RD&D-Programme 2004 - SKB

A limitation in the evaluation of the welds up to 2004 has been that it was only possible to detect
the presence of volumetric defects by means of nondestructive testing (radiography), since the
technology for ultrasonic testing was not fully developed. Ultrasonic testing of the welds has
been possible since the beginning of 2004. Destructive testing (macrosection, see Figure 6-20)
is used, but since a lid weld is more than three metres long only random samples can be taken.
Additional destructive testing has been carried out. Tensile specimens (45 millimetres wide)
from lid welds show no reduction in tensile strength compared with the parent metal. The test
bars rupture in an area outside the weld zone in the heat-affected zone, see Figure 6-21. The
tensile test results for the lid welds confirm the good quality of the welds.
One problem that arose when real lids and not rings were to be welded was too much flash
on the canister side. This was a result of the large mass of copper in the lid, the high thermal
conductivity of copper and the lid clamping/cooling unit. Together, they caused the canister to
be hotter than the lid. Since copper has a relatively high coefficient of thermal expansion
(17.7·10 –6 per °C), the canister expanded more than the lid. The extra flash gave rise to surplus
heat, which had an adverse effect on the stability of the welding process. This problem was
solved by mechanically locking the lid and the canister with a male-female rebate. This modification
is now used with success at the Canister Laboratory. The joint line was also moved to a
position equivalent to that used in electron beam welding, i.e. higher up in the lid. This reduces
the asymmetric heating and expansion of the lid and canister, resulting in a more stable process.
The procedure also improves the possibilities for inspection by nondestructive testing.
As mentioned previously, the exit hole is positioned at a distance from the weld line. This
concept was transferred to the Canister Laboratory during 2003 and entails that welding starts
above the joint line, see Figure 6-18, for the purpose of isolating welding defects, but also to
enable the process to be interrupted at an early stage if it does not converge towards a stable
process. It is possible to start a new weld in a new start point without discarding the piece,
since the affected area is machined away after welding when the lid is machined to its final
dimensions.
Figure 6-20. Macrosection of FSW weld showing the fine-grained microstructure that is obtained.
80 RD&D-Programme 2004