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Wear 262 (2007) 292–300

Dry sliding wear behaviour of aluminium–lithium alloysreinforced with SiC particles

J. Rodrıguez ∗, P. Poza, M.A. Garrido, A. RicoDepartamento de Ciencia e Ingenierıa de Materiales, Universidad Rey Juan Carlos, Tulipan s/n, E28933 Mostoles, Madrid, Spain

Received 9 September 2005; received in revised form 7 April 2006; accepted 11 May 2006Available online 27 June 2006

bstract

Several wear tests were carried out at different pressures and temperatures on Al-8090 and Al-8090 + 15 vol.% SiCp. Worn specimens and debrisere also examined using SEM and EDX techniques to identify the dominant wear mechanisms. Wear rate increases about two orders of magnitudehen temperature is above a critical one. The transition from mild to severe wear is dependent on nominal pressure. The composite transition

emperature is higher than that of the unreinforced alloy. Within the mild wear regime, the wear rates for both materials exhibit a minimum over

00 ◦C and are higher for the composite material than for the Al-8090 below the transition temperature. It has been also observed that the presencef mechanically mixed layers (MML) on the wear surface with varying morphology and thickness influenced the wear rate. The morphology andomposition of the wear debris also change with the wear mechanism.

2006 Elsevier B.V. All rights reserved.

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eywords: Sliding wear; Metal–matrix composites (MMCs); Mechanically mi

. Introduction

The dispersion of ceramic particles in aluminium alloys leadso considerable improvements in many mechanical properties.enefits have been reported in stiffness, temperature perfor-ance and wear resistance, while other aspects mainly related

o ductility and fatigue behaviour remain controversial [1,2].he combination of cost and performance makes these discon-

inuously reinforced aluminium matrix composites (DRAMC)ppealing for components in the automotive and aerospacendustries.

Under operating conditions where the contact between solidss expected, the tribological behaviour may become criti-al. Although in many circumstances lubrication is employed,ry sliding can be considered as the limit case. It has beenointed out [2] that wear resistance is not a material propertynd, consequently, the wear performance of DRAMC should

e evaluated considering all the contributing factors (load,peed, temperature, materials involved, environment, geometry,tc.).

∗ Corresponding author. Tel.: +34 914887159; fax: +34 914888150.E-mail address: jesus.rodriguez.perez@urjc.es (J. Rodrıguez).

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043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2006.05.006

yer (MML); Temperature effect

The tribological behaviour of aluminium alloys in dry slid-ng against steel counterfaces has been widely studied. The wear

aps methodology developed by Lim and Ashby [3] has beenuccessfully applied to aluminium alloys identifying severalominant mechanisms depending on load, speed and temper-ture [4]. At low loads and speeds, the mild wear regime isssociated with an oxidational process. The formation of tribo-ayers seems to be crucial in this regime, although the classicalear models neglect it. As the load is increased, a transition to a

evere wear regime is observed and the operating wear mecha-isms changes to delamination with severe plastic deformation.

DRAMC behave quite similar, but the presence of the rein-orcement particles can alter the range where the differentechanisms become dominant. In spite of the great number of

apers published treating the wear behaviour of DRAMC, gen-ral trends are difficult to identify. In fact, the matrix compositionas a critical effect. Nevertheless, it is generally accepted thatithin the mild wear regime, the role of the reinforcement par-

icles is to support the contact stresses preventing high plasticeformations. If the load is increased over a critical value, the

articles will be fractured and comminuted, loosing their role asoad supporters. In the severe wear regime, composites wear rateakes values similar or even worse than those of the unreinforcedlloys.

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J. Rodrıguez et al. / W

Unlike load and speed, studies regarding the influence of theemperature are not common. Several studies showed that the

ild to severe wear transition occurred at higher temperaturesn the composites [5,6].

It is remarkable that, although it is not usually reported, nomprovement is observed after adding reinforcement particles ineveral cases. For example, Alpas and Zhang studied the effectf particle size concluding that wear rates of composites exhibitimilar wear resistance to that of the unreinforced alloys whenhe particle size is of the order of surface roughness [7]. Anotheregative effect may be the abrasive action of the hard reinforce-ent phase on the counterbody wear.In the last 15 years, most of the traditional aluminium alloys,

uch as 2000, 5000, 6000 and 7000 have been studied fromhe tribological point of view [1]. To the authors’ knowledge, noesults are available about wear behaviour of aluminium–lithiumlloys and DRAMC using these alloys as matrices.

The presence of lithium increases the elastic modulus of alu-inium alloys by approximately 6% for every 1 wt.% of lithium

nd the density is reduced by 3%, which improves significantlyhe specific modulus [8]. The addition of ceramic reinforcementsrovides further improvements in this direction and Al/Li–SiComposites exhibit elastic modulus over 100 GPa with relativeensities around 2.6 leading to specific stiffness 50% abovehat of standard aluminium and titanium alloys [9,10]. In addi-ion, the presence of lithium in the aluminium matrix helpso strengthen these materials through the precipitation of therdered �′ (Al3Li) phase coherent with the matrix [11,12], andhe Al–Li/SiC system has an excellent performance/cost ratioor applications where the stiffness is critical, as in aeronauticalnd aerospace applications.

In this work, a systematic experimental study is performed tonalyse the effect of load and temperature on the wear behaviourf an aluminium–lithium alloy Al-8090 and the same alloyeinforced with 15 vol.% of SiC particulates. The aim is to eval-ate if the tendencies observed in more conventional aluminiumlloys and their composites are also followed in this material.icrostructural and mechanical behaviour have been previously

nd extensively studied, which allows this work to be focusedn the tribological performance.

. Materials

The 8090 Al alloy reinforced with 17 vol.% of SiC particlesnd the unreinforced alloy were supplied by Cospray Inc. (Ban-ury, United Kingdom) in the form of extruded rectangular bars

f 25.4 mm × 62.5 mm cross-section. Composition of the alloynd the composite is given in Table 1. Both materials were pro-uced by spray codeposition of the matrix and the particles ontosubstrate [8]. They were extruded at 420 ◦C into rectangular

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able 1hemical composition of Al-8090 and the composite

aterial Li Cu Mg Zr T

l-8090 2.38 0.99 0.81 0.12 0l–Li/SiC 2.32 1.21 0.82 0.10 0

62 (2007) 292–300 293

ars, with an extrusion ratio of 25:1. The bars were solution heatreated at 530 ◦C for 2 h, water quenched, and cold stretched up to% to relieve the residual stresses introduced during quenching.fterward, they were artificially aged at 170 ◦C, during 32 and8 h for the composite and the unreinforced alloy, respectively,o reach the peak-aged condition (T651). The grain structure ofhe unreinforced alloy was highly anisotropic and grains largerhan several hundreds of micrometers were found in the longi-udinal direction. The average grain sizes in the long and shortransversal directions, perpendiculars to the extrusion axis, were1 and 8 �m, respectively. The ceramic particles homogenizedhe grain dimensions in the composite and the average grain sizef the matrix was 12 �m in the longitudinal direction and 6 �mn the long and short transversal directions. The size of the rein-orcements was 7.5 ± 2.4 �m with an aspect ratio of 2.4 ± 1.2.he particles were oriented with the longer axis in the extru-ion direction. It is worth noting that very few (<4%) brokeniC particles were found and particle–matrix descohesion wasever observed in the as-received material. The main precipitateound in the unreinforced alloy was the metastable and ordered′ (Al3Li) phase, which is coherent with the aluminum matrixnd nucleates homogeneously, with an average diameter around5 nm. In order to refine the grain size, Zr is added to the 8090lloy, leading to the development of �′ (Al3Zr) dispersoid. S′Al2CuMg) phase was also observed. This precipitate, semico-erent with the matrix, has an orthorhombic crystal structure ands observed as needles oriented along the 〈1 0 0〉 directions in the

atrix. The main difference between the composite and the unre-nforced alloy was the development of a �′ precipitate-free zonePFZ), around 200 nm thick, along the matrix–reinforcementnterface. Further details about the microstructure have beenescribed elsewhere [10].

The stress–strain curves of these materials as a function ofemperature have been recently reported [13] showing a degra-ation of the mechanical performance as the temperature isncreased up to 190 ◦C. The elastic modulus of the unreinforcedlloy was found around 80 GPa at room temperature, while theeramic reinforcements increased this value up to 101 GPa forhe composite. The yield stress of both materials was quiteimilar being reported 537 MPa for the unreinforced alloy and07 MPa for the composite at room temperature. The elasticodulus for the unreinforced alloy and the composite at 190 ◦Cere 71 and 91 GPa, respectively and the yield stress of bothaterials at this temperature was around 360 MPa.

. Experimental techniques

Tests were carried out in a wear testing machine with a pin onisc configuration under dry sliding conditions without elimi-ating the debris formed (see Fig. 1). Specimen and counterbody

i Fe Si Al SiC (wt.%)

.023 0.04 0.03 Balance –

.036 0.05 0.06 Balance 17.3

294 J. Rodrıguez et al. / Wear 2

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Fig. 1. Schematic view of the pin-on-disc test configuration.

ere cleaned using methanol to avoid the presence of humiditynd other non-desirable films such as grease. Most requirementsf the ASTM standard G99-04 were followed. Nevertheless,everal modifications were introduced, mainly regarding thein shape. Prismatic pins were made of the material undertudy (Al-8090 and its composite) with rectangular section of.5 mm × 6.3 mm. With this geometry, the nominal contact areaas maintained constant during the tests in spite of the wearrocess. The disc, made of carbon steel (SAE 1045), rotatesorizontally at sliding speed of 0.1 m/s. A dead weight loadingystem was used to perform the tests at nominal normal pressuresf 6.3, 12.5, 16.5 and 50 MPa. The bulk test temperature was alsoodified from 20 to 300 ◦C for the Al-8090 alloy and from 20 to

50 ◦C for the composite material. Different temperatures werepplied by means of a furnace designed to accommodate theain elements of the test: the pin and the disc. The temperatureas measured with a thermocouple located at the centre of thein support, providing, thus, a bulk temperature measurement.he furnace was programmed to ensure that a thermal steadytate was reached before the beginning of the wear test (±5 ◦C).he coefficient of friction was obtained by means of a torque

ransducer. The variation of the pin height was registered using

LVDT with ±1 �m of precision. The wear rate (�m/m) was

alculated as the slope of the sample height versus sliding dis-ance. Both, friction coefficient and wear rate were continuouslyecorded during the test.

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Fig. 2. Relative wear rates of the unreinforced alloy (a

62 (2007) 292–300

Finally, worn specimens were cross-sectioned using a dia-ond saw. To analyse the microstructure changes in the nearest

one at the pin contact surface, samples were prepared withonventional metallographic techniques using SiC papers up to000 grit finish, followed by polishing in a diamond slurry (upo 1 �m) and, finally, on SiO2. These metallographic samplesere observed in an Environmental Scanning Electron Micro-

cope Philips XL 30 (ESEM) equipped with energy dispersive-rays microanalysis (EDX). To contribute to the wear mecha-isms identification, debris formed was also analysed by ESEMnd EDX.

Additionally, nanoindentation tests were carried out in someross-sectioned specimens to evaluate the extent of plastic defor-ation in both materials. A MTS XP nanoindenter was used to

pply a maximum load of 100 mN.

. Results

.1. Wear rate and friction coefficient

The relative wear rates versus temperature are shown at dif-erent pressures in Fig. 2a and b using a logarithmic scale. Thealue of the wear rate is normalised by the wear rate measuredt room temperature to better capture the tendency.

As it can be appreciated in the figures, wear rate increasedbout two orders of magnitude when temperature is above aritical value, indicating a change in the mechanism from mildo severe wear. Although this effect is observed regardless ofhe pressure applied, the transition temperature is dependent ont. As it can also be seen in Fig. 2a and b, this temperature wasigher for the composite material than for the unreinforced alloy,l-8090. On the other hand, it has been observed that in bothaterials, the higher the normal pressure, the lower the transition

emperature. Furthermore, for the lower nominal pressures (6.3nd 12.5 MPa), there was an initial decrease in wear rate with

emperature, taking minimum values at 100 ◦C. In fact, wear rateid not recover the room temperature values up to 200 ◦C.

Although the relative representation included in the last fig-res may be convenient to make the results easy to analyse, it

) and the composite material (b) vs. temperature.

J. Rodrıguez et al. / Wear 262 (2007) 292–300 295

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Fig. 3. Wear rates and friction coefficients of

oes not allow a direct comparison between both materials. Thebsolute wear rate, determined from the height loss of the pin ishown in the upper part of Fig. 3. As it can be seen, the unrein-orced alloy exhibited higher wear resistance than the compositeaterial below the transition temperature. However, the begin-

ing of the severe wear regime is shifted to higher temperaturesn the composite.

Additionally, in the lower part of Fig. 3, the average fric-ion coefficients are included. The composite material presentedigher values than those of the unreinforced alloy. An exceptiono this trend seems to appear when both materials are subjectedo very demanding conditions, that is, within the severe wearegime.

.2. Surface morphology

Figs. 4 and 5 show cross-sections of the specimens observedy ESEM, corresponding to the aluminium alloy and the com-osite, respectively. The nearest zone of the surface presented aifferent microstructure than both the wearing material and the

teel counterbody. It seems to be a mechanically mixed layerMML) composed of debris particles, probably fractured andomminuted, coming from both sides of the contact. The pres-nce of oxygen is also detected indicating oxidational processes.

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atrix and reinforced material vs. temperature.

he MML is also characterized by a high density of defects andicrocracks.As an example, Figs. 4 and 5 show the different morpholo-

ies observed in the MML generated under conditions clearlyorresponding to mild wear regime: 20, 100 and 200 ◦C for aormal pressure of 6.3 MPa (the lowest used in this work).

The MML exhibits significant differences as the temperaturencreases from room to higher values. At 20 ◦C and 6.3 MPaFig. 4a) the layer was discontinuous, without covering thehole surface. The maximum thickness was over 30 �m. When

emperature was increased to 100 ◦C and the pressure was main-ained at 6.3 MPa (Fig. 4b), the MML covered completely theear surface, forming a continuous and protective layer. How-

ver, the thickness was not regular, ranging from 40 �m, asaximum value, to around 5 �m. Finally, subsequent increases

n temperature (Fig. 4c) provided a MML with more regularhickness, but with the same maximum values of 40 �m.

The observations performed in the composite materialFig. 5) resulted in similar conclusions as the unreinforcedlloy, but with thinner layers. MML measurements should be

aken into account only as qualitative information because of thexperimental difficulties associated with their obtaining. Smallhanges on the surfaces during the specimen preparation couldodify the experimental data.

296 J. Rodrıguez et al. / Wear 262 (2007) 292–300

Fig. 4. SEM micrographs showing the MML formed on the contact surface ofthe unreinforced alloy: (a) 20 ◦C, 6.3 MPa; (b) 100 ◦C, 6.3 MPa; (c) 200 ◦C,6

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Some examples of the morphology of the composite debrisparticles are shown in Fig. 7a (20 ◦C and 6.3 MPa) and

.3 MPa.

.3. Wear debris

SEM and EDX analyses of the wear debris were performedo identify its morphology and composition. As we have pointedut before, there are very marked differences in wear rate withinhe mild and severe wear regimes.

Figs. 6 and 7 show some micrographs of the debris obtained

uring wear tests of the unreinforced alloy and the composite,espectively.

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ig. 5. SEM micrographs showing the MML formed on the contact surface ofhe composite: (a) 20 ◦C, 6.3 MPa; (b) 100 ◦C, 6.3 MPa; (c) 200 ◦C, 6.3 MPa.

Fig. 6a (Al-8090 tested at 20 ◦C and 6.3 MPa) shows aimodal size distribution of fine particles and larger plate-likeebris with sizes up to 200 �m. In tests carried out under severeonditions, 200 ◦C and 16.5 MPa (Fig. 6c), large plate-like debrisecome dominant. These particles, with sizes beyond the mm,xhibited marked signals of plastic deformation as wear surfacerooves and edge cracks.

ig. 7c (350 ◦C and 6.3 MPa). The situation is similar to thatointed out in Fig. 6 in both cases. The wear debris morphol-

J. Rodrıguez et al. / Wear 262 (2007) 292–300 297

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ig. 6. (a) Wear debris from Al-8090 tested under mild wear conditions (20 ◦nder severe wear conditions (200 ◦C, 16.5 MPa); (d) corresponding EDS analy

gy was consistent with the wear rates measured during theests.

From EDX analysis of the particles (see Figs. 6b and d and7b and d), it can be derived that the debris under mild wear con-itions was composed of aluminium and iron in the case of the

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ig. 7. (a) Wear debris from Al–Li/SiC tested under mild wear conditions (20 ◦C, 6.3nder severe wear conditions (350 ◦C, 6.3 MPa); (d) corresponding EDS analysis.

MPa); (b) corresponding EDS analysis; (c) wear debris from Al-8090 tested

nreinforced alloy. However, the large particles associated withhe severe wear regime were composed mainly of aluminium. In

he case of the composite, the results were very similar, exceptor the presence of broken silicon carbide particles coming fromhe sub-surface zone.

MPa); (b) corresponding EDS analysis; (c) wear debris from Al–Li/SiC tested

2 ear 262 (2007) 292–300

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Fig. 8. (a) Al-8090 tested under mild wear conditions (200 ◦C and 6.3 MPa). Itis showed the three zones observed: mechanically mixed layer (MML), hard-ened zone (HZ) and not hardened zone (NHZ) (b) Al–Li/SiC tested under mildwm(

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98 J. Rodrıguez et al. / W

. Discussion

According to the methodology developed by Lim and Ashby3], the variables controlling wear mechanism are the normalisedressure, F , and the normalised velocity, v, defined as

¯ = F

AnHand v = vr

d

here F is the normal force, An, the nominal contact area, H theaterial hardness, v the sliding velocity, d, the thermal diffusiv-

ty and r is the radius of the circular nominal contact area. In theests performed at room temperature, normalised pressures areower than 3 × 10−2 and normalised velocities lower than 2.4.hese values are associated with mild wear, but another fac-

ors like temperature softening or surface roughness may alterhe conditions, placing the high temperature tests in the severeegime. The measured wear rates confirm this situation.

As it has been pointed out in Section 4, variables that seem toontrol the transition from mild to severe wear are the load andhe bulk temperature. A recent work on the mechanical proper-ies of these aluminium lithium alloys reports deformation andailure mechanisms depending on temperature [13]. Actually,he ratio between the severity of the contact (pressure, frictionoefficient and velocity) and the material resistance (hardness athe test temperature) controls the wear behaviour. This is con-rmed by the experiments which show a clear dependency of

he transition temperature with the normal pressure. Zhang andlpas [14] showed that while the reinforcement particles support

he loads, the wear rate is maintained at lower values. Mechanicalredictions have been carried out in different metal matrix com-osites from different approaches: uniaxial models and finitelement calculations [15]. In spite of the differences, the sameonclusion can be established from the qualitative point of view:f the reinforcement particles fracture, the wear rate is consider-bly increased. This situation is also confirmed in this work forhe Al-8090 + 15 vol.% SiC composite. The presence of the SiCarticles provides a higher thermal stability and, consequently,he transition temperature from mild to severe wear increases

ore than 50 ◦C in the composite. Within the mild wear regime,he reinforcement particles constraint the extent of plastic defor-

ation. In Fig. 8a (Al-8090 tested at 200 ◦C and 6.3 MPa) andig. 8b (composite tested at 200 ◦C and 6.3 MPa), hardness mea-urements obtained from nanoindentation show hardening effectp to higher distance from the worn surface in the unreinforcedlloy.

Wear debris is another relevant aspect to be considered. Atow pressures, it is mainly equiaxed with a composition mixedrom the wearing material and the counterbody. When the loads increased, the dominant wear mechanism becomes to delam-nation and severe plastic deformation. With the morphology ofhe wear debris collected there is not doubt about this question,ecause the large wear particles show evident signal of grosslastic deformation (see Figs. 6c and 7c).

On the other hand, the results obtained revealed thatnder conditions of mild wear, the wear rate is minimum atemperatures around 100 ◦C. Initially, the wear rate decreasesith temperature, but once the minimum is reached, subsequent

abwo

ear conditions (200 ◦C and 6.3 MPa). It is showed the three zones observed:echanically mixed layer (MML), hardened zone (HZ) and not hardened zone

NHZ).

ncrements are observed up to the transition temperature fromild to severe wear. This behaviour has been previously

bserved in other materials [14,16]. This type of variationhould be due to a simultaneous effect of two competitivehenomena. The very well known material softening at ele-ated temperatures should be compensated by another effectddressed in the opposite direction. It is well established thatithin the mild wear regime, wear rate of aluminium and its

omposites is controlled by the formation of mechanicallyixed layers rather than by the bulk strength of the material.nfortunately, the actual role played by the MML is still under

ontroversy. Some authors have indicated that the MML is

protective layer [17], while others have not observed any

enefit. Ghazali et al. have recently published a systematicork focused on the effect of the aluminium alloy compositionn the formation and resulting properties of the MML [18].

J. Rodrıguez et al. / Wear 262 (2007) 292–300 299

Fig. 9. Micrographs showing: (a) backscattered electron image of the Al-8090 MML (20 ◦C, 6.3 MPa); (b) corresponding EDS analysis of Al-8090 MML.

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ig. 10. Micrographs showing: (a) backscattered electron image of the composi

esults are disappointing because no clear tendencies could bestablished and more fundamental research is needed.

In the particular case of the aluminium lithium alloy studiedn this work, the presence of this MML seems to be beneficial. Noppreciable differences have been observed in the microstructurend composition of the MML of the specimens analysed. Theicrostructure of this layer is not uniform and consists of an

gglomeration of smaller particles with a high level of porosity.hese characteristics indicates that MML has a lubrication effect

ather than to be a hard layer. As an example, Fig. 9 (Al-8090ested at 20 ◦C and 6.3 MPa) and Fig. 10 (Al-8090 + SiCp testedt 100 ◦C and 6.3 MPa) show details of the MML formed in thenreinforced alloy and in the composite, respectively. Measuredriction coefficients also show a minimum at temperatures higherhan 20 ◦C (see Fig. 3).

Finally, there is a remaining question to be resolved: whys the wear rate of the composite higher than that of the unre-nforced alloy in the mild wear regime? Although there is noxperimental proof, a plausible explanation may be related to the

brasive role played by the reinforcement particles when com-ng to the contact surface by the wear process. As Fig. 3 shows,riction coefficient is always higher in the composite than in thenreinforced alloy.

A

c

L (100 ◦C, 6.3 MPa); (b) corresponding EDS analysis of the composite MML.

. Conclusions

Tribological behaviour of Al–Li and Al–Li + 15% SiC com-osite have been experimentally analysed, leading to the follow-ng conclusions:

A temperature dependency transition from mild to severe wearhas been observed for both materials, leading to changesof two orders of magnitude in wear rate. The temper-ature transition exhibits a clear dependency on nominalpressure.The reinforcement benefit is limited to shift the transitiontemperature to higher values. Within the mild wear regime,composite wear rates are even higher than those of the rein-forced alloy.The formation of a mechanically mixed layer seems to be akey factor controlling the mild wear of these materials.

cknowledgement

Authors are indebted to Comunidad de Madrid for the finan-ial support of this work through grant 07N/0013/2002.

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