Inertia drive welding is the most popular method for the joining of axisymmetrical parts in which one workpiece is rotated as the two workpieces are brought together under friction pressure. In inertia friction welding, according to some researchers, one of the workpieces is connected to flywheel while the other is restrained from rotating. The flywheel is accelerated to a predetermined rotational speed to store the required energy. The drive motor is disengaged and the workpieces are forced together by a friction welding force. This causes the faying surfaces to rub together under pressure. The kinetic energy stored in the rotating flywheel is dissipated as heat through friction at the weld interface thus welding the two surfaces together as the flywheel speed decreases. An increase in friction welding force may be applied (forge force) before rotation stops. The forge force is maintained for a predetermined time after rotation ceases. Inertia Drive Friction Welding has nearly unlimited applications and has been used to weld cutting tools, agricultural machinery, automotive parts, oil field pieces, military equipment, spindle blanks and bimetallic materials. For more information can read my review as follows:

Article Advances in friction welding process: A review