2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
59
high rotational speed along with low travel speed is another possible cause of flash
formation. This defect leads to material thinning. Selecting appropriate plunge depth
coupled with a tilt angle or concave shaped shoulder should help in eliminating it
[78].
Kissing Bond They are formed at the interface of the pin offset and the workpiece
or towards root side where the material is not bonded [79]. This is due to insufficient
stirring and low heat input within the workpiece reducing the flowability of plasticized material resulting in poor mechanical properties as shown in Fig. 2.11b. The
identification of this type of defect is challenging. It is investigated by employing
non-destructive tests like ultrasound. Also, it needs careful observations by using
optical or SEM scanning through a series of careful polishing followed by etching
of the sample [80, 81]. The kissing bond can be eliminated by selecting the proper
combination of tool rotation and welding speed to generate sufficient heat for bond
formation.
Tunnel Defect This defect is generated due to the improper combination of input
parameters. Generally, lower rotation speed and high welding speed yields tunnel
defects as shown in Fig. 2.11b. In such cases, the workpiece nugget zone is not
properly stirred, resulting in a tunnel formation along the weld. To avoid these a
proper combination of rotational and welding speed should be given to the tool
[29, 81].
Microcracks These are as shown in Fig. 2.11c and is observed along the welding
direction between the plunge and steady-state region. The reason could be attributed
to a lack of heat dissipation coming from the lower tool rotation speed. Also, an
unexpected increase in welding speed, which results in a lack of material flow forming
these microcracks. The proper combination of tool rotation speed and welding speed
can result in the elimination of these defects [80].
Void It is formed due to the lack of material stirring because of high tool welding
speed. Also, it was found that higher welding speed results in the faster cooling of the
material leading to an insufficient stirring of the material resulting in void formation
[82]. Figure 2.11d shows a void formed in the weld region. Welding speed should be
optimized according to the thickness of the base metal to be welded to reduce void
formation [80].
Root Flaws It starts from the bottom of the workpiece near the backing plate. These
are the irregular planes toward the welded zone that forms when there is an incorrect
plunge depth of the tool or pin height. It generally occurs towards the advancing
side of the workpiece. Due to the root flaw, ductility decreases [83]. Bobbin tool
eliminates this defect or selecting a proper plunge depth or pin height can reduce this
defect in the workpiece [84, 85].
Surface Defects These appear as groves, irregularity, or surface galling at the
faying/top surface of the weld after processing. These can arise due to an incorrect set
of input process parameters. This defect is formed as a result of excessive/deficient
59
high rotational speed along with low travel speed is another possible cause of flash
formation. This defect leads to material thinning. Selecting appropriate plunge depth
coupled with a tilt angle or concave shaped shoulder should help in eliminating it
[78].
Kissing Bond They are formed at the interface of the pin offset and the workpiece
or towards root side where the material is not bonded [79]. This is due to insufficient
stirring and low heat input within the workpiece reducing the flowability of plasticized material resulting in poor mechanical properties as shown in Fig. 2.11b. The
identification of this type of defect is challenging. It is investigated by employing
non-destructive tests like ultrasound. Also, it needs careful observations by using
optical or SEM scanning through a series of careful polishing followed by etching
of the sample [80, 81]. The kissing bond can be eliminated by selecting the proper
combination of tool rotation and welding speed to generate sufficient heat for bond
formation.
Tunnel Defect This defect is generated due to the improper combination of input
parameters. Generally, lower rotation speed and high welding speed yields tunnel
defects as shown in Fig. 2.11b. In such cases, the workpiece nugget zone is not
properly stirred, resulting in a tunnel formation along the weld. To avoid these a
proper combination of rotational and welding speed should be given to the tool
[29, 81].
Microcracks These are as shown in Fig. 2.11c and is observed along the welding
direction between the plunge and steady-state region. The reason could be attributed
to a lack of heat dissipation coming from the lower tool rotation speed. Also, an
unexpected increase in welding speed, which results in a lack of material flow forming
these microcracks. The proper combination of tool rotation speed and welding speed
can result in the elimination of these defects [80].
Void It is formed due to the lack of material stirring because of high tool welding
speed. Also, it was found that higher welding speed results in the faster cooling of the
material leading to an insufficient stirring of the material resulting in void formation
[82]. Figure 2.11d shows a void formed in the weld region. Welding speed should be
optimized according to the thickness of the base metal to be welded to reduce void
formation [80].
Root Flaws It starts from the bottom of the workpiece near the backing plate. These
are the irregular planes toward the welded zone that forms when there is an incorrect
plunge depth of the tool or pin height. It generally occurs towards the advancing
side of the workpiece. Due to the root flaw, ductility decreases [83]. Bobbin tool
eliminates this defect or selecting a proper plunge depth or pin height can reduce this
defect in the workpiece [84, 85].
Surface Defects These appear as groves, irregularity, or surface galling at the
faying/top surface of the weld after processing. These can arise due to an incorrect set
of input process parameters. This defect is formed as a result of excessive/deficient