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MicrostructuresandmechanicalpropertiesofAl/Al2O3surfacenano-compositelayerproducedbyfrictionstirprocessing

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Materials Science and Engineering A 500 (2009) 84–91

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Microstructures and mechanical properties of Al/Al2O3 surfacenano-composite layer produced by friction stir processing

A. Shafiei-Zarghani ∗, S.F. Kashani-Bozorg, A. Zarei-HanzakiSchool of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e i n f o

Article history:Received 11 June 2008Received in revised form 28 August 2008Accepted 24 September 2008

Keywords:Friction stir processingSurface nano-compositeAluminumAl2O3

HardnessWear

a b s t r a c t

In this study, a new processing technique, friction stir processing (FSP) was attempted to incorporatenano-sized Al2O3 into 6082 aluminum alloy to form particulate composite surface layer. Samples weresubjected to various numbers of FSP passes from one to four, with and without Al2O3 powder. Microstruc-tural observations were carried out by employing optical and scanning electron microscopy (SEM) ofthe cross sections both parallel and perpendicular to the tool traverse direction. Mechanical propertiesinclude microhardness and wear resistance, were evaluated in detail. The results show that the increas-ing in number of FSP passes causes a more uniform in distribution of nano-sized alumina particles. Themicrohardness of the surface improves by three times as compared to that of the as-received Al alloy. Asignificant improvement in wear resistance in the nano-composite surfaced Al is observed as comparedto the as-received Al.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Aluminum and its alloys are used extensively in aerospace andautomotive industries because of its low density and high strengthto weight ratio [1]. However, a poor resistance to wear and erosionis of serious concern for prolonged use [2]. Metal matrix compos-ites are a new class of materials that exhibit good wear and erosionresistance properties, higher stiffness and hardness at a lower den-sity as compared to the matrix [3]. However, the presence of theceramic particles in the metallic matrix makes the matrix brittle[4]. In this regard, it may however be noted that wear is a surfacedependent degradation mode, which may be improved by a suit-able modification of surface microstructure and/or composition [5].Hence, instead of bulk reinforcement, if the ceramic particles wouldbe added to the surface, it could improve the wear and erosionresistance without sacrificing the bulk properties [5]. Dispersionof ceramic particles on metallic substrate surface and the controlof its distribution are difficult to achieve by conventional surfacetreatments [6].

For most isotropic composites, the micro-sized whiskers orparticulates with a reinforcement volume fraction of 15–35% arecommonly added into metallic alloys. In terms of grain size refine-

∗ Corresponding author. Fax: +98 21 8800 6076.E-mail addresses: azizshafiei@yahoo.com (A. Shafiei-Zarghani),

fkashani@ut.ac.ir (S.F. Kashani-Bozorg), zareih@ut.ac.ir (A. Zarei-Hanzaki).

ment and particle strengthening, one of the critical microstructureparameters is the particle interspacing L, which can be roughly esti-mated from L = (d/2)(2�/3Vf)1/2 [7] where d is the average particlediameter and Vf is the particle volume fraction. With reinforcingparticles of Vf = 20% and d = 20 �m in typical aluminum based com-posites, L will be around 32 �m. According to the above equation,as the reinforcement size d decreases to the 10–50 nm range, thecomposites only need a small volume percentage to result in theinterspacing being in the 100–1000 nm range.

Dispersion of the nano-reinforcements in a uniform manner isa critical and difficult task. It should be pointed out that the exist-ing processing techniques for forming surface composites are basedon liquid phase processing at high temperatures. In this case, it ishard to avoid the interfacial reaction between reinforcement andmetal matrix and the formation of some detrimental phases. Fur-thermore, critical control of processing parameters is necessary toobtain ideal solidified microstructure in surface layer. Obviously,if processing of surface composite is carried out at temperaturesbelow melting point of substrate, the problems mentioned abovecan be avoided.

Recently, much attention has been paid to a new surface mod-ification technique named friction stir processing (FSP) [8–11].FSP is a solid state processing technique to obtain a fine-grainedmicrostructure. This is carried out using the same approach as fric-tion stir welding (FSW), in which a non-consumable rotating toolwith a specially designed pin and shoulder is plunged into the inter-face between two plates to be joined and traversed along the line of

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Table 1Chemical composition of 6082 Al alloy (in wt.%).

Si Mg Mn Fe Cr Cu Zn Al

1.2 0.78 0.5 0.33 0.14 0.08 0.05 Bal.

the joint. Localized heating is produced by the friction between therotating tool and the workpiece to raise the local temperature of thematerial to the range where it can be plastically deformed easily. Asthe rotating tool traverses along the joint line, metal is essentiallyextruded around the tool before being forged by the large downpressure [12–14]. It is well known that the stirred zone consists offine and equiaxed grains produced due to dynamic recrystallization[14]. Though FSP has been basically advanced as a grain refinementtechnique, it is a very attractive process for also fabricating com-posites. Mishra et al. [15] fabricated the Al/SiC surface compositesby FSP, and indicated that SiC particles were well distributed in theAl matrix, and good bonding with the Al matrix was generated.

In the present study, an attempt has been made to enhance thewear resistance of 6082 Al alloy by dispersion of nano-sized Al2O3particulates on the surface using FSP technique. Followed by FSP,a characterization of the stirred zone in terms of distribution ofreinforcement particles has been undertaken to optimize the FSPparameters. Finally, the mechanical properties (mainly microhard-ness and wear resistance) of the composite surfaced layer have beenevaluated in details and correlated with the number of FSP passes.

2. Experimental procedures

Commercial 6082 Al extruded bar with a thickness of 7 mm andnano-sized Al2O3 powder with an average diameter of ∼50 nmwere used as substrate and reinforcement particulates, respec-tively. The alumina powder with a purity of 99.8% was suppliedby Buehler and 6082 Al alloy received from Alcoa Europe.

The nominal chemical composition of the alloy is shown inTable 1. The simplified FSP unit was a modified form of a con-ventional miller machine. The hardened H-13 tool steel pin was5 mm in diameter and its length was about 4 mm. The shoulderdiameter was 16 mm and the tilt angle of fixed pin tool was 3◦. Thepin rotation was set to be 1000 rpm, and its advancing speed was135 mm/min. To insert nano-size Al2O3 powder, a groove with adepth and width of 4 and 1 mm was machined out of workpieces,in which the desired amount of Al2O3 powder was filled in. In orderto prevent sputtering of Al2O3 powder and its ejection from grooveduring the process, groove’s gap initially was closed by means ofa tool that only had shoulder and no pin. Samples were subjectedto various numbers of passes from one to four, with and withoutAl2O3 powder. After each pass an ambient air cooling was applied.Next pass was started when the workpiece reached room temper-ature.

Microstructural observations were carried out by employingoptical and scanning electron microscopy (SEM) of the cross sec-tions both parallel and perpendicular to the tool traverse direction.Mechanical properties of the surface composite layers were eval-uated by microhardness profiles across the stirred zone, using aVickers hardness tester utilizing a 200 g load for 15 s. The aver-age microhardness measurements were made along a line whichis 2 mm under the surface, transferring the entire stirred zone, intothe base material.

The wear tests were conducted with a pin-on-disk tribometer asshown schematically in Fig. 1. Most requirements of the ASTM stan-dard G99-04 were followed. Nevertheless, several modificationswere introduced, mainly regarding the pin shape. Prismatic pinswere made of the materials under study (6082 Al and its surfacenano-composite layer produced by four FSP passes) with rectan-

Fig. 1. Scheme of the testing configuration of the pin-on-disc machine.

gular section of 11 mm × 5 mm. With this geometry, the nominalcontact area was maintained constant during the tests in spite ofthe wear process. The counterpart discs were made of GCr15 steel,hardness of about 60 HRC and surface roughness of Ra = 0.2 �m. Thediameter of the sliding track on the disk surface was 22 mm. Beforethe wear test, each pin specimens was ground down to 1000 gritabrasive paper. The wear tests were performed under dry slidingconditions for 2000 s with a constant load (60 N) and sliding veloc-ity (0.5 m/s). All wear test specimens were cleaned in acetone andweighed to an accuracy of ±1 mg prior to testing and at differentsliding distances during the test. The coefficient of friction betweenthe pin specimen and the disk was determined by measuring thefrictional force with a stress sensor.

3. Results and discussion

Fig. 2 shows the surface appearances of surface nano-compositelayer produced by four FSP passes. Defects, such as voids and cracks,are not observed on the surface. The top surface shows very smoothquality and there are almost no prominences or depressions, inspite of the tool’s stirring.

3.1. Microstructures

The friction stirred zones were typically about the size of therotating pin, namely width and depth of 5 and 4 mm, respectively.Nano-sized Al2O3 particles were found to be distributed within thisregion due to the occurrence of vigorous stirring during the process.Fig. 3 shows optical and SEM micrographs of interface zone between

Fig. 2. Upper surface of FSP zone of Al/Al2O3 surface nano-composite layer producedby four FSP passes.

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Fig. 3. (a) Optical and (b) SEM micrographs showing perfect bonding between sur-face composite and aluminum alloy substrate.

surface nano-composite layer and aluminum alloy substrate afterfour FSP passes. The surface composite layer appears to be verywell bonded to the aluminum alloy substrate, and no defects werevisible.

Fig. 4 shows SEM images obtained from the surface nano-composites fabricated by the FSP, respectively. The dark regions inFig. 4(a) are agglomerated Al2O3 particles which have been ver-ified using energy dispersive spectroscopy (EDS), and the whiteparticles are strengthening precipitates of 6082 Al which are dis-persed in the Al matrix. The dispersion of the Al2O3 particles in the6082 Al matrix was related to the number of FSP passes. Agglom-erated Al2O3 particles could be observed in the surface compositelayer produced by one FSP pass. The observed clustering particle

size is frequently 0.1–2 �m (Table 2), much larger than the indi-vidual Al2O3 size (∼50 nm). The situation after two to four passesappears to have further reduced the clustering Al2O3 size, as shownin Fig. 4 for the surface composite layers produced by three and fourFSP passes. In comparison to the surface composite layer producedby one FSP pass, the surface composite layer produced by threeFSP passes showed a better dispersion of Al2O3 particles. Therewere just a few regions which included the aggregated nano-sizedAl2O3 particles. On the other hand, a good dispersion of nano-sizedAl2O3 particles, which were separated from each other, could beobserved for the surface composite layer produced by four FSPpasses.

The grain size of the 6082 Al matrix was obviously refined by theFSP as shown in Fig. 5. As reported by other researchers [16,17], itseems that the grain refinement was caused due to dynamic recrys-tallization during the FSP. However, the FSP with the nano-sizedAl2O3 particles more effectively reduced the grain size of the 6082Al matrix in which some grains were less than 300 nm as shownin Fig. 5(c) and (d). It is considered that the pinning effect by thenano-sized Al2O3 particles retarded the grain growth of the 6082Al matrix. The fine grain seems to be very useful for enhancing themechanical properties.

Quantitative results of the average Al2O3 cluster size and theaverage 6082 Al matrix grain size for surface composite layers pro-duced by various number of FSP passes are listed in Table 2. Theresulting grain size is significantly refined from the initial grainsize of 120 �m for the 6082 Al extruded bar. Assuming a completelyuniform distribution of clustering nano-sized Al2O3 particles, theZener limiting grain size (dz) can be calculated by the equationdz = 4r/3Vf, where r and Vf are the radius and volume fraction ofclustering Al2O3 particles, respectively [18]. The calculated Zenerlimiting grain sizes are listed in Table 2 (in this study the vol-ume fraction of Al2O3 particles is equal to 0.15). The measured Almatrix grain size can be related to the Zener limiting grain size.This suggests that the presence of finely dispersed Al2O3 parti-cles can limit the grain growth and result in an ultrafine grainsize in the Al matrix. If all of the nano-sized Al2O3 particles areseparated, and completely and uniformly dispersed, the theoreti-cally estimated grain size should vary within 0.22 �m. It followsthat a certain level of local clustering is inevitable, and not allnano-particles can restrict grain boundary migration. Note that thegrain size of the 6082 Al alloy (without nano-sized Al2O3 parti-cles) after the one, two, three, and four FSP passes was measuredto be around 6, 3.7, 2.3, and 2 �m, respectively. The microstruc-tures in the friction stir processed specimens with nano-sizedAl2O3 fillers can be refined to a much smaller scale than thefriction stir processed parent alloy without nano-sized aluminaparticles.

3.2. Hardness measurement

The typical microhardness readings, Hv, in the central cross-sectional zones of the friction stir processed specimens are depictedin Fig. 6. Almost a three times increment of the hardness of theparent Al alloy was achieved, especially for the surface compositelayer produced by four FSP passes, as compared in Table 2. AfterFSP, the scattering of Hv within the FSP stirred zone is consideredto be minor, implying that the pin stirring efficiently dispersedthe nano-sized Al2O3 powders in a reasonably uniform manner,especially after more than tow passes. The possible strengthen-ing mechanisms which may operate in particle-reinforced metalmatrix composites are [19]

(1) Orowan strengthening.(2) Grain and substructure strengthening.

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Fig. 4. SEM images of the Al/Al2O3 surface composite layers produced by (a) one, (b) three, and (c) four FSP passes.

Table 2Summary of the average Al2O3 cluster size, the average Al matrix grain size and microhardness for surface composite layers produced by various number of FSP passes.

Number of FSP passes Average Al2O3 cluster size (nm) Al grain size (�m) Zener limiting grain size (�m) Microhardness (Hv)

1 550 4.84 2.44 1652 290 2.53 1.29 1963 145 0.89 0.64 2534 90 0.66 0.40 295

The individual Al2O3 size is 50 nm and the initial 6082 Al extruded bar grain size is 120 �m.

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Fig. 5. Optical micrographs showing grain size of (a) as-received 6082 Al and (b) Al 6082 after four FSP passes. SEM images (c) and (d) showing the microstructure of theAl/Al2O3 surface composite layer produced by four FSP passes, (d) is enlargement inside a circle for (c).

(3) Quench hardening resulting from the dislocations generated toaccommodate 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 majorcontributions to the hardness of the surface composite layers fab-ricated by FSP are (1) the fine grain size of the Al matrix, and (2)the Orowan strengthening due to the fine dispersion of nano-sizedAl2O3 particles.

For the 6082 Al alloy with no alumina powder, after four FSPpasses, microhardness profile shows a general softening and reduc-tion of hardness in the stirred zone in contrast to that of theas-received Al. The hardness of as-received Al has an average of110 Hv. However, the hardness of stirred zone shows variable valuefrom the 56 to 73 Hv with an average of 67 Hv. The stirred zone hasa lower hardness than that of the as-received Al in spite of smallergrain size. The result means that the hardness distribution of stirredzone was independent with grain size considering Hall–Petch rela-tion. Therefore, other factors may govern the mechanical propertiesof stirred zone such as precipitations and dislocation density. Asreported in literature studies [14], it seems that the softening of the

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Fig. 6. Typical variation of the microhardness Hv distributions of the friction stirprocessed 6082 Al alloy (no Al2O3) and surface composite layers.

stirred zone was result of coarsening (and dissolution) of strength-ening precipitates during the FSP of heat-treatable (precipitationhardenable) Al alloys.

3.3. Wear

Fig. 7 compares the kinetics of wear in terms of the weight loss(as a function of sliding distance) using the specimen as the pin andGCr15 steel as the disk material. From Fig. 7 it is seen that the wearweight loss increased with sliding distance. For the as-received Althe wear rate (weight loss/sliding distance) was low during theinitial period of wear after which it increased, but for the surfacenano-composite layer produced by four FSP passes the wear rate

Fig. 7. Change in the reduction in pin weight with sliding distance for as-receivedAl and surface nano-composite layer produced by four FSP passes.

Fig. 8. Change in the rate of wear with sliding distance for as-received Al and surfacenano-composite layer produced by four FSP passes.

was roughly constant during sliding time (Fig. 8). Close compari-son of the graphs in Figs. 7 and 8 also reveals that both the extent ofwear and the rate of wear are significantly lower in the surfacenano-composite layer as compared to the as-received substrate.The improved wear resistance due to FSP composite surfacing isattributed to improved hardness achieved by dispersion of ceramicparticles and grain refinement of Al matrix. A detailed study of thescanning electron micrograph of the worn surfaces and the varia-tion of the coefficient of friction for surface nano-composite layerproduced by four FSP passes and as-received 6082 Al alloy wereundertaken to understand the mechanism of wear.

Fig. 9 shows the variation of the coefficient of friction with slid-ing distance for surface nano-composite layer and as-received Al.From Fig. 9 it is relevant that the coefficient of friction of surfacecomposite layer is fairly low (with an average of 0.35) and does notchange with sliding distance. The low coefficient of friction implies

Fig. 9. Change in the coefficient of friction with sliding distance for as-received Aland surface nano-composite layer produced by four FSP passes.

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Fig. 10. Scanning electron micrograph of the worn out track of (a) as-received Al, and (c) surface nano-composite layer produced by four FSP passes against steel disk. (b)and (d) are high magnified view of (a) and (c), respectively.

that the mechanism of wear is predominantly abrasive in naturedue to the harder (steel disk) surface scratching over the softer (pin)surface. On the other hand, the coefficient of friction was found toincrease to a very high value (initially at a very high rate followedby a gradual steady state) during the wear of as-received Al. Theincreased coefficient of friction is attributed to a localized weld-ing of the worn debris to the Al surface. From this behavior, it maybe concluded that wear mechanism in as-received Al is abrasiveas the predominant wear initiation mechanism, which converts toadhesive at a much latter stage.

A detailed study of the microstructures of the worn debris wasundertaken to identify the genesis of wear. Fig. 10(a) and (b) showsthe scanning electron micrographs of the worn tracks of as-receivedAl against steel disk. Fig. 10(a) shows evidence of the presence ofdeep scratches in the worn surface. Hence, wear progresses byan abrasive mechanism. On the other hand, patches of materialremoval are seen at the worn surfaces (which is evident from thepresence of groove in Fig. 10(b)). The material removal in patchessignifies local welding at the interface and subsequent ruptureof the welded joint with the formation of grooves. The increased

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material removal at the latter stage can also be explained by thehigher magnitude of the coefficient of friction. Hence it may beconcluded that the mode of wear is a combination of adhesive andabrasive mechanisms.

Fig. 10(c) and (d) shows the worn tracks observed in the surfacenano-composite layer produced by four FSP passes against a steeldisk. A comparison between Fig. 10(a) and (c) reveals that the weartrack width decreased in surface composite layer as compared toas-received Al. It is evident that the extent of wear in the surfacecomposite layer is lower than that of the as-received Al. The highermagnified view of the wear track shows the presence of small pitspredominantly because of local removal of ceramic particles duringwear (Fig. 10(d)). Though fine micro-scratch marks were observedat the worn track at low magnification no deep scratch marks wereobserved at high magnification. Hence, it may be concluded thatthe extent of wear is significantly lower in surface composite layeras compared to as-received Al. Furthermore, the appearance of asmooth black surface is because of the presence of accumulativewear debris (which is mainly softer Al in this case, due its selec-tive removal from the surface and its presence at the interface).The presence of worn out particles convert the wear mode fromtwo body to three body wear and reduces the rate of wear fur-ther by acting as a lubricant, which is also evident from the lowercoefficient of friction in surface nano-composite layer (Fig. 9).

From the detailed observation of the coefficient of friction andthe microstructure of the worn surfaces, it may be concluded thatthe improved wear resistance of surface nano-composite layer pro-duced by four FSP passes as compared to that of as-received Al isattributed to increased hardness (due to presence of fined grainsof Al matrix and fine dispersion of nano-sized Al2O3 particles). Themode of wear in the as-received Al and surface nano-compositelayer is both adhesive and abrasive. The rate of wear increasedwith increasing sliding distance in as-received Al mainly becauseof increased coefficient of friction. On the other hand, in surfacenano-composite layer due to a comparatively lower coefficient offriction, wear due to decrease in adhesive mode. Furthermore, theabrasive mode of wear is lower in the surface nano-composite layerbecause of a higher hardness.

4. Conclusions

In the present investigation, the Al/Al2O3 surface nano-composites were successfully fabricated by the FSP. The microstruc-ture, microhardness and tribological behavior were evaluated by aview of observing the matrix grain size and the dispersion of thenano-sized Al2O3 particles. The obtained results can be summa-rized as follows:

1. Increasing in number of FSP passes cause a more uniform dis-persion of nano-sized Al2O3 particles. A good dispersion ofAl2O3 particles, which are separated from each other, can beobtained for the surface nano-composite layer produced by fourFSP passes.

2. The FSP with the nano-sized Al2O3 particles more effectivelyreduce the grain size of the 6082 Al matrix in which some grainsare less than 300 nm for the surface nano-composite layer pro-duced by four FSP passes. It is considered that the pinning effectby the nano-sized Al2O3 particles retarded the grain growth ofthe dynamically recrystallized grains of Al matrix.

3. The microhardness of the Al/Al2O3 surface nano-compositesincreases significantly with increasing number of FSP passes. Themaximum microhardness for the surface composites is 295 Hv,while that of the samples treated by the FSP without Al2O3 par-ticles and the as-received Al are 67 and 110 Hv, respectively.

4. The high microhardness of Al/Al2O3 surface nano-compositecan be attributed to the presence of nano-sized Al2O3 parti-cles, which contribute significantly to the strength through theOrowan mechanism, as well as the ultrafine grain size of the Almatrix.

5. Wear resistance against a steel disk is significantly improved(two to three times) in Al/Al2O3 surface nano-composite layerproduced by four FSP passes compared to as-received Al. Themechanism of wear is a combination of abrasive and adhe-sive. Improved wear resistance of surface composite layer isattributed to a lower coefficient of friction and an improvedhardness.

Acknowledgement

Special thanks must go to the Iranian Nanotechnology Initia-tive for partial financial support. Also, the provision of researchfacilities in the Tribology Lab. by Prof. H.R. Ghasemi is gratefullyacknowledged.

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