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((Short description|Welding process in which joining occurs without melting or heating the interface))
((Distinguish|cold fusion))
((More citations needed|date=October 2010))
[[File:HidegHegesztes.jpg|thumb|Cross-section of cold welding((snd))before and after the weld]]

'''Cold welding''' or '''contact welding''' is a [[solid]]-state [[welding]] process in which joining takes place without [[melting|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#Van_der_Waals_forces_between_macroscopic_objects|Van der Waals force]]). Newly discovered micro-<ref>((cite journal |jstor=2879122 |title=Contact Adhesion of Thin Gold Films on Elastomeric Supports: Cold Welding Under Ambient Conditions |journal=Science |volume=253 |issue=5021 |pages=776–778 |last1=Ferguson |first1=Gregory S. |last2=Chaudhury |first2=Manoj K. |last3=Sigal |first3=George B. |last4=Whitesides |first4=George M. |s2cid=10479300 |year=1991 |doi=10.1126/science.253.5021.776 |pmid=17835496 ))</ref> and nano-scale cold welding<ref name="gold-nanowires">((cite journal |doi=10.1038/nnano.2010.4|pmid=20154688|title=Cold welding of ultrathin gold nanowires|journal=Nature Nanotechnology|volume=5|issue=3|pages=218–224|year=2010|last1=Lu|first1=Yang|last2=Huang|first2=Jian Yu|last3=Wang|first3=Chao|last4=Sun|first4=Shouheng|last5=Lou|first5=Jun))</ref> has shown potential in [[nanofabrication]] processes.

((Quote|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 connector]]s and [[wire wrap]] connections).

== In space ==
Mechanical problems in early [[satellites]] were sometimes attributed to cold welding.

In 2009 the [[European Space Agency]] published a [[peer reviewed]] paper detailing why cold welding is a significant issue that spacecraft designers need to carefully consider.<ref name="esa2009">((cite book|title=Assessment of Cold Welding between Separable Contact Surfaces due to Impact and Fretting under Vacuum |id=ESA STM-279|year=2009|publisher=European Space Agency |isbn=978-92-9221-900-0|url=http://esmat.esa.int/Publications/Published_papers/STM-279.pdf|author=A. Merstallinger|author2=M. Sales |author3=E. Semerad |author4=B. D. Dunn |accessdate=24 February 2013|oclc=55971016|issn=0379-4067))</ref> The paper also cites a documented example<ref>((cite journal |url= https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940028813.pdf |title= The Galileo High Gain Antenna Deployment Anomaly |last= Johnson |first= Michael R.|journal= Nasa. Lewis Research Center, the 28th Aerospace Mechanisms Symposium |publisher= NASA Jet Propulsion Laboratory |year= 1994 |hdl= 2014/32404 |accessdate= 1 December 2016 |archive-url= https://web.archive.org/web/20180208192609/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19940028813.pdf |archive-date= 8 February 2018))</ref> from 1991 with the [[Galileo spacecraft]] high-gain antenna.

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]], [[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 (or fretting 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 ultra-thin gold [[nanowire]]s (diameters less than 10&nbsp;nm) can be cold-welded together within seconds by mechanical contact alone, and under remarkably low applied pressures.<ref name="gold-nanowires" /> 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]].<ref name="gold-nanowires" />

== See also ==
* ((Annotated link|Abutment (dentistry)))
* ((Annotated link|Gauge block#Wringing|Gauge block wringing))
* ((Annotated link|Intermolecular force))
* ((Annotated link|Nanoimprint lithography))
* ((Annotated link|Optical contact bonding))
* ((Annotated link|Spot welding))
* ((Annotated link|Tribology))
* ((Annotated link|Vacuum cementing))

== References ==
((Reflist))

== Further reading ==
* ((Cite journal |doi=10.1080/01694243.2014.891349|title=Influence of fabrication parameters on bond strength of adhesively bonded flip-chip interconnects|journal=Journal of Adhesion Science and Technology|volume=28|issue=12|pages=1167–1191|year=2014|last1=Sinha|first1=K.|last2=Farley|first2=D.|last3=Kahnert|first3=T.|last4=Solares|first4=S.D.|last5=Dasgupta|first5=A.|last6=Caers|first6=J.F.J.|last7=Zhao|first7=X.J.|s2cid=136894947 ))
* ((Cite book|last=Kalpakjian |first=Serope |year=2005 |title=Manufacturing Engineering and Technology |edition=5th |publisher=Prentice Hall |page=981 |isbn=978-0-13-148965-3))

== External links ==
* ((Wikisource-inline|Advanced Automation for Space Missions/Appendix 4C))
[[Category:Welding]]

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Abutment (dentistry) (edit)
Cold welding (edit)
Gauge block (edit)
Intermolecular force (edit)
Nanoimprint lithography (edit)
Optical contact bonding (edit)
Spot welding (edit)
Tribology (edit)
Vacuum cementing (edit)
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