6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
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favourable-orientated grains grow for a considerable distance, while the growth of
less favourable-oriented grains is blocked by fast-growing grains [28]. For this reason,
the columnar microstructure is seen in weld metal where relatively longer grains are
present, and these grains grow parallel with the direction of heat flow. These grains
grow due to favourable crystal orientation and solidification. The initial grains to
solidify are the ones that are close to the unmelted parent material. The unmelted
parent metal nucleates these grains to maintain equal crystal orientation. In metals
having body-centred cubic (BCC) and FCC crystal structures, solidification growth
occurs in preference to <100> crystallographic direction. As solidification is most
prominent in this crystallographic orientation, this direction is sometimes called as
the direction of “easy growth”. In metal having hexagonal closed pack (HCP) crystal
structure, grain growth occurs in the <1010> direction parallel to the basal plane.
The grain growth becomes prominent if this direction of easy growth comes parallel
with the direction of heat flow over the S–L interface [29].
The crystallographic orientations in the weld regions obtained in different fusion
welding techniques have been discussed in the following sections.
6.3.1.1 Resistance Spot Welding (RSW)
The values of current and voltage largely affect the grain size and the texture in
RSW as found in [30]. RSW at higher values of the time and current (300 ms–
9.5 kA), produces randomly textured microstructure in the weld zone. In reverse,
strongly textured microstructures are obtained at a low value of time and current (i.e.
200 ms–8 kA). The major texture components in RSWed samples can be classified
as deformation and recrystallization textures. Later one includes Q {013} <123> ,
Goss {110} <001> , Cube {001} <100> and RCND1{001} <310> , whereas the
deformation texture components include S {123} <634> , Brass {110} <112> ,
Shear1{001} <110> and Shear2 {111} <110> . The volume fractions of these texture
components depend on process parameters of the welding. Welding at a lower range
of time and current (200 ms–8 kA) produces dominating volumetric percentage of
cube texture in the weld zone and higher volume fraction of deformation texture
components at an increased value of time and current (i.e. 300 ms–9.5 kA).
6.3.1.2 Gas Metal Arc Welding Process (GMAW)
In GMAW, directional solidification and preferred grain growth are largely affected
by Lorenze and Marangoni force [31]. Magnitudes of the current and voltage determine the value of Lorenze and Marangoni force. In GMAW of austenitic stainless
steel, the preferred grain growth occurs along <001> and <101> directions parallel to
the RD in the weld at a higher value of current and welding speed. In contrary to this,
at lower value of the current and welding speed, the direction of grain growth aligns
to <101> , i.e. parallel to RD. Later, in the cooling stage of the welding process,
the grain growth further changes, either along <001> or <111> . This change in the
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favourable-orientated grains grow for a considerable distance, while the growth of
less favourable-oriented grains is blocked by fast-growing grains [28]. For this reason,
the columnar microstructure is seen in weld metal where relatively longer grains are
present, and these grains grow parallel with the direction of heat flow. These grains
grow due to favourable crystal orientation and solidification. The initial grains to
solidify are the ones that are close to the unmelted parent material. The unmelted
parent metal nucleates these grains to maintain equal crystal orientation. In metals
having body-centred cubic (BCC) and FCC crystal structures, solidification growth
occurs in preference to <100> crystallographic direction. As solidification is most
prominent in this crystallographic orientation, this direction is sometimes called as
the direction of “easy growth”. In metal having hexagonal closed pack (HCP) crystal
structure, grain growth occurs in the <1010> direction parallel to the basal plane.
The grain growth becomes prominent if this direction of easy growth comes parallel
with the direction of heat flow over the S–L interface [29].
The crystallographic orientations in the weld regions obtained in different fusion
welding techniques have been discussed in the following sections.
6.3.1.1 Resistance Spot Welding (RSW)
The values of current and voltage largely affect the grain size and the texture in
RSW as found in [30]. RSW at higher values of the time and current (300 ms–
9.5 kA), produces randomly textured microstructure in the weld zone. In reverse,
strongly textured microstructures are obtained at a low value of time and current (i.e.
200 ms–8 kA). The major texture components in RSWed samples can be classified
as deformation and recrystallization textures. Later one includes Q {013} <123> ,
Goss {110} <001> , Cube {001} <100> and RCND1{001} <310> , whereas the
deformation texture components include S {123} <634> , Brass {110} <112> ,
Shear1{001} <110> and Shear2 {111} <110> . The volume fractions of these texture
components depend on process parameters of the welding. Welding at a lower range
of time and current (200 ms–8 kA) produces dominating volumetric percentage of
cube texture in the weld zone and higher volume fraction of deformation texture
components at an increased value of time and current (i.e. 300 ms–9.5 kA).
6.3.1.2 Gas Metal Arc Welding Process (GMAW)
In GMAW, directional solidification and preferred grain growth are largely affected
by Lorenze and Marangoni force [31]. Magnitudes of the current and voltage determine the value of Lorenze and Marangoni force. In GMAW of austenitic stainless
steel, the preferred grain growth occurs along <001> and <101> directions parallel to
the RD in the weld at a higher value of current and welding speed. In contrary to this,
at lower value of the current and welding speed, the direction of grain growth aligns
to <101> , i.e. parallel to RD. Later, in the cooling stage of the welding process,
the grain growth further changes, either along <001> or <111> . This change in the