144
P. Sarkar et al.
4.2 Defects in FSW Process
Being a solid-state welding process, the defects generated during FSW process are
very few and can be controlled effectively. The defects e.g. flash, voids, wormhole,
kissing bond, etc. can be seen in the weld region if the selection of process conditions
is not proper. Any material can be deformed mechanically when the resistance offered
by the material to deformation is exceeded by supplying sufficient energy. In FSW,
this energy is in the form of heat. Again, the resistance offered is dependent on the
material properties and these properties are related to the composition of the material.
Thus, one should always consider the material and choose the process conditions that
are compatible with the material to form a strong joint. Conditions like excessive
or insufficient heat generation and also improper mixing of the material due to tool
geometry or improper positioning of the tool on the workpiece can lead to the formation of the following defects that severely deteriorate the joint strength. However,
defects can be completely eliminated by choosing proper process parameters.
4.2.1 Defects Generated Due to Excessive Heat Generation
Excessive heat generation occurs if ω is high and v is low. The increase in PD also
adds to the heat generation. Combined effect of all these factors softens the material
excessively which lead to the following defects during the process:
Nugget collapse Although the excessive heat generation can lead to the softening of
the material around the immediate neighborhood of the pin forming the nugget zone
with recrystallized grain, the shoulder continues to apply a torsional effect on the
material. However, due to the local drop in temperature for high thermal conductivity
materials and also because of the insufficiency of this torsion to recrystallize the
material beneath the shoulder, coarser grains are formed than that in the recrystallized
NZ. This broadens the TMAZ zone at the cost of collapsing the nugget [32, 33]. This
defect reduces the joint strength.
Surface galling Due to severe plasticization of the material, some small blisters start
to appear on the surface of the weld line which is called surface galling [34]. This
hampers the welding appearance. These blisters can also act as a site for corrosion.
Surface galling can be seen in Al alloys.
Onion ring formation The onion ring forms as a band that matches with the forward
motion of the tool in a single rotation. Too high ω can increase the spacing between
the bands, and at hotter process condition it was observed that fracture occurs along
with the band in the weld line [35].
Flash Flashes are generated when the material is over plasticized due to high heat
input and the shoulder cannot retain the material under it, expelling them as a form
of flash. Too high PD also leads to the formation of the excessive flash generation
P. Sarkar et al.
4.2 Defects in FSW Process
Being a solid-state welding process, the defects generated during FSW process are
very few and can be controlled effectively. The defects e.g. flash, voids, wormhole,
kissing bond, etc. can be seen in the weld region if the selection of process conditions
is not proper. Any material can be deformed mechanically when the resistance offered
by the material to deformation is exceeded by supplying sufficient energy. In FSW,
this energy is in the form of heat. Again, the resistance offered is dependent on the
material properties and these properties are related to the composition of the material.
Thus, one should always consider the material and choose the process conditions that
are compatible with the material to form a strong joint. Conditions like excessive
or insufficient heat generation and also improper mixing of the material due to tool
geometry or improper positioning of the tool on the workpiece can lead to the formation of the following defects that severely deteriorate the joint strength. However,
defects can be completely eliminated by choosing proper process parameters.
4.2.1 Defects Generated Due to Excessive Heat Generation
Excessive heat generation occurs if ω is high and v is low. The increase in PD also
adds to the heat generation. Combined effect of all these factors softens the material
excessively which lead to the following defects during the process:
Nugget collapse Although the excessive heat generation can lead to the softening of
the material around the immediate neighborhood of the pin forming the nugget zone
with recrystallized grain, the shoulder continues to apply a torsional effect on the
material. However, due to the local drop in temperature for high thermal conductivity
materials and also because of the insufficiency of this torsion to recrystallize the
material beneath the shoulder, coarser grains are formed than that in the recrystallized
NZ. This broadens the TMAZ zone at the cost of collapsing the nugget [32, 33]. This
defect reduces the joint strength.
Surface galling Due to severe plasticization of the material, some small blisters start
to appear on the surface of the weld line which is called surface galling [34]. This
hampers the welding appearance. These blisters can also act as a site for corrosion.
Surface galling can be seen in Al alloys.
Onion ring formation The onion ring forms as a band that matches with the forward
motion of the tool in a single rotation. Too high ω can increase the spacing between
the bands, and at hotter process condition it was observed that fracture occurs along
with the band in the weld line [35].
Flash Flashes are generated when the material is over plasticized due to high heat
input and the shoulder cannot retain the material under it, expelling them as a form
of flash. Too high PD also leads to the formation of the excessive flash generation