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D. Sen et al.
tool. Special holding devices are required during welding of tubular structures which
needs to be designed according to the diameter of the tubular structure [41, 47].
In FSW, large vertical force is exerted by the tool during the welding process.
This can damage the surface of the tubular structure, and hence compromise its
circularity. Thus, during FSW of tubular components, a mandrel is provided inside
the tube which provides a counter balancing force to the force employed by the tool
on the tube. In other words, the mandrel bears the vertical force applied by the tool
and prevents distortion of the tube. Thus, the mandrel should be designed in such a
way that it fits properly with the inner diameter of the tubular structure to guarantee
a good welded joint. Also, the mandrel should easily mantle and dismantle without
damaging the tube before and after the welding. In this context, many researchers
tried to design and fabricate an expandable mandrel which could be easily expanded
by fastening screws to be used for various tubes with varying inner diameter. Again,
on unscrewing it, the mandrel can easily come out from the tube without any damage
to the interior surface of the tube. Moreover, this mandrel could be adjusted to apply
the desired degree of support to the tube [41, 48, 49].
7.3.2 Tool Design
The tool design is a significant parameter in attaining a flawless joint in welding
tubular components by FSW. In FSW, the heat required to plasticize the material
is produced because of friction among the tool and the material. The shoulder of
the tool rubs against the curved surface of the tube and the generated heat softens
the material enabling the material to flow. Unlike in flat butted plates, in tubular
structures, the tool shoulder makes contact with the curved outer surface of the tube.
Because of the curvature of the tube, the tool shoulder does not have full contact with
the tube surface, as shown in Fig. 7.6. As a result of this, the heat generated is less
in case of tube welding. This, in turn, results in less softening of the material and
improper material flow leading to a defective joint. It has been reported that welding
tubes using flat shoulder tool decreased the tensile strength of the joint by 65% with
respect to the tube material due to ineffective material flow [37]. Thus, researchers
suggested that joining tube with a double pass can lead to a defect-free weld where
the first pass can improve the shoulder contact with the tube by flattening the curved
surface during welding, and the second pass can improve the surface quality of the
welded tube [50].
Tool shoulder diameter is an important parameter in FSW of tubular components.
Tools having smaller shoulder diameter will result in a defective welded tubular joint
since it will not generate enough frictional heat required for the material softening. It
may result in defects such as tunnels or wormholes in the welded tube. Conversely,
tools with large shoulder diameter cannot be used because it will not increase the
heat generation further. This is because the contact of the shoulder of the tool will
not increase due to the curvature of the tube. Thus, selection of an optimum shoulder
D. Sen et al.
tool. Special holding devices are required during welding of tubular structures which
needs to be designed according to the diameter of the tubular structure [41, 47].
In FSW, large vertical force is exerted by the tool during the welding process.
This can damage the surface of the tubular structure, and hence compromise its
circularity. Thus, during FSW of tubular components, a mandrel is provided inside
the tube which provides a counter balancing force to the force employed by the tool
on the tube. In other words, the mandrel bears the vertical force applied by the tool
and prevents distortion of the tube. Thus, the mandrel should be designed in such a
way that it fits properly with the inner diameter of the tubular structure to guarantee
a good welded joint. Also, the mandrel should easily mantle and dismantle without
damaging the tube before and after the welding. In this context, many researchers
tried to design and fabricate an expandable mandrel which could be easily expanded
by fastening screws to be used for various tubes with varying inner diameter. Again,
on unscrewing it, the mandrel can easily come out from the tube without any damage
to the interior surface of the tube. Moreover, this mandrel could be adjusted to apply
the desired degree of support to the tube [41, 48, 49].
7.3.2 Tool Design
The tool design is a significant parameter in attaining a flawless joint in welding
tubular components by FSW. In FSW, the heat required to plasticize the material
is produced because of friction among the tool and the material. The shoulder of
the tool rubs against the curved surface of the tube and the generated heat softens
the material enabling the material to flow. Unlike in flat butted plates, in tubular
structures, the tool shoulder makes contact with the curved outer surface of the tube.
Because of the curvature of the tube, the tool shoulder does not have full contact with
the tube surface, as shown in Fig. 7.6. As a result of this, the heat generated is less
in case of tube welding. This, in turn, results in less softening of the material and
improper material flow leading to a defective joint. It has been reported that welding
tubes using flat shoulder tool decreased the tensile strength of the joint by 65% with
respect to the tube material due to ineffective material flow [37]. Thus, researchers
suggested that joining tube with a double pass can lead to a defect-free weld where
the first pass can improve the shoulder contact with the tube by flattening the curved
surface during welding, and the second pass can improve the surface quality of the
welded tube [50].
Tool shoulder diameter is an important parameter in FSW of tubular components.
Tools having smaller shoulder diameter will result in a defective welded tubular joint
since it will not generate enough frictional heat required for the material softening. It
may result in defects such as tunnels or wormholes in the welded tube. Conversely,
tools with large shoulder diameter cannot be used because it will not increase the
heat generation further. This is because the contact of the shoulder of the tool will
not increase due to the curvature of the tube. Thus, selection of an optimum shoulder