2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
61
of the precipitates at that rotational speed with the significant amount of grain refinement. This continues to soften till TMAZ/HAZ due to the softening of the material.
Thereafter, the reoccurrence of fine precipitates results in an increase in hardness till
the base metal. The tensile properties of the weld seam decrease in comparison to the
base material due to the softening effect at HAZ leading to failure of material from
that region. The region next to TMAZ is HAZ where the effect of plastic deformation
is very less. It only experiences a significant amount of thermal cycle. Due to thermal
effects, it results in the occurrence of fine precipitates in an alloy. Also, a reduction
in hardness profile investigated while increasing the tool shoulder diameter. It is due
to higher heat dissipation resulted in grain growth and coarser microstructure [88].
Hardness distribution reveals that with an increase in rotational speed hardness of
the weld decrease.
2.6 Metallurgical Aspect (Microstructure and Grain Size)
Fine grains are produced due to continuous dynamic recrystallization during the
welding stage. Circular onion ring continuously forms due to the rotationaltranslation tool motion in the entire welding phase [8]. Finer grains are possible
as a result of at low rotational speed and high welding speed or because of faster
cooling [29].
A base metal microstructure is always a mixture of coarse and fine grains. After
FSW, the weld nugget zone contains fine equiaxed grains. Grains have undergone a
dissolution of around half of precipitates present in alloy into the base metal matrix
during the welding process. In the stirred zone a continuous grain boundary misorientation occurs along the weld line because of the continuous dynamic recrystallization
process which is the primary mechanism for the formation of fine equiaxed nugget
grains [89]. In HAZ, grains do not undergo plastic deformation but it only undergoes
thermal cycle this results in the growth of grains without a change in shape during
FSW. In the intervening time, the grains in TMAZ show a significant change in
shape indicating the possibility of plastic deformation caused by the tool. Also, the
thermomechanical affected zone (TMAZ) has experienced a similar but lesser extent
in the dissolution of precipitates whereas it was observed that significant coarsening
of precipitates in the HAZ takes place [83].
61
of the precipitates at that rotational speed with the significant amount of grain refinement. This continues to soften till TMAZ/HAZ due to the softening of the material.
Thereafter, the reoccurrence of fine precipitates results in an increase in hardness till
the base metal. The tensile properties of the weld seam decrease in comparison to the
base material due to the softening effect at HAZ leading to failure of material from
that region. The region next to TMAZ is HAZ where the effect of plastic deformation
is very less. It only experiences a significant amount of thermal cycle. Due to thermal
effects, it results in the occurrence of fine precipitates in an alloy. Also, a reduction
in hardness profile investigated while increasing the tool shoulder diameter. It is due
to higher heat dissipation resulted in grain growth and coarser microstructure [88].
Hardness distribution reveals that with an increase in rotational speed hardness of
the weld decrease.
2.6 Metallurgical Aspect (Microstructure and Grain Size)
Fine grains are produced due to continuous dynamic recrystallization during the
welding stage. Circular onion ring continuously forms due to the rotationaltranslation tool motion in the entire welding phase [8]. Finer grains are possible
as a result of at low rotational speed and high welding speed or because of faster
cooling [29].
A base metal microstructure is always a mixture of coarse and fine grains. After
FSW, the weld nugget zone contains fine equiaxed grains. Grains have undergone a
dissolution of around half of precipitates present in alloy into the base metal matrix
during the welding process. In the stirred zone a continuous grain boundary misorientation occurs along the weld line because of the continuous dynamic recrystallization
process which is the primary mechanism for the formation of fine equiaxed nugget
grains [89]. In HAZ, grains do not undergo plastic deformation but it only undergoes
thermal cycle this results in the growth of grains without a change in shape during
FSW. In the intervening time, the grains in TMAZ show a significant change in
shape indicating the possibility of plastic deformation caused by the tool. Also, the
thermomechanical affected zone (TMAZ) has experienced a similar but lesser extent
in the dissolution of precipitates whereas it was observed that significant coarsening
of precipitates in the HAZ takes place [83].