7 Tubular Structures: Welding Difficulty and Potential …
239
all the movements during welding. A non-consumable tool is used for welding which
comprises of a shoulder and a pin, designed according to the thickness and the type of
the tube material to be joined. The tool is rotated with a certain speed and plunged into
the abutting edges of the tube and is subsequently traversed along the longitudinal
joint line. High amount of heat generation takes place because of frictional heating
among the tool shoulder and the tube material. The material undergoes severe plastic
deformation which also contributes to the total heat generation. This localized heating
softens the material and because of the rotation of the tool the material gets stirred
and mixed by the pin, which leads to the joint formation. FSW typically exhibits
four main types of microstructural region, namely NZ, the thermo-mechanically
affected zone (TMAZ), the HAZ and the BM. One of the significant benefits of
the FSW process is that fully recrystallized, equiaxed and fine grain microstructure
is formed in the NZ [43]. The properties of the welded tubular structure possess
excellent mechanical properties, good corrosion resistance, better fatigue properties,
enhanced formability, etc., than its BM [44].
There are various factors involved in obtaining a good joint by FSW process, but
in case of tubular structures those factors are even more challenging. Some of those
key factors are mentioned below.
7.3.1 Fixture
In FSW, a fixture is required for holding and clamping the workpiece. It plays an
important role in getting a sound weld, as it bears the forces during the plunging
phase and maintains the position of the workpiece. It also absorbs all the vibrations
generated during welding. In FSW of tubular structures, the design of the fixture is
a bit complex than that required for flat sheets. Due to the curvature of the tubular
structure, the clamping system required should be such that it can hold the rolled
material firmly with the outer surface of the tube. Any gap between the tube and
the clamp can lead to distortion or buckling of the tube because of higher forces
generated during welding. Various researchers developed different types of fixtures
in order to weld tubular structures of different diameter and thickness. Researchers
developed a fixture where a die of semicircular shape was placed at the bottom as a
basic support for the bent sheets to be positioned. Two clamps were utilized to grip the
bent sheets from sideways to restrict the movement during welding [45]. Similarly,
another fixture was developed where the sheets can be rolled and then longitudinally
welded by FSW so as to obtain a tubular structure. The fixture consisted of three main
systems: the clamping system which could form the sheet into an open tube; the fixing
system which restricted the movement of the open tube; and the supporting system
which provided proper support from inside of the hollow tube [46]. Thus, designing a
fixture in FSW of tube is a complex yet an essential process for obtaining a flaw-free
tube. Another complexity arises in FSW while welding tubes with smaller diameter,
because of less contact area available to hold it and resist the forces applied by the
239
all the movements during welding. A non-consumable tool is used for welding which
comprises of a shoulder and a pin, designed according to the thickness and the type of
the tube material to be joined. The tool is rotated with a certain speed and plunged into
the abutting edges of the tube and is subsequently traversed along the longitudinal
joint line. High amount of heat generation takes place because of frictional heating
among the tool shoulder and the tube material. The material undergoes severe plastic
deformation which also contributes to the total heat generation. This localized heating
softens the material and because of the rotation of the tool the material gets stirred
and mixed by the pin, which leads to the joint formation. FSW typically exhibits
four main types of microstructural region, namely NZ, the thermo-mechanically
affected zone (TMAZ), the HAZ and the BM. One of the significant benefits of
the FSW process is that fully recrystallized, equiaxed and fine grain microstructure
is formed in the NZ [43]. The properties of the welded tubular structure possess
excellent mechanical properties, good corrosion resistance, better fatigue properties,
enhanced formability, etc., than its BM [44].
There are various factors involved in obtaining a good joint by FSW process, but
in case of tubular structures those factors are even more challenging. Some of those
key factors are mentioned below.
7.3.1 Fixture
In FSW, a fixture is required for holding and clamping the workpiece. It plays an
important role in getting a sound weld, as it bears the forces during the plunging
phase and maintains the position of the workpiece. It also absorbs all the vibrations
generated during welding. In FSW of tubular structures, the design of the fixture is
a bit complex than that required for flat sheets. Due to the curvature of the tubular
structure, the clamping system required should be such that it can hold the rolled
material firmly with the outer surface of the tube. Any gap between the tube and
the clamp can lead to distortion or buckling of the tube because of higher forces
generated during welding. Various researchers developed different types of fixtures
in order to weld tubular structures of different diameter and thickness. Researchers
developed a fixture where a die of semicircular shape was placed at the bottom as a
basic support for the bent sheets to be positioned. Two clamps were utilized to grip the
bent sheets from sideways to restrict the movement during welding [45]. Similarly,
another fixture was developed where the sheets can be rolled and then longitudinally
welded by FSW so as to obtain a tubular structure. The fixture consisted of three main
systems: the clamping system which could form the sheet into an open tube; the fixing
system which restricted the movement of the open tube; and the supporting system
which provided proper support from inside of the hollow tube [46]. Thus, designing a
fixture in FSW of tube is a complex yet an essential process for obtaining a flaw-free
tube. Another complexity arises in FSW while welding tubes with smaller diameter,
because of less contact area available to hold it and resist the forces applied by the