Cold or contact 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 under vacuum.
Cold welding is a solid-state welding process in which joining takes place without fusion at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no liquid or molten phase is present in the joint.
In cold welding, pressure is applied to the workpieces through dies or rolls. Because of the plastic deformation involved, it is necessary that at least one (but preferably both) of the mating parts be ductile. Prior to welding, the interface is degreased, wire-brushed, and wiped to remove oxide smudges. Cold welding can be used to join small workpieces made of soft, ductile metals.
It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the workpieces, or any combination of the above. Research has shown that even for very smooth metals, only the high points of each surface, called asperities, touch the opposing piece. Perhaps as little as a few thousandths of a percent of the total surface is involved. However, these small areas of taction develop powerful molecular connections; electron microscope investigations of contact points reveal that an actual welding of the two surfaces takes place after which it is impossible to discern the former asperitic interface. If the original surfaces are sufficiently smooth, attractive van der Waals forces between contact points eventually draw the two pieces completely together and eliminate even the macroscopic interface.
Exposure to oxygen or certain other reactive compounds produces surface layers that reduce or completely eliminate the cold welding effect. This is especially true if, for example, a metal oxide has mechanical properties similar to those of the parent element (or softer), in which case surface deformations do not crack the oxide film. The reason cold welding does not normally occur between metals on earth is because there is a very fine layer of oxidized metal due to the atmosphere. Even when a metal is put into a vacuum, this layer does not disappear without wire-brushing.
Applications include wire stock and electrical connections (such as Insulation-displacement connectors).
Mechanical problems in early satellites were sometimes attributed to cold welding. However, in 2006, Henry Spencer stated that the phenomenon of spontaneous cold welding in outer space is "basically a myth", pointing out that "there are no documented cases of it actually occurring in orbit, except in experiments deliberately designed to provoke it (with susceptible materials, great care to avoid contamination, and deliberate mechanical removal of oxide layers, etc.)."