Materials and Design 35 (2012) 290–295

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Materials and Design

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Short Communication

Effect of wire brushing on warm roll bonding of 6XXX/5XXX/6XXX aluminumalloy clad sheets

Su-Hyeon Kim ⇑, Hyoung-Wook Kim, Kwangjun Euh, Joo-Hee Kang, Jae-Hyung ChoStructural Materials Division, Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam 642-831, Republic of Korea

a r t i c l e i n f o

Article history:Received 28 July 2011Accepted 9 September 2011Available online 16 September 2011

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.09.024

⇑ Corresponding author. Tel.: +82 55 280 3538; faxE-mail address: shawnkim@kims.re.kr (S.-H. Kim).

a b s t r a c t

Three-layered 6XXX/5XXX/6XXX aluminum alloy clad sheets were fabricated by warm roll bonding andthe effect of wire brushing on the threshold reduction and bonding strength was studied. When thesheets were wire-brushed before rolling, the threshold reduction for successful bonding was reduced,and bonding strengths were lower than those of the non-brushed sheets. The difference between thethreshold reduction of the brushed and non-brushed sheets decreased with increasing rolling tempera-ture. The bonding strength of the non-brushed sheets was higher than that of the brushed sheets becausethe bonding area was larger. In the non-brushed sheets, deformation between the clad and the core maybe responsible for waviness and crack formation, causing interfacial sliding at the bonding interface,enlarging the bonding area, and increasing the bonding strength.

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1. Introduction

Roll bonding, solid-state welding of similar or dissimilar metalsby rolling, is a cost-effective fabrication technique for the mass pro-duction of multi-layered clad sheets. Roll bonding can be used to joinvarious combinations of metal sheets, such as Al/Al sheets, Cu/Albimetals, and stainless steel/Al sheets for automotive, electrical,and kitchen utensil applications. Typical roll bonding includes a se-quence of chemical or mechanical surface treatments of the metalsheets, stacking of the sheets, and rolling. Rolling can be conductedat room temperature or at elevated temperatures. Threshold reduc-tion, the minimum value of single-pass reduction for successfulbonding, can be reduced by increasing the rolling temperature [1–3]. Cold roll bonding has been applied to various combinations ofmetals [4], but it requires a high-power rolling mill for thick andwide strips because high reduction is required for bonding. Somehard metals require heat to achieve high reduction by rolling. A cladsheet composed of ultrahigh carbon steel and mild steel has beenfabricated by hot roll bonding [5]. Kong et al. prepared aTi–6Al–4 V/TiAl clad sheet by hot roll bonding at 1180 �C [6]. Liuet al. investigated the effect of surface roughness on the hot rollbonding process used to prepare 1100 Al/2024 Al/1100 Al sheets[7]. A high-strength 7075 Al alloy has also been roll-bonded to purealuminum at the solution temperature of the alloy [8].

Warm or hot roll bonding is often used to join dissimilar metals.An aluminum clad steel plate and titanium clad aluminum sheethave been successfully roll-bonded when they were heated to differ-ent temperatures [9]. The bonding strength of warm roll-bonded

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1350 Al/low carbon steel clad sheet increases with the pre-heatingtemperature of the sheet [1]. Interfacial microstructures of hotroll-bonded Al/Cu and Ag/Cu clad sheets have been investigated[10,11]. Zhang et al. examined the bond criterion and microstructureevolution of 7075Al/AZ31B Mg/7075Al clad sheets prepared by hotroll bonding [12,13]. Movahedi et al. reported that the thresholdreduction of 3003 Al/Zn clad sheet substantially decreased withthe rolling temperature [14]. It has also been reported that increasedrolling temperatures can increase bonding strength between similarand dissimilar metals [1,2,14,15].

To achieve successful bonding, it is essential to clean the bond-ing surfaces. Cold roll bonding often requires wire brushing prior torolling because chemical degreasing is not sufficient to prepare thesurfaces. Wire brushing removes surface layers composed of con-taminated layers and oxide layers. More importantly, it forms ahardened cover layer on the surface [16] and increases surfaceroughness. Wire brushing increases the hardness and roughnessbecause the brushed surface, exposed to air, becomes oxidizedand contaminated quickly after wire brushing. In cold rolling, wirebrushing is a prerequisite for good bonding properties [17]. How-ever, few studies have focused on the effect of wire brushing onwarm rolling.

Multi-layered aluminum alloy sheets are used for a variety ofapplications. Different alloys with different properties can bejoined together by roll bonding. In the present study, three-layered6XXX/5XXX/6XXX aluminum alloy clad sheets were fabricated bywarm rolling. The combination of 5XXX and 6XXX aluminumalloys exhibited the multiple functionality of high formability(5XXX alloys) and high corrosion resistance (6XXX alloys). Theobjective of this study was to investigate the effect of wirebrushing on the threshold reduction and bonding strength of warm

Table 1Chemical compositions of 5XXX and 6XXX sheets.

Alloy Si Fe Cu Mn Mg Cr Zn Ti Al

5XXX 0.03 0.06 0.30 0.00 5.54 0.00 0.01 0.02 Bal.6XXX 1.04 0.18 0.01 0.07 0.59 0.02 0.02 0.01 Bal.

Heat treatment(5XXX: O, 6XXX: T4)

Degreasing(Acetone)

Wire brushing

Stacking(6XXX/5XXX/6XXX)

Rolling

Heating(Infrared heating)

Stacking(6XXX/5XXX/6XXX)

Rolling

Heating(Infrared heating)

Fig. 1. Flow chart of warm roll bonding of 6XXX/5XXX/6XXX clad sheets.

Table 2Mechanical properties of heat-treated 5XXX and 6XXX sheets.

Alloy Tensile strength (MPa) Yield strengtha (MPa) Elongation (%)

5XXX-O 285 109 536XXX-T4 205 88 39

a 0.2% offset strength.

Sheet (width: 100mm)

Wire brush

165N

0.2mpm

300rpm

Fig. 2. Schematic illustration of wire brushing.

Side polishing

Hole drilling Compressive testSpecimen

Hole size: 2.5 mm

Punch size: 1.5 mmMoving speed: 1 mm/min

Size: 15 15 mm

Fig. 3. Bonding strength test of roll-bonded 6XXX/5XXX/6XXX clad sheets.

Table 3Success or failure of 6XXX/5XXX/6XXX clad sheets roll-bonded with wire brushing.

Samplenumber

Heatingtemperature (�C)

Heating time(s)

Reduction(%)

Bonding

B1 Room temperature – 44 Notbonded

B2 Room temperature – 47 Notbonded

B3 Room temperature – 54 BondedB4 100 150 42 Not

bondedB5 100 150 48 Not

bondedB6 100 150 50 BondedB7 150 210 42 Not

bondedB8 150 220 46 BondedB9 150 230 55 BondedB10 150 250 57 BondedB11 200 350 36 Not

bondedB12 200 280 39 Not

bondedB13 200 320 44 BondedB14 200 310 45 Bonded

Table 4Success or failure of 6XXX/5XXX/6XXX clad sheets roll-bonded without wirebrushing.

Samplenumber

Heatingtemperature (�C)

Heating time(s)

Reduction(%)

Bonding

N1 Room temperature – 60 Notbonded

N2 100 160 40 Notbonded

N3 100 170 41 Notbonded

N4 100 170 50 Notbonded

N5 100 190 56 BondedN6 150 240 37 Not

bondedN7 150 190 44 Not

bondedN8 150 240 50 BondedN9 150 220 56 BondedN10 200 310 42 Not

bondedN11 200 310 43 BondedN12 200 310 45 BondedN13 200 340 48 Bonded

S.-H. Kim et al. / Materials and Design 35 (2012) 290–295 291

roll-bonded sheets and to elucidate a bonding mechanism with re-spect to surface preparation.

2. Experimental procedure

Commercial 5XXX Al and 6XXX Al sheets were used for the fab-rication of three-layered 6XXX/5XXX/6XXX clad sheets. Chemicalcompositions of the sheets were measured by inductively coupledplasma (ICP; Table 1). The thicknesses of the 5XXX and 6XXX

sheets, measured by a micrometer, were 2.00 mm and 0.99 mm,respectively. Several 100 � 250 mm (W � L) sheets were cut fromthe materials for roll bonding. Fig. 1 shows the roll bonding pro-cess, including heat treatment, degreasing, wire brushing, heating,and rolling. The 6XXX alloy sheets were solution-treated for

0 50 100 150 20030

35

40

45

50

55

60

65

70

Bonded Not bonded

Thic

knes

s re

duct

ion

(%)

Heating temperature (oC)

Not brushed

0 50 100 150 20030

35

40

45

50

55

60

65

70

Bonded Not bonded

Thic

knes

s re

duct

ion

(%)

Heating temperature (oC)

Brushed

Fig. 4. Threshold reduction for roll bonding of 6XXX/5XXX/6XXX clad sheets as afunction of heating temperature.

0

10

20

30

40

50

60

70

80

90 Total reduction Reduction of clad Reduction of core

Thic

knes

s re

duct

ion

(%)

Not brushed, 150oC, 56%Brushed, 150oC, 57%

0

10

20

30

40

50

60

70 Total reduction Reduction of clad Reduction of core

Thic

knes

s re

duct

ion

(%)

Not brushed, 200oC, 45%Brushed, 200o

o

o

C, 45%

Heating temperature: 150 C

Heating temperature: 200 C

Fig. 6. Thickness reduction of the clad, core, and total layers of roll-bonded 6XXX/5XXX/6XXX clad sheets.

292 S.-H. Kim et al. / Materials and Design 35 (2012) 290–295

3.5 min at 550 �C in an air atmosphere of a convection furnace andquenched in water. The 5XXX alloy sheets were annealed at 400 �Cfor 1 h in an air atmosphere of a convection furnace and cooled in

Fig. 5. Photographs of roll-bonded

air. Table 2 shows the mechanical properties of the heat-treated5XXX and 6XXX alloy sheets measured by a room-temperatureuniaxial tensile test. After heat treatment, the surfaces of thesheets were degreased with acetone.

Fig. 2 shows the wire-brushing conditions. Wire brushing wascarried out using a 200-mm-diameter circumferential brush with0.3-mm-diameter brass-coated steel wires. The sheet was movedalong the rolling direction at 0.2 m per min (mpm), and the wirebrush was rotated at 300 revolutions per min (rpm). A force of165 N was applied vertically to the surface of the sheets. Both sur-faces of the 5XXX alloy sheets and one surface of the 6XXX alloysheets were wire-brushed.

6XXX/5XXX/6XXX clad sheets.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Bond

ing

stre

ngth

(MPa

)

Not brushed, 200oC, 45%Brushed, 200oC, 45%

0.00.10.20.30.40.50.60.70.80.91.01.11.21.3

Bond

ing

stre

ngth

(MPa

)

Not brushed, 150o

o

o

C, 56%Brushed, 150oC, 57%

Heating temperature: 150 C

Heating temperature: 200 C

Fig. 7. Bonding strengths of roll-bonded 6XXX/5XXX/6XXX clad sheets.

S.-H. Kim et al. / Materials and Design 35 (2012) 290–295 293

The brushed 6XXX, 5XXX, and 6XXX sheets were stacked andheld in an infrared lamp heater until the temperature of the sheetsreached the target temperature. Then, the sheets were bonded byrolling, which was carried out in a single pass without lubrication.A two-high rolling mill with 210-mm-diameter rolls was used, andthe rolls were heated at 100 �C. The rolling speed was 2.4 rpm. Coldroll bonding was also conducted using the same rolling mill butwithout heating the sheets or the rolls. Fourteen different cladsheets were fabricated by altering the heating temperature andthe rolling reduction. For comparison, 13 different clad sheets werealso fabricated by the same procedure using the same materialsbut without wire brushing.

Bonding strengths were measured by a modified peel test (Fig. 3).This method was inspired by a test geometry suggested by Zhangand Bay [18]. A 15 � 15 mm sample was cut from the roll-bondedsheet, and the sides of the sample were polished to an inclinationof about 20�. A 2.5-mm-diameter hole was drilled at the center ofthe outer clad sheet with a depth greater than its thickness. The sam-ple was fixed by gripping its sides. To measure the force needed topeel off the clad sheet, a 1.5-mm-diameter punch was placed inthe center hole and was moved down at a speed of 1 mm/min. Peelstrength was defined as peel force divided by the sample area. Thepeeled surface was observed using scanning electron microscopy(SEM).

3. Results and discussion

Tables 3 and 4 show the success or failure of the rolled sheets.Hereafter, the outer sheets of the 6XXX aluminum alloy and the cen-ter sheet of the 5XXX aluminum alloy are referred to as the clad and

the core, respectively. The threshold reduction for successful bond-ing was determined and plotted as a function of the sample heatingtemperature (Fig. 4). The threshold reduction decreased withincreasing sample heating temperature, regardless of whether thesheets were brushed or not brushed before rolling. During rolling,the surface layers composed of the adsorbed layer and oxide layerwere fractured, and the formation of surface cracks exposed theunderlying virgin surface. As the cracks expanded into fissures, thevirgin surface was extruded through the surface cracks and con-tacted the virgin metal extruded from the other surface, formingmetallic bonds [3,18]. When the rolling temperature increased, soft-ening of the material promoted the extrusion of the virgin layer,reducing the threshold reduction [1–3]. Several researchers havenoted that the microstructural changes that occur during rollingare responsible for the bonding strength of the clad sheets roll-bonded at elevated temperature [11,13] Chemical reactions at thebonding interface can also influence the bonding strength[6,19,20]. In the present research, microstructural changes andlong-range diffusion at the interface during rolling were negligiblebecause the heating temperature was not high.

At low heating temperatures, the threshold reduction of the non-brushed sheets was larger than that of the brushed sheets, but thedifference decreased with increasing heating temperature. Wirebrushing is often used to prepare a surface prior to roll bonding. Wirebrushing cleans the surface by removing the surface layers, includ-ing the adsorbed layer and oxide layer, and also applies severe plas-tic deformation to form a hardened layer [3,16,18,21,22]. In an airatmosphere, it is impossible to completely remove the adsorbedlayer and the oxide layer by wire brushing because the wire-brushedsurface becomes oxidized and contaminated immediately after wirebrushing. It is believed that formation of a hardened layer can reducethe threshold reduction for cold roll bonding [3,18,21]. Wire brush-ing causes a hardened, brittle layer to form on the surface; this layeris easily broken [17]. Eizadjou et al. [3] and Kim et al. [22] obtainedvery high hardnesses of the wire-brushed aluminum alloy sheets,confirming the existence of the hardened layers. During rolling,the brittleness of the hardened layer can promote the formation ofsurface cracks, exposing the underlying virgin surface. As mentionedabove, softening of the material by increasing the rolling tempera-ture promotes the extrusion of the virgin layer, reducing the thresh-old reduction. However, increasing the rolling temperature can alsosoften the surface-hardened layer. As a result, the hardened layerformed by wire brushing may not favor warm roll bonding. Fig. 4shows that wire brushing prior to rolling did not affect thresholdreduction when the heating temperature reached 200 �C.

Fig. 5 shows images of the roll-bonded sheets. The wire-brushedsamples were smooth with a few side cracks. However, wavinessand severe side cracks were observed in the samples roll-bondedwithout wire brushing. Waviness and crack formation in the non-brushed samples may be related to incompatible deformation be-tween the clad and the core. We examined the thickness reductionof each clad and core by observing longitudinal sections of the sam-ples after roll bonding and compared them with the overall thick-ness reduction (Fig. 6). Thickness reduction of the wire-brushedsamples did not differ among each layer and the total thickness.On the contrary, thickness reduction of the non-brushed sampleswas inhomogeneous; the thickness of the clad layers was less de-formed than that of the core. Inhomogeneous deformation is evita-ble during rolling. With heavy reductions, the center regiontended to expand laterally more than the surfaces did [23]. Differ-ences in lateral deformation between the clad and the core can resultin dimensional misfit. Waviness and crack formation can be attrib-uted to dimensional misfit in the lateral direction. Different reduc-tion between the clad and the core sheets also indicates thatinterfacial sliding occurred at the contacting interface. Interfacialsliding due to movement of the clad and the core can influence bond-

Fig. 8. SEM micrographs of peeled surfaces after bonding strength tests of 6XXX/5XXX/6XXX clad sheets roll-bonded at 150 �C.

Fig. 9. SEM micrographs of peeled surfaces after bonding strength tests of 6XXX/5XXX/6XXX clad sheets roll-bonded at 200 �C.

294 S.-H. Kim et al. / Materials and Design 35 (2012) 290–295

ing strength, as discussed later. The brushed samples did not exhibitincompatible deformation between the clad and the core, possiblydue to interfacial interlocking. Because wire brushing increasesroughness and forms hills and valleys on the surface, mechanicalinterlocking of the contacting surface can prevent interface slidingduring rolling. Interfacial interlocking can suppress incompatibledeformation between the clad and core.

Fig. 7 shows the bonding strengths of the roll-bonded sheets.Under similar rolling reduction conditions, the bonding strengthsof the non-brushed sheets were higher than those of the brushedsheets when the sample heating temperature was 150 or 200 �C.Figs. 8 and 9 show the peeled surfaces after bonding strength tests

of the brushed and non-brushed samples roll-bonded at 150 and200 �C, respectively. In the wire-brushed samples, the bondedregions are seen as bright asperities perpendicular to the rollingdirection. Asperities are often found in the peeled surface ofwire-brushed and roll-bonded sheets [3,15,24]. Metallic bondingoccurred between the asperities of the opposing surfaces by a se-quence of crack formation, crack expansion, and extrusion of thevirgin surface [1]. The fractured surface suggests that the deforma-tion of the asperity tops was limited, and the bonding area of thewire-brushed samples was very small; thus, the bonding strengthswere low. The peeled surface of the non-brushed samples differedsignificantly from that of the brushed samples. The bonding area

S.-H. Kim et al. / Materials and Design 35 (2012) 290–295 295

was large and elongated along the rolling direction, and no asper-ities perpendicular to the rolling direction were found. Liu et al.demonstrated bonding areas elongated along the rolling directionof aluminum clad sheets warm-rolled without wire brushing [7].Enlargement of the bonding area is direct evidence of higher bond-ing strengths [3] and can be attributed to interfacial sliding andmutual rubbing caused by incompatible deformation betweenthe clad and the core. The relative movement of the sheets duringbonding is evidenced by asperity deformation in the rolling direc-tion [15]. Interfacial sliding can occur when dissimilar metals areroll-bonded [25]. Interfacial sliding due to movement at the bond-ing interface is beneficial to roll bonding [24–26] because it can rubthe contacting surfaces and expand the fissures of the surface lay-ers along the rolling direction, enlarging the bonding area.

4. Conclusions

Three-layered 6XXX/5XXX/6XXX aluminum alloy clad sheetswere fabricated by warm rolling. The threshold reduction of thewire-brushed sheets was lower than that of the non-brushedsheets but the difference decreased with increasing rolling temper-ature. The hardened layer formed by wire brushing may not favorwarm roll bonding because it can be softened by increasing therolling temperature.

The sheets roll-bonded without wire brushing exhibited wavi-ness and crack formation, which may have resulted from incom-patible deformation between the clad and the core. The wire-brushed sheets did not exhibit incompatible deformation betweenthe clad and the core, possibly due to interfacial interlocking.

The bonding strength of the non-brushed sheets was higherthan that of the brushed sheets because the bonding area was lar-ger. The fractured surface of the wire-brushed sheets suggests thatthe deformation of the asperity tops was limited, and the bondingarea was small. In the non-brushed sheets, interfacial sliding at thecontacting interface expanded the bonding area, increasing thebonding strength.

Acknowledgment

This work was supported by a grant from the Korea Institute ofMaterials Science (Project No. PNK2460), Republic of Korea.

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