232
D. Sen et al.
In arc welding processes, the tube welding is carried out mostly by using a filler
material. Filler material, even if is of the same material as the tube, forms an in
homogenity in the microstructural characteristics. Also, in arc welding, always a
weld reinforcement is present, both internally and externally, in the surface of the
tube which deteriorates its surface quality. Welding of thicker tubular structures needs
multiple passes in arc welding. Due to these multi-passes, the residual stresses in the
joint increase, which in turn reduce the life of the component. To avoid these problems, manufacturers have started joining tubes with resistance welding technique. In
this method, no filler material is required resulting in microstructural homogeneity
and good surface finish. However, in this technique, the edges of the tube are melted
for joining, thus in some materials, the alloying elements tend to combine with atmospheric oxygen and form high melting point oxides. These oxides persist in the seam
and generates welding defects such as inclusions, incomplete joining, lack of fusion
and hook crack, and reduces the long sustainability of the tubes [10]. In arc welding,
as well as resistance welding, a high heat input is required since the melting of the
tube material takes place for joining. Due to this, a wider heat-affected zone (HAZ) is
created in the welded tubes. This changes the microstructure of this zone and results
in softening which reduces its strength and leads to failure. In some industries, tubular
structures are used in critical places, like in nuclear reactors, where no compromise
with the desired properties is allowed to avoid a miserable disaster. Therefore, very
precision welding of tubes is required with minimum defects and lower HAZ to minimize the risk. In such cases, high energy density welding technique is used where
the energy required for welding is focused into a very small area which reduces the
overall heat input for welding. This results in minimal degradation of the base material with narrow HAZ and low residual stress [13]. There are a variety of materials
used for tubing which include stainless steels, ferritic steels and nickel base alloys
to name a few. These materials are generally welded by using conventional welding
techniques. However, there are many applications where welding cannot be used
owing to environmental concerns or where non-weldable materials must be used due
to the necessity of specific material properties. Also, as mentioned earlier, lightweight
structures are preferred in automotive and aerospace industries; thus, materials like
aluminium alloys having high strength-to-weight ratio are generally preferred, but
cannot be welded by the above-mentioned welding techniques. Aluminium metal has
high thermal conductivity and high reflectivity, causing it difficult to be welded. Also,
they have high hydrogen solubility at higher temperature which leads to defects, such
as porosity and blow holes during fusion welding [14]. Thus, to overcome these difficulties, and to manufacture tubes of aluminium alloys, solid-state welding technique
is used. In this welding technique, no melting of the tubular material takes place,
only softening occurs due to temperature and pressure, which on cooling creates a
solid phase bond. Since solid-state welding occurs below the melting point of the
tube material, a very low heat input is required in welding. This reduces the solidification defects and distortion of the weld and improves the dimensional accuracy
of the tubes. In solid-state welding, the weld zone comprises of fine recrystallized
grains which improves the mechanical as well as microstructural properties of the
tubes. The heat input in solid-state welding is lesser in comparison to the fusion
D. Sen et al.
In arc welding processes, the tube welding is carried out mostly by using a filler
material. Filler material, even if is of the same material as the tube, forms an in
homogenity in the microstructural characteristics. Also, in arc welding, always a
weld reinforcement is present, both internally and externally, in the surface of the
tube which deteriorates its surface quality. Welding of thicker tubular structures needs
multiple passes in arc welding. Due to these multi-passes, the residual stresses in the
joint increase, which in turn reduce the life of the component. To avoid these problems, manufacturers have started joining tubes with resistance welding technique. In
this method, no filler material is required resulting in microstructural homogeneity
and good surface finish. However, in this technique, the edges of the tube are melted
for joining, thus in some materials, the alloying elements tend to combine with atmospheric oxygen and form high melting point oxides. These oxides persist in the seam
and generates welding defects such as inclusions, incomplete joining, lack of fusion
and hook crack, and reduces the long sustainability of the tubes [10]. In arc welding,
as well as resistance welding, a high heat input is required since the melting of the
tube material takes place for joining. Due to this, a wider heat-affected zone (HAZ) is
created in the welded tubes. This changes the microstructure of this zone and results
in softening which reduces its strength and leads to failure. In some industries, tubular
structures are used in critical places, like in nuclear reactors, where no compromise
with the desired properties is allowed to avoid a miserable disaster. Therefore, very
precision welding of tubes is required with minimum defects and lower HAZ to minimize the risk. In such cases, high energy density welding technique is used where
the energy required for welding is focused into a very small area which reduces the
overall heat input for welding. This results in minimal degradation of the base material with narrow HAZ and low residual stress [13]. There are a variety of materials
used for tubing which include stainless steels, ferritic steels and nickel base alloys
to name a few. These materials are generally welded by using conventional welding
techniques. However, there are many applications where welding cannot be used
owing to environmental concerns or where non-weldable materials must be used due
to the necessity of specific material properties. Also, as mentioned earlier, lightweight
structures are preferred in automotive and aerospace industries; thus, materials like
aluminium alloys having high strength-to-weight ratio are generally preferred, but
cannot be welded by the above-mentioned welding techniques. Aluminium metal has
high thermal conductivity and high reflectivity, causing it difficult to be welded. Also,
they have high hydrogen solubility at higher temperature which leads to defects, such
as porosity and blow holes during fusion welding [14]. Thus, to overcome these difficulties, and to manufacture tubes of aluminium alloys, solid-state welding technique
is used. In this welding technique, no melting of the tubular material takes place,
only softening occurs due to temperature and pressure, which on cooling creates a
solid phase bond. Since solid-state welding occurs below the melting point of the
tube material, a very low heat input is required in welding. This reduces the solidification defects and distortion of the weld and improves the dimensional accuracy
of the tubes. In solid-state welding, the weld zone comprises of fine recrystallized
grains which improves the mechanical as well as microstructural properties of the
tubes. The heat input in solid-state welding is lesser in comparison to the fusion