Enhancing friction stir welding through process instrumentation and closed-loop control

The objectives of this work are to determine an accurate temperature measurement strategy and to develop a closed-loop feedback control system for Friction Stir Welding (FSW). FSW is a novel joining technology enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings. Numerous parameter and condition variations are present in the FSW production environment that can adversely affect weld quality. To enable large scale automation while maintaining weld quality, techniques to control the FSW process in the presence of unknown disturbances must be developed. Using a custom wireless data acquisition system, two thermocouples were placed in through-holes directly at the interface of tool and workpiece so that the tips are in contact with the workpiece material. This measurement strategy reveals temperature variations within a single rotation of the tool in real-time. It was found that the temperatures correlate with weld quality (mechanical testing and weld cross-sections) and that the highest temperatures are at the shoulder interface between the advancing side and the trailing edge of the tool, closer to the advancing side. The dynamic temperature measurements obtained with the current system are of unmatched resolution, fast and reliable and are likely to be of interest for fundamental studies, process monitoring and control of FSW. A linear multi-input-multi-output process model with transport delay has been developed that captures the dynamic relations between two process inputs (spindle speed and vertical tool position) and two process outputs (interface temperature and axial force). A closed-loop temperature control system was implemented on a CNC mill and applied to welding of 6061-T6 aluminum. The control system was also transferred to a robotic FSW system control in a real manufacturing environment and combined with axial force control. Desired axial forces and interface temperatures are achieved by automatically adjusting the spindle speed and the vertical tool position at the same time. The axial force and interface temperature are maintained during both thermal and geometric disturbances and thus weld quality can be increased for a variety of conditions in which each control strategy applied independently would fail.