Wear Behaviour of AA6061 Aluminium Alloy and Its Composites

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WEARWear 188 (1995) 185-191

Short CommunicationWear behaviour of AA 6061 aluminium alloy and its composites

A.B. Gurcan, T.N. Baker *Department of Metall urgy and Engin eeri ng Materi als Uni versiry of Strathclyde Glasgow GI IXN UK

Received 12 October 1994; accepted 2 1 March 1995

Abstract

This study investigates the wear resistance of four AA6 061 MM Cs together with the monolithic AA6 061 alloy, all in the T6 condition,using a pin-on-disc test. In addition to the widely studied 20 vol.% Saffil MMCs, the present investigation considered a hybrid of 11%Saffil + 20% Sic, and a high volume fraction Sic, MM C. AA 6061 + 60% Sic,. The wear behaviour against P40 0 Sic grit adhesive bondedpaper and against B S817 M4 0(EN 24) steel were explored under an applied load of 9.8 N with a nominal contact pressure of 0.5 MPa.

It was found that after testing against Sic grit, AA 6061 + Saffil showed little advantage over the monolithic alloy, but the other threecomposites had a significant improvement in wear resistance. The hybrid and the AA6 061 + 60% Sic showed the best performance. Onlysmall improvements were noted for AA 6061 + Saffil and AA 6061 + 20% Sic over the monolithic alloy, when tested against steel. However,the addition of 11% Saffil to 20% Sic in the hybrid, resulted in this composite and the AA6 061 + 60% Sic being retained in regime I of theAlpas and Zhang classification, and recording low wear rates.Keywords: Abrasive; Sliding; W ear; A A606 1 Al alloy; SIC + Saffil; Metal matrix composites

1 Introduction

Metal matrix composites have a potential for enhancedwear resistance over the unreinforced alloy. This expectationhas normally been found by experiment to be the case [ 11.While both sliding [ 2,3] and abrasive [ 4-81 wear situationshave been explore d, with few exceptions, direct comparisonof data within these different regimes is not very meaningfulbecause different experimental conditions were employ edduring testing on different reinforcements in the MM Cs.

It has been reported that the abrasive wear resistance ofparticle reinforced MM Cs increases with the volume fractionof particles, under b oth high and low stress abrasive wearconditions [4-71. Under low stress abrasion, the larger theparticle size, the higher th e relative wear resistance (RW R)[ 6-81, which is defined as the wear rate of the unreinforcedmatrix divided by that of the MM C under the same test con-ditions [ 11.

How ever, inconsistencies exist in the literature. M anyexamp les are known of an increase in the hardness of a mate-rial being reflected in improved wear resistance [ 11. This isgenerally the case for abrasive [ 91 and lubricated sliding wea r

* Corresponding author.

[ 51, but for unlubricated sliding situations, there are conflict-ing reports rega rding the role of carbides and oxides andbetween different morphologies of these ceramics [ lo]. Themetal matrix compo site may exhibit similar [ 111 or eveninferior [ 121 wear resistance to that of the unreinforced alloy.

Wang and Rack [ 131 and Zhang and Alpas [ lo] havedemo nstrated that the rate-controlling wear mechanism s maychange ab ruptly at certain sliding velocities and contact loads,leading to abrupt increases in wear rate. It is suggeste d thatmany of the conflicting results in the wear literature can beexplained by the three regimes which have been identified,when the wear rate is considered as a function of test load[ 10,131.

Aluminium based metal matrix composites have been fab-ricated by several routes. The liquid rou te, whic h includessqueeze-casting and liquid pressure infiltration techniquesoften employs a ceramic preform. In some cases, the preformconsists of short fibres or whiske rs and may be combinedwith particulate. It is of interest to comp are the wear prop-erties of a metal matrix compo site which has this type ofhybrid ceram ic reinforcement with those of a single type ofreinforcement. This is the subject of the present researchwhich was conducted on AA6061 reinforced with SIC par-ticles, Al,O, whiske rs (Saffil) and a mixture of both.

0043 -1648 /95/ 09.50 0 199 5 Elsevier Science S.A. All rights reservedSSDIOO43-1648(95)06639-X


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186 A.B. Gurcan. T.N. Baker/ Wear 188 (1995) 185-191

2 Experimental methodDetails of the AA6061 (l.l%Mg, 0.76% Si, 0.30% Cu,

0.20% C r, 0.32% Fe, 0.02% Zn) comp osites and route ofmanufacture are given below.

Material A (AA60 61 alloy) w as manufactured at Strath-Clyde University from prealloyed powd er, b lended in a Tur-bula T2C m ixer in dry conditions at 42 rev min- for 20 min,placed in aluminium (comme rcial purity) cans, 50 mmdia X 70 mm, degassed and hot pressed in a closed die to 50%reduction in height using a 200 ton capacity Fielding hydrau-lic press.

Material B (AA6061 + 20 vol.% Saffil) was vacuum sin-tered and isostatically pressed at the National EngineeringLaboratory, Glasgow NEL) . The billets were then extrudedat the National Physical Laborato ry, Teddington. The Osprayprocess was used to manufacture material C(AA6061 +20 vol.% SIC,) whilst D (AA6061 + 11 vol.%Saffil+ 20% SIC,) and E (AA6061 + 60 vol.% SIC,) wereproduced by Advanced Materials Systems, formerly CrayAdvanced Materials, Yeovil, w ith the liquid pressure infiltra-tion (LPI) process.

The a-alumina fibres (RF Saffil, ICI plc) w ere originally150 pm in length and 3 p,m in diameter with a final lengthafter fabrication around 10 pm; Sic, (7 km) was purch asedfrom Washington Mills for materials D and E. Sic, ( 10 pm)was used in material C.

All the material was given a T6 heat treatment of 520 Cfor two h, water quenched to room temperature, followed byageing at 160 C for 18 h and water quenched. Subsequentstudies show ed that the use of the T6 conditions, well estab-lished for AA60 6 1, resulted in some o ver-ageing of the com-posites [ 141. A Vickers hardness tester with a 10 kg load wasused for the hardness data, which w as averaged from fiveindentations. The results are given in Table 1. The wearresistance was determined for all materials using a pin-on-disc test in air under dry sliding conditions and at roomtemperatu re, with an applied load of 9.8 N, giving a nominalcontact pressure of 0.5 M Pa and a disc velocity of 0.14 m s - The pin, which had a diameter of 5 mm _t 0.1 mm, wasmachined after the T6 heat treatment using a cooled lubricantand slow machining speed s to avoid influencing the micros-tructure.Table 1Wear rates of 6061 alloy and composites

Two disc materials were investigated, P40 0 SIC grit adhe-sive bonded paper and B S8 17M4 0( EN24, AISIE4340)0.36C, 1.65 Ni 1 OCr 0.3Mo (all weight percent) steel, theformer as an example of a very hard material which might bepresent as an abrasive particle in a lubricant and the latter asa surface which might, in practice, be in contact with MM Cs.The pin-on-disc tester was of a simple design and the pin wasin contact with the same disc track. D uring a single test, theSic, grit paper w as replaced every 25 m.

The steel was heat treated at 850 C for 30 min followedby oil quenching, and tempering at 250 C for 1 h whichraised the HVlo value from 300 to about 600. Hardness meas-urements were taken across the 70 mm diameter of the steeldisc to check the efficiency of the heat treatment in producinga homog eneous hardness level. Two indentations were madein each of twelve positions and gave a hardness of590 k 23 VPN. The steel discs were cleaned prior to testing,and before and after each weighing. A new disc was used foreach material tested. Th e weight loss of the pin was deter-mined to an accuracy of + 2 X 10e5 g as a function of thesliding distance, p roportional to the number of revolutions ofthe disc. Two readings were made for each weighing and themean taken. A ll the tests were duplicated for the SIC grit, butowing to shortag e of material only one set of tests was carriedout using the steel disc. The pin was ultrasonically cleanedby immersing in methanol for five min before and after eachmeasuremen t followed by remounting in the tester in the samelocation. The test duration was up to 4 h with a total slidingdistance of approximately 2100 m.

3. Metallographic examinationThe microstructures of both the pin and the discs were

examined by optical metallograph y and by a scanning elec-tron microsco pe with facilities for chemical analysis EDX) .

The prior particle boundaries of the aluminium 6061 pow-der were clearly visible in material A and wered ecorate d withthe remaining fragments of the aluminium oxide skin [ 151.In material B, the Saffil fibres were aligned along the directionof extrusion, and while the overall distribution of Saffil wasfairly uniform, some clustering was noticeable. The orienta-tion of the fibres was such that their cross-section was subject

Material Vickers hardness at10 kg

Density (Mg mm ) Wearrate (mm3 mm)

P400 SiCX IO- BS817M40 steel X 10m4

A 115 2.712 120 7.1B 114 2.96 70 5.5C 113 2.81 6 6.2D 140 2.95 6 0.9E 156 3.00 1 o 0.2Al, _ 3.96Sic _ 3.2


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A.B. Gurcan, T.N. Baker/ Wear 188 (1995) 185-191 187

to wear testing. E xcept for some exp ected porosity in materialC produ ced by the Osprey process , a well distributed partic-ulate was observed. The hybrid comp osite, material Dshow ed a random orientation of fibres, but areas with a lowerdensity of particulate were noted. M aterial E, with a highloading of Sic, did have evidence of contiguity and a littleporosity was found in these regions owing to poor infilling.

4. Hardness resultsThe hardness data of the T6 aged alloy and composites is

given in Table 1. Surprisingly, the average h ardness valuesof material A, B and C was almost identical, but significantincreases in hardness were recorded from the higher volumefraction reinforcement comp osites, D and E.

5. Wear results5 1 P400 Sic grit paper

It can be seen from Fig. 1 that increasing the volume per-cent of reinforcement, including the hybrid compo site,decrea sed the weigh t loss. Furthermore, AA60 61 alloy rein-forced with 20 vol.% Sic particulate was superiorto the com-posite containing 20 vol.% S affil which reco rded a weightloss similar to that of the monolithic alloy. Bo th the comp os-ites containing 20 vol.% Sic, and 20 vol.% Sic, with 11%Saffil had an almost identical wear resistance. The 60 vol.%Sic, composite showed the lowest weight loss for the grouptested in this study.

The ranking of AA60 61 and the four comp osites is clearlyshown in Table 1 which allows comparison in terms of thesteady state wear rates, defined as the slope of the wear-distance plot divided by the density.5.2. BSBI 7M4O EN24) steel

Table 1 indicates a significant change in the ranking of thealloy and comp osites, comp ared with sliding against Sic grit.

+ 6061 +20 Saff I l- 6061 +2O S1Ca 6061 + 11 Sai+zO SI~

500 1000 1500 2000Sliding Distance m)

Fig. 1. Graph of weight loss vs. sliding distance against P400 Sic grit forAA6061 alloy and composites.

500 1000 1500 2000Shding Distance m)

Fig. 2. Graph of weight loss vs. sliding distance against BS817M40 steel forAA6061 alloy and composites.

Materials A, B and C have similar wear resistance, as seen inFig. 2. The weight of material remove d by the steel disc isabout two orders o f magnitude less than that of the SIC grit.Further, the slopes of the graphs showed considerable differ-ences for the two counterface materials, but the influence ofpercent reinforceme nt on the wear resistance was maintainedand material E was again superior in this respect. How ever,the hybrid composite showed a better wear resistance thaneither materials B and C, which were similar to the alloy.

6 SEM observations6 1 P400 Sic grit paper

After testing the AA60 61 unreinforced alloy, the surfacewas characterise d by long and continuing groov es as seen atA in Fig. 3(a). Material B show s a heavily deform ed surface(Fig. 3 (b) ) which may be associated with Sic pick up at Aand B, while the 20% Sic, MM C surface (Fig. 3(c) ) con-tains heavy sc ars at A and cracking of crater-shap ed areas atB. The hybrid surface (Fig. 3 (d) ) show s signs o f discontin-ued grooving and possible pick up of debris at A and B. Incontrast, the surface of the 60 vol.% Sic com posite showedonly a slight tendency to grooving (Fig. 3 (e) ) owing to theresistance offered by the high volume fraction of hard partic-ulate.The EDX analysis on surfaces of material A and Cobserved in the SEM indicated that in the many areas exam-ined, at least ten per specimen, there w ere no discernablesigns o f pick u p of SIC by the pin from the P40 0 Sic gritpape r. A strong silicon peak was obtained from material B(20% Saffil) , and as expec ted, from m aterials D and E.6.2. BS8I 7M4O EN24) steel disc

The AA606 1, material A, revealed patches of highly dam-aged regions, su h as those re corded at A in Fig. 4(a). Avery uneven heavy da mag e occurred in the 20 vol.% Saffil


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(a)

Fig. 3. SEM micrographs: (a) AA6061 alloy; (b) AA6061+20% Saffil; (c) AA6061+20 % Sic,; (ii) hybrid compo site; (e) AA 6061+60% Sic,. Allmaterials abraded against P400 Sic grit.comp osite (Fig. 4(b) ) which h as been considered as micro-cutting, microploughing and microchipping, producing longand continuous grooving (A). Also the presence of featureswhich could be Saffil fibres parallel to the groov es can beobserved. The 20 vol.% SIC MM C again show s evidence ofgrooving. The uneven region above A in Fig. 4(c) show s abroken fine groov ed area with evidence of fine flakes, pos-sibly displaced iron, iron oxide or aluminium matrix. Bycomparison, the surface of the hybrid contains only fine shal-low clusters, A, Fig. 4(d). The surface of the 60 vol.% SICMM C contained no evidence of grooving, but regions indi-cated as A, Fig. 4(e) , which might have b een iron or ironoxide p icked up from the steel disc, were noted.

EDX analysis of the worn surface s indicated a pick up ofiron during the course of testing. MaE erial B had the lowestpick up, material E the highest.

7. Discussion7 1 Materials

The metal matrix com posites studied in this project w ereall based on AA6061, containing two of the most common

reinforcements used for aluminium based comp osites, SICparticulates and RF Saffil whiskers. The SIC particulates weretoward the small end (7-10 p,m) of the size range currentlybeing assessed for comm ercial exploitation. 20 vol.% isclose to the upper limit of the reinforcement volume fractionwhich can be incorporated by the powder route. For highervolume fractions, a casting route has been used, and theliquid pressure infiltration metho d allowed compositeswith mixtures o f reinforcement types (hybrids), andthose with greater than 20 vol.% reinforcement to be manu-factured.

On metallograp hic examination, all the pin materials (Ato E) showed some defects. The former powder surfaces inAA60 61 were outlined by small particles, p robably oxides.Microporosity was observed to a small degree in all thecomp osites. In the LPI comp osites, particularly material D ,some areas of segregation of the reinforcements werenoted and in the 60 vol.% SIC MM C, microporosity betweengroups of particulates occurred . Howe ver, a fairlyhomo geneou s distribution of SIC was observed. MaterialsB and D showed evidence of fragmentation of Saffilwhiskers.


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e)Fig, 4. SEM micrographs: (a) AA6061 alloy; (b) AA6061 +20 Saffil; (c) AA6061 +20 Sic,; (d)materials sliding against BS817M40 steel.

7.2. HardnessThe almost identical h ardness data (Table 1) obtained

after a T6 heat treatment for materials A, B and C was unex-pected . This is in contrast to the results of others. For exampleWang and Hutchings [9] found a linear increase in micro-hardness (500 g) with increasing Saffil volume fraction inan AA6061 composite manufactured by liquid metal squeezeinfiltration and in the T4 condition. Derby and Walker [ 16 ]also observed increases in hardness with increase of volumefraction of Sic from 5 to 30 vol.% after quenching fromabove 2 50 C. Howe ver, there have been several studies inwhich little difference in the T6 hardness has been recordedbetween AA6061 alloy and AA6061 containing 20 vol.%Saffil [ 17-21 1. This observation is now recognised as havingits origin in the chemical reaction between Saffil and mag-nesium, which is removed from the alloy matrix to form afine-scale reaction p roduct. Th erefore the magnesiumavailable to precipitate as B-M g,Si and age-harden the mate-rial is reduced . Initially it appe ared that the situation wasconfined to comp osites manufactured by a liquid metal route,but a similar situation has been recorde d in some liquid p hasesintered composites [ 221.

hybrid composite; (e) AA6061 +60 Sic,. All

7.3. Abrasive wearIt can been seen from th e data on the abrasive wear of the

materials against SIC grit, Fig. 1 that the AA60 61 alloy,(material A) and material B appe ar to fall in one group, whilematerials C, D and E, with a similar wear resistance form asecond group. Ho wever, materials A, B and C have, withinexperimental error, identical hardness values (Table 1).Thus there would ap pear to be no simple correlation betweenthe hardness of these materials and their abrasive wear resis-tance, an observation which h as also been reported in previ-ous work on aluminium MM Cs [9] although others havereported a linear increase in abrasive wear resistance withhardness [ 51.

Extensive plastic grooving and ploughing has been widelyobserved for AA6061 Al matrix alloy when abraded with SICgrit, for examp le Refs. [ 91, [ 231, and the extent of this typeof dama ge increases with decreasing SIC grit size [ 91. Wangand Hutchings [ 93 observed for pin-on-disc tests against 600Sic grit that with AA6061 10 vol.% Saffil, the worn surfacesshowed evidence that both the matrix and the fibres deformedplastically during we ar and that the Saffil reinforcement hada beneficial effect in reducing wear. While less evidence of


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190 A.B. Gurcan. T.N. Baker/ Wear I88 1995) 185-191

grooving was seen on material B, the surface contained fea-tures similar to previous work [ 91, such as unfractured fibres,but also fractured fibres together with regions of plastic defor-mation, are present.

Zongyi et al. [ 23) have compared, also with a pin on disctester, the abrasive wear behaviour of a commercial AA6061alloy with AA60 61 compo sites containing discontinuous SICas whiske rs, particulates (3.5 pm mean size) and fibres, fab-ricated by a powder metallurgy route, against P400 and 600Sic adhesively bonded paper. They suggested that the reasonfor the higher wear rate found with AA6061 Al compositescontaining Sic, than with those containing Sic, is that theceramic in the former has a greater tendency to agglom erate,and in these regions is readily worn by the SIC grit. In contrastto these observations, Fig. 3(c) show s that for material C, inregions such as B, craters due to the disintegration of the wornsurface into angular sections, lo-20 p,m in size, are foundtogether w ith grooves separated by broader widths ofdeform ed material. This surface had a significantly lowerwear rate than material B, in agreemen t with the conclusionsof Zongyi et al. [ 23 J

The compo sites they studied were also in the T6 condition,and contrary to work on other aluminium alloys, such asAA7005 [ 251, it was found that the peak aged condition ofthe AA6061 Sic, gave the lowest w ear rate. The peak hard-ness of this composite was 135 Hv compared with 150 Hvfor material C. The smaller size of particulate and lower pe akhardness for their composite may account for the higher wearrate found [ 231 compared with the present w ork.7.4. Dry sliding against BS817A 44O E24) steel

Fig. 2 show s that, as in the testing against SIC grit, thematerials slid against the steel discs fall into two groups. T hedata in Table 1 indicates that materials D and E have signif-icantly lower ste ady state wear rates than materials A, B andC. Thus the presence of 11 vol.% Saffil incorporated fromthe preform used in the liquid pressure infiltration proces stogether with 20 vol.% SIC (material D) h as the effect ofconferring a significant reduction in wear comp ared withAA6061 composites with 20 vol.% additions of SIC alone or20 vol.% Saffil alone, when tested against the steel disc.

Long et al. [24] have investigated the wear, by a pin onreciprocating steel (SKH-5 1) plate imme rsed in an ethanolbath at room temperature under a load of 49 N, of AA6061alloy reinforced with a hybrid of Saffil fibres and Sic whisk-ers, produced by a PM route, hot pressed and heat treated toT6 condition. Using somewh at different test conditions, theyfound a similar result to that of the present work, where theaddition of SIC to Saffil/6061 comp osites gave rise to aremarkable improvement in the wear resistance comp aredwith that of SaffiV6061 or Sic/6061 composites. The greaterwear of the Saffil/6061 composite was considered to be dueto some o f the Saffil fibres, wh ich were originally dispersedrandomly, being displaced parallel to the wear surface inwhich they formed deep grooves. In the hybrid, the SIC

reduced considerably the change in orientation of the Saffilfibres, resulting in less wear. A close examination of materialB in the SEM show s fibrous features lying on the worn su rfaceof the pin, as seen in Fig. 4(b), which tentatively suppo rtsthe explanation of Long et al [24] for the superior wearresistance of a Saffil/SiC hybrid 606 1 comp osite. Anotherappro ach to understanding the superior wear resistance of thehybrid, material D is through a consideration of the classifi-cation produc ed by Zhang and Alpas [ lo]. The presentresults given in Table 1 and Fig. 2, imply that the additionof Saffil ( 11 vol.%) to a 20 vol.% A A6061 SIC hybrid resultsin this compo site being retained in the Zhang and Alpasregime I, a low wea r rate region (as is material E , whereasmaterials A, B and C are, even under a load of 9.8 N, in thefaster wear region, regime II. While the details of the metho dsof testing in the present case and that of Zhang and Alpasdiffer, as does the reinforcement, the general appro ach issimilar and the order o f the wear rates in both studies com-parable.

The high wear resistance in regime I was attributed [ 10,251to the load bearing capacity of the hard reinforcement. Duringtesting, the expo sed portions of the reinforcements are con-sidered to create a local milling action on the steel counterfacewhich leads to detach ed steel fragments being transferred onto the surfaces of the comp osite and forming transfer layersrich in iron and iron oxide. In the present work , scanningelectron microscopy and energy dispersive analysis obser-vations on materials D and E show ed evidence for the for-mation of a surface film and considerable pick-up of iron.The iron rich film has low coefficient of friction which con-tributes to the high we ar resistance of the compo sitematerialsat low loads, such as the 9.8 N used in this wo rk [lo]. Inregime II, the wear mechanisms of the composites were con-sidered to involve fracture and fragmentation of reinforce-ment, resulting in the comp osite matrix coming into directcontact with the counterface. This situation in turn produ cesa significantly higher rate of wear, and may be aggrav ated bythe pull-out of the reinforcements, leading to three body abra-sions. The presence of cavities, probably due to particulateremoval, can be observed in Fig. 4(c) for examp le. The freeparticulate could ac t as third body abrad ers and be mainlyresponsible for the higher wear rate of material C comparedwith D and E.

8. onclusions1. Pin-on-disc wear tests on AA6 061 comp osites containing

Saffil, Sic, or mixtures of both ceramics against P400 SICgrit and BS8 17M 40 (EN24 ) steel counterfaces, showedthat comp osites containing only Saffil had inferior w earresistance to those containing the same volume fractionof SIC,.

2. A small increase in the wear resistance over 20% Sic,AA6061 composites was found for a 11 Saffil + 20%Sic, hybrid compo site when tested against SIC grit.


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The greatest w ear resistance was observed in the compos-ite containing 60% Sic,, which had a wear rate over fivetimes lower than the 20% Sic, composite when testedagainst SIC grit and over thirty times lo wer when te stedagainst on the steel.The wear resistance conferred by the combination of Sicand Saffil in material D, when tested ag ainst the steel wassignificantly superior to the corresponding wear resistanceof either materials B or C. The weight loss of material Dapproached that of the 60% Sic composite. It is worthnoting tha t while the manufacturing route used for thehybrid in this work was a liquid route, th e volume fractionof the ceramic of 30% is just within the limit of manufac-ture by a pow der metallurgy route.

References

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[2] F.M. Hosking, F. Folgar-Portillo, R. Wunderlin and R. Mehrabian,Composites of aluminium alloys: Fabrication and wear behaviour, J.Mater. Sci., 17 ( 1982) 477498.

[3] K. Anand and Kishore, On the wear of aluminium-corundumcomposites, Wear, 85 (1983) 163-169.

[4] S.V. Prasad and P.K. Rohatgi, Tribological properties of Al alloyparticle composites, J. Met, 39 (1987) 22-26.

[5] A. Banejee, S.V. Prasad, M.K. Surappa and P.K. Rohatgi, Abrasivewear of cast aluminium alloy-zircon particle composites, Wear, 82(1982). 141-151.

[6] S.V. Prasad, P.K. Rohatgi and T.H. Kosel, Mechanisms of materialremoval during low stress and high stress abrasion of aluminiumalloys-zircon particle composites, Mater. Sci. Eng., 80 ( 1986) 213-220.

[7] K.J. Bransali and R. Mehrabian, Abrasive wear of aluminium matrixcomposites, J. Met., 32 (1982) 30-34.

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Wear Behaviour of AA6061 Aluminium Alloy and Its Composites

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