Periodic variation of torque and its relations to interfacial sticking and slipping during friction stir welding

The fraction of slipping between the tool and the workpiece during friction stir welding was calculated based on the measured torque and axial force with a sampling frequency of 1000 Hz. The results showed that the torque and fraction of slipping present a variation with the same periodicity that is equal to the time of one tool rotation. A model was proposed to present the average slipping fraction under different process parameters. The variation of the contact condition between the tool and the workpiece in one rotation was explained.