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Materials Science and Engineering A 527 (2010) 4858–4863

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

ffect of Al2O3 nano-particles on the bond strength in CRB process

oohollah Jamaati ∗, Mohammad Reza Toroghinejadepartment of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

r t i c l e i n f o

rticle history:eceived 19 January 2010eceived in revised form 1 April 2010ccepted 2 April 2010

a b s t r a c t

Layered alloy and composite materials have become an increasingly interesting topic in industrial devel-opment. Cold roll bonding (CRB) process, as a solid phase method of bonding, has been widely used inthe manufacturing of large layered sheets and foils. In this paper, the CRB process characteristics of as-received commercial pure aluminum (AA 1100) strips, such as bond strength and threshold deformationin the presence of nano-sized Al2O3 particles are carried out and compared to that of layered pure alu-minum. The bond strength was evaluated by the peeling test. The results show that the presence of Al O

eywords:luminumluminaold roll bondingond strength

2 3

particles reduces the bond strength of layered strips when compared to layered pure aluminum for thesame deformation reduction applied through CRB process. However an increase in bond strength wasachieved by pre- and post-rolling annealing the layered strips with and without Al2O3. But, it was foundthat the alumina particles have important effect on the pre- and post-rolling annealed strips. Finally, inorder to evaluate the surface conditions of peeled surfaces optical and scanning electron microscopes

were used.

. Introduction

Aluminum based metal matrix composites (AMCs) are promis-ng materials for automotive, aerospace, and other structuralpplications because of their low density, high stiffness and lowear rate [1–4]. Discontinuously reinforced AMCs containing par-

iculates are of particular interest to industrial community owingo their isotropic behavior [5]. Due to its unique service perfor-

ance features, cold roll bonding (CRB) process has also exhibitedrapid growth and development in recent years among compositeaterial technologies for the production of strips [6–10].CRB is a solid phase welding process, where the bonding is estab-

ished by joint plastic deformation of the metals to be bonded.onding is obtained when the surface expansion breaks the oxide

ayers and the roll pressure bonds the surfaces together causing thextrusion of material through any cracks of fractured oxides thatay be present [11–15].CRB is also referred as cold pressure welding by rolling

6–10,16], bonding by cold rolling [17], clad sheet by rolling [18],nd cold roll bonding [11,19]. This process can be used on a largeumber of materials. In addition, materials that cannot be bonded

y traditional fusion, often respond well to CRB process [6]. Inomparison with other methods, CRB is simple and can be easilyutomated. CRB is also considered as one of the promising methodsf bonding materials from foils [20–22].

∗ Corresponding author. Tel.: +98 911 2124023; fax: +98 311 3915726.E-mail address: r.jamaatikenari@ma.iut.ac.ir (R. Jamaati).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2010.04.020

© 2010 Elsevier B.V. All rights reserved.

Many research studies on the parameters governing bondinghave been carried out to understand the complex nature of thebonding mechanisms, and the conditions of the process have beenwell defined. It has been reported that CRB of metals is affectedby various factors, such as, reduction in thickness during rolling[7,10–16,23–30], bonding temperature [31], annealing treatmentafter CRB process [32], rolling speed [29,33] and the presence ofparticles between strips [34,35]. Alizadeh and Padyar [34] haveused two A1050 aluminum strips and TiH2 powder to manufac-ture the closed-cell aluminum foam. They have found that TiH2particles decrease the bond strength between aluminum strips.Also, Lu et al. [35] have used two A6060 aluminum strips and SiO2powder for the fabrication of composite. But, they have found thatSiO2 particles increase the bond strength between aluminum strips.Also, in these papers, the influence of process parameters in rela-tion to the amount of powder and annealing treatment on bondstrength is not evaluated. In any case, there is no research on theeffects of Al2O3 nano-particles on the strength of strip bondingwhen CRB is used. The Al2O3 particles reinforced aluminum matrixexhibits high hardness, superior wear resistance, good thermalstability, high stiffness and high strength at elevated tempera-ture and are promising materials for automotive and aerospace[1,3–5].

The recent expansion of CRB to a range of materials has

prompted the present study on the influence of alumina parti-cles on bonding quality. Also, in this paper the influence of pre-and post-rolling annealing treatments and percentage reductionon bond strength in the presence and absence of alumina particlesare examined.

R. Jamaati, M.R. Toroghinejad / Materials Science and Engineering A 527 (2010) 4858–4863 4859

Fp

2

2

aa(C

2

esoaibitTantito

2

tssGksrltCTa

Fig. 2. Typical plot of the peeling force versus the peel distance.

ig. 1. Schematic illustration showing the principle of CRB in the presence of Al2O3

articles.

. Experimental procedures

.1. Materials

As-received commercial purity aluminum sheets (specificationsre given in Table 1) were cut into 150 mm × 30 mm × 1 mm par-llel to the sheet rolling direction. Nano-sized Al2O3 particles<100 nm) were then uniformly spread between two strips beforeRB process.

.2. Surface preparation

To produce a satisfactory metallurgical bond by CRB process, it isssential to remove any contaminations that may be present on theurfaces of the metals to be joined. These surfaces are composed ofxides, adsorbed ions, greases, humidity, and dust particles. Severaluthors have stated that the best method for surface preparations degreasing followed by scratch brushing with a rotating steelrush [8,36,37]. Therefore, the preparation processes for samples

ncluded degreasing in an acetone bath and then scratch brushinghe surfaces with a stainless steel brush of 0.26 mm wire diameter.he initial surface roughness of the samples was 0.5 �m, which,fter scratch brushing, became to about 4.2 �m in the longitudi-al and transverse rolling direction. It is important not to touchhe cleaned surfaces, because grease or oil on the faying surfacesmpairs the formation of a strong joint. To avoid any oxide forma-ion and interference with bonding, CRB process must be carriedut immediately, after degreasing and scratch brushing.

.3. Cold roll bonding (CRB) process

After surface preparation, the samples were carefully handledo avoid renewed contamination. To prevent the formation of anyurface oxides and formation of a strong bond between layer strips,amples were rolled as soon as surface preparation was complete.enerally, the time between surface preparation and rolling, wasept to less than 120 s. Attention was made to align the two stripurfaces properly, prior to rolling. The CRB experiments were car-ied out with no lubrication, using a laboratory rolling mill, with a

oading capacity of 20 tons. The roll diameters were 125 mm, andhe rolling speed was set at 2 m/min. The schematic illustration ofRB process with the presence of Al2O3 particles is shown in Fig. 1.he high reduction by this process generates a great amount of heatnd bonds the two surfaces together. For the prepared samples,

Fig. 3. Variation of the peeling force in as-received strips in the presence of 0.5 wt.%Al2O3 particles versus the peel distance.

a series of rolling experiments were carried out using the rollingreductions between 20 and 90%. The amount of Al2O3 particles var-ied between 0 and 1.0 wt.%. A number of samples were annealed at643 K for 2 h before or after the CRB process in order to investigatethe effect of pre- and post-annealing on the bond strength.

2.4. Peeling test

The bond strength of the strips, was evaluated using a peelingtest according to ASTM-D1876-01. The peel tests were performedusing a Hounsfield H50KS tensile testing machine with a 50 kg loadcell and a crosshead speed of 20 mm/min. In this test the breakingoff forces were measured as shown in Fig. 2 and the average peelstrength was determined using the following equation:

average peel strength = average loadbond width

(N

mm

)(1)

After peeling, the fracture surfaces of the samples were exam-ined by optical microscopy (OM) in conjunction with a PHILIPS XL30scanning electron microscopy (SEM).

3. Results and discussion

3.1. Effect of reduction in thickness

Fig. 3 presents the effect of reduction on the peeling force ofAl/Al2O3 composite strips (with 0.5 wt.% Al2O3) at ambient tem-perature. Also the results of peeling tests obtained for different

4860 R. Jamaati, M.R. Toroghinejad / Materials Science and Engineering A 527 (2010) 4858–4863

Table 1Specifications of the commercial purity aluminum.

Material Chemical composition (wt.%) Tensile strength (MPa) Yield strength (MPa) Elongation (%) Hardness (HV)

As-received Al 1100 99.11Al, 0.17Si, 0.49Fe, 0.12Cu, 0.02Mn, 0.09 others 186.4 169.1 6.02 57

Fr

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ematfiaMaatmtt

aifidp

spmAtbblt

ig. 4. Variation of the average peel strength of as-received aluminum strips versuseduction.

eductions with or without Al2O3 particles are shown in Table 2.s can be seen from Table 2, for low thickness reductions, the per-

ormed bonding was not strong but for high reductions, the bondsreated were so strong that the specimens broke in the base metal.

Over the past decades, many attempts have been made toxplain the mechanism of CRB by OM, SEM and others analyticalethods. According to the technique used to establish atom-to-

tom bond between layers, so far four theories have been proposedo explain the mechanisms involved in CRB process. These include,lm [10–14,23–25], energy barrier [23,25], diffusion bonding [38]nd joint recrystallization [39] theories. Vaidyanath et al. [10],ohamed and Washburn [23] have stated that the film theory ismajor mechanism in CRB process since low rolling temperaturesre involved. They found that during rolling, two opposing brit-le surface layers, break up coherently, exposing the underlying

etals, and are extruded under the action of normal roll pressurehrough widening cracks in the surface layers from both sides ofhe interfaces.

Referring to Fig. 3 an increase in reduction of thickness, causesn improvement in peeling force and therefore bond strength. Thiss mainly due to the enhancement of the rolling pressure, sur-ace expansion, fraction of crack formation and the extension ofnterface bonded areas. Also by increasing the amount of plasticeformation, the number of cracks on the surface layers increased,roviding new extruded metals to be joined.

The effect of reduction in thickness on bond strength of CRBedtrips of Al/Al at ambient temperature with and without Al2O3articles is illustrated in Fig. 4. It can be seen that threshold defor-ation for the formation of bonding, increases with the presence ofl2O3 particles. Also, it is evident that by increasing the Al2O3 par-

icles between strips, the average peel strength and therefore, theond strength, decrease. Using the film theory [10–14,23–25], it can

e suggested that, Al2O3 particles, prevent the bonding of under-

ying surfaces due to the presence of interfacial cracks. Therefore,he bonding area and consequently bond strength decrease.

Fig. 5. Variation of threshold deformation (Rt) in as-received, pre- and post-rollingannealed strips including 0–1.0 wt.% Al2O3 particles.

3.2. Combination of alumina, pre- and post-rolling annealingeffects

3.2.1. Threshold deformationThe influence of the alumina quantities on the threshold defor-

mation of CRBed strips for as-received, pre- and post-rollingannealed strips is illustrated in Fig. 5. The virgin metal surfacescannot form a strong bond when deformed below the thresholdvalue (Rt). From Fig. 5 the threshold deformation for the formationof bonding increases with increasing of Al2O3 particles. The reasonis elucidated by the energy barrier theory [23,25], which suggeststhat an energy barrier must be overcome before two virgin metalsurfaces can meet and bond together. Above these values, the bondstrength increases rapidly. This phenomenon is due to an increasein contact pressure, and the overlapping surface exposure at theinterface. Furthermore, when deformation is below its threshold,there is only a few surface cracks and therefore, the amount of virginmetals in contact with one another, is very slight at the interface.Therefore, new metal surfaces cannot bond together sufficientlysince contaminations such as air and absorbed vapor delay thebond formation. As shown in this figure, threshold deformationfor bonding, compared to as-received strips, decreases by pre-rolling annealing treatment. But, threshold deformation does notchange by post-rolling annealing. In other words, the post-rollingannealing treatment has no effect on the threshold deformation.As mentioned before, it is necessary to overcome the activationenergy of bond formation to initiate the bonding process in the CRBprocess. This energy is certainly provided by pre-rolling annealingprocess. By this treatment more atoms obtain the activation energyfor bond formation. Therefore, the deformation role to overcomethe energy barrier decreases. Thus, with the pre-rolling anneal-ing treatment, the amount of threshold deformation of bondingdecreases. In other word, this treatment leads to successful bondingpossible at the lower thickness reduction.

3.2.2. Average peel strengthFigs. 6 and 7 indicate the variation of the average peel strength

of CRBed aluminum strips for as-received, pre- and post-rolling

R. Jamaati, M.R. Toroghinejad / Materials Science and Engineering A 527 (2010) 4858–4863 4861

Tab

le2

Ave

rage

pee

lin

gfo

rce

(FA)

inva

riou

sco

nd

itio

ns

for

as-r

ecei

ved

stri

ps.

Red

uct

ion

(%)

2530

3540

4550

6065

7075

8085

90

F A(N

)—w

ith

out

Al 2

O3

par

ticl

esB

0(+

5)50

(±5)

115

(±10

)16

5(±

10)

240

(±10

)47

0(±

15)

FF

FF

FF

F A(N

)—w

ith

0.1

wt.

%A

l 2O

3p

arti

cles

BB

0(+

5)80

(±5)

110

(±10

)14

5(±

10)

280

(±10

)35

0(±

15)

440

(±15

)F

FF

FF A

(N)—

wit

h0.

5w

t.%

Al 2

O3

par

ticl

esB

BB

0(+

5)35

(±5)

85(±

5)23

5(±

10)

270

(±10

)30

5(±

15)

360

(±15

)41

5(±

15)

FF

F A(N

)—w

ith

1.0

wt.

%A

l 2O

3p

arti

cles

BB

BB

0(+

5)35

(±5)

160

(±10

)19

5(±

10)

235

(±10

)27

0(±

15)

310

(±15

)40

0(±

15)

F

B:

the

sam

ple

s,w

hic

hBo

nd

ing

was

not

succ

essf

ul;

F:w

ere

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ture

din

base

met

al.

Fig. 6. Comparison between as-received, pre- and post-rolling annealed strips,without alumina particles.

Fig. 7. Comparison between as-received, pre- and post-rolling annealed strips, with1.0 wt.% alumina particles.

annealed strips in the presence and absence of the alumina par-ticles, respectively. The figures show the pre-rolling annealingtreatment increases the bond strength of the strips, in the presenceand absence of Al2O3 particles. The pre-rolling annealing processreduces Al 1100 strips hardness and therefore, increases the bondtoughness [40]. In the peel test, a crack propagates through thebonded area and separates two bounded strips. Improvement of thebond toughness raises the required force for the crack propagationand so, increases the bond strength.

Again, with reference to Figs. 6 and 7, the post-rolling annealingtreatment increases the bond strength of the strips, with or withoutAl2O3 particles. Three phenomena may influence the bond strengthduring the post-rolling annealing treatment. First, post-rollingannealing treatment reduces the hardness of pure aluminum strips

which induces an increase in bond toughness. Improvement of thebond toughness increases the force required for the crack propaga-tion in the peel test and therefore, the bond strength is increased.Second, enhancement of the bond strength after annealing may berelated to the atomic diffusion phenomenon at the interface. Third,

4862 R. Jamaati, M.R. Toroghinejad / Materials Science and Engineering A 527 (2010) 4858–4863

Fig. 8. (a) OM and (b) SEM micrographs of the scratch brushed areas of the stripsurface, before CRB process.

Fig. 9. The fracture surfaces of as-received strips after the peeling test for: (a) R = 50%and (b) R = 60%.

Fig. 10. The fracture surfaces of as-received strips after the peeling test, with 50% reduction, at two magnifications, in the (a and c) absence and (b and d) the presence ofAl2O3 particles.

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R. Jamaati, M.R. Toroghinejad / Materials Sc

nnealing treatment after the CRB process can remove or reducehe residual stresses between the solid-state bonded materials.

In attention to Figs. 6 and 7, there is an important and interestingoint. In Fig. 7, for pre- and post-rolling annealed strips comparedo as-received strips, beyond the 60% (for pre-rolling annealing) and5% (for post-rolling annealing) reduction, a sudden increase in theverage peel strength is observed. But, in the absence of aluminaarticles (Fig. 6) this effect is not for both pre- and post-rollingnnealed samples. However, this effect at pre-rolling annealedtrips is more pronounced. It can be suggested that beyond a spe-ific reduction, the alumina particles can break the oxide layer thatxists at the surfaces of the both strips, because the alumina par-icles are harder than the aluminum matrix. In other words, thelumina particles cause fresh surfaces to be exposed and, withoutufficient time to re-form an oxide layer, the surfaces are bondedore effectively. Whatever these fresh surfaces be softer, bond-

ng between them is stronger. Therefore, this effect at pre-rollingnnealed strips is more pronounced.

.3. Fractured topology of bonded surfaces

OM and SEM micrographs of scratch brushed areas before CRBrocess of the strips are shown in Fig. 8. The brush caused someearing of the surface. It can be seen that the scratch brushed layerhich is relatively hard and brittle consists of striations in the wire

rushing direction.Fig. 9 shows the fracture surfaces of the specimens after the

eeling test at different reductions in thickness. These figures showracks along with extruded areas of the virgin metal on the peeledurface. The extruded areas of the virgin substrate facilitate the inti-ate contact and ultimately the bonding between the two strips.n increase in total reduction of thickness increases the size andumber of these cracks.

The fractured surfaces of the specimens with or without Al2O3articles, produced by 50% reduction, at two magnifications arehown in Fig. 10. Fig. 10 demonstrates that, Al2O3 particles preventhe extrusion and so bonding of the virgin metals and consequentlyhe bond strength decreases. The difference between the bondingtrength in the presence and absence of Al2O3 particles is notice-ble. Much smaller contact area as well as much less conformancef the surfaces is noted when the process is applied in the pres-nce of Al2O3 particles. The reason for this phenomenon is clarifiedy the energy barrier theory [23,25]. As mentioned before, it isecessary to overcome the activation energy of bond formation

n order to initiate bonding in CRB process. In fact, the requirednergy for rearrangement of the surface atoms to achieve a properonding configuration [25], dispersal of any surface contaminant

ayers [25,28] and activation energy for atom-to-atom bond forma-ion [20,41], markedly increase, when Al2O3 particles are presentetween the layers of the strips.

. Conclusions

The following conclusions are summarized as follows:

. The bond strength between the aluminum strips increases withincrease in reduction of thickness. A point is reached where the

[

[[[

and Engineering A 527 (2010) 4858–4863 4863

bond strength reaches the base metal strength in strips of purealuminum and those of containing nano-sized Al2O3 particles.

2. The threshold deformation of aluminum strips in the absenceof Al2O3 particles is about 30%, while this is 35%, 40% and45% for strips containing 0.1, 0.5 and 1.0 wt.% Al2O3 particles,respectively. In other words, the bond strength between stripsdecreases with an increase in the amount of Al2O3 particles.

3. With pre-rolling annealing treatment, the average peel strengthas well as the bond strength of strips increases. Also, this treat-ment decreases threshold deformation in constant reductionwhen compared to as-received strips.

4. With post-rolling annealing treatment, an increase in the aver-age peel and bond strength is achieved. But, effect of thistreatment on the threshold deformation is not pronounced whencompared to as-received strips.

5. The alumina particles beyond a specific reduction have impor-tant effect on the pre- and post-rolling annealed strips. This effectconsists of sudden increase in the average peel strength.

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