72
A. K. Choudhary and R. Jain
the tool and faying surface at high temperature. The shoulder surface was highly
influenced by abrasion and adhesion wear. The porous and brittle oxide was formed
on the tool due to oxidation.
2.8.1.5 Titanium Alloys
An investigation has been done on the welding of titanium alloy with the help of
FSW. Different literature has been discussed based on the FSW approach. Application of titanium alloys in the automobile and aerospace sector is due to its superior strength-to-weight ratios, its capability to withstand high melting point, and
outstanding corrosion resistance. Apart from these titanium alloys are also used
because of their smaller thermal expansion coefficient, fire resistance, non-magnetic
nature, and short radioactive half-life. Important applications include the aerospace
industry for the manufacturing of turbine jet engines as it can withstand high temperatures and non-corrosive nature of the titanium alloy [139]. The difficulties associated
with FSW of titanium are its high strength and sufficiently high hardness. This makes
titanium difficult to soften and undergo appreciable deformation to form joint. The
tool needed for titanium alloy is of a higher grade as compared to the soft alloys like
aluminum and magnesium [139].
Gianluca Buffa et al. [140] investigated different tool materials for titanium alloy
based on refractory metals with cobalt added as a binder. As it significantly reduced
the cost and has a wide industrial application. It also limits due to the high wear
and deformation rate during the processing of titanium alloy. The best results were
obtained with the W25Re and WC tool in terms of both tool life and joint quality
without any evidence of degradation during the entire process. A special machine is
to be developed for the welding of titanium alloy because of very high reactive loads
in a range of 85–140 kN.
Sanders et al. [141] conducted experiments with a combination of higher rotational and lower welding speed. It leads to a coarse grain microstructure because
of high temperature and higher exposure time. FSW of Ti–6Al–4V sheets has been
investigated to reveal microstructural change, hardness variation, and mechanical
properties. Microstructure at the SZ results in high grain refinement and it increases
with the depth of the weld. During microstructural evolution of FSW at the SZ results
in the formation of fine equiaxed alpha grains surrounded by zigzag grain boundaries.
It is a result of beta to alpha (β → α) allotropic transformation during the cooling
cycle. Fine alpha grains showed an increase in hardness as related to the base metal.
The hardness values at the stir zone vary with depth along the transverse direction.
It changed by about 5 HV for each successive traverse depth.
Seighalani et al. [53] studied FSW of titanium alloy assisted with compressed
air jet to provide a cooling medium to lower the temperature of the tool. It is due
to the higher heat generation between the tungsten carbide tool and the titanium.
This will lead to lower degradation and wear of the tool. In addition, a 1-degree tilt
angle was reported as an optimized parameter and increasing the tilt angle up to 3°
resulted in surface defect because of the increase in forging action. FSW of titanium
A. K. Choudhary and R. Jain
the tool and faying surface at high temperature. The shoulder surface was highly
influenced by abrasion and adhesion wear. The porous and brittle oxide was formed
on the tool due to oxidation.
2.8.1.5 Titanium Alloys
An investigation has been done on the welding of titanium alloy with the help of
FSW. Different literature has been discussed based on the FSW approach. Application of titanium alloys in the automobile and aerospace sector is due to its superior strength-to-weight ratios, its capability to withstand high melting point, and
outstanding corrosion resistance. Apart from these titanium alloys are also used
because of their smaller thermal expansion coefficient, fire resistance, non-magnetic
nature, and short radioactive half-life. Important applications include the aerospace
industry for the manufacturing of turbine jet engines as it can withstand high temperatures and non-corrosive nature of the titanium alloy [139]. The difficulties associated
with FSW of titanium are its high strength and sufficiently high hardness. This makes
titanium difficult to soften and undergo appreciable deformation to form joint. The
tool needed for titanium alloy is of a higher grade as compared to the soft alloys like
aluminum and magnesium [139].
Gianluca Buffa et al. [140] investigated different tool materials for titanium alloy
based on refractory metals with cobalt added as a binder. As it significantly reduced
the cost and has a wide industrial application. It also limits due to the high wear
and deformation rate during the processing of titanium alloy. The best results were
obtained with the W25Re and WC tool in terms of both tool life and joint quality
without any evidence of degradation during the entire process. A special machine is
to be developed for the welding of titanium alloy because of very high reactive loads
in a range of 85–140 kN.
Sanders et al. [141] conducted experiments with a combination of higher rotational and lower welding speed. It leads to a coarse grain microstructure because
of high temperature and higher exposure time. FSW of Ti–6Al–4V sheets has been
investigated to reveal microstructural change, hardness variation, and mechanical
properties. Microstructure at the SZ results in high grain refinement and it increases
with the depth of the weld. During microstructural evolution of FSW at the SZ results
in the formation of fine equiaxed alpha grains surrounded by zigzag grain boundaries.
It is a result of beta to alpha (β → α) allotropic transformation during the cooling
cycle. Fine alpha grains showed an increase in hardness as related to the base metal.
The hardness values at the stir zone vary with depth along the transverse direction.
It changed by about 5 HV for each successive traverse depth.
Seighalani et al. [53] studied FSW of titanium alloy assisted with compressed
air jet to provide a cooling medium to lower the temperature of the tool. It is due
to the higher heat generation between the tungsten carbide tool and the titanium.
This will lead to lower degradation and wear of the tool. In addition, a 1-degree tilt
angle was reported as an optimized parameter and increasing the tilt angle up to 3°
resulted in surface defect because of the increase in forging action. FSW of titanium