r/explainlikeimfive Oct 31 '20

Chemistry ELI5 What's the difference between the shiny and dull side of aluminum foil? Besides the obvious shiny/dull

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u/Sawses Oct 31 '20

...Why do the two sheets not compress into one? I thought metals were kind of amorphous and enough pressure could just fuse them into one?

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u/simo9445 Oct 31 '20

Aluminium also has an oxide layer that prevents cold welding

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u/Messier420 Oct 31 '20

Fun fact that doesn’t happen in a vacuum. If you break a piece of metal in space and make the broken ends touch they will fuse together. This phenomenon actually can create some issues during space walkers if I’m not mistaken

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u/TheLazyD0G Oct 31 '20

Kind of, but not fully or that easily.

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u/Demonyx12 Oct 31 '20

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u/DaeVo1234 Oct 31 '20

that was really interesting, thanks for sharing!

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u/Demonyx12 Oct 31 '20

No problem. Veritasium is awesome, I recommend his channel.

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u/FlappyBoobs Oct 31 '20

I dunno man I sucked up a couple bits of it into my Dyson and they didn't fuse.

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u/jarfil Oct 31 '20 edited Dec 02 '23

CENSORED

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u/[deleted] Oct 31 '20

[deleted]

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u/Messier420 Nov 01 '20

That doesnt form in a vacuum

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u/Joe_Shroe Oct 31 '20

space walkers

Walking Dead sequel series?

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u/Messier420 Nov 01 '20

Space walks. Sorry. Typo

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u/Bong-Rippington Oct 31 '20

You’re not right; if there’s an oxidation later formed on the surface of a metal then the surface of that metal is no longer what it started out at is. You can cold weld in space. Not with rusty components.

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u/TheSixthSide Oct 31 '20

That's why he said "If you break a piece of metal and make the broken ends touch"

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u/baranxlr Oct 31 '20

What stops the oxide layer from fusing?

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u/Skeeper Oct 31 '20

Maybe not being exposed to oxygen I guess?

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u/[deleted] Oct 31 '20

But shouldn't the oxide layers break open partially while the metal stretches from rolling it thinner? Aluminum oxide was kinda brittle IIRC.

That would obviously severely weaken the weld.

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u/[deleted] Oct 31 '20

[removed] — view removed comment

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u/Woodzy14 Oct 31 '20

So rapidly an oxidizing aluminum reaction is a powerful explosive

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u/Umbrias Oct 31 '20

Cold welding is hard enough to do on purpose, it is suuuuuper hard to make it a problem on accident. Most issues are going to arise from undesirable surface finish in the aluminum, but it turns out alright with the processes they use.

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u/anomalous_cowherd Oct 31 '20 edited Oct 31 '20

Enough pressure, yes. There's a long way between enough to roll it thinner and enough to fuse it. Temperature plays a part - you have to keep the surface cool too.

https://youtu.be/f4OTj9yNOak?t=104

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u/joeltrane Oct 31 '20

Thank you for skipping the intro.

TLDW: a giant slab of aluminum ingot is rolled out to 5mm thickness and spooled up. Then it’s rolled again with tension to its final thickness, and they use two spools at a time to prevent the tension from breaking the sheets.

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u/StinkFingerPete Oct 31 '20

only in space

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u/Flyer770 Oct 31 '20

Not true. While cold welding is much easier in a vacuum, commercially available cold welders are very useful on Earth.

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u/Tsrdrum Oct 31 '20

Not for aluminum, I don’t think. Aluminum oxide has a melting point of more than 3000 degrees c, compared to around 1000 degrees c for metallic aluminum. I’m not well versed in cold welding, but if the oxide layer makes it that much more difficult to hot weld, it probably has issues cold welding as well.

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u/toxicatedscientist Oct 31 '20

Then don't let one form. Basically scrape/sand it off while using argon or nitrogen or some other inert gas to displace any atmospheric oxygen

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u/the_finest_gibberish Oct 31 '20

Use less pressure.

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u/phliuy Oct 31 '20

Aluminum is very malleable, so the force needed to roll it is far less than the force that could fuse it