6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
197
The FZ consists of two regions. The first zone is the composite zone (CZ), where
the parent metal and filler metal are mixed in a composite composition. Along the
fusion boundary, surrounding CZ, the second region is present, which is known as
the unmixed zone (UMZ). This zone comprises of melted and resolidified parent
material which never mixes with the filler material. Between CZ and UMZ, another
zone named transition zone (TZ) exists, and it shows a microstructure different
from its surrounding regions, especially in case of heterogeneous welds. The HAZ
is subdivided into two zones, named as true HAZ (T-HAZ) and partially melted
zone (PMZ). PMZ is present in most of the fusion welding processes because of the
transition from 100% liquid to 100% solid along the FZ boundary [4]. T-HAZ is a
part of the BM and is differentiated from HAZ, where no melting occurs, and all the
metallurgical events happen in the solid state.
6.2.1.2 Metallurgical Processes Influencing Microstructure in Fusion
Welding
The BM is usually manufactured after casting, followed by rolling, heat treatment
and other secondary manufacturing processes. Heat in the fusion welding modifies
the microstructure of material. The difference in microstructures of the base and the
weld material is dictated by the differences in the mechanical and thermal histories.
The following sections will give a brief knowledge of the metallurgical events that
occur in FZ and HAZ of a fusion weld responsible for change in microstructure.
Metallurgical Events in FZ
Three most important metallurgical events occur in FZ such as gas–metal reactions,
i.e. reaction of weld metal with gases, liquid–metal reactions, i.e. reaction of weld
metal with any liquid phase and solid-state reaction, which occurs during and after
solidification in FZ.
Melting and Solidification
These are two important processes to achieve acceptable joints in fusion welding
processes. Microstructure development in FZ is mostly dependent on solidification
behaviour. The ideal substrate for solidification is the unmelted part of the grain
in HAZ at solid–liquid (S–L) interface. Various solidification parameters are useful
to describe microstructure development in fusion welding. Those are: (a) partition
coefficient, (b) liquid temperature gradient, (c) solidification rate, and (d) cooling
rate. Solidification parameter is generally defined by a ratio of temperature gradient
and square root of solidification growth rate (how fast S–L interface moves). To start
solidification, it is mandatory to have the nucleation (homogeneous/heterogeneous)
of solids inside the liquid phase in the molten weld pool, and the S–L interface
197
The FZ consists of two regions. The first zone is the composite zone (CZ), where
the parent metal and filler metal are mixed in a composite composition. Along the
fusion boundary, surrounding CZ, the second region is present, which is known as
the unmixed zone (UMZ). This zone comprises of melted and resolidified parent
material which never mixes with the filler material. Between CZ and UMZ, another
zone named transition zone (TZ) exists, and it shows a microstructure different
from its surrounding regions, especially in case of heterogeneous welds. The HAZ
is subdivided into two zones, named as true HAZ (T-HAZ) and partially melted
zone (PMZ). PMZ is present in most of the fusion welding processes because of the
transition from 100% liquid to 100% solid along the FZ boundary [4]. T-HAZ is a
part of the BM and is differentiated from HAZ, where no melting occurs, and all the
metallurgical events happen in the solid state.
6.2.1.2 Metallurgical Processes Influencing Microstructure in Fusion
Welding
The BM is usually manufactured after casting, followed by rolling, heat treatment
and other secondary manufacturing processes. Heat in the fusion welding modifies
the microstructure of material. The difference in microstructures of the base and the
weld material is dictated by the differences in the mechanical and thermal histories.
The following sections will give a brief knowledge of the metallurgical events that
occur in FZ and HAZ of a fusion weld responsible for change in microstructure.
Metallurgical Events in FZ
Three most important metallurgical events occur in FZ such as gas–metal reactions,
i.e. reaction of weld metal with gases, liquid–metal reactions, i.e. reaction of weld
metal with any liquid phase and solid-state reaction, which occurs during and after
solidification in FZ.
Melting and Solidification
These are two important processes to achieve acceptable joints in fusion welding
processes. Microstructure development in FZ is mostly dependent on solidification
behaviour. The ideal substrate for solidification is the unmelted part of the grain
in HAZ at solid–liquid (S–L) interface. Various solidification parameters are useful
to describe microstructure development in fusion welding. Those are: (a) partition
coefficient, (b) liquid temperature gradient, (c) solidification rate, and (d) cooling
rate. Solidification parameter is generally defined by a ratio of temperature gradient
and square root of solidification growth rate (how fast S–L interface moves). To start
solidification, it is mandatory to have the nucleation (homogeneous/heterogeneous)
of solids inside the liquid phase in the molten weld pool, and the S–L interface