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A study of microstructure and tribological behaviour ofAl–4.5% Cu/Al3Ti composites

Karabi Das⁎, L.K. NarnawareDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +91 3222 283254;E-mail address: karabi@metal.iitkgp.ernet.i

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.01.012

A B S T R A C T

Article history:Received 5 June 2008Received in revised form11 December 2008Accepted 22 January 2009

Al–4.5% Cu/Al3Ti composites with the varying amount of in-situ Al3Ti were prepared in aninduction furnace. The prepared in-situ composites were subjected tomechanical stirring at700 °C. The composites after stirring have been defined asmelt processed composites. Weartests have been performed to study the influence of Al3Ti on the sliding wear behavior of in-situ and melt processed composites. The effect of ageing on the wear behavior of both in-situ and melt processed composites has been studied. Wear has been simulated in twodifferent conditions, namely mild and severe. It has been found that the wear resistance ofthe composites improves with the increase in amount of Al3Ti as well as with ageing. It hasbeen observed that mainly oxidative and abrasive wear coexist in severe condition, whereasa combination of oxidative, abrasive and adhesive wear exists in mild condition.

© 2009 Elsevier Inc. All rights reserved.

Keywords:Aluminium matrix compositeIn-situMicrostructureWear

Table 1 – Chemical analysis of in-situ and melt processed(after stirring) composites.

Materialdesignation

Nominal Composition Wt%Al

Wt%Cu

Wt%Ti

S1 Al–4.5% Cu 95.5 4.5 0S2 Al–4.5%Cu–2%Al3Ti

(before stirring)95.0 4.5 0.6

S3 Al–4.5% Cu–5% Al3Ti(before stirring)

93.8 4.5 1.8

S4 Al–4.5% Cu–10% Al3Ti(before stirring)

91.4 4.5 4.1

1. Introduction

The presence of hard reinforcement phases, either particu-lates, fibres or whiskers improves the tribological properties ofaluminiummatrix composites (AMCs). An extensive review onthe dry sliding wear of aluminium based composites has beendone by Deuis et al. [1]. They have discussed in great detailabout the influence of various factors such as reinforcementsize and fraction, load, sliding speed and temperature.Information related to the effect of artificial ageing on wearproperties of AMCs is also available [2–5].

It has been shown by different researchers that differentreinforcements in different wear conditions result in differentwear mechanisms. As reviewed by Sannino et al., thetribological parameters involve some mechanical (e.g., loadapplied, surface of material, sliding velocity) as well as somematerial parameters (e.g., reinforcement type, shape, size) [6].Among the mechanical parameters insufficient studies arereported on the effect of counterpart material on wear whichcan be a useful parameter in determining the wear mecha-nism. The usual way of testing abrasive wear is to slide the

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material against an abrasive medium. These abrasive mediaare generally hard ceramics like SiC particles embedded on apaper or cloth. In most of the studies volume loss and wearrates are reported based on the sliding of material againstthese abrasive papers, but the wear of abrasive medium isgenerally not accounted. Since most of the compositematerials comprise of hard dispersoids in the matrix, the

S5 Al–4.5% Cu–5% Al3Ti(after stirring)

94.0 4.5 1.5

S6 Al–4.5% Cu–10% Al3Ti(after stirring)

92.0 4.5 3.7

.

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wear of these composites becomes the function of wear of thecounterpart material, i.e. abrasive paper. In this study,tribological properties of in-situ and melt processed Al–Al3Ticomposites in both as-cast and peak aged conditions havebeen studied and the effect of wear of abrasive medium isaccounted.

2. Experimental Procedure

Al–4.5% Cu/Al3Ti composites with varying amount of Al3Tiwere prepared by melting commercially available aluminium,titanium and Al–30 wt.% Cu master alloy in an inductionfurnace followed by solidification in metal mould. The aimed

Fig. 1 – SEM micrographs of (a) as-cast alloy S1 and

wt.% of Al3Ti was 2, 5 and 10%. A part of the in-situ compositewas then re-melted at 700 °C in a furnace attached to a stir castassembly and stirred at 550 rpm for 30 min followed bysolidification in a metal mould. The composites have beencharacterized by scanning electron microscopy (SEM), andenergy dispersive X-ray (EDX) analysis.

Abrasive wear tests were carried out on 10×10 mm crosssection samples having a thickness of 10 mm, against 220 gritSiC paper affixed to a rotating flat disc of 250 mm diameter.The rotation speed was fixed at 350 rpm. All the experimentswere carried out at a single load i.e., 9.8 N (≈10 N). An electronicbalance was used to measure the weight loss and prior toweighing the sample surface was cleaned properly. Experi-ments were carried out in two different ways i.e., severe andmild wear conditions. In mild condition, sliding was done at

composites (b) S2, (c) S3, (d) S4, (e) S5, (f) S6 [7].

Fig. 3 – SEM micrograph of the worn surface of composite S4.

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6 cm from the center of the rotation disc and continued on thesame circumference for a sliding distance of 800 m. Weightloss was measured after every 65 m (i.e., after every 30 s). Insevere condition sliding was done for the same distance, butabrasive paper was changed after every 65m to create a severecondition of wear. In both cases sliding was done for 800 mand experiment was carried out at an ambient environment(room temperature and normal humidity). Weight loss hasbeen measured carefully and then converted into the volumeloss. The wear rates have been calculated from the magnitudeof the slope of the cumulative volume loss versus slidingdistance plot. Analysis of wear track on the samples surface,abrasive paper and debris were done by using scanningelectron microscopy (SEM) and energy dispersive X-ray (EDX)analysis.

3. Results and Discussion

3.1. Microstructure

Chemical compositions of the as-cast alloy S1 and compositesS2, S3, S4, S5 and S6 are given in Table 1. From the chemicalcomposition of the composites, stoichiometric calculation hasbeen done to estimate thewt.% of Al3Ti assuming no solubilityof Ti in Al matrix. The estimated amount of Al3Ti incomposites S2, S3, S4, S5 and S6 are 1.6, 4.8, 11, 4, and 10 wt.%, respectively. The microstructure of as-cast alloy S1showsthe presence of white phase along the interdendritic regionwhich has been identified as Al2Cu on the basis of EDS andXRD analysis (Fig. 1a). Fig. 1b shows the microstructure ofcomposite S2. The Al3Ti phase is not clearly visible in themicrostructure. There may be two reasons for this; either thewhole amount of Ti is in solution or the amount and size ofAl3Ti are too small to be detected through SEM. Microstruc-tures of composite S3 and S4 show the presence of large

Fig. 2 – Variation of cumulative volume loss with increasingsliding distance for in-situ composites S2, S3 and S4 andalloy S1 in severe wear condition.

elongated plates of Al3Ti with dimensions of about 150 to200 µm long and 10 µm wide along with Al2Cu along the grainboundary (Fig. 1(c) and (d)). Microstructures of composite S5and S6 are shown in Fig. 1(e) and (f), respectively. It is observedthat needle shaped Al3Ti with a dimension of about 10 to20 µm long and 1 to 2 µm wide are present in themicrostructure. The details of the microstructures have beenreported in our previous paper [7].

3.2. Effect of Al3Ti on Abrasive Wear of In-Situ Composites

3.2.1. Severe WearFig. 2 shows the variation of cumulative volume loss withsliding distance for in-situ composites S2, S3 and S4 andmonolithic alloy S1 in severe wear condition. It can beobserved that the volume loss in monolithic alloy is higherthan that of as-cast in-situ composites. It is also clear from thefigure, that the volume loss curve has a linear relation shipwith sliding distance. Since abrasive paper is changedfrequently and abrasion by blunt SiC particles is restricted,

Fig. 4 – Variation of cumulative volume loss with increasingsliding distance for in-situ composites S2, S3 and S4 andalloy S1 in mild wear condition.

Fig. 6 – Variation of wear rate of in-situ composites with

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volume loss after every consecutive reading remains almostthe same and hence shows the linear relation ship.

Fig. 3 shows the SEM micrograph of the worn surface ofcomposite S4 in severe condition. Wear tracks with narrowwidth can be observed in the composite which is due to theploughing action of sharp SiC abrasive particles. Some brightparticles adhered to the tracks can be observed in Fig. 3. It iswell known that there is a formation of oxide tribochemicallayer on the worn surface during wear of metals. This layercontrols the wear rates and volume loss at higher slidingdistances. Since the blunting of SiC is avoided by changing thepaper frequently, there are chances of rupture of the layer dueto severe action of sharp abrasive particles. The brightparticles adhered to the trackmay be the resultant of rupturedlayer. There is no evidence of particle pull out from thematrix.Presence of Al3Ti after such severity indicates that theinterfacial strength between thematrix and the reinforcementis high in this composite.

sliding distance in different abrasion condition.

3.2.2. Mild WearFig. 4 shows that the variation of cumulative volume loss withsliding distance is not linear in mild wear condition. Abrasionduring initial sliding is due to the sharp SiC particles. Thismaybe the reason for high volume loss at initial sliding. Afterinitial sliding Al3Ti particles protrude out from the matrix. Athigher sliding distances abrasive particles interact with theseprotruded Al3Ti. Since sliding was done at a single circumfer-ence, therefore repeated sliding of SiC particles against hardprotruded Al3Ti particles results in blunting and decrease intheir cutting efficiency. As a result, material removal becomesless at higher sliding distances resulting non linear curve. Thevolume loss for initial sliding is high, but it becomes almostconstant at higher sliding distances for composites with highamount of Al3Ti. This can be seen clearly from volume losscurve of composite S4. With the increase in the wt.% of Al3Tithere is an increase in volume fraction of protrusion. Moreprotrusion embedded inmatrix results in high rate of bluntingof SiC particles and hence the cutting efficiency of abrasivebecomes constant. Fig. 5 shows the micrograph of the wornsurface of composite S4. It can be observed that the grooves

Fig. 5 – SEM micrograph of the worn surface of S4 compositeabraded in mild wear condition.

Fig. 7 – Effect of aging on abrasive wear of in-situ compositesabraded in severe condition a) variation of volume loss withsliding distance b) variation of wear rate with slidingdistance.

Fig. 8 – Effect of aging on abrasive wear of in-situ compositesabraded in mild condition a) variation of volume loss withsliding distance b) variation of wear rate with slidingdistance.

Fig. 9 – Effect of abrasion condition onwear of melt processedcomposites a) variation of volume loss with sliding distanceb) variation of wear rate with sliding distance.

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are wide and shallow owing to the action of blunt SiC particlesand their low cutting efficiency.

3.2.3. Comparison ofWear Rates in Severe and Mild ConditionsWear rate curves of both the conditions reveal that the wearrates are more or less the same at initial sliding distances,except composite S4 abraded in mild condition has shownvery less wear rates at initial sliding distance (Fig. 6). This maybe because of the high hardness of the composite. When theworn microstructure of composite S4 abraded in mild condi-tion is compared with the worn microstructure of the samecomposite abraded in severe condition, it is found that theamount of bright particles has reduced drastically. Thisreduction may be because of the mild action of abrasive.Presence of bright particles changes the two body abrasioninto three body abrasion. Therefore, there are chances thatcomposite surfaces have interacted with two types of abrasiveparticles rather than one during severe wear condition

resulting a large difference between the wear rates in mildand severe conditions.

It is also observed that the constant wear rate in severecondition reaches within a short sliding distance, whereas thewear rate decreases throughout the tested sliding distance forcomposite S2 and S3 and becomes constant for composite S4under mild wear condition. Abrasion at initial sliding distanceis governedby thewearofmatrixmaterial. As it proceeds to thehigher sliding distances strain hardening of the matrix takesplace and wear rate decreases. However, after some slidingreinforcement protrudes out from the surface and controls thewear rate. During severe wear the matrix gets strain hardenedwithin a short sliding distance. At higher sliding distancessharp SiC particles are rubbing against Al3Ti particles givingconstant wear rate. The wear rate is continuously decreasingfor composites with low volume fraction of Al3Ti during mildwear condition. There may be two reasons. Under the mildcondition the matrix is yet to be completely strain hardenedwithin the tested sliding distance. It can be also due to the factthat Al3Ti particles are rubbing against blunt SiC particles

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(since SiC paper is not changed) making them progressivelyless efficient with increasing sliding distance.

3.2.4. Effect of Ageing on WearFigs. 7 and 8 show the effect of ageing on volume loss ofcomposites in severe and mild conditions of wear, respec-tively. Wear resistance of aged composites is higher than thatof as-cast composites. This is due to the higher hardness of theaged composites than that of the as-cast composites. It is alsoobserved that the constantwear rate is achieved earlier in caseof aged composite both in severe and mild wear conditions.This is perhaps due to the fact that thematrix is already strainhardened due to the precipitation of Al2Cu.

3.3. Effect of Al3Ti on Abrasive Wear of Melt ProcessedComposites

Fig. 9 shows the effect of abrasion condition on wear of meltprocessed composites. The nature of both the curves are verysimilar to as that have been observed for in-situ composites.

Fig. 10 – Effect of ageing on wear of melt processedcomposites abraded in severe wear condition a) variation ofvolume loss with sliding distance b) variation of wear ratewith sliding distance.

Fig. 11 – Effect of ageing on wear of melt processedcomposites abraded in mild wear condition a) variation ofvolume loss with sliding distance b) variation of wear ratewith sliding distance.

Figs. 10 and 11 show the effect of ageing on the wearbehaviour of melt processed composite under severe andmildconditions. Again the observations are same as that have beenobserved for in-situ composites.

3.4. Effect of Stirring on Wear

It has been reported in our previous paper that stirringdrastically reduces the size of Al3Ti [7]. Fig. 12 shows theabrasive wear behaviour of S4 and S6 composites in severecondition. It can be observed that the wear resistance of bothin-situ and melt processed composites is higher than that ofmonolithic alloy. In spite of having higher hardness, meltprocessed composite has shown lower wear resistance thanthat of the in-situ composite. This may be because ofincapability of small Al3Ti particles to carry normal loadbecause of poor interfacial property which results in dislodg-ing of Al3Ti particles. Semisolid stirring of in-situ compositewas done to form themelt processed composites. At this stage

Fig. 12 – Effect of stirring on wear of Al–4.5% Cu/10% Al3Ticomposites a) variation of volume loss with sliding distanceb) variation of wear rates with sliding distance.

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solid Al3Ti can interact with atmospheric air whichmay resultin deterioration of interfacial properties. Fig. 13 shows theworn microstructure of S6 composite. The microstructure is

Fig. 13 – SEMmicrograph of the worn surface of composite S6abraded in severe condition.

similar to that of S5 composite, the only addition is thepresence of large craters on the surface. These craters may bebecause of dislodging of Al3Ti reinforcement from the matrix.To confirm this Al3Ti particles are tested for hardness in boththe types of composites, it is found that the hardness of Al3Tiin in-situ composite is higher than that of the same in meltprocessed composite. Since any oxide formation along theinterface results in the cushioning effect to the applied loadduring hardness test, the hardness of that reinforcement willbe less than that of the same reinforcement with a clearinterface.

Fig. 12(b) shows the effect of stirring on the wear rates of S4and S6 composites. One can observe that the wear rate of S6composite is decreasing up to 300m and becoming constant at

Fig. 14 – a) As received 220 grit SiC abrasive paper b) abrasivepaper after 800 m sliding in mild wear condition showingmatrix deposition c) abrasive paper after 65 m sliding insevere wear condition.

Fig. 15 – EDX spectrum showing presence of Al matrix on abrasive paper.

Fig. 16 – Wear debris showing ribbon type, plate type andwelded matrix on SiC particles.

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higher sliding distances, whereas more or less constant wearrate is achieved at a sliding distance of more than 600m for S4composite. This may be because smaller reinforcementparticles are easy to worn out than that of massive. Thisleads to the exposure of soft matrix to the abrasive particles.As explained earlier the constantwear ratesmay be because ofwork hardening of matrix. Exposure of large matrix area inmelt processed composite brings the work hardening stagenear to the initial sliding distances, whereas massive particlesof Al3Ti prevent the exposure of matrix.

3.5. Wear Track on Abrasive Paper

Fig. 14a shows the SEM micrograph of as received 220 grit SiCabrasive paper and Fig. 14b shows the abrasive paper aftersliding of 800 m in less severe condition. Fig. 14b revels thatthere is a layer of matrix (which was confirmed by EDXanalysis, shown in Fig. 15) on paper. EDX spectrum confirmsthe presence of aluminium which might have come from therepeated rubbing of matrix to the abrasive. This layer wasfound to be welded and cannot be removed by washing withalcohol or water. This suggests that there has been aninteraction between the composite surface and mechanicallymixed layer of matrix and SiC particles. This confirms thechange in wear mechanism during mild wear condition andalso accounts for the change in the slope of volume loss vs.sliding distance curves for mild condition. Generally interac-tion between ceramic abrasive particles and matrix results inthe plastic deformation of surface like ploughing and cutting.But interaction betweenmechanically mixed layer of abrasiveand metal and the composite surface may result in a differentmechanism and to understand this some wear debris out ofmildly worn samples have been collected and analyzed.

3.6. Wear Debris

Micrograph of debris is shown in Fig. 16. Three major types ofmatrix debris can be seen 1) ribbon type 2) plate type and 3)weldedmatrix on themassive particles. As explained by someresearchers ribbon type debris are characteristics of cuttingand ploughing action of hard abrasive particles, plate types are

characteristics of adhesion between the metal–metal surfaceand some welding of matrix may occur because of thecontinuous rubbing action of samples on wear track. Al3Tiwas found to be missing in the debris. As studied by Wu et al.[8] this combination can be seen in the adhesive wearcondition, but above result shows that similar combinationof oxidative, abrasive and adhesive wear can exist in mildwear condition also.

4. Summary

The wear resistance of the in-situ and melt processedcomposites in as-cast and peak aged condition has beeninvestigated in both mild and severe wear conditions. Wearresistance of in-situ and melt processed composites increaseswith the increase in the amount of reinforcement and ageingin both mild and severe wear conditions. The in-situcomposites shows better wear resistance than that of themelt processed composites. Mainly oxidative and abrasivewear co-exists in severe condition, whereas a combination ofoxidative, abrasive and adhesive wear is observed in mildwear condition.

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[7] Das K, Narnaware LK. Synthesis and characterization ofAl–4.5%Cu/Al3Ti composites: microstructure and ageingbehaviors. Mater Sci Eng A 2008;497:25–30.

[8] Wu JM, Li ZZ. Contributions of the particulate reinforcement todry sliding wear resistance of rapidly solidified Al–Ti alloys.Wear 2000;244:147–53.