Cold welding is a solid-state welding process that joins two metal surfaces at room temperature by pressure alone, with no heat and no molten metal. It works because the atoms of a clean metal surface bond to any identical atoms brought close enough; heavy plastic deformation breaks up the oxide film that normally keeps them apart, pushes fresh metal into contact, and a metallic bond forms across the joint.
Cold welding is used to join aluminium and copper wires, to make aluminium-to-copper electrical joints, and to seal containers that must not be heated. It is also a failure mode in space, where parts can weld themselves together by accident. It sits in the solid-state group of the types of welding, alongside friction, diffusion and ultrasonic welding.
How does cold welding work?
A piece of metal is a regular array of atoms held together by metallic bonding: the outer electrons are shared among all the atoms. An atom sitting at the surface has neighbours only on one side. If another piece of the same metal is brought within about one atomic spacing, the surface atoms cannot tell which piece they belong to, and the two pieces become one. As Richard Feynman put it, when the atoms in contact are all of the same kind, there is no way for them to “know” that they are in different pieces.
So why do two copper coins not stick together in your pocket? Two reasons:
- Surface films. In air every metal is covered within moments by an oxide layer, plus adsorbed water, grease and dirt. These films sit between the metal atoms and stop them bonding.
- Roughness. Even a polished surface is covered in microscopic peaks, so flat-looking surfaces touch only at a tiny fraction of their area.
Cold welding overcomes both with pressure and deformation:
- The surfaces are cleaned to remove grease and as much oxide as possible.
- They are pressed together hard enough to make the metal flow plastically.
- As the joint area stretches, the brittle oxide film cannot stretch with it. It cracks into islands, and fresh, clean metal is extruded through the cracks.
- The clean metal from both sides meets at atomic distance, and metallic bonds form. With enough deformation the original interface disappears and the joint can be as strong as the parent metal.
No diffusion or melting is needed; the bond is made by contact. That is why the process is so fast.
Cold welding in space
In the vacuum of space the process can happen by accident. There is no oxygen to rebuild an oxide film once it has been rubbed off, so two clean metal parts pressed or rubbed together can bond. The effect was first recognised as a general materials phenomenon in the 1940s and became a serious concern for spacecraft mechanisms.
The best-known case is the Galileo spacecraft. According to the European Space Agency’s report STM-279 (2009), the umbrella-like high-gain antenna could not be fully deployed in 1991, and investigations showed that fretting during transport and lift-off had caused its ribs to cold weld together in the stowed position. ESA now tests pairs of materials for cold welding under impact and fretting in vacuum, and designers prevent it with coatings, dry lubricants and dissimilar material pairs.
Which metals can be cold welded?
The metal must be ductile enough to deform heavily without cracking, and it helps if its oxide is hard and brittle so that it breaks up. TWI lists aluminium, 70/30 brass, copper, gold, nickel, silver and silver alloys, and zinc as metals commonly cold welded. Most of these are face-centred cubic metals, which have many slip systems and flow easily at room temperature.
- Good: aluminium, copper, gold, silver, nickel, zinc, lead, 70/30 brass.
- Dissimilar pairs: aluminium to copper is the most important, since fusion welding these two forms brittle intermetallic compounds.
- Not suitable: steels and other hard or work-hardened metals. TWI notes that metals containing carbon cannot be joined this way, and that material which has been severely hardened will not weld.
Types of cold welding process
Butt cold welding of wires and rods
The two wire ends are gripped in dies with a short length projecting, and pushed together. The projecting metal is upset (squeezed sideways) into a flash, carrying the surface oxide out of the joint with it. Commercial wire cold welders repeat the upset several times, commonly four, so all the contaminated metal is expelled. The flash is then trimmed off. This is how wire drawing and cable plants join aluminium and copper wire coils end to end without a heat-affected zone.
Lap cold welding
Two overlapping sheets are squeezed between shaped indenters. The joint must be deformed heavily; practical guides say to allow for at least half the thickness to be lost at the weld. That thinning is why lap cold welds are used on thin sheet, foil and tabs rather than structural plate.
Roll bonding and cladding
Two or more strips are passed together through a rolling mill with a large reduction in one pass, which bonds them along their whole length. It is used to make clad and bimetal strip.
Cold welding process step by step
- Surface preparation. Degrease with solvent, then wire brush (stainless steel brush) or scratch the surfaces to remove the oxide and roughen them. Weld straight away before the oxide re-forms.
- Fixturing. Clamp the parts in dies or between indenters so they cannot slide.
- Pressure. Apply pressure well above the yield strength so the metal flows. For wires this is an axial upset; for sheet it is an indentation.
- Deformation. Keep squeezing until the oxide is broken up and fresh metal is in contact over the whole joint area.
- Finishing. Trim the flash (wire) and inspect. A good wire weld can be bent and pulled without separating.
Applications of cold welding
- Joining aluminium and copper wires and rods in wire drawing, cable and transformer winding plants.
- Aluminium-to-copper transition joints in busbars, lugs and motor windings, avoiding brittle intermetallics.
- Electrical connections such as insulation-displacement and wire-wrap connections, which rely on cold welding at the contact points.
- Clad and bimetal strip by roll bonding.
- Sealing containers whose contents must not be heated, and working in places where a spark or flame is a hazard.
- Research: gold nanowires under 10 nm across have been cold welded within seconds by contact alone, a route to joining at the nanoscale.
Advantages and limitations
| Advantages | Limitations |
|---|---|
| No heat, so no heat-affected zone or change of temper | Only ductile metals; steels and hardened alloys cannot be joined |
| Joins dissimilar metals like Al-Cu without brittle intermetallics | Surfaces must be very clean; hard to keep so in production |
| No filler, flux, gas or electricity at the joint | Heavy deformation thins and distorts the joint |
| No sparks or fumes; safe near flammable material | Needs regular shapes: wires, rods, sheets and tabs |
| Fast; wire joints are made in seconds | High forces need rigid dies and tooling |
| Joint can match parent metal strength and conductivity | In service, unwanted cold welding causes galling and seizure |
Cold welding vs “cold weld” epoxy
Products sold as “cold weld” in hardware shops are two-part epoxy adhesives, often filled with steel powder. They glue parts together by adhesion; no metal-to-metal bond forms and nothing is deformed. They are useful for repairs, but a joint made with them is far weaker than a genuine weld. When an engineering text says cold welding, it means the solid-state pressure process on this page.
Cold welding vs diffusion bonding vs friction welding
| Point | Cold welding | Diffusion bonding | Friction welding |
|---|---|---|---|
| Heat | None; room temperature | Furnace heat, below melting | Frictional heat from rubbing, below melting |
| Pressure | Very high, enough for heavy plastic flow | Moderate, only to close the gaps | High forging force at the end |
| Deformation | Heavy | Very small | Moderate, with flash |
| Bonding mechanism | Fresh metal forced into atomic contact | Atoms diffuse across the interface over time | Hot plastic metal forged together, oxides expelled in flash |
| Time | Seconds | Minutes to hours, often in vacuum | Seconds |
| Typical parts | Wires, rods, foil, electrical joints | Aerospace titanium parts, precision assemblies | Shafts, axles, bimetal pipe joints |
For joining methods that do melt a filler but not the base metal, see brazing; for fusion welding of aluminium, see welding aluminium; and for arc, gas and resistance equipment, see welding machine types. Early research on the adhesion of ductile metals appeared in the journal Wear, and standards for welding practice are published by the American Welding Society (AWS).
FAQs
How does cold welding work?
Heavy pressure makes the metal flow, which cracks the oxide film on the surfaces and pushes fresh, clean metal from both parts into contact at atomic distance. The atoms then form metallic bonds across the joint, with no heat or melting.
What metals can be cold welded?
Ductile metals such as aluminium, copper, gold, silver, nickel, zinc and 70/30 brass. Aluminium to copper is a common dissimilar pair. Steels and hardened alloys are not suitable.
Why does cold welding happen in space?
In vacuum there is no oxygen to re-form the oxide film once rubbing or impact has removed it, so clean metal parts in contact can bond. ESA reports that fretting caused the ribs of Galileo’s high-gain antenna to cold weld in 1991.
Is cold weld epoxy the same as cold welding?
No. “Cold weld” epoxy is an adhesive that glues surfaces together. Real cold welding forms a metallic bond between the metals themselves by pressure.
Is a cold weld as strong as a normal weld?
A properly made cold weld in a suitable metal can match the strength of the parent metal, because the joint is the same metal with no heat-affected zone. A poorly prepared joint with oxide left in it will be weak.