Cold welding

Cold or contact 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.

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 under vacuum.

Process

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 faying surfaces are de-greased, 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[1], 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.[2]

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

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

Many gold nuggets are formed by cold welding in nature since gold has no protective oxide skin to inhibit junction formation. They form in streams and rivers by impact action from stones carried by the flow of water on the tiny flakes of gold washed from deposits upstream. Owing to the high density of gold, they tend to remain in pockets in the bed of the stream where further flakes are thrown and aggregate with the growing nuggets. Such nuggets can grow to very large sizes and are highly prized by prospectors.

In space

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.).

Cold welding at nanoscale

Unlike cold welding process at macro-scale which normally requires large applied pressures, scientists discovered that single-crystalline ultrathin gold Nanowires (diameters less than 10 nm) can be cold-welded together within seconds by mechanical contact alone, and under remarkably low applied pressures [4]. High-resolution transmission electron microscopy and in situ measurements reveal that the welds are nearly perfect, with the same crystal orientation, strength and electrical conductivity as the rest of the nanowire. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface diffusion. Nanoscale welds were also demonstrated between gold and silver, and silver and silver, indicating that the phenomenon may be generally applicable and therefore offer an atomistic view of the initial stages of macroscopic cold welding for either bulk metals or metallic thin film