FSW & P: Friction Stir Welding & Processing
Friction-stir welding (FSW) is a solid-state joining process (meaning the metal is not melted during the process) and is used for applications where the original metal characteristics must remain unchanged as far as possible. This process is primarily used on aluminum, and most often on large pieces which cannot be easily heat treated post weld to recover temper characteristics. It was invented and experimentally proven by Wayne Thomas and a team of his colleagues at The Welding Institute UK in December 1991.
In FSW, a cylindrical-shouldered tool, with a profiled threaded/unthreaded probe (nib or pin) is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces of sheet or plate material, which are butted together. The parts have to be clamped rigidly onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. The length of the nib is slightly less than the weld depth required and the tool shoulder should be in intimate contact with the work surface. The nib is then moved against the work, or vice versa
During welding a number of forces will act on the tool:
- A downwards force is necessary to maintain the position of the tool at or below the material surface. Some friction-stir welding machines operate under load control but in many cases the vertical position of the tool is preset and so the load will vary during welding.
- The traverse force acts parallel to the tool motion and is positive in the traverse direction. Since this force arises as a result of the resistance of the material to the motion of the tool it might be expected that this force will decrease as the temperature of the material around the tool is increased.
- The lateral force may act perpendicular to the tool traverse direction and is defined here as positive towards the advancing side of the weld.
- Torque is required to rotate the tool, the amount of which will depend on the down force and friction coefficient (sliding friction) and/or the flow strength of the material in the surrounding region (sticking friction).
As commented in previous description FSW needs of big forces (above all in Z direction) to maintain the tool against the material and be able to move it along it. The parallel configuration of tricept modules permits that these forces can be applied easily to the process. Articulated robots are limited in this task because of their poor stiffness, which limits them in being able to weld reliably in industrial conditions. The Tricept robots have become one of the preferred solutions among different companies for this type of applications due to its natural configuration being able to handle big forces with a very robust robot and maintaining the flexibility of being able to perform weldings in 3D surfaces.
There are a number of applications that can benefit from this relatively new technology, e.g. welding together long panels for railroad industry or for aeronautic applications. The use of a Tricept configuration for this applications can make it possible to have a dual machine, which can do CNC machinings while maintaining the possibility to FSW wherever is needed. This type of dual machine was selected by Riftec, which is using the tricept module T9000 sucessfully in production for some Audi parts.