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Definition
In late 1991 a very novel and potentially world beating welding method was
conceived at TWI. The process was duly named friction stir welding (FSW), and
TWI filed for world-wide patent protection in December of that year. TWI (The
Welding Institute) is a world famous institute in the UK that specializes in materials
joining technology. Consistent with the more conventional methods of friction
welding, which have been practiced since the early 1950s, the weld is made in
the solid phase, that is, no melting is involved. Compared to conventional friction
welding, FSW uses a rotating tool to generate the necessary heat for the process.
Since its invention, the process has received world-wide attention and today two
Scandinavian companies are using the technology in production, particularly for
joining aluminium alloys. Also, FSW is a process that can be automated. It is
also a cleaner and more efficient process compared to conventional techniques. WORKING
PRINCIPLE In
friction stir welding (FSW) a cylindrical, shouldered tool with a profiled probe
is rotated and slowly plunged into the joint line between two pieces butted together.
The parts have to be clamped onto a backing bar in a manner that prevents the
abutting joint faces from being forced apart. Frictional heat is generated between
the wear resistant welding tool and the material of the work pieces. This heat
causes the latter to soften without reaching the melting point and allows traversing
of the tool along the weld line. The maximum temperature reached is of the order
of 0.8 of the melting temperature of the material. The plasticized material is
transferred from the leading edge of the tool to the trailing edge of the tool
probe and is forged by the intimate contact of the tool shoulder and the pin profile.
It leaves a solid phase bond between the two pieces. The process can be regarded
as a solid phase keyhole welding technique since a hole to accommodate the probe
is generated, then filled during the welding sequence
DESCRIPTION OF THE ROTATING TOOL PIN The
non-consumable tool has a circular section except at the end where there is a
threaded probe or more complicated flute; the junction between the cylindrical
portion and the probe is known as the shoulder. The probe penetrates the work
piece whereas the shoulder rubs with the top surface. The tool has an end tap
of 5 in 6 mm diameter and a height of 5 to 6 mm (may vary with the metal thickness).
The tool is set in a positive angle of some degree in the welding direction. The
design of the pin and shoulder assembly plays a major role on how the material
moves during the process. MICROSTRUCTURE
CLASSIFICATION The
first attempt at classifying microstructures was made by P L Threadgill (Bulletin,
March 1997). This work was based solely on information available from aluminium
alloys. However, it has become evident from work on other materials that the behavior
of aluminium alloys is not typical of most metallic materials, and therefore the
scheme cannot be broadened to encompass all materials. It is therefore proposed
that the following revised scheme is used. This has been developed at TWI, but
has been discussed with a number of appropriate people in industry and academia,
and has also been provisionally accepted by the Friction Stir Welding Licensees
Association
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