Abrasive Wear of In Situ AlB2/Al-4Cu Composite MaterialProduced by Squeeze Casting Method

FERIT FICICI1,2

1.—Faculty of Technology, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey.2.—e-mail: fficici@sakarya.edu.tr

The wear behavior of a weight fraction of particles with up to 30 wt.% in situAlB2 flakes reinforced in Al-4Cu matrix alloy composites and fabricated by asqueeze casting method was investigated in a pin-on-disk abrasion testinstrument against different SiC abrasives at room conditions. Wear testswere performed under the load of 10 N against SiC abrasive papers of 80, 100,and 120 mesh grits. The effects of sliding speed, AlB2 flake content, andabrasive grit sizes on the abrasive wear properties of the matrix alloy andcomposites have been evaluated. The main wear mechanisms were identifiedusing an optical microscope. The results showed that in situ AlB2 flake rein-forcement improved the abrasion resistance against all the abrasives used,and the abrasive wear resistance decreased with an increase in the slidingspeed and the abrasive grit size. The wear resistances of the composites wereconsiderably bigger than those of the matrix alloy and increased with in-creases in in situ AlB2 flake contents.

INTRODUCTION

Aluminum alloys are widely used in the aerospaceand automobile industries as a result of their lowdensities, good mechanical properties, high thermalconductivities, and good corrosion properties.1 Therelatively poor wear resistance of aluminum alloyshas limited their use in certain tribological appli-cations. Aluminum metal matrix composites (AlMMCs) have shown significant improvements inwear properties compared with conventional alu-minum alloys.2 AMCs are usually used reinforcedwith Al2O3, SiC, SiO2, TiO2, AlN, Si3N4, TiC, B4C,TiB2, and ZrB2.3–6

The reinforcement–matrix interface plays animportant role in the mechanical properties ofAMCs, and the improvement mainly depends on thestrength of the interface between matrix and rein-forcement. Most of the Al-based ex situ compositesexhibit poor adhesion and hence low bondingstrength between the main matrix and the rein-forcement phase. In situ AMCs, however, possessstronger interfacial bonding at the reinforcement–matrix interface, one of the significant parameters,which leads to better mechanical properties.7

The main factors that control the abrasive wearperformance of a particle-reinforced metal matrix

composite can be classified into two categories:8 oneincludes intrinsic factors such as the mechanicalproperties of both the reinforcing phase and thematrix8–15 as well as the microstructure of thecomposites, i.e., the volume fraction,8–10,16–18 parti-cle size,8,10,16–21 size distribution, shape, and shapedistribution of the reinforcement. The other in-cludes external factors, i.e., the loading condi-tion,8,9,22,23 shape and size of the abrasive,8,9,16,24–27

surface finish, temperature, and the environment.Among the above-mentioned factors, mechanical

properties, size and volume fraction of reinforce-ment, size of the abrasive, and the loading conditionhave been extensively studied.

Sahin28 carried out the abrasive wear test on Al2011 alloy with 5–10 wt.% SiCp content with 32–64-lm particle size, using factorial designs of experi-ments. The wear rate increased with increases inthe abrasive size, load, and sliding distance whenSiC paper was used. However, the wear rate in-creased with increases in the abrasive size and loadand decreased with sliding distance when Al2O3

emery paper was selected.Lee et al.29 investigated the effects of sintered

porosity, volume fraction, and particle size of siliconcarbide particles on the abrasive wear resistance ofpowder metallurgy Al alloy 6061 matrix composites.

JOM, Vol. 66, No. 5, 2014

DOI: 10.1007/s11837-014-0949-4� 2014 The Minerals, Metals & Materials Society

(Published online April 22, 2014) 711

Results showed a significant beneficial effect of hardSiCp addition on wear resistance for the P/M com-posites. For the composites containing the sameamount of SiCp reinforcements, the wear rates de-creased with increases in SiCp size. Al alloy com-posites reinforced with a large SiCp size were moreeffective against abrasive wear than those rein-forced with a smaller SiCp size. However, for thehigher volume fraction of SiCp composite, and forthe composite with a large SiCp size, the aging ef-fect on the wear rate of the composites was found tobe small.

Axen et al.30 studied the abrasion resistance ofalumina fiber-reinforced aluminum using a pin-on-drum abrasion wear tester. The composites weremanufactured by a hot liquid infiltration technique.They concluded that fiber reinforcement signifi-cantly improved the abrasion resistance in milderabrasive situations, i.e., small and soft abrasivesand low applied loads. However, in severe abrasivesituations, the abrasion resistance of the compositeswas equal to or, in some cases, even lower than thatthe unreinforced materials.

Diler and Ipek31,32 first investigated the flexuralstrength and then investigated the main andinteraction effects of matrix particle size, rein-forcement particle size, and volume fraction on thewear characteristics of Al-SiCp composites usingcentral composite design. They have asserted thatthe wear loss decreased with the increase of per-centage volume fraction up to 17.5%. Also, theyhave indicated that the decrease in particle size ofthe matrix and the increase in the reinforcementparticle size reduce the wear loss.

Das et al.33 examined the synergic effect of SiCparticle reinforcement and heat treatment on thetwo body abrasive wear behavior of an Al-Si alloyunder varying loads and abrasive sizes. They havenoted that the alloy suffers from a higher wear ratethan that of composites either in cast or heat-trea-ted conditions, irrespective of applied load andabrasive size. Furthermore, in most cases, the wearrate of composite decreased with an increase in SiCparticle content.

In light of the above, this research is aimed atstudying the abrasive wear behavior of aluminumcomposites reinforced with in situ developed AlB2

flakes.

EXPERIMENTAL DETAILS

Fabrication of Composite Materials

As matrix material, Etial8 Al alloy was used to pro-duce composites materials. xAlB2/Al-4Cu (x = 0 wt.%,

5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.%) in situ com-posites were prepared by adding boron oxide (B2O3)salt into the molten matrix to facilitate chemicalreactionat 1400�C. At this temperature,an unresolvedportion of boron oxide forms viscous glassy liquids thatare not miscible with molten aluminum. This glassyliquid is less dense than aluminum and floats on top ofthe melt as slag material (aluminum = 2.69 g/cm3,B2O3 = 1.844 g/cm3).

The chemical composition of Al-4Cu alloy used inthis study is provided in Table I.

The composite samples were shaped by using asqueeze casting method so as to provide a densermicrostructure and to control the amount of thereinforcement phase.

Microstructural Characterizationand Mechanical Properties

Density and hardness were determined accordingto the ASTM D 792 and ASTM D 2240 standards,respectively. The hardness tests were carried outusing a standard Brinell tester, with a 10-kg loadand 1-mm steel ball as an indenter. Hardnessmeasurements were obtained from three differentpoints on each sample, and then the arithmeticaverage was taken into account.

The phase structure identification was conductedwith an x-ray diffraction spectrometer with Cu Karadiation of the wavelength of 1.7902 A and be-tween 20� and 100� 2h values. The microstructureand wear surface properties of the composite sam-ples were analyzed by a Jeol JSM-5410 type scan-ning electron microscope (SEM).

Table I. Chemical composition of the matrix alloy

Element Si Fe Cu Mn Mg Cr Ni Zn Ti Al

% 0.01 0.012 4.042 0.003 0.002 0.02 0.001 0.01 0.004 Base

Fig. 1. Schematic diagram of the abrasive wear test apparatus.

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Abrasive Wear Test

A pin-on-disk with emery paper apparatus wasemployed to evaluate the wear characteristics ofcomposites and the aluminum matrix alloy, asshown in Fig. 1. The emery paper was fixed to a 20-mm-thick and 175-mm-diameter steel disk to serveas the abrasive medium. Wear test samples werecut from the composite disk and shaped into theform of a cylinder 6 mm in diameter and 10 mm inlength.

Before the abrasion wear tests, each specimenwas ground up to grade 800 abrasive paper to makesure that the wear surface was in complete contactwith the surface of the abrasive paper. Samples forwear testing were loaded against the abrasivemediums by a cantilever mechanism. Wear testswere carried out at room-temperature dry condi-tions. The test parameters were as follows: normalload on the pin, 10 N, equivalent to nominal normalstress of 0.37 MPa; three different sliding velocitiesof 1 m/s, 1.5 m/s, and 2 m/s; and total sliding dis-tance, 50 m.

The matrix alloy and composites were testedagainst SiC abrasive papers, and each test was per-formed with a new abrasive paper. Before and afterevery test, the pins and the disk were cleaned in an

Fig. 2. XRD patterns of in situ AlB2/Al-4Cu composite.

Fig. 3. SEM microstructure of in situ 30 wt.% AlB2/Al-4Cu composite.

Abrasive Wear of In Situ AlB2/Al-4Cu Composite Material Produced by Squeeze Casting Method 713

ultrasonic bath with acetone and then dried. Duringthe wear tests, the end of the pin specimen waspressed against the abrasive paper on the disk,rotating at a fixed speed under the applied load. Thewear losses were obtained from the differences inweight of the pin specimens measured before andafter the tests using an electronic balance with asensitivity of 0.1 mg. For each test condition, at leastthree tests were performed and the average was used.

RESULTS AND DISCUSSION

Phase Identification and Microstructures

The XRD pattern in Fig. 2 clearly indicates thepresence of AlB2 in the composite that contains30 wt.% AlB2 reinforcement. The XRD analysis alsoconfirms the absence of any other intermetallicphase or compounds.

A typical microstructure obtained from an Al-Bmaster alloy is given in Fig. 3a. The microstructureshows the in situ AlB2 phases formed in the Al-4Cumatrix alloy. The composite samples were shaped byusing a squeeze casting method so as to provide adenser microstructure and to control the amount ofthe reinforcement phase. Figure 3b shows a densermicrostructure of composite material obtainedafter the squeezing process. An SEM image of thedeep-etched composite containing 30 wt.%AlB2

reinforcement is shown in Fig. 3c. As observed inFig. 3c, the AlB2 particles within the aluminum ma-trix have a high aspect ratio and a finer flake shape.The formation of such high-aspect-ratio boride flakeswithin the aluminum matrix is in good agreementwith previous reports on AlB2/Al type alloys.34,35

Hardness

Figure 4 shows the hardness of Al-4Cu alloy withdifferent amounts of AlB2 particles. It is observedthat hardness increases with increases in the per-centage of AlB2 flakes in the matrix alloy.

Hard reinforcement in a soft and ductile matrixalloy always increases the hardness of the matrixalloy in general.36,37 Aluminum boride (AlB2), beinga hard phase, contributes significantly to the im-proved hardness of the composites. Uniformity inthe distribution is the key success in enhancing thehardness of composites. In this regard, the pro-cessed composites have exhibited very high degrees

Fig. 4. Hardness of Al-4Cu alloy with different amounts of AlB2

flakes.

Fig. 5. Density of matrix alloy and in situ AlB2/Al-4Cu compositematerials.

Fig. 6. Effect of sliding speed on abrasive weight loss.

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of uniformity in the distribution of reinforcement asevidenced in Fig. 3b and c.

Density

Figure 5 shows the mean density values of thematrix alloy and composite samples. It is evidentthat the density of the matrix alloy is less than thoseof all composite specimens.

From Fig. 5, it is also observed that 30% (wt.)AlB2/Al-4Cu composite has the highest density

among all the other composite materials. The valuesin Fig. 5 also suggest that the density of compositematerials tends to increase with the increasing rateof AlB2 reinforcement.

Abrasive Wear Properties

Effect of the Sliding Speed

The effect of the sliding speed on the abrasiveweight loss is shown in Fig. 6. It can be observed

Fig. 7. Effect of weight fraction on abrasive weight loss.

Abrasive Wear of In Situ AlB2/Al-4Cu Composite Material Produced by Squeeze Casting Method 715

that the abrasion weight loss is also stronglydependent on the sliding speed. As the sliding speedincreases, the weight loss also increases.

At a high sliding speed, the density of the abra-sive particle that comes into contact with the slidingsurface increases, thus, resulting in production offrictional heat. As a result, micro thermal softeningof matrix alloy takes place, which in turn lowers thebonding effect of in situ AlB2 with that of matrixmaterial. Because of its lower bonding strength,AlB2 can easily be pulled out from the matrix alloyunder such a sliding condition. From Fig. 6, it isalso clear that as the weight fraction of the AlB2

increases, the weight loss decreases with referenceto the sliding speed. The increase in weight loss athigh speed is attributed to the thermal softening ofthe matrix alloy; therefore, the addition of in situAlB2 to the Al matrix increases the thermal stabilityof the matrix material. The degree in thermal sta-bility also depends on the matrix alloy.

At 1 m/s, 80 mesh, there were about 13.38%,16.88%, 21.66%, and 27.07% reductions in weightloss with the additions of 5 wt.%, 10 wt.%, 20 wt.%,and 30 wt.% of in situ AlB2, respectively. At 1.5 m/s,80 mesh, there were about 55.99%, 64.29%, 69.99%,and 70.56% reductions in weight loss with theadditions of 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.%of in situ AlB2, respectively. At 2 m/s, 80 mesh,there were about 44.77%, 56.75%, 59.31%, and65.04% reductions in weight loss with the additionsof 5 wt.%, 10 wt.%, 20 wt.%, and 30 wt.% of in situAlB2, respectively.

Effect of Weight Fraction on Abrasive Weight Loss

Figure 7 shows the influence of the addition ofin situ AlB2 on the abrasive wear behavior of Al-4Cumatrix alloy. It is clear from this figure that theunreinforced matrix alloy wore much more rapidlythan the reinforced composite materials, and incomparison with the matrix alloy, the weight loss ofthe composites substantially reduced with increas-ing in situ AlB2 weight fraction from 5% to 30%.That is, an increase in the wt.% of in situ AlB2 in-creases the wear resistance of Al-4Cu matrix alloy.This high wear resistance of composite materials isa result of the presence of in situ AlB2, which acts asa load-supporting element.

Most studies38–41 have shown that the weight lossof MMC decreases with increases in the weightfraction of reinforcement. On the other hand, Wangand Hutching41 claimed that over a certain size ofabrasive SiC particles, the wear resistance of thecomposites decreased with increases in the rein-forcement above a certain level.

Effect of Abrasive Grit Size on Abrasive Weight Loss

Figure 8 shows the variation of abrasive weightloss of matrix alloy and its composites with abrasivegrit size. It is observed that with a decrease in theabrasive grit size, both matrix alloy and its compos-

ites exhibited increased weight loss. However, in allthe cases studied, when compared with matrix alloyand its composites, composite materials possessed

(a)

(b)

(c)

Fig. 8. Effect of abrasive size on abrasive weight loss.

Fig. 9. Effect of hardness on abrasive weight loss.

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reduced abrasive weight loss. Abrasive grit size hasmore of an effect on the weight loss for the composites.This might be a result of the penetration abilityincreasing with increasing abrasive grit size and

effective stress on the abrasive. These resulted inincreasing the cutting efficiency of the abrasive par-ticles. This is in good agreement with Das et al.33 anda recent study carried out by Mondal et al.23

Fig. 10. Optical micrographs of worn surfaces by SiC abrasive papers of 100 abrasive mesh.

Abrasive Wear of In Situ AlB2/Al-4Cu Composite Material Produced by Squeeze Casting Method 717

Effect of Hardness on Abrasive Weight Loss

Figure 9 of the present study shows that weightloss reduces with hardness. It can be observed thatthe composites show lower weight loss, indicatingthe beneficial effect of the addition of AlB2 flakes. Itmay be attributed to the hardness of the material,which is a dominating factor affecting the wearresistance. The decrease in wear weight loss mayalso be attributed to the higher load-bearingcapacity of hard reinforcing material.42

Wear Mechanisms

Optical micrographs of the worn surfaces of thematrix alloy and the composites given in Fig. 10show the evidence of grooves running parallel tothe sliding direction for all wear samples. Theworn surface of the unreinforced Al-4Cu matrixalloy was defined by extensive plastic deformationand very clear evidence of ploughing, cutting, andsmearing (Fig. 10a). Because the unreinforcedmatrix alloy was much softer than the harder silicaparticles, these particles could penetrate and cutdeeply into the softer wearing surface, causingextensive plastic deformation of the wearing sur-face (Fig. 10a), resulting in a great amount ofmaterial loss. The worn surface of the sample5 wt.% AlB2/Al-4Cu composite was considerablysimilar to that of the unreinforced matrix alloy,and it was described by plastic deformation andwith some ploughing and cutting (Fig. 10b). Whenthe amount of in situ AlB2 flakes was increasedfrom 5 wt.% to 30 wt.%, the worn surface consistedof localized grooves and fine scratches (Fig. 10c).The plastic deformation on the worn surface wassignificantly reduced with increases in the in situAlB2 flake weight fraction from 10 wt.% to20 wt.%, and the worn surface was characterizedby localized shallow grooves and very fine scrat-ches (Fig. 10d). Very little plastic deformation wasobserved on the worn surface of the sample30 wt.% AlB2/Al-4Cu (Fig. 10e), which was char-acterized by very fine scratches, and in situ AlB2

flakes that protruded on the worn surface wereground by the hard SiC particles.

CONCLUSION

The present study can be concluded as follows:

1. The wear properties of the Al-4Cu matrix alloywere considerably improved by the addition ofin situ AlB2 flakes, and the wear resistance of thecomposites was much higher than that of theunreinforced Al-4Cu matrix alloy.

2. The wear resistance of the composite materialsincreased with increases in the in situ AlB2 flakecontent. The AlB2 flakes possess a high hardnessvalue, and a strong particle–matrix interfacialbonding is formed.

3. The weight loss decreases linearly with increasesin hardness. This signifies that the hardness of

the materials plays an important role in control-ling their wear resistance.

4. The excellent wear resistance of the compositematerials was mainly dependent on the effec-tive resistance of in situ AlB2 flakes to pene-tration, cutting, and grinding by the SiC emerypapers.

5. The main wear mechanism operating on theworn surfaces of the composites was the plasticdeformation (micro cutting and micro plough-ing).

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