6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
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stresses and creates residual stress cracking (similar to solidification cracking), partial
melting zone cracking and distortion. Aluminium has a higher thermal conductivity
than steels hence making the melting difficult because whenever heat is applied
on abutting surfaces of the BMs to be joined, heat is dissipated towards BM. It
produces wider HAZ. Another important concern related to aluminium is solidification temperature range. For this reason, aluminium shows cracking tendencies
and softening in HAZ. To overcome these problems, it is always essential to use
special processes rather than conventional welding processes (e.g. gas welding and
SMAW) to weld aluminium because in both the cases, shielding is not that much
effective to prevent oxidation. Gas-based welding methods like GMAW, GTAW
and plasma arc welding are more effective to weld aluminium alloys. But these
processes cause porosity, solidification cracks, liquation cracks, etc. Thermal softening is another major problem in aluminium. It can be possible to get rid of this
problem of aluminium by various strengthening mechanisms applied to aluminium.
To weld thick aluminium plates, preheating is required [26]. When compared with
the alloys of steel, aluminium alloys will require high heat input in fusion welding,
high current and small welding time in RSW. Alloying with various materials like
lithium, zinc, silicon, manganese, magnesium and copper is performed to obtain the
required properties and for grain refinement. To get optimum mechanical properties, heat treatment, in addition to welding, must be carried out for aluminium. The
typical microstructure of the heat-treatable aluminium alloy is given in [27]. Solidstate welding such as FSW is proved to be one of the best joining technologies to
weld aluminium.
6.3 Texture in Welding
As mentioned in the introduction section, texture in a polycrystalline material is
the preferred orientation of grains in regard to a reference co-ordinate system. This
reference system is of two types: (a) crystal reference system and (b) sample reference
system. In the first case, the reference is three mutually perpendicular axes of a unit
cell. For all the crystals present in the sample, this reference is considered the same.
In the second one, three mutually perpendicular axes: [(a) rolling direction (RD), (b)
transverse direction (TD) and (c) normal direction (ND)] of the sample are taken as
reference.
There are two broad categories of texture present known as: (a) macrotexture and
(b) microtexture. Macrotexture is a bulk measure of the orientations of all grains
present in a sample. It does not give any idea about any particular grain within the
sample. It is measured by X-ray diffraction (XRD). Microtexture gives information
about the orientation of a particular grain along with the neighbouring ones as well.
It is measured by electron back scattered diffraction (EBSD).
Texture changes in all stage of processing. Based on it, there are four kinds of
texture present, i.e. (a) solidification texture, (b) recrystallization texture, (c) deformation texture and (d) transformation texture. The first one develops during melting and
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stresses and creates residual stress cracking (similar to solidification cracking), partial
melting zone cracking and distortion. Aluminium has a higher thermal conductivity
than steels hence making the melting difficult because whenever heat is applied
on abutting surfaces of the BMs to be joined, heat is dissipated towards BM. It
produces wider HAZ. Another important concern related to aluminium is solidification temperature range. For this reason, aluminium shows cracking tendencies
and softening in HAZ. To overcome these problems, it is always essential to use
special processes rather than conventional welding processes (e.g. gas welding and
SMAW) to weld aluminium because in both the cases, shielding is not that much
effective to prevent oxidation. Gas-based welding methods like GMAW, GTAW
and plasma arc welding are more effective to weld aluminium alloys. But these
processes cause porosity, solidification cracks, liquation cracks, etc. Thermal softening is another major problem in aluminium. It can be possible to get rid of this
problem of aluminium by various strengthening mechanisms applied to aluminium.
To weld thick aluminium plates, preheating is required [26]. When compared with
the alloys of steel, aluminium alloys will require high heat input in fusion welding,
high current and small welding time in RSW. Alloying with various materials like
lithium, zinc, silicon, manganese, magnesium and copper is performed to obtain the
required properties and for grain refinement. To get optimum mechanical properties, heat treatment, in addition to welding, must be carried out for aluminium. The
typical microstructure of the heat-treatable aluminium alloy is given in [27]. Solidstate welding such as FSW is proved to be one of the best joining technologies to
weld aluminium.
6.3 Texture in Welding
As mentioned in the introduction section, texture in a polycrystalline material is
the preferred orientation of grains in regard to a reference co-ordinate system. This
reference system is of two types: (a) crystal reference system and (b) sample reference
system. In the first case, the reference is three mutually perpendicular axes of a unit
cell. For all the crystals present in the sample, this reference is considered the same.
In the second one, three mutually perpendicular axes: [(a) rolling direction (RD), (b)
transverse direction (TD) and (c) normal direction (ND)] of the sample are taken as
reference.
There are two broad categories of texture present known as: (a) macrotexture and
(b) microtexture. Macrotexture is a bulk measure of the orientations of all grains
present in a sample. It does not give any idea about any particular grain within the
sample. It is measured by X-ray diffraction (XRD). Microtexture gives information
about the orientation of a particular grain along with the neighbouring ones as well.
It is measured by electron back scattered diffraction (EBSD).
Texture changes in all stage of processing. Based on it, there are four kinds of
texture present, i.e. (a) solidification texture, (b) recrystallization texture, (c) deformation texture and (d) transformation texture. The first one develops during melting and