What’s the difference that makes melted iron particles stick together into a solid shape vs simply gathering a pile of loose iron particles into a shape that can easily scatter apart?
In: Physics
Metals are actually rather sticky, especially towards similar metals. Metal parts have been known to seize together or gall in the vacuum conditions in orbit around the earth. In the early days of space exploration in the 1950s and 60s this fact caused some notable mechanical failures. Therefore metal items in space usually need to receive some kind of nonmetallic coating any area they’re liable to rub or slide against other metal parts. Hinges for example. However using a plastic or ceramic spacer as a bearing works even better.
You can heat two metal plates to dull red hot in a vacuum, then apply pressure. They will slowly fuse into a homogenous solid provided the vacuum is high quality. This is called “diffusion bonding”
So a more instructive question is, what **stops** metals from bonding and sticking to each other on earth, under normal atmosphere?
The reason is that basically all metals have a thin layer of oxides, hydroxides, and absorbed water vapor and nitrogen atoms on their surface. Even gold, while it doesn’t react with the air and form oxide compounds, will still attract legions of air and water molecules onto it’s surface. So you can’t cold weld gold despite it’s softness and unreactivity. Oxides in particular tend to have strong negative surface charges. So they repel each other and prevent metals from bonding.
Even in space you need some kind of process that works to scub off the layer of surface oxdes and other impurities that was formed down in the atmosphere when a spacecraft was being built. But two parts rubbing against each other repeatedly will do.
Since before recorded history, smiths have taken advantage of this kind of solid state welding. You can apply a *flux* to two iron or copper objects when they’re red hot. This is a mixture of salts such as fluorite, borax, table salt, some fine sand, and burned lime that can dissolve metal oxides like iron or copper oxide, and also slowly etch the metal surface. The etching action removes metal atoms from the surface before a layer of oxides can reform. Once the flux is applied, it melts, after more heating in the forge, the two items are vigorously hammered together and fuse well below the melting point of the metal.
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