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?


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

That is pretty normal behavior of most solids – hardly unique to iron particles. If you take a bunch of ice cubes melt them down and then freeze it, you’ll get a big piece of ice. There are some materials that in the presence of air, will burn before it melts. Others might sublimate from solid to gas and therefore not reform into one big block when cooled. Some chemical compounds will decompose into other compounds under heat.

Without reference, it is hard to know why you believe iron particles are different.


Atoms are basically clumps of positive electric charge surrounded by a shell of negative electric charge. Since those charges are equal in magnitude, at a long enough distance, they cancel each other out and the atom can be considered neutral.

But once you get atoms close enough, the relative distances between the negative electrons closest to two atoms, the positive protons in the nucleus, and the negative electrons on the other side, forms some interesting competing forces between like charges repelling and different charges attracting.

So basically at a distance, you have no real force, as they get closer there is a repulsive force that keeps them from apart, but if you overcome that force with enough energy, you can force them to bond, then repulsion kicks in again, keeping them at bonds length, but with even more energy you enter into fusion territory (generally, but not with iron specifically).

Where does that energy come from to overcome that initial repulsion? Heat.

Metal particles are made up of smaller crystals. When you put them in a jar, the crystals don’t bond together, the metal powder stays a powder.

Heat can soften the crystals and make them stick to each other better under pressure. This is called “sintering”, and it’s like making a snowball. The snowball isn’t a uniform solid, but the internal structures have been forced to link with each other.

Enough heat/pressure can cause the crystals to merge, a process called “forging”, where new crystal boundaries can be formed. This is like soaking your snowball in water and letting it refreeze. It’s much more solid.

Add enough heat, and you can melt the metal. The liquid can then be formed into any shape you want through casting, like wax or plaster. To overdo the analogy, this is melting the snow and freezing a new ice-sphere.

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.