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The word “friction” is often used to depict something negative – friction between two people, two vehicles, thoughts of two people, etc. In engineering, however, changing the way friction happens, can lead to something very miraculous. Friction brought about between two metals in a controlled condition can actually bind the two metals together. Friction Welding – a boon to mankind.

Broadly there are two types of welding: Fusion Welding, which relies of the formation of molten bridge, and Pressure Welding which relies on the deformation of the metals. Pressure welding takes place without any melting of the metal. As a matter of fact, all ductile metals can be cold pressure welded, but the forces required would be massive and impractical. Hence, to improve the ductility materials were heated using flame, induction, resistance or friction.

By definition, friction welding is a class of solid-state welding processes that generates heat through mechanical friction between a moving work-piece and a stationary component, with the addition of a lateral force called “upset” to plastically displace and fuse the materials.

In effect, one of the metal components (A) spins around its axis or is kept in lateral motion, while the other metal component (B) is brought closer to the former. Once the two surfaces (in motion) undergo friction, heat is generated and the surface points of the two components go into a plastic state. Here the motion of the components is stopped and a pressure is applied between them. In this state, the two components are so close to each other that there is an exchange of electrons between the two which facilitates binding at molecular level. This process is called friction welding.

Illustration

Despite its apparent simplicity, friction welding consists of complex of interrelated process:


Heat generation & dissipation
Abrasion of common surfaces due to friction
Plastic deformation
Cold working & re-crystallization of metal heated to high temperature
Inter-diffusion of metal & penetration of macroscopic metal parts from one piece into other
Continuous formation & destruction of connections between the friction surfaces

The quality of welded junction, and particularly the efficiency of the friction welding process, depends on the relative velocity of the friction surfaces and on the axial force applied during the heating period. All other conditions being equal, these factors determine the intensity of the heat introduction for the welding of the given pair of metals.

Following are the variables and parameters that govern the quality of the welded component:

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