In short, cold welding (aka contact welding) is a solid-state welding process that bonds 2 pieces of material together by applying pressure to them both. The pressure applied to the pieces of metal breaks up the oxide layers and allows the two pieces of material to stick to one another. You don’t have to worry about applying heat to or melting materials with cold welding.
Pros
- Reduces the necessary skills for welding exotic metals
- Makes it possible to join a number of metals that are dissimilar and would be otherwise difficult to weld
- Gives a near perfect welding joint without brittle intermetallic compounds, microfractures, and other joint weaknesses
- Eliminates most heat affected zone (HAZ) problems since there is no concentrated heat and because of this, no HAZ, from the welding arc
- Is a perfect process for welding aluminum, especially in joining copper with aluminum, two metals that would typically be a challenge to join with other welding processes
Cons
- It is difficult to weld irregular shapes, and the best results are gotten with flat surfaces
- Difficult to achieve in industrial settings because of the debris that float in the air
- Carbon steel and hardened metal can’t be cold welded, this only works with non-ferrous ductile metal like copper, gold, aluminum, lead, etc.
- Surfaces must be cleaned pristinely; meaning that multiple steps of cleaning may have to be taken and the metal may have to be prepped
- Surface irregularities, contamination, and nanoscale molecular structures can hurt the results
How does cold welding work?
When you have 2 pieces of material without a solid oxide layer, the atoms on the exposed sides of the materials collapse into one another. This is essentially how cold welding works. The union of the two materials can fail if they are in either a reactive or oxygen intense environment. For cold welding to be effective both surfaces must be cleaned thoroughly and free of contaminants.
Video credits to Cody’sLab. It demonstrates (or attempts to demonstrate) cold welding in a vacuum chamber.
For cold welding to work the two surfaces being welded must essentially be both clean and flat (anatomically so). The bond produced will be equal to the strength of the parent metal.
What metals can be cold welded?
Cold welding can only be used on non-ferrous metals or soft iron with no carbon content. The most common metals used for cold welding are aluminum and copper. However, there are other metals one can use such as:
- Alloys (Aldrey, Triple E, Constantan)
- 70/30 Brass
- Zinc
- Silver
- Silver Alloys
- Nickel
- Gold
Why use cold welding?
Cold welding is typically used to weld wires together, especially between metals that are dissimilar. This method of welding is great when one is laying underground wires and there is the danger of flammable gasses catching fire from welding processes that are heat inducing.
On top of this, this method of welding is used to seal containers that’re sensitive to heat, like containers with explosives for example.
Cold welding is generally used when heat will damage the product or may present a danger.
Requirements for cold welding
The main requirements for cold welding are prepping the joint’s geometry and a clean metal surface. Joint surfaces that are flat work best, so flattening any irregularities in the shape is what’s recommended.
The oxide layer and any other impurities can be removed by degreasing, wire brushing, or chemical or mechanical methods. There will typically be oil and grease on the surface of the metal that will have to be removed before wire brushing. This is necessary since the brush can force the impurities deeper into the metal. Thanks to the wire brush’s sharp bristles, soft metal like silver, gold, copper, aluminum, and many others are more susceptible to getting surface oils embedded below the surface.
After the oils are cleaned, one can continue to strip away the oxide layer itself. Varying with the metal, different brush types and bristle materials may be recommended. It’s always a good idea to check the metal’s specification sheet.
Is cold welding strong?
Colded welded joints are as strong as the parent metal if it is properly prepped. Joint strength varies with the metal’s properties. Unlike with other welding methods, cold welding joint strength can’t outperform the original metal strength.
Joint strength will be compromised if the joining surfaces aren’t cleaned sufficiently enough or are irregularly shaped. For typical cold-welding applications such as joining wire, the maximum bond isn’t very hard to achieve.
What are the possible weld joints?
Because cold pressure welding is best done on large contact surfaces, it’s best to use lap and butt joints.
Butt welds are typically used when welding pipes and wires together. This is because it’s simple to trim the ends, put clean metal onto the contact surface, and press the two wires together.
When you’re making a butt weld, the distance between the clamping points and the contact surface should not be too large since soft metals can always bend sideways rather than join together.
Cold lap joints can be tricky. By pressing sheet metal together you’ll reduce the thickness of it because of the applied pressure. Meaning that one should account for at least a 50 percent thickness when you’re prepping the project. Without that, the final part won’t meet the requirements of the project.
The project may be perfect, but the thinned part isn’t acceptable. One should consider the metals softness and ductility and perform a few test welds to determine the thickness that will occur as a result.
Machines that cold weld and can join wires
Cold welders are hand operated for wires with small diameters. But larger diameters need a pneumatic or electric-pneumatic operation. Many of these machines are portable and can handle strips, wires, and rods.
Making use of an air hydraulic intensifier, cold welders generate extreme pressure. On the operator’s side, there is a welding head. This is located on the top of the machine and serves to accept a welding die, control the applied pressure, and offer stability.
Once the die is placed and secured into the die pocket, rods/wires are then fed in on both sides. By applying pressure, one will cause the die to grip the wires near the endpoints and to push them tightly together. As a result of this, small impurities that remain on the wires cross-section surfaces are squeezed outwards from their cores. This is why cold-welding wires will create a better joint bond than when welding sheet metal. This is mainly due to the wires having a small joining surface area unlike with sheet metal.
Pressure is applied at least 4 times to get rid of all the impurities. This is considered the multi upset principle. Once the wires have bonded, one can remove them from the machine while chipping away the residue from around the joint area.
Hot welding vs Cold welding
Hot welding methods involve the use of an electrical arc, an active flame, or internal resistance to melt and meld the metal. Cold welding is better suited to specific applications and non-ferrous metal, while hot welding can be used for many more things.
Which metals can be cold welded?
The metals that can be cold welded include 70/30 brass alloy, silver alloys, silver, nickel, lead, zinc, aluminum, copper, gold and platinum. This method can also weld 7xxx and 2xxx series of aluminum alloy. These types of metals can’t be fusion welded because they will likely crack when under heat, and they are hard to weld together with welding methods outside of cold welding.
It’s impossible to cold weld carbon steel or any other metal that contain carbon. This will limit the cold-welding application because carbon steel is welded far more than most other metals.
Cold welding will work best with metals that have a face-centered cubic arrangement of atoms that won’t harden quickly. All of the metals that harden quickly tend to crack under the pressure felt when cold welding. This explains why only highly ductile metals can be cold welded.
The different methods of cold welding
There are only 3 similarly named methods, no different cold welding types.
JB Weld
There is a brand epoxy bonding system that’s used with metal, fiberglass, brick, and concrete named JB weld. It doesn’t actually create a weld between metals but it’s called the original cold weld formula.
Unlike with the cold-welding process, there is no interatomic attraction, and the metals don’t fuse to because a single homogenous mass.
JB weld works well to adhere metal, but the metals won’t be welded together. The product is a 2-component epoxy, base, and an activator. When this product is mixed and applied to metal parts one should secure them with clamps and start the curing process.
This method has a tensile bonding strength of 5020 PSI, which is compared to the typical E6010 stick electrode that has a 60,000 PSI bond strength.
This is no substitute for an actual weld unless you plan on making small repairs around the house.
Cold Metal Transfer
CMT or Cold Metal Transfer is a fusion welding process that makes use of a welding arc to make a joint. It’s often improperly titled cold welding. CMT is a MIG welding process that needs about 90 percent less input of heat than regular MIG welding processes.
Because this arcwelding process is so “cold” it has solved many problems like with the actual cold welding process.
CMT shouldn’t be mistaken for cold welding though, as it uses an electrical arc, filler metal wire, and it can be used with metal with which cold pressure welding isn’t an option. CMT relies on a precise filler wire retraction arc initiation to control the heat input.
Because only a robot is able to do this, it isn’t economical if cold pressure welding is a viable option.
TIG Cold Welding
As with CMT, TIG cold welding is of no relation to the method of cold pressure welding. There are some TIG welding machines that have a cold setting that limits the heat input. It does so by applying an electric arc to a small spot for a fraction of a second.
Temperature is minimal since any generated heat will dissipate quickly, especially with metals that are highly conductive like aluminum.
This is especially useful when one is welding thin sheets of wires and metal. One could achieve something similar with an advanced TIG welder by using the pulse settings.
You will achieve low heat TIG weld by setting a low pulse current and a high time dilation between the pulses. Low heat isn’t always good enough, so if cold pressure welding is an option, it will produce a better joint.
History of Cold Welding
Cold welding started in the Bronze Age, around 700 BC but back then it wasn’t anywhere as sophisticated as it is today. Archaeologists have excavated a number of utensils and tools from that time period that were made using an older cold welding process.
The first documented scientific experiment on cold welding was done in 1724 by Reverend J.T. Desaguilers. He found that if he pressed and twisted two lead balls together, they would make a solid joint. He then tested the bond strength on a steelyard with positive results.
The next notable point in history was the second World War when light alloy aircraft parts were made using cold welding in Germany. As industry made progress, cold welding became more advanced and became what is today a well understood process used in specialized settings.
