The solid-state welding technique known as “cold welding,” also known as “contact welding,” joins two or more metals together with little to no heat or fusion. Instead, pressure is used as the energy source to form a weld. A liquid or molten phase is not present in the joint during the cold welding process, in contrast to fusion welding processes, as is the case with other techniques like arc welding, friction welding, or laser welding.
The method of joining metals without heat, also known as cold pressure welding, was first recognized in the 1940s, though cold welding has a much longer history. This process had numerous industrial uses, including joining wires and joining two metals together in space.
What Exactly Is Cold Welding And How Does It Work?
Most people are familiar with how welding works in a workshop. The two pieces of metal that need to be welded are heated until they melt, then we let them combine. various welding processes, including laser, TIG, and arc welding., cannot be done without using heat energy. In the welding process, heat is used to soften the two metals so that they can diffuse with one another or with a third metal (filler).
On the other hand, cold welding can join two metal parts without needing any heat energy; how does it happen?
In everyday life, nothing may have happened when we rubbed or touched two flat pieces of the same metal (let’s say aluminum). The two metal aluminum parts will, however, adhere to one another if you thoroughly clean both of their surfaces and place them in contact while under vacuum. Cold welding is the term for this phenomenon.
Two flat pieces of a single metal, let’s say aluminum, form an oxide layer on the metal surface when exposed to atmospheric oxygen. Even if you bring the two flat pieces into contact with one another and apply pressure to them, welding won’t happen due to the layer of oxide and other surface impurities like grease or oil.
However, when you thoroughly clean the surfaces (to remove the oxide layer and other impurities) and bring the two parts into contact under vacuum (no oxygen), the two pieces get cold-welded because the atoms contained in both the aluminum parts cannot differentiate and jump into each other creating a physical bond. By appropriately applying pressure to both components to completely make contact with one another’s surfaces, this bond’s strength will equal that of the parent metal (applicating pressure does not require a vacuum). Applying the proper amount of pressure to both metal components allows for 100% surface contact.
How Does It Work?
The oxide layers on the materials’ surfaces must be scraped away before two or more metals can be joined together using cold welding. Under normal circumstances, the surface of the majority of metals has an oxide layer that acts as a barrier, preventing the metal atoms from bonding. After removing this oxide layer, the metals can be pressed together at high pressure to form metallurgical bonds. Wire brushing, degreasing, or other chemical or mechanical methods can be used to remove the oxide layer.
Metals that have been cleaned can be pressed together, but the materials must be ductile and shouldn’t have undergone significant hardening. As a result, softer metals are frequently chosen for cold welding.
Early satellites and other spacecraft had mechanical issues due to the cold welding process’ inability to completely eliminate the relative motion between surfaces that needed to be joined. This implies that adhesion, fretting, galling, and sticking can overlap; as an illustration, fretting and cold welding can happen simultaneously. On the plus side, though, the ability to fuse metals without a liquid or molten phase enables astronauts to work swiftly and efficiently outside a spacecraft to complete any necessary repair work.
Nanoscale cold welding is also possible; experiments have shown that single-crystalline ultrathin gold nanowires (with diameters under 10 nm) can be mechanically joined in a matter of seconds. The outcomes were demonstrated to be almost perfect, sharing the same crystal orientation, electrical conductivity, and strength as the rest of the nanowire. Due to the nanoscale sample dimensions, mechanically assisted surface diffusion, and oriented attachment mechanisms, this high-quality welding is possible. A silver to silver and gold to silver cold weld has been demonstrated at the nanoscale level.
Explaining how cold welding works, Richard Feynman noted in his ‘Feynman Lectures’ that, “There is no way for atoms in contact to “know” that they are in different pieces of copper when they are all of the same kind, which is the cause of this unexpected behavior. The atoms “know” when they are not on the same part when there are additional atoms present in the oxides, greases, and more intricate thin surface layers of contaminants in between.”
History
In spite of the fact that cold welding techniques have a much longer history than the 1940s, when the phenomenon first became widely known.
Although cold welding has been used to join tools dating back to the Bronze Age, the first scientific test of the technique was not carried out until 1724 by Reverend John Theophilus Desaguliers, who held two lead balls together and twisted them until he noticed that they had fused together. The results of additional testing showed that the bond formed had the same strength as the parent metal.
Advantages
Cold welding delivers a number of advantages over other welding procedures, including:
1. No Haz
The risk of damaging chemical or mechanical changes to the base materials being joined is significantly reduced by cold welding because it doesn’t produce a heat affected zone (HAZ).
2. Strong, Clean Welds
The weakest of the parent materials can be at least as strong as the cleanest welds produced by cold welding. Brittle intermetallic compounds are not created at the joint during this welding process.
3. Joining Dissimilar Materials
Cold welding can be used to join dissimilar metals like copper and aluminum that are challenging to join with other methods.
4. Aluminium Welding
The advantages of cold welding aren’t limited to joining copper and aluminum; the process can also be used to weld aluminum in the 2xxx and 7xxx series, which cannot be done with any other method of metal joining.
Disadvantages
While the technique of cold welding has some distinct benefits, it also has some drawbacks. Because of these shortcomings, cold welding is rarely used as the primary joining technique. Cold welding, however, has its advantages in some situations, as was previously demonstrated. The problems and challenges of cold welding include:
1. Cleanliness
The main issue with cold welding is that for it to produce an effective weld, the materials must be free of oxide. In a high-volume production setting, this can be costly, challenging to manage, and difficult to achieve.
2. Material Types
There are restrictions on the kinds of materials that can be cold welded together because the metals need to be ductile and cannot have undergone intense hardening processes. Furthermore, this method cannot be used to join any metals that contain carbon in any form.
3. Material Shape
Even after all other precautions have been taken, metal surfaces may have irregularities that make joining them challenging. Materials for cold welding must have a consistent shape and no surface imperfections. Flat, regular surfaces enable the strongest cold welds to be made.
Is Cold Welding Robust?
The final weld joint will be just as sturdy as the parent metal if all cold welding procedures are followed. You must keep in mind, though, that brittle (non-ductile) metals, metals with hard surfaces, and metals containing carbon cannot be joined using the cold welding technique. Cleaning the surfaces will be beneficial but may not be necessary in today’s cold welding machines for butt joints because the oxide layer is removed during diffusion. But for lap joints, cleaning is essential.
Weld Joints
Cold welding can be used to create butt and lap joints. Butt joints are frequently used for welding copper and aluminum wires with a diameter of at least 10 mm and 0.5 mm. Many cold welding machines have an option to trim the ends of the wire or flat before loading into the machine. A neat, flat end is ensured by trimming. The pressure is typically applied to both ends. However, since the oxide layer is removed during fusion, cleaning the surfaces before butt joints may not be necessary.
Lap joints may be found between sheets or between sheets and rods. The designer should account for at least 50% of one sheet’s thickness being lost due to pressure when performing the strength calculations.
Applications
Despite all of the difficulties the method presents, cold welding has many different industrial applications.
This technique is most frequently used to weld wires because thermal energy can be an issue. Cold welding is frequently used with aluminum, 70/30 brass, copper, gold, nickel, silver, silver alloys, and zinc to ensure quick and reliable joins in wires.
Additionally, cold welding is useful for joining metals that would otherwise be challenging to successfully weld together. This technique can be used to weld together the 2xxx and 7xxx material series of aluminum, which are particularly useful for joining copper and aluminum together.
Cold welding is frequently used to make butt joints or lap joints, and it is employed in a variety of industries, including aerospace and automotive.
Cold Welding Machines
Using a cold welding machine, you can cold-pressure weld the majority of non-ferrous ductile metals. These devices can work with wires and rods as thin as 0.5 mm and as thick as 15 mm. For smaller wire diameters, a hand-operated cold welding machine can be used, and for larger wire diameters, pneumatic or electric pneumatic machines can be used. Usually portable, these devices can handle both wires and strips.
To hold wires and strips, you must fit the machine’s specific dies, which can accommodate a narrow range of wire diameters or strip sizes. After being loaded from opposite sides of the machine, the two pieces of the wire or strip that need to be cold-welded come into contact with one another under pressure, become diffused and are then joined together by welding. The ends of the metal to be cold-welded can often be trimmed using a guillotine on cold welding machines.
Faqs
What Types Of Metal Can Be Cold Welded?
The joining material must be ductile in order to be cold welded, but other metals frequently joined using this method include nickel, copper, zinc, silver and silver alloys, silver and silver alloys, gold, and even unweldable grades of aluminum like the 7XXX series.
Under extreme pressure, cold welding can also be used to join metals like stainless steel.
Cold welding is ineffective when used to join carbon-containing metals.
Are Cold Welds Durable?
If the circumstances are ideal, cold welding can produce a bond that is as strong as the parent materials themselves. This calls for the metals to be ductile, free of surface oxidation, and ideally of a uniform shape. The materials can’t be very hardened or have carbons in them.
Despite these obstacles, cold welding can produce some of the strongest welds imaginable.
Is Cold Welding A Permanent Process?
Under the right circumstances, cold welding can produce permanent welds. If done correctly, the join cannot be undone without damaging the workpieces. However, the joins run the risk of failing if the cold welding is not performed under the proper circumstances.
Conclusion
Solid-state diffusion causes the joining or cold-welding of two non-ferrous ductile metal parts when they are pressed into contact and have flat, clean surfaces, like aluminum or copper. Cold pressure welding and contact welding are additional names for cold welding.
Without the use of heat, cold welding is a unique bonding method that can produce incredibly strong bonds. Although it has been used since the Bronze Age, scientific understanding of it did not really start to develop until the 16th century.
Cold welding presents some difficulties, but when done properly, it can join disparate materials and even some aluminum grades that are considered “unweldable.” Cold welding has applications in a variety of sectors, including the aerospace and automotive industries. It is typically used to join wires.
