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Wear 271 (2011) 18281832

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Evaluation ofthe tribological properties ofan AlMgSi alloyprocessed by severe plastic deformation

E. Ortiz-Cuellar, M.A.L. Hernandez-Rodriguez, E. Garca-Sanchez

Universidad Autnoma de Nuevo Len, Facultad de IngenieraMecnica y Elctrica, Mexico

a r t i c l e i n f o

Article history:

Received 2 September 2010

Received in revised form26 December 2010

Accepted 29 December 2010

Keywords:

ECAP

Severe plastic deformation

Wear, Al6XXX

a b s t r a c t

Aluminum alloys are widely used as structural materials in various types of applications due to light

weight, excellent strength, corrosion resistance, formability. However, problems may arise in those appli-

cations that require high wear resistance. With the aim to study the effect ofthe severe plastic deformation

on the tribological properties in an aluminum alloy; in this work a commercial AlMgSi alloy under two

initial microstructural conditions has been deformed at room temperature by multi-pass 90 equal chan-

nel angular pressing (ECAP). After the processing each sample was evaluated by means ofmicrohardness

measurements and the microstructural condition was determinate by electron microscopy.

Subsequently sliding wear test was performed in a ball on disk configuration under lubricated condi-

tions.

Itwas found a wear resistance increase with the rise in the deformation promoted by number ofextru-

sion passes and the initial microstructural condition. The dominant wear mechanisms were identified

and correlated with mechanical properties, microstructure and the level ofdeformation.

2011 Elsevier B.V. All rights reserved.

1. Introduction

Nanostructured materials processed by severe plastic deforma-

tion (SPD) are porosity free, contamination-free, and sufficiently

large for use in real commercial structural applications like:

aerospace, transportation, sport and construction [1,2]. These

materials can exhibit high strength, good ductility, superior super-

plasticity, and low coefficient of friction; high wear resistance

and improved fatigue life in high cycles [3]. The SPD processes

are excluded from conventional forming operations like: uniaxial

tension, compression, unidirectional extrusion and lamination or

pressing [4].

Numerous studies have shown that some types of SPD cause a

decrease in the grains sizes and also affects particles and precip-

itates into the material, some SPD methods promotes formation

of a disperse and more homogeneous structure. Indeed, promisingmethods for increasing the strengthand plasticityby the creationof

submicron and nanocrystalline structures in the material by severe

plastic deformation are; equal-channel angular pressing (ECAP),

accumulative roll bonding (ARB)and shear deformation under high

pressure torsion (HPT) [35].

Corresponding author. Tel.: +52 81 14920378x1624; fax: +52 81 10523321.

E-mail addresses: ogarcia@gama.fime.uanl.mx, egs7710@gmail.com

(E. Garca-Sanchez).

The strength and hardness of aluminum alloys can increase due

to segregation of fine particles from supersaturated solid solution.Thus, aluminum alloys could have a structure favorable for wear

resistance; hard second phase particles are distributed in a relative

soft matrix [6,7]. Recent works have shown that a composite struc-

ture consisting of hard particles distributed in a relatively plastic

matrix is preferred for the improvements of wear resistance. The

wear process is affected not only by the amount and uniformity

on distribution of particles of the second phase over the volume

but also by the strength of the particle/matrix boundary and the

mechanical proprieties of the matrix [6,8].

There are several mechanisms of wear which include seizure,

melting, oxidation, adhesion, abrasion, delaminating, fatigue, fret-

ting, corrosion, and erosion. Wear can be reduced normally by

using a lubricant with appropriate anti-wear additives, changing

the materials and/or the operating parameters affecting the wearrate [7].

Due to the technological importance of wear [9] and the lack

of information concerning to these phenomena in materials pro-

cessed by SPD, the topic of theevaluation of thewear properties on

an aluminum alloy processed by the combination of ECAP and heat

treatments in this work is justified.

2. Experimental procedure

In order to evaluate the effect of the initial microstructure on

the tribological properties, two set of samples in form of bars

0043-1648/$ seefront matter 2011 Elsevier B.V. All rights reserved.

doi:10.1016/j.wear.2010.12.082

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Fig. 1. (a) Diagram of thethermal and mechanical procedure, (b) samples cut along longitudinal plane in theinternal section foranalysis.

(12mm12mm40mm) of commercial Al6060 alloy in two dif-

ferent conditions (C1 and C2) was submitted to ECAP followed by

an aging heat treatments (Fig. 1a). The C1 condition corresponds

to T6 aging heat treatment and C2 solution heat treatment, (530C

by 2h water quenched) [10,11]. ECAP was performed using a 90

channels intersection die, from 1 to 5 passes at room temperature

and graphite as lubricant using Bc route, which is one of the most

effective route to obtain refinement of the microstructure in sev-

eral materials [35]. The aging heat treatment selected was 170 C

for 60min and cooling air [12].AfterECAP andagingheat treatment thesampleswere sectioned

longitudinally (Fig. 1b), Microhardness profiles were performed on

the internal plane according to the ASTM E384 using a load of

200 g by 15s. In order to analyze the initial conditions and refined

microstructure after ECAP process; optical and transmission elec-

tron microscopy was undertaken in a JEOL model JEM1020. Forthis

purpose the samples were mechanical polished until 0.150.2mm

in thickness and then electropolished with a 75% methanol, 25%

HNO3solution and 25 V by 510min at room temperature.

Sliding wear tests were performed on ball-on-disc tribometer.

The discs were the samples processed by different number ECAP

passes and the pin was a steel ball according to the ASTM G99

standard. The parameters used for tribological test were: speed of

264 RPM, load of 12.5N, a distance of 500m and distilled wateras lubricant. Prior to wear test, the samples were polished using

a set of SiC grids until mirror like shape. Gravimetric method was

used to assess the mass lost. Examinations of the wear scars were

undertaken by means of scanning electron microscopy (SEM).

3. Results and discussions

Fig. 2 presents microhardness profiles in C1 and C2 conditions,

is observed that C1, without solution heat treatment has a high

increment in the microhardness level after the first ECAP pass, and

slightlytendency to decrease in subsequent passes. In C2 condition

is observed an increase in the microhardness superior than C1, and

these values in C2 are maintained in subsequent number of extru-

sion passes. The stabilization of the hardness with the increase in

the deformation has been associated with the dislocation satura-

tion after three passesin ECAP and with the level of microstructural

refinement possible [13,14].

Fig. 3 presents TEM evidence of the microstructural condition

after four passes in ECAP previous to ball on disc test, the grain

size was estimated near to 200nm, is possible to observe some

grains withhigh density of deformation accumulated, also is exhib-

ited the tendency to form equiaxed microstructure, in this figure

there is not clear evidence of second phase particles, however isexpected a homogenous distribution of these, due to the solution

heat treatment and the aging post-ECAP [12,21].

Sliding wear resistance in samples processed by ECAP is

observed in Fig. 4. The condition C2 presented an improvement

in wear resistance compared with C1, it could be associated with

the hardness values reported, which in the condition C1 for all

the deformation levels remains below of C2. For both conditions

the increase of the deformation level, promote an improvement

Fig. 2. Microhardness profiles in samples processed by ECAP.

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1830 E. Ortiz-Cuellar et al./ Wear271 (2011) 18281832

Fig. 3. Microstructure:Al6060 alloy, (a) condition T6 aging heat treatment(grain size47m), (b) solution heattreatment (grainsize 60m), (c)afterfour passesin ECAP

for C2 condition(grainsize 0.2m), TEM.

in the wear resistance, related with the combined effect of the

microstructural refinement and the second phase particles distri-

bution [12,1520]. Fig.5 showsthe frictioncoefficient forC1, C2 and

a sample without ECAP as reference, it is observed that the values

in the two conditions are smaller than obtained in Al6060 without

processing by ECAP, it was attributed to high adhesion achieved

between aluminum films on thesteel ball and the sampledisc. Also

was noted that values of friction coefficient for C2 are smaller and

homogenous than C1. This behavior of friction between C1 and C2coefficients depends mainly of the distribution and concentration

of second phase particlesin the microstructure [21]. The processing

by ECAP and heat treatments makes a positive effect in tribological

properties (sliding wear and friction coefficient) due to increase in

the grain boundaries density associated with significant grain size

reduction and the re-distribution of second phase particles during

heat treatments [22]. It is important to note that after the 4th pass

the wear response wasalike forbothconditions. It can be attributed

Fig. 4. Lose weights in sliding wear for samples deformed in different number of

passesby ECAP.

Fig. 5. Friction coefficients forECAP from 1 to 5 passes, (a)C1 and reference, (b)C2

condition.

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Fig. 6. (a,b) Sliding wear inAl6060alloy conditionT6 aging heat treatment, (c,d) sliding wear in Al6060processed by ECAP onepassesin conditionC1, (e,f) Al6060processed

by ECAP one passes in condition C2, (g, h)ECAPfive passes in condition C1, (i, j) ECAP five passesin condition C2.

to the similar grain refinement achieved as a result of cumulative

deformation.

Representative SEM images of wear surface for all condi-

tions are presented in Fig. 6; the wear surfaces of the reference

Al6060 showed adhesion mechanism on some zones and fatigue

mechanism by delamination due to the interaction between the

ball and the material surface. It can be observed in agreement

with the scale (see arrows Fig. 6a, e, i) that the width of the

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track decrease as increased the number of extrusion passes. The

fatigue wear mechanism by delamination was present for all

conditions (Fig. 6). However it was possible to observe that in

fine microstructures the high density of grain boundaries pro-

moted less surface damage leading smaller laminateddebris, minor

width wear track and less wear. The de-bonding of big second

phase particles is observed in the reference sample (Fig. 6b);

this can be explained by the fracture and detachment of big sec-

ond phase particles. This phenomenon was minor on the samples

after the ECAP processing where the second phase particles were

re-distributed.

4. Conclusions

Room temperature equal channel angular pressing can produce

materials with an improved wear resistance that depends mainly

of the level of strain produced in each extrusion pass. From one to

four passes, the previous solution heat treatment had an impor-

tant effect in mechanical and tribological properties due to the

redistributions of second phase particles achieved during the aged

treatment. After five passes the tribological behavior was similar

for condition C1 and C2 but its wear resistance increased near to

100% regarding to sample reference.

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