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Al bimetal composite fabricated by cold rolling
Composites: Part B 42 (2011) 1468–1473
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Influence of heat treatment on interface of Cu/Al bimetal composite fabricated by cold rolling L.Y. Sheng a,b,⇑, F. Yang c, T.F. Xi a, C. Lai b, H.Q. Ye d a
Peking University, Beijing 100871, China PKU-HKUST ShenZhen-Hong Kong Institution, Shenzhen 518057, China c Shenzhen Airlines, Shenzhen Bao’an International Airport, Shenzhen 518128, China d Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China b
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 16 April 2011 Accepted 30 April 2011 Available online 3 May 2011 Keywords: A. Laminates B. Microstructures B. Interface B. Mechanical properties E. Heat treatment
a b s t r a c t A copper/aluminum/copper sandwich clad sheet was fabricated by means of cold rolling process and heat treated with different temperature and time. The Al/Cu interface and its bond strength were investigated by SEM, TEM and peeling test. The results reveal that low temperature heat treatment can improve the morphology of Al/Cu interface and increase its bond strength. However high temperature and long time result in the formation of Al2Cu intermetallic compound layer, which is detrimental to the bond strength, and moreover, small Al2O3 particles precipitate along the Al2Cu and Al interface. When the interlayer along Al/Cu interface grows to a certain thickness, the effect of heat treatment temperature and time become weak. For the present study, the reasonable heat treatment may be 423 K and 20 h. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction With the development of modern industry, it is difficult to meet the varied demands such as superior electrical and thermal properties for a single material. Therefore, clad metals, consisting of two or more metals, have been developed due to their unique properties [1–3]. Till now, many kinds of techniques have been developed to fabricate the clad metals, such as explosive welding, rolling bonding, diffusion bonding, extrusion and friction-stir welding [4–9]. Among these methods, rolling bonding technique has and been extensively investigated, because of its efficiency and economy. Roll bonding is a solid state welding process to join dissimilar metals, which imply that one of the basic parameters is the degree of deformation [10–13]. There is a certain threshold deformation, and only the deformation exceeding this value could the different metals bond together. In addition, the removal of contamination layers from the surfaces by chemical and mechanical treatments is also very importance [14,15]. Previous investigations have revealed that the function of scratch-brushing is not only to clean, but also to form a brittle layer on the metal surfaces by work hardening the surface layers [16]. Since the rolling bond is based on the high deformation, there will be great stress in the metals and their ⇑ Corresponding author at: PKU-HKUST ShenZhen-Hong Kong Institution, Shenzhen 518057, China. Tel.: +86 0186 65806226; fax: +86 0755 26987080. E-mail address: [email protected] (L.Y. Sheng). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.04.045
interface. Therefore, the clad sheet must be annealed in order to obtain good formability for the next deep drawing process. In recent years, the roll bonding technique has been widely applied to fabricate many kinds of multi-layer composites, such as aluminum/copper, steel/aluminum/copper, and titanium/steel. [17–19]. Copper and aluminum clad composites have been widely studied, because of their advantages and widely application. For example, a two-layer clad sheet of aluminum/copper can almost reduce 40% in weight, with the equivalent conductivity and heat conductivity as a copper alloy. But the cost is just the 60% of a 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 investigations on the Al/Cu composite fabricated by cold rolling are still few, especially its interface. Therefore in the present work, a three-layer clad sheet comprised of copper (TU2), aluminum (1350) and copper (TU2) was fabricated by cold rolling process. The effect of heat treatment on the mechanical properties and interface diffusion of the three-layer clad sheet were investigated. 2. Experimental In this research, annealed aluminum plate (1350) and copper strip (TU2) with thickness of 12 mm and 0.6 mm, respectively, were used. The surface of aluminum plate and copper strip were scratch brushed by a 3 cm diameter stainless steel brush with
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0.3 mm wire, in order to degrease, remove the oxide film, and form a hard surface. Then the Cu/Al/Cu clad sheet was fabricated by cold rolling on a laboratory rolling mill with a loading capacity of 150 tons. The roll diameter was 400 mm and the rolling speed (x) was 5 rpm. The thickness reduction of Cu was 30% and the thickness reduction of Al 60%. Some Al/Cu clad sheet were investigated at as fabricated state, and the others were heat treated at 423 K, 573 K, 673 K and for 2 h, 5 h, 20 h, 50 h, respectively. The samples for microstructure observation and peeling test were cut from as fabricated and heat treated Al/Cu clad sheets Microstructural characterization of all samples was carried out by OLYMPUS GX41 Optical microscope (OM) and S-3400 scanning electron microscope (SEM) with energy dispersive spectrometer (EDS). The compositions of constitute phases were detected by EPMA-1610 electronic probe microanalysis (EPMA). The samples for transmission electron microscope (TEM) observation were cut from the clad sheet with different state by electro-discharge machining (EDM). The foils were mechanically ground from both sides to 30 lm and then thinned by ion milling. The TEM observation was performed by a JEM-2010 transmission electron microscope operated at 200 kV. In order to evaluate the effect of heat treatment on the interface, the bond strength of the Cu clad Al sheets were measured using the peeling test according to ASTMD1876-72 [18].
3. Results and discussion 3.1. Microstructure The interface morphology of Cu/Al/Cu clad sheets with different heat treatment temperature is shown in Fig. 1. It is clear that the Al/Cu interface of as fabricated clad sheet is not smooth, and the Al and Cu are squeezed into each other. The low temperature heat treatment causes the Al/Cu interface to become smooth, as shown in Fig. 1b. Such a phenomenon should be attributed to the diffusion between Al and Cu. With the increase of heat treatment temperature, the transition layer appeared along the Al/Cu interface, as shown in Fig. 1c and d. Moreover the width of the transition layer was increased significantly. TEM observations on the as fabricated clad sheet reveal that the Al layer experiences greater deformation than the Cu layer, as shown in Fig. 2. The deformation results in many dislocations in the Cu layer and some of the Cu grains are elongated, as shown in Fig. 2a. The inset selected area electron (SAED) pattern also shows that the spots are elongated, which indicates that there is great stress inside. In addition, a lot of twin lamellae are found in the Cu strip, as shown in Fig. 2b. The tangled dislocations can be observed along the twinned lamella and the inset SAED pattern also indicates the existence of stacking fault. The corresponding HRTEM of the twin lamella shows that many dislocations gather on the twin boundary and some exist in twin lamella. Moreover, some stacking faults forms along the twin boundary, which resulted by the movement of Shockley partial dislocations. The observations on Al layer reveal that the layer experiences a greater deformation than the copper layer and it consists of some fine microstructure with hundreds of nanometer in size, as shown in Fig. 2d. The Al grains are all greatly elongated along the rolling direction. High-density dislocations are generated along the grain boundary and inside grains. Such changes should attribute to the great deformation during clad process. The former investigations [20,21] reveal that there is a threshold reduction between 40% and 80% to Al and Cu layers in the cold roll bonding process. While in the present study the threshold reduction is more than 60%. Moreover, the Cu and Al layers experience compressive and tensile deformation during the cold rolling, which is beneficial to the
Fig. 1. Interfacial morphology of Cu/Al/Cu clad sheets with different heat treatments; (a) as fabricated, (b) 423 K/2 h, (c) 573 K/2 h, (d) 673 K/2 h.
microstructure refinement. However, such a great deformation can lead to a lot of crystal defects, such as twinned crystal, stacking faults and dislocations. But according to the recent research of Lu et al. [22], the copper with high density of nanoscale twins can have both high strength and high electrical conductivity. So one can ascertain that the Cu/Al/Cu clad sheet should have a good electrical conductivity.
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Fig. 2. (a) Bright field TEM micrograph of Cu layer in as fabricated Cu/Al/Cu clad sheets (inset picture shows a SAED pattern of the deformed Cu), (b) enlarged image of twin lamella in (a) (inset picture shows a SAED pattern of the twin lamellae), (c) HREM images observed along [0 1 1] axis in (b) (dislocations on twin boundary are marked by black ‘‘\’’ and dislocations in twin lamella are marked by black arrows), (d) bright field TEM image of Al layer with nano grains (Inset picture showing the SAED pattern of the nanocrystalline Al).
The clad sheet was heat treated at different temperature for different time, in order to study the change of Cu/Al interface during heat treatment. The concentration profiles of Al and Cu elements
Fig. 3. EPMA profiles of Al Cu elements across the interface of the Cu/Al/Cu clad sheet with different heat treatment; (a) 423 K/5 h, (b) 423 K/150 h, (c) 573 K/2 h, (d) 573 K/20 h.
across the interface at 423 K and 573 K are shown in Fig. 3. At the temperature of 423 K, the 5 h heat treatment has led to the formation of interlayer. The width of interlayer increases obviously, when the time is increased to 150 h. Moreover, the interlayer
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changes into several layers, as shown in Fig. 3b. At 573 K, the 2 h heat treatment has caused the formation of interlayer, but the thickness is not uniform. When the heating time increase to 20 h, the thickness of interlayer grows to two times as the 2 h treated one. However, the thickness of interlayer almost has no change, when the heating time increases further. The morphology of interlayer and its composition in the clad sheet heat-treated at 573 K for 20 h are shown in Fig. 4 and Table 1 respectively. The observations indicate that the interlayer consists of several kind of layers, as shown in Fig. 4. The EDS tests on the interlayer reveal that the layer adjacent to the Al has many Cu, and the proportion of Al to Cu is about 2:1, as shown in Fig. 4b. When the test position deviates from the Al, the EDS tests show that the Al concentration in the layer increases again, as shown in Fig. 4c and d. That is very different from the previous research [7], which shows that the layers from Al to Cu are Al2Cu, AlCu, Al(Cu) solid solution, Cu4Al3 and Cu3Al, respectively. However, in research [23] a heat treatment time of 1000 h has been used which is much larger than the heat treatment of 20 h used in present work. The multilayer intermetallic compounds were found in that study. In fact, one may conclude that the magnitude of heat treatment time influences considerably the number of layers as well as the chemical composition of the intermetallics forming on the interface during the heat treatment. The TEM observation on the interlayers of clad sheet treated at 423 K for 1 h is shown in Fig. 5a. There are still a lot of dislocations in Cu and Al layers, and no other layers forms on the Cu/Al interface.
Table 1 Compositions of different positions along interlayer in the Cu/Al/Cu clad sheet (at.%). Test position
Cu
Al
A B C
32.21 18.52 23.77
67.79 81.48 76.23
The corresponding SAED pattern also shows that there are many fine grains in Cu layer. With heat treatment time increasing to 20 h, the Al2Cu layer forms along the Cu/Al interface, as shown in Fig. 5b. HRTEM image of the Al and Al2Cu layer interface viewed along the [0 0 1] direction of Al2Cu phase is shown in Fig. 5c. It can be seen that the Al and Al2Cu phase interface is not straight. The FFT of Al layer reveals that the Al crystal structure is also along the [0 0 1] direction. The Al2Cu phase has a tetragonal crystal structure with a = 0.6067 nm and c = 0.4887 nm. While Al has a Facecenter crystal structure with a = 0.4049. So along the [0 0 1] crystal direction, the lattice parameters of Al and Al2Cu differ greatly. In order to decrease the effect of lattice difference, a transition layer forms along the Al and Al2Cu interface. In addition, some small particles (as indicated by white arrows) with decades nanometers in size also have been observed in clad sheet with 573 K 10 h treatment, as shown in Fig. 5d. These particles should be Al2O3, which is resulted by the surface oxide or residual oxygen during fabrication. The previous studies [7,24] exhibit that the surface treatment on the Al may lead to a thin oxide film, which will be segmented
Fig. 4. (a) SEM micrograph of Cu/Al interface heat-treated at 573 K for 1 h, (b) EDS of A point, (c) EDS of B point, (d) EDS of C point.
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Fig. 6. Peeling force of Cu/Al/Cu clad sheet with different heat treatment.
segregation of Al2O3 along the Al/Al2Cu phase interface is harmful to the bond strength of the Cu/Al/Cu clad sheet. 3.2. Bond strength In order to evaluate the effect of heat treatment on the bond strength of Cu/Al/Cu clad sheet, the peeling force test was used. The variation of peeling force with heat treatment is shown in Fig. 6. It is clear that the appropriate heat treatment can increase the bond strength of the clad sheet. The clad sheet has the highest bond strength at 423 K 20 h treatment. The clad sheet with 573 K 20 h heat treatment has the lowest bond strength, which indicates the high temperature and long treating time decrease the bond strength. According to the observations above, the high temperature and long time heat treatment can result in the fast growth of intermetallic compound along the Cu and Al interface. According to the former researches [25–27], the intermetallic compound has high strength but low ductility. In addition, the intermetallic compound has big lattice difference with Al and Cu crystal, which lead to high stress along the interface. Such changes are all detrimental to the bond strength. Therefore, one can believe that relative low temperature and short time heat treatment is helpful to improve the bond strength of the Cu/Al/Cu clad sheet. 4. Conclusions
Fig. 5. (a) Bright field TEM image of Cu/Al interface of clad sheet with 423 K 1 h treatment (Inset picture showing the SAED pattern of Cu layer), (b) formation of Al2Cu layer along Cu/Al interface after 423 K 20 h (Inset picture showing the SAED pattern of Al2Cu layer), (c) HRTEM image of Al2Cu with the electron beam parallel to the [0 0 1] direction, (d) precipitates of Al2O3 along Al/Al2Cu interface in clad sheet with 573 K 10 h treatment (as the arrow pointed).
into pieces during cold roll bonding process. The long time and high temperature heat treatment promotes the elements diffusion. Due to the low interfacial energy of the Al/Al2Cu phase interface, the fine Al2O3 particles precipitate along the interface. However the
(1) The heat treatment can lead to the formation of interlayer, and the temperature has a greater effect on the growth of the interlayer than the heat treatment time. (2) When the interlayer grows to a certain thickness, the effect of temperature and time become weak. (3) The Al2Cu intermetallic compound layer forms adjacent to Al layer. In addition, small Al2O3 particles precipitate along the Al2Cu and Al interface at high temperature treatment. (4) The relative low temperature and short time heat treatment can well improve the bond strength of the Cu/Al/Cu clad sheet significantly. Acknowledgements The work is financially supported by the National Basic Research Program (973 Program) of China (2009CB930004), the National High Technology Research and Development Program (863 program) of China (2011AA030104) and the Science and Technology Research Foundation of Shenzhen Bureau of Science and Technology & Information (JC200903170498A).
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References [1] Cowan GR, Bergmann OR, Holtzman AH. Mechanism of bond zone wave formation in explosion-clad metals. Metall Mater Trans B 1971;2:3145–55. [2] He P, Yue X, Zhang JH. Hot pressing diffusion bonding of a titanium alloy to a stainless steel with an aluminum alloy interlayer. Mater Sci Eng A 2008;486:171–6. [3] Manesh HD, Taheri AK. Bond strength and formability of an aluminum-clad steel sheet. J Alloys Compd 2003;361:138–43. [4] Yang Y, Xinming Z, Zhenghua L, Qingyun L. Adiabatic shear band on the titanium side in the Ti/mild steel explosive cladding interface. Acta Mater 1996;44(2):561–5. [5] Han JH, Ahn JP, Shin MC. Effect of interlayer thickness on shear deformation behavior of AA5083 aluminum alloy/SS41 steel plates manufactured by explosive welding. J Mater Sci 2003;38:13–8. [6] Mori T, Kurimoto S. Press-formability of stainless steel and aluminum clad sheet. J Mater Process Technol 1996;56:242–53. [7] Abbasi M, Taheri AK, Salehi MT. Grow rate of intermetallic compounds in Al/Cu bimetal produced by cold roll welding process. J Alloys Compd 2001;319:233–41. [8] He P, Yue X, Zhang JH. Hot pressing diffusion bonding of a titanium alloy to a stainless steel with an aluminum alloy interlayer. Mater Sci Eng A 2008;486:171–6. [9] Lee W, Bang K, Jung S. Effects of intermetallic compound on the electrical and mechanical properties of friction welded Cu/Al bimetallic joints during annealing. J Alloys Compd 2005;390:212–9. [10] Pan D, Gao K, Yu J. Cold roll bonding of bimetallic sheet and strips. Mater Sci Technol 1989;5:934–9. [11] Danesh Manesh H, Karimi Taheri A. Study of mechanisms of cold roll welding of aluminum alloy to steel strip. Mater Sci Technol 2004;20(8):1064–8. [12] Yahiro A, Masui T, Yoshida T, Doi D. Development of nonferrous clad plate and sheet by warm rolling with different temperature of materials. ISIJ Int 1991;31(6):647–54. [13] Cave JA, Williams JD. The mechanisms of cold pressure welding by rolling. J Inst Met 1973;101:203–7.
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[14] Le HR, Stucliffe MPF, Wang PZ, Burstein GT. Surface oxide fracture in cold aluminum rolling. Acta Mater 2004;52:911–20. [15] Tylecote RF, Howd D, Furmidge JR. The influence of surface films on the pressure welding of metals. Br Weld J 1958;5:21–38. [16] Bay N. Cold welding: part I. Characteristic, bonding mechanisms, bond strength. Met Construct 1986;18(6):369–72. [17] Eizadjou M, Kazemi Talachi A, Danesh Manesh H, Shakur Shahabi H, Janghorban K. Investigation of structure and mechanical properties of multilayered Al/Cu composite produced by accumulative roll bonding (ARB) process. Compos Sci Technol 2008;68:2003–9. [18] Danesh Manesh H, Karimi Taheri A. The effect of annealing treatment on mechanical properties of aluminum clad steel sheet. Mater Des 2003;24:617–22. [19] Fukuda T, Seino Y. Bonding strength and microstructure of bonding interface of hot rolled titanium clad steel. J Iron Steel Inst Jpn 1989;75:1162–9. [20] Bay N. Cold pressure welding; characteristics, bonding mechanism, bond strength. J Metal Constr 1986;18:369–72. [21] Clemensen C, Julstarp O. Cold welding-influence of surface preparation on bond strength. J Metal Constr 1986;18:625–9. [22] Lu L, Shen YF, Chen XH, Qian LH, Lu K. Ultrahigh strength and high electrical conductivity in copper. Science 2004;304:422–6. [23] Abbasi M. Investigation on properties of cold roll welding of aluminum and copper. PhD thesis Iran University of Science and Technology. Fac Metall Mater Sci 2001:164–8. [24] Vaidyanath LR, Nicholas MG, Milner DR. Pressure welding by rolling. Br Weld J 1959;6:13–28. [25] Sheng LY, Zhang W, Guo JT, Wang ZS, Ye HQ. Microstructure evolution and elevated temperature compressive properties of a rapidly solidified NiAl– Cr(Nb)/Dy alloy. Mater Des 2009;30:2752–5. [26] Miyazaki S, Kumai S, Sato A. Plastic deformation of Al–Cu–Fe quasicrystals embedded in Al2Cu at low temperatures. Mater Sci Eng A 2005;400– 401:300–5. [27] Sheng LY, Guo JT, Tian YX, Zhou LZ, Ye HQ. Microstructure and mechanical properties of rapidly solidified NiAl–Cr(Mo) eutectic alloy doped with trace Dy. J Alloys Compd 2009;475:730–4.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Influence of heat treatment on interface of Cu/Al bimetal composite fabricated by cold rolling L.Y. Sheng a,b,⇑, F. Yang c, T.F. Xi a, C. Lai b, H.Q. Ye d a
Peking University, Beijing 100871, China PKU-HKUST ShenZhen-Hong Kong Institution, Shenzhen 518057, China c Shenzhen Airlines, Shenzhen Bao’an International Airport, Shenzhen 518128, China d Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China b
a r t i c l e
i n f o
Article history: Received 15 January 2011 Received in revised form 16 April 2011 Accepted 30 April 2011 Available online 3 May 2011 Keywords: A. Laminates B. Microstructures B. Interface B. Mechanical properties E. Heat treatment
a b s t r a c t A copper/aluminum/copper sandwich clad sheet was fabricated by means of cold rolling process and heat treated with different temperature and time. The Al/Cu interface and its bond strength were investigated by SEM, TEM and peeling test. The results reveal that low temperature heat treatment can improve the morphology of Al/Cu interface and increase its bond strength. However high temperature and long time result in the formation of Al2Cu intermetallic compound layer, which is detrimental to the bond strength, and moreover, small Al2O3 particles precipitate along the Al2Cu and Al interface. When the interlayer along Al/Cu interface grows to a certain thickness, the effect of heat treatment temperature and time become weak. For the present study, the reasonable heat treatment may be 423 K and 20 h. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction With the development of modern industry, it is difficult to meet the varied demands such as superior electrical and thermal properties for a single material. Therefore, clad metals, consisting of two or more metals, have been developed due to their unique properties [1–3]. Till now, many kinds of techniques have been developed to fabricate the clad metals, such as explosive welding, rolling bonding, diffusion bonding, extrusion and friction-stir welding [4–9]. Among these methods, rolling bonding technique has and been extensively investigated, because of its efficiency and economy. Roll bonding is a solid state welding process to join dissimilar metals, which imply that one of the basic parameters is the degree of deformation [10–13]. There is a certain threshold deformation, and only the deformation exceeding this value could the different metals bond together. In addition, the removal of contamination layers from the surfaces by chemical and mechanical treatments is also very importance [14,15]. Previous investigations have revealed that the function of scratch-brushing is not only to clean, but also to form a brittle layer on the metal surfaces by work hardening the surface layers [16]. Since the rolling bond is based on the high deformation, there will be great stress in the metals and their ⇑ Corresponding author at: PKU-HKUST ShenZhen-Hong Kong Institution, Shenzhen 518057, China. Tel.: +86 0186 65806226; fax: +86 0755 26987080. E-mail address: [email protected] (L.Y. Sheng). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.04.045
interface. Therefore, the clad sheet must be annealed in order to obtain good formability for the next deep drawing process. In recent years, the roll bonding technique has been widely applied to fabricate many kinds of multi-layer composites, such as aluminum/copper, steel/aluminum/copper, and titanium/steel. [17–19]. Copper and aluminum clad composites have been widely studied, because of their advantages and widely application. For example, a two-layer clad sheet of aluminum/copper can almost reduce 40% in weight, with the equivalent conductivity and heat conductivity as a copper alloy. But the cost is just the 60% of a 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 investigations on the Al/Cu composite fabricated by cold rolling are still few, especially its interface. Therefore in the present work, a three-layer clad sheet comprised of copper (TU2), aluminum (1350) and copper (TU2) was fabricated by cold rolling process. The effect of heat treatment on the mechanical properties and interface diffusion of the three-layer clad sheet were investigated. 2. Experimental In this research, annealed aluminum plate (1350) and copper strip (TU2) with thickness of 12 mm and 0.6 mm, respectively, were used. The surface of aluminum plate and copper strip were scratch brushed by a 3 cm diameter stainless steel brush with
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0.3 mm wire, in order to degrease, remove the oxide film, and form a hard surface. Then the Cu/Al/Cu clad sheet was fabricated by cold rolling on a laboratory rolling mill with a loading capacity of 150 tons. The roll diameter was 400 mm and the rolling speed (x) was 5 rpm. The thickness reduction of Cu was 30% and the thickness reduction of Al 60%. Some Al/Cu clad sheet were investigated at as fabricated state, and the others were heat treated at 423 K, 573 K, 673 K and for 2 h, 5 h, 20 h, 50 h, respectively. The samples for microstructure observation and peeling test were cut from as fabricated and heat treated Al/Cu clad sheets Microstructural characterization of all samples was carried out by OLYMPUS GX41 Optical microscope (OM) and S-3400 scanning electron microscope (SEM) with energy dispersive spectrometer (EDS). The compositions of constitute phases were detected by EPMA-1610 electronic probe microanalysis (EPMA). The samples for transmission electron microscope (TEM) observation were cut from the clad sheet with different state by electro-discharge machining (EDM). The foils were mechanically ground from both sides to 30 lm and then thinned by ion milling. The TEM observation was performed by a JEM-2010 transmission electron microscope operated at 200 kV. In order to evaluate the effect of heat treatment on the interface, the bond strength of the Cu clad Al sheets were measured using the peeling test according to ASTMD1876-72 [18].
3. Results and discussion 3.1. Microstructure The interface morphology of Cu/Al/Cu clad sheets with different heat treatment temperature is shown in Fig. 1. It is clear that the Al/Cu interface of as fabricated clad sheet is not smooth, and the Al and Cu are squeezed into each other. The low temperature heat treatment causes the Al/Cu interface to become smooth, as shown in Fig. 1b. Such a phenomenon should be attributed to the diffusion between Al and Cu. With the increase of heat treatment temperature, the transition layer appeared along the Al/Cu interface, as shown in Fig. 1c and d. Moreover the width of the transition layer was increased significantly. TEM observations on the as fabricated clad sheet reveal that the Al layer experiences greater deformation than the Cu layer, as shown in Fig. 2. The deformation results in many dislocations in the Cu layer and some of the Cu grains are elongated, as shown in Fig. 2a. The inset selected area electron (SAED) pattern also shows that the spots are elongated, which indicates that there is great stress inside. In addition, a lot of twin lamellae are found in the Cu strip, as shown in Fig. 2b. The tangled dislocations can be observed along the twinned lamella and the inset SAED pattern also indicates the existence of stacking fault. The corresponding HRTEM of the twin lamella shows that many dislocations gather on the twin boundary and some exist in twin lamella. Moreover, some stacking faults forms along the twin boundary, which resulted by the movement of Shockley partial dislocations. The observations on Al layer reveal that the layer experiences a greater deformation than the copper layer and it consists of some fine microstructure with hundreds of nanometer in size, as shown in Fig. 2d. The Al grains are all greatly elongated along the rolling direction. High-density dislocations are generated along the grain boundary and inside grains. Such changes should attribute to the great deformation during clad process. The former investigations [20,21] reveal that there is a threshold reduction between 40% and 80% to Al and Cu layers in the cold roll bonding process. While in the present study the threshold reduction is more than 60%. Moreover, the Cu and Al layers experience compressive and tensile deformation during the cold rolling, which is beneficial to the
Fig. 1. Interfacial morphology of Cu/Al/Cu clad sheets with different heat treatments; (a) as fabricated, (b) 423 K/2 h, (c) 573 K/2 h, (d) 673 K/2 h.
microstructure refinement. However, such a great deformation can lead to a lot of crystal defects, such as twinned crystal, stacking faults and dislocations. But according to the recent research of Lu et al. [22], the copper with high density of nanoscale twins can have both high strength and high electrical conductivity. So one can ascertain that the Cu/Al/Cu clad sheet should have a good electrical conductivity.
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Fig. 2. (a) Bright field TEM micrograph of Cu layer in as fabricated Cu/Al/Cu clad sheets (inset picture shows a SAED pattern of the deformed Cu), (b) enlarged image of twin lamella in (a) (inset picture shows a SAED pattern of the twin lamellae), (c) HREM images observed along [0 1 1] axis in (b) (dislocations on twin boundary are marked by black ‘‘\’’ and dislocations in twin lamella are marked by black arrows), (d) bright field TEM image of Al layer with nano grains (Inset picture showing the SAED pattern of the nanocrystalline Al).
The clad sheet was heat treated at different temperature for different time, in order to study the change of Cu/Al interface during heat treatment. The concentration profiles of Al and Cu elements
Fig. 3. EPMA profiles of Al Cu elements across the interface of the Cu/Al/Cu clad sheet with different heat treatment; (a) 423 K/5 h, (b) 423 K/150 h, (c) 573 K/2 h, (d) 573 K/20 h.
across the interface at 423 K and 573 K are shown in Fig. 3. At the temperature of 423 K, the 5 h heat treatment has led to the formation of interlayer. The width of interlayer increases obviously, when the time is increased to 150 h. Moreover, the interlayer
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changes into several layers, as shown in Fig. 3b. At 573 K, the 2 h heat treatment has caused the formation of interlayer, but the thickness is not uniform. When the heating time increase to 20 h, the thickness of interlayer grows to two times as the 2 h treated one. However, the thickness of interlayer almost has no change, when the heating time increases further. The morphology of interlayer and its composition in the clad sheet heat-treated at 573 K for 20 h are shown in Fig. 4 and Table 1 respectively. The observations indicate that the interlayer consists of several kind of layers, as shown in Fig. 4. The EDS tests on the interlayer reveal that the layer adjacent to the Al has many Cu, and the proportion of Al to Cu is about 2:1, as shown in Fig. 4b. When the test position deviates from the Al, the EDS tests show that the Al concentration in the layer increases again, as shown in Fig. 4c and d. That is very different from the previous research [7], which shows that the layers from Al to Cu are Al2Cu, AlCu, Al(Cu) solid solution, Cu4Al3 and Cu3Al, respectively. However, in research [23] a heat treatment time of 1000 h has been used which is much larger than the heat treatment of 20 h used in present work. The multilayer intermetallic compounds were found in that study. In fact, one may conclude that the magnitude of heat treatment time influences considerably the number of layers as well as the chemical composition of the intermetallics forming on the interface during the heat treatment. The TEM observation on the interlayers of clad sheet treated at 423 K for 1 h is shown in Fig. 5a. There are still a lot of dislocations in Cu and Al layers, and no other layers forms on the Cu/Al interface.
Table 1 Compositions of different positions along interlayer in the Cu/Al/Cu clad sheet (at.%). Test position
Cu
Al
A B C
32.21 18.52 23.77
67.79 81.48 76.23
The corresponding SAED pattern also shows that there are many fine grains in Cu layer. With heat treatment time increasing to 20 h, the Al2Cu layer forms along the Cu/Al interface, as shown in Fig. 5b. HRTEM image of the Al and Al2Cu layer interface viewed along the [0 0 1] direction of Al2Cu phase is shown in Fig. 5c. It can be seen that the Al and Al2Cu phase interface is not straight. The FFT of Al layer reveals that the Al crystal structure is also along the [0 0 1] direction. The Al2Cu phase has a tetragonal crystal structure with a = 0.6067 nm and c = 0.4887 nm. While Al has a Facecenter crystal structure with a = 0.4049. So along the [0 0 1] crystal direction, the lattice parameters of Al and Al2Cu differ greatly. In order to decrease the effect of lattice difference, a transition layer forms along the Al and Al2Cu interface. In addition, some small particles (as indicated by white arrows) with decades nanometers in size also have been observed in clad sheet with 573 K 10 h treatment, as shown in Fig. 5d. These particles should be Al2O3, which is resulted by the surface oxide or residual oxygen during fabrication. The previous studies [7,24] exhibit that the surface treatment on the Al may lead to a thin oxide film, which will be segmented
Fig. 4. (a) SEM micrograph of Cu/Al interface heat-treated at 573 K for 1 h, (b) EDS of A point, (c) EDS of B point, (d) EDS of C point.
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Fig. 6. Peeling force of Cu/Al/Cu clad sheet with different heat treatment.
segregation of Al2O3 along the Al/Al2Cu phase interface is harmful to the bond strength of the Cu/Al/Cu clad sheet. 3.2. Bond strength In order to evaluate the effect of heat treatment on the bond strength of Cu/Al/Cu clad sheet, the peeling force test was used. The variation of peeling force with heat treatment is shown in Fig. 6. It is clear that the appropriate heat treatment can increase the bond strength of the clad sheet. The clad sheet has the highest bond strength at 423 K 20 h treatment. The clad sheet with 573 K 20 h heat treatment has the lowest bond strength, which indicates the high temperature and long treating time decrease the bond strength. According to the observations above, the high temperature and long time heat treatment can result in the fast growth of intermetallic compound along the Cu and Al interface. According to the former researches [25–27], the intermetallic compound has high strength but low ductility. In addition, the intermetallic compound has big lattice difference with Al and Cu crystal, which lead to high stress along the interface. Such changes are all detrimental to the bond strength. Therefore, one can believe that relative low temperature and short time heat treatment is helpful to improve the bond strength of the Cu/Al/Cu clad sheet. 4. Conclusions
Fig. 5. (a) Bright field TEM image of Cu/Al interface of clad sheet with 423 K 1 h treatment (Inset picture showing the SAED pattern of Cu layer), (b) formation of Al2Cu layer along Cu/Al interface after 423 K 20 h (Inset picture showing the SAED pattern of Al2Cu layer), (c) HRTEM image of Al2Cu with the electron beam parallel to the [0 0 1] direction, (d) precipitates of Al2O3 along Al/Al2Cu interface in clad sheet with 573 K 10 h treatment (as the arrow pointed).
into pieces during cold roll bonding process. The long time and high temperature heat treatment promotes the elements diffusion. Due to the low interfacial energy of the Al/Al2Cu phase interface, the fine Al2O3 particles precipitate along the interface. However the
(1) The heat treatment can lead to the formation of interlayer, and the temperature has a greater effect on the growth of the interlayer than the heat treatment time. (2) When the interlayer grows to a certain thickness, the effect of temperature and time become weak. (3) The Al2Cu intermetallic compound layer forms adjacent to Al layer. In addition, small Al2O3 particles precipitate along the Al2Cu and Al interface at high temperature treatment. (4) The relative low temperature and short time heat treatment can well improve the bond strength of the Cu/Al/Cu clad sheet significantly. Acknowledgements The work is financially supported by the National Basic Research Program (973 Program) of China (2009CB930004), the National High Technology Research and Development Program (863 program) of China (2011AA030104) and the Science and Technology Research Foundation of Shenzhen Bureau of Science and Technology & Information (JC200903170498A).
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