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CHAPTER 5
EFFECT OF MULTIPLE-PASS FSP
5.1 INTRODUCTION
This chapter discusses the effect of number of passes on the
microstructure and mechanical properties of friction stir
processed AS7U3G
aluminum alloy. Threaded cylindrical pin profiled tool with
concave shoulder
is used to process the alloy. Double pass (with 100% overlapping
on the top
of the first pass) and triple pass (with 100% overlapping on the
top of the
second pass) FSP experiments were performed. The tensile
tests,
metallographic examinations and hardness measurements of the
FSPed
samples were carried out as per the section 3.4 mentioned in the
third chapter.
5.2 RESULTS
5.2.1 Tensile properties
The longitudinal tensile properties, such as yield strength,
tensile
strength, percentage of elongation of the friction stir
processed (FSP)
materials were evaluated by testing three specimens in each
condition. Data
comparing the tensile behavior of as cast and FSPed materials
are presented in
Table 5.1 and Figure 5.1.Significant microstructural refinement,
homogeneity
and densification by multiple- FSP in AS7U3G casting resulted in
remarkable
improvement in the tensile properties. The tensile strengths of
the single pass,
double pass, and triple pass FSPed materials are significantly
higher than that
of the as cast alloy. The tensile strength of triple pass FSPed
material is 273
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MPa which is 2.25 times higher compared to that of the as cast
alloy of 121
MPa. Friction stir processing of this alloy enhanced the
elongation from 1.8%
to 10%. Fourth pass has slightly reduced the UTS and the
Percentage of
elongation. As friction stir process is a stress induced solid
state process,
macro cracks were observed in the fourth pass.
Table 5.1 Mechanical properties of unprocessed and friction
stir
processed materials
Condition Yieldstrength(MPa)
Ultimate tensile
strength(MPa)
Elongation in 25mm gauge length (%)
Unprocessed basemetal
109 121 1.8
Single pass FSP 196 218 5
Double pass FSP 232 253 8
Triple pass FSP 268 273 10
Fourth pass 239 258 7.5
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Figure 5.1 Load Vs Displacement diagram of cast and friction
stir
processed materials
5.2.2 Microstructure
Scanning electron microscopy examination was carried out to
study
the influence of the number of overlapping passes on the
microstructure of
FSP region. Figure 5.2 illustrates typical SEM micrographs of as
cast base
alloy (a), single pass FSP stir zone (b), double pass FSP stir
zone (c) and
triple pass FSP stir zone (d). These SEM micrographs distinctly
reveal the
significant favorable effect of friction stir processing on the
size, shape, and
distribution of Si particles with increasing number of passes.
The stirring
action of the FSP at the nugget zone fragmented the large
primary and
secondary phase particles to very fine particles. The stirring
action also
compacted the porosities followed by solid state fusion which
closed all
voids. However the size of the particles in the nugget zones is
different due to
0
1
2
3
4
5
0 0.4 0.8 1.2 1.6 2 2.4 2.8 3.2 3.6Displacement, mm
Load Vs Displacement
1Pass
2Pass
3Pass
4Pass
BM
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the influence of number of passes. The fineness of particles is
in the
increasing order with the increasing number of passes. This
proves that
overlap of various passes continues to homogenize the
microstructure and its
properties. The nugget microstructure of the triple pass FSPed
material
consists of very fine eutectic Si particles which are uniformly
distributed
throughout the aluminum matrix (Figure 5.2d) than double pass
and single
pass FSPed materials.
Figure 5.2 SEM micrographs of as cast alloy (a), single pass FSP
stir zone (b), double pass FSP stir zone (c) and triple pass FSP
stir zone (d).
Table 5.2 shows that the size of the Si particle in the cast
AS7U3G
aluminum alloy is in the range of 1.12 to 23.36 µm, with high
aspect ratio of
3.70. Table 5.2 also shows that after single pass of FSP, the
average Si
particle size and aspect ratio values were drastically reduced
to 1.17± 1.00 µm
and 1.45±1.33, respectively. Further reduction in the average Si
particle size
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values to 0.86 µm and0.80 µm were obtained in the case of double
pass and
triple pass FSP samples.
Table 5.2 Microstructural characterization (size and aspect
ratio of Si
particles) of base metal and FSP zones
ConditionSilicon Particle diameter, µm Aspect Ratio
Min Max Mean Min Max Mean
Base Metal 1.12 23.36 5.55±3.73 1.08 17.50 3.70±2.45
Single pass 0.28 11.89 1.17±1.00 1.02 11.67 1.45±1.33
Doublepass
0.14 9.43 0.86±0.78 1.05 12.22 1.43±1.14
Triple Pass 0.09 4.23 0.80±0.63 1.02 10.12 1.35±1.36
Figure 5.3 shows the energy- dispersive spectra of base
metal
and the stir zones of single-pass, double-pass and triple-pass
FSPed samples.
The energy- dispersive spectra of single-pass FSP zone showed
the presence
of all the alloying elements, but the amount of magnesium
quantified is very
less. In the stir zones of double-pass and triple-pass samples,
magnesium was
not detected by EDS. The heat produced during FSP might have
caused the
evaporation of magnesium.
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Figure 5.3 Energy-dispersive spectra of stir zones in (a) Base
metal,
(b) single-pass
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Figure 5.3 (Continued) (c) double-pass, and (d) triple-pass
FSPed samples
In order to study the composition of the particles in the stir
zone of
double-pass FSPed sample, one large particle in the matrix was
selected and
subjected to EDS (Figure 5.4). The EDS analysis showed the
existence of Al-
Si and CuAl2 phases in the aluminum matrix. The spectra also
showed the
absence of magnesium and hence the absence of Mg2Si.
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Figure 5.4 EDS of a marked particle (large particle) in the stir
zone of
double-pass FSPed sample
Similarly one of the fine particles in the matrix was selected
and
subjected to EDS (Figure 5.5). The spectra showed the existence
of Al-Si,
CuAl2 phases and absence of Mg2Si phase.
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Figure 5.5 EDS of a marked particle (fine particle) in the stir
zone of
double-pass FSPed sample
5.2.3 Hardness
The effect of number of passes on the distribution of
hardness
across FSP region is presented in Figure 5.6. Soft spots found
in the casting
due to porosity and aluminum dendrite cores were eliminated by
friction stir
processing. The distribution of hardness values also appears
narrower in the
stir zone. This is consistent with its more uniform
microstructure. The average
hardness values in single pass, double pass, and triple pass
FSPed AS7U3G
alloy are lower than that of the as cast alloy. Microhardness
values indicate a
softening of the processed material at the friction stir
processed zone due to
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inherent nature of the process. With an increase in number of
passes, an
increase in the microhardness values was observed in the
processed materials.
Figure 5.6 Microhardness profile across the FSP region
5.2.4 Fracture analysis
Figure 5.7a-d displays the fractographs of the unprocessed and
the
processed materials. In all conditions studied, the fracture
surfaces of the
processed specimens reveal a ductile fracture. It is known that
the presence of
flakes promote a tendency towards brittle fracture (Nakata et al
2006).As can
be seen from Figure 5.7 a, that the fracture surface of the base
metal reveals
the presence of flakes like structure. In addition, large
lamellar structures
were observed containing possibly Mg2Si and Al-Si eutectic.
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Figure 5.7 SEM fractographs of tensile specimens for (a) as cast
alloy, (b) single pass FSP, (c) double pass FSP, (d) triple pass
FSP samples
Fracture in the processed material is transgranular with
medium
to good developed surfaces. Figure 5.7d reveals fine ductile
fracture with a
few featureless regions. Fine dimples are a characteristic
feature of highly
ductile materials. Consistent with these observations, triple
pass specimen
displayed highly ductile behavior before fracture. In contrast,
Figure 5.7b, and
c represents a less ductile fracture.
5.3 DISCUSSION
5.3.1 Effect of multiple-pass FSP on tensile properties
From the experimental results (Table 5.1), it is very clear
that
multiple-pass FSP resulted in remarkable improvement in the
longitudinal
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tensile properties of AS7U3G aluminum alloy. The factors
increasing the
tensile strength for the multiple-pass FSPed materials can be as
follows: (1)
elimination of casting defects (such as porosity); (2) uniform
distribution of
the fine Si particles; (3) grain refinement of aluminum matrix.
In all the
conditions studied, the specimens showed a considerable increase
in ultimate
tensile strength and ductility values. The triple pass FSPed
material displayed
superior tensile properties than the other materials.
Double pass FSP with a 100% overlap produced a better effect
on
stir zone microstructure (Ma et al 2006c). Five-pass FSP sample
(not 100%
overlap, but the overlap between the passes was one-half of the
pin diameter)
in various microstructural regions exhibited strength and
ductility values
comparable to those achieved in the single-pass FSP sample (Ma
et al
2006b).They also reported that multiple-pass FSP with a 50%
overlap is a
feasible route to perform microstructural modification on
large-sized
aluminum castings.
The material flow behavior will be different for cast alloys
and
wrought alloys due to large differences in ductility. Usually
cast alloys will
have lower ductility compared to wrought alloys due to the
presence of
porosity, inclusions and higher amount of silicon. This will
reduce the
ductility of cast alloys and subsequently affect the material
flow behavior
under the action of rotating FSP tool. All the processed
specimens invariably
showed considerable increase in ductility compared to the base
metal. The
elongation of triple pass FSPed sample is 10%, which is 5.5
times higher than
that of the as cast unprocessed alloy. This could have been due
to grain
refinement in the stir zone. The ultimate tensile strength and
ductility
generally improve as porosity levels and the microstructure
scale decrease. In
the present set of experiments increasing the FSP pass resulted
in enhanced
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mechanical properties, due to the reduced Si particle size and
porosity level,
and increased dissolution of CuAl2 and Mg2Si.
5.3.2 Effect of multiple-pass FSP on microstructure
Multiple-pass FSP resulted in significant refinement in the
microstructure of the alloy. Virtually all traces of dendritic
solidification
microstructure were eliminated throughout the stir zone. With
increasing
number of FSP passes, the refinement of Si particles increased.
Double pass
FSP produced a pronounced effect in refining the stir zone
microstructure and
the break-up of the Si particles is further intensified in the
triple pass FSP.
Triple pass FSP produced a stupendous effect on microstructural
refinement,
homogeneity and densification of Si particles. The reduction in
the average
size of Si particles after single pass FSP of AS7U3G alloy is
79%, which
further refined to 84.6% and 85.6% after double pass and triple
pass FSP
respectively.
In the case of hypoeutectic A356 aluminum alloy, Ma et al
(2006b)
have reported about 86% reduction in the average size of Si
particles after one
pass FSP. They have also reported that the 50% overlapped
multi-pass FSP
did not influence the size, aspect ratio and distribution of Si
particles. Rao et
al (2009) reported that double pass FSP with 100% overlapping on
the top of
the first pass itself had a pronounced effect on size, shape,
and distribution of
Si particles. The reduction in the average size of the Si
particles after single
pass FSP is 98%., which further refined to 99% after second
pass.
Similarly, Nakata et al (2006) produced a fine grain structure
of 2 –
3 µm in ADC12 die casting alloy via multi-pass FSP. For AA2219
Al alloy,
single-pass FSP resulted in an average grain size of 6.2 µm, but
in the
subsequent passes (two-pass, three-pass) the average grain size
showed a
marginal increase (Surekha et al 2008). Nascimento et al (2009)
reported
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that for AA7022- T6 alloy, single-pass FSP reduced the grain
size from 160
µm to an average grain size of 7.1 µm and this remained
constant
independently of the number of passes and overlap ratios tested.
The present
results indicate that, with increase in number of passes, a
decrease in size of
Al-Si eutectic particles (densification of particles is in
increasing order) is
obtained. This proves that 100% overlap of various passes
continues to
homogenize the microstructure in the stir zone. In comparison,
for the sample
processed with three passes with 100% overlapping, a more
homogeneous
processed area was obtained with average Si particle size of 800
nm.
5.3.3 Effect of multiple-pass FSP on microhardness
It was found that the hardness in the FSPed region increased
with
an increase in number of passes. The observed increase in
microhardness
values can be attributed to the reduced grain sizes. By the
Hall-Petch
relationship, Hv= Ho+kH d -1/2, where Ho and kH are appropriate
constants.
Because Hv is proportional to d -1/2, the finer the grain size
is, the higher the
hardness value is. The effect of frictional heat on
microstructure during single
pass FSP results in generation of dynamic recrystallized grains
having low
dislocation density (Santella et al 2005 and Karthikeyan et al
2009), reduction
of dislocation density having greater effect on softening than
the hardening
effect of increased grain-boundary area, through dynamic
recrystalization.
The subsequent increase in hardness upon two pass FSP and three
pass FSP
could be due to the significant increase in dislocation density
and frequency
of sub micron silicon particles in the stir zone.
Rao et al (2009) reported that the average hardness values in
both
single pass FSP and two pass FSP friction stir processed Al-30Si
alloy were
lower than that of the base metal. Surekha et al (2008) studied
the effect of
multiple-pass FSP on AA2219 aluminum alloy and reported that the
hardness
in the nugget region increased with increase in number of
passes. They also
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reported that in all the studied conditions, the nugget showed a
lower hardness
compared to the base material. The microhardness behavior of
friction stir
processed AS7U3G aluminum alloy is consistent with this behavior
pattern.
5.4 CONCLUSION
In this investigation, cast AS7U3G (Al-Si-Mg (Cu)) aluminum
alloy was friction stir processed (FSP) with multiple passes
(100% overlap)
and the following important conclusions are derived;
(i) Triple-pass (3P) FSP produced a stupendous effect on
microstructural refinement, homogeneity and densification of
Si particles.
(ii) It was found that the hardness in the friction stir
processed
region increased with increase in number of passes.
(iii) The tensile strength of triple pass friction stir
processed
material is 273 MPa which is 2.25 times higher compared to
that of the as cast alloy.