Many investigations have been reported for the mechanism of friction welding process of similar materials. However, the mechanism has not been clarified precisely up to now. A series of studies has been carrying out to clarify the joining mechanism of friction welding process. As for the first step, the present paper describes the joining phenomena in the first phase of friction welding process, i.e., friction stage. The materials joined were mild steels (same materials), and a brake type friction welding equipment was used throughout this experiment. Main conclusions are obtained as follows. (1) When the weld faying surfaces of both substrates contacted each other, the friction torque rapidly increased and maintained at the nearly constant. Following this, the friction torque reached to the initial peak torque. (2) The wear of both surfaces started at the periphery portion (outer surface) of joint, and moved to the center portion (center axis). Then, seizure and joining began at the center portion, and they extended toward the periphery portion. (3) The friction torque reached to the initial peak torque when the welded interface was joined completely and upsetting of both substrates started. (4) The joining models at the first phase in wear and the seizure stages were made from the observation results. As a conclusion, the first phase of friction stage was composed of the wear and the seizure stages.
