Cold welding

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Cold welding or contact welding is a solid-state welding process in which joining takes place without fusion or heating at the interface of the two parts to be welded. Unlike in fusion welding, no liquid or molten phase is present in the joint.

Cold welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact while in a vacuum (see Van der Waals force). Newly discovered micro-^[1] and nano-scale cold welding^[2] has shown potential in nanofabrication processes.

The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to "know" that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms "know" when they are not on the same part.

— Richard Feynman, The Feynman Lectures on Physics, 12–5 Friction

Applications include wire stock and electrical connections (such as insulation-displacement connectors and wire wrap connections).