[namibj]

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.

[grogenaut]

Is the grain boundaries wander and it touches on the atomic level is it not mixing?

And thanks for the additional details

[pfdietz]

> No, you don't need to melt the metal to mix; it's just almost always easier to do it that way.

Except for welding aluminum, where friction stir welding does exactly that to solid metal.

[potato3732842]

The point of the friction is to generate heat.

[pfdietz]

Yes, but not to the point of melting in this case.

https://en.wikipedia.org/wiki/Friction_stir_welding

> Friction stir welding (FSW) is a solid-state joining process that uses a non-consumable tool to join two facing workpieces without melting the workpiece material.[1][2] Heat is generated by friction between the rotating tool and the workpiece material, which leads to a softened region near the FSW tool. While the tool is traversed along the joint line, it mechanically intermixes the two pieces of metal, and forges the hot and softened metal by the mechanical pressure, which is applied by the tool, much like joining clay, or dough.