RD&D-Programme 2004 - SKB

Conclusions in RD&D 2001 and its review
The development of the alternative method for sealing copper canisters by joining copper by
means of friction stir welding and SKB’s cooperation with TWI were described in detail in
RD&D 2001. In its review, SKI emphasized the importance of examining the joint, see sections
6.3.3 and 6.4.
Newfound knowledge since RD&D 2001
The design and manufacture of a probe, in accordance with the stipulated requirements, was the
subject of an intensive development programme during the period 2001–2003. The technical
breakthrough came with the development of a probe capable of welding 50 millimetres thick
copper. The probe is a variant of MX-Triflute. The effectiveness and reliability of the probe
is the basis of a granted patent (SKB/TWI) /6-5/.
The choice of suitable materials for the manufacture of the FSW tool proved to be a challenge.
A large number of materials with sufficient strength at high temperatures have been evaluated as
the material in the probe. A nickel-based superalloy (Nimonic 105) was chosen, since it proved
capable of producing canister welds without traces of abrasion or rupture.
Programme
Nimonic 105 will be used in SKB’s development programme during 2004. There are, however,
other materials that have certain physical properties that are superior to those of Nimonic 105.
The evaluation of alternative materials will therefore continue. A surface treatment method
has been used with some success to increase the surface hardness of probes made of the alloy
Nimonic 105. Other surface treatment methods will be evaluated with respect to their ability to
increase the service life of the FSW probe and thereby reduce the risk of defects in the process.
The geometry developed for the probe permits the production of welds with excellent properties.
The work of improving the durability, geometry and size of the probe continues. No great
changes in the probe are expected during 2004–2005, however.
The tool shoulder is made of a machined tungsten alloy with a high melting point that rapidly
reaches high temperature. The tool shoulder generates most of the friction heat, and it is unlikely
that the shape or dimensions of the shoulder will be significantly changed in the near future.
6.3.2 Welding equipment
Two welding machines have been used in the development of friction stir welding since RD&D
2001: One development machine at TWI was used in 2001–2002 and was phased out in January
2003, and one production-adapted machine was commissioned in April 2003 in the Canister
Laboratory.
The specifications for the welding machine that was described in RD&D 2001 and was
developed by TWI were determined in conjunction with early attempts to weld together
50 millimetres thick copper plates with a rebuilt milling machine. Important design parameters
were adequate strength and stiffness to counteract the forces that arise in the FSW process
without the machine being deformed.
During the development programme conducted with the welding machine at TWI,
100 millimetres high rings (taken from a canister) were mounted two by two on top of
each other and welded together. A milestone for the programme was when a real lid was
welded to a ring in January 2003.
76 RD&D-Programme 2004