What Is Cold Welding and Its Types?

Cold welding is a fascinating solid-state process that creates a nearly perfect metallurgical bond between two metals without using any heat, electricity, or filler material. Instead, extreme pressure is applied at ambient temperature. This technique, often used for ductile metals like aluminum and copper, relies on eliminating surface barriers to allow the atoms of the two workpieces to fuse into a single, homogeneous mass.

What is Cold Welding and How Does It Work?

Cold welding is a solid-state welding technique that joins two or more metals using pressure as the sole source of energy. No metal is liquefied or even noticeably heated during the procedure.

The Mechanism: Eliminating the Oxide Layer

The process of cold welding involves pressing the metal atoms together.  Nearly every metal has a thin, imperceptible layer of oxide on its surface under normal conditions. This metal oxide acts as a barrier, preventing the metal atoms in the materials from pressing against one another and fusing.

1. Surface Preparation: The oxide layer is removed through various mechanical and chemical techniques (wire brushing, degreasing) to ensure the surface is immaculate and oxide-free.
2. Pressure Application: Industrial machinery generates the extreme pressure needed. This pressure forces the two workpiece surfaces together as tightly as possible.
3. Interatomic Bonding: Once the surface barrier is removed and the pieces are pressed together, the atoms achieve nanoscale proximity. Metallic bonding occurs as free electrons quickly move between the lattices, fusing the two different pieces of metal into a single, homogeneous mass.

The Role of Ductility and FCC Structure

Cold welding is severely constrained by the alloys that can be welded because at least one of the metals must be highly ductile (flexible).

• Ideal Candidates: Metals having a face-centered cubic (FCC) arrangement of atoms, like copper, aluminum, nickel, silver, and gold, are the most outstanding candidates. They deform easily under pressure without immediately breaking.
• Excluded Metals: Carbon steel and other metals containing carbon cannot be cold-welded. Fast-working metals often break before the required pressure can form the bond, severely limiting the usage of the process.

Purpose and Applications of True Cold Welding

Cold welding is one of the most common applications when combining dissimilar metals (e.g., aluminum to copper). Since the metals are never heated, they will not form brittle intermetallic compounds, resulting in a strong, pure bond that fusion welding cannot achieve.

Industrial Methods for Cold Welding

While the core concept is pressure-based, there are specific industrial techniques used to apply that pressure at scale:

• Roll Bonding: Used to create layered products, such as coinage or bimetallic strips. Heavy industrial rollers squeeze two or more metal sheets together to fuse their surfaces.
• Explosive Welding: A method used to join large dissimilar metal plates (e.g., steel to aluminum). A controlled detonation forces the two surfaces together at extremely high velocity and pressure, creating the necessary atomic bond across a large area.
• Ultrasonic Welding: Uses high-frequency (ultrasonic) vibration combined with minimal pressure. The vibration scrubs the oxide layer away, and the remaining pressure fuses the metal. This is ideal for small, delicate parts, thin foils, and microelectronic wiring.

Clarifying the "Types" (Common Misnomers)

There is only one true solid-state process of cold welding. However, three processes are often confusingly labeled with similar names:

The confusion around the word "cold" in welding highlights a communication gap; while engineers refer to the true solid-state process, ground-floor workers may refer to any process that runs at a low heat input as a "running cold machine."