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 Journal of Materials Processing Technology 210 (2010) 560–563

Contents lists available at ScienceDirect

 Journal of Materials Processing Technology

 j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j m a t p r o t e c

Effects of processing parameters on the bond strength of Cu/Cu roll-bonded strips

Mohsen Abbasi, Mohammad Reza Toroghinejad ∗

Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e i n f o

 Article history:

Received 19 July 2009

Received in revised form 26 October 2009

Accepted 8 November 2009

Keywords:

Copper

Cold roll bonding

Peel strength

a b s t r a c t

Roll bonding, widely used in manufacturing large layered composite sheets, is a solid phase method for

bonding similar or dissimilar metals by rolling. In this study, the effects of process parameters such as

rolling reduction, rolling temperature, rolling speed, initial thickness of strip, and surface roughness on

the bond strength between two-layer strips of Cu/Cu were investigated. The strips were subjected tochemical and mechanical cleaningprior to rolling, andafterrolling, bond strengthswere measured using

the peeling test. It was observed that increased reduction, rolling temperature, strip width, and surface

roughness led to an increase in peeling strength while increased rolling speed and initial thickness of 

strips caused peeling strength to decrease. Results also showed that increasing the initial thickness of 

strips would increase threshold deformation.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Roll bonding is a solid state welding process used to join sim-

ilar and/or dissimilar metals. According to Pan et al. (1989), it is

the most economical and productive manufacturing process that

can be used to produce flat clad metal sheets and foils. In this pro-

cess (Fig. 1), two or more metal/alloy strips are stacked together

and passed through a pair of rolls. After proper deformation, a

solid state joint between the original individual metal pieces will

be produced. Before roll bonding, the surfaces to be bonded must

be cleaned and prepared. As expressed by Bay and Zhang (1994),

the two common methods used to remove contaminants and sur-

face oxides are chemical and mechanical cleaning. It has been

reported that cold welding of metals is affected by such param-

eters as percent of deformation according to Danesh Manesh and

Karimi Taheri (2004), strain rate which is considered as the time

of welding as reported by Vaidyanath et al. (1959), and welding

temperature as mentioned by Pan et al. (1989). Vaidyanath et al.

(1959) and Mohamed and Washburn (1975) have claimed that the

principal mechanism involved in rolling is what is called “the film

theory”. They have also expressed that the film theory has been

the major mechanism in cold roll welding because of low tem-

peratures. According to this theory, two opposing brittle surface

layers break up during rolling and the underlying base metal is

extruded through cracks of the broken layers. Zhang andBay (1997)

identified a threshold value for plastic deformation (Rth) which

is necessary for cold welding to initiate. At deformations higher

than Rth, uncontaminated regions are extruded through cracks so

∗ Corresponding author. Tel.: +98 311 3915726; fax: +98 311 3912752.

E-mail address: toroghi@cc.iut.ac.ir (M.R. Toroghinejad).

that welding takes place. Also Mohamed and Washburn (1975) and

Parks (1953) have put forward different theories to account for the

mechanism of bonding between strips in the roll bonding process

such as the energy barrier theory and the joint recrystallization

theory, respectively.

Our literature review revealed studies that had been conducted

on the roll bonding process of Al (alloys), Ti (alloys), steel, and

dissimilar layers such as Al/Zn, Al/steel, etc. However, we came

across no study so far investigating the roll bonding process of 

Cu/Cusheets.Therefore,this studywas conductedto investigatethe

effects of process parameters including rolling reduction, rolling

temperature, rolling speed, initial thickness of strip, and surface

roughness on bonding.

2. Materials and experimental procedure

 2.1. Materials and roll bonding process

The chemical composition of the copper sheet used is presented

in Table 1. The initial surface roughness of the copper sheets was

Ra = 0.25m. Cold roll welding experiments were carried outusing

a laboratory rolling mill with a loading capacity of 20 tons. The

rolling diameter was 127 mm. The samples for the two-layered

bonding were prepared as follows: stripswith initial dimensions of 

120 mmin length, 30mm inwidth,and 0.5 mmthickwerecut from

a cold rolledsheet,parallel to the original rolling direction. The sur-

faces were degreased in acetone for300 s andthen scratch brushed

using a stainless steel circumferential brush 90 mm in diameter

with wires0.3 mmacross running ata rotational speedof 2000rpm.

After surface preparation, the handling of the strips was performed

carefully to avoid renewed contamination. Two pieces of the strips

0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmatprotec.2009.11.003

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M. Abbasi, M.R. Toroghinejad / Journal of Materials Processing Technology  210 (2010) 560–563 561

 Table 1

Chemical composition of copper sheets.

Element Cu (wt%) O (wt%) Pb (ppm) Fe (ppm) Sn (ppm) S (ppm) Si (ppm) Ni (ppm) Al (ppm) Te (ppm) Sb (ppm) Bi (ppm)

Amount 99.97 0.002 86 32.11 18.27 11.81 10.45 9.24 7.28 7.11 6 6

Fig. 1. Schematic illustration of the roll bonding process.

were stacked together by a soft copper wire. To investigate the

effects of different parameters, a series of rolling experiments were

carried out on this metal combination using the rolling reductions

of 55%, 60%, 70%, 80%, and 90%. In order to study the tempera-

ture and rolling speed effects on the bond strength between layers,

rolling was carried out at temperatures of 373K, 473K, 573K, and

673 K and at rolling speeds of 150 rpm, 300 rpm, 400 rpm, 600 rpm,800 rpm, and 1000rpm. Also, the initial thickness of the strips was

varied from 0.5 mm to 1 mm and the surface roughness was varied

from Ra =1.1m to Ra =6.67m to determine the effects of other

parameters.

 2.2. Peel test 

Bond strengths were measured for roll-bonded sheets using the

peel test according to ASTM D903-93 standard. In this test, the

breaking-off forces were measured as shown in Fig. 2 and the aver-

age peel strengths are calculated from Eq. (1).

Average peel strength =

average load

bond width(N/mm) (1)

Peel tests were performed using an Instron tensile testing machinewith a crosshead speed of 20 mm/min. Fig. 3 illustrates the clamp-

ing configuration used in this study.

3. Results and discussion

 3.1. Effect of thickness reduction on bond strength

Fig. 4 presents the effect of total reduction in thickness on the

bond strength of Cu/Cu sheets. The bond strength increased rapidly

with total thickness reduction. In cold welding processes like

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

Fig. 3. Schematic illustration of peeling test fixture.

rolling, the total thickness reduction is one of the most important

parameters that affect weld formation. According to Bay (1986),

there are two basic bonding mechanisms for scratch brushed sur-

faces during roll bonding including:

1. The scratch brushing forms a heavy work hardened surface layer

on part of surface. This brittle cover layer fractures at small

surface expansions and reveals virgin metals which extrudes

through the cracks of the cover layer and meets virgin metal

of the opposing surface to form a metallic bond.

2. Where no brittle cover layer is present bonding occurs when a

threshold surface expansion causing fracture of the contaminantfilm has been exceeded.

Accordingto filmtheory mechanismin roll bonding, thebrittle sur-

face layers which result from brushing during the rolling of two

strips are subjected to increasing normal pressure to deform in the

rolling direction.Therefore, somesurface cracks are producedin the

surface layer of the strips. Then, the underlying metals are exposed

through these cracks so that virgin metal surfaces are extruded.

On the other hand the high normal roll pressure on the surfaces

leads to produce cracks in the brittle surface layers, so extrusion of 

these virgin metals is convenient for metal-to-metal and atom-to-

Fig. 4. Variation in peel strength of two-layer strips of Cu/Cu versus total thickness

reduction.

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562 M. Abbasi, M.R. Toroghinejad / Journal of Materials Processing Technology  210 (2010) 560–563

Fig. 5. Variation in peelstrength of two-layer strips of Cu/Cu versus rolling temper-

ature.

atom bonding, thus producing metallic bonds. According to Danesh

Manesh and Karimi Taheri (2004), increasing the bond strength by

increasing total thickness reduction is due to the increase of con-

tact mean pressure and the overlapping surface exposure at the

interface with increasing the reduction in thickness. Moreover byincreasing the deformation, the number of cracks increased and

therefore more virgin metal surfaces were exposed in contact sur-

faces.Then the areaavailable for atom-to-atombond was extended,

andaccordinglythe bond strength between layers increased. It can,

therefore, be maintained that increasedbond strength as a resultof 

increasing total reduction in thickness is due to the enhancement

of the rolling pressure, surface expansion, area fraction of cracks,

and weld area percentage. Yong et al. (2000) in an analytical model

of roll bonding predicted that by increasing the reduction in thick-

ness, the initial bonded area shifts to the entrance of the roll gap.

This means that at the time of loading a normal pressure on the

Cu/Cuinterface, the interfacebecomes larger and,thus, the bonding

is easier to initiate.

 3.2. Effect of rolling temperature on bond strength

As shown in Fig. 5, enhancement of the rolling temperature

increases the bond strength for certain values of thickness reduc-

tion. At higher rolling temperatures, the flow stress of metals

decreases, which, in turn, increases the ductility and formability of 

virgin metals in the underlying surfaces and enhances their extru-

sion through more cracksin thecontact surfaces.On theotherhand,

an increase in the rolling temperature may lead to a greater degree

of recovery and recrystallization of the metals to soften the mate-

rial. According to Bay (1986), when the soft work piece metal is

deformed plastically, friction against the hard brittle cover layer

causes a buildup of tangential stress,  t, in this film. When  t

reaches the tensile strength of the cover layer, this will fractureand the tangential stress drops to zero next to the point of fracture.

 t is built up in the same way in the rest of the cover layer. As the

metals become softer, the effect of deformation increases, result-

ing in fragmentation of the brittle layer and formation of a stronger

bond between the metals. In the warm roll bonding stage of the

present study, the metals will react with atmospheric oxygen to

form an oxide filmon themetalsurface which restrictsthe physical

contact of the two surfaces. On the other hand, plastic deformation

wouldfragmentthe surface oxidefilm and develop physical contact

between the metal surfaces. The formation and fragmentation of 

the surface oxide film is indeed a dynamic process occurring in the

rolling stage. According to Yan and Lenard (2004) at high tempera-

tures,the strengthof thehardened work andtheoxidelayersas well

as the bond between the oxide and the parent metal reduces, caus-

Fig. 6. Variation in peel strength of two-layer strips of Cu/Cu versus sample thick-

ness.

ing cracks to form more easily and the area fraction of the cracks

thus formed to increase. Peng et al. (1999) suggested that under

the combined action of pressure and heat over short periods, the

reactions between the metal laminates involve a three-stage pro-cess of (1) development of physical contact, (2) activation of the

surfaces in contact, and (3) interaction within the materials being

 joined. It is believed that a similar principle can be applied to the

roll bonding process and a strong mechanical bonding should be

the major contribution to the higher bond strength of the as-rolled

metal laminates.

 3.3. Effect of initial thickness of strips on bond strength

According to Figs. 6 and 7, by decreasing the initial thickness of 

Cu strips, the bond strength increases and the threshold reduction

decreases. When theinitial thicknessof stripsreduces,the pressure

requiredto reach a certain reductionmayincrease, which isequalto

increasing the threshold deformation. When the total deformationis less than the threshold deformation, fewer cracks, and thereby a

negligible number of virgin metal surfaces, are created so that no

bonding occurs between the layers in the contact surfaces. Accord-

ing to Yong et al.(2000) the initial thickness of the layers affects the

location of thebond point thereby changing bond strength.The ini-

tial thickness may also be related to the position of the bond point

and may approach the entrance of the roll gap when the initial

thickness is reduced.

As expressed by Hwang and Kiuchi (1992), the mean contact

pressure (P ) for multilayer strip rolling was calculated using the

Fig. 7. Variation in threshold deformation in Cu strips versus sample thickness.

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Fig. 8. Variation in peel strength of two-layer strips of Cu/Cu versus rolling speed.

following equation:

P =F 

WL(2)

where F , W , and L are the rolling force (monitored by the rolling

machine), width of strip,and projection of roll-strip contact length,

respectively. By increasing the mean contact pressure, average peel

strength and bond strength increase. Increased initial thickness

increases the roll-strip contact length (L) for a constant reduction

thereby reducing the mean contact pressure. Thus, average peel

strength and bond strength decrease.

 3.4. Effect of rolling speed on bond strength

Fig. 8 shows the variations in average peel strength with respect

to reduction in thickness for different rolling speeds. It is clear that

increasing therolling speedcausesslightlylower valuesof thebond

strength at interfaces. The bond strength is observed to decrease

as the speed is increased, leading to shorter times of contact. The

increased bond strength can further be related to the insufficient

extrusion of virgin metals through cracks resulting from the frac-

ture of the oxide film or from the work-hardening layer in a shorttime; thus, it is difficult to bring two surfaces with large areas into

contact. In addition, high speeds can result in width changes on

the top and in other parts of the specimens. Rolling speed has a

twofold effect on the bond. It affects the interface temperature;

high speed causes high temperature that is good for the bond. It

simultaneously affects the effective time for bonding. The bond

strength is observed to decrease as thespeed increases,leading to a

shorter time of contact. Our results indicate that the time is a more

important parameter for rolling speed effects than is temperature.

 3.5. Effect of surface roughness on bond strength

Fig.9 shows theeffect of surface roughnesson thebondstrength

of Cu/Custrips.The initial surface roughness (prior to scratch brush-ing)ofthesampleswasRa = 0.25malongboththerollingdirection

and the transverse direction. Surface roughening by scratch brush-

ing greatly improved the bonding quality, reduced the pressure

required to initiate bonding, and gave some of the highest bond

strengths. By increasing surface roughness of the sheets, the aver-

age peel strength or bond strength increased. This is because work

Fig. 9. Variation in peel strength of two-layer strips of Cu/Cu versus surface rough-

ness.

hardening of the sheets increased and caused a more brittle layer

to form on the surface that could be broken more easily so that the

virgin metal could be extruded more easily as well.

4. Conclusions

The bond strength between two-layered Cu/Cu strips in the roll

bonding process was measured using the peeling test. The follow-

ing conclusions may be drawn from the present work:

1. Bond strength is improved by increasing thickness reductions,

rolling temperature, and surface roughness of the strips.

2. Bond strength in Cu/Cu stripsdecreases by increasing the rolling

speed and the initial thickness of strips.

3. Increasing the initial thickness of strips decreases the value of 

threshold deformation to accomplish the bonding between the

layers.

References

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