Solid State Welding Processes

In these processes, the base materials to be joined are heated to a temperature below or just up to the solidus temperature and then continuous pressure is applied to form the welded joint. No filler metal is used in solid-state welding processes.

The various solid-state welding processes are-

Forge Welding
Cold Pressure Welding
Friction Welding
Explosive Welding
Welding by Diffusion
Welding via Thermo-compression
Welding using Friction Stir

The heat for welding is generated by mechanically induced sliding motion between rubbing surfaces of workpieces in this process.

In friction welding, one part is kept firmly in place while the other (typically cylindrical) is rotated while axial pressure is applied simultaneously.

Friction heats these parts when they are forced to rub against each other under pressure.

When the appropriate forging temperature is reached, the rotation is halted and the axial pressure (up to 10 MN) is increased to achieve forging action and, as a result, a welded junction.

Friction welding can be used to join most metals and their dissimilar combinations, such as aluminium and titanium, copper and steel, aluminium and steel, and so on.

Welding using Friction Stir (FSW)

A revolving tool is fed down the joint line between two work components, generating friction heat and mechanically stirring the metal to form the weld seam. Friction stir welding (FSW) is a solid state welding procedure.

This swirling or mixing movement gives the procedure its name.

Friction heat is generated by a separate wear-resistant tool rather than by the parts themselves, which distinguishes FSW from traditional FRW.

The Welding Institute in Cambridge, UK, invented FSW in 1991.

A stepped rotating tool with a cylindrical shoulder and a smaller probe (pin) extending under it.

During welding, the shoulder scrapes against the top surfaces of the two pieces, generating much of the friction heat, while the probe mechanically mixes the metal along the butt surfaces, generating additional heat.

The probe was built with a shape that makes mixing easier.

The heat generated by friction and mixing does not melt the metal, but rather softens it to a highly plastic state.

The leading surface of the rotating probe drives the metal around it and into its wake as the tool moves forward along the joint, creating forces that forge the metal into a weld seam.

The shoulder keeps the plasticized metal flowing around the probe in check.

The extreme plastic deformation in the solid state, which involves dynamic recrystallization of the work material, facilitates the welding of the material.

Advantages

The weld junction has good mechanical qualities.
Toxic fumes, warping, shielding concerns, and other concerns connected with arc welding are avoided.
Because there are no hazardous vapours or molten material splatter, there is a higher level of safety.
Environmentally friendly.
There is little deformation or shrinkage, and the process is simple to carry out.
The process is simple to set up and requires little operator experience.
Simple milling machines can simply automate the process.
Because no weld pool forms, the process can be carried out in any orientation (horizontal, vertical, etc.).
The quality of the welds made is excellent. Welds have an excellent look, thus no expensive machining is required after welding.
Since the procedure is done at a low temperature, there is no microstructural change in the work material.
Only a tool steel threaded pin is needed to weld over 1000 metres of aluminium.

Disadvantages

When the tool is withdrawn, it leaves an exit hole.
Clamping the plates together necessitates a great deal of force.
Manual arc welding procedures are less flexible (difficulties with thickness fluctuations and non-linear welds).
Some fusion welding processes have a slower traverse rate.