4 An Application from a Defect—A Friction …
139
Further developments focused on increasing the deformation and on providing
local turbulent flow of the plasticized material. To achieve this, a machined flat area
was incorporated onto a threaded pin, and a truncated coned pin was developed. The
addition of flats increased the weld nugget area and reduced the transverse force
and tool torque. Further, whorl pins were developed by adding helical ridges on the
pin. The introduction of the helical ridge enhanced the downward motion of the
material. The use of this pin also decreased the traverse loads and increased the v
by reducing the displaced volume. Later, by adding flutes on the helical ridges to
enhance the material mixing, MX triflute pins were developed. A helical ridge with
three flutes machined into the ridge is the main feature of the pin. The flutes improved
the deformation at the weld line. A modification of the triflute pin is the trivex pin.
Trivex pins were found to reduce the traverse and the forging forces further compared
to the MX triflute pins [14].
Thread-less pins are most commonly used for FSW of thin sheets, such as 0.4 mm
thick Mg AZ31, to avoid the weakening of the tool due to the presence of threads.
Another version of the thread-less pin is the square pin. Out of five tool profiles
studied (straight cylindrical, tapered cylindrical, threaded cylindrical, square, and
triangular) for welding AA 6061 aluminium alloy, it was found that the tool with
square pin produced defect-free welds for different axial forces used [14].
A void-like defect called the wormhole defect can be seen at the AS of the FSWed
workpiece. It forms at low processing temperatures that cause insufficient material
flow [9–11]. To avoid this defect, the ‘restir’ tool was invented by TWI. This tool is
capable of reversing its direction of rotation periodically. The latest modification in
the FSW tool is the use of the twin tool concept [17]. In the twin tool set up, two
tools are mounted on the same spindle, offering better control over mixing. They
have different variations, where the rotation can be reversed, and the gap between
the tools can be varied as per requirement [18]. These tools are proposed to provide
high productivity.
4.1.5.2 Influence of Process Parameters
The process parameters, mainly ω, v, and F z play essential roles in generating the
heat by friction, mechanical deformation, and material mixing. It was stated that the
increase in ω increases the heat input causing the material to soften for better mixing.
Nevertheless, it was also seen that for a particular material increasing the value of ω
beyond a specific limit can lead to higher heat generation that is detrimental for the
joint strength. v, on the other hand, mainly dominates the cooling rate. Too low a v
can increase the time of contact of the tool and workpiece, generating more frictional
heat and also causing a slow cooling rate giving rise to coarser grains. This can cause
voids to appear along the joint line. Again, too high v leads to inappropriate material
mixing that can lead to void formation. Thus, a perfect range of v must be selected
for greater joint strength.
F z is another critical factor that affects the joint strength significantly. The higher
F z causes the value of friction factor to increase, which increases the frictional heat
139
Further developments focused on increasing the deformation and on providing
local turbulent flow of the plasticized material. To achieve this, a machined flat area
was incorporated onto a threaded pin, and a truncated coned pin was developed. The
addition of flats increased the weld nugget area and reduced the transverse force
and tool torque. Further, whorl pins were developed by adding helical ridges on the
pin. The introduction of the helical ridge enhanced the downward motion of the
material. The use of this pin also decreased the traverse loads and increased the v
by reducing the displaced volume. Later, by adding flutes on the helical ridges to
enhance the material mixing, MX triflute pins were developed. A helical ridge with
three flutes machined into the ridge is the main feature of the pin. The flutes improved
the deformation at the weld line. A modification of the triflute pin is the trivex pin.
Trivex pins were found to reduce the traverse and the forging forces further compared
to the MX triflute pins [14].
Thread-less pins are most commonly used for FSW of thin sheets, such as 0.4 mm
thick Mg AZ31, to avoid the weakening of the tool due to the presence of threads.
Another version of the thread-less pin is the square pin. Out of five tool profiles
studied (straight cylindrical, tapered cylindrical, threaded cylindrical, square, and
triangular) for welding AA 6061 aluminium alloy, it was found that the tool with
square pin produced defect-free welds for different axial forces used [14].
A void-like defect called the wormhole defect can be seen at the AS of the FSWed
workpiece. It forms at low processing temperatures that cause insufficient material
flow [9–11]. To avoid this defect, the ‘restir’ tool was invented by TWI. This tool is
capable of reversing its direction of rotation periodically. The latest modification in
the FSW tool is the use of the twin tool concept [17]. In the twin tool set up, two
tools are mounted on the same spindle, offering better control over mixing. They
have different variations, where the rotation can be reversed, and the gap between
the tools can be varied as per requirement [18]. These tools are proposed to provide
high productivity.
4.1.5.2 Influence of Process Parameters
The process parameters, mainly ω, v, and F z play essential roles in generating the
heat by friction, mechanical deformation, and material mixing. It was stated that the
increase in ω increases the heat input causing the material to soften for better mixing.
Nevertheless, it was also seen that for a particular material increasing the value of ω
beyond a specific limit can lead to higher heat generation that is detrimental for the
joint strength. v, on the other hand, mainly dominates the cooling rate. Too low a v
can increase the time of contact of the tool and workpiece, generating more frictional
heat and also causing a slow cooling rate giving rise to coarser grains. This can cause
voids to appear along the joint line. Again, too high v leads to inappropriate material
mixing that can lead to void formation. Thus, a perfect range of v must be selected
for greater joint strength.
F z is another critical factor that affects the joint strength significantly. The higher
F z causes the value of friction factor to increase, which increases the frictional heat