RD&D-Programme 2004 - SKB

Figure 6-21. Tensile specimens from lid weld.
After one lid weld at TWI and eleven lid welds in the Canister Laboratory’s new welding
machine, the FSW process can be summarized as robust and stable. However, it is not fully
developed in the sense of full automation of the process to eliminate the human factor. But
experience has shown that it is possible to weld with high repeatability and reliability despite
this limitation. The reason is that the only parameter that is changed during a welding cycle is
the downward force, and it has a relatively high tolerance.
The problem with the exit hole has been solved by parking the tool above the joint line, which
means the hole is machined away when the lid is machined to its final dimensions. The start
sequence also takes place at this height above the joint line, which means that any start defects
are machined away.
Programme
An important phase of the development programme is optimizing the welding parameters
and establishing tolerances for parameter setting. A programme for this has been devised that
includes a test series with three levels of selected parameters. Permissible deviations from
specified parameters can then be determined and serve as a basis for the preparation of a WPS
(Welding Procedure Specification). This part of the programme is expected to be carried out
during 2004, see also /7-1/.
In the subsequent demonstration phase, which will take place under production-like forms, the
method’s potential as a production method will be demonstrated on approximately 20 welds and
the resulting weld quality will be investigated for further statistical processing to provide input
data for the safety assessment SR-Can.
Material evaluation including nondestructive and destructive testing will be carried out to fully
evaluate the integrity of FSW lid welds. Metallurgical investigations will be performed in a
structured manner so that the integrity of the welds can be related to the welding conditions.
Much of this evaluation will be carried out by Swedish research and development centres and
Swedish universities. In parallel with these studies, mechanical tests, corrosion tests, creep tests
and chemical analyses of the weld will be performed.
RD&D-Programme 2004 81