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Journal of Materials Processing Technology 211 (2011) 1614– 1619

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

j o ur nal ho me p age : www.elsev ier .com/ locate / jmatprotec

novel technique for development of A356/Al2O3 surface nanocomposite byriction stir processing

. Mazaheri ∗, F. Karimzadeh, M.H. Enayatiepartment of Materials Engineering, Nanotechnology and Advanced Materials Institute (NAMI), Isfahan University of Technology, Isfahan 84156-83111, Iran

r t i c l e i n f o

rticle history:eceived 8 October 2010eceived in revised form 18 April 2011ccepted 30 April 2011

a b s t r a c t

A356/Al2O3 surface nanocomposite was produced by friction stir processing (FSP) method. X-ray diffrac-tometery, optical and scanning electron microscopy, microhardness and nanoindentation tests were usedto characterize the samples. The results indicated that the uniform distribution of Al2O3 particles in A356matrix by FSP process can improve the mechanical properties of specimens. The hardness and elastic

vailable online 8 May 2011

eywords:356urface nanocompositeriction stir processing

modulus of the as-received A356, the sample treated by the FSP without Al2O3 particles, surface micro-and nanocomposite specimens were about 75 Hv and 74 GPa, 69 Hv and 73 GPa, 90 Hv and 81 GPa, 110 Hvand 86 GPa, respectively.

© 2011 Elsevier B.V. All rights reserved.

anoindentation

. Introduction

Conventional Al matrix composites (AMCs) reinforced witheramic particulates, especially Al2O3 exhibit high strength, hard-ess and elastic modulus (Tjong, 2007). AMCs are one of thedvanced engineering materials that have been developed foreight-critical applications in the aerospace, and more recently

n the automotive industries due to their excellent combination ofigh specific strength and better wear resistance as demonstratedSurappa et al., 1982). AMCs have been widely studied since the920s (Sethi, 2007). A survey of the previous studies indicates that

homogenous dispersion of fine particles in a fine grained matrix iseneficial to the mechanical properties of AMCs (Shorowordi et al.,003).

Dispersion of the nano-reinforcements in a uniform manner is critical and difficult task. There are several methods to fabri-ate particulate reinforced Al or Mg based composites, includingtir casting, squeeze casting, molten metal infiltration, and powderetallurgy (Lee et al., 2006). It should be pointed out that the exist-

ng processing techniques for forming surface composites are basedn liquid phase processing at high temperatures. In this case, it isard to avoid the interfacial reaction between reinforcement and

etal matrix and the formation of some detrimental phases. Fur-

hermore, critical control of processing parameters is necessary tobtain ideal solidified microstructure in surface layer. Obviously,

∗ Corresponding author. Tel.: +98 9173047580; fax: +98 3113912752.E-mail addresses: y.mazaheri@ma.iut.ac.ir, yoosef mazaheri64@yahoo.com

Y. Mazaheri).

924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2011.04.015

if processing of surface composite is carried out at temperaturesbelow melting point of substrate, these problems can be avoided.Recently, a lot of attention has been paid to friction stir process-ing (FSP) as a new surface modification technique (Su et al., 2005).Though FSP has been basically advanced as a grain refinement tech-nique, it can be readily used for fabricating surface composites. FSPinduces intense plastic deformation and mixing of material in theprocessed zone in which the true strain can be as high as 40 (Ma,2008). In this way, it is possible to incorporate the ceramic particlesinto the metallic substrate plate, to form the surface composites.Mishra et al. (2003) reported the first result on the fabrication of Al-SiCp surface composite via FSP. In this way SiC powder was addedinto a small amount of methanol and mixed, and then applied to thesurface of Al plates to form a thin layer of SiC particle. The coatedaluminum plates were then subjected to FSP. Additional researchefforts were dedicated to fabricating the surface/bulk compositesvia FSP with improved particle predeposition methods. One of themethods is to produce one or two grooves along the FSP direc-tion to pour the particles inside. Morisada et al. (2006a,b) reportedthe fabrication of SiC particles and multiwalled carbon-nanotubes-reinforced AZ31 surface composites via FSP, using this method.With deeper grooves being cut, Lee et al. (2006) demonstrated asuccessful fabrication of bulk composites via FSP. More recently,Dixit et al. (2007) successfully dispersed nitinol (NiTi) particles inAl1100 matrix via FSP. They used four small holes drilled below thesurface of the Al1100 plate to load the NiTi powders. Subjecting

the powder-filled plate to FSP produced an Al1100/NiTi compositewith improved mechanical properties. It is important to note thatthe fabrication of the surface/bulk composites is achieved under asolid-state condition. No interfacial reaction occurs between the

Y. Mazaheri et al. / Journal of Materials Processing Technology 211 (2011) 1614– 1619 1615

Table 1Chemical composition of A356 bars.

Element Al Si Mg Fe Mn Cu Ti

rccmbas

watw

2

2

1paf

Table 2HVOF parameters for A356–Al2O3 composite powder.

Parameters Unit Value

Oxygen flow rate l min−1 750Fuel gas flow rate l min−1 175Spray distance mm 350

Composition (wt.%) Rem 7.44 0.44 0.26 0.07 0.05 0.02

einforcing particles and the metallic matrix, and good interfa-ial bonding is achieved. According to above researches, surfaceomposites were attempted by changing base materials, reinforcedaterials and processing way. However, some drawbacks have

een revealed when each of these techniques are applied. Thegglomeration of the reinforced particles especially in case of nano-ized powder is the major limitation of these methods.

To overcome this drawback, in this research a new techniqueas used to incorporate nano-sized Al2O3 into A356 aluminum

lloy to form particulate composite surface layer. The microstruc-ure and mechanical properties of the composite surfaced layerere evaluated in details.

. Experimental procedure

.1. Materials

The specimens used for the FSP experiments were A356-T6

0 mm × 50 mm × 250 mm bars. The chemical composition of sam-les is given in Table 1. Residual machining chips of A356 and micrond nanosized �-Al2O3 powders with purity of 99.9% were usedor producing A356/Al2O3 composite powders. Scanning electron

Fig. 1. Morphology of: (a) A356 chips, (b) microsized

Powder rate g min−1 35Number of passes 3

microscopy (SEM), transmission electron microscopy (TEM) micro-graphs and Corresponding selected area diffraction pattern (SADP)of as-received materials are shown in Fig. 1.

A356 chips were irregular in shape with a size distribution of200–300 �m. Microscaled (�-Al2O3) and nanoscaled alumina (n-Al2O3) powder particles had an angular and nearly spherical shapewith a size distribution of 50–100 �m and 20–40 nm, respectively.Several ring patterns and the absence of preferred orientation inthe SADP confirmed that the used alumina particles are nanosized.See Fig. 1(c).

2.2. Samples preparation

The A356 chips and Al2O3 powder particles were mixed toachieve A356–5 vol.% Al2O3 composition. Mechanical milling was

carried out in a high energy planetary ball mill (Retsch PM100),nominally at room temperature and under Ar atmosphere upto 12 h. The milling media consisted of twenty 20 mm diameterballs confined in a 500 ml volume vial. The ball and vial materials

alumina, (c) TEM image of nanosized alumina.

1616 Y. Mazaheri et al. / Journal of Materials Processing Technology 211 (2011) 1614– 1619

m) an

wt3w

2(gM

tedbdi

2

ifirtm

Fig. 2. (a) Dimensions (in m

ere hardened high carbon and chromium steels, respectively. Ballo powder weight ratio was 6:1 and rotation speed of vial was00 rpm. The total powder mass was 200 g and 0.3 wt.% stearic acidas added as a process control agent (PCA).

The as-milled powders were sieved in order to separate5–63 �m fraction which is suitable for high velocity oxy-fuelHVOF) spraying. The composite powders were deposited onto therit blasted A356-T6 substrates by HVOF spraying (Metallizationet Jet III). The spraying parameters are presented in Table 2.Then plates with preplaced composite coatings were subjected

o FSP. The FSP tool was made of H13 steel (Fig. 2). In all FSPxperiments tool rotation rate, traverse speed and tilt of the spin-le towards trailing direction were kept constant in this study toe 1600 rpm, 200 mm min−1 and 2◦, respectively. The concept ofevelopment of A356/Al2O3 surface composites design in this study

s schematically shown in Fig. 3.

.3. Analysis techniques

Cross-sectioning of the friction stir processed (FSPed) samplesn planes perpendicular to the processing direction was performedor metallographic analysis. The samples were prepared accord-

ng to standard metallographic practice and etched with Keller’seagent (1 ml 48% HF, 1.5 ml HCl and 10 ml nitric acid in 87.5 ml dis-illed water for 30 s). Transverse sections were examined by optical

icroscopy (OM) and SEM in a Philips XL30.

Fig. 3. The concept of development design

d (b) picture of the FSP tool.

The phase composition of the as-received and as-milled pow-ders were analysed by X-ray diffraction (XRD, Philips X’Pert-MPD)using Cu K� radiation (� = 0.15406 nm) generated at 40 kV and30 mA. The XRD patterns were recorded in the 2� range of 20–100◦

(step size of 0.05◦ and time per step of 1 s).The hardness profile along the cross-section of FSPed samples

was also determined by microhardness test using a Vickers inden-ter at the load of 100 g and dwell time of 5 s. The average of threemeasurements for each point was calculated and reported as micro-hardness value.

The hardness and elastic modulus of as-received A356, FSPedsample (no Al2O3) and surface composites were evaluated fromthe load-penetration depth curves obtained in nanoindentationtests using a nanoindentation tester (NHTX S/N: 01-03119, CSMInstruments) with a Berkovich diamond indenter (B-J87). Thismethod is capable of measuring elastic moduli of the surfacecomposites because of the relatively small volume being tested.Thus, any adverse effects of porosity, commonly obtained in bulkproperty measurements, can be avoided. Indeed, the propertiesmeasured from nanoindentation are the true properties of thesurface composite layers. The indentations were made to a max-imum load of about 70 mN and under loading and unloading rateof 140 mN/min. In order to take the repeatability into account,

the test results were acquired from the average of four inden-tations. The hardness and elastic modulus were calculated bya standard procedure according to the method of Oliver andPharr (1992).

of A356/Al2O3 surface composites.

Y. Mazaheri et al. / Journal of Materials Process

Fc

3

3

mmacs

to the aluminum alloy substrates, and no defects were visible.

FA

ig. 4. The XRD patterns of as-received �-Al2O3, A356 and as-milled A356–Al2O3

omposite powders for 12 h.

. Results and discussions

.1. Structural evolution

A356–Al2O3 composite powder was prepared by 12 h ballilling of A356 machining chips and nanosized as well asicrosized Al2O3 powder. The XRD patterns of powder particles

s-received and after 12 h of milling time are shown in Fig. 4. Theomposites consist only of �-Al2O3 and A356 peaks. Because of thetability of alumina no other reaction product in the compositions

ig. 5. Cross-sectional microstructure of: (a) A356–�Al2O3 composite coating, (b) A3356/Al2O3 surface nanocomposite.

ing Technology 211 (2011) 1614– 1619 1617

was found. So that after 12 h milling, Al2O3 peaks was difficult to bedetected due to the decrease in intensity and/or overlapping withhigher intensity peaks related to A356. All the diffraction peaksfrom the A356 matrix are clearly seen; however, all the expecteddiffraction peaks of Al2O3 are not clearly observed. In comparisonto the diffraction peaks from A356, these peaks have a low inten-sity in the diffraction pattern, which is attributed to the low volumefraction of the Al2O3 phase and the fine size of the powder in theA356–Al2O3 powder mixture.

The as-milled powders were sieved in order to separate25–63 �m fraction and used for spraying. Composite coatings weredeposited by HVOF process on A356-T6 substrates. After 3 passes, acoating thickness of about 200 �m was achieved. The coated spec-imens were then subjected to FSP. The friction stirred zones weretypically about the size of the rotating pin, namely width and depthof 4 and 5 mm, respectively. Nanosized Al2O3 particles were foundto be distributed within this region due to the occurrence of vig-orous stirring during the process. Fig. 5 shows the cross-sectionalimages of the coatings and surface composites. As can be observedin Fig. 5(a) and (c), micrographs of the coatings revealed goodhomogeneity and uniformity and high-quality contact with sub-strates. Fig. 5(b) and (d) shows optical micrographs of interface zonebetween surface composite layers and A356-T6 substrates afterFSP. The surface composite layers appeared to be very well bonded

Fig. 6 shows SEM images obtained from the surface micro andnanocomposite fabricated by the FSP. The dark particles in Fig. 6(a)and (b) are the broken Si particles dispersed in the Al matrix and

56/Al2O3 surface microcomposite, (c) A356–nAl2O3 composite coating and (d)

1618 Y. Mazaheri et al. / Journal of Materials Processing Technology 211 (2011) 1614– 1619

Fig. 6. SEM micrograph of: (a) A356–�Al2O3 and (b) A356–nAl2O3 surface composite.

Fa

tp

3

s1aAomzt(Ksr

TM

ig. 7. The hardness profiles along the cross section of the FSPed A356-T6 (no Al2O3)nd surface composite layers.

he white particles are Al2O3 particles identified using energy dis-ersive spectroscopy (EDS).

.2. Mechanical characterization

Fig. 7 shows the hardness profile along the cross-ection of samples under the same FSP condition (� = 2◦,600 rpm—200 mm min−1). The hardness profile was measuredlong the centerline of the cross-section of the processed zone.ccording to the previous microstructural results, the effect of FSPn the distribution of hardness on the any sample is justifiable. Theicrostructure of FSPed samples are composed of four primary

ones: the base metal (BM), the heat affected zone (HAZ), thehermo-mechanically affected zone (TMAZ) and the stir zone

SZ). The information presented in the publications (Chen andovacevic, 2003; Ma et al., 2003) indicate that, FSP generatesufficiently high temperatures, at least in some locations, to affecte-solutionizing of the hardening phases in heat treatable Al

able 3echanical characterizations of as-received A356, FSPed sample (no Al2O3) and surface c

Parameter Value

As-received A356 FSPed sample (no Al2O3

HIT 783 750

HV (nanoindentation) 74.7 ± 0.9 68.9 ± 1.2

HV (microhardness) 79.6 ± 1.1 66.8 ± 0.9

EIT 74 73

Fig. 8. Load versus penetration depth curves of as-received A356, FSPed sample (noAl2O3) and surface composite layers.

alloys reducing the hardness value. On the other hand FSP causesintense plastic deformation resulting in significant microstructuralrefinement of the processed zone increasing the hardness value.For the FSPed A356-T6 with no alumina powder, microhardnessprofile shows a general softening and reduction of hardness in thestirred zone in contrast to that of the as-received A356-T6 in spiteof smaller grain size. The average hardness values of the BM andSZ were about 80 and 67 Hv, respectively. Others have also foundthat FSP had a softening effect on A356. As reported in literaturestudies (Ma et al., 2007, 2008), it seems that the softening of thestirred zone was result of dissolution of strengthening precipitatesduring FSP. In addition, the heat generated by FSP can increase

the temperature of TMAZ and HAZ leading to the full/partialdissolution of the hardening precipitates in heat treatable alloys.Therefore, some softening could also take place in these zones.As can be seen the decrease in hardness for TMAZ and HAZ was

omposite layers using nanoindentation technique.

Dimension

) Microcomposite Nanocomposite

978 1196 MPa89.6 ± 1.4 109.8 ± 1.1 Vickers89.8 ± 2.6 109.7 ± 2.5 Vickers81 86 GPa

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reater than SZ. The different extent of softening in HAZ and SZepends on the temperature in each zone, which affects precipitateoarsening, resolution and re-precipitation. But it seems that theess softening in the SZ is due to the fact that SZ additionallyncludes grain refinement effect.

The average microhardness values for A356–�Al2O3 and356–nAl2O3 surface composites were about 90 and 110 Hv,espectively, which are higher than that of the as-received andSPed A356-T6. The HV of the surface layer of aluminum substratesas significantly enhanced with the incorporation of Al2O3 parti-

les, and increased as the Al2O3 particle size was decreased. Al2O3articles increased the resistance of aluminum matrix to indenta-ion. The increase in the hardness of the surface composite layer cane explained by the hardening effect of the dispersoids (pinning ofislocations and also of grain boundaries). The mechanical perfor-ances of nanoparticle reinforced AMCs are far superior to those

f microparticle strengthened composites with a similar volumeontent of particulate (Tjong, 2007).

Fig. 8 shows the load–penetration depth curves for differentamples obtained from nanoindentation test. The difference inardness of the materials is apparent from the difference in theeak depth. It is clear from the graphs that the hardness of theomposites is higher than that of the non-reinforced samples. Theechanical properties of samples obtained from the analysis of

oad/unload curves, are summarized in Table 3. As mentioned ear-ier, decreasing microhardness of FSPed sample in comparison withs-received A356 can be related to the precipitation dissolution. The356/Al2O3 surface composites showed better microhardness andlastic modulus values in comparison with as-received A356 andSPed sample (no Al2O3) due to presence of hard alumina particles.

The possible strengthening mechanisms which may operate inarticle-reinforced MMCs are (Lloyd, 1994):

1) Orowan strengthening.2) Grain and substructure strengthening.3) Quench hardening resulting from the dislocations generated to

accommodate the differential thermal contraction between thereinforcing particles and the matrix.

4) Work hardening, due to the strain misfit between the elasticreinforcing particles and the plastic matrix.

According to the characteristics of the microstructure, the bet-er mechanical properties of A356–Al2O3 nanocomposite can bettributed to (1) the nano grain size of the Al matrix following thelassical Hall-Petch relationship, and (2) the Orowan strengtheningue to the fine dispersion of Al2O3 particles. This may be explainedy the rule of mixtures, applied to composite materials (Dieter,976). The hardness values obtained by nanoindentation techniquere in good agreement with values measured using Vickers hard-ess tester (Table 3).

. Conclusions

In the present investigation, the FSP technique has been suc-essfully used for producing the A356/Al2O3 surface composites.

ing Technology 211 (2011) 1614– 1619 1619

From the experiments and analyses performed, some conclusionscan be drawn:

(1) The microstructural study of A356/Al2O3 surface compositelayers fabricated by FSP indicated that Al2O3 particles werewell distributed in the Al matrix, and good bonding with theAl matrix was generated.

(2) The FSP with Al2O3 particles obviously increased the micro-hardness of the substrates. The microhardness values forA356–�Al2O3 and A356–nAl2O3 surface composite were about90 and 110 Hv, respectively, while that of the sample treated bythe FSP without Al2O3 particles and the as-received A356 wereabout 67 and 80 Hv, respectively.

(3) The results obtained from nanoindentation techniqueshowed better microhardness and elastic modulus valuesfor A356/Al2O3 surface composites in comparison withas-received A356 and FSPed sample (no Al2O3).

(4) The better mechanical properties of A356/Al2O3 surfacenanocomposite can be attributed to the presence of nanosizedAl2O3 particles, which contribute significantly to the strengththrough the Orowan mechanism.

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