Welding is an important material processing technology.  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, which has a variety of structural forms, and complex, heterogeneous materials to achieve integrated connection, the manufacturing process is simple.  It has been widely used in the petrochemical industry, electric power, aerospace, ocean engineering, nuclear power engineering, microelectronics technology, Bridges, ships, submarines and various metal structures, and other industrial sectors.  Welding technology will play an important role in promoting economic construction and the development of scientific undertakings in our country.  Now a brief introduction to welding-related basic knowledge. 

1. Relevant definitions of welding 

Welding: A method of uniting workpieces by heating or pressurizing, or both, and with or without filling materials. 

Welding method: refers to a specific welding method, such as submerged arc welding, gas welding, etc., which includes the metallurgical, electrical, physical, chemical, and mechanical principles involved in the method. 
 

Welding process: the process and implementation requirements related to the manufacturing of weld parts, including welding preparation, material selection, welding method selection, welding parameters, operation requirements, etc. 

Welding Process Specification (Code): A detailed document of the process and practice requirements associated with the manufacture of weld parts to ensure reproducibility of quality when performed by a skilled welder or operator. 

Welding skill: The ability of a hand welder or welding operator to follow the specifications of the welding process. 

2. Classification of welding methods 

Common welding methods are divided into the following three categories: 

Fusion welding (fusion welding): a welding method in which the base metal to be welded is melted to form a weld.  During the welding process, the weldment is heated to the melting state, and the welding method is called fusion welding. 

Press welding: A method of welding in which pressure (heated or not heated) must be applied to the weldment to complete the welding.  Including solid-state welding, hot-pressing welding, forging welding, diffusion welding, pressure welding, and cold-pressing welding. 

Brazing: General term for hard and soft brazing.  The metal material with a lower melting point than the base metal is used as the solder, the solder and the solder is heated to a temperature higher than the melting point of the solder and lower than the melting temperature of the base metal, and the liquid solder is used to moisten the base metal, fill the joint gap and spread with the base metal to realize the connection of the solder. 

3. Physical nature of the welding process 

The materials of the welded parts (same or different) are permanently connected by heating, pressurizing, or combining both, and with or without filler materials, so that the materials of the workpiece reach an interatomic bond.  It not only forms a permanent joint on the macro level but also establishes an internal connection on the micro level. 

For metals, the bonding force between welded metals depends on the joint action of the gravitational and repulsive forces between them. When the distance between atoms is rA, the bonding force is the maximum.  Theoretically speaking, when two solid metal surfaces 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, only the theoretical conditions, in fact, even after fine processing of the surface, there will be uneven in the micro, not to mention the surface of the general metal often with an oxide film, oil, and water adsorption layer.  In this way, the metal surface will be prevented from close contact. 

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