Guide to Cold Welding

The word “welding” conjures images of heat, glowing red hot steel, molten metal, and sparks flying. However, there is a type of welding that occurs with no heat: cold welding.

What Is Cold Welding?

Cold welding, commonly referred to as cold pressure welding, contact welding, or cold forge welding, is the process of joining two pieces of metal together with pressure to achieve the desired bond. There are various ways to apply pressure to achieve the desired connection.

Cold Welding vs. Hot Welding

Welding in most processes involves the use of heat from either an electric arc, the flame of a torch, or a furnace to bring the base metal, filler metals, or wires to a molten state. With cold welding, since there is no heat, the metal is not heated to a molten state.

There are many instances where the term “cold welding” has been used in more traditional arc welding applications to mean welding at lower amperages and heat inputs to minimize distortion, burn-through, and other flaws. However, those types of welding still use heat to take the base metal to a molten state and are not true cold welding.

Applications of Cold Welding

Cold welding is often used to bond dissimilar metal parts, like copper wires to aluminum fittings in electrical applications. Cold welding is also used in the aerospace industry to bond metals like stainless steel or copper to aluminum. Trying to join dissimilar metals through more conventional welding processes would result in heat-affected zones making a brittle bond, which would be subject to failure.

Understanding the Cold Welding Process

The cold welding concept dates back to the Bronze Age (circa 700 B.C.), when metal workers would simply hammer metals together, typically gold. Archeologists have discovered gold boxes and tools made with the cold welding process.

The first recorded use of cold welding took place in 1724, when Reverend John Theophilus Desaguiliers tested the theory of cold welding by twisting two lead balls together. In a further investigation of the bond he had achieved, Reverend Desaguiliers was surprised to find that it was of the same strength as the parent metal.

See image below for a simplified illustration of how the cold welding process works.

Materials Suited for Cold Welding

Cold welding is ideal for joining metals and alloys when the high heat of traditional welding methods could potentially compromise the integrity of the materials. The process is particularly effective for joining gold, zinc, silver, brass, copper, and aluminum, especially grades that are traditionally deemed non-weldable.

Cold Welding Condition Requirements

To cold weld effectively, it is essential that both the metals intended for joining and the surrounding environment are devoid of oxygen. The existence of oxygen and surface contaminants can lead to the formation of an oxide layer on the surfaces of the metals.

For this reason, it is necessary to first clean the metals to completely remove the oxide layer. In some applications, materials to be cold welded are then placed in a vacuum chamber to protect them from oxygen contamination in the atmosphere.

Advantages and Limitations of Cold Welding

Like with any welding technique, the cold welding process comes with advantages and disadvantages.

Advantages of Cold Welding

Cold welding offers the advantage of joining dissimilar metals, which is often tricky with conventional welding.

One practical aspect of cold welding is its capability to work in clean environments, unlike many other welding methods that produce fumes, sparks, and debris. Unlike traditional welding, cold welding doesn't create a heat-affected zone, therefore avoiding changes to the metallurgical structure of the metals being joined that can lead to stress and potential failure.

Cold welding is also useful for joining small, thin wires and small plate and sheet sections — tasks that might be challenging with welding processes that generate a lot of heat. This makes it suitable for applications where precision and delicacy are crucial.

Limitations of Cold Welding

Since cold welding only works with metals that are highly ductile, the technique isn’t suitable for hardened or high-carbon content metals.

Prepping the joint is crucial in cold welding. The surfaces within the joint have to be almost perfectly flat and smooth. Since any irregularities or gaps can result in a weak or defective bond, attention to detail is key.

Cold welding also has its limitations. You can only pull off lap joints and simple butt joints, since the technique presses metals together. Welding large parts or anything with varying shapes is a tough or nearly impossible task with cold welding.

Cold Welding Equipment and Tools

One of the distinct advantages of cold welding is it doesn’t rely on machines that require large amounts of electrical output to generate an electric arc. In more primitive applications, a hammer and a flat surface were all that were necessary. Modern cold welding applications use tremendous amounts of pressure applied in a vacuum. The most common usage of cold welding involves a portable, sometimes even handheld, device to join or cold weld copper wire.

Cold Welding Machines and Devices

Cold welding machines types vary greatly depending on the material being used and the size and shape of the parts to be welded. For instance, a cold welding machine to join copper wires to aluminum parts may be a handheld device. For cold welding larger parts, a large stationary machine may be necessary. In these instances, the metal parts are placed in a die on one side and pressed tightly against the other piece being joined. Under enough pressure, a tight bond at atomic level is formed between the two metals.

Consumables and Tooling

One of the advantages of cold welding is the relatively small number of tools required. A wire brush (power or handheld), sanding pads or grinding discs, and possibly some cleaning solvents and rags are typically all that is necessary to perform the material preparation to achieve a cold pressure welded joint.

Safety Precautions

Since there are fewer sparks, fumes, and other hazards associated with cold welding, the personal protective equipment (PPE) required is less extensive than the PPE for more conventional welding processes.

If you are using power tools in the materials preparation stage of the cold welding process, you must wear safety glasses to protect your eyes. If you use cleaning solvents, make sure to check the manufacturer’s recommendations for the PPE required. Rubber gloves, safety glasses, and respiratory protection may be advised.

A Step-by-Step Guide to Cold Welding

To make a welded joint with the cold welding process, follow these steps.

Prepare Your Work Area and Materials

Material cleanliness is a vital step in the cold welding method. You must carefully clean and remove the oxide layer typically found on the surface of many materials with a wire brush, fine grit sanding pads or grinding discs, or even degreasers or solvents like acetone.

Operate the Cold Welder

Cold welding machines — used to weld wire, for example — are fed into a machine from the sides and placed in a die. The pressure brings the metal together so firmly that any gaps in the connection are effectively pressed out and the wires are so tightly fused together that there is an almost total fusion bond between the two.

Post-Welding Procedures

Once the bond has been established, you can remove the cold welded wires from the machine. Any surface residue or abnormalities that the welding operation has created should be chipped away. In most cases a small, handheld wire brush will be sufficient for this.

Cold Welding Troubleshooting and Tips

There are some inherent challenges in cold welding. If you struggle to achieve the desired bond or if the weld fails, consider the following.

Common Challenges in Cold Welding

One of the more common causes of failure in cold welding is insufficient prep. If oxides or other impurities are present in the joint, these impurities will prevent a solid metallurgical bond from forming.

Solutions to Common Cold Welding Issues

Prepping the material properly — making sure all pieces to be joined are free from oxides and impurities — will solve most problems encountered with cold welding.

As with any other process, ensure the equipment you are using is in proper working order. Any dirt, debris, or other contaminants that keep the pieces from being held in firm contact will lead to an unsatisfactory weld.

FAQs

Is cold welding suitable for all metals?

Cold welding is not suitable for hardened metals and metals with a high carbon content.

In addition, cold welding is not suitable for joining metal parts of odd shapes and sizes or those with challenging joint and fit-up configurations. Typically, cold welding joints use simple lap joints to be able to press the metal together and make a good cold weld.

What is the difference between cold welding and traditional welding?

Traditional welding processes use heat to bring the pieces being joined and the appropriate welding electrode/filler metal or filler wire into a molten state to achieve the welded joint.

Cold welding, however, uses little to no heat and therefore the metal parts being joined do reach a molten state. Cold welding uses pressure to press the metals together after careful prep and cleaning to achieve the desired weld joint.

Does cold welding require gas and welding rods?

A distinguishing feature of cold welding is its independence from the use of shielding gas. This is unlike many traditional welding processes, where a shielding gas is crucial to protect the molten weld pool from atmospheric contaminants. This difference arises because neither the base metal nor the filler metal undergoes liquefaction, eliminating the need for shielding gas to safeguard against atmospheric impurities during the welding process.

Likewise, cold welding does not use welding rods, filler metals, filler wires, welding wires, and electrodes, as there is no molten weld puddle that would require the use of a separate filler metal.

Are the welds made with the cold welding process strong?

Welds made by cold welding can be just as strong as the parent metal, if welded properly with good technique and sufficient material prep. Cold pressure welding can achieve a strong metallurgical bond.