Sam: Huh, that’s backwards from what I’d expect.
06:07 Dan: Right? Most metals get brittle in the cold. This one doesn’t.
06:11 Sam: Okay. Now the one I think most people have heard of. Six-thousand series.
06:16 Dan: Magnesium plus silicon. And sixty sixty-one, T6, might be the most popular aluminum alloy on the planet. About ninety-five Brinell. It’s not the strongest, it’s not the most corrosion resistant… but it’s good at everything. It machines well, it welds okay, it anodizes nicely, and it’s affordable.
06:35 Sam: The jack of all trades.
06:37 Dan: Exactly. Bike frames, camera rigs, scuba tanks, machine parts. And then its sibling, sixty sixty-three, is the extrusion king. A bit softer, but it squeezes through really complicated shapes with a gorgeous finish. So, window frames, railings, and those heat sinks with all the little fins.
06:55 Sam: And phones and laptops, right?
06:57 Dan: Very often, yeah. Actually, there’s a famous story there. A major phone came out with a thin six-thousand series body, and people found out it could bend in their back pocket.
07:07 Sam: Oh, I remember that!
07:09 Dan: And the next year’s model switched to a seven-thousand series alloy. Which brings us to the heavyweights.
07:15 Sam: Seven-thousand series. Zinc.
07:18 Dan: Zinc, plus magnesium, and usually some copper. These are the strongest aluminum alloys you can buy. Seventy seventy-five, T6, comes in around one hundred fifty Brinell.
07:28 Sam: Hold on. That’s harder than our mild steel number.
07:32 Dan: It is. And its strength rivals a lot of steels, at about a third of the weight. So you’ll find it in aircraft wing spars, high-end bike parts, climbing carabiners, race car parts.
07:42 Sam: Okay, so what’s wrong with it? There’s always something.
07:46 Dan: There’s always something. It’s more expensive. Its corrosion resistance is just okay, and some versions can crack under stress over time. So engineers often use a slightly different temper, T73, which gives up a little strength for a lot more durability. And you basically can’t weld it with normal methods. It’s more of a, carve it out of a solid block, kind of alloy.
08:06 Sam: And just to finish the list, eight-thousand series?
08:09 Dan: That’s the “everything else” bucket. Alloys with iron, or lithium, or other stuff. A lot of foil and packaging lives there.
08:17 Sam: Okay, let’s zoom out. Say I’m a listener and I actually need to pick an aluminum for a project. How do I think about it?
08:24 Dan: I’d ask myself a few questions. One: do I need to weld it? If yes, go with five-thousand or six-thousand series. Stay away from two and seven.
08:33 Sam: Two?
08:34 Dan: Is it going to see salt water or bad weather? Five-thousand is your friend. Six-thousand is fine if it’s anodized or painted.
08:42 Sam: Three, how strong does it need to be?
08:44 Dan: If you need the absolute best strength for the weight, seven-thousand. If you want good strength and easy everything, sixty sixty-one. And if strength doesn’t really matter and you just want to bend it or form it, three-thousand or one-thousand.
08:57 Sam: And if it’s a complicated shape?
08:59 Dan: Then you’re probably extruding it, so sixty sixty-three, or casting it, so something like A356.
09:07 Sam: Can I underline one thing for people? The temper really, really matters. Sixty sixty-one in the soft “O” temper is around thirty Brinell. The exact same alloy in T6 is about ninety-five.
09:19 Dan: Same metal. Three times harder.
09:21 Sam: Just from heating and cooling it the right way. That blew my mind.
09:25 Dan: And it goes the other way too. If you weld sixty sixty-one T6, the area right next to the weld loses a big chunk of its strength, because you’ve basically undone that heat treatment. Good designers plan for that.
09:38 Sam: Any last myths before we wrap up?
09:40 Dan: Just one. People think aluminum doesn’t corrode. It does! It just instantly grows this super thin, tough oxide layer that protects the metal underneath. And anodizing is basically growing that layer thicker on purpose. That’s also how you get those colorful aluminum parts.
09:56 Sam: Oh, that’s so cool. Okay, quick recap. One-thousand: pure and soft, for wires and foil. Two-thousand: copper, strong, for airplanes, but needs corrosion protection. Three-thousand: manganese, for cans and cookware. Four-thousand: silicon, for welding wire and castings. Five-thousand: magnesium, for boats and tanks. Six-thousand: the all-rounder. And seven-thousand: zinc, the strongest, for aerospace and high-performance stuff.
10:27 Dan: And always…
10:29 Sam: Always check the temper.
10:31 Dan: There it is.
10:33 Sam: That’s it for this episode of Grain Boundary. Thanks so much for listening, and we’ll see you next time.
10:38 Dan: Take care, everyone.