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Cu composite plates
Materials and Design 47 (2013) 590–598
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Effect of heat treatment on the bending behavior of tri-layered Cu/Al/Cu composite plates In-Kyu Kim, Sun Ig Hong ⇑ Department of Nanomaterials Engineering, Chungnam National University, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Accepted 27 December 2012 Available online 5 January 2013 Keywords: Layered composite Interfacial fracture Intermetallics Bending Work hardening
a b s t r a c t Bending 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. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The demand for advanced materials with enhanced properties and functions increased as the technology advanced. Clad composite 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 combination 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 between 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 studied because of their advantages associated with high conductivity, low density and price competitiveness over copper and copper alloys. 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 in
Cu/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 CuxAly intermetallic compound at elevated temperature could deteriorate the mechanical and electrical 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], electroplating [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 rollbonding and their bending behaviors were studied. The bending properties of Cu/Al/Cu composite plates should be understood for further 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 details ⇑ Corresponding author. Tel.: +82 42 8216595; fax: +82 822 5850. E-mail address: [email protected] (S.I. Hong). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.070
The Cu/Al/Cu clad-composite was fabricated using roll-bonding process. Materials used in this study were OFHC copper and
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Fig. 1. Cross section of the Cu/Al/Cu clad metal (a) and schematic of the test setup for three-point bending (b).
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 2.0 mm and that of Cu and Al layer was 0.2 mm and 1.6 mm respectively (Fig. 1a). The bending specimen with a length 40 mm and the width 5.0 mm were used for three-point bending test (Fig. 1b). In Fig. 1b, top Cu layer experiences the overall compressive stress while the bottom Cu layer experiences the overall tensile stress. The clad metal was annealed at temperatures at 200 °C, 300 °C 400 °C 450 °C and 500 °C for 3 h. To examine the bendability, the 3-point bending test were performed using a Universal Materials Testing Machine (UNITED, US/SSTM) at room temperature. The crosshead speed was maintained at the speed of 1 mm/min and 10 mm/min. Interfacial structures of Cu/Al/Cu clad metal were examined by an optical microscope (OM) and a scanning electron microscope (SEM). The chemical compositions of
Fig. 3. Thickness of interfacial reaction layer plotted against the annealing temperature (for 3 h).
intermetallic compounds formed at the Cu/Al interface were examined 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.
3. Results and discussion Fig. 2 shows the optical micrographs of Cu/Al interfaces for clad composite materials; as-rolled (a) and annealed at 300 °C (b), 400 °C (c), and 500 °C (d) for 3 h. For the as-rolled clad metal, no interface intermetallic layer was observed as shown in Fig. 2a.
Fig. 2. Optical micrographs of Cu/Al interface region in Cu/Al/Cu clad composites; as-roll-bonded (a) and annealed at 300 °C for 3 h (b), 400 °C for 3 h (c), and 500 °C for 3 h (d).
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Fig. 4. Back-scattered electron image (a) and EDX spectra (b–d) from layers marked by numbers 1 (b), 2 (c) and 3 (d) in the Cu/Al interface region (a).
After annealing at 200 °C, no visible change of the interface structure was observed by OM. When annealing temperature was raised to 300 °C, intermetallic reaction layer was formed with a thickness of about 2 lm (b). With the increase of annealing temperature to 400 °C, and 450 °C, the reaction layer continues to thicken to 12 lm, and 17 lm respectively. When the annealing temperature reached 500 °C, the interface increased more rapidly to a thickness of 28 lm. At higher temperatures, interfacial reaction layers appeared to consist of two or three layers. The multi-layered interfacial reaction products are more clearly visible in Fig. 2d. The thickness of the interfacial reaction layer was measured and plotted as a function annealing temperature in Fig. 3. Fig. 4 shows the back-scattered electron image (a) and EDX spectra (b, c, d) from regions 1, 2, and 3 marked in Fig. 4a. The region 2 in this figure appears to consist of two separate layers, but EDS spectra was obtained from the center of layer 2 because it was too thin. Chen and Hwang [1] reported that four layers of intermetallic compounds, Al4Cu9, Al3Cu4, AlCu, Al2Cu, were formed at the interface of Cu/Al at 500 °C. The intermetallic phases can be identified based on XRD analyses and the equilibrium phase diagram analyses of Al–Cu system [17]. In this study, in order to identify the thin intermetallic layers more accurately, tri-layered Cu/Al/ Cu composite plates annealed at 500 °C were separated along the brittle intermetallic layers and the fractured interfaces of two separated layers, Cu side and Al side were examined by the XRD analysis. Fig. 5a and b displays the XRD peaks from the fractured interfaces of the Cu side (a) and Al side (b). It should be noted that the peaks from the intermetallic compounds are strong because the separated layer fractured along the brittle intermetallic layers. Fig 5a and b demonstrate the presence of Cu9Al4, Cu3Al2 and CuAl (Cu3Al4) on the Cu side and CuAl2 and CuAl. It seem to be clear that intermetallic layers 1 and 3 are suggested to be CuAl2, and Cu9Al4 based on the results of in Figs. 4 and 5. On the other hand, the
Fig. 5. XRD peaks from the separated Cu and Al layers (a) Cu side and (b) Al side.
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region 2 in Fig. 4 is thought to consist of two layers Cu3Al2 and CuAl, which is supported by the XRD analysis. Cu3Al4 was not identified in the XRD analyses of the present study. The room temperature load–displacement curves of as-rollbonded and annealed Cu/Al/Cu clad composite at the displacement rate of 1 mm/min and 10 mm/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 composite annealed at 300 °C and up to 450 °C, on the other hand, exhibited the lower bending load due to the softening associated with recovery/recrystallization in Cu/Al/Cu clad metals, but the higher work hardening for an extended period before gradual softening took place. Cu/Al/Cu clad composite annealed at 500 °C exhibited the high work hardening behavior initially, but rather faster softening at the final stage of bending. All specimens exhibited the elastic bending behavior initially accompanied by plastic bending [8]. The elastic to plastic bending transition load, Py was 280 N for the as-roll-bonded clad composite and that annealed at 200 °C and approximately 40 N for Cu/Al/Cu clad composite annealed at 300 °C and up to 500 °C. Fig. 7a–f displays the low magnification images of bent Cu/Al/ Cu clad composites after bending displacement to 7 mm; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c),
Fig. 6. Load–displacement curves of as-rolled clad metal and annealed clad metals at various temperatures at the displacement rate of 1 mm/min (a) and 10 mm/min (b).
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400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. In the as-rolled and annealed clad composites at 200 °C which exhibited the extended load plateau and work softening, top Cu layers showed the significantly 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 at 300 °C and 400 °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. 7 were taken after displacement up to 7 mm at which work softening already took effect (Fig. 6). Cu/Al/Cu clad composite plate annealed at 500 °C with the extended softening (or weakening) at the final 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 at 500 °C. The hardening rate in bending can be defined to be the load increase rate as a function of displacement, dP/dD, where P is the bending load and D is the bending displacement. The relative hardening rate compensated by strength is thought to be more meaningful in predicting the uniformity of deformation. Therefore, the elastic load compensated hardening rate in bending, dP/(Py dD) at the displacement rate of 1 mm/min was calculated from Fig. 6a and plotted as a function of displacement in Fig. 7. Here, Py is the elastic to plastic bending transition load (Fig. 6). The elastic load compensated hardening rate will be called relative hardening 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 at 200 °C decreased very rapidly and dropped to nearzero at the displacement of 0.5 mm and remained around zero for an extended period before it went into the negative region. Cu/Al/Cu clad composite annealed at 300–450 °C exhibited the higher relative hardening rates, which gradually dropped zero at the displacement of 3.5 mm. The relative hardening rates of Cu/ Al/Cu clad composite annealed at 500 °C was observed to be similar to those of 300–450 °C initially, but decreased more rapidly and dropped to zero at the displacement of 2.6 mm. One interesting observation for Cu/Al/Cu clad composite annealed at 500 °C is that the hardening rate decreased down to 1.0/mm, which suggests the drastic softening (or weakening). The drastic decrease of the bending load Cu/Al/Cu clad composite annealed at 500 °C at the displacement of 3.0–4.2 mm can be linked to the interfacial cracks between Al and bottom Cu layer, which is most pronounced after annealing at 500 °C. The rapid drop of the load after the displacement of 2.5 mm 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 (at 200 °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 negligible, leading to the localized fracture. For the Cu/Al/Cu composites annealed at 300 °C and up to 450 °C, the pronounced work hardening in the localized bent region tends to spread the localized deformation and distribute the bending deformation uniformly, leading to the rather uniform bending. Cu/Al/Cu clad annealed at 500 °C is likely to behave similarly to those annealed at high temperatures in the initial stage because of the initial high work hardening, but in the final stage it is likely to behave similarly to the as-roll-bonded clad composite
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Fig. 7. Low magnification images of bent Cu/Al/Cu clad plates after bending displacement to 7 mm. (a) As-roll-bonded; (b–f) annealed at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
because of the rapid softening (or weakening). The faster load drop after the displacement of 3 mm in the Cu/Al/Cu clad composite annealed at 500 °C may be linked to the more severe interfacial debonding and interfacial cracks compared to those samples heattreated at 300–450 °C. ˇ The displacement rate sensitivity defined by the D log P/D log D were calculated using the data in Fig. 6a and b at the displacement of 1.5 mm at various temperatures and plotted as a function of ˇ is the displacement rate used in this temperature in Fig. 9. Here D 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 developed into well-defined interfacial cracks along interface intermetallic 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. 7 was measured along the distance from the center of Cu/Al/Cu plate and plotted in Fig. 10. Since the shape and morpholy of bent composites can be grouped into three groups, low (asroll-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 and their
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Fig. 8. Relative hardening rate in bending compensated by the elastic to plastic bending transition load, dP/(Py dD) plotted as a function of displacement at the displacement rate of 1 mm/min.
ˇ at the displacement of 1.5 mm Fig. 9. Displacement rate sensitivity, D log P/D log D plotted as a function temperature.
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 at 500 °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 increases 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 at 200 °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 at 500 °C.
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Fig. 10. Curvature of the interface between top Cu layer and Al layer (shown in Fig. 7) measured along the distance from the center of Cu/Al/Cu plate.
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/ Cu clad composites; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. Since the overall compressive stress is applied in this region, no significant fracture took place especially for clad composites annealed at 300 °C and 400 °C which exhibited the higher strain hardening in the initial stage. For as-rolled and annealed clad composites at 200 °C, small cracks (marked by arrows) around the small round particles were observed in Al, suggesting the presence of localized tensile stress. Panahi et al. [20] suggested that many round particles in Al–0.3Fe–0.45Si was a-AlFeSi. The round intermetallic particles observed in the commercial purity Al in this study are supposed to be a-AlFeSi pericles. As shown in Fig. 10a, the more severe localized bending in the top Cu layer compared to the region below can induce the localized tensile stress, resulting in the cracks around the interface of particles or particle cracks in Al. For annealed clad composites at 300 °C, both Cu and Al deformed uniformly with neither interfacial cracks nor debonding between top Cu layer and Al layer. For annealed clad composites at 400 °C and 450 °C, the typical compressive cracks in the intermetallic layer were observed. In Cu/Al/Cu annealed at 500 °C, some 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 tensile stress. It is interesting to note that intermetallic layer with the thickness of 28 lm 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 interface region between middle Al layer and bottom Cu layer in the bent Cu/Al/Cu clad composites; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. Since the overall tensile stress is applied in this region, more noticeable cracks were observed especially in as-rolled clad composites and those annealed at 200 °C which exhibited the low hardening rate and pronounced work softening. For as-rolled and annealed clad composites at 200 °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 at 300 °C, small
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Fig. 11. High 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 annealed clad plates at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
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 emanating 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 28 lm 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 associated with appreciable work softening in the late stage of bending.
4. Conclusions As a result of the study on the bending behaviors of the tri-layered Cu/Al/Cu composites, the following conclusions were obtained: (1) The as-roll-bonded 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. 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 at 200 °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 localized bending.
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Fig. 12. High 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 and annealed clad plates at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
(3) For the Cu/Al/Cu composites annealed at 300 °C and up to 450 °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 of large crack along the interface intermetallic layer act against the tendency for uniform deformation induced by high hardening rate and high strain rate sensitivity at the initial stage of bending. (5) For the as-roll-bonded Cu/Al/Cu composite and that annealed 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 periodic cracks perpendicular to the interface were observed in the intermetallic layer and the localized slip developed both
in 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. Acknowledgments This 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 of aluminum–copper joints. Mater Trans 2007;48:1938–47. [2] Abbasi M, Karimi Taheri A, Salehi MT. Growth rate of intermetallic compounds in 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 annealing treatment on the interfacial properties of explosive-welded Al/Cu/Al multilayer. Mater Des 2012;37:122–7.
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[4] Peng XK, Heness G, Yeung WY. Effect of rolling temperature on interface and bond strength development of roll bonded copper/aluminium metal laminates. J Mater Sci 1999;34:277–81. [5] Peng XK, Wuhrer R, Heness G, Yeung WY. On the interface development and fracture behaviour of roll bonded copper/aluminium metal laminates. J Mater Sci 1999;34:2029–38. [6] Lee WB, Bang KS, Jung SB. Effects of intermetallic compound on the electrical and mechanical properties of friction welded Cu/Al bimetallic joints during annealing. J Alloy Compd 2005;390:212–9. [7] Rhee KY, Han WY, Park HJ, Kim SS. Fabrication of aluminum/copper clad composite using hot hydrostatic extrusion process and its material characteristics. Mater Sci Eng A 2004;383:70–6. [8] Khosravifard A, Ebrahimi R. Investigation of parameters affecting interface strength in Al/Cu clad bimetal rod extrusion process. Mater Des 2010;31: 493–9. [9] Manesh HD, Taheri AK. The effect of annealing treatment on mechanical properties of aluminum clad steel sheet. Mater Des 2003;24:617–22. [10] Sheng LY, Yang F, Xi TF, Lai C, Ye HQ. Influence of heat treatment on interface of Cu/Al bimetal composite fabricated by cold rolling. Composites Part B 2011;42:1468–73. [11] Li XP, Zhao ZJ, Seet HL, Heng WM, Oh TB, Lee JY. Magnetically controlled electroplating of NiFe/Cu composite wires. Electrochem Solid-State Lett 2004;7:C1–3.
[12] Venkateswara Rao N, Madhusudhan Reddy G, Nagarjuna S. Weld overlay cladding of high strength low alloy steel with austenitic stainless steel – structure and properties. Mater Des 2011;32:2496–506. [13] Sudha C, Shankar P, Subba Rao RV, Thirumurugesan R. Microchemical and microstructural studies in a PTA weld overlay of Ni–Cr–Si–B alloy on AISI 304L stainless steel. Surf Coat Technol 2008;202:2103–12. [14] Findik F. Recent developments in explosive welding. Mater Des 2011;32: 1081–93. [15] Gulenc B. Investigation of interface properties and weldability of aluminum and copper plates by explosive welding method. Mater Des 2008;29:275–8. [16] Hwang YM, Hsu HH, Lee HJ. Analysis of sandwich sheet rolling by stream function method. Int J Mech Sci 1995;37:297–315. [17] Funamizu Y, Watanabe K. Interdiffusion in the aluminum–copper systems. Jpn Inst Met 1971;12:147–52. [18] Lee KW, Kim SK, Kim KT, Hong SI. Ductility and strain rate sensitivity of zircaloy-4 nuclear fuel claddings. J Nucl Mater 2001;295:21–6. [19] Hong SI, Kim PH, Choi YC. High strain rate superplasticity of deformation processed Cu–Ag filamentary composites. Scripta Mater 2004;51:95–9. [20] Panahi D, Malakhov DV, Gallerneault M, Marois P. Influence of cooling rate and composition on formation of intermetallic phases in solidifying Al–Fe–Si melts. Can Metall Q 2011;50:173–80. [21] Hong SI, Laird C. Mechanisms of slip mode modifications in f.c.c. solid solutions. Acta Mater 1990;38:1581–94.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of heat treatment on the bending behavior of tri-layered Cu/Al/Cu composite plates In-Kyu Kim, Sun Ig Hong ⇑ Department of Nanomaterials Engineering, Chungnam National University, Daejeon, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 September 2012 Accepted 27 December 2012 Available online 5 January 2013 Keywords: Layered composite Interfacial fracture Intermetallics Bending Work hardening
a b s t r a c t Bending 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. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The demand for advanced materials with enhanced properties and functions increased as the technology advanced. Clad composite 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 combination 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 between 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 studied because of their advantages associated with high conductivity, low density and price competitiveness over copper and copper alloys. 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 in
Cu/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 CuxAly intermetallic compound at elevated temperature could deteriorate the mechanical and electrical 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], electroplating [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 rollbonding and their bending behaviors were studied. The bending properties of Cu/Al/Cu composite plates should be understood for further 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 details ⇑ Corresponding author. Tel.: +82 42 8216595; fax: +82 822 5850. E-mail address: [email protected] (S.I. Hong). 0261-3069/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.12.070
The Cu/Al/Cu clad-composite was fabricated using roll-bonding process. Materials used in this study were OFHC copper and
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Fig. 1. Cross section of the Cu/Al/Cu clad metal (a) and schematic of the test setup for three-point bending (b).
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 2.0 mm and that of Cu and Al layer was 0.2 mm and 1.6 mm respectively (Fig. 1a). The bending specimen with a length 40 mm and the width 5.0 mm were used for three-point bending test (Fig. 1b). In Fig. 1b, top Cu layer experiences the overall compressive stress while the bottom Cu layer experiences the overall tensile stress. The clad metal was annealed at temperatures at 200 °C, 300 °C 400 °C 450 °C and 500 °C for 3 h. To examine the bendability, the 3-point bending test were performed using a Universal Materials Testing Machine (UNITED, US/SSTM) at room temperature. The crosshead speed was maintained at the speed of 1 mm/min and 10 mm/min. Interfacial structures of Cu/Al/Cu clad metal were examined by an optical microscope (OM) and a scanning electron microscope (SEM). The chemical compositions of
Fig. 3. Thickness of interfacial reaction layer plotted against the annealing temperature (for 3 h).
intermetallic compounds formed at the Cu/Al interface were examined 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.
3. Results and discussion Fig. 2 shows the optical micrographs of Cu/Al interfaces for clad composite materials; as-rolled (a) and annealed at 300 °C (b), 400 °C (c), and 500 °C (d) for 3 h. For the as-rolled clad metal, no interface intermetallic layer was observed as shown in Fig. 2a.
Fig. 2. Optical micrographs of Cu/Al interface region in Cu/Al/Cu clad composites; as-roll-bonded (a) and annealed at 300 °C for 3 h (b), 400 °C for 3 h (c), and 500 °C for 3 h (d).
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Fig. 4. Back-scattered electron image (a) and EDX spectra (b–d) from layers marked by numbers 1 (b), 2 (c) and 3 (d) in the Cu/Al interface region (a).
After annealing at 200 °C, no visible change of the interface structure was observed by OM. When annealing temperature was raised to 300 °C, intermetallic reaction layer was formed with a thickness of about 2 lm (b). With the increase of annealing temperature to 400 °C, and 450 °C, the reaction layer continues to thicken to 12 lm, and 17 lm respectively. When the annealing temperature reached 500 °C, the interface increased more rapidly to a thickness of 28 lm. At higher temperatures, interfacial reaction layers appeared to consist of two or three layers. The multi-layered interfacial reaction products are more clearly visible in Fig. 2d. The thickness of the interfacial reaction layer was measured and plotted as a function annealing temperature in Fig. 3. Fig. 4 shows the back-scattered electron image (a) and EDX spectra (b, c, d) from regions 1, 2, and 3 marked in Fig. 4a. The region 2 in this figure appears to consist of two separate layers, but EDS spectra was obtained from the center of layer 2 because it was too thin. Chen and Hwang [1] reported that four layers of intermetallic compounds, Al4Cu9, Al3Cu4, AlCu, Al2Cu, were formed at the interface of Cu/Al at 500 °C. The intermetallic phases can be identified based on XRD analyses and the equilibrium phase diagram analyses of Al–Cu system [17]. In this study, in order to identify the thin intermetallic layers more accurately, tri-layered Cu/Al/ Cu composite plates annealed at 500 °C were separated along the brittle intermetallic layers and the fractured interfaces of two separated layers, Cu side and Al side were examined by the XRD analysis. Fig. 5a and b displays the XRD peaks from the fractured interfaces of the Cu side (a) and Al side (b). It should be noted that the peaks from the intermetallic compounds are strong because the separated layer fractured along the brittle intermetallic layers. Fig 5a and b demonstrate the presence of Cu9Al4, Cu3Al2 and CuAl (Cu3Al4) on the Cu side and CuAl2 and CuAl. It seem to be clear that intermetallic layers 1 and 3 are suggested to be CuAl2, and Cu9Al4 based on the results of in Figs. 4 and 5. On the other hand, the
Fig. 5. XRD peaks from the separated Cu and Al layers (a) Cu side and (b) Al side.
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region 2 in Fig. 4 is thought to consist of two layers Cu3Al2 and CuAl, which is supported by the XRD analysis. Cu3Al4 was not identified in the XRD analyses of the present study. The room temperature load–displacement curves of as-rollbonded and annealed Cu/Al/Cu clad composite at the displacement rate of 1 mm/min and 10 mm/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 composite annealed at 300 °C and up to 450 °C, on the other hand, exhibited the lower bending load due to the softening associated with recovery/recrystallization in Cu/Al/Cu clad metals, but the higher work hardening for an extended period before gradual softening took place. Cu/Al/Cu clad composite annealed at 500 °C exhibited the high work hardening behavior initially, but rather faster softening at the final stage of bending. All specimens exhibited the elastic bending behavior initially accompanied by plastic bending [8]. The elastic to plastic bending transition load, Py was 280 N for the as-roll-bonded clad composite and that annealed at 200 °C and approximately 40 N for Cu/Al/Cu clad composite annealed at 300 °C and up to 500 °C. Fig. 7a–f displays the low magnification images of bent Cu/Al/ Cu clad composites after bending displacement to 7 mm; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c),
Fig. 6. Load–displacement curves of as-rolled clad metal and annealed clad metals at various temperatures at the displacement rate of 1 mm/min (a) and 10 mm/min (b).
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400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. In the as-rolled and annealed clad composites at 200 °C which exhibited the extended load plateau and work softening, top Cu layers showed the significantly 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 at 300 °C and 400 °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. 7 were taken after displacement up to 7 mm at which work softening already took effect (Fig. 6). Cu/Al/Cu clad composite plate annealed at 500 °C with the extended softening (or weakening) at the final 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 at 500 °C. The hardening rate in bending can be defined to be the load increase rate as a function of displacement, dP/dD, where P is the bending load and D is the bending displacement. The relative hardening rate compensated by strength is thought to be more meaningful in predicting the uniformity of deformation. Therefore, the elastic load compensated hardening rate in bending, dP/(Py dD) at the displacement rate of 1 mm/min was calculated from Fig. 6a and plotted as a function of displacement in Fig. 7. Here, Py is the elastic to plastic bending transition load (Fig. 6). The elastic load compensated hardening rate will be called relative hardening 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 at 200 °C decreased very rapidly and dropped to nearzero at the displacement of 0.5 mm and remained around zero for an extended period before it went into the negative region. Cu/Al/Cu clad composite annealed at 300–450 °C exhibited the higher relative hardening rates, which gradually dropped zero at the displacement of 3.5 mm. The relative hardening rates of Cu/ Al/Cu clad composite annealed at 500 °C was observed to be similar to those of 300–450 °C initially, but decreased more rapidly and dropped to zero at the displacement of 2.6 mm. One interesting observation for Cu/Al/Cu clad composite annealed at 500 °C is that the hardening rate decreased down to 1.0/mm, which suggests the drastic softening (or weakening). The drastic decrease of the bending load Cu/Al/Cu clad composite annealed at 500 °C at the displacement of 3.0–4.2 mm can be linked to the interfacial cracks between Al and bottom Cu layer, which is most pronounced after annealing at 500 °C. The rapid drop of the load after the displacement of 2.5 mm 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 (at 200 °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 negligible, leading to the localized fracture. For the Cu/Al/Cu composites annealed at 300 °C and up to 450 °C, the pronounced work hardening in the localized bent region tends to spread the localized deformation and distribute the bending deformation uniformly, leading to the rather uniform bending. Cu/Al/Cu clad annealed at 500 °C is likely to behave similarly to those annealed at high temperatures in the initial stage because of the initial high work hardening, but in the final stage it is likely to behave similarly to the as-roll-bonded clad composite
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Fig. 7. Low magnification images of bent Cu/Al/Cu clad plates after bending displacement to 7 mm. (a) As-roll-bonded; (b–f) annealed at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
because of the rapid softening (or weakening). The faster load drop after the displacement of 3 mm in the Cu/Al/Cu clad composite annealed at 500 °C may be linked to the more severe interfacial debonding and interfacial cracks compared to those samples heattreated at 300–450 °C. ˇ The displacement rate sensitivity defined by the D log P/D log D were calculated using the data in Fig. 6a and b at the displacement of 1.5 mm at various temperatures and plotted as a function of ˇ is the displacement rate used in this temperature in Fig. 9. Here D 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 developed into well-defined interfacial cracks along interface intermetallic 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. 7 was measured along the distance from the center of Cu/Al/Cu plate and plotted in Fig. 10. Since the shape and morpholy of bent composites can be grouped into three groups, low (asroll-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 and their
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Fig. 8. Relative hardening rate in bending compensated by the elastic to plastic bending transition load, dP/(Py dD) plotted as a function of displacement at the displacement rate of 1 mm/min.
ˇ at the displacement of 1.5 mm Fig. 9. Displacement rate sensitivity, D log P/D log D plotted as a function temperature.
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 at 500 °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 increases 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 at 200 °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 at 500 °C.
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Fig. 10. Curvature of the interface between top Cu layer and Al layer (shown in Fig. 7) measured along the distance from the center of Cu/Al/Cu plate.
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/ Cu clad composites; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. Since the overall compressive stress is applied in this region, no significant fracture took place especially for clad composites annealed at 300 °C and 400 °C which exhibited the higher strain hardening in the initial stage. For as-rolled and annealed clad composites at 200 °C, small cracks (marked by arrows) around the small round particles were observed in Al, suggesting the presence of localized tensile stress. Panahi et al. [20] suggested that many round particles in Al–0.3Fe–0.45Si was a-AlFeSi. The round intermetallic particles observed in the commercial purity Al in this study are supposed to be a-AlFeSi pericles. As shown in Fig. 10a, the more severe localized bending in the top Cu layer compared to the region below can induce the localized tensile stress, resulting in the cracks around the interface of particles or particle cracks in Al. For annealed clad composites at 300 °C, both Cu and Al deformed uniformly with neither interfacial cracks nor debonding between top Cu layer and Al layer. For annealed clad composites at 400 °C and 450 °C, the typical compressive cracks in the intermetallic layer were observed. In Cu/Al/Cu annealed at 500 °C, some 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 tensile stress. It is interesting to note that intermetallic layer with the thickness of 28 lm 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 interface region between middle Al layer and bottom Cu layer in the bent Cu/Al/Cu clad composites; (a) as-rolled and annealed clad composites at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h. Since the overall tensile stress is applied in this region, more noticeable cracks were observed especially in as-rolled clad composites and those annealed at 200 °C which exhibited the low hardening rate and pronounced work softening. For as-rolled and annealed clad composites at 200 °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 at 300 °C, small
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Fig. 11. High 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 annealed clad plates at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
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 emanating 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 28 lm 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 associated with appreciable work softening in the late stage of bending.
4. Conclusions As a result of the study on the bending behaviors of the tri-layered Cu/Al/Cu composites, the following conclusions were obtained: (1) The as-roll-bonded 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. 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 at 200 °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 localized bending.
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Fig. 12. High 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 and annealed clad plates at 200 °C (b), 300 °C (c), 400 °C (d), 450 °C (e) and 500 °C (f) for 3 h.
(3) For the Cu/Al/Cu composites annealed at 300 °C and up to 450 °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 of large crack along the interface intermetallic layer act against the tendency for uniform deformation induced by high hardening rate and high strain rate sensitivity at the initial stage of bending. (5) For the as-roll-bonded Cu/Al/Cu composite and that annealed 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 periodic cracks perpendicular to the interface were observed in the intermetallic layer and the localized slip developed both
in 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. Acknowledgments This 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 of aluminum–copper joints. Mater Trans 2007;48:1938–47. [2] Abbasi M, Karimi Taheri A, Salehi MT. Growth rate of intermetallic compounds in 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 annealing treatment on the interfacial properties of explosive-welded Al/Cu/Al multilayer. Mater Des 2012;37:122–7.
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