Influence of microstructure and microgeometry of the probe on friction stir welding of AA 6060 T66

Increasing demands on joining technology in terms of lightweight construction and component complexity require the further development of technologically suitable welding processes. Friction stir welding is a promising alternative to conventional fusion welding processes. The central element of this technology is a tool consisting of a shoulder and a probe, which generates a friction-induced heat input through rotation and pressure. The process is used in automotive, shipbuilding and aerospace applications due to its excellent mechanical material properties, which are similar to those of the base material. However, FSW is subject to process-specific challenges, including comparatively high process forces, high clamping requirements and tribological stresses during welding, as the process involves constant contact between the tool and the workpiece. As the welding process progresses, the tribochemical stresses cause the tool shape and material to change. The result is premature tool failure and reduced process reliability.
While previous investigations have focused on increasing wear resistance by selecting a suitable coating system and varying tool geometries, the aim of this study is to adapt the surface of the welding probe by changing the microgeometry and microstructure. Based on the current state of the art, the present work investigates the influence of the welding probe by adapting the microgeometry and microstructure and shows the extent to which tool wear, weld seam quality and behaviour in the welding process can be influenced. By analogy with machining, individual welding probes will be prepared by drag finishing and the effect on tool geometry and wear will be demonstrated. In addition, a feasibility study will be carried out on the thread rolling of welding probes with metric threads. This process, which is used to adjust the microstructure, will show whether production by rolling is feasible and to what extent the surface finish of the rolled welding probes differs from that of conventionally produced ones. The investigations were carried out using a force-controlled robotic welding setup to weld 5 mm thick AA 6060 T66 sheets. A Kistler multicomponent dynamometer type 9139AA was used to measure the cartesian forces in the x-, y- and z-directions. The weld seam properties of the initial and the optimized tool designs were determined by visual and metallographic inspection and tensile tests to DIN EN ISO 25239-5.