1 Friction Stir Welding
13
influence of the welding. Changing the shoulder from convex to concave may change
the mode of failure of the weld as well as its microstructure. Similar phenomena may
occur when flat tool changes to a concave one. Hirasawa et al. [18] studied friction
stir welding with cylindrical, triangular, tapered and triangular taper pin geometries
and flat, concave, and convex tool shoulders. Their studies show that a concave triangular pin resulted in spot welds of enhanced strength. Li et al. [31] explained that the
angle of the shoulder surface influences the vertical force, based on the radius of the
tool pin. They reported that convex shoulder with a scroll results in higher stability
of the FSW process.
1.4.2.2 Pin Geometry and Features
In FSW, pin penetrates the workpiece and mixes the softened material and hence plays
an important role in the process. Thus, shape and size of the pin would influence the
quality of the weld. The pin length is selected based on the thickness of the workpiece
to be welded. As material softening occurs to some extent beyond the tip of the pin,
the pin length is normally taken a bit shorter than the actual workpiece thickness.
As a convention, the pin length is taken 0.85–0.95 of the thickness of the workpiece
[38]. In case of the diameter of the pin, a smaller diameter is normally preferred
because it helps in reducing the HAZ in size. But again a tool with smaller diameter
is weak and is vulnerable to failure especially when the tool is plunging down and
the vertical force acting on it is at its peak. The pin can also fracture if it faces a
higher shear load due to insufficient softening of the material during welding. As a
thumb rule, the diameter of the pin is taken as 1/3 of the diameter of the shoulder.
A pin diameter of approximately 1/3 the shoulder diameter is generally used.
The shape of the pin and the features present on it is found to affect the material
flow. Cylindrical and truncated conical shapes are the well-known pin shapes [37].
There is no downward push for cylindrical pin when the materials flow from the
front to the back of the tool. When the pin is truncated conical, some download
force component is generated leading to material flow in downward direction. This
is important while welding high-temperature materials. The addition of features to
the pin aids the vertical flow of material along with breaking up the flow. Figure 1.6
shows additional features, viz. flutes and flats, which disrupt the material flow further.
Various features and geometry of the PCBN tool system are illustrated in Fig. 1.7.
Variety of tool designs and their features along with their advantages are summarized
in Table 1.1.
1.5 Workpiece Materials Suitable for Friction Stir Welding
Welding aluminum and its alloys are the primary reasons behind the development of
FSW technique. This welding technique can be also applied for joining other nonferrous materials such as copper, magnesium and titanium. Later, FSW is developed
13
influence of the welding. Changing the shoulder from convex to concave may change
the mode of failure of the weld as well as its microstructure. Similar phenomena may
occur when flat tool changes to a concave one. Hirasawa et al. [18] studied friction
stir welding with cylindrical, triangular, tapered and triangular taper pin geometries
and flat, concave, and convex tool shoulders. Their studies show that a concave triangular pin resulted in spot welds of enhanced strength. Li et al. [31] explained that the
angle of the shoulder surface influences the vertical force, based on the radius of the
tool pin. They reported that convex shoulder with a scroll results in higher stability
of the FSW process.
1.4.2.2 Pin Geometry and Features
In FSW, pin penetrates the workpiece and mixes the softened material and hence plays
an important role in the process. Thus, shape and size of the pin would influence the
quality of the weld. The pin length is selected based on the thickness of the workpiece
to be welded. As material softening occurs to some extent beyond the tip of the pin,
the pin length is normally taken a bit shorter than the actual workpiece thickness.
As a convention, the pin length is taken 0.85–0.95 of the thickness of the workpiece
[38]. In case of the diameter of the pin, a smaller diameter is normally preferred
because it helps in reducing the HAZ in size. But again a tool with smaller diameter
is weak and is vulnerable to failure especially when the tool is plunging down and
the vertical force acting on it is at its peak. The pin can also fracture if it faces a
higher shear load due to insufficient softening of the material during welding. As a
thumb rule, the diameter of the pin is taken as 1/3 of the diameter of the shoulder.
A pin diameter of approximately 1/3 the shoulder diameter is generally used.
The shape of the pin and the features present on it is found to affect the material
flow. Cylindrical and truncated conical shapes are the well-known pin shapes [37].
There is no downward push for cylindrical pin when the materials flow from the
front to the back of the tool. When the pin is truncated conical, some download
force component is generated leading to material flow in downward direction. This
is important while welding high-temperature materials. The addition of features to
the pin aids the vertical flow of material along with breaking up the flow. Figure 1.6
shows additional features, viz. flutes and flats, which disrupt the material flow further.
Various features and geometry of the PCBN tool system are illustrated in Fig. 1.7.
Variety of tool designs and their features along with their advantages are summarized
in Table 1.1.
1.5 Workpiece Materials Suitable for Friction Stir Welding
Welding aluminum and its alloys are the primary reasons behind the development of
FSW technique. This welding technique can be also applied for joining other nonferrous materials such as copper, magnesium and titanium. Later, FSW is developed