Page 79 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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usually carried out in custom heavy-duty machine tool equipment, where weld
joints are frequently limited to straight lines or two-dimensional contours. The use
of industrial robots increases the flexibility of this process, providing the ability to
weld three-dimensional contours [46].

Nowadays FSW is used in the welding of pieces in aluminum alloys ranging in
thickness from 0.5 to 75 mm. It is being used in the shipbuilding and the marine
industries, for manufacturing of panels, platforms and heavy profiles, in the
aerospace industry for production of fuel tanks, wings and fuselages, in the railway
industry for high speed trains, in the automotive industry, for production of panels
and other components, etc.

2.5.2 Welding Equipment

In the beginning mainly high stiffness machines were developed, specifically
tailored to meet client needs, but nowadays standardized, flexible and modular
systems suited to several industry segments are being produced too, for welding
nonferrous metals. These latter systems consist of a sturdy basic framework, a set
of safety stops, a welding carriage assembly, a welding head assembly, a control
system, a hydraulic unit and the welding tools [47]. These machines can have
several basic designs, providing vertical down forces ranging from 6 to 200 kN,
welding speeds up to 2 m/min, though an option up to 6 m/min exists, and tool
rotation speed between 500 and 2000 rpm.

As referred above these machines have low flexibility producing welds in simple
two or three-dimensional pathways. Robotic systems allow the improving of
flexibility but need to be able to apply and maintain a large and constant axial force
during the welding operation, which is not simple in these systems. This is done
using high payload robots that sense the force directly and use feedback to
maintain the force during the welding operation. The axial force decreases with
increasing tool rotation speed but increases with increasing travel speed, and
therefore for robotic FSW a compromise may need to be established between travel
speed and axial force requirements [46].

The appropriate tool type is a key factor of the quality of friction stir welded joints.
For butt welding aluminum alloys of thickness up to 12 mm cylindrical threaded
pin probes are recommended, while for thicker plates the Whorl and MX-Triflute
probes should be used [48], see Figure 2.25. These latter probe types allow welding
speeds that exceed largely those achievable with threaded pin probes: at least by a
factor of 2. In addition they have flat or re-entrant features or oval cross section,
which reduce the probe volume (static volume), allowing one to achieve a suitable
swept volume (dynamic volume) to static volume ratio. The greater this ratio, the
greater the path for material flow and the efficiency of the probe [49].