RD&D-Programme 2004 - SKB

6.2.2 The welding process
In parallel with development of the equipment for electron beam welding we have also developed
the actual welding process. It was not until 2003 that all important process parameters
could be identified and controlled, so that the occurrence of unacceptable defects in the weld
can be prevented. An important part of the work has been to systematically try different combinations
of process parameters.
Conclusions in RD&D 2001 and its review
Kasam says in its review that a great deal of work remains to be done on sealing of the canister.
Further studies should be conducted of FSW and EBW until one of the methods gives completely
satisfactory results. A thorough understanding of the mechanisms that give rise to defects
in the canister must be built up.
Newfound knowledge since RD&D 2001
The welding process has been developed by means of trial weldings, where full-scale lids are
joined to shortened cylinders. Trial weldings have also been performed in copper blocks in
connection with the development and testing of new welding parameters. Through the end of
February 2004, 29 lids and 172 copper blocks had been welded. All welds have been examined
by means of NDT in the form of radiographic, ultrasonic and visual inspection. Furthermore, the
microstructure of the weld has been examined in etched macrospecimens from selected parts.
Test bars from two lid welds have also been tensile tested (L026 and L028).
New welding parameters have been tried after installation of the new type of cathode. This has
made it possible to weld at high beam powers and high welding speeds. Under such conditions
the weld pool solidifies faster. At welding speeds higher than 300 millimetres per minute, the
problems with run-out of the weld pool and internal cavities virtually disappeared. The internal
weld quality is now very homogeneous. After installation of the new equipment for oscillating
the beam, a number of different beam patterns have been tried. The purpose is to control how
the surface of the weld solidifies, see further below.
Trial welding with an oscillating beam has been done in a number of copper blocks. The trials
were performed with a portable machine from TWI that was connected to SKB’s welding
machine. After a number of different parameter settings were tried, good weld quality was
achieved. The weld was homogeneous and the weld surface was smooth with a concave
contour, see Figures 6-6 to 6-8.
By using an oscillating beam we can obtain a homogeneous weld metal. The weld surface is
treated during the process by remelting, which results in a smoother surface. Four different
beam patterns have been tried with an energy distribution of about 91 percent for welding and
Figure 6-6. Weld TB 122. Welding test with oscillating beam.
70 RD&D-Programme 2004