Classification and mechanisms of welding

Welding can be broadly classified into three groups according to the joining method used. This page introduces the classification and mechanisms of welding.

Classification of welding

Welding can be broadly classified into three groups according to the joining method used:
Fusion welding melts base materials or a base material and a welding rod used for joining the base material (filler material); pressure welding melts base materials mechanically through friction, pressure or electric current; and brazing/soldering uses a filler material (brazing paste) applied on the joining sections.

There are many different welding methods in each group that allow selection of the optimum method depending on the base materials to be joined and other conditions.

Metallurgical joining (welding) Fusion welding
Electrical energy
  • Arc welding
  • Electron beam welding
Chemical energy
  • Gas welding
Light energy
  • Laser welding
Pressure welding
Electrical energy
Resistance welding:
  • Resistance spot welding
  • Projection welding
  • Seam welding
  • Upset welding
  • Flash welding
Chemical energy
  • Explosion welding
Mechanical energy
  • Cold pressure welding
  • Friction welding
  • Friction stir welding (FSW)
  • Ultrasonic welding
  • Diffusion welding
Electrical energy
  • Induction heating brazing (soft brazing = soldering)
Chemical energy
  • Torch brazing (flame brazing)
Light energy
  • Light beam brazing
  • Laser brazing

The classifications above are just an example. There are various ways to classify the types, and some may differ from the table above.

Fusion welding

Fusion welding is the most common welding method.
Fusion welding is a process of welding by melting one or both of a base material and a filler material.
Arc welding is a common example of fusion welding. Arc welding and laser welding are generally used for automatic welding using robot arms. In complicated product assembly lines, such as for automobile parts, robot and human welding are used depending on the characteristics or conditions of the process.

Arc welding
Arc welding
  1. Arc
  2. Weld

Pressure welding

Pressure welding can be classified into three types: friction welding takes advantage of the property whereby when a material is deformed by a certain force, the deformation remains even when the force is removed (plasticity). Gas pressure welding joins two base materials by bringing them into contact under pressure and heating them with gas. Resistance spot welding joins two base materials by holding them together and conducting electric current to heat them with the heat generated by electrical resistance.
Since friction and resistance spot welding can be automated without human intervention, they are widely used in automatic pressure welding machines on FA (factory automation) sites.

Resistance spot welding
Resistance spot welding
  1. Pressure force
  2. Flow of electric current
  3. Electrodes
  4. Weld materials


Brazing/soldering is a method of joining base materials by using a filler material (brazing paste) that has a lower melting temperature (melting point). Filler materials have been blended with flux to prevent the base material from melting and to ensure proper joins with the base material.
Filler materials are required to not only have a melting temperature lower than that of the base material but also provide high affinity to ensure atomic bonding between the molten filler material and base materials.

A variety of filler materials are used depending on the materials to be joined, such as aluminum, silver, copper phosphorus, and brass. Soft filler materials with low melting points, such as zinc, lead, tin, and tin-lead alloy, are generally called solders.
Since brazing and soldering can join metals easily, it has been used widely for commodities, fine arts and crafts, and dental applications. Soldering that takes advantage of the conductivity of filler materials is used for electronic circuits or other devices in various industries from home appliances, aviation, nuclear power, and chemical equipment.

  1. Filler material or solder
  2. Joined section