6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
205
6.2.2.2 Metallurgical Processes Influencing Microstructure
in Solid-State Welding
In friction welding, frictional heat and pressure are the main reasons for welding due
to the relative movement of the workpieces. In diffusion welding, interdiffusion takes
place due to heating. Here, the two metals to be joined are maintained at close vicinity
with each other and are heated up to an elevated temperature, followed by application
of pressure. A local deformation occurs at the interface of the two metals, followed
by boundary migration, recrystallization and reduction in pore size, along with bulk
diffusion phenomena [20]. Explosion welding is a kind of collision welding process
which happens instantaneously. Due to collision and impact, the heat generation
occurs and then huge plastic flow occurs at the interface. These two facilitate the
development of the metallurgical bond between the materials to be joined. As it is
an instantaneous process, many metallurgical reactions are suppressed, and a very
narrow or undetectable HAZ is seen in this welding process [21]. Metals which are
metallurgically compatible, for example, stainless steel–titanium, titanium–copper,
aluminium–steel, stainless steel–carbon steel, etc., can be joined with this process.
In ultrasonic welding, very less heat is generated at the interface (30–50% of melting
point temperature of base material) by friction. By using ultrasonic vibratory energy
(frequency more than 20 kHz), the joint is made through localized plastic deformation
at the interface. The generated heat helps in softening of metal and facilitates plastic
flow at the interface. This heat is high enough for recrystallization and the formation
of the new grains. Refinement of grains at the interface occurs due to the elastic
plastic flow along with work-hardening due to the plastic flow of the metal. No
melting occurs as well as no heat related issues, i.e. solidification cracking, porosity,
blow holes, etc., are there in this welding, heat being generated over a small volume
of the metal, so very less HAZ is found. There is no evidence of grain growth in this
kind of welding [22]. In all the cases, there exists a different bonding mechanism
which in other ways affects the weld microstructure.
Since the authors have considered aluminium to stainless steel welding in the case
study, therefore, a brief overview of the microstructure and weldability of steel, as
well as aluminium, is stated below.
6.2.3 Microstructure of Steel and Its Weldability
Steel is primarily iron with few carbon and other alloying elements. Properties of steel
get affected depending on these alloying elements. The main alloying element for the
formation of microstructure in steel is carbon. Iron–carbon equilibrium diagram is
a convenient way to understand the microstructure of steel at different temperatures
and carbon content. Various structures are austenite, pearlite, cementite, ferrite, etc.
The typical microstructure of steel can be understood in one of the literature [23].
Most commercial steels can be divided into low-alloy steel (ferrite + small grains
of pearlite), high-alloy steel (pearlite at 0.8% c, above 0.8% c pearlite + cementite),
205
6.2.2.2 Metallurgical Processes Influencing Microstructure
in Solid-State Welding
In friction welding, frictional heat and pressure are the main reasons for welding due
to the relative movement of the workpieces. In diffusion welding, interdiffusion takes
place due to heating. Here, the two metals to be joined are maintained at close vicinity
with each other and are heated up to an elevated temperature, followed by application
of pressure. A local deformation occurs at the interface of the two metals, followed
by boundary migration, recrystallization and reduction in pore size, along with bulk
diffusion phenomena [20]. Explosion welding is a kind of collision welding process
which happens instantaneously. Due to collision and impact, the heat generation
occurs and then huge plastic flow occurs at the interface. These two facilitate the
development of the metallurgical bond between the materials to be joined. As it is
an instantaneous process, many metallurgical reactions are suppressed, and a very
narrow or undetectable HAZ is seen in this welding process [21]. Metals which are
metallurgically compatible, for example, stainless steel–titanium, titanium–copper,
aluminium–steel, stainless steel–carbon steel, etc., can be joined with this process.
In ultrasonic welding, very less heat is generated at the interface (30–50% of melting
point temperature of base material) by friction. By using ultrasonic vibratory energy
(frequency more than 20 kHz), the joint is made through localized plastic deformation
at the interface. The generated heat helps in softening of metal and facilitates plastic
flow at the interface. This heat is high enough for recrystallization and the formation
of the new grains. Refinement of grains at the interface occurs due to the elastic
plastic flow along with work-hardening due to the plastic flow of the metal. No
melting occurs as well as no heat related issues, i.e. solidification cracking, porosity,
blow holes, etc., are there in this welding, heat being generated over a small volume
of the metal, so very less HAZ is found. There is no evidence of grain growth in this
kind of welding [22]. In all the cases, there exists a different bonding mechanism
which in other ways affects the weld microstructure.
Since the authors have considered aluminium to stainless steel welding in the case
study, therefore, a brief overview of the microstructure and weldability of steel, as
well as aluminium, is stated below.
6.2.3 Microstructure of Steel and Its Weldability
Steel is primarily iron with few carbon and other alloying elements. Properties of steel
get affected depending on these alloying elements. The main alloying element for the
formation of microstructure in steel is carbon. Iron–carbon equilibrium diagram is
a convenient way to understand the microstructure of steel at different temperatures
and carbon content. Various structures are austenite, pearlite, cementite, ferrite, etc.
The typical microstructure of steel can be understood in one of the literature [23].
Most commercial steels can be divided into low-alloy steel (ferrite + small grains
of pearlite), high-alloy steel (pearlite at 0.8% c, above 0.8% c pearlite + cementite),