2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
53
The reason is higher welding speed results in lower heat input causing the lack of
material transportation and mixing and low welding speed cause higher temperature
and slow cooling rate resulting in grain growth and formation of precipitates [46].
Jha et al. [47] investigated the influence of welding speed on the microstructure.
Increasing welding speed from 40 to 100 mm/min led to a decrease in the grain size at
the SZ. The nugget zone corresponding to the highest welding speed of 100 mm/min
showed a very fine microstructure of size 5 μm. At low welding speed, more time is
available for the recrystallization process to form larger grains [48]. Microstructure
observed at a fixed tool welding speed of 40 mm/min with varying rotational speeds
of 800, 1000, and 1200 rpm. It shows the extent of grain growth is directly proportional to the tool rotation. It resulted in a coarser microstructure (20 μm) at 1200 rpm
and smaller grains (3-4 μm) for 800 rpm. They concluded that larger grain growth
at higher rotational speed is because of a higher heat generation. Dynamic recrystallization happens as a result of excessive plastic deformation and thermal cycle.
This results in the development of equiaxed grains through nucleation followed by
grain growth. So, recrystallized grain size depends on the peak temperature, thermal
cycle, and the degree of deformation. When the rotational speed increases, both the
factors increase resulting in larger grain growth [49].
2.3.5 Tool Tilt Angle and Plunge Depth
Plunge depth is also an important parameter for the successful weld. This is provided
to have proper forging load and contact area which helps to further increase the
heat generation. An excessive plunge depth could lead to higher flash formation
and deteriorates the weld quality due to defects [50]. Experiments were conducted
for plunge depth varying from 0.15 to 0.3 mm. Increasing plunge depth from 0.15
to 0.2 mm, the forming limit of FSWed specimen has increased. It is due to the
evolution of the thickness gradient in the FSW sample [51]. Zheng et al. [52] studied
the effect of plunge depth for 0, 0.1, 0.3, and 0.5 mm. Its effect on grain size and
mechanical properties were studied for 1200 rpm and 40 mm/min. At a zero mm
plunge depth, the joint failed during the sample preparation. For 0.1 mm, the layer
beneath the shoulder skinned off forming two hooks towards the AS and RS. This is
very important as it responsible for mechanical interlocking between the two joining
surfaces and improves weld quality. As the plunge depth was increased further to
0.3 mm the layer was worn out into pieces due to plastic deformation. This resulted
in a non-uniform distribution of these broken particles in the SZ. When the depth was
increased, the number of shattered particles increased. When plunge depth increased
to extreme 0.5 mm, the interface of the weld could not be filled leading to the
formation of the void. It resulted in flash formation throughout the weld length
without an appreciable weld formation. Results show that an increase in plunge
depth increased in weld strength of FSW specimen. The weld strength of 7.9 kN was
obtained for a plunge depth of 0.3 mm.
53
The reason is higher welding speed results in lower heat input causing the lack of
material transportation and mixing and low welding speed cause higher temperature
and slow cooling rate resulting in grain growth and formation of precipitates [46].
Jha et al. [47] investigated the influence of welding speed on the microstructure.
Increasing welding speed from 40 to 100 mm/min led to a decrease in the grain size at
the SZ. The nugget zone corresponding to the highest welding speed of 100 mm/min
showed a very fine microstructure of size 5 μm. At low welding speed, more time is
available for the recrystallization process to form larger grains [48]. Microstructure
observed at a fixed tool welding speed of 40 mm/min with varying rotational speeds
of 800, 1000, and 1200 rpm. It shows the extent of grain growth is directly proportional to the tool rotation. It resulted in a coarser microstructure (20 μm) at 1200 rpm
and smaller grains (3-4 μm) for 800 rpm. They concluded that larger grain growth
at higher rotational speed is because of a higher heat generation. Dynamic recrystallization happens as a result of excessive plastic deformation and thermal cycle.
This results in the development of equiaxed grains through nucleation followed by
grain growth. So, recrystallized grain size depends on the peak temperature, thermal
cycle, and the degree of deformation. When the rotational speed increases, both the
factors increase resulting in larger grain growth [49].
2.3.5 Tool Tilt Angle and Plunge Depth
Plunge depth is also an important parameter for the successful weld. This is provided
to have proper forging load and contact area which helps to further increase the
heat generation. An excessive plunge depth could lead to higher flash formation
and deteriorates the weld quality due to defects [50]. Experiments were conducted
for plunge depth varying from 0.15 to 0.3 mm. Increasing plunge depth from 0.15
to 0.2 mm, the forming limit of FSWed specimen has increased. It is due to the
evolution of the thickness gradient in the FSW sample [51]. Zheng et al. [52] studied
the effect of plunge depth for 0, 0.1, 0.3, and 0.5 mm. Its effect on grain size and
mechanical properties were studied for 1200 rpm and 40 mm/min. At a zero mm
plunge depth, the joint failed during the sample preparation. For 0.1 mm, the layer
beneath the shoulder skinned off forming two hooks towards the AS and RS. This is
very important as it responsible for mechanical interlocking between the two joining
surfaces and improves weld quality. As the plunge depth was increased further to
0.3 mm the layer was worn out into pieces due to plastic deformation. This resulted
in a non-uniform distribution of these broken particles in the SZ. When the depth was
increased, the number of shattered particles increased. When plunge depth increased
to extreme 0.5 mm, the interface of the weld could not be filled leading to the
formation of the void. It resulted in flash formation throughout the weld length
without an appreciable weld formation. Results show that an increase in plunge
depth increased in weld strength of FSW specimen. The weld strength of 7.9 kN was
obtained for a plunge depth of 0.3 mm.