Journal of Materials Processing Technology 161 (2005) 381–387

Production and mechanical properties of Al2O3 particle-reinforced2024 aluminium alloy composites

M. Kok∗

Department of Mechanical Program, Vocational College Education, Kahramanmaras Sutcu Imam University,46100 Kahramanmaras, Turkey

Received 25 August 2003; received in revised form 16 July 2004; accepted 20 July 2004

Abstract

2024 aluminium alloy metal matrix composites (MMCs) reinforced with three different sizes and weight fractions of Al2O3 particles up to30 wt.% were fabricated by a vortex method and subsequent applied pressure. The effects of Al2O3 particle content and size of particle on themechanical properties of the composites such as hardness and tensile strength were investigated. The density measurements showed that thesamples contained little porosity, and the amount of porosity in the composites increased with increasing weight fraction and decreasing sizeof particles. Scanning electron microscopic observations of the microstructures revealed that the dispersion of the coarser sizes of particlesw the tensiles©

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as more uniform while finer particles led to agglomeration of the particles and porosity. The results show that the hardness andtrength of the composites increased with decreasing size and increasing weight fraction of particles.2004 Elsevier B.V. All rights reserved.

eywords:Metal matrix composites; Al2O3 particle; Particle-reinforced composite; Aluminium alloy; Vortex

. Introduction

Metal matrix composites (MMCs) represent a new gen-ration of engineering materials in which a strong ceramiceinforcement is incorporated into a metal matrix to improvets properties including specific strength, specific stiffness,ear resistance, excellent corrosion resistance and high elas-

ic modulus[1,2]. MMCs combine metallic properties of ma-rix alloys (ductility and toughness) with ceramic propertiesf reinforcements (high strength and high modulus), lead-

ng to greater strength in shear and compression and higherervice-temperature capabilities[3,4]. Thus, they have sig-ificant scientific, technological and commercial importance.uring the last decade, because of their improved properties,MCs are being used extensively for high performance ap-lications such as in aircraft engines and more recently in theutomotive industry[2,4]. Al2O3 and SiC fibres and parti-les are the most commonly used reinforcements in MMCs

∗ Tel.: + 90 344 2512315/325; fax: +90 344 2512312.E-mail address:metinkok@ksu.edu.tr.

and the addition of these reinforcements to aluminium ahas been the subject of a considerable amount of reswork [1,5]. The application of Al2O3 or SiC reinforced aluminium alloy matrix composites in the automotive andcraft industries is gradually increasing for pistons, cylinheads, etc., where the tribological properties of the mrial are very important[6–12]. Therefore, the developmeof aluminium matrix composites is receiving consideraemphasis in meeting the requirements of various indusIncorporation of hard second phase particles in the alloytrix to produce MMCs has also been reported to be mbeneficial and economical[13–15].

Ceramic particle-reinforced MMCs have always psented machining problems. Therefore, much researchvoted to eliminating this step by perfecting near-net-shproduction techniques. Due to the increased use and imtance of MMCs, their fabrication techniques have beenjected to continuous development during the last few yA variety of methods for producing MMCs have recenbecome available. The particle-reinforced aluminium acomposites which are among the most widely used c

924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2004.07.068

382 M. Kok / Journal of Materials Processing Technology 161 (2005) 381–387

posites materials, can be produced by various fabricationprocesses including melt processing (liquid-phase process-ing) and powder metallurgy (solid-phase processing)[3,16].Compared with powder metallurgy, melt processing whichinvolves the stirring of ceramic particles into melts, has someimportant advantages, e.g., better matrix-particle bonding,easier control of matrix structure, simplicity, low cost of pro-cessing and nearer net shape[1,3,16,17]. Moreover, the wideselection of materials for this fabrication method is also anadvantage[17]. However, the melting process has two majorproblems which are firstly, the ceramic particles are generallynot wetted by the liquid metal matrix, and secondly, the parti-cles tend to sink or float depending on their density relative tothe liquid metal and so that the dispersion of the ceramic par-ticles are not uniform, whereas powder metallurgy makes theuniform dispersion of the reinforcements less of a problem[18–21]. In this study, these problems have been overcomeby melt stirring at high mixing speed (900 rev min−1) and bypreheating of the ceramic particles to improve the wettabilitybefore incorporation into the metal matrix alloy. In terms ofboth processing and properties, the main concern that mustbe investigated is that of obtaining uniform dispersion of theceramic particles in MMCs[22].

It is still costly and difficult to manufacture the metal ma-trix composites because of the poor wetting between matrixalloys and some reinforcements[23–26]. However, amongt cedM od)i po-n eses thev them r tos f them me-t ilitya ltenm dingi tem-p rk,tm inetp ef-f andt 4a

2

2

cald ile� 16,

Table 1Chemical composition of Al2O3 particles

�-Alumina (wt.%) ±93Fe2O3 (wt.%) ±0.8TiO2 (wt.%) ±1.8CaO (wt.%) ±1.1Other magnetic materials (wt.%) ±0.2

32 and 66�m, and a density of 3950 kg/m3 were used as thereinforcements. The Al2O3 particles supplied by Treibacher,are short particles with a white colour. The grain sizesof Al2O3 particles were determined using a Malvern lasersize analyser. The chemical analysis of the Al2O3 particlesand the 2024 Al alloy used in this study are presented inTables 1 and 2, respectively. For manufacturing of the MMCs,10, 20 and 30 wt.% Al2O3 particles were used. The Al2O3particle-reinforced 2024 Al alloy metal matrix compositeshave been produced by using a vortex method and a sub-sequently applied pressure[16,27]. The composites wereshaped in the form of cylinder of 40 mm outer diameter andheight of 160 mm, using a 2 kW power resistance-heated fur-nace designed especially for this investigation under protec-tive argon gases.

The melting process is carried out in a graphite cruciblewith upper diameter of 102 mm and lower diameter of 70 mmwhile the mixing process is conducted with a 55 mm diametergraphite mixer having four channels, which combines with asteel mandrel driven by a variable speed AC motor. The steelmandrel was enclosed in a graphite sleeve to prevent contactwith the molten aluminium alloy. The outside of this produc-tion unit was insulated with glass fibres as shown inFig. 1. Inthis unit, argon gas is divided into two channels; one is sentover the crucible in order to prevent molten metal interactingw rce-m Thet etali ostath uplesw theirt

2

ndh -p ru-c to the

TC

CMSMZOA

he various fabrication methods of the particle-reinforMCs, the molten metal stirring method (vortex meth

s very promising for manufacturing near-net-shape coments at a relatively low cost. A vortex method includes thteps which are incorporation of ceramic particles intoortex formed by stirring of the molten metal, stirring ofixture after the completion of the particle feeding in orde

upply uniform dispersion of the particles and casting oolten mixture. In this method, the manufacturing para

ers in homogenous mixing are the crucible size, the abnd the size of the impeller, the temperature of the moetal, the stirring time, the stirring speed, the particle fee

nto the mixture at a continuous and uniform rate, and theerature of the mold[3,27]. The purpose of the present wo

herefore, was to: (a) produce the Al2O3 particle-reinforcedetal matrix composites by a vortex method; (b) exam

he wettability problem during the incorporation of Al2O3articles into the aluminium alloys; (c) investigate the

ect of Al2O3 particle content and size on the porosityhe mechanical properties of Al2O3 particle-reinforced 202luminium alloy composites.

. Experimental

.1. Materials and equipment

In this study, 2024 aluminium alloy with the theoretiensity of 2800 kg/m3 was used as the matrix material wh-Al2O3 (alumina) particles with various particle sizes of

ith the atmosphere while the other is fixed on the reinfoent unit to control the flow rate of the reinforcements.

emperature control of the electric furnace and molten ms carried out by an NR911 type thermostat. This thermas a special control unit and thermocouples. Thermocoere inserted into the melt and the furnace to measure

emperature[16,27].

.2. Procedure

Initially, 2024 Al alloy was charged into the crucible, aeated to about 700◦C, which is above the liquidus temerature of the Al alloy. After the entire alloy in the cible was melted, the pre-heated impeller was attached

able 2hemical composition of 2024 Al alloy matrix

u (wt.%) ±3.23g (wt.%) ±0.81i (wt.%) ±0.74n (wt.%) ±0.54n (wt.%) ±0.13ther (wt.%) ±0.05–0.20luminium Balance

M. Kok / Journal of Materials Processing Technology 161 (2005) 381–387 383

Fig. 1. Schematic diagram of the experimental apparatus for producing the MMCs.

motor shaft, turned on and set to the pre-determined speed.Then, the mixer was lowered into the melt slowly to stir themolten metal, while the Al2O3 particles, which were heated at400◦C for 10 min and air-cooled to room temperature (about25◦C) before incorporation, were added into the uniformlyformed vortex using a funnel-shaped pipe with flowing ar-gon. The clearence of the impeller from the bottom of thecrucible was approximately 10 mm with the melt depth be-ing about 90 mm. Argon gas was also blown into the crucibleduring the operation. After the completion of particle feed-ing, the mixing was continued for a further 5 min. Then, themixer was turned off, and the molten mixture was poured inthe pre-heated mold by tipping the furnace. The mold wastaken to a hydraulic press and subsequently 6 MPa pressurewas applied to the mixture in order to reduce the porosity inthe composites and improve the bonding force between theAl alloy/Al 2O3 particles. The time of applied pressure wasabout 30 s. Finally, the mold was opened after 5 min and thefabricated billets were air-cooled to room temperature. Un-reinforced matrix alloy bars were also produced by the samemethod.

2.3. Density measurement and porosity

The experimental density of the composites was obtainedby the Archimedian method of weighing small pieces cutf ter,w ture

rule according to the weight fraction of the Al2O3 particles.The porosities of the produced composites were evaluatedfrom the difference between the expected and the observeddensity of each sample.

2.4. Hardness and tensile strength tests

The hardnesses of the composites and matrix alloy weremeasured after polishing to a 1�m finish. The Brinell hard-ness values of the samples were measured using a ball largeenough (2.5 mm diameter at a load of 187.5 kg) to obtain anindentation which would be representative of the macrostruc-ture of the material. In order to eliminate possible segregationeffects, the mean of at least five tests was taken for each spec-imen.

Tensile tests were used to assess the mechanical behaviourof the composites and matrix alloy. The composite and ma-trix alloy rods were machined to tensile specimens with adiameter of 6 mm and gauge length of 30 mm. The surfaceof the samples was polished on 600 grit sand paper. The ten-sile strength was tested on a Hounsfield testing machine at across-head speed of 0.5 mm s−1.

2.5. Microstructures

The composite billets fabricated in the preliminary exper-i f theu ere

rom the composite cylinder first in air and then in wahile the theoretical density was calculated using the mix

ments were first sectioned to enable an estimation oniformity of dispersion. After that, sectioned samples w

384 M. Kok / Journal of Materials Processing Technology 161 (2005) 381–387

polished with 120, 400 and 600 grit sandpapers, respectively.Finally, the polishing was finished on cloth using diamondpaste of 6 and 1�m. Unreinforced 2024 Al matrix alloy wasalso polished in the same way. Microscopic examinationsof the composites and matrix alloy were carried out using ascanning electron microscope (SEM).

3. Results and discussion

3.1. Production of composites

In the present work, 2024 Al alloy MMCs reinforced withvarying size (16, 32 and 66�m), and weight fraction (10,20 and 30 wt.%),�-Al2O3 particles have been successfullyproduced using a vortex method and subsequently appliedpressure. As a result of various trials, in the production stageof investigation, the optimum process parameters were foundto be as follows—pouring temperature: 700◦C, pre-heatedmold temperature: 550◦C, stirring speed: 900 rev min−1, stir-ring time: 5 min after the completion of particle feeding, par-ticle addition rate: 5 g min−1 and applied pressure: 6 MPa.Approximately, the same values of process parameters havebeen found in some previous studies[1,3,28–30]. At higherpouring temperature, Al2O3 particles tended to sink, whereasat lower pouring temperature and stirring speed, and higherp intot t thes rings cru-c wasn d, thec intot plied.T sed.A ot ratea . Asa ya bove.

3

andp tionso .F pos-i res).A l den-s ensi-t ositesca asedw thep us in-v he

Fig. 2. The variation of theoretical and experimental density with Al2O3

particle content and size.

Fig. 3. The variation of porosity with Al2O3 particle content and size.

MMCs, some porosity is normal, because of the long particlefeeding duration and the increase in surface area in contactwith air caused by decreasing the particle size[27,31]. How-ever, in this study, the pressure immediately applied after thecasting, has reduced this porosity in the composites, and im-proved the bonding force between the Al alloy and Al2O3particles and the wettability of the particles.

3.3. Microstructures

SEM micrographs of the 30 wt.% Al2O3 particle-reinforced composites with 66, 32 and 16�m particle size,fabricated under the optimum production conditions, areshown inFig. 4. The most important factor in the fabrica-tion of MMCs is the uniform dispersion of the reinforce-ments. As shown inFig. 4, uniform dispersion of the parti-cles was achieved in the composites reinforced with 66�mparticles, whereas the distribution of 16 and 32�m particleswas not uniform and some of these particles agglomerated.Figs. 5 and 6present the SEM micrographs of the 10 wt.%Al2O3 particle-reinforced composites with 32 and 16�m par-ticle sizes in which the particle clustering and agglomerationare clearly shown. Also,Fig. 7shows the SEM micrograph ofthe 30 wt.% Al2O3 particle-reinforced composite with 16�mparticle size where the porosity is clearly indicated by the darkblack regions.

article addition rate, the particles were not incorporatedhe molten metal and particle agglomeration occurred aurface of the molten metal. In addition, at higher stirpeed some of the particles were dispersed out of theible by the wind of the impeller and so particle additionot achieved. When the mold temperature was decreaseomposite mixture solidified immediately it was pouredhe mold and the necessary pressure could not be aphus, the amount of porosity in the composite increalso, at higher mold temperature, Al2O3 particles sunk t

he bottom of the mold because of the low solidificationnd uniform dispersion of the particles was not achievedresult, the uniform distribution of Al2O3 particles was onlchieved under the optimum process conditions given a

.2. Density and porosity

The graphs of theoretical and experimental densitiesorosities of the composites according to the weight fracf Al2O3 particles are shown inFigs. 2 and 3, respectivelyig. 2shows that the theoretical density values of the com

tes increase linearly (as expected from the rule of mixtulthough a linear increase was seen in the experimentaities, the values are lower than that of the theoretical dies. The density measurements showed that the compontained some porosity, and as shown inFigs. 2 and 3, themount of porosity and density in the composites increith increasing weight fraction and decreasing size ofarticles. These results have been observed in previoestigations[1,27,31]. During the production process of t

M. Kok / Journal of Materials Processing Technology 161 (2005) 381–387 385

Fig. 4. SEM micrographs of composites reinforced with 30 wt.% Al2O3

particles, black regions are Al2O3 particles: (a) 66, (b) 32, (c) 16�m.

As a result, SEM observations of the microstructures re-vealed that the dispersion of the coarser sizes was more uni-form while the finer particles led to agglomeration and seg-regation of the particles, and porosity. The reason for theparticle segregation is proposed as follows: the Al dendritessolidify first during solidification of the composite, and theparticles are rejected by the solid–liquid interface, and hence,

Fig. 5. SEM micrograph of 10 wt.% Al2O3 particle-reinforced compositewith 32�m particle size, which shows particle segregation, black regionsare Al2O3 particles.

Fig. 6. SEM micrograph of 10 wt.% Al2O3 particle-reinforced compositewith 16�m particle size, which shows particle agglomeration, black regionsare Al2O3 particles.

Fig. 7. SEM micrograph of 10 wt.% Al2O3 particle-reinforced compositewith 32�m particle size, which shows porosity, black regions are porosity.

386 M. Kok / Journal of Materials Processing Technology 161 (2005) 381–387

Fig. 8. The variation of hardness with Al2O3 particle content and size.

are segregated to the inter-dendritic region. This event oc-curred more easily with the finer particles[32].

3.4. Hardness and tensile strength

Hardness tests were performed on a Brinell Hardness ma-chine and the results of the tests are shown inFig. 8. Asshown, hardness increases with the amount of Al2O3 parti-cles present and decreasing particle size.

The results of the tensile strength tests are given inFig. 9,which shows the variation of tensile strength with the weightfraction and the size of Al2O3 particles. As shown here, thetensile strength of the MMCs increased with decreasing sizeand increasing amount of particles like the hardness. How-ever, there is a discrepancy in the tensile strength of the com-posite reinforced with 10 wt.% of 32�m Al2O3 particles.The tensile strength of this composite is a little bit greaterthan that of the composite reinforced with 10 wt.% of 16�mAl2O3 particles. The elongation measured for the compositeslies with a very low range, below 3%, depending on the Al2O3particle size and content. The higher the particle content andthe lower the particle size, the lower the elongation.

Among all of the MMCs, the composites reinforced with16�m Al2O3 particles have the maximum hardness and ten-sile strength, and the minimum elongation. Compared witht nesso

F e.

has increased the tensile strength and hardness of the Al alloy[27,33].

4. Conclusions

The optimum process conditions for production of Al2O3particle-reinforced aluminium alloy composites by the vortexmethod and subsequently applied pressure were found in thepresent work. SEM microstructures, density and porosity, andthe tensile strength and hardness of MMCs were investigated.The following conclusions have been drawn:

1. 2024 Al alloy MMCs reinforced with different sizes andweight percentages of�-Al2O3 particles (up to 30 wt.%)have been successfully fabricated by the vortex methodand subsequently applied pressure. The optimum con-ditions of the production process were that the pouringtemperature was 700◦C, preheated mold temperature was550◦C, the stirring speed was 900 rev min−1, the stirringtime after the completion of particle feeding was 5 min,the particle addition rate was 5 g min−1 and the appliedpressure was 6 MPa.

2. SEM observations of the microstructures showed that thecoarser particles were dispersed more uniformly, while

on of

3 singoros-and

4 Ales-eased

5 d butsize

R

ring. 28

ing. 34

ticle-tirringol. 55

be-d by

sites,fax,

he 2024 Al matrix alloy, the tensile strength and hardf the MMCs are greater, and the addition of Al2O3 particles

ig. 9. The variation of tensile strength with Al2O3 particle content and siz

the finer particles led to agglomeration and segregatiparticles, and porosity.

. The density of the composites increased with increaweight percentage and size of particles, whereas the pity of the composites increased with decreasing sizeincreasing weight percentage of particles.

. The wettability and the bonding force betweenalloy/Al2O3 particles were improved by the applied prsure after the casting and the porosity was also decrbecause of this pressure.

. The tensile strength and hardness of MMCs increasethe elongation of them decreased, with decreasingand increasing weight percentage of the particles.

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. Kok is an assistant professor at the Department of Mechanicaram, Vocational Education College of Kahramanmaras Sutcu Imamersity, Kahramanmaras, Turkey. He received the MSc and PhD derom Firat University, Elazıg, Turkey, in 1996 and 2000, respectively.orked on production and machinability of composites.