2 Fundamentals of Friction Stir Welding, Its Application, and Advancements
71
Generally, polycrystalline cubic Boron nitride or tungsten carbide could be employed
as a good tool material. Tool material should retain a high-temperature strength and
toughness, excellent wear resistance and good microstructural stability at elevated
temperature [32]
Al-Moussawi and Smith [80] studied defects generated during FSW of steel.
Steel grades of DH36 and EH46 were used for joining. Experiments were conducted
at different combinations of welding parameters on as-received material. It yields a
microcrack due to lower linear speed between the plunge and the steady-state region.
Welding speed for steel was optimized, as higher speed resulted in inadequate material flow to form joint. The formation of microcrack was attributed to the elemental
precipitation of alloying elements in steel due to higher rotational speed. At high rotational speed, the peak temperature rises and formed TiN. Therefore, optimum speed
was found between 200 and 500 rpm. These elemental precipitation/segregation
resulted in the formation of microcracks which later act as the initiation of the stress
concentration region leading to the reduction of mechanical properties of the weld.
Dinda and Ramakrishnan [136] have studied FSW of 12.7 mm high-strength
martensitic steel plates. FSW tools were made of W-25Re with 2% HfC. FSW process
was carried out in water-assisted cooling. The tilt angle for the tool was maintained
at 2° for all experiments. An optimized tool rotation speed of 740 rpm and a welding
speed of 12.7 mm/min resulted in the defect-free weld. Also, increasing the welding
speed further resulted in a wormhole in the stir zone. An exhaustive microstructural
characterization of all the regions was investigated by the author. SZ had 45% bainite,
48% martensite, and rest retained austenite by volume %age resulting in the higher
hardness of the SZ as compared to other regions of the weld. The TMAZ revealed
a combination of 35% ferrite, 34% bainite, and rest as martensite phases. TMAZ
consists of alternate layers of bright equiaxed ferrite phase due to continuous dynamic
recrystallization of the pro eutectoid ferrite phase into the ferrite and austenite region.
The tempered martensite in HAZ was formed from martensite steel due to the FSW
process.
Li and Yang [137] investigated the FSW of modified 9Cr–1Mo steel. The tool was
made of W–Re alloy. It resulted in defect-free weld at optimized welding parameters
of 300 rpm and 400 rpm for a fixed welding speed of 50 mm/min, respectively.
Microstructural observation revealed the development of quenched martensite within
SZ and HAZ along with M23C6 carbide precipitate. It showed a significant drop in
the grain size of SZ as compared to the base metal. Hardness value has increased to
twice that of base metal of the SZ. A weld efficiency of 98% was obtained in the
experiment. Post weld heat treatment showed a substantial effect on the hardness
profile and microstructure of the FSW joints.
Avinish Tiwari et al. [138] investigated FSW of DH36 a shipbuilding grade steel.
Tungsten carbide tools have been used for welding in butt configuration. Two different
grades of WC tools were used with 6 wt% (Tool A) and 10 wt% Co (Tool B). Their
wear properties were analyzed. The wear mechanism depends on tool configuration
and the input welding parameters. Tool A has undergone intergranular failure of
tungsten carbide grains, which resulted in cracks initiation. Progressive wear was
investigated in tool B. Adhesion at the edge was due to compression load between
71
Generally, polycrystalline cubic Boron nitride or tungsten carbide could be employed
as a good tool material. Tool material should retain a high-temperature strength and
toughness, excellent wear resistance and good microstructural stability at elevated
temperature [32]
Al-Moussawi and Smith [80] studied defects generated during FSW of steel.
Steel grades of DH36 and EH46 were used for joining. Experiments were conducted
at different combinations of welding parameters on as-received material. It yields a
microcrack due to lower linear speed between the plunge and the steady-state region.
Welding speed for steel was optimized, as higher speed resulted in inadequate material flow to form joint. The formation of microcrack was attributed to the elemental
precipitation of alloying elements in steel due to higher rotational speed. At high rotational speed, the peak temperature rises and formed TiN. Therefore, optimum speed
was found between 200 and 500 rpm. These elemental precipitation/segregation
resulted in the formation of microcracks which later act as the initiation of the stress
concentration region leading to the reduction of mechanical properties of the weld.
Dinda and Ramakrishnan [136] have studied FSW of 12.7 mm high-strength
martensitic steel plates. FSW tools were made of W-25Re with 2% HfC. FSW process
was carried out in water-assisted cooling. The tilt angle for the tool was maintained
at 2° for all experiments. An optimized tool rotation speed of 740 rpm and a welding
speed of 12.7 mm/min resulted in the defect-free weld. Also, increasing the welding
speed further resulted in a wormhole in the stir zone. An exhaustive microstructural
characterization of all the regions was investigated by the author. SZ had 45% bainite,
48% martensite, and rest retained austenite by volume %age resulting in the higher
hardness of the SZ as compared to other regions of the weld. The TMAZ revealed
a combination of 35% ferrite, 34% bainite, and rest as martensite phases. TMAZ
consists of alternate layers of bright equiaxed ferrite phase due to continuous dynamic
recrystallization of the pro eutectoid ferrite phase into the ferrite and austenite region.
The tempered martensite in HAZ was formed from martensite steel due to the FSW
process.
Li and Yang [137] investigated the FSW of modified 9Cr–1Mo steel. The tool was
made of W–Re alloy. It resulted in defect-free weld at optimized welding parameters
of 300 rpm and 400 rpm for a fixed welding speed of 50 mm/min, respectively.
Microstructural observation revealed the development of quenched martensite within
SZ and HAZ along with M23C6 carbide precipitate. It showed a significant drop in
the grain size of SZ as compared to the base metal. Hardness value has increased to
twice that of base metal of the SZ. A weld efficiency of 98% was obtained in the
experiment. Post weld heat treatment showed a substantial effect on the hardness
profile and microstructure of the FSW joints.
Avinish Tiwari et al. [138] investigated FSW of DH36 a shipbuilding grade steel.
Tungsten carbide tools have been used for welding in butt configuration. Two different
grades of WC tools were used with 6 wt% (Tool A) and 10 wt% Co (Tool B). Their
wear properties were analyzed. The wear mechanism depends on tool configuration
and the input welding parameters. Tool A has undergone intergranular failure of
tungsten carbide grains, which resulted in cracks initiation. Progressive wear was
investigated in tool B. Adhesion at the edge was due to compression load between