Why Clean Metal Can Join Without Heat
A metal surface is not a perfectly smooth plane. Under a microscope, it has peaks and valleys, and its outermost atoms are usually covered by a thin film of oxide, adsorbed gas, or grime. That film keeps the atoms in one piece from getting close enough to bond with atoms in another.
In a vacuum, fresh metal surfaces are less likely to acquire a new coating from air. If the oxide and contamination are also absent or removed, and the surfaces are pressed into intimate contact, atoms at the boundary can form metallic bonds across it. The two pieces then behave more like one continuous piece of metal. This is called cold welding: the joining happens without melting the metal.
Contact Is the Crucial Part
Vacuum alone does not fuse ordinary metal objects that happen to drift together. Their microscopic roughness may leave only a few points touching, and even a very clean surface needs close contact for atoms to bond across the boundary. Pressure, sliding, or repeated contact can help bring more of the surfaces together; heat is not the essential ingredient.
The effect is especially associated with matching metals because their atoms fit readily into the same metallic structure, though clean surfaces of different metals can also bond under suitable conditions. In practical equipment, surface films and protective coatings are therefore useful barriers: they keep moving or touching metal parts from sticking when they should remain separate.
Space does not supply a welding force. Its vacuum helps preserve the conditions for bonding by removing the surrounding air that would otherwise coat exposed metal. The surprising consequence is that, in the right circumstances, two solid pieces can join at their boundary without ever being heated until they glow.
