
Why don't metals bond when touched together? - laurent123456
http://physics.stackexchange.com/a/87109/17365
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InclinedPlane
This is a major reason why the practice of welding, and soldering, is so
complicated.

A lot of what makes welding and soldering complicated is the need to remove,
or avoid creation of, the oxide layers that inevitably form on the surfaces of
metals. More so, these oxides form faster at higher temperatures. That's why
solder wire and stick welding electrodes have flux cores or coatings, why
MIG/TIG welding uses a shielding gas, and so on.

~~~
toomuchtodo
I highly recommend everyone take a Welding 101 and 102 class at their local
community college. While my primary trade is IT, I'm certified to weld, and I
personally think its a lot of fun. It's about not only knowledge, but skill
and experience.

You'll be satisfied when you're able to successfully move the "puddle" of
molten metal down your weld line, the weld looks beautiful, and then you find
out your weld is extremely strong when its destructively tested.

~~~
gte910h
I've been wanting to do this. How expensive is the equipment to keep and
maintain?

~~~
toomuchtodo
Used welders in good condition can be had for under $1000. Depends on if its a
MIG, TIG, or "stick" (flux). Traditional welding relies on the shielding gas
being generated from a solid material (flux) wrapped around the consumable
electrode. Other methods (MIG and TIG) rely on inert gasses being provided by
separate support equipment to a "gun", which is also feeding electrode wire.

To take the class should only cost you book and tuition fees. My total out of
pocket at a community college in the Chicago suburbs was ~$600 for three
classes (Welding 101, 102, 110).

EDIT: I have a whole box of welding books. If you're serious, I'm more than
happy to ship them to you.

~~~
tptacek
Which suburb? My wife Erin took welding at Triton in Tyne near west suburbs
and liked it, but they shut the program down before she could take the
advanced class.

~~~
toomuchtodo
My classes were at Elgin Community College (I was living in Saint Charles at
the time). Unfortunately, its quite the haul from Chicago.

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ChuckMcM
This was the primary source of 'stiction' in the disk drive business. Platters
became optically smooth, read write heads similarly. When the heads "landed"
on a spot where the lubricant had migrated away from they would bind so
tightly the motor couldn't break them free. (old SparcStation drives you could
take them out. give them a sharp twist along the platter's rotational access,
and if you were lucky move the heads to a space they wouldn't bind to.)

~~~
careersuicide
Many summers years ago, during an internship, I got tasked with setting up a
development machine from old parts lying around the office. One of those parts
was an, even at the time, ancient hard drive. When I first powered the
computer up everything turned on just fine, but it couldn't recognize the
disk. I'd run into bad hard drives before, no big deal.I went to my bosses
boss, an older engineer who had seen a thing or two, and told him we'd need
another drive. He took the thing out and gave it a good hard twist, and told
me to try again. Lo and behold, it worked!

He explained that sometimes when older hard drives have been powered off for
many years the head some times sticks to the platter and doing what he did can
fix it. I could barely believe that the phenomenon was actually real. I've
spoken to a few people over the years about heads sticking to platters and it
turns out, while it isn't (or wasn't?) exactly common, it's not exactly rare
either.

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IvyMike
You're just not pushing hard enough.

[https://en.wikipedia.org/wiki/Explosive_welding](https://en.wikipedia.org/wiki/Explosive_welding)

~~~
elnate
As I understand it's not bonding chemically.

"The process does not melt either metal, instead it plasticizes the surfaces
of both metals, causing them to come into intimate contact sufficient to
create a weld. "

~~~
icegreentea
You don't need to melt the metal for there to be "chemical bonding". And now
for me to probably mangle what I remember from material science.

When you're dealing with metals (and alloys) you're pretty much dealing with a
lattice of atoms. It's not very useful to talk about individual atoms
"chemically bonding" with each other, or at least not the way you would talk
about say... bounding sodium and chlorine to get table salt, or nitrogen with
oxygens to get nitrates.

Instead, you have this lattice of metal atoms, within which you can
incorporate other atoms. Say you had a lattice of iron. By incorporating other
metals into the lattice (say magnesium, or nickle), or even other non-metallic
elements (most famously carbon) into the lattice, then you get different
alloys. But at no time are you actually bonding say an iron atom with a carbon
atom making a "steel molecule".

So when you weld dissimilar metals/alloys, what you're trying to do is to get
two dissimilar lattices to "blend" into each other. When you have a crappy
weld, one of the things that happen is you get a crappy boundary layer
(potentially of stuff like metal oxides) sitting between the two lattices.
Whereas if you have a "perfect" weld, the two lattices will seamlessly merge
into each other - you'll get this diffusion zone. In the case of two similar
metals, then the two lattices can actually perfectly merge - that is what
Feynman is trying to get at there.

So, back to the point. Creating a good weld is in many ways as close to
"chemically bonding" as you can get with metals / lattice structured
materials. Or at least, in a useful sense.

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tedsanders
Fun fact: ultrasonic vibration can weld metals together (and this is used in
modern wire bonders).

[https://en.wikipedia.org/wiki/Wire_bonding](https://en.wikipedia.org/wiki/Wire_bonding)

~~~
extempo
Another fun fact: there's a company using ultrasonic metal welding for 3D
printing. They call it Ultrasonic Additive Manufacturing.

[http://www.fabrisonic.com/](http://www.fabrisonic.com/)

------
tantalor
[http://en.wikipedia.org/wiki/Cold_welding](http://en.wikipedia.org/wiki/Cold_welding)

------
Balgair
[https://en.wikipedia.org/wiki/Cold_welding](https://en.wikipedia.org/wiki/Cold_welding)

Cold welding is a big problem, and a debatable one, in spacecraft engineering
due to the lack of oxides and the out-gassing of the oils in space vacuum.

------
meersoup
There is wringing:

[http://en.wikipedia.org/wiki/Gauge_block#Wringing](http://en.wikipedia.org/wiki/Gauge_block#Wringing)

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seanalltogether
Correct me if I'm wrong but isn't this why panning for gold in rivers was so
prevalent through history? Flecks of gold would naturally cold weld over a
millennium of being knocked around in the water.

~~~
wavefunction
I can't speak for all of history but at least in the US West gold panning was
used to locate rich ore veins upstream. The idea is that you pan for gold,
steadily moving up the stream until the relative amount of gold flakes drops,
meaning you've gone just a bit too far and you probably now have a great idea
of where a lot of gold is.

------
xd
Guage blocks[1] due to their near perfect flatness are thought[1] to adhere by
"Molecular attraction".

[1][http://en.wikipedia.org/wiki/Gauge_block](http://en.wikipedia.org/wiki/Gauge_block)

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IgorPartola
Is this not an obvious answer? I am genuinely curious why the impurities and
oxidation are not the answer the OP jumped to.

~~~
michaelt
When you know you have basic but not expert knowledge you can make an educated
guess (or several), but in the absence of empirical evidence you acknowledge
there could be other factors you haven't thought of.

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samatman
There is galling:

[http://en.wikipedia.org/wiki/Galling](http://en.wikipedia.org/wiki/Galling)

and galvanic corrosion:

[http://en.wikipedia.org/wiki/Galvanic_corrosion](http://en.wikipedia.org/wiki/Galvanic_corrosion)

In short, they do, given friction and/or moisture, and time.

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netman21
The hatches and frames on the Mercury and Gemini capsules had to be well
painted because the exposure to vacuum raised the danger of like metals
welding together.

