Cold welding
Encyclopedia
Cold or contact welding is a solid
-state welding
process in which joining takes place without fusion
/heating 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
. Newly discovered micro- and nano-scale cold welding has already shown great potential in latest nanofabrication process.
s).
Many gold nugget
s 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.
s 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.)."
Three years following this 2006 post the European Space Agency published a peer reviewed paper detailing why cold welding is a significant issue that spacecraft designers need to carefully consider. The conclusions of this appropriately titled study can be found on page 25 of "Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum". The paper also cites a documented example from 1991 with the Galileo spacecraft high-gain antenna (see page 2; the technical source document from NASA regarding the Galileo spacecraft is also provided in a link here).
One source of difficulty is that cold welding does not exclude relative motion between the surfaces that are to be joined. This allows the broadly defined notions of galling
, fretting
, sticking, stiction
and adhesion
to overlap in some instances. For example, it is possible for a joint to be the result of both cold (or "vacuum") welding and galling (and/or fretting and/or impact). Galling and cold welding, therefore, are not mutually exclusive.
s (diameters less than 10 nm) can be cold-welded together within seconds by mechanical contact alone, and under remarkably low applied pressures . 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
[1].
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
-state welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
process in which joining takes place without fusion
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
/heating at the interface of the two parts to be welded. Unlike in the fusion-welding processes, no liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
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
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
would strongly adhere if brought into contact under vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. Newly discovered micro- and nano-scale cold welding has already shown great potential in latest nanofabrication process.
Applications
Applications include wire stock and electrical connections (such as Insulation-displacement connectorInsulation-displacement connector
An Insulation-displacement connector, insulation-displacement technology/termination or insulation-piercing connector is an electrical connector designed to be connected to the conductor of an insulated wire or cable by a connection process which forces a selectively sharpened blade or blades...
s).
Many gold nugget
Gold nugget
A gold nugget is a naturally occurring piece of native gold. Watercourses often concentrate and grow the nuggets. Nuggets are recovered by placer mining, but they are also found in residual deposits where the gold-bearing veins or lodes are weathered...
s 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
Impact
Impact may refer to:In science :* Impact , a high force or shock over a short time period* Impact crater, a meteor crater caused by an impact event* Impact event, the collision of a meteoroid, asteroid or comet with Earth...
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 satelliteSatellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s were sometimes attributed to cold welding. However, in 2006, Henry Spencer
Henry Spencer
Henry Spencer is a Canadian computer programmer and space enthusiast. He wrote "regex", a widely-used software library for regular expressions, and co-wrote C News, a Usenet server program. He also authored The Ten Commandments for C Programmers. He is coauthor, with David Lawrence, of the book...
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.)."
Three years following this 2006 post the European Space Agency published a peer reviewed paper detailing why cold welding is a significant issue that spacecraft designers need to carefully consider. The conclusions of this appropriately titled study can be found on page 25 of "Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum". The paper also cites a documented example from 1991 with the Galileo spacecraft high-gain antenna (see page 2; the technical source document from NASA regarding the Galileo spacecraft is also provided in a link here).
One source of difficulty is that cold welding does not exclude relative motion between the surfaces that are to be joined. This allows the broadly defined notions of galling
Galling
Galling usually refers to the adhesive wear and transfer of material between metallic surfaces in relative converging contact during sheet metal forming and other industrial operations....
, fretting
Fretting
Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...
, sticking, stiction
Stiction
Stiction is the static friction that needs to be overcome to enable relative motion of stationary objects in contact. The term is a portmanteau of the term "static friction", perhaps also influenced by the verb "stick"....
and adhesion
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....
to overlap in some instances. For example, it is possible for a joint to be the result of both cold (or "vacuum") welding and galling (and/or fretting and/or impact). Galling and cold welding, therefore, are not mutually exclusive.
Nanoscale
Unlike cold welding process at macro-scale which normally requires large applied pressures, scientists discovered that single-crystalline ultrathin gold NanowireNanowire
A nanowire is a nanostructure, with the diameter of the order of a nanometer . Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important —...
s (diameters less than 10 nm) can be cold-welded together within seconds by mechanical contact alone, and under remarkably low applied pressures . High-resolution transmission electron microscopy
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
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
Nanowire
A nanowire is a nanostructure, with the diameter of the order of a nanometer . Alternatively, nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important —...
. The high quality of the welds is attributed to the nanoscale sample dimensions, oriented-attachment mechanisms and mechanically assisted fast surface diffusion
Surface diffusion
Surface diffusion is a general process involving the motion of adatoms, molecules, and atomic clusters at solid material surfaces. The process can generally be thought of in terms of particles jumping between adjacent adsorption sites on a surface, as in figure 1...
.
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
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
[1].
Further reading
- Kalpakjian, Manufacturing Engineering and Technology 5th Edition. 981.