Friction welding is a solid-state bonding process. When the energy for a friction weld is supplied by a rotating flywheel the process is called inertia welding. Inertia welding is characterized by short weld time, high power, and a dwell period of low torque followed by quite high torque which contributes to interface forging at the end of the weld. The low-torque stage of inertia welding is attributed to adiabatic shear.The flywheel adds versatility to the friction weld process by supplying power to the interface as demanded by the material, by the size and shape. There is a minimum speed for inertia welding of any material. This speed is equated to the physical properties of the material-namely, the thermal conductivity, density, specific heat, and melting temperature. Excellent welds between similar or dissimilar materials are obtained by means of this process, as shown by fatigue data.