Friction stir welding (FSW) is an innovative joining technology, which offers several advantages over conventional fusion welding technologies. During FSW a rotating non-consumable tool induces heat into the processing zone due to friction and plastic deformation. Thereby, the material in the vicinity of the tool is softened and stirred generating the weld seam. In contrast to fusion welding technologies, in which melting and subsequent solidification of the material in the processing zone occurs, the workpiece material is only plasticized during FSW. Because friction stir welds are formed in the solid state, defect-free, joints of aluminum alloys, which usually suffer from hot cracking or the formation of gas pores if welded by conventional fusion welding technologies, can be made. The weld quality of friction stir welds is governed by the welding temperature. The welding temperature is mainly influenced by the process parameters and the thermal boundary conditions of the welding task. This means that it changes as the thermal boundary conditions change throughout the weld line. This is especially the case at complex workpiece geometries, where heat accumulation occurs. Thereby, the material in the process zone is overly softened, which leads to the formation of defects such as excessive flash. Furthermore, the mechanical properties are compromised if overheating of the workpiece material occurs.
The project aims the development of a closed loop temperature control system for friction stir welding. The purpose of the control system is to negate the influence of varying thermal boundary conditions and thereby ensure uniform weld properties throughout the weld line.
In the first project phase, adequate temperature measuring systems were identified. For this, the accuracy and the measuring dynamics of a single channel pyrometer and a tool with integrated thermocouples were examined. Furthermore, the correlation between process torque and welding temperature has been investigated to determine if an indirect temperature measurement is possible. After selecting a suitable temperature measuring system, the general effects of heat input and heat conduction during FSW were investigated. Based on this, a control algorithm was deduced and implemented on a real time system. Finally, the effectiveness of the closed loop temperature control was evaluated experimentally. Hereby, welding experiments with a systematic variation of the welding temperature were performed to determine the influence of the welding temperature on the weld quality. The weld quality was quantified by the static uniaxial tensile strength and the resulting micro structure in the weld. The result was the welding temperature, which leads to the highest weld quality.
The feasibility of a closed-loop temperature control during FSW has already been proven. The temperature controller was able to negate the influence of varying boundary conditions on complex workpiece geometries. The welding temperature was measured by a thermocouple in the tool or a single channel pyrometer and the rotational spindle speed was adjusted to ensure a constant welding temperature. Consequently, the formation of flash could be reduced and a uniform weld quality throughout the weld line was ensured. Furthermore, it was shown that lattice defects that are introduced during the FSW process have a significant influence on the tensile strength of friction stir welded samples.
Gratitude is given to the German Research Foundation (DFG) for funding the project "Temperature Control in FSW" and to the iwb e.V. for the financial support to acquire the real-time system.