6 Microstructure and Texture in Welding: A Case Study on Friction Stir Welding
213
pin, having diameters of 18 mm and 5 mm, respectively. Welding was performed at
a welding speed (v), rotational speed (ω), plunge depth (PD) and tilt angle (α) of
125 mm/min, 900 rpm, 0.2 mm and 1°, respectively. The experiment was performed
in a computer numerical control (CNC) FSW machine (ETA Technology Bangalore,
WS004).
To determine the tensile strength of the parent material (AA6061) and weld joint,
tensile samples were cut in a rectangular shape in the CNC wire electrical discharge
machining (EDM) machine (Electronica, Maxicut 5231) [40]. The cut samples were
tested in a 5 kN universal testing machine (UTM), (Instron, 3325). SEM observations
were performed to study the fracture surface of failed samples.
The cross sections of the BM and weld sample (from the welded region) were
cut having a length, width and thickness of 20 mm, 7 mm and 1.8 mm, respectively, in a low-speed saw machine (Buehler, Isocut) and then polished mechanically
with silicon carbide emery papers of various grit sizes followed by diamond cloth
polishing. For EBSD characterization, the cut samples were electro-polished in a
solution of 1000 ml (200 ml perchloric acid and 800 ml ethanol) for 15 V and at
10 s only. EBSD scan was performed on different weld regions at a scan rate of 0.5
step size in a field emission gun scanning electron microscope (ZEISS, AURIGA
with oxford detector). Obtained data from EBSD were cleaned and analysed in a
software (TSLOIM Analysis 7.0) by following the standard grains dilation method
in a single iteration. OIM means orientation imaging microscopy which is a tool
to analyse local texture in polycrystalline material. Spatial distribution of crystallographic orientation can be visualized through OIM measurements. It also provides a
tool to link orientation as well as microstructure.
Different maps are used to allow colours to be assigned to points in an OIM scan
based on some parameter associated with the point such as orientation or image
quality (IQ). IQ defines the quality of an EBSD. IQ depends on the materials and
their state. It is a function of parameters used and the technique to index the pattern.
Authors have shown grain orientation spread (GOS) map, kernel average misorientation (KAM), ODF map and IPF map in the following sections to describe texture.
In KAM, the misorientation is calculated between a grain at the kernel core and all
points at kernel perimeter. The average of these misorientations is the locale misorientation value assigned to the centre point. In GOS, each point is shaded according
to the orientation spread of the grain to which the point belongs. This can also be
normalized to the area of the grain; however, this tends to make the large grains have
smaller spreads than those observed in the small grains. Instead of plotting crystal
orientations with regard to an external frame of reference, IPFs can be produced
which shows the RD, TD and ND directions with reference to the crystallographic
axes. These are plotted in a standard stereographic triangle shown in Fig. 6.6a as well
as in the bottom left portion of Fig. 6.7. ODFs are developed by the combination of
data from various pole figures. ODFs define the texture of each crystal with regard to
three Euler angles [defines the variation with three orientation axes (RD, ND, TD)].
213
pin, having diameters of 18 mm and 5 mm, respectively. Welding was performed at
a welding speed (v), rotational speed (ω), plunge depth (PD) and tilt angle (α) of
125 mm/min, 900 rpm, 0.2 mm and 1°, respectively. The experiment was performed
in a computer numerical control (CNC) FSW machine (ETA Technology Bangalore,
WS004).
To determine the tensile strength of the parent material (AA6061) and weld joint,
tensile samples were cut in a rectangular shape in the CNC wire electrical discharge
machining (EDM) machine (Electronica, Maxicut 5231) [40]. The cut samples were
tested in a 5 kN universal testing machine (UTM), (Instron, 3325). SEM observations
were performed to study the fracture surface of failed samples.
The cross sections of the BM and weld sample (from the welded region) were
cut having a length, width and thickness of 20 mm, 7 mm and 1.8 mm, respectively, in a low-speed saw machine (Buehler, Isocut) and then polished mechanically
with silicon carbide emery papers of various grit sizes followed by diamond cloth
polishing. For EBSD characterization, the cut samples were electro-polished in a
solution of 1000 ml (200 ml perchloric acid and 800 ml ethanol) for 15 V and at
10 s only. EBSD scan was performed on different weld regions at a scan rate of 0.5
step size in a field emission gun scanning electron microscope (ZEISS, AURIGA
with oxford detector). Obtained data from EBSD were cleaned and analysed in a
software (TSLOIM Analysis 7.0) by following the standard grains dilation method
in a single iteration. OIM means orientation imaging microscopy which is a tool
to analyse local texture in polycrystalline material. Spatial distribution of crystallographic orientation can be visualized through OIM measurements. It also provides a
tool to link orientation as well as microstructure.
Different maps are used to allow colours to be assigned to points in an OIM scan
based on some parameter associated with the point such as orientation or image
quality (IQ). IQ defines the quality of an EBSD. IQ depends on the materials and
their state. It is a function of parameters used and the technique to index the pattern.
Authors have shown grain orientation spread (GOS) map, kernel average misorientation (KAM), ODF map and IPF map in the following sections to describe texture.
In KAM, the misorientation is calculated between a grain at the kernel core and all
points at kernel perimeter. The average of these misorientations is the locale misorientation value assigned to the centre point. In GOS, each point is shaded according
to the orientation spread of the grain to which the point belongs. This can also be
normalized to the area of the grain; however, this tends to make the large grains have
smaller spreads than those observed in the small grains. Instead of plotting crystal
orientations with regard to an external frame of reference, IPFs can be produced
which shows the RD, TD and ND directions with reference to the crystallographic
axes. These are plotted in a standard stereographic triangle shown in Fig. 6.6a as well
as in the bottom left portion of Fig. 6.7. ODFs are developed by the combination of
data from various pole figures. ODFs define the texture of each crystal with regard to
three Euler angles [defines the variation with three orientation axes (RD, ND, TD)].