Friction rotary welding is a solid phase welding process without melting the metal. A rotating tool (spindle), specially designed for the PCT, is pressed with force against the surface of the joint and moves along it, releasing heat and deforming the material, transferring it to a superplastic state.
On the working end of the tool of cylindrical shape there is a protruding central cylindrical part of a smaller diameter. This protrusion (spike), penetrating when the tool rotates into the joint, creates around itself a thin layer of metal in a superplastic state. This layer closes behind the spike as the tool moves along the joint and forms a strong connection of the ends of the joint. Thus, the connection process occurs due to the deformation and superplasticity of the material formed in the rotating tool.
The thermal cycle created by the rotation of the tool at different speeds is a factor in controlling the microstructure of the joint and the zone of thermal influence. The temperature difference at the surface of the joint and at its root affects the deformation process of the superplastic metal.
With an increase in rotational speed (and, consequently, with an increase in energy input), the hardness along the cross-section of the core of the welded point is more uniform, which leads to an increase in the size of the grains.
At a very high speed of rotation of the instrument, the nuclei of the welded point begin to collapse due to crystallization around large grains. For each material and its thickness, there is an optimal ratio between the speed of rotation of the tool and the speed of movement at the joint.
The research began with one-sided welding, in which the distance between the end of the tool and the root of the joint has an important impact on welding results. Next, two-sided welding was investigated: with two heads and a reel tool on a solid base and with two heads on flat profiles.
