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This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg)Nanyang Technological University, Singapore.
Friction stir processing of Al‑CNT composites
Du, Zhenglin; Tan, Ming‑Jen; Guo, Jun‑Feng; Wei, Jun
2015
Du, Z., Tan, M. ‑J., Guo, J. ‑F., & Wei, J. (2016). Friction stir processing of Al‑CNT composites.Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Designand Applications, 230(3), 825‑833.
https://hdl.handle.net/10356/82911
https://doi.org/10.1177/1464420715571189
© 2015 Institution of Mechanical Engineers (IMechE). This is the author created version of awork that has been peer reviewed and accepted for publication by Proceedings of theInstitution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications,IMechE. It incorporates referee’s comments but changes resulting from the publishingprocess, such as copyediting, structural formatting, may not be reflected in this document.The published version is available at: [http://dx.doi.org/10.1177/1464420715571189].
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Special Issue
Friction stir processing of Al–CNTcomposites
Zhenglin Du1, Ming-Jen Tan1, Jun-Feng Guo2 and Jun Wei2
Abstract
Friction stir processing (FSP) is a solid-state process with the
ability to refine grain sizes and uniformly disperse particles
to improve the mechanical properties of the base material. In
this study, FSP was performed on AA6061-T6 with and
without additions of multi-walled CNTs. For FSP on monolithic Al
plates, dendrites were broken down and dispersed
uniformly with the increase in number of passes. As for FSP of
Al–CNT composites, the CNTs have been successfully
dispersed with three FSP passes. Dispersion is more uniform with
increasing number of passes. The Vickers hardness and
tensile yield strength were found to have improved after
performing FSP with the addition of CNTas compared to FSP of
AA6061-T6 without CNT.
Keywords
Friction stir processing, nanocomposites, carbon nanotubes,
mechanical properties
Date received: 28 August 2014; accepted: 18 December 2014
Introduction
Friction stir welding and friction stir processing
Friction stir welding (FSW) was invented by TheWelding Institute
(TWI) of UK in 1991.1 It is asolid-state joining technique used to
weld two piecesof metal together without melting. Much research
hasbeen done on the welding of aluminum due to its rela-tive low
melting pointing and low weldability usingtraditional welding
techniques.
The basic working principle of FSW involves anonconsumable tool
with a threaded pin and shoulderbeing plunged into the abutting
edge of two sheetsand transverse along the direction of the line
ofjoint. The friction between the tool and the plate gen-erates
heat which softens the work piece. The rotationof the tool moves
the material from the front to theback.2
The working principle of friction stir processing(FSP) is based
on FSW. Work is done on a singlework piece instead of joining two
pieces together(Figure 1). Friction stir processing technique
wasfirst reported by Mishra et al.3 for localized micro-structure
modification to achieve certain desirableproperties and has
attracted much attention eversince.
For friction stir processing, the constituent phasematerial in
the process zone is being mixed and refinedby the tool due to the
intense plastic deformationduring the FSP. The true strain during
FSP isapproximately 40.4 Mishra et al.3 studied FSP withthe
addition of SiC and observed an increase in
surface hardness as well as uniform distribution ofSiC particles
in Al matrix. An investigation on theeffect of rotation speed on
FSPed AZ31–Al2O3 com-posites, found that an increase in rotation
speed led toenhancement in the particle distribution and
createdfiner nanoparticle agglomeration.5
Metal matrix composites
High elastic modulus and wear resistance of
particu-late-reinforced metal matrix composites (MMCs)have drawn
the attention of the aerospace, automo-bile and defence industries.
Furthermore, additions ofsmall amount of nano-sized particles
significantlyenhanced the material properties.6–8
Guo et al.9 studied the evolution of grain structureand
mechanical properties of AA6061 alloy reinforcedwith nano-Al2O3.
Slurry of nano-sized Al2O3 particlesand a volatile solvent was used
to preplace reinforcingparticles in an array of cylindrical holes
on the surfaceof AA6061 plate. Multiple FSP passes were applied
toimprove the dispersion of the particles. In the study,particle
dispersion improved with increasing number
1School of Mechanical & Aerospace Engineering, Nanyang
Technological
University, Singapore2Singapore Institute of Manufacturing
Technology (SIMTech), Singapore
Corresponding author:
Ming-Jen Tan, School of Mechanical & Aerospace Engineering,
Nanyang
Technological University, 50 Nanyang Avenue, Singapore
639798.
Email: [email protected]
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of FSP passes, and finer grain size was produced withthe
addition of composite. Also, the final grain size forone pass and
three passes were similar, as the finalgrain size is dependent on
welding temperature.10
Liu et al.11 studied FSP of Mg–Li–Al–Zn underwater and reported
fine equiaxed, recrystallizedalpha (hcp), and beta (bcc) grains.
Superplasticitywith ductility of 300% at 100 �C and more than400%
under high strain rate at 225–300 �C wasachieved.
To the best of the author’s knowledge, no one hasreported on
CNTs reinforcement using friction stirprocessing. However, a study
by Liao and Tan12
was conducted on the addition of CNT to aluminummatrix. CNTs
were mixed with aluminum powder andsintered before hot extrusion
and hot rolling. The spe-cimens were then tested for mechanical
properties. Itwas observed that the presence of CNTs in the
alumi-num matrix slowed fatigue crack propagation bycrack-bridging,
CNT frictional pull-out, and breakagemechanism. Al–CNT composites
showed significantlyimproved densification, nano-indentation
modulus,hardness, tensile strength, and fatigue resistance.However,
the CNTs were not well dispersed andagglomerated in clusters. FSP
is able to achieve a uni-form dispersion of many particles, hence
the aim ofthis study is to use FSP to obtain uniform dispersionof
the CNTs in the aluminum matrix and study itsmicrostructure and
properties.
Experimental details
There are various methods of applying particles onthe substrate
before performing FSP. Mishra et al.3
prepared Al–SiC surface composites by applying amixture of SiC
powder suspended in methanol ontorolled 5083 aluminum Alloy
surface. Holes, orgrooves can also be made on the surface of the
base
material to contain the reinforcement particles.13
Billets made from cold compacting and sintering amixture of
metal powder and composites can also bedone prior to FSP.14 Cross
rolling is then done on thecast alloys to obtain a flat surface for
FSP.15
In this study, CNTs were applied onto an AA6061-T6 rolled plate
of 300mm length and 100mm width(rolling direction). An array of 960
cylindrical holeswith diameter of 1mm and depth of 2mm weremachined
in an area of 240mm� 50mm (Figure 2).Acetone was used to degrease
the plates before airdrying. The multi-walled carbon
nanotubes(MWCNTs) are of outer diameter ranging from 10to 20 nm,
length ranging from 10 to 30 mm, andpurity of at least 95%. The
nominal volume fractionof CNTs produced by FSP is 0.5%. A friction
stirwelding robot capable of generating a maximumdownward force of
12 kN was used to carry outFSP. The tool used to conduct FSP was a
threadedconical probe welding tool with three flats. The toolhas a
shoulder diameter of 12.5mm, probe length of2mm, and a probe base
diameter of 5mm. An add-itional 1mm thin sheet of AA6061-T6 was
placedabove it prior to performing FSP, using a rotationalspeed of
1800 r/min, travel speed of 8mm/s, and tiltangle of 3�.
Metallographic samples were then sectioned trans-versely from
the plates after FSP was performed.They are then polished using
conventional mechanicalpolishing method and viewed under field
emissionscanning electron microscope (FESEM) equippedwith electron
backscattered diffraction (EBSD).EBSD was performed with step size
of 0.5lm andmaps were used to plot misorientation angle histo-grams
using Channel 5 software by HKLTechnology. A minimum of five points
were sampledfrom the cross section surface using Vickers
hardnesswith 50 gf loading. Tensile testing was conducted in
Figure 1. Schematic and actual illustration of FSP.
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accordance to ASTM E08-04 standards at test speedof 1mm/min. A
minimum of three ‘‘I’’-shaped rect-angular sub-sized samples with
gage length of 25mmand width of 6mm in the reduced section were
used(Figure 3). A fractography was done using scanningelectron
microscope (SEM) on the fractured sites.
Results and discussions
Particle dispersion
Large dendrites as well as Si particles were observed inthe
as-received AA6061-T6 (Figure 4(a)). FSP hasbroken up some Si
particles and aluminum dendritesand dispersed them uniformly into
the aluminummatrix. Finer particles as well as a more uniform
dis-tribution of the particles were observed with increasednumber
of passes (Figure 4(b) and (c)). The stirredzones of the FSP were
free of porosities.
Similar observations were seen in the Al–CNTSEM images. From
SEM, clusters of CNTs werefound in the sample with one pass (Figure
4(d)). Inthe sample with additions of CNT and three
passescondition, there were no visible CNT clustersobserved (Figure
4(d) and (e)). It is believed that theCNTs were broken up by the
stirring and mixing byFSP.
Grain structure evolution
The EBSD results of the friction stir processed sam-ples showed
significant grain refinement compared tothe base material (Figure
5). It was also observed tohave sub-grain boundaries present in
several slightlyelongated grains. More sub-grain boundaries
wereseen in the stirred zone. This is in agreement with
the continuous dynamic recrystallization, whichoccurred with the
introduction of continuous straincoupled with rapid recovery and
migration of sub-grain boundaries during friction stir
processing.16–18
Hence, severe deformation was caused by the intensedislocation
generation experienced by the material inthe stir zone. The stored
energy in the dislocationresulted in the dynamic recovery and
recrystallizationprocess.
For samples without CNT, the average grain size is4.49lm for
samples that underwent one pass and5.04lm for samples that
underwent three passes(Table 1). This slight variation is in
agreement witha previous study that found grain sizes are similar
andindependent on the number of passes.19 And,
dynamicrecrystallization is a strong function of the flow stressand
not the temperature during deformation.20
Hence, the grain sizes were somewhat similar here.However,
studies have also showed that grain sizesof the stir zone are a
function of the welding tempera-ture.10,19,21 In this study, the
processing parameterswere kept constant for the different passes,
resultingin a constant flow stress and processing temperature.
For the samples with CNT, the average grain size is2.11lm for
samples that underwent one pass and4.67lm for samples that
underwent three passes(Table 1). Interestingly, samples with CNT
thatwent through three passes had larger grain sizes ascompared
those that went through one pass. Thiscould be due to higher
temperature experienced withthe increase in number of passes with
the presence ofCNTs. For Al-CNTs with one pass, the average
grainsize is smaller than those without CNT. This couldonly be the
result of adding CNT as the processingparameters are the same;
particle stimulated nucle-ation may occur when Al-based metal
matrix
Figure 2. (a) AA6061-T6 plate with an array of 720 holes with 1
mm diameter, 1 mm in depth, and spacing of 4.2 mm; (b)
AA6061-T6
plate after FSP.
Du et al. 3
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composites were friction stir processed.22–24 However,particle
stimulated nucleation did not occur as it isonly possible when
dislocations accumulate at theparticles during deformation and CNTs
are smallerthan 1 lm. Therefore, it is believed that Zenerpining
effect by CNT has resulted in finer grain sizesby retarding the
grain growth of the matrix. The rate
of grain growth in the recrystallization of metalswith dispersed
second phase particles can bedescribed using equation (1) according
toHumphreys et al20
dR
dt¼M P� Pzð Þ ¼Mð
��bR� 3FV�b
2rÞ ð1Þ
Here, Fv is the volume fraction, M is the boundarymobility, P is
the driving pressure from the curvatureof the grain boundaries, Pz
is the Zener pinning pres-sure, R is the radius of the grain, r is
the radius of thepinning particles, a is a small geometric
constant, and�b is the boundary energy.
When P¼Pz, grain growth will stop
��bR¼ 3FV�b
2rð2Þ
Figure 4. FESEM image of AA6061-T6 samples (a) as received with
�1000 magnification; (b) with one pass condition with
�1000magnification; and (c) three passes with �1000 magnification;
FSP Al–CNT composite with (d) one pass with �5000 magnification
and(e) three passes with �5000 magnification.
Figure 3. Dimensions of the samples used for tensile test in
millimeters.
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Zener limiting grain size (a¼ 1) can be obtainedwhen the mean
grain radius (D) and the radius ofcurvature (R) are taken to be the
same
Dz ¼4r
3Fvð3Þ
Tweed et al.25 suggested that the interactionbetween the pinning
particles and grain boundariesis very complicated. Their study
suggested that thehigh energy of the grain boundary of the
high-angleboundaries may have curved the boundary planewhen it
touches a second phase inclusion. Thisresult in a bypass long
before the boundary bentinto a semi-circle as a whole. However, low
angleboundaries has lower energy and are more flexibleresulted in a
more perturbed plane.
EBSD analysis was used to further investigate thepinning effect
of the CNTs (Figure 6).
For friction stir processing of Al–CNT with threepasses, the
mean boundary misorientation and thenumber of high angle boundaries
(>15�) was observedto have slightly decreased with a slight
increase in thenumber of low angle boundaries (415�) when com-pared
to AA6061-T6 that underwent three passes(Table 2). The reason for
this observation is that theCNTs were randomly oriented and very
small in size.Hence, there were no significant pinning effect
anddifferences in the results.
Micro-hardness
The Vickers’ micro-hardness values were measuredon the base
material, samples without CNT, and
Figure 5. Typical grain structures of EBSD image of AA6061-T6
samples (a) as received, (b) with one pass condition and (c)
three
passes; FSP Al–CNT composite with (d) one pass and (e) three
passes. For the boundary misorientation: white lines: between 1�
and5�, grey lines: between 5� and 15�, black lines: >15�. (The
reader is referred to the web version of the article for
interpretation of thereferences to colour in this figure
legend.)
Du et al. 5
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samples with CNTs added (Table 3). The decreasein hardness value
was observed when FSP was con-ducted on AA6061-T6. It is also
observed thatincreasing the number of passes did not influencethe
hardness of the samples without CNT. Thiscould be due to the
dissolution of the hardeningprecipitates.26
For Al–CNT composites, significant improvementin the hardness
value was observed when comparingwith those friction stir processed
samples withoutCNTs. This is due to the finer grain sizes and
theOrowan strengthening caused by the addition ofCNTs. An increase
in the hardness value was
observed with the increase in the number of passes.This could be
due to the improved dispersion of theCNTs under three pass
conditions.
Tensile testing
The tensile stress strain verses strain curves wereplotted as
shown in Figure 7. The tensile results ofFSP samples without CNTs
showed that a decreasein yield strength compared to a non-FSP
sample(Table 4). There is also a significant increase in
per-centage elongation-to-failure, indicating an improve-ment in
ductility. This is mainly due to the grain
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Fra
ctio
n
Misorientation (°)
Misorientation HistogramAA6061-T6 1 pass
AA6061-T6 3 passes
Al-CNT 1 Pass
Al-CNT 3 Passes
Figure 6. Histogram showing the distribution of grain/sub-grain
misorientation angle by EBSD.
Table 2. Grain/ sub-grain boundary misorientations.
Material and
process
Mean grain
misorientation (�)
Fraction of
high-angle grain
boundaries (>15�)
Fraction of low angle grain
boundaries (415�)Number of
samples(1–5�) (0–15�)
AA6061-T6 1 Pass 23.11 0.50 0.29 0.50 49,871
AA6061-T6 3 Passes 24.00 0.58 0.26 0.42 39,629
Al-CNT 1 Pass 31.14 0.74 0.13 0.26 71,873
Al-CNT 3 Passes 22.75 0.48 0.30 0.52 50,795
Table 3. Micro-hardness values measurement of the base material,
FSP of base material and FSP of Al–CNT composite.
Materials and
process AA6061-T6 AA6061-T6 1 Pass AA6061-T6 3 Passes Al–CNT 1
Pass Al–CNT 3 Passes
HV 109.5 61.7 66.2 80.1 90.6
Standard deviation 1.38 1.70 0.72 0.93 0.87
Table 1. Grain size measurements using EBSD.
Material and process AA6061-T6 AA6061-T6 1 Pass AA6061-T6 3
Passes Al-CNT 1 Pass Al-CNT 3 Passes
Average grain size (lm) 70.04 4.49 5.04 2.11 4.67
Standard deviation, SD 39.04 3.37 3.53 1.45 3.16
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Figure 8. SEM image of the fracture site of the FSP monolithic
Al plates with (a) one pass condition with �1000 magnification;(b)
one pass with �10,000 magnification; (c) three passes with �1000
magnification; and (d) three passes with �10,000 magnification.
0
50
100
150
200
250
0 5 10 15 20 25 30 35
Stre
ss (
Mpa
)
Strain (%)
Stress vs Strain
AA6061-T6 1 Pass
AA6061-T6 3 Passes
Al-CNT 1 Pass
Al-CNT 3 Passes
1
2
3
4
1
2
3 4
Figure 7. Stress versus strain plot of friction stir processed
samples.
Table 4. Tensile results.
Tensile properties of Al base metal and Al–CNT composites
produced by FSP
Materials and process Ultimate tensile strength (MPa) Yield
strength (MPa) Elongation (%) Young’s modulus (GPa)
AA6061-T627 290 240 8 69
AA6061-T6 1 Pass 220� 4 110� 4 27� 1 71� 1AA6061-T6 3 Passes
206� 2 96� 9 29� 1 71� 1Al–CNT 1 Pass 157� 4 120� 5 3� 1 57�
2Al–CNT 3 Passes 178� 28 112� 2 10� 5 65� 12
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refinement (Figure 4). A reduction in the yieldstrength was
observed when comparing one passand three passes; this could be due
to the dissolutionof the hardening precipitates.26
The tensile results from the FSP with CNTsshowed no improvement
in yield strength as com-pared to the parent material (AA6061-T6)
withoutFSP (Table 4). However, the yield strength of Al–CNT was
superior when compared to FSP withoutany addition of CNTs. For
Al–CNT samples, therewas a significant reduction in the percentage
elonga-tion-to-failure in both one and three passes.The CNTs could
have acted as defects andstress concentrators during the tensile
test. Thestrengthening of the material could be attributedto the
grain size differences from Hall–Petchequation.9
Fractography
The fracture sites of the FSP samples without CNTswere observed
under SEM and dimpled appearancesat the fracture sites were
observed indicating ductilefracture. In addition, smaller dimples
were observedin FSP samples that underwent three passes(Figure
8).
For the FSP samples with CNTs, larger dimpleswere found on the
samples with one pass conditioncompared to three passes. This is in
agreement with
the tensile results earlier indicating three passes ismore
ductile than one pass. Some CNTs wereobserved in the SEM (Figure
9).
Comparing the percentage elongation-to-failure ofthe specimens
with and without CNT, the specimenwith CNT had smaller percentage
elongation-to-fail-ure. This observation implies that specimens
withCNT are less ductile. In addition, some CNTs wereobserved at
the fracture site, indicating the CNTscould have acted as defects
in the material under ten-sile load. The interaction between the
CNTs and theAl matrix may not be strong enough in enhancing ofthe
mechanical properties. The appearance of CNTsat the fracture sites
of the samples that underwentthree passes also suggested that crack
bridging couldhave occurred.12,28
Conclusion
1. In this work, FSP on monolithic AA 6061 platesand Al–CNT
composites were studied togetherwith the effects of different
number of passes.For the monolithic Al 6061, the dendrites
werebroken down and dispersed uniformly with theincrease in the
number of passes. In the study ofFSP of Al-CNT composite, the CNTs
have beensuccessfully dispersed with three FSP passes; dis-persion
is more uniform with the increasingnumber of passes.
Figure 9. SEM image of the fracture site of the FSPed Al–CNT
samples: (a) one pass condition under �1000; (b) one pass
under�10,000; (c) three passes under �1000; (d) three passes under
�10,000.
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2. The Vickers hardness number decreased withincreasing number
of passes for the AA6061 spe-cimens that underwent FSP due to
dissolution ofthe hardening precipitates. For specimens withCNTs,
the Vickers hardness increased withincreased number of passes.
Overall, the speci-mens with CNT have superior hardness valuesthan
specimens without CNT that underwentFSP, but inferior to the
as-received AA6061-T6specimens.
3. Grain refinement was achieved using FSP. Theaddition of CNT
resulted in further refinementof the grains, improved tensile yield
strength aswell as provided crack bridging in the material.The
ductility of the material improved withincreased number of
passes.
Conflict of interest
None declared.
Funding
This research received no specific grant from any fundingagency
in the public, commercial, or not-for-profit sectors.
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