arise when applying the complex phenomena that occur
accompanying melting and damage at the HAZ to joining
of high tensile strength steels and dissimilar materials.
Therefore, FSW, which is a non-fusion joining
method, has attracted attention as a new technology20).
Because tool durability is a problem in this joining
technology, to date, practical application had centered
on low melting point metals such as Al and the like.
However, with progress in tool development, research
has now advanced also to steel materials. In particular,
studies have been carried out on application of FSW to
high tensile strength steel sheets for automotive applications,
and expansion of the range of joining conditions
under which strength can be obtained stably can be seen.
On the other hand, in the field of thick plates, study of
application to the girth-welded joints of UOE pipes has
begun. Butt-welded joints have been fabricated with the
plate thickness of 12 mm, as shown in Photo 1, and
detailed study of the microstructure, toughness evaluation,
etc. is underway. Since FSW is a non-fusion
method, it has large merits from the viewpoint of weldability,
in that defects such as blowholes, residual stress,
and welding distortion, etc. are suppressed, and the
welding process does not generate spatter, fumes, or
slag. High expectations are placed on future practical
application in joining of dissimilar materials, including
suppression of the formation of intermetallic compounds,
which have a large negative effect that can be
seen at the bonded interface.