When you think of a welding procedure, the very first thing that springs to mind is probably the use of heat. Techniques like arc welding, friction welding, ultrasonic welding, and laser welding, all involve heat in some way or another. In fact, heat is considered synonymous with welding and is, in the above examples, critical to joining two metals together.
However, these are not the only way. You can, believe it or not, actually, fuse metals together in a process called cold welding.
Commonly used in aviation and electrical engineering, it is widely considered one of the best ways to bond metals (and other materials) together.
It might sound impossible, but it is actually one of the most popular welding methods out there. Let's find out a little more about it.
How can we join metals without heat?
Heat-based welding works by effectively making the parts plastic enough so that diffusing of the atoms can take place, either between the two workpieces or with a different medium in the middle. While this is traditionally done by applying heat, there are other ways to coax the atoms to diffuse.
Cold welding (also known as cold pressure welding and contact welding) uses pressure, under vacuum conditions, instead of heat, to join two materials, through a process called solid-state diffusion.
It can also be used to bond other materials, like plastics, together too.
However, this begs the question "is cold welding strong?" As it turns out yes.
The resulting bond is usually as strong as that of the parent materials once the process is complete.
During the process, no metal is liquified, and the materials are usually not heated to a notable degree. The process does, however, rely on the need for any oxide layers to first be removed from the metals in question.
This is mainly due to the fact that the metals usually contain a surface oxide layer, which acts as a thin barrier on the surface of the materials, preventing the diffusion of metal atoms between the metal pieces.
Most metals will, under normal conditions, have some type of oxide layer on their exposed surfaces -- even if it is not visible to the naked eye. They may also pick up layers of other contaminants like grease, dust, etc.
Cold welding overcomes this issue by preparing the metals before they are welded. The preparation process involves cleaning or brushing the metals to such an extent that the top oxide, or barrier, layer is removed.
This usually involves a mixture of chemical and mechanical methods. Degreasing, wire brushing. and other techniques are used to ensure that any metal surfaces are oxide-layer-free as much as possible.
What is needed for a cold weld?
As previously mentioned, any metals that will be subject to cold welding must first be free of any oxide layers.
Once the desirable surface cleanliness is obtained, both materials are pressed together mechanically, using just the right amount of force. This amount of force depends on the material itself, as some materials may only weld at high pressures.
But there are other requirements.
One of the conditions required for cold welding is that at least one of the materials must be ductile and must not have undergone severe hardening. This obviously narrows down the list of materials that can be the candidate for cold welding.
Soft metals, like aluminum or copper, are the best choices for cold welding.
The most common joints that are possible with cold welding are:
- Butt joint
- Lap joint
In a Butt joint, removing the barrier layer of the metal is not often required as the plastic deformation that happens during the joining process breaks up the barrier automatically. This kind of joint is most commonly applied to metals like aluminum or copper wires with diameters between 0.02 inches (0.5 mm) and 0.4 inches (10 mm).
Lap joints, on the other hand, do require special treatment because otherwise, the material won’t adhere to each other. Lap joints are more commonly used when welding sheets together, or sheets to rods.
Cold welding is also commonly used with wire, including aluminum, copper, zinc, 70/30 brass, nickel, silver, silver alloys, and gold.
The history of cold welding
Cold welding was first officially recognized back in the 1940s, but there is some evidence it may have an even earlier origin.
In 1724, for example, Reverend J I Desaguliers appears to have successfully cold-welded two metals using cold welding techniques. He showed that when he pressed and twisted two lead balls of the same diameter together, they would stick to each other. The joints were somewhat erratic but appeared to be as strong as those of the parent lead balls.
The limitations of cold welding
As useful as cold welding is, it is far from without its limitation -- like any form of welding.
A perfect cold weld is very hard to achieve. This is due to several reasons, including oxide layers that form on top of the metal in atmospheric conditions, surface irregularities, surface contamination, and more. Perfect conditions can be difficult to achieve, and costly, especially for large-scale welding projects.
Optimum cold welds only happen when the two surfaces that are being pressed together are clean and free from any form of contaminants. This requires additional preparation steps and can take some time to achieve.
Also, the more flat and regular the surface is, the easier and more uniform the weld is going to be. A perfect flat and smooth surface is not always possible -- especially at the micro-, and nano-scale.
Another limitation is the types of metals that can be cold-welded. At least one of them must be ductile, with non-ferrous soft metals the only real candidates suitable for cold welding. Copper and aluminum are the two most commonly cold-welded metals.
Metals that contain carbon are usually excluded from the possibility of being cold-welded.
Advantages of using cold welding
The most noteworthy advantage of cold welding is that the resultant welds have the same bond strength, or very close to, that of the parent material. This feat is very hard to recreate in other forms of metalworking without completely melting down and recasting.
Cold working can also be used to weld aluminum alloys in the 2xxx and 7xxx series, which cannot be fusion welded due to their tendency for hot cracking, and which can be very difficult to join with other forms of welding.
In industry, cold welding is known for its ability to weld aluminum and copper together which are also often hard to weld with other forms of weld techniques. However, the bond created between the two materials by cold welding is very strong.
Cold welding provides clean and strong welds without the formation of brittle intermetallic compounds.
Applications of cold welding
Cold welding is mostly applied in welding wires. Since there is no heat involved and the process can be done quickly, cold welding can ensure perfectly welded wires, mainly with aluminum, copper, 70/30 brass, zinc, silver and silver alloys, nickel, and gold.
There are even handheld tools available that you can use to cold weld wires, making them very portable and easy to use -- once the metal surfaces have been sufficiently cleaned, of course.
Cold welding is also used in cases where dissimilar metals have to be joined, like between copper and aluminum, for example.
Cold welding provides one of the most solid welds for creating parent metal-like bonds. It doesn’t require heat energy nor special tools. Among the most popular weld techniques, cold weld shows that heat isn’t necessary if you are joining specific types of materials.