Solid-state welding or Pressure welding process refers to joining processes in which coalescence or joining of metals results from application of pressure alone or a combination of heat and pressure. If heat is used in solid-state welding, the temperature in the process is below the melting point of the metals which is being welded. In solid-state welding processes, no filler metal is utilized.
Solid-state welding is a joining process which is done
- Without any liquid or vapor phase,
- With the use of pressure, and
- With or without the aid of temperature.
Solid-state welding is done over a wide range of pressure and temperature (if required), with appreciable deformation and solid-state diffusion. In solid-state welding, the cohesive forces between metal atoms are utilized.
Need of Solid-State Welding
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- Solid-state welding is used for joining of dissimilar metals and alloys
- When joining is difficult to weld metals like aluminum, super alloys.
- To obtain Stronger & Intermediate free welds
In fusion welding, the films are dissolved or burned away by high temperatures, and atomic bonding is established by the melting and solidification of the metals. But in solid-state welding, the films and other contaminants must be removed by other means to allow metallurgical bonding to take place.
In some cases, a thorough cleaning of the surfaces is done just before the welding process; while in other cases, the cleaning action is accomplished as an integral part of bringing the part surfaces together. To summarize, the essential ingredients for a successful solid-state weld are that the two surfaces must be very clean, and they must be brought into very close physical contact with each other to permit atomic bonding.
Classification of the Solid-State Welding Processes
Solid-state welding processes have been classified into the following:-
- Solid state bonding mechanism
- Cold welding (CW)
- Ultrasonic welding (USW)
- Friction Welding (FW)
- Friction stir welding (FSW)
- Diffusion Welding (DW)
- Explosive Welding (EW)
- Electromagnetic Pulse Welding (EMPW)
- Forge Welding (FRW)
- Roll Welding (RW)
- Pressure Welding (PW)
Advantages of Solid-State Welding Processes
- Because of no melting and solidification, these processes provide comparatively lower lever of distortion and residual stress generation.
- Heat affected zone (HAZ) is also narrower.
- It provides excellent weld appearance.
- Mechanical properties of parent materials remain intact as no fusion takes place (As no melting occurs generally and the heat input is lower than in fusion processes, there is less disruption to the microstructures of the materials being joined and hence less effect on their properties.)
- Easy to join dissimilar metals.
Welding processes that do not involve melting have several advantages over fusion welding processes. If no melting occurs, then there is no heat-affected zone, and so the metal surrounding the joint retains its original properties. Many of these processes produce welded joints that comprise the entire contact interface between the two parts, rather than at distinct spots or seams, as in most fusion-welding operations. Also, some of these processes are quite applicable to bonding dissimilar metals, without concerns about relative thermal expansions, conductivities, and other problems that usually arise when dissimilar metals are melted and then solidified during joining.
Disadvantages of Solid-State Welding Processes
- It requires special type of joint design, edge preparation and/or surface finish.
- Primary shape of the components is crucial factor as pressure is needed to apply (a suitable shape is required to apply pressure uniformly).
- Joining more than two components at a time is difficult; in some cases it is impossible.
- Application of filler material is not possible. So wider root gap cannot be filled.
It is interesting to note that some of the oldest welding processes ever used are solid state processes (forge welding) as also some of the newest (friction stir welding). In this section of solid-state welding process, we examine briefly the solid state welding processes that are commonly used in the joining of aluminium alloys.