Welding traditionally involves heating at least one piece of metal to its melting temperature, then placing the melted material into contact with another piece of metal to fuse the two parts together. However, not all manufacturing processes require extreme heat to fuse two metals together. In fact, it’s possible to weld at or near room temperature. This is called cold welding, and it’s actually more fascinating than it sounds.
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What is cold welding?
Cold welding, also known as contact welding, involves joining two or metal parts made from the same material, which are in direct contact while under the pressure of a vacuum.
There’s no welding arc, torch, or heating of any kind involved. It’s all done with pressure and with some help from chemistry. However, some careful preparation is required.
How cold welding works
When exposed to atmospheric oxygen, metals like aluminum form an oxide layer on the metal’s surface. This layer of rust, along with other impurities like oil and grease, acts as a barrier that keeps two metals from fusing.
This is why the two or more pieces of metal that need to be cold-fused together have to be thoroughly cleaned and brushed to remove the rust — it’s there even if you can’t see it with the naked eye — and other impurities. Besides cleaning, another requirement is that the metals have to be ductile (the ability of a metal to withstand permanent deformation without fracturing). Non-ferrous metals like copper, lead, gold, and aluminum are some of the most commonly used ductile metals for cold welding.
Once prepared, the two metal parts are brought into contact at high pressure under a vacuum and at ambient temperature. The vacuum is to prevent the formation of a new oxide layer, however, contact welding can be performed in the atmosphere as well (i.e. directly at the construction site), although the strength of the bond is not as great as you’d get in a vacuum.
In this environment, the two pieces made of the same metal material get cold-welded because the atoms in both materials “jump ship” between each other, creating a chemical bond. Technically, this process is known as solid-state diffusion, meaning the atoms of two metals are joined and diffused together. Ultimately, the junction between the two metals forms a new bond as if they had always been a single, homogenous piece.
The two metals are typically put into contact using pressure welding machines, although in some cases it is possible to form a bond simply by pressing manually.
“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,” famous physicist Richard Feynman explained.
What is cold welding good for?
Arc welding is fairly straightforward, involving two simple electrodes that create an electric arc between them that melt and join metals. An operator can use an arc welding machine virtually anywhere besides underwater. Cold welding involves quite a bit of extra steps and a special vacuum machine. All of this requires trained technicians and can be expensive. It is well worth the effort though in some instances.
Cold welding is great when you need to join dissimilar metals, meaning metals and alloys that have widely different properties. When dissimilar metals are melted in points to fuse them, they don’t actually join well and can thus lead to weak welds and cracks. You definitely don’t want this to happen inside sensitive parts, such as those inside an airplane or automobile.
This is not an issue for cold welding because the process fuses metals solely through atomic bonding formed through free electrons. When done right, a cold weld is at least as strong as the weakest material in the junction. Cold welding is often used in the aerospace and automotive industries, for advanced fabrication applications, and in the labs of research institutes.
Cold welding is also preferable when laying underground wires inside dangerous hot spots such as tunnels brimming with inflammable gas that may catch fire from head-inducing welding. For the same reason containers holding explosives are sealed with cold welding.
An engineer might decide to choose contact welding because too much heat introduced into the material might cause warping or weakening of the material.
In general, cold welding is used when heat welding is either ineffective due to the properties of the materials that have to be fused or when heat will cause too much damage or present a danger.
How strong is cold welding?
If cold welding is done by the book, meaning the materials are thoroughly cleaned of impurities and the welding joints are nice and flat, one can expect the welded joint to be as strong as the parent material. Some other welding methods can result in joint strength that can be greater than the original material’s strength, but this is not the case with cold welding.
For intended applications, however, contact welding is strong enough. In fact, sometimes the strength of cold welding can be a real nuisance, such as during space applications, as NASA had to learn it the hard way during the 1960s and 1970s. YouTuber Veritasium posted a great vblog explaining cold welding and the accidental problems it caused in the space industry. A famous example is the 1989 Galileo spacecraft which, upon reaching Jupiter, couldn’t use couldn’t use 3 of its 18 special antennas – they were supposed to unfurl but had spontaneously cold welded together.
Perhaps now is the time to mention JB welding. JB Weld is the brand name for an epoxy bonding system, which basically involves a mixture of two components, an activator and a base. Mixing the two together and applying the resulting paste to the areas that need bonding. When cured, the JB Weld bond can be as strong as steel, so it can be used on anything from old bar stools to trailer hitches. But although JB weld is often called the ‘original cold welding’, it is not technically cold welding because there’s no molecular contact between the pieces to be welded. So don’t confuse JB welding with cold welding — they’re not the same thing at all.
The pros and cons of cold welding
Cold welding has the advantage of not having to use heat to create a bond between materials. This reduces the risk of warping or weakening the original material. Cold welding can be performed with virtually no training, unlike arc welding which requires some skill and dexterity.
However, press welding machines can be expensive, as well as the entire cold welding process for that matter. Carefully cleaning and brushing materials can take a lot of time, whereas arc welding can be performed in a jiffy. Also, cold welding isn’t practical in an industrial setting because of all the dust and debris hovering in the atmosphere.
Cold welding typically only works on ductile materials, whereas arc welding can be performed on virtually any type of metal as long as their boiling point isn’t too low, as is the case with gallium, for instance.
Irregular shapes are very difficult to bond with cold welding, whose best results come from using flat surfaces. Arc welding, on the other hand, can be used to fuse metals of all shapes and sizes.
Which metals are suitable for cold welding?
As mentioned earlier, cold welding works best with ductile materials that have similar metallic structures and non-ferrous properties. Some of the most commonly used metals in cold welding include:
- Aluminum (including the 2xxx and 7xxx grades which are generally deemed unweldable with heat)
- 70/30 Brass
- Silver alloys
- Stainless steel
Although cold welding sounds like an oxymoron, it’s actually a very practical technique for bonding metals. However, its limitations — including the prerequisites of removing oxide layers and the presence of vacuum for best performance — make cold welding practical for a relatively narrow range of applications in the aerospace and automotive industries.