Before cold welding can join two or more metals together, the oxide layers on the surfaces of the materials need to be removed. Most metals (under normal conditions) have an oxide layer on the surface that forms a barrier preventing the metal atoms from bonding. Once this oxide layer is removed, the metals can be pressed together under high pressures in order to create metallurgical bonds. The oxide layer can be removed using wire brushing, degreasing, or other chemical or mechanical techniques.
Once cleaned, the metals can be pressed together, but the materials need to be ductile and should not have undergone severe hardening. As a result, softer metals are often preferred for cold welding.
The cold welding process has caused mechanical problems in early satellites and other spacecraft as the process does not exclude the relative motion between surfaces to be joined. This means that adhesion, fretting galling, and sticking can overlap so, for example, cold welding and fretting can occur at the same time. However, on the plus side, being able to fuse metals together without a liquid or molten phase allows astronauts to work quickly and effectively outside a spacecraft to carry out any necessary repair work.
Cold welding can also be performed on a nanoscale, with demonstrations showing that single-crystalline ultrathin gold nanowires (with diameters under 10 nm) can be joined within seconds through mechanical contact. The results have been shown to be near-perfect, with the same crystal orientation, electrical conductivity and strength as the rest of the nanowire. This high-quality welding is due to the nanoscale sample dimensions, mechanically assisted surface diffusion and oriented attachment mechanisms. Nanoscale cold welding has been demonstrated for joining gold to silver and silver to silver.
Explaining how cold welding works, Richard Feynman noted in his ‘Feynman Lectures’ that, “The reason for this unexpected behaviour 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.”
Cold welding was first recognised as a phenomenon in the 1940s, but the history behind cold welding techniques goes back a lot further.
Archaeologists have found Bronze Age tools that were joined using cold welding, but the first scientific experiment into the method was not conducted until 1724 when Reverend John Theophilus Desaguliers used two lead balls to test the concept by holding them together and twisting them, at which point he noticed that they had stuck together. Further testing demonstrated that the bond that was formed had the same strength as the parent metal.
Cold welding delivers a number of advantages over other welding procedures, including:
1. No HAZ
Cold welding doesn’t create a heat affected zone (HAZ), which greatly reduces the risk of negative chemical or mechanical changes to the base materials being joined.
2. Strong, Clean Welds
Cold welding can offer clean welds that are at least as strong as the weakest of the parent materials. This welding process doesn’t form brittle intermetallic compounds at the join.
3. Joining Dissimilar Materials
Dissimilar metals that are difficult to join using other techniques, such as aluminium and copper, can be joined using cold welding.
4. Aluminium Welding
It isn’t just when joining copper to aluminium when cold welding shows its benefits, as the technique can also be used for welding aluminium 2xxx and 7xxx series, which is not possible using any other metal welding technique.
While cold welding offers some notable advantages, there are also limitations associated with the technique. These drawbacks make it difficult for cold welding to be considered as a primary joining method in most instances. However, as shown above, cold welding can still be beneficial in some circumstances. The problems and challenges of cold welding include:
The main problem with cold welding is that the materials need to be clean and oxide free to create a satisfactory weld. This can be hard to achieve as well as expensive and difficult to manage in a high-volume production environment.
2. Material Types
There are limitations to the types of material that can be cold welded together as the metals must be ductile and can’t have undergone severe hardening processes. In addition, metals that contain any form of carbon cannot be joined using this technique.
3. Material Shape
Irregularities on metal surfaces can make it difficult to join them, even if all other steps have been taken. Cold welding requires the materials to be of a regular shape and to have no surface irregularities. The strongest cold welds are achieved with flat, regular surfaces.
For all of the challenges that the technique poses, cold welding has a range of different applications across industry.
The most common application for this method is for welding wires, where thermal energy can be a problem. Cold welding can ensure fast and strong joins in wires and is commonly used with aluminium, 70/30 brass, copper, gold, nickel, silver, silver alloys, and zinc.
Cold welding is also good for joining dissimilar metals that can otherwise be difficult to weld effectively. Particularly useful for joining copper and aluminium together, this method can also join welding aluminium 2xxx and 7xxx material series together.
Used in industries including aerospace and automotive, cold welding is often used to create butt joints or lap joints.
What Metals Can Be Cold Welded?
Metal to be cold welded need to be ductile but the technique is commonly used to join aluminium (including unweldable grades like the 7XXX series), 70/30 brass alloys, copper, zinc, silver and silver alloys, nickel, and gold, especially as wires.
Cold welding can also be used to join metals such as stainless steel under great pressure.
Metals containing carbon cannot be cold welded together.
Is Cold Welding Strong?
Cold welding can provide a bond as strong as the parent materials themselves if the conditions are correct. As mentioned above, this means that the metals should be ductile, cleaned of oxides on the surface and ideally of a regular shape. The materials cannot be severely hardened or contain carbons.
Despite these factors, cold welding can create some of the strongest welds possible.
Is Cold Welding Permanent?
Cold welding can create permanent welds under the right conditions. If done correctly, the join can only be reversed with damage to the workpieces. However, if the cold welding is not done under the right conditions, the joins will be liable to fail.
Cold welding is a unique bonding technique that can create very strong bonds without the use of heat. It has been used since the Bronze Age, yet only really began to be understood scientifically in the 16th Century.
While there are challenges with cold welding, when performed correctly it can bond dissimilar materials and even some ‘unweldable’ grades of aluminium. Typically used for joining wires, cold welding also find use in industries such as aerospace and automotive.
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