Applied torque control of friction stir welding using motor current as feedback

When controlling friction stir welding, effort must be given to maintaining proper tool shoulder contact with the workpiece in order to achieve consolidation of the parent materials. Axial force control has been used prior with some success. The research presented in this article examines the controlling of welding torque as an alternative to force control. A mathematical model of welding torque was enhanced for the design and study of convex shoulder profiles. Focus was placed on linearizing the response between plunge depth and torque. The model predicted that a spherical profiled shoulder was preferred for a more linear response. In conjunction with the spherical shoulder, a closed-loop torque controller was implemented and its performance evaluated. Welding torque for feedback control was sensed indirectly through the spindle motor current using a commercially available clamp-on current meter. The system produced 1/4 in (6 mm) bead-on-plate welds that were 10 ft (3 m) in length. Over the course of the welds, the torque controller responded to workpiece elevation changes of 1/8 in (3 mm) and 1/4 in (6 mm). Results show that the tool maintained a near constant plunge depth into the workpiece as the tool traversed along the workpiece. It was concluded that the presented method of torque control is a reliable and less complex alternative to axial force control of friction stir welding.