2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
45
Dwelling In this stage, the already plunged tool continues to rotate about its axis
to further enhance the temperature for efficient stirring and material mixing. The
schematic of the dwelling stage is shown in Fig. 2.3b. The material underneath the
tool shoulder is further plastically deformed and softened by the generated heat due
to tool rotation. This is an optional stage and its duration depends on the requirement
of heat generation.
Welding In this stage, the rotating tool translates along the abutting edge to transport
the material from leading to the trailing edge of the tool to produce a joint. A schematic
of the same is shown in Fig. 2.3c. In this stage, actual welding is performed and it is
reported that axial force, spindle torque data attains a steady-state value and hence
an average value of this stage can be calculated for the analysis [8].
2.2 Fundamentals of FSW
2.2.1 Material Flow and Mechanism of Bond Formation
Researchers have conducted various experiments to study the material flow behavior
using various techniques like marker material [9, 10], tracer material [11], steel
balls [12], and Cu foils [10, 13] in the faying surface. Generally, Cu foil, steel
balls, or tungsten is employed as a tracer material to reveal the material flow history
by metallography inspection of the flow patterns. Copper is preferred as a marker
material because its particles entering in Al matrix can be very easily seen as a
difference of color contrast [14]. Also, tungsten material is easy to inspect during
material flow visualization. Material flow is a complex phenomenon and most of
the research has been carried out on cylindrical or threaded pins. Apart from these
square [15], triangular [16], trivex/triflat [17], and conical pins [17, 18] have also
been studied for material flow behavior.
The material flow along with all the phases of FSW are studied. When the tool
impinges into the material, the tool pin has the initial contact at the faying surface.
As the pin impinges into the material, it starts flowing in an upward direction till it
eventually gets compressed by the shoulder and finally, the material partially spills
out around the shoulder as it plunges into the workpiece. The spilled material is
known as flash. It is formed to compensate for the volume of the shoulder occupied
in the workpiece [10]. During welding, the material flow takes place due to the
relative motion between the rotating-translating tool and the workpiece and the flow
path is circular around the pin. The material encountering the pin rotates along with
it and follow a circular trajectory before getting deposited on the trailing edge of the
tool. The material flow patterns are different in AS and RS [19]. The difference in
the relative velocity on both the side has an impact on the flow patterns. Colligan
[13] and Jain et al. [20] studied the mechanism of material flow in the different
regions using threaded cylindrical and conical pins, respectively. Colligan used steel
45
Dwelling In this stage, the already plunged tool continues to rotate about its axis
to further enhance the temperature for efficient stirring and material mixing. The
schematic of the dwelling stage is shown in Fig. 2.3b. The material underneath the
tool shoulder is further plastically deformed and softened by the generated heat due
to tool rotation. This is an optional stage and its duration depends on the requirement
of heat generation.
Welding In this stage, the rotating tool translates along the abutting edge to transport
the material from leading to the trailing edge of the tool to produce a joint. A schematic
of the same is shown in Fig. 2.3c. In this stage, actual welding is performed and it is
reported that axial force, spindle torque data attains a steady-state value and hence
an average value of this stage can be calculated for the analysis [8].
2.2 Fundamentals of FSW
2.2.1 Material Flow and Mechanism of Bond Formation
Researchers have conducted various experiments to study the material flow behavior
using various techniques like marker material [9, 10], tracer material [11], steel
balls [12], and Cu foils [10, 13] in the faying surface. Generally, Cu foil, steel
balls, or tungsten is employed as a tracer material to reveal the material flow history
by metallography inspection of the flow patterns. Copper is preferred as a marker
material because its particles entering in Al matrix can be very easily seen as a
difference of color contrast [14]. Also, tungsten material is easy to inspect during
material flow visualization. Material flow is a complex phenomenon and most of
the research has been carried out on cylindrical or threaded pins. Apart from these
square [15], triangular [16], trivex/triflat [17], and conical pins [17, 18] have also
been studied for material flow behavior.
The material flow along with all the phases of FSW are studied. When the tool
impinges into the material, the tool pin has the initial contact at the faying surface.
As the pin impinges into the material, it starts flowing in an upward direction till it
eventually gets compressed by the shoulder and finally, the material partially spills
out around the shoulder as it plunges into the workpiece. The spilled material is
known as flash. It is formed to compensate for the volume of the shoulder occupied
in the workpiece [10]. During welding, the material flow takes place due to the
relative motion between the rotating-translating tool and the workpiece and the flow
path is circular around the pin. The material encountering the pin rotates along with
it and follow a circular trajectory before getting deposited on the trailing edge of the
tool. The material flow patterns are different in AS and RS [19]. The difference in
the relative velocity on both the side has an impact on the flow patterns. Colligan
[13] and Jain et al. [20] studied the mechanism of material flow in the different
regions using threaded cylindrical and conical pins, respectively. Colligan used steel