5.7 Solid-state welding
In highly deformable materials, such as metals and thermoplastics, the above aims can be achieved by solid-state welding, that is, forcing the two surfaces together so that plastic deformation makes their shapes conform to one another. At the same time the surface layers are broken up, allowing the intimate contact needed to fuse the materials without necessarily melting them. This was the principle of the first way known to weld metals – by hammering the pieces together whilst hot. It is not always essential for the parts to be hot; ductile metals such as gold can be pressure welded at ambient temperatures. Processes that join without melting the material are called solid-state welding.
Solid-state welding can be carried out in various ways. For example, two metal sheets laid over each other can be welded together by rolling – 'roll bonding'. Bimetallic strips for thermostats are made this way. Deformation of the surfaces can also be done in more exotic ways such as rubbing the two surfaces against one another (friction welding) or by using explosives to fold one sheet of metal against another (explosive welding).
In solid-state welding, there is a small melted zone, but the melted matter is usually 'squeezed out' when the parts are pushed together, so the join is a solid-state one. Commonly polyethylene gas pipes are welded using heat and pressure. In this case a heated plate is used to generate the heat on the two surfaces. Once hot, the plate is removed and the pipes are forced together under pressure, thereby forming a welded joint. The heat sealing of thermoplastics works on the same principle. Two layers of plastic sheet that are to be joined are overlapped and compressed between heated tools. This forces the materials together and the joint is made because of the intimate contact between the surfaces.