What is cold pressure welding?

Cold pressure welding is a form of solid phase welding that is unique in that it is carried out at ambient temperature.

In a nutshell, cold welding is a process in which workpieces are joined under high pressure. They do not lose their solid state in the process. This means that no molten pool is created. Instead, the layers on the surface of the materials are destroyed and an atomic bond is achieved, which creates a stable connection between the workpieces.

Little or no heat is added to create the connection. Instead, the energy required is only generated in the form of pressure. This not only eliminates the molten phase, but also the heat distortion and cooling after the actual welding.

In principle, cold pressure welding has been known for thousands of years. Unlike the Fire welding the process is used with the aid of pressure. In the 1940s, it is described as an upset welding process. Heat is no longer used here.

Today, this process is used specifically for joining wires. It is also used in other areas of industry. However, it is interesting to note that it can also occur unintentionally or undesirably. In space travel, for example, it can be observed as an unintentional phenomenon of the vacuum.

How does cold pressure welding work?

There are two very important main requirements for cold pressure welding. Firstly, the materials must be brought within a few nanometers of each other. Secondly, the surface layers must be free of impurities and oxide layers so that direct contact can be established.

In other words, the materials must be prepared. Under normal conditions, metals often form an oxide layer on their surface. However, this would prevent the materials, and therefore their metal atoms, from successfully bonding with each other. It is therefore essential to remove this layer from the workpieces involved.

Both chemical and mechanical methods can be used to remove the oxide layers. These include, for example, degreasing and the use of wire brushes. Only then can they be pressed together under high pressure.

Metals

Metals that are soft and pure are particularly suitable for the process. They should not be cold-worked or hardened. Only then can they be pressed together sufficiently strongly and, above all, evenly after cleaning.

Areas of application for cold pressure welding

For space travel itself, cold welding is a simple and easy-to-use process, but it can also pose a problem. The earlier satellites and spacecraft experienced problems when uncoated metal surfaces moved relative to each other. This led to adhesion. In other words, the surfaces began to stick to each other. However, as the relative movement did not stop, cold welding was accompanied by friction and thus destruction of the material surfaces.

On the other hand, this phenomenon makes cold welding a process that is relatively easy to use in the vacuum of space. Nothing needs to be melted or liquefied. Astronauts can easily carry out the various repair work required after leaving the spacecraft in space.

Cold pressure welding can also be used in the nanometer range. This means, for example, that ultra-thin gold nanowires with a diameter of less than 10 nanometers can be joined in seconds through mechanical contact. The resulting connections are almost perfect. The orientation of the crystals, the strength and the conductivity are identical to the rest of the nanowire.

Richard Feynman describes the simple functioning of cold pressure welding by saying that atoms, when they touch each other and are of the same type, have no awareness that they are in different workpieces. They therefore act in the same way. However, if there are oxides, impurities or fats between the materials, the atoms know that they do not belong to the same part and behave differently.

How did cold pressure welding evolve?

While cold pressure welding was first described as a phenomenon in the 1940s, the technology can be traced back to before this time. Archaeologists have found tools dating back to the Bronze Age that were produced using cold pressure welding. This means that even back then, people were able to weld bronze using this relatively simple process.

A scientific experiment was also recorded in 1724. Two lead balls were held together and twisted. The result was that they could be joined in this way. The resulting joint had the same strength as the material itself.

What are the advantages of cold pressure welding?

The cold pressure welding process has a number of advantages over other welding processes. These are as follows:

• High quality weld seams: The weld seams that are produced are of a very high quality. They are extremely clean and have a minimum strength that corresponds to the weakest of the base materials used. There are no brittle joints between the metals at the weld seam.
• No heat supply: No additional heat is usually supplied during the work. Consequently, no heat-affected zone is created. This in turn means that there are no chemical or mechanical changes in the materials.
• Dissimilar materials: Metals that are dissimilar to each other and are very difficult to weld using other processes can be joined. For example, aluminum and copper can be welded together.
• Aluminum: Aluminum from the 2xxx and 7xxx series can also be welded, which is not possible with fusion welding processes.

What are the disadvantages of cold pressure welding?

Of course, there are also some disadvantages associated with cold pressure welding. It is therefore not always possible to use this process as the primary method for joining materials. The advantages of the process still apply, without question, but the right conditions must always be met. Here are the disadvantages that cold pressure welding faces:

• The material types: While material types can be welded where other processes have difficulties, this does not mean that cold pressure welding can join all materials. Instead, it can only be used with ductile metals that have not been heavily hardened. It is also not possible to join metals that have a carbon content.
• Cleanliness: It is absolutely essential that the workpieces are cleaned before starting work. They must be clean and free of oxide layers. This is the only way to produce a flawless weld seam. Cleaning itself can be time-consuming and expensive. Depending on the production environment, this can make it difficult to process larger quantities.
• The shape: The workpieces must have a smooth surface for welding. Any irregularity makes joining more difficult. This is the case even if all other preparatory work has been carried out correctly. The surfaces must be able to be pressed together evenly and without irregularities.

Where is cold pressure welding used?

While the cold pressure welding process has its challenges, its advantages outweigh its disadvantages in some areas. Accordingly, it is also used in industry. This is particularly the case when welding wires. These are difficult to weld using a fusion welding process, for example. The heat introduced would lead to complications. Cold pressure welding is ideal here and ensures strong connections between the wires. This is particularly true for 70/30 brass, gold, silver, nickel, copper, aluminum and zinc.

Cold pressure welding can also be used to join dissimilar metals that can hardly be joined effectively using other methods. In particular, copper can be welded to aluminum or aluminum from the 2xxx and 7xxx series can be joined.

The process is also used in the aerospace, aviation and automotive industries. Here, overlap and butt joints can be joined perfectly.