The authors have clarified the joining mechanism of the first phase of friction stage during friction welding process for similar materials. The present paper describes the simulated friction torque in the first phase during friction stage for joining of mild steels. The joining model of the wear stage and the seizure stage were constructed from the joining phenomenon that was obtained by experiment. In actual simulation, the real contact area and the thermal stress at welded interface in the wear stage, and the seizure temperature and the relative speed of welded interface in the seizure stage were taken into consideration. The calculations of the friction torque and the temperature of welded interface were carried out under the conditions that were coupled with the friction pressure, the relative speed and the yield strengths of base metals. The following are concluded.
(1) The calculated values of friction torque, initial torque and elapsed time for initial torque were in approximate agreement with the experimental values. The calculated values of temperature at welded interface of center portion were higher than that of periphery portion at the time that the friction torque reached to the initial torque.
(2) It was clarified that the initial torque value depends on the seizure temperature at welded interface, and the yield strength that correspond to the seizure temperature.
(3) Simple calculation for required level to capacities of the friction welding machine can be estimated the initial torque by the seizure temperature.
