Force control of friction stir welding
Longhurst, William Russell
As an emerging technology in the field of material joining, friction stir welding (FSW) has the potential to be automated through robotics. Due to the relatively large forces associated with FSW, force control is needed for robotic applications in order to compensate for robot linkage deflection. Prior research has determined the axial force associated with FSW is a function of the tool plunge depth, traverse speed and rotation speed. The research presented in this dissertation examines the how each of these parameters affects the force. A closed-loop force control architecture was implemented on an FSW system. The controller was tuned using a Ziegler-Nichols tuning method and experiments were conducted by butt welding ¼ inch by 1 ½ inch by 8 inch long samples of aluminum 6061. The results indicate that force control via plunge depth adjustment can be achieved with an error of less than 3%. However this mode of force control is vulnerable to stability issues. For successful implementation of force control via plunge depth, four key enablers are identified. The results indicate for force control via traverse speed a much smaller error of less than ½ % results. An energy model predicts the weld power to be constant but due to the changing traverse speed the energy deposited along the weld seam varies. It is hypothesized that the feedback of axial force provides a measure of relative temperature. Thus heat distribution control along the weld seam is achieved as a byproduct of force control via traverse speed. Force control via rotation speed has similar results but with greater error. It was discovered that torque control provides an attractive alternative to force control. Results show that torque provides a better indication of plunge depth. This is supported by an equation that shows plunge depth to be directly proportional to the torque. It is concluded that using torque as a feedback signal to control robotic FSW is viable. Controlling torque has great potential for the advancement of FSW in manufacturing and automation because the feedback signal could be obtained from the spindle motor current.