What You Need To Know About Cold Welding

Cold welding, or contact welding, is a solid-state welding process that requires little or no heat or fusion to join two or more metals together. Instead, the energy used for creating a weld comes in the form of pressure. During the cold welding process, unlike with fusion welding processes, no liquid or molten phase is present in the joint as can be seen in other techniques including arc welding, friction welding or laser welding.

Also known as cold pressure welding, this process to join metals without heat was first recognised in the 1940s, although the history of cold welding goes back much further. Widely used for joining wires as well as for joining two metals together in space, this process had a range of applications across industry.

(Cross-section of cold welding – before and after the weld)

History of Cold Welding

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.

How Does Cold Welding Work

The reason behind why cold welding can bond two metals together without heat is because of the removal of the oxide layers on the surfaces of the materials being joined.

Almost all metals in normal conditions have some type of oxide layer on them, even though it may not be visible to the naked eye. These metal oxides form a barrier that prevents the metal atoms on the materials from being pressed together and bonding with one another. However, once the oxide layer is removed, the metal atoms are able to join with one another with enough pressure.

To remove the oxide layer, various mechanical and chemical methods are used. Wire brushing, degreasing, and other techniques are used to ensure that the metals surface is free from oxides. The metals must also be somewhat ductile. Industrial machinery is then used to create the substantial amount of pressure needed to create the metallurgical bonds.

What is Cold Welding Used For

One of the most popular instances of cold welding is when joining dissimilar metals. This is because when dissimilar metals are melted together, they do not join well. This can result in the metals not joining together, or can lead to weak welds or welds with cracks. Cold welding avoids this problem as it relies solely on the atomic bonds formed through free electrons.

Typically, cold welding is used to create butt or lap joints. Industries include aerospace, automotive, advanced fabrication applications, and laboratory experiments often use cold welding. It is also often used for joining wires together.

What Metals Can be Cold Welder

Since ductile materials are typically required, metals that are commonly cold welded include:

• Aluminum (including unweldable grades such as the 7XXX series)
• Copper
• Brass alloys

Metals that contain carbon are not able to be joined using cold welding.

What are the Advantages and Disadvantages of Cold Welding?

Advantages:

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.

Disadvantages:

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:

1. Cleanliness

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.

There is a significant difference between the two when operating in a normal pulse circuit or pulse mode. In this mode, the arc strikes and then rises to its peak amperage before falling to a low amperage known as background amperage, after which it rises and falls again. Most of these parameters are programmable; we instruct the machine on how we want it to operate.

(Pulse Welding)    

 (Cold Welding)               

In contrast, when operating in cold welding mode, the arc completely terminates instead of rising to peak amperage and then returning to background amperage. This is the most notable distinction. In a conventional pulse circuit, the arc remains lit throughout the process, merely oscillating between two different levels of amperage. In cold weld mode, the arc reaches the peak amperage and then shuts off completely.

Conclusion

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.