Friction stir welding at three-dimensional contours

Melt based welding processes of spatially curved aluminum structures are state-of-the-art. However, these processes cannot be applied for alloys prone to crack formation and material combinations with a low capacity for handling cyclic loads. In these cases friction stir welding presents a promising alternative.

A challenge, however, is the capability to weld along a three-dimensional path. The process requires a very high control of the welding position and a very stiff welding machine, which makes it difficult to weld large parts. The objective therefore is to develop a cost efficient machine concept for the industrial friction stir welding of larger parts.

The technical approach was to extend the typical application range of a so-called Pentapod milling machine center by collaborating with the equipment manufacturer. Such machines are based on the parallel kinematic principle. This substantially reduces the required machine weight (typical to about 10%) for stable operation while maintaining the workspace, stiffness and positioning precision compared to conventional CNC machine tools.

Friction stir welding typically requires control algorithms, which can be adapted for complex three-dimensional welding tasks. This is possible by developing internal process control systems, which can be activated and adapted by the user depending on the particular application. A successful friction stir welding process was, for example, demonstrated using aircraft fuselage segments which were provided for research purposes.

The transition of the system to the user is also simplified. The typically stationary machines can be modularized for mobile deployment. This way it is feasible to apply friction stir welding processes at construction sites. Especially when applying the technique to very large parts this concept enables completely new possibilities.

Additionally it is possible to envision a similar deployment of a laser beam welding system, which was successfully tested. Thus it is possible to combine processes such as laser beam and friction stir welding with milling of complex parts in a single setup. To increase the protection of parts or weld seams from crack formation it is possible to introduce compressive stresses via three-dimensional deep rolling processes, which were successfully implement as well.