it is a solid state welding..
two parts are held in a rotating device(like chuck) and rotated against each other…
due to this plastic deformation takes place at contact surface and temperature increases to a large extent so that the contact portion melts…then the rotation is stopped and axial force is applied in the parts so the the bond becomes stronger
It is also know as resistance welding…. when the two parts are subjected to rotate against each other..There will be a heat formation.. That heat will bond the parts which are subjected to be make a joint….