I know nothing about welding, what is the distinction?
I clicked on that Wiki link and it said cold welding is "welding process", and the hyperlink to the regular welding page includes solid-state welding, which mentions cold welding.
Cold welding is unintentional, spontaneous joining of two metal parts in vacuum. You don't want that to happen, especially if the parts are meant to move.
Normal welding is intentional application of heat to partially melt two parts at the seam, so that they "mix" in semi-liquid state and become one part when they solidify. Welding may or may not use a third material (solder) to aid the process.
>Cold welding is unintentional, spontaneous joining of two metal parts in vacuum.
Is the only distinction the intention, though?
Because I saw some examples of industrial applications of cold welding, so I'm still not quite getting why cold welding isn't considered welding (I have been googling since my original comment, but not finding anyone making this same distinction).
my understanding, welding is where the metals mix and bond directly. Solder is where they flow and fill the gaps completely, but they're not mixing. According to the webs welding is where both metals melt and mix. Soldering is where only the solder melts. I think you can imagine that if you flow a solder completely between two piece of metal then there'd be no air and it'd basically a really good cold weld. Imagine sticking two pieces of wood together with peanut butter. The PB doesn't actually chemically bond to the wood nor does it merge with the wood.
Cold welding in my understanding is where the metals are very very flat against each other and held togehter by that. like when you have 2 sheets of paper against each other and try to pull them apart without sliding them off of each other or grabbing corners.
In a good weld you actually make 2 (or 3) pieces of metal into one.
No, you don't need to melt the metal to mix; it's just almost always easier to do it that way.
As soon as you cause conditions under which grain boundaries wander, you get welding if the two pieces happen to touch at an atomic/molecular level.
Note explosion welding: you use a shockwave to hold the pieces together while aggressively dislocating grain boundaries. The result is a (very good) weld. Many glues involve welding behavior, especially if they are used without waiting minutes to hours for the bond to harden before loading it.
For example, "contact cement" (polychloroprene glue) works by precipitating a polychloroprene layer from a solvent into the surface pores of both to-be-bonded parts, letting all the solvent dry, and then forcing such prepared surfaces together to cause intimate interaction of the polymer chains on the surfaces to weld into a single layer of polychloroprene that's solvent-soaked into both it's sides (which wouldn't be possible unless the materials are extremely porous).
However, solids are very bad at wetting surfaces, so you will have a hard time getting the needed atomic contact.
Welding isn't really applicable to composites like wood or paper.
Normal welding needs heat to melt the metals. Cold welding happens without heat. Two metal parts will cold-weld on any smooth, touching faces if the air molecules that keep the two separated disappear.
Sure, I mean I read both the wikipedia pages so I've understood that. And I'm not trying to be obtuse here, honestly, but that still doesn't really help me understand why "cold welding, despite the name, _is not welding_".
Your description, and wiki's, both sound like it is welding.
And after spending an embarrassing amount of time last night going down a welding rabbit hole, I have not seen anyone else claim that "cold welding is not welding". I'm pretty sure it is.
Cold welding is basically galling like you get with stainless on stainless under high pressure. Uniformity and consistency is hard to achieve so besides accidentally sticking things that shouldn't be stuck and the lightest duty applications the usefulness is limited.
I wonder if cold welding could be useful in space, though. Make the two surfaces totally flat, clean them in vacuum, and then press them together to weld them.
Ok, "clean them in vacuum" is kind of a "now draw the rest of the owl" type of thing. But I wonder what's possible when you don't have an atmosphere to mess up the surface. Could you scrape off the oxide layer of aluminum, for example, and get it smooth enough to cold weld without worrying about it re-oxidizing because there's no air?
This is the hard part. They need to be flat to less than atom size over whole area, at least several square centimeters. We can't do that yet economically. If they are not so flat, your weld will be pretty weak.
BUT if you could make them flat enough and then wiggle them ultrasonically so that those almost flat surfaces rub the rest of bumps, that would probably not require a lot of heat and energy to make a pretty good connection.
In order to make a surface properly flat you need to have the cleave surface line up with crystal boundaries, but crystals are randomly oriented in three dimensions meaning the surface can't be truly flat.
And that's even assuming the solid you're working with is crystalline of the sort that can do this. Many materials are alloys meaning that cold welding would be further difficult.
I clicked on that Wiki link and it said cold welding is "welding process", and the hyperlink to the regular welding page includes solid-state welding, which mentions cold welding.