warm dynamic compaction of al6061/sic nanocomposite...

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

376 Proc IMechE Part L: J Materials: Design and Applications 230(2)



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.

Majzoobi et al. 377



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




(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.

Majzoobi et al. 379



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.




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.

Majzoobi et al. 381



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

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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.




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.

Majzoobi et al. 383



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




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



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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