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Original Article
Warm dynamic compaction of Al6061/SiCnanocomposite powders
GH Majzoobi1, H Bakhtiari1, A Atrian1, MK Pipelzadeh2 andSJ
Hardy2
Abstract
Powder dynamic compaction is one of the new methods for the
production of nanocomposites. In this paper, Al6061/
SiCnp nanocomposite is compacted using warm dynamic compaction
by simultaneous application of heat and dynamic
compressive waves. A comparison between the results of this
study and those reported in the literature confirms that
the warm dynamic compaction methods are superior to cold dynamic
and quasi-static compaction method in densifi-
cation of nanocomposites especially for high volume fractions of
nano particles reinforcement. Mechanical and micro-
structural characterization of the samples is carried out to
investigate the effects of temperature and content level of
reinforcement. The results indicate that the increase of nano
reinforcement content in warm dynamic compaction leads
to reduction of the relative density and increase of hardness
and the compressive strength. Moreover, higher compaction
temperatures result in enhanced density and lower hardness. It
is shown that samples compacted using warm dynamic
compaction exhibit lower spring back and ejection force and also
the distribution of mechanical properties is significantly
more homogeneous. Sensitivity analysis showed that temperature
increase has the most effect on homogeneity improve-
ment and reducing dimensional change. Microscopic analyses
verified that higher compaction temperature leads to lower
porosity and improved metal particle bonding. It seems that
agglomeration of nanoparticles and destructive phenomena
such as capping and delamination are the main reasons for loss
of compressive strength at room temperature. These
issues are resolved in warm dynamic compaction by increasing the
compaction temperature which leads to better
bonding between particles.
Keywords
Al6061/Sic nanocomposite, dynamic powder compaction, relative
density, quasi-static powder compaction, mechanical
characterization
Date received: 21 January 2014; accepted: 8 December 2014
Introduction
In recent decades, the need to produce materials withhigher
performance and lowweight and cost has drawnthe attention of
various industries to metal matrixcomposites (MMCs). Also, advances
in technologyand production of nanoparticles have made metalmatrix
nanocomposites a suitable option that yieldshigher performance. Due
to excellent properties suchas low weight, good strength, and high
wear resistance,SiC reinforced aluminum MMCs have found
wideapplications in aerospace and automotive indus-tries.1–5
Aluminum-based nanocomposites are pro-duced using liquid state
(e.g. casting) or solid state(e.g. powder metallurgy) methods. Low
wet abilitybetween molten aluminum and SiC, undesirable reac-tions
between them, and restriction in adding high vol-umes of
nanoparticles are some limitations of theliquid state production
method. Thus, many research-ers have preferred using powder
metallurgy for theproduction of mechanically alloyed
nanocomposites.6
Dynamic compaction is one of the powder metal-lurgy methods
which presents significant advantagesover other conventional
methods. Benefits of thismethod include better density, higher
hardness andsurface quality, and more homogenous
mechanicalproperties which are achieved with lower ejectionforce
and dimensional changes.7 In this method, theforce required for
compression of the powder is pro-vided by creating dynamic
compressive waves.Dynamic compressive waves can be generated
byexplosion (explosive compaction),8 creating shockmagnetic field
(magnetic compaction)9 or collision of
1Mechanical Engineering Department, Bu-Ali Sina University,
Hamedan,
Islamic Republic of Iran2College of Engineering, Swansea
University, Swansea, UK
Corresponding author:
MK Pipelzadeh, College of Engineering, Swansea University,
Swansea
SA2 8PP, UK.
Email: [email protected]
Proc IMechE Part L:
J Materials: Design and Applications
2016, Vol. 230(2) 375–387
! IMechE 2015
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DOI: 10.1177/1464420714566628
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two objects at a high speed.10 This wave is dispersedthroughout
the powder in milliseconds and breaks theoxide layers between
particles, creating a bond betweenthem.11 Because of producing
strong shock waves, thedynamic compaction method is very suitable
for com-paction of ceramics, MMCs, nanocomposites, andmetals that
have high work hardening (such as titan-ium alloys12). Several
studies have been carried out ondynamic compaction of ceramic
particles,13 aluminumalloys,10 and aluminum matrix
composites.14
Fredenburg et al.10 successfully used a gas gun to com-press
nanocrystalline Al 6061-T6 powders to a relativedensity of 98–99%
while retaining initial microstruc-ture. Sivakumar et al.15–17
blended different aluminumalloy powders with SiC micro particles
and dynamic-ally compacted the mixture. They were able to com-press
composite powder Al 2024-40%SiC up to 96%ofits theoretical density
utilizing explosive compaction.Yan et al.12 compressed titanium
alloy powders to rela-tive green density of 96.2% by high velocity
compac-tion technique (Hydropulsor device). Volger et al.13
investigated the static and dynamic compaction oftungsten
carbide ceramic powders under the pressures1.6 and 5.9GPa,
respectively. Based on their results, amaximum density of nearly 14
g/cm3 was achieved bystatic compaction compared to 14.99 g/cm3
densityachieved by dynamic compaction (gas gun device),which was
not far from the crystal density of WC of15.7 g/cm3. Bond and
Inal14 could consolidate alumi-num–boron carbide composite powders
at 10–12GPapressure using explosive materials. Recently, Atrianet
al.18 compressed Al7075-SiC nanocomposite pow-ders using warm
dynamic compaction. Despite ofreduction in hardness, they concluded
that warmdynamic compaction caused higher green density com-pared
with the samples produced by dynamic compac-tion at room
temperature. In another study,19 the sameauthors compared
Al7075-SiC nanocomposite sam-ples consolidated by warm dynamic and
quasi-staticcompaction methods. Their results revealed
highercompressive strength and density for quasi-static hotpressed
samples than those produced under dynamiccompaction. Nevertheless,
it has been shown that byreducing particle size of SiC
reinforcement down tonano scale, its compressibility drops
significantly.20
In addition, a reduction in particle size causes difficul-ties
in breaking oxide layers and bonding betweenparticles.21
In most dynamic compression methods, strongcompressive waves are
required to achieve high
density. Hence, equipment used to create highspeed (like gas
gun) or strong explosion are verycostly and their service time span
is short due toexperiencing extremely high stresses. In this
paper,warm dynamic compaction method using a low-cost drop hammer
device is employed to compressnanocomposite powders. In this
method, simultan-eous utilization of collision and thermal
energiesallows for ultra-high density compaction of
particles.Higher temperatures result in thermal softening
inparticles and facilitate their compressibility. In fact,in this
method, higher temperature replaces highspeed or explosion to
enhance density and mechan-ical properties.
The production of Al 6061-SiCnp nanocompositesby dynamic
compaction method has not been inves-tigated in the literature.
Previous studies have onlyfocused on cold dynamic compaction of
compositepowders. In this paper the emphasis is put on thestudy of
simultaneous effects of temperature andshock waves on compaction of
nanocomposite pow-ders. Hence, Al 6061-SiCnp nanocomposite is
pro-duced using cold and warm dynamic compactionmethods and the
effects of temperature and nanoreinforcement fraction on mechanical
propertiesand microstructure of produced samples are studied.In
‘‘Experiments’’ section, the process of the experi-ment is
explored. ‘‘Results and discussion’’ sec-tion analyzes the
mechanism and effect oftemperature and volume fraction of
nanoparticleson properties (density, ejection force, spring
back,microhardness, homogeneity and compressivestrength) of green
and sintered samples. Sensitivityanalysis is also performed in this
section to investi-gate the effect of temperature on various
parametersof the process.
Experiments
Preparing materials
The nanocomposite powder was prepared from theas-received Al
6061-T6 powder (specifications areprovided in Table 1) blended with
5 and 10 vol%of SiC nanoparticles (average size of 50 nm
andspherical morphology). In order to prevent agglom-eration of
nanoparticles and also to obtain a uniformdispersion, the mixture
was then suspended in etha-nol and was subjected to ultrasonic
vibration for20min. Ethanol serves as a neutral environment for
Table 1. Spectrometry analysis of Al6061 (gas atomized, –100 mm,
irregular morphology)-unit is weight percentage (wt%).
Si Fe Cu Mn Mg Cr Ni Zn Ti Be Ca Li
0.58 0.36 0.21 0.01 0.88 0.19 � 0 0.01 0.007 � 0 � 0 � 0Pb Sn Sr
V Na Bi Co Zr B Ga Cd Al
� 0 5 0:005 � 0 0.009 0.002 5 0:004 5 0:002 � 0 5 0:001 0.004 �
0 Base
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uniform distribution of SiC nanoparticles and wasused to reduce
oxidation of aluminum. Afterdrying, the mixture was milled using
0.5wt% of ste-aric acid in a planetary ball mill (with 3:1 ball
topowder ratio and a speed of 300 r/min) underargon gas to properly
distribute nanoparticles.Figure 1 shows SEM images of Al 6061-T6
powdermorphology before and after blending with SiCnanoparticles.
As the figure indicates, nanoparticlesare properly distributed on
the surface of aluminumparticles after milling.
Warm dynamic compaction
Dynamic compaction is conventionally performedusing devices such
as a gas gun10 or hydropulsordevice.7 However, procurement and
maintenance ofthese devices are very costly and they suffer short
ser-vice life span due to experiencing extremely high
stresslevels.22 Dynamic compaction in this study is carriedout
using a simple and low-cost drop hammer device.Figure 2 illustrates
a schematic of this device in whichthe compaction force is supplied
by the free drop of a
Figure 2. Warm dynamic compaction device.
Figure 1. SEM images of (a) initial Al 6061 powder and (b)
Al6061-5%SiCnp composite powder after 2 h of ball milling.
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weight of 60 kg from a height of 3.5m. The impactvelocity of the
weight is about 8m/s and its energy is2 kJ. The weight of the punch
is very small comparedto the impacting weight. Therefore, according
to theconservation of momentum principle, compactionspeed (velocity
of punch at collision) is much higherthan the velocity of the
colliding weight. The diedesigned for the experiments is shown
schematicallyin Figure 2. In order to resist the thermal energy
andthe applied impact, the die is made of 1.2344 heat-treated
hot-work steel and the 15mm diameterpunch is made of 1.2542
shock-resisting steel.
Two tablets made of the punch material andhaving a length of 5mm
are placed at the top andthe bottom of powder to hold the powder
bed, toreduce the spring back effects, and to maintain thesurface
quality of compacts.23 The required tempera-ture for the die is
provided by a 400W thermal elem-ent which is wrapped around the
outer surface of thedie. Powder temperature is measured by a
thermom-eter (temperature difference between the die andpowder is
about 5�C). After that powder temperaturereaches a steady state,
the powder is subjected todynamic loading. After compaction and
ejection ofthe sample out of the die, it is cooled down to
ambienttemperature.
In this study, the die wall is lubricated using MoS2spray
lubricant. The reason for this selection is thatinternal
lubrication of the powder itself is not allow-able due to the
reduction in final density and the pos-sibility of vaporization at
high temperatures.22,24 Thetest conditions are as given in Table 2.
The testarrangement was designed to investigate the effectsof
temperature and the content of nano reinforcementand also to
compare the cold and warm dynamic com-paction methods. In each
experiment, 5 g of powder ispoured into the die and compacted. For
the purposeof final strengthening, sintering of green samples
isperformed at 630�C for 30min in a vacuum furnace.
Results and discussion
Density
Relative density specifies the porosity inside thematerial and
is an important parameter in evaluatingthe performance of the
compaction method. Thereason is that the final properties of the
compactedsample are directly related to relative density.25
In this paper, density of sintered samples was mea-sured using
the water displacement method. Densityof green samples was
calculated through dimensionalmeasurement due to higher porosity
and risk of fluidinflux. This was accomplished by polishing surface
ofsamples and measuring diameter and height of thesample at three
different points using a micrometer.The arithmetic average of the
measurements was con-sidered. The mass of samples was measured
using adigital weighing scale with an accuracy of 0.001 g.
Relative green and final densities of samples areshown in Figure
3(a) and (b), respectively.Furthermore, to compare warm dynamic
compactionand quasi-static compaction methods, the highestreported
green densities for Al 6061-SiCnp nanocom-posite20 compacted in
approximately similar condi-tions (the same die diameter and the
same size ofnanoparticles) using quasi-static method at 400MPaare
shown in Figure 3(a). It can be seen that increas-ing compaction
temperature leads to an enhancedrelative density in all conditions.
Also, as volume frac-tion of nanoparticles rises, it reduces the
relative dens-ity. Hence, temperature and volume fraction
ofnanoparticles present opposite effects on density.This is a
result of the powder compressibility charac-teristics. It has been
shown that as the temperatureincreases, the yield stress and the
work hardening ofthe powder reduce.26 Conversely, adding ceramic
par-ticles leads to increasing work hardening of powderand
decreasing compressibility of metal particles.20,27
In addition, high specific surface area and stronginter-particle
friction between nanoparticles makesthe compaction difficult.
Because of all these factorscombined, adding SiC nanoparticles
results in lowergreen density.
It is clear that the processing technique is animportant factor
that affects the green density of thecompact. For example,
Fredenburg et al.10 couldreach the relative density of 98–99% for
Al6061-T6powders using a single stage light gas gun which ishigher
than the densities obtained by quasi-staticcompaction20 and that
obtained in the presentstudy. A comparison between cold dynamic,
warmdynamic, and quasi-static compactions at 400MPaindicates that
warm dynamic compaction yields thehighest green density. Another
point that can beinferred is that at higher volume fractions of
nanopar-ticles, warm dynamic compaction provides better
per-formance and produces higher density compared toother methods.
This issue will be discussed in moredetail in ‘‘Sensitivity
analysis’’ section. It is alsoobserved that relative density
increases between 1and 3% after sintering. Samples with lower
greendensity experience higher density increase. The
highestrelative density for monolithic Al 6061 reinforced witha 5
and 10 vol% of SiC nanoparticles at 425�C is 96.5,96.2, and 96%,
respectively. A comparison betweenthe methods shows that the
density of Al6061-10%SiC compacted using warm dynamic
compaction
Table 2. Test conditions.
Dynamic compaction Temperature (�C) SiC content (vol%)
Cold Room 0,5,10
Warm 125 0,5,10
225 0,5,10
425 0,5,10
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(at 425�C) increases by 4.6 and 9% greater than colddynamic
compaction and quasi-static compaction,respectively. These results
contradict the resultsreported by Hafizpour et al.20 which indicate
thatquasi-static compaction of Al6061-SiC nanocompo-site at
elevated temperatures up to 110�C does notimprove compressibility
of the powder.
Optical images of monolithic and nanocompositesamples are shown
in Figure 4. Marked areas in theimages indicate the empty space and
weak bondbetween the particles. As it is observed, rising
com-paction temperature from room temperature to 225�Cenhances the
formability of monolithic samples andleads to filling empty spaces
between particles andstronger bonds between them (Figure 4(a) and
(b)).Similar to monolithic samples, the porosity of nano-composite
samples is reduced with increasing tem-perature to 425�C (Figure
4(c) and (d)). It should benoted that most metals show more work
hardening athigher strain rates, hence the increase in impact
vel-ocity makes the metal powders more resistant againstcompaction.
Some researchers have reported similaror even lower density when
using the dynamic com-paction method compared to the
quasi-staticmethod.28,29 Therefore, warm dynamic compactioncauses
thermal softening of metal particles which inturn compensates for
the effect of work hardeningcaused by the high velocity impact. On
the whole,warm dynamic compaction results in higher relativedensity
and better distribution of the microstructurethroughout the
material.
Figure 5 shows a comparison between the surfacestructure of the
samples compacted by cold and warmdynamic compaction methods. In
order to investigatethe effects of temperature and compaction
method onporosity and its distribution uniformity, images were
analyzed using ImageJ software. Figure 5(a) and (c)shows SEM
images of two nanocomposite samples.Figure 5(b) and (d) illustrates
micro porosity distribu-tion maps of these two samples,
respectively. As canbe observed, higher compaction temperature not
onlyreduces surface porosity, but also creates a more uni-form
distribution of microporosity.
Ejection force and spring back
Ejection force is the maximum force required toremove the
compacted part out of the die. Thisforce is influenced by powder
specification, frictionbetween sample and die, ratio of height to
diameterof the compacted sample, and radial residual
stressproduced.7 Another effect of radial residual stress isthe
accumulation of elastic energy in the compactedsample which is
released after ejection of the sampleand is manifest as radial
expansion. This phenomenonis called spring back. Radial spring back
is calculatedusing equation (1)
� ¼ d� d0d0� 100 ð1Þ
where � is radial spring back percentage, d is diameterof sample
after ejection, and d0 is internal diameter ofthe die.
In this paper, an Avery hydraulic device isemployed to eject
compacted samples. Figure 6(a)and (b) displays ejection force and
spring back forvarious samples at four different temperatures. As
itis seen, an increase in temperature significantlyreduces ejection
force and spring back of thesample. Since radial residual stress
produced in thesample is equal to its yield stress,30 higher
91
92
93
94
95
96
97
0 5 10
Rela
�ve
den
sity
(%)
SiC (Vol %)
T=25°C T=125°CT=225°C T=425°C
85
87
89
91
93
95
97
0 5 10
Rela
�ve
Gre
en d
ensi
ty (%
)
SiC (Vol %)
T=25°C(a) (b)
T=125°CT=225°C T=425°CHafizpour et.al
Figure 3. Effect of temperature and SiCnp content on relative
density (a) before and (b) after sintering.
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temperatures result in lower yield stress and conse-quently
lower radial residual stress. As a result ofdecrease in radial
residual stress, ejection force andspring back are reduced. In
addition, it is shownthat viscosity of lubricant decreases at
higher tem-peratures (below vaporization temperature) and
con-sequently friction between the sample and diereduces.31
Furthermore, with the increase in volumefraction of SiC
nanoparticles, less elastic energy isaccumulated in the sample,
which is probably due tolarger brittle phase and spring back is
reduced. It canbe seen in Figure 6 that by increasing
temperaturefrom room temperature to 425�C spring back andejection
force for Al6061-10vol%SiC decrease about70 and 78%,
respectively.
Micro hardness and homogeneity
In this paper, the hardness of samples was measuredusing a micro
hardness tester manufactured byBuehler Company Ltd. The surface of
samples waspolished and then Vickers hardness at six points(three
on the top and three on the bottom surface)was measured by applying
a load of 100 gf in 15 s.The arithmetic average of readings was
consideredas the hardness value of the sample.
One of the complications in nanocomposites pro-duction is the
formation of gradients of mechanical
properties in the material. This gradient results in
aheterogeneous structure and also causes internaldefects and
microscopic cracks in the material.22
Experimental and numerical results indicate that sam-ples
compacted using dynamic compaction demon-strate a more homogenous
structure compared toother methods.7 In this paper, the effect of
tempera-ture on homogeneity of sample properties is evaluatedusing
dispersion of hardness values as an index.Choosing hardness to
evaluate heterogeneity ofmaterial is because measurement of density
orstrength of material at different points is difficult
orimpractical and also hardness is directly proportionalto
mechanical and structural properties.2 Hardnessstandard deviation
as an index of dispersion is speci-fied as equation (2)
S:D ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPNi¼1
HVi �HVaveð Þ
2
N
s
HVave ¼PN
i¼1 HVið ÞN
ð2Þ
where HVi is hardness at ith point, N the number ofall points,
HVave average hardness, and S.D is hard-ness standard deviation.
Lower S.D values mean lessdispersion of hardness values and more
homogenousstructure of material.
Figure 4. The effect of compaction temperature on porosity
reduction of dynamic compacted samples: (a) Al6061-T¼ 25�C,(b)
Al6061-T¼225�C, (c) Al6061-5%SiC-T¼25�C, and (d)
Al6061-5%SiC-T¼425�C.
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Figure 5. SEM images and micro porosity distribution maps of
(a), (b) cold dynamic compacted sample (Al6061-5%SiC-T¼25�C)
and(c), (d) warm dynamic compacted sample
(Al6061-5%SiC-T¼425�C).
00.10.20.30.40.50.60.70.80.9
1
0 5 10
Rad
ial s
prn
ig-b
ack
(%)
SiC (Vol %)
T=25°C T=125°C T=225°C T=425°C
0
2
4
6
8
10
12
14
16
0 5 10
Ejec
tion
forc
e (K
N)
SiC (Vol %)
T=25°C T=125°C T=225°C T=425°C(a) (b)
Figure 6. Effect of temperature and SiCnp content on (a)
ejection force and (b) radial spring back.
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Figure 7(a) and (b) illustrates average hardness andhardness
standard deviation at different temperaturesfor various volume
content of SiC nano reinforce-ment. It is observed that by
increasing the SiC nano-particles to 5% hardness reduces slightly,
but if wekeep increasing the SiC nanoparticles to 10%, hard-ness
starts to increase again. It is also seen that highercompaction
temperature leads to hardness decrease.Adding nanoparticles has
considerably less effect onhardness than compaction temperature.
Due to theinherent hardness of ceramic particles and their highwork
hardening, the addition of SiC nanoparticlesincreases hardness.
Moreover, increasing temperaturesoftens metal particles and
consequently the resistanceof particles to indentation drops
significantly. As aresult, hardness is decreased. Wojtaszek
andDudek32 also reported a similar result for Al–Si–Fe–Cu alloy
powder. They concluded that Brinell hard-ness of investigated
materials decreased with increas-ing the temperature of the
compaction. Generally, itcan be stated that addition of SiC
nanoparticlesincreases work hardening and high temperaturecauses
thermal softening. These two have conflictingeffects on
hardness.
In Figure 7(b), it can be seen that hardness distri-bution at
different points tends to be more homogen-ous as temperature rises.
Conversely, the addition ofnanoparticles leads to increasing
heterogeneity.Hence, another statement is that higher green
density(increase in temperature) results in a more homogen-ous
structure. This fact confirms previously reportedresults.22 The
most important parameters in the for-mation of property gradients
in composites are het-erogeneous distribution of particles
(regarding shape,material, and size) and friction. Friction leads
to aheterogeneous distribution of pressure and this is afactor in
the development of property gradients inthe sample. By adding more
reinforcement particles,the possibility of clustering and
heterogeneous
distribution of them in the matrix increases and thisleads to
more hardness heterogeneity in the sample.Moreover, friction
between die wall and the samplereduces as a result of lower
viscosity of the lubricantat high temperature.31 This leads to a
more homoge-neous distribution of pressure. Figure 7 also
indicatesthat by increasing the temperature from room tem-perature
to 425�C, micro hardness of Al6061-10vol%SiC sample decreases
approximately by 40%,while hardness heterogeneity reduces around
41%.
Sensitivity analysis
The effect of temperature on the mechanical proper-ties of
compacted samples was studied by sensitivityanalysis on the
experimental results using MATLAB2010 software. Sensitivity
analysis determines theeffect of uncertainty in independent input
variableson uncertainty in output response.33 In this
paper,normalized sensitivity factor is defined as follows
S ¼ @Y@X� XY
ð3Þ
where S is normalized sensitivity factor and X and Yare the
input and the output variables, respectively.Interested readers are
referred to the literature33 formore information. Generally, higher
values of S meangreater effectiveness of the studied parameter
onoutput, so that variation in input variables results inmore
variation in output value.
In this paper, sensitivity analysis was performed toinvestigate
the effect of temperature (independentvariable) on green density,
micro hardness, hardnesshomogeneity, ejection force, and spring
back (outputvariables) of the compacted sample. Figure 8(a)
showsthe sensitivity factor for the output variables for vari-ous
volume fractions of nano reinforcement.Moreover, the average
sensitivity factor for each
0
2
4
6
8
10
12
14
0 100 200 300 400 500
Har
dn
ess
STD
Temp (°C)
0%SiC 5%SiC 10%SiC
0
20
40
60
80
100
120
0 5 10
Mic
ro H
ard
nes
s(H
V)
SiC (Vol %)
T=25°C T=225°C T=425°C(a) (b)
Figure 7. Effect of temperature and SiCnp content on (a) micro
hardness and (b) hardness heterogeneity.
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parameter is shown in Figure 8(b). In this figure, (þ)shows a
direct relationship and (–) indicates an inverserelationship
between output variable and the increasein temperature.
It is evident that variation in compaction tempera-ture affects
homogeneity and spring back of thesample. Furthermore, an increase
in temperatureinfluences ejection force and micro hardness
significantly. It should be noted that although tem-perature has
a low effect on relative green density, a4% increase in density can
be quite significant, espe-cially in higher densities. It is
observed that at highervolume fraction of nanoparticles, the effect
of tem-perature increase on relative density and spring backis more
pronounced. It can be concluded that withhigher reinforcement, warm
dynamic compaction
Spring back Homogeneity Ejec�on force Hardness Rela�ve
density0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Nor
mal
ized
sen
siti
vity
fact
or
0%SiC(a)
(b)
5%SiC 10%SiC
Ave
rage
nor
mal
ized
sen
siti
vity
fa
ctor
Spri
ng b
ack
(-)
Hom
ogen
eity
(+)
Ejec
�on
forc
e (-
)
Har
dnes
s (-
)
Rela
�ve
den
sity
(+)
Figure 8. Effect of temperature increase on various parameters
using dimensionless sensitivity factor (a) in different volume
frac-
tions of reinforcement phase and (b) average sensitivity factor
for various parameters.
Figure 9. True stress–true strain compression curves of (a)
conventional dynamic and (b) warm dynamic compacts.
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presents better performance in increasing the densityand
reducing the spring back effect. However, thistrend is inversed for
the homogeneity parameter.This implies that the mechanism of
homogeneityincrease in warm dynamic compaction depends onplastic
deformation in metal particles and reductionof porosity.
Yield strength
Uniaxial compression tests at a constant speed of5mm/min and
strain rate of 0.01 s–1 were carriedout using SANTAM device at room
temperature toinvestigate compressive strength of compacted
sam-ples. In order to minimize the friction coefficient, thetop and
bottom surface of the samples were polishedand contact surfaces
were lubricated. Prior to any test,the device was calibrated and
compressioncontinued up to the fracture of the sample.Figure 9(a)
and (b) displays true stress–strain curvesfor samples compacted at
room temperature and425�C, respectively.
In order to determine the compressive strength ofsamples, the
0.2% offset proof stress was computedand is given in Table 3. At
room temperature, asvolume fraction of nano reinforcement
increases,compressive strength is reduced (Figure 9(a))
whileconversely at 425�C, increasing reinforcement fractionleads to
higher compressive strength (Figure 9(b)).Compressive strength
obtained at this temperaturefor the Al6061-10%SiCnp sample shows a
56.5%increase compared to monolithic sample.
The reason behind the decrease in compressiveyield strength at
room temperature is the weak bond-ing between the particles. With
the increase in volumefraction of nano reinforcement, the
possibility ofagglomeration and weakening of particle
bondingarises. Moreover, stress concentration and crackgrowth
usually occur at reinforcement particle clus-ters.5,34 Akbarpour et
al.35 showed that increasingvolume fraction of SiC nanoparticles
from 4 to 6%in Cu matrix particles reduces yield strength.
Theybelieved that the reason was due to weak bondingbetween
particles at nanoparticle clusters. Alba-Baena et al.36
investigated yield strength of Al-SiCcomposite samples compacted
using the explosive
compaction method. From their tensile test resultsand SEM
images, they concluded that agglomerationof SiC particles was
responsible for particles debond-ing and consequently strength
reduction. Hence, ifproper bonding is not created at nanoparticle
clusters,the increase in volume fraction of reinforcement phasewill
give rise to a reduction in the yield strength ofthe material.
Non-uniform dispersion of reinforcingparticles is another factor
causing the crack initiationboth at and near the particulate–matrix
interfaces andin regions of particulate agglomeration and
conse-quently, failure of particles at lower stresses.37
Figure 10(a) and (b) shows SEM images of interfaceregions
between matrix particles and reinforcementparticles in compacted
Al6061-5%SiCnp nanocompo-site at 425�C and EDX analysis of SiC
nanoparticles.It is observed in the figures that there is rigidity
andproper bonding between particles. However, agglom-eration of
nano reinforcement is evident in both coldand warm dynamic
compacted samples. Figure 10(c)and (d) illustrates SEM images of
Al6061-5%SiCnpcompacted at room temperature and 425�C,
respect-ively. Agglomeration of nanoparticles is clear in
bothimages. Therefore, the main reason behind higherstrength in
samples compacted using the warmdynamic compaction method is
probably maintainingrigidity at the interface between reinforcement
andmatrix particles.
Weak bonding between particles in dynamic com-paction at room
temperature can be recognized by theoccurrence of phenomena such as
capping and delam-ination at the surface of compacted sample.
Thesephenomena usually happen due to dispersion ofreflective stress
waves inside the sample and morespring back. Figure 11 provides a
comparisonbetween nanocomposite samples compacted at
roomtemperature (Figure 11(a)) and 425�C (Figure 11(b)).As it is
seen, the aforementioned phenomena areobserved in cold dynamic
compaction, but no appar-ent defect is found in the same conditions
in warmdynamic compaction method. Lateral cracks anddelamination in
dynamic compaction result in theloss of rigidity and strength. Some
other studies alsohave reported this phenomenon as a challenge
indynamic compaction.23
If the bonding between particles is perfectly shaped,the
addition of nanoparticles enhances strength. Incompression tests,
nanoparticles act as obstacles formore deformation of the matrix.38
Zhang and Chen39
showed that thermal mismatch stresses are the mostimportant
cause of increasing strength of nanocompo-sites with higher volume
fractions of nanoparticles.Due to the difference in coefficients of
expansion ofmatrix and reinforcement particles, while
temperaturechange is occurring, deformations of particles are
notequal and ceramic particles prevent more deformationsof the
matrix particles. These thermal stresses atparticulate–matrix
interfaces lead to plastic deforma-tion of the matrix and increase
of dislocation density.
Table 3. Compressive yield strength of cold and warm
dynamic compacted samples.
0.2% offset
Proof stress �y(MPa)Compaction
temp. (�C) Sample
182.5 Room Al6061
117.8 425
114.86 Room Al6061-5 wt% SiC
143.28 425
65 Room Al6061-10 wt% SiC
184.4 425
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230(2)
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Figure 10. (a) SEM micrograph of matrix-reinforcement interface,
(b) EDX analysis of SiCnp particles contained between Al6061
particles, nanoparticles agglomeration of (c) cold and (d) warm
dynamically compacted Al6061-5%SiCnp samples.
Figure 11. Al6061-10vol% n-SiCp samples dynamically compacted at
(a) room temperature and (b) 425�C. It can be observed that
warm dynamic compaction can eliminate capping and delamination
phenomena.
Majzoobi et al. 385
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Consequently, the strength of matrix and that of thecomposite
are enhanced. With higher temperaturevariation and greater volume
fraction of nanoparticles,dislocation density and yield strength
increase. Theresistance of nanoparticles to movement of
disloca-tions (Orowan strengthening mechanism40) is
anotherimportant reason for increased strength of nanocom-posites
with higher volume fractions of nanoparticles.In fact, dislocations
are forced to create an Orowanbowing around particles to be able to
pass them.Assuming that a proper bonding is formed betweenparticles
in both cold and warm dynamic compactionmethods, composite samples
produced using warmdynamic compaction experience higher
temperaturedifference. Hence, as a result of increasing
dislocationdensity created by thermal mismatch, these
samplespresent higher strength compared to samples com-pacted using
the cold dynamic compaction method.The same effect is clearly
observed in the casting ofcomposites where thermal mismatch
stresses causedby cooling of the molten material result in an
increasein dislocation density and consequently, leads togreater
strength of samples.34
Summary and conclusion
In this paper, warm dynamic compaction was used forthe
compaction of aluminum matrix nanocomposites.Al6061-SiCnp nano
composites with different volumefractions of reinforcement
particles were preparedusing cold and warm dynamic compaction and
theresults were compared. From the results, the follow-ing
conclusions may be derived:
1. Warm dynamic compaction of Al6061 at 425�Cyielded highest
density, lowest radial spring backand ejection force, and the most
homogeneity ofall samples tested in this work. Green
densityobtained at this temperature for Al6061-10%SiCnp samples
shows a 4.6 and 9% increasecompared to cold dynamic compaction and
quasi-static compaction (at 400MPa) methods, respect-ively.
Moreover, radial spring back, ejection force,and hardness
heterogeneity for the same condi-tions reduced 78, 70, and 40%,
respectively.
2. Increasing compaction temperature reduced microhardness of
compacted samples. By increasing thetemperature from room
temperature to 425�C,Vickers hardness of Al6061-10%SiCnp
sampledecreases by approximately 40%.
3. Microscopic analyses reveal that higher formabil-ity of metal
particles and reduction of their yieldstrength are the main reasons
for lower porosity,higher homogeneity, and less dimensional
changein warm dynamic compaction.
4. Sensitivity analysis of experimental results showedthat the
increase in compaction temperature is themost effective factor in
homogeneity improvement
and reduction of the dimensions of compactedsamples. The results
of this analysis indicate thatthe performance of the warm dynamic
compactionmethod in connection with the increase of densityand
dimensional change is improved for highervolume fractions of
nanoparticles. At the sametime, the simultaneous effects of
temperature andnanoparticle fraction on hardness homogeneityare not
as significant.
5. Undesirable phenomena such as capping anddelamination which
may occur in cold dynamiccompaction are eliminated in warm
dynamiccompaction.
6. Warmdynamic compaction provides higher strengthcompared to
cold dynamic compaction.Compressive yield strength of samples
produced bycold dynamic compaction reduces by increasing thevolume
fraction of nano reinforcement. This trend iscompletely inversed
for warm dynamic compaction.
7. SEM reveals that agglomeration of nanoparticlesmay be
observed in samples obtained using bothmethods. Therefore, by
maintaining rigidity andproper bonding between matrix and
reinforcementparticles and eliminating destructive phenomenasuch as
capping or delamination, yield strengthof the warm and dynamically
produced samplescan be increased more easily.
Funding
The author(s) received no financial support for the
research,authorship, and/or publication of this article.
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