Effect of heat treatment on the bending behavior of tri-layered Cu-A-lCu composite plates

Effect of heat treatment on the bending behavior of tri-layered Cu/Al/Cu composite platesIn-Kyu Kim,Sun Ig Hong ⇑Department of Nanomaterials Engineering,Chungnam National University,Daejeon,Republic of Koreaa r t i c l e i n f o Article history:Received 26September 2012Accepted 27December 2012Available online 5January 2013Keywords:Layered composite Interfacial fracture Intermetallics BendingWork hardeninga b s t r a c tBending and fracture behaviors of tri-layered Cu/Al/Cu composite plates were processed by roll-bonding and the effects of work hardening and displacement rate sensitivity during bending on the overall bending behavior and fracture were investigated.As-roll-bonded composite exhibited the extensive load plateau before a relatively rapid load drop.The more localized bending in the as-roll-bonded Cu/Al/Cu clad composite can be attributed to the near-zero work hardening rate in bending.For the Cu/Al/Cu composites annealed at 300°C and up to 450°C,the pronounced work hardening during bending tends to distribute the bending deformation uniformly.For the as-roll-bonded Cu/Al/Cu composite,a fatal crack perpendicular to the Cu/Al interface through the bottom Cu layer was formed by the large tensile stress associated with the severely localized bending.A large crack parallel to the interface adjacent to the fatal crack through the bottom Cu layer appeared to have propagated in Al layer,not along the interface between Al and the bottom Cu layer,suggesting the excellent bonding between Al and Cu in the as-roll-bonded Cu/Al/Cu.For annealed clad composites at 500°C,the localized bending is thought to be caused by the growth of cracks along the interface reaction layer,resulting in the fracture of bottom Cu layer.Ó2013Elsevier Ltd.All rights reserved.1.IntroductionThe demand for advanced materials with enhanced properties and functions increased as the technology advanced.Clad compos-ite materials in which different metals and alloys with various properties were joined have been developed to meet these demands and used in various industrial fields.The design and development of clad composite materials with the optimum com-bination of various properties include the selection of component materials to be joined,the stacking structure of different materials with various thicknesses and interface structure/properties be-tween different materials.In selecting different materials with various properties,not only the properties of individual materials but also the interfacial reaction and interface structure between them should be taken into account.Stacking structure should also be designed and determined in a way to maximize and exploit the useful properties of individual component materials.Copper and aluminum clad composites have been widely stud-ied because of their advantages associated with high conductivity,low density and price competitiveness over copper and copper al-loys.For example,a two-layer clad sheet of aluminum/copper can almost reduce 40–50%in weight,with the equivalent electrical and thermal conductivity to those of some copper alloys.But the cost inCu/Al clad can be reduced by 30–50%compared to copper alloy.For these reasons,Al/Cu clad is frequently used for armored cables,yoke coils in TV sets,air-cooling fin and bus-bar conductor joint.However,the formation of brittle Cu x Al y intermetallic compound at elevated temperature could deteriorate the mechanical and elec-trical reliability of Cu/Al clad composites.Interfacial structure and properties of Cu/Al clad composites have been studied by many investigators [1–5].However,the investigations on the effect of interfacial intermetallic on the mechanical properties of Al/Cu composite fabricated by cold rolling are still few [6].Several method such as extrusion [7,8],rolling [9,10],electro-plating [11],overlay welding [12,13]and explosive welding [14,15]have been used for the clad materials production.Among these methods,the rolling is one of the useful processes because of its low-cost and good productivity [9,16].In this study,tri-layered Cu/Al/Cu composite plates were designed and fabricated by roll-bonding and their bending behaviors were studied.The bending properties of Cu/Al/Cu composite plates should be understood for fur-ther shaping and forming of clad composite plates.The objective of this research was to examine the bending properties of Cu/Al/Cu clad composite in relation to the interfacial properties.2.Experimental detailsThe Cu/Al/Cu clad-composite was fabricated using roll-bonding process.Materials used in this study were OFHC copper and0261-3069/$-see front matter Ó2013Elsevier Ltd.All rights reserved./10.1016/j.matdes.2012.12.070Corresponding author.Tel.:+82428216595;fax:+828225850.E-mail address:sihong@cnu.ac.kr (S.I.Hong).aluminum of the commercial purity (0.20%Si,0.25%Fe,0.05%Cu,balance Al).Cu/Al/Cu clad-metal used in this study have a total thickness of 2.0mm and that of Cu and Al layer was 0.2mm and mm respectively (Fig.1a).The bending specimen with a length mm and the width 5.0mm were used for three-point bending test (Fig.1b).In Fig.1b,top Cu layer experiences the overall com-pressive stress while the bottom Cu layer experiences the overall tensile stress.The clad metal was annealed at temperatures 200°C,300°C 400°C 450°C and 500°C for 3h.To examine the bendability,the 3-point bending test were performed using a Uni-intermetallic compounds formed at the Cu/Al interface were exam-ined by an energy dispersive X-ray analysis (EDX).Bent specimens were also observed by an optical microscope (OM)and a scanning electron microscope (SEM)to examine the bent morphology and cracking behaviors.Results and discussionFig.2shows the optical micrographs of Cu/Al interfaces for clad composite materials;as-rolled (a)and annealed at 300°C (b),1.Cross section of the Cu/Al/Cu clad metal (a)and schematic of the test setup three-point bending (b).of Cu/Al interface region in Cu/Al/Cu clad composites;as-roll-bonded (a)and annealed at 300°C for 3h (b),400°C Fig. 3.Thickness of interfacial reaction layer plotted against the annealing temperature (for 3h).4.Back-scattered electron image(a)and EDX spectra(b–d)from layers marked by numbers1(b),2(c)and3(d)in the Cu/Al interface regionannealing at200°C,no visible change of the interface struc-observed by OM.When annealing temperature was raisedintermetallic reaction layer was formed with a thicknessl m(b).With the increase of annealing temperature to450°C,the reaction layer continues to thicken to17l m respectively.When the annealing temperature500°C,the interface increased more rapidly to a thicknessAt higher temperatures,interfacial reaction layers ap-consist of two or three layers.The multi-layered interfa-reaction products are more clearly visible in Fig.2d.Thethe interfacial reaction layer was measured and plot-function annealing temperature in Fig.3.shows the back-scattered electron image(a)and EDXc,d)from regions1,2,and3marked in Fig.4a.The re-thisfigure appears to consist of two separate layers,butspectra was obtained from the center of layer2because itthin.Chen and Hwang[1]reported that four layers ofintermetallic compounds,Al4Cu9,Al3Cu4,AlCu,Al2Cu,were formedinterface of Cu/Al at500°C.The intermetallic phases can bebased on XRD analyses and the equilibrium phase dia-analyses of Al–Cu system[17].In this study,in order to iden-Fig.5.XRD peaks from the separated Cu and Al layers(a)Cu side and(b)Al side.region2in Fig.4is thought to consist of two layers Cu3Al2and CuAl,which is supported by the XRD analysis.Cu3Al4was not iden-tified in the XRD analyses of the present study.The room temperature load–displacement curves of as-roll-bonded and annealed Cu/Al/Cu clad composite at the displacement rate of1mm/min and10mm/min are displayed in Fig.6a and b, respectively.The as-rolled clad composite and that annealed at 200°C exhibited the initially high bending load with the extensive load plateau before a relatively rapid load drop.Cu/Al/Cu clad com-posite annealed at300°C and up to450°C,on the other hand, exhibited the lower bending load due to the softening associated 400°C(d),450°C(e)and500°C(f)for3h.In the as-rolled and an-nealed clad composites at200°C which exhibited the extended load plateau and work softening,top Cu layers showed the signif-icantly localized bent region,making an acute angle and bottom Cu layers showed the fracture resulting from the localized tensile stress.Bending appeared to occur more uniformly in the Cu/Al/ Cu clad composites annealed at300°C and400°C which exhibited the rather smooth load–displacement curve with a remarkable strain hardening in bending.It should be noted that the images of bent clad composites in Fig.7were taken after displacement up to7mm at which work softening already took effect(Fig.6). Cu/Al/Cu clad composite plate annealed at500°C with the ex-tended softening(or weakening)at thefinal stage displayed the bent morphology similar to that of as-roll-bonded composite(a); significantly localized bending of top Cu layer and fracture of bottom Cu layer.One major difference is the presence of extensive interfacial cracks between Al and bottom Cu layer along interfacial reaction intermetallic layer after annealing at500°C.The hardening rate in bending can be defined to be the load in-crease rate as a function of displacement,dP/dD,where P is the bending load and D is the bending displacement.The relative hard-ening rate compensated by strength is thought to be more mean-ingful in predicting the uniformity of deformation.Therefore,the elastic load compensated hardening rate in bending,dP/(P yÁdD) at the displacement rate of1mm/min was calculated from Fig.6a and plotted as a function of displacement in Fig.7.Here, P y is the elastic to plastic bending transition load(Fig.6).The elas-tic load compensated hardening rate will be called relative harden-ing rate hereafter.In Fig.8,the relative hardening rate (compensated by the elastic to plastic bending transition load)in bending for the as-roll-bonded Cu/Al/Cu clad composite and that annealed at200°C decreased very rapidly and dropped to near-zero at the displacement of0.5mm and remained around zero for an extended period before it went into the negative region.Cu/Al/Cu clad composite annealed at300–450°C exhibited the higher relative hardening rates,which gradually dropped zero at the displacement of3.5mm.The relative hardening rates of Cu/ Al/Cu clad composite annealed at500°C was observed to be simi-lar to those of300–450°C initially,but decreased more rapidly and dropped to zero at the displacement of2.6mm.One interesting observation for Cu/Al/Cu clad composite annealed at500°C is that the hardening rate decreased down toÀ1.0/mm,which suggests the drastic softening(or weakening).The drastic decrease of the bending load Cu/Al/Cu clad composite annealed at500°C at the displacement of3.0–4.2mm can be linked to the interfacial cracks between Al and bottom Cu layer,which is most pronounced after annealing at500°C.The rapid drop of the load after the displacement of2.5mm for the as-roll-bonded Cu/Al/Cu clad composite and that annealed at 200°C is likely to be associated with the low work-hardenability of as-rolled and low-temperature heat-treated(at200°C)Cu/Al/ Cu clad composites in bending.The more localized bending in the as-roll-bonded Cu/Al/Cu clad composite and that annealed at 200°C can be attributed to the near-zero and negative hardening rate in bending over the whole displacement.In this case,once bending occurs,bending continue to occur in the localized region because the work hardening due to the localized bending is negli-gible,leading to the localized fracture.For the Cu/Al/Cu composites annealed at300°C and up to 450°C,the pronounced work hardening in the localized bent re-gion tends to spread the localized deformation and distribute the bending deformation uniformly,leading to the rather uniform bending.Cu/Al/Cu clad annealed at500°C is likely to behave sim-ilarly to those annealed at high temperatures in the initial stage because of the initial high work hardening,but in thefinal stage it is likely to behave similarly to the as-roll-bonded clad compositeFig.6.Load–displacement curves of as-rolled clad metal and annealed clad metalsvarious temperatures at the displacement rate of1mm/min(a)and10mm/min(b).I.-K.Kim,S.I.Hong/Materials and Design47(2013)590–598593because of the rapid softening (or weakening).The faster load drop after the displacement of 3mm in the Cu/Al/Cu clad composite an-nealed at 500°C may be linked to the more severe interfacial deb-onding and interfacial cracks compared to those samples heat-treated at 300–450°C.The displacement rate sensitivity defined by the D log P /D log Dˇwere calculated using the data in Fig.6a and b at the displacement of 1.5mm at various temperatures and plotted as a function oftemperature in Fig.9.Here Dˇis the displacement rate used in this study.The displacement sensitivity was found to increase with increasing annealing temperature.It is generally accepted that the higher rate sensitivity promotes the more uniform deformation [18,19].Therefore not only the high hardening rate in bending but also high displacement rate sensitivity would enhance the uniform bending.More uniform bending after annealing above 200°C up to 450°C is thought to be attributed to the high hardening rate and high displacement rate sensitivity.One interesting observation is the interfacial debonding started slightly from 400°C and devel-oped into well-defined interfacial cracks along interface interme-tallic layer at 500°C.The development and presence of large crack along the interface intermetallic layer is likely to act against the tendency for uniform deformation induced by high hardening rate and high strain rate sensitivity at the initial stage of bending.Therefore,the localized bending after annealing at 500°C is thought to be associated with the growth of interfacial cracks along the interface reaction layer between Al and bottom Cu layer.The curvature of the interface between top Cu layer and Al layer as shown in Fig.7was measured along the distance from the center of Cu/Al/Cu plate and plotted in Fig.10.Since the shape and morp-holy of bent composites can be grouped into three groups,low (as-roll-bonded and annealed at 200°C),intermediate (300–450°C)and high (500°C)annealing temperature,the roll-bonded Cu/Al/Cu composites and Cu/Al/Cu composites annealed at 300°C and 500°C were used for the curvature measurements andtheirmagnification images of bent Cu/Al/Cu clad plates after bending displacement to 7mm.(a)As-roll-bonded;(b–f)annealed at 200°C (b),(f)for 3h.curvature values were plotted as a function of distance from the center in Fig.10.As expected,the curvature is high in the center line of the bending and decreased with distance away from the center line.The curvature measured for as-roll-bonded Cu/Al/Cu composite exhibited the peak at the center and diminished.On the other hand,the curvature in Cu/Al/Cu composite annealed at 300°C exhibited the relatively uniform values,supporting the uniform deformation during bending.After annealing at500°C the curvature at the center increased again,inducing localized bending which may be associated with the presence of large interfacial cracks(Fig.7f).The tensile stress and strain experienced by the bottom Cu layer during bending in-creases with the increase of the bending curvature,resulting in the fracture of bottom Cu layer when the curvature reaches the critical value.It should be noted in Fig.7a and b that the fracture of the bottom Cu layer induce the propagation of the crack into Al layer (marked by an arrow)in as-roll-bonded Cu/Al/Cu composite and that annealed at200°C.In Fig.7f,however,the fracture of the bottom Cu layer did not induce the propagation of the crack into Al layer probably due to the presence of interfacial cracks along the interfacial reaction layer already developed in Cu/Al/Cu clad composite annealed at500°C.Fig.11a–f display the high magnification images of the interface region between top Cu layer and middle Al layer in the bent Cu/Al/ clad composites;(a)as-rolled and annealed clad composites 200°C(b),300°C(c),400°C(d),450°C(e)and500°C(f)for3 Since the overall compressive stress is applied in this region, significant fracture took place especially for clad composites an-nealed at300°C and400°C which exhibited the higher strain hardening in the initial stage.For as-rolled and annealed clad com-posites at200°C,small cracks(marked by arrows)around the small round particles were observed in Al,suggesting the presence cracks were wide open,suggesting the presence of large localized tensile stress.As discussed,the significantly high curvature by localized bending of top Cu layer developed a large localized ten-sile stress.It is interesting to note that intermetallic layer with the thickness of28l m fractured parallel to the interface and were separated by two intermetallic layers.More cracks were observed in the intermetallic layer adjacent to Al layer than that adjacent to top Cu layer.Fig.12a–f displays the high magnification images of the inter-face region between middle Al layer and bottom Cu layer in the bent Cu/Al/Cu clad composites;(a)as-rolled and annealed clad composites at200°C(b),300°C(c),400°C(d),450°C(e)and 500°C(f)for3h.Since the overall tensile stress is applied in this region,more noticeable cracks were observed especially in as-rolled clad composites and those annealed at200°C which exhibited the low hardening rate and pronounced work softening. For as-rolled and annealed clad composites at200°C,Cu layer fractured due to the severely localized bending induced by strain softening.A large crack parallel to the interface was observed adjacent to the main crack,but it appeared to have propagated in Al layer,not along the interface between Al and the bottom Cu layer as shown in Fig.12a and b,suggesting the excellent bonding between Al and Cu.For annealed clad composites at300°C,smallFig.8.Relative hardening rate in bending compensated by the elastic to plastic bending transition load,dP/(P yÁdD)plotted as a function of displacement at the displacement rate of1mm/min.Fig.9.Displacement rate sensitivity,D log P/D log Dˇat the displacement of1.5mm plotted as a function temperature.10.Curvature of the interface between top Cu layer and Al layer(shown 7)measured along the distance from the center of Cu/Al/Cu plate.cracks were developed in the thin intermetallic layer and slip lines were observed to be emanated from these small cracks as shown in Fig.12c.For annealed clad composites at 400°C and 450°C,the periodic cracks perpendicular to the interface were observed and it appears clearer that the localized slip developed both in Cu and Al emanat-ing from the open cracks in the intermetallic layer as shown in Fig.12d and e.The localized slip marking was much clearer in Cu than in Al,reflecting the lower stacking fault energy in Cu [21].After annealing at 500°C,intermetallic layer with the thickness of 28l m fractured parallel to the interface and were also separated by two intermetallic layers.The localized slip developed both in Cu and Al emanating from the open cracks in the intermetallic layer also in the bent region at the bottom,more noticeable interface cracks and intermetallic cracks were observed in Cu/Al/Cu clad composites annealed at 500°C due to the strain localization asso-ciated with appreciable work softening in the late stage of bending.4.ConclusionsAs a result of the study on the bending behaviors of the tri-lay-ered Cu/Al/Cu composites,the following conclusions were obtained:(1)The as-roll-bonded clad composite and that annealed at200°C exhibited the initially high bending load with the extensive load plateau before a relatively rapid load drop.The more localized bending in the as-roll-bond Cu/Al/Cu clad composite and that annealed at 200°C can be attributed to the near-zero and negative work hardening rate over the whole displacement.(2)For as-roll-bonded clad composite and that annealed at200°C,fatal crack perpendicular to the Cu/Al interface through the bottom Cu layer was formed by the large tensile stress associated with the severely localizedbending.magnification images of the interface region between top Cu layer and middle Al layer in the bent Cu/Al/Cu clad plates.(a)As-rolled and 400°C (d),450°C (e)and 500°C (f)for 3h.(3)For the Cu/Al/Cu composites annealed at 300°C and up to450°C,the pronounced work hardening in the localized bent region tends to spread the localized deformation over the volume and distribute the bending deformation uniformly,leading to the rather uniform bending.(4)After annealing at 500°C,the development and presence oflarge crack along the interface intermetallic layer act against the tendency for uniform deformation induced by high hard-ening rate and high strain rate sensitivity at the initial stage of bending.(5)For the as-roll-bonded Cu/Al/Cu composite and thatannealed clad composites at 200°C,a fatal crack parallel to the interface was observed,but the crack appeared to have propagated in Al layer,not along the interface between Al and the bottom Cu layer,suggesting the excellent bonding between Al and Cu.(6)For annealed clad composites at high temperatures,the peri-odic cracks perpendicular to the interface were observed in the intermetallic layer and the localized slip developed bothin Cu and Al emanating from the open cracks.The localized slip marking was much more evident in Cu than in Al,reflecting the lower stacking fault energy in Cu.AcknowledgmentsThis work was supported by the Fundamental R&D Programs for Core Technology of Materials funded by Ministry of Knowledge Economy,Republic of Korea (10037275).References[1]Chen CY,Hwang WS.Effect of annealing on the interfacial structure ofaluminum–copper joints.Mater Trans 2007;48:1938–47.[2]Abbasi M,Karimi Taheri A,Salehi MT.Growth rate of intermetallic compoundsin Al/Cu bimetal produced by cold roll welding process.J Alloys Compd 2001;319:233–41.[3]Honarprisheh M,Asemabadi M,Sedighi M.Investigation of annealingtreatment on the interfacial properties of explosive-welded Al/Cu/Al multilayer.Mater Des2012;37:122–7.magnification images of the interface region between middle Al layer and bottom Cu layer in the bent Cu/Al/Cu clad plates.(a)As-rolled °C (c),400°C (d),450°C (e)and 500°C (f)for 3h.[4]Peng XK,Heness G,Yeung WY.Effect of rolling temperature on interface andbond strength development of roll bonded copper/aluminium metal laminates.J Mater Sci1999;34:277–81.[5]Peng XK,Wuhrer R,Heness G,Yeung WY.On the interface development andfracture behaviour of roll bonded copper/aluminium metal laminates.J Mater Sci1999;34:2029–38.[6]Lee WB,Bang KS,Jung SB.Effects of intermetallic compound on the electricaland mechanical properties of friction welded Cu/Al bimetallic joints during annealing.J Alloy Compd2005;390:212–9.[7]Rhee KY,Han WY,Park HJ,Kim SS.Fabrication of aluminum/copper cladcomposite using hot hydrostatic extrusion process and its material characteristics.Mater Sci Eng A2004;383:70–6.[8]Khosravifard A,Ebrahimi R.Investigation of parameters affecting interfacestrength in Al/Cu clad bimetal rod extrusion process.Mater Des2010;31: 493–9.[9]Manesh HD,Taheri AK.The effect of annealing treatment on mechanicalproperties of aluminum clad steel sheet.Mater Des2003;24:617–22.[10]Sheng LY,Yang F,Xi TF,Lai C,Ye HQ.Influence of heat treatment on interface ofCu/Al bimetal composite fabricated by cold posites Part B 2011;42:1468–73.[11]Li XP,Zhao ZJ,Seet HL,Heng WM,Oh TB,Lee JY.Magnetically controlledelectroplating of NiFe/Cu composite wires.Electrochem Solid-State Lett 2004;7:C1–3.[12]Venkateswara Rao N,Madhusudhan Reddy G,Nagarjuna S.Weld overlaycladding of high strength low alloy steel with austenitic stainless steel–structure and properties.Mater Des2011;32:2496–506.[13]Sudha C,Shankar P,Subba Rao RV,Thirumurugesan R.Microchemical andmicrostructural studies in a PTA weld overlay of Ni–Cr–Si–B alloy on AISI304L stainless steel.Surf Coat Technol2008;202:2103–12.[14]Findik F.Recent developments in explosive welding.Mater Des2011;32:1081–93.[15]Gulenc B.Investigation of interface properties and weldability of aluminumand copper plates by explosive welding method.Mater Des2008;29:275–8.[16]Hwang YM,Hsu HH,Lee HJ.Analysis of sandwich sheet rolling by streamfunction method.Int J Mech Sci1995;37:297–315.[17]Funamizu Y,Watanabe K.Interdiffusion in the aluminum–copper systems.JpnInst Met1971;12:147–52.[18]Lee KW,Kim SK,Kim KT,Hong SI.Ductility and strain rate sensitivity ofzircaloy-4nuclear fuel claddings.J Nucl Mater2001;295:21–6.[19]Hong SI,Kim PH,Choi YC.High strain rate superplasticity of deformationprocessed Cu–Agfilamentary composites.Scripta Mater2004;51:95–9. [20]Panahi D,Malakhov DV,Gallerneault M,Marois P.Influence of cooling rate andcomposition on formation of intermetallic phases in solidifying Al–Fe–Si melts.Can Metall Q2011;50:173–80.[21]Hong SI,Laird C.Mechanisms of slip mode modifications in f.c.c.solidsolutions.Acta Mater1990;38:1581–94.598I.-K.Kim,S.I.Hong/Materials and Design47(2013)590–598。