The influence of gaps and misalignment on friction stir welded butt joints of medium-sized parts

In order to industrialize friction stir welding (FSW) processes, not only the machine concept itself needs to be evaluated, but also the robustness of the process application being carried out on the machine. Especially for FSW of medium-sized and larger parts, a small degree of misalignment can have an increasing influence on the weld quality. Therefore an exemplary tolerance study for friction stir welded butt joints was conducted. The intentional introduction of gaps between the sheets to be welded can limit the welding process and thus the weld quality. However, for the considered experimental set-up it can be shown that the introduction of a well defined gap can support the welding process and weld quality. The experimental procedure was carried out on a parallel kinematic machine - a so-called Pentapod. This machine is suitable for large and complex three-dimensional structures. Although the machine is able to record the process forces (Fx, Fy and Fz) acting on the tool-work piece interface, the forces acting perpendicular to the clamping system are still unknown. Therefore additional load cells were integrated into the clamping system to measure the in-process reaction forces. The combined results of the force measurements give a nearly complete overview of the internal loads during the process. In conjunction with knowledge about gaps and misalignment, the data gained in this study can help to understand and predict the clamping behaviour, and thus design rules for future clamping systems can be derived.