RD&D-Programme 2004 - SKB

A programme has been devised for optimizing the welding parameters and establishing
tolerances for parameter setting. The programme comprises a test series with two levels
(high and low). The difference from the normal parameter level will be set at about five percent.
Permissible deviations from specified parameters can then be determined and serve as a basis
for the preparation of a WPS (Welding Procedure Specification).
6.3 Friction Stir Welding (FSW)
SKB has been developing friction stir welding for sealing copper canisters for spent nuclear
fuel in cooperation with TWI since 1997. FSW was invented in 1991 at TWI and is a thermomechanical
solid-state process, i.e. not a fusion welding method. This means that the problems
encountered in fusion welding, for example unfavourable grain structure and size and segregation
phenomena, can be avoided. The microstructure in copper resulting from FSW resembles
the microstructure resulting from hot forming of the copper components in the canister. FSW
can also be used to join bottoms to the copper canisters.
One reason for the rapidly increasing use of FSW in industry is that the method has few process
parameters. This means that the welding process is simple to control. The welding tool rotates at
a specific number of revolutions per minute and moves along the joint at a constant speed. The
tool also has a constant angle to the workpiece. The leading side of the tool shoulder is located
above the canister surface, so that the tool shoulder “surfs” on the surface. The position of the
tool shoulder in relation to the canister surface is then controlled with a specific downward
force, as shown in Figure 6-15.
6.3.1 Tool
The FSW tool consists of two parts: a tapered pin (or probe) and a shoulder, see Figure 6-16.
The function of the probe is to heat up the material by means of friction and, by virtue of its
shape, force the material to flow around it and create a joint. The function of the shoulder is to
heat up the material by means of friction and prevent it from being squeezed out of the joint.
There is no doubt that the tool is the most important part of the FSW process. The parts of
the tool must be able to withstand the high process temperature (800–900°C) as well as the
complex forces to which they are subjected when a weld of up to four metres is produced in
approximately 45 minutes.
Figure 6-15. Schematic drawing of the FSW process.
Figure 6-16. The FSW tool.
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