Materials Science and Engineering A268 (1999) 104–108

A comparative wear study on heat-treated aluminium–lithiumalloy and pure aluminium

Md. Aminul Islam, A.S.M.A. Haseeb *Department of Materials and Metallurgical Engineering, Bangladesh Uni6ersity of Engineering and Technology, Dhaka 1000, Bangladesh

Received 15 September 1998; received in revised form 8 February 1999

Abstract

Wear behaviour of solution-treated and aged aluminium–lithium alloy (2.5% Li, 2% Cu, 1% Mg and 0.15% Zr) was comparedwith that of pure aluminium. Wear tests were carried out in ambient air in a pin-on-disc type apparatus under dry slidingconditions using hardened steel disc as the counterbody. A normal load of 10 N and a linear speed of 0.98 m s−1 were used duringthe tests. Microscopic investigation, microhardness measurement on the subsurface of wear scars on pins, X-ray diffraction studyand morphological examination of wear debris were done to elucidate wear mechanism. It has been found that the wear resistanceof the aluminium–lithium alloy is about two and half times higher than that of pure aluminium. Subsurface deformation,delamination, etc. have been found during the wear of the aluminium–lithium alloy, while severe adhesive wear has been identifiedas the main mechanism in pure aluminium. © 1999 Elsevier Science S.A. All rights reserved.

Keywords: Wear; Aluminium–lithium alloy; X-ray diffraction

1. Introduction

There has been a growing interest in the wear be-haviour of aluminium alloys as more and more of thesealloys, both in type and quantity, are being used indifferent areas of technology. Among the aluminiumbased alloys, probably the aluminium–silicon alloyshave been investigated most for their tribological be-haviour [1–3]. In recent years aluminium–lithium al-loys have drawn a great deal of attention because oftheir attractive properties, i.e. high stiffness, low den-sity, etc. and their ability to offer weight savings of upto 10–15% which is of great value in the transportationand aerospace applications. Although a number ofstudies have been devoted in recent years to differentmechanical properties of aluminium–lithium alloys [4–6], the wear behaviour of these alloys is not wellstudied.

In the present work, the wear behaviour of an alu-minium–lithium based alloy has been studied and com-pared with that of pure aluminium. This paper also

attempts to explain the wear behaviour of the alloy interms of the wear mechanism involved.

2. Experimental

Wear tests were conducted on aluminium–lithiumbased alloy (2.5 wt.% Li, 2 wt.% Cu, 1 wt.% Mg and0.15 wt.% Zr) and commercially pure aluminium. Thealuminium–lithium alloy was received as extruded barwhich was solution-treated at 520°C for 4 h followed bywater quenching and subsequent ageing at 175°C for 8h to a hardness of RB 78. Commercially pure alu-minium bar used in the study had a hardness of RB 56.Wear tests were performed in a pin-on-disc type ap-paratus as depicted in Fig. 1. Wear specimens were inthe form of cylindrical pins of diameter 8 mm andlength 6.5 mm, and were pressed against a hardeneddisc of medium carbon steel (RC 50) which acted as acounter body. The cylindrical pins of the aluminium–lithium alloy and pure aluminium were machined tosize and subsequently wet polished with 1 mm aluminapowder. This yielded a surface roughness, Ra of 0.25mm. The hardened steel counterbody was ground to anRa value of 0.7 mm. During the tests, the curved surface

* Corresponding author. Tel.: +880-864-64044; fax: +880-2863-026.

E-mail address: haseeb@mme.buet.edu (A.S.M.A. Haseeb)

0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII: S0921 -5093 (99 )00079 -9

M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108 105

Fig. 1. Schematic of the pin-on-disc type wear testing apparatus.

of the cylindrical pin made contact with the flat surfaceof the counter body which was 80 mm in diameter and10 mm in thickness. The counter body was rotated at aconstant rpm of 380 that gave a linear speed of 0.98 ms−1 at the wear track. All the tests were conductedusing a constant load of 10 N. Tests were carried outfor three sliding distances, i.e. 900, 1800 and 3600 m.Each of the tests was conducted using a new counterbody.

Both the pins and the counter body were cleanedthoroughly in water and degreased with acetone priorto the tests. The weight of the pin was measured beforeand after the test and the volume loss due to wearcalculated. After each test, the wear scar morphology ofboth pin and counter body was examined under anoptical microscope. Subsurface damage below the wornsurface was also investigated on the cross-sectionalsamples which were prepared using standard metallo-graphic techniques. Microhardness below the wear scarwas measured on cross-sectional samples in a Shimudzumicrohardness tester using a load of 50 g applied for 10s. Wear debris generated during the tests was collectedand examined under a stereoscope. The debris was alsoinvestigated by means of X-ray diffraction in a JEOLJDX-8P X-ray diffractometer employing CuKaradiation.

3. Results and Discussion

Fig. 2 shows the rate of wear of both aluminium–lithium alloy and pure aluminium as a function ofsliding distance. The wear rate versus sliding distancecurves for both materials show a similar trend. Afterthe initial transient period, the wear rate of both sam-ples tends to attain a steady state value. It is evidentfrom the graph that in the steady state condition, thewear rate of pure aluminium is about two and halftimes higher than that of the aluminium–lithium alloy.The morphology of the wear scar of aluminium–lithium alloy and pure aluminium tested for a slidingdistance of 3600 m is shown in Fig. 3. Pronouncedsliding marks containing deep groves and ridges areobserved in the case of the aluminium–lithium alloy.The grooves and ridges are seen to run continuouslyalong the wear scar. Evidence of spalling of materialwas also observed in some areas of the wear scar.Plastic flow of material was also seen. The wear trackon pure aluminium tested for 3600 m is seen to exhibitlarge plastic flow. Aluminium smeared massively on thesurface. At a shorter sliding distance, i.e. 900 m, bothaluminium–lithium alloy and aluminium showed simi-lar wear track morphology which consisted of continu-ous sliding marks as is seen in Fig. 3a. In the case of thealuminium–lithium alloy, the wear scar morphologydid not change much while for aluminium the morphol-

Fig. 2. Wear rate as a function of sliding distance.

Fig. 3. Morphology of wear scar of (a) aluminium–lithium alloy and(b) pure aluminium tested for a sliding distance of 3600 m.

M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108106

Fig. 4. Micrographs of wear debris of (a) aluminium–lithium alloyand (b) pure aluminium.

debris of both aluminium–lithium alloy and pure alu-minium were found to possess only the peaks of alu-minium. No other peaks such as those of aluminiumoxides, iron from counterbody, etc. could be identified.The absence of peaks of aluminium oxide, iron, etc.does not necessarily imply that these are absent in thedebris because the X-ray diffraction technique can notdetect a certain phase if it is present below a fewpercentage. The peak characteristics of each debrismaterial were slightly different. The full width halfmaximum (FWHM) of the (220) peak of aluminium–lithium alloy and aluminium original bulk samples andtheir wear debris is given in Table 1. The FWHM ofwear debris was subtracted from that of the bulksample and the difference is shown in the fifth columnof Table 1. This difference in FWHM represents thepeak broadening caused by the wear process. The ex-tent of peak broadening is seen to be higher in the caseof the debris of the aluminium–lithium alloy. Peakbroadening in a diffraction pattern can occur as a resultof finer grains and/or residual stress in the sample. Aswill be shown later (Fig. 5a), the grains of aluminium–lithium alloy at the surface are heavily strained which isindicated by their elongated appearance in the slidingdirection. This material, before escaping as debris,therefore undergoes a great deal of deformation whichinduces residual stress as well as causes fragmentationof grains. Greater peak broadening in the debris of thealuminium–lithium alloy suggests that the extent ofdeformation and damage suffered by this debris ishigher. On the other hand, smaller amount of peakbroadening suggests that the debris of pure aluminiumcould escape before undergoing much deformation.

Cross-sectional micrographs of the aluminium–lithium worn pin is shown in Fig. 5. Three distinctregions can be identified in the general view of thecross-section (Fig. 5a). A mixed top layer is followed bya deformed region beneath. Below the deformed regionlies the parent metal. Cracks and voids etc. were foundto nucleate in the deformed region. The evidence ofdeformation and damage in the wear debris of thealuminium–lithium alloy provided by X-ray line broad-ening is consistent with the presence of a deformedlayer below the wear scar of the alloy. Rice et al. [7]summarised the changes that may occur below the wear

ogy changed with sliding distance. Microscopic exami-nation showed that the wear track on the counterbodies rotated against pure aluminium contained amassive and continuous transfer layer of aluminium.The transfer layer on the counter bodies rotated againstthe aluminium–lithium alloy was rather thin anddiscontinuous.

Fig. 4a and b show the micrographs of wear debris ofthe aluminium–lithium alloy and pure aluminium re-spectively. The debris in both cases is found to consistof irregularly shaped particles. The wear debris ofaluminium is rather large, typically 30–50 mm in size.In comparison, the wear debris of the aluminium–lithium alloy is smaller in size. Close examination ofwear debris of the aluminium–lithium alloy revealedthat these particles were in fact composite mixtures ofeven smaller particles. X-ray diffraction patterns of the

Table 1Full width half maximum of X-ray peaks of original bulk samples and wear debrisa

2u (220)Material FWHM

Wear debrisOriginal bulk sample Difference in FWHM

44.78°Al–Li alloy 0.34° 0.58° 0.24°44.78°Pure Al 0.44° 0.46° 0.02°

a FWHM, full width half maximum.

M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108M. Aminul Islam, A.S.M.A. Haseeb / Materials Science and Engineering A268 (1999) 104–108 107

Fig. 5. Cross-sectional micrographs of aluminium–lithium alloy be-low the wear scar. (a) A general view and (b) evidence of delamina-tion.

tal conditions. The wear debris of aluminium wasrather large (30–50 mm). Robinowicz [8] classified adhe-sive wear based on the size of the wear debris. Severegalling wear was characterized by a debris size of20–200 mm whereas moderate wear was characterizedby debris of 2–20 mm. It can thus be concluded thatunder the present experimental conditions, wear suf-fered by aluminium is of the severe adhesive type.

In the case of the aluminium–lithium alloy, the wearscar morphology typified by continuous grooves andridges was found both in the initial as well as the steadystate conditions. Initially, groves and ridges wereformed on the aluminium–lithium alloy surface by therubbing action of the hard asperities of the steel coun-ter body. The hard asperities of the counter bodycaused the deformation of the aluminium–lithium alloyand induced nucleation of cracks and voids, whicheventually led to fragmentation and formation of debrisparticles. Some adhesive transfer of the aluminium–lithium alloy to the counter body also occurred whichpartially covered the wear track on the counter body.Later on, hardened debris particles acted as asperitieswhich along with the asperities on the uncovered sur-face of the counter body continued the sliding/plough-ing process. It appears that adhesion plays a relativelyminor role in the wear of aluminium–lithium alloyagainst steel as compared with the case of pure alu-minium. Similar results were also obtained with analuminium–copper alloy [9]. It was suggested [9] thatthe lower wear rate of aluminium–4.5% copper alloy ascompared with pure aluminium was the result of re-duced adhesion with the steel counter body. The pres-ence of copper in the alloy was suggested to reduce theadhesion. Rabinowicz [10] showed that the mutual solidsolubility of two contacting bodies can be taken as acriterion for their tendency of adhesion. The higher thesolubility, the higher is the adhesion. In the presentaluminium alloy, most of the alloying elements includ-ing copper have a lower solid solubility in iron thanthat of aluminium [11]. This is believed to be the reasonfor reduced adhesion between the aluminium–lithiumalloy and the steel counter body. This factor as well asthe higher hardness of the alloy contributed to thelower wear rate of the aluminium–lithium alloy.

4. Conclusion

1. The wear resistance of the aluminium–lithium alloyis found to be two and half times higher than that ofpure aluminium.

2. Severe adhesive wear has been identified as the mainwear mechanism for pure aluminium tested againststeel counter body under the present experimentalconditions.

scar of metallic materials. Based on a number of studieson different alloys, they suggested the existence of threezones below the wear scar, as has been observed in thepresent case. They observed that the top layer is acompositional mix of original specimen materials andchemical species from the counter body as well asenvironment. It was found that the cracks that nucle-ated in the deformed region of the aluminium–lithiumworn sample extended to the surface and/or ran parallelto the surface (Fig. 5b). This led to the delamination ofmaterial and formation of debris. No change in hard-ness at the subsurface region was observed in the caseof either of the materials. The subsurface hardness ofthe samples was more or less the same as the bulksample. It was reported [7] that both increase anddecrease in the hardness of the subsurface region canoccur depending upon alloy composition and test con-ditions. Two competitive processes, i.e. softening due tofrictional heating and work hardening occur simulta-neously during the test. In the present case, neither ofthe processes seems to dominate over the other.

The morphology of the wear scar on aluminium inthe steady state and the formation of a heavy andcontinuous transfer layer of aluminium on the steelcounter body suggest that adhesive wear is the mainmechanism for aluminium under the present experimen-

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3. Subsurface deformation, delamination etc. havebeen found during the wear of the aluminium–lithium alloy. The extent of adhesion in the alu-minium–lithium alloy is lower due to the presenceof alloying elements.

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