Welding: Definition, Classification And Physical Essence

It is an important welding process.

With the pursuit of material civilization in human society, the continuous development of various new materials and the continuous development of science and technology, welding technology has become an independent discipline.

It has the characteristics of diverse and complex structural forms, integrated connection between different materials and simple manufacturing process.

It has been widely used in various industries such as marine electronics, nuclear power, marine engineering, nuclear power, submarine and so on.

Welding technology will play an important role in promoting China’s economic construction and scientific development.

Now let me briefly introduce the basic knowledge related to welding.

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1. Relevant definitions of welding

Welding: a method of combining workpieces by heating or pressurizing, or both, with or without filler material.

Welding method: refers to specific welding methods, such as submerged arc welding, gas shielded welding, etc.

Its meaning includes the metallurgical, electrical, physical, chemical and mechanical principles involved in this method.

Welding process: processing methods and implementation requirements related to the manufacture of weldments, including welding preparation, material selection, welding method selection, welding parameters, operation requirements, etc.

Welding procedure specification (procedure): detailed documents of processing and practice requirements related to the manufacture of weldments, which can ensure the reproducibility of quality when operated by skilled welders or operators.

Welding skills: the ability of manual welders or welding operators to implement the welding process rules.

2. Classification of welding methods

Common welding methods are divided into the following three categories:

Fusion welding (fusion welding): a welding method that melts the base metal to be welded to form a weld.

In the welding process, the weldment is heated to the melting state, and the welding method completed without pressure is generally called fusion welding.

Pressure welding: during the welding process, pressure must be applied to the weldment (heated or unheated) to complete the welding method.

Including solid-state welding, hot pressing welding, forging welding, diffusion welding, pneumatic welding and cold pressing welding.

Brazing: the general term for brazing and soldering.

The method of connecting weldments is realized by using metal materials with lower melting point than the base metal as solder, heating weldments and solder to higher than the melting point of solder and lower than the melting temperature of base metal, wetting the base metal with liquid solder, filling the joint gap and diffusing with the base metal.

Classification of welding methods

3. Physical essence of welding process

The material of the workpiece to be welded (the same or different) forms a permanent connection by heating or pressurizing or both, and using or without filling material to make the material of the workpiece achieve the combination between atoms.

It not only forms a permanent joint in the macro, but also establishes the internal relationship in the microstructure.

For metals, the bonding force between welding metals depends on metal bonds, which is determined by the joint action of gravitational force and repulsive force between them.

When the distance between atoms is rA, the bonding force is the largest.

For most metals, rA ≈ 0.3 ~ 0.5nm.

When the distance between atoms is greater than or less than rA, the binding force decreases significantly.

Theoretically speaking, when two solid metal surfaces to be welded are close to the distance rA, physical and chemical processes such as diffusion and recrystallization can be carried out on the contact surface, so as to form metal bonds and achieve the purpose of welding.

However, it is only a theoretical condition.

In fact, even the finely processed surface will have irregularities on the micro level, not to mention the adsorption layers such as oxide film, oil stain and water on the surface of general metals.

In this way, it will hinder the close contact of the metal surface.

Physical essence of welding process
welding process

In the welding process, the measures of applying pressure (destroying the oxide film on the contact surface and increasing the contact area) and heating (destroying the oxide film, reducing the metal deformation resistance and promoting diffusion, chemical reaction and crystallization / recrystallization) are generally adopted to overcome various factors that hinder the close contact of the metal surface.

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