- Email: [email protected]
Fabrication and characterization of Ti–Cu clad materials by indirect extrusion
Journal of Materials Processing Technology 187–188 (2007) 653–656
Fabrication and characterization of Ti–Cu clad materials by indirect extrusion J.S. Lee ∗ , H.T. Son, I.H. Oh, C.S. Kang, C.H. Yun, S.C. Lim, H.C. Kwon Materials & Part team, Korea Institute of Industrial Technology (KITECH), Wolgye-dong, Gwangsan-gu, Gwangju 506-824, Republic of Korea
Abstract This study was to investigate the effects of extrusion condition (extrusion temperature, extrusion ratio, dies angle, initial Ti thickness) on the fabrication of Ti–Cu clad material for high conductivity and high corrosion resistance, using indirect extrusion method in which there is no friction between container and billet. The range of extrusion temperature, extrusion ratio, dies angle and initial Ti thickness were changed from 700 to 900 ◦ C, from 10 to 38, from 20◦ to 50◦ , and from 1.8 to 4.8 mm, respectively. Extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. Extrusion pressure increased with increasing extrusion ratio and initial Ti thickness. Extrusion pressure was also affected by dies angle. Namely, the maximum extrusion pressure was increased with smaller dies angle because the friction between billet and dies increased with decreasing of dies angle. The thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. The interface layer composed of hard inter-metallic phases which may act a reducer of bonding strength depending upon its thickness. © 2006 Elsevier B.V. All rights reserved. Keywords: Clad material; Indirect extrusion; Ti–Cu clad; Interface; Bonding strength
1. Introduction Clad material is a variant of the typical composites which is composed of two or more materials joined at their interface surfaces. The advantage of clad material is that the combination of different properties of materials can satisfy both the need of good mechanical properties and the demand of user such as electrical and corrosion properties simultaneously [1,2]. There are some processes to the fabrication methods of clad materials such as extrusion and hot rolling, etc. In recently, application part of copper alloy for electric and electron industry widely increased, but it has several problems such as heavy weight and poor corrosion resistance and so on. Therefore, to overcome these problems, the necessary of clad materials using copper as a core material is enhanced such as Al/Cu and Ti/Cu clad material, etc. Park et al. [3] performed finite element analysis on rod extrusion of Al/Cu clad material with hydrostatic extrusion method. Yamaguchi et al. [4] suggested the extrusion limit for variation of the volume fraction of clad material and flow stress ratio with hydrostatic extrusion. Matsushita et al. [5] reported ∗
Corresponding author. Tel.: +82 62 600 6290; fax: +82 62 600 6149. E-mail address: [email protected] (J.S. Lee).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.144
hot hydrostatic extrusion of Ti–Cu alloy. Kang and Kwon [6] reported finite element analysis considering fracture strain of sheath material and die lubricant in extrusion process of Al/Cu clad composites and its experimental investigation. For instance, titanium–copper clad material composed of high electrical property and good corrosion resistant material can be used effectively as an electrode material. It is well known that the hydrostatic extrusion and indirect extrusion process is particularly suitable for the extrusion of composite billets because there is no friction between container and billet, comparing to the direct extrusion process. This study was to investigate the effects of extrusion condition such as extrusion temperature, extrusion ratio, dies angle, initial Ti thickness on the fabrication of Ti–Cu clad material for high electrical conductivity and high corrosion resistance, using indirect extrusion method in which there is little friction between container and billet. Also, we were to investigate the effects of inter-metallic compound on the interface layers and bonding strength between titanium and copper interface layers. 2. Experimental setup The billet consists of pure titanium (CP grade I, outer material) and ETP copper (inner material). A horizontal indirect extruder having a loading capacity of 400 t was equipped with variable RAM speed and a pressure transducer.
654
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
Fig. 1. Compression test method for measuring bonding strength of Ti/Cu clad materials. The size of a container, which was designed to make it possible to control the desired temperature by silicon heater, is 73φ × 300 mmL. After surface treatment, the mechanically machined inner copper material is inserted into the outer titanium tube. To increase the material flow, the billet shapes were mechanically machined to the same angle shape of dies angle. The extrusion conditions such as extrusion temperature, extrusion ratio, dies angle and initial Ti thickness were changed from 700 to 900 ◦ C, from 10 to 38, from 20◦ to 50◦ , and from 1.8 to 4.8 mm, respectively. The carbon oil and boron nitride (BN) were used as lubricants to reduce the friction between container and billet. The maximum extrusion pressures are measured by A/D converter and recorder using pressure transducer (model; PT 3000) with strain gauge. The microstructure was observed by optical microscopy (OM) and scanning electron microscopy (SEM) for thickness of the interface layers between titanium and copper clad material. The existence of inter-metallic compound was conducted by X-ray diffraction analysis (XRD). The bonding strength of interface layer on the titanium–copper clad materials was conducted by compression test. Fig. 1 shows the measuring method of bonding strength between titanium and copper for extruded clad material. Generally, in metal forming process, bonding strength is expressed as follows: σ=
Fmax S
(1)
where σ is bonding strength, Fmax is maximum compression strength and S is bonding area [7,8].
Fig. 2. The variation of maximum extrusion pressure with extrusion temperature in Ti/Cu clad extrusion.
indirect extrusion process is important to control the range of dies angle from 40◦ to 50◦ and using carbon oil as a lubricant. The variation of thickness of interface layer with extrusion temperature in titanium–copper clad extrusion is shown in Fig. 3. The thickness of extruded interface layer of titanium–copper clad material at 920 ◦ C was about 370 m, whereas the thickness of extruded interface layer at 750 ◦ C was about 3.2 m. As shown in figure, the thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. Resulting from the XRD analysis on interface layer, the interface layer composed of hard inter-metallic phases which may
3. Results and discussions Fig. 2 shows the variation of maximum extrusion pressure with extrusion temperature in titanium–copper clad extrusion. The extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. In the case of variation of extrusion ratio, the extrusion pressure increased with increasing extrusion ratio. It is well known that the increment of extrusion ratio is caused not increase of friction surface between billet and dies, but increase of flow stress and deformation rate in the titanium–copper clad materials [9]. Namely, extrusion pressure was increased with smaller dies angle because the friction between billet and dies increased with decreasing of dies angle. In the case of dies angel from 40◦ to 50◦ , it is found that the extruded materials with carbon oil as a lubricant were obtained without crack on the surface of titanium–copper clad material. However, in the case of dies angles from 20◦ to 30◦ and same other condition, it could be observed the surface crack of titanium–copper clad material. It is considered that the fabrication of titanium–copper clad materials by
Fig. 3. The variation of thickness of interface layer with extrusion temperature in Ti/Cu clad extrusion.
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
655
Fig. 4. The microstructures and EDS results of interface layer between titanium and copper materials.
act a reducer of bonding strength depending upon its thickness. The intermetallic compound was mainly consists of Ti3 Cu4 and Ti3 Cu. Fig. 4 shows the microstructures and EDS results of interface layer between titanium and copper materials. The difference of thickness on the interface layer with variation of extrusion temperature was attributed to the phase transformation of titanium, which was occurred the phase transformation from HCP structure (␣-Ti) to BCC structure (-Ti) at 882 ◦ C. It is found that in the case of extrusion temperature at 900 ◦ C the thickness of interface layer between titanium and copper clad material is
drastically increased because titanium material of BCC structure is easily promoted the diffusion comparing to that of HCP structure [5]. The variation of bonding strength with extrusion temperature in titanium–copper clad extrusion was shown in Fig. 5. The bonding strength was no difference until extrusion temperature of 800 ◦ C, whereas it was drastically decreased over extrusion temperature of 800 ◦ C. Generally, it is reported that the bonding shear strength of extruded clad materials without the fracture of sheath material in the optimum extrusion conditions is similar to the shear strength of soft material in the clad material [10–12].
Fig. 5. The variation of bonding strength with extrusion temperature in Ti/Cu clad extrusion.
Fig. 6. The variation of bonding strength with extrusion ratio in Ti/Cu clad extrusion.
656
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
In the present study, it is found that the bonding shear strength of titanium–copper clad materials is similar to that of copper material. Fig. 6 shows the variation of bonding strength with extrusion ratio in titanium–copper clad extrusion. As shown in figure, the bonding strength increased gradually with increasing extrusion ratio. In the case of extrusion ratio of 10.0 and 38.0, the bonding strength was about 145 and 199 MPa, respectively. In the case of optimum extrusion temperature without fracture of sheath titanium materials, the inner temperature of clad materials increased because of friction between billet and dies. However, it is found that the thickness of interface layers with variation of the extrusion ratio has no difference in the titanium–copper clad materials. It is concluded that the fabrication of high conductivity and high bonding strength titanium–copper clad materials without surface crack could be obtained to control the extrusion conditions such as extrusion temperature up to 800 ◦ C, extrusion ratio below 38, initial titanium thickness 2.0 mm and dies angle over 40, respectively. 4. Conclusions Fabrication and characterization of titanium–copper clad materials by indirect extrusion have been carried out. The obtained results are as follow: (1) The extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. (2) The thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. Resulting from the XRD analysis on interface layer, the interface layer composed of hard inter-metallic phases which may act a reducer of bonding strength depending upon its thickness.
(3) The difference of thickness on the interface layer with variation of extrusion temperature was attributed to the phase transformation of titanium from HCP structure (␣-Ti) to BCC structure (-Ti) at 882 ◦ C. (4) The optimum fabrication conditions of high conductivity and high bonding strength titanium–copper clad materials without surface crack could be obtained to control the extrusion conditions such as extrusion temperature up to 800 ◦ C, extrusion ratio below 38.0, initial titanium thickness 2.0 mm and dies angle over 40, respectively. References [1] N.H. Kim, C.G. Kang, H.C. Kwon, Extrusion process analysis of Al/Cu clad composite materials by finite element method, J. Korean Soc. Compos. Mater. 43 (1999) 1507–1520. [2] J.M. Story, B. Avitzur, W.C. Hahn Jr., Criterion for the prevention of core fracture during extrusion of bimetal rods, J. Eng. Ind. 104 (1982) 293– 299. [3] H.J. Park, K.H. Na, N.S. Cho, Y.S. Lee, Hydrostatic extrusion of copperclad aluminum rod, J. Korean Soc. Technol. Plast. 4 (1994) 123–130. [4] Y. Yamaguchi, M. Noguchi, T. Matsushita, M. Nishihara, Hydrostatic extrusion of clad materials, J. JSTP 15 (1974) 723–729. [5] T. Matsushita, M. Noguchi, K. Arimura, Hot hydrostatic extrusion of Ti–Cu alloy, J. Mater. Sci. 37 (1988) 413–417. [6] C.G. Kang, H.C. Kwon, Finite element analysis considering fracture strain of sheath material and die lubricant in extrusion process of Al/Cu clad composites and its experimental investigation, Int. J. Mech. Sci. 44 (2002) 247–267. [7] S. Ikeda, S. Saito, Manufacturing process and properties of clad metals by plastic working, J. Mater. Process. Technol. 45 (1994) 395–400. [8] M. Nakamura, S. Miki, N. Takahashi, Cladding of rod or pipe by drawing making use of interfacial slide, J. JSTP 30 (1987) 1051–1057. [9] T. Kawanami, Fabrication technique of clad sheet material, J. JSTP 32 (1991) 360. [10] M. Kiuchi, M. Hoshino, Two-billets extrusion with modified shape for bonding interface, J. JSTP 36 (1995) 410. [11] T.S. Lee, I.K. Kim, C.H. Lee, Effect of bonding temperature of Ti/Al bond interface and bonding strength, J. Korean Inst. Met. Mater. 36 (1998) 1678. [12] F. Yoshida, H. Iwagaki, Analysis of stretch bending and unbending for clad sheet metal, J. JSTP 35 (1994) 399.
Fabrication and characterization of Ti–Cu clad materials by indirect extrusion J.S. Lee ∗ , H.T. Son, I.H. Oh, C.S. Kang, C.H. Yun, S.C. Lim, H.C. Kwon Materials & Part team, Korea Institute of Industrial Technology (KITECH), Wolgye-dong, Gwangsan-gu, Gwangju 506-824, Republic of Korea
Abstract This study was to investigate the effects of extrusion condition (extrusion temperature, extrusion ratio, dies angle, initial Ti thickness) on the fabrication of Ti–Cu clad material for high conductivity and high corrosion resistance, using indirect extrusion method in which there is no friction between container and billet. The range of extrusion temperature, extrusion ratio, dies angle and initial Ti thickness were changed from 700 to 900 ◦ C, from 10 to 38, from 20◦ to 50◦ , and from 1.8 to 4.8 mm, respectively. Extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. Extrusion pressure increased with increasing extrusion ratio and initial Ti thickness. Extrusion pressure was also affected by dies angle. Namely, the maximum extrusion pressure was increased with smaller dies angle because the friction between billet and dies increased with decreasing of dies angle. The thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. The interface layer composed of hard inter-metallic phases which may act a reducer of bonding strength depending upon its thickness. © 2006 Elsevier B.V. All rights reserved. Keywords: Clad material; Indirect extrusion; Ti–Cu clad; Interface; Bonding strength
1. Introduction Clad material is a variant of the typical composites which is composed of two or more materials joined at their interface surfaces. The advantage of clad material is that the combination of different properties of materials can satisfy both the need of good mechanical properties and the demand of user such as electrical and corrosion properties simultaneously [1,2]. There are some processes to the fabrication methods of clad materials such as extrusion and hot rolling, etc. In recently, application part of copper alloy for electric and electron industry widely increased, but it has several problems such as heavy weight and poor corrosion resistance and so on. Therefore, to overcome these problems, the necessary of clad materials using copper as a core material is enhanced such as Al/Cu and Ti/Cu clad material, etc. Park et al. [3] performed finite element analysis on rod extrusion of Al/Cu clad material with hydrostatic extrusion method. Yamaguchi et al. [4] suggested the extrusion limit for variation of the volume fraction of clad material and flow stress ratio with hydrostatic extrusion. Matsushita et al. [5] reported ∗
Corresponding author. Tel.: +82 62 600 6290; fax: +82 62 600 6149. E-mail address: [email protected] (J.S. Lee).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.144
hot hydrostatic extrusion of Ti–Cu alloy. Kang and Kwon [6] reported finite element analysis considering fracture strain of sheath material and die lubricant in extrusion process of Al/Cu clad composites and its experimental investigation. For instance, titanium–copper clad material composed of high electrical property and good corrosion resistant material can be used effectively as an electrode material. It is well known that the hydrostatic extrusion and indirect extrusion process is particularly suitable for the extrusion of composite billets because there is no friction between container and billet, comparing to the direct extrusion process. This study was to investigate the effects of extrusion condition such as extrusion temperature, extrusion ratio, dies angle, initial Ti thickness on the fabrication of Ti–Cu clad material for high electrical conductivity and high corrosion resistance, using indirect extrusion method in which there is little friction between container and billet. Also, we were to investigate the effects of inter-metallic compound on the interface layers and bonding strength between titanium and copper interface layers. 2. Experimental setup The billet consists of pure titanium (CP grade I, outer material) and ETP copper (inner material). A horizontal indirect extruder having a loading capacity of 400 t was equipped with variable RAM speed and a pressure transducer.
654
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
Fig. 1. Compression test method for measuring bonding strength of Ti/Cu clad materials. The size of a container, which was designed to make it possible to control the desired temperature by silicon heater, is 73φ × 300 mmL. After surface treatment, the mechanically machined inner copper material is inserted into the outer titanium tube. To increase the material flow, the billet shapes were mechanically machined to the same angle shape of dies angle. The extrusion conditions such as extrusion temperature, extrusion ratio, dies angle and initial Ti thickness were changed from 700 to 900 ◦ C, from 10 to 38, from 20◦ to 50◦ , and from 1.8 to 4.8 mm, respectively. The carbon oil and boron nitride (BN) were used as lubricants to reduce the friction between container and billet. The maximum extrusion pressures are measured by A/D converter and recorder using pressure transducer (model; PT 3000) with strain gauge. The microstructure was observed by optical microscopy (OM) and scanning electron microscopy (SEM) for thickness of the interface layers between titanium and copper clad material. The existence of inter-metallic compound was conducted by X-ray diffraction analysis (XRD). The bonding strength of interface layer on the titanium–copper clad materials was conducted by compression test. Fig. 1 shows the measuring method of bonding strength between titanium and copper for extruded clad material. Generally, in metal forming process, bonding strength is expressed as follows: σ=
Fmax S
(1)
where σ is bonding strength, Fmax is maximum compression strength and S is bonding area [7,8].
Fig. 2. The variation of maximum extrusion pressure with extrusion temperature in Ti/Cu clad extrusion.
indirect extrusion process is important to control the range of dies angle from 40◦ to 50◦ and using carbon oil as a lubricant. The variation of thickness of interface layer with extrusion temperature in titanium–copper clad extrusion is shown in Fig. 3. The thickness of extruded interface layer of titanium–copper clad material at 920 ◦ C was about 370 m, whereas the thickness of extruded interface layer at 750 ◦ C was about 3.2 m. As shown in figure, the thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. Resulting from the XRD analysis on interface layer, the interface layer composed of hard inter-metallic phases which may
3. Results and discussions Fig. 2 shows the variation of maximum extrusion pressure with extrusion temperature in titanium–copper clad extrusion. The extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. In the case of variation of extrusion ratio, the extrusion pressure increased with increasing extrusion ratio. It is well known that the increment of extrusion ratio is caused not increase of friction surface between billet and dies, but increase of flow stress and deformation rate in the titanium–copper clad materials [9]. Namely, extrusion pressure was increased with smaller dies angle because the friction between billet and dies increased with decreasing of dies angle. In the case of dies angel from 40◦ to 50◦ , it is found that the extruded materials with carbon oil as a lubricant were obtained without crack on the surface of titanium–copper clad material. However, in the case of dies angles from 20◦ to 30◦ and same other condition, it could be observed the surface crack of titanium–copper clad material. It is considered that the fabrication of titanium–copper clad materials by
Fig. 3. The variation of thickness of interface layer with extrusion temperature in Ti/Cu clad extrusion.
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
655
Fig. 4. The microstructures and EDS results of interface layer between titanium and copper materials.
act a reducer of bonding strength depending upon its thickness. The intermetallic compound was mainly consists of Ti3 Cu4 and Ti3 Cu. Fig. 4 shows the microstructures and EDS results of interface layer between titanium and copper materials. The difference of thickness on the interface layer with variation of extrusion temperature was attributed to the phase transformation of titanium, which was occurred the phase transformation from HCP structure (␣-Ti) to BCC structure (-Ti) at 882 ◦ C. It is found that in the case of extrusion temperature at 900 ◦ C the thickness of interface layer between titanium and copper clad material is
drastically increased because titanium material of BCC structure is easily promoted the diffusion comparing to that of HCP structure [5]. The variation of bonding strength with extrusion temperature in titanium–copper clad extrusion was shown in Fig. 5. The bonding strength was no difference until extrusion temperature of 800 ◦ C, whereas it was drastically decreased over extrusion temperature of 800 ◦ C. Generally, it is reported that the bonding shear strength of extruded clad materials without the fracture of sheath material in the optimum extrusion conditions is similar to the shear strength of soft material in the clad material [10–12].
Fig. 5. The variation of bonding strength with extrusion temperature in Ti/Cu clad extrusion.
Fig. 6. The variation of bonding strength with extrusion ratio in Ti/Cu clad extrusion.
656
J.S. Lee et al. / Journal of Materials Processing Technology 187–188 (2007) 653–656
In the present study, it is found that the bonding shear strength of titanium–copper clad materials is similar to that of copper material. Fig. 6 shows the variation of bonding strength with extrusion ratio in titanium–copper clad extrusion. As shown in figure, the bonding strength increased gradually with increasing extrusion ratio. In the case of extrusion ratio of 10.0 and 38.0, the bonding strength was about 145 and 199 MPa, respectively. In the case of optimum extrusion temperature without fracture of sheath titanium materials, the inner temperature of clad materials increased because of friction between billet and dies. However, it is found that the thickness of interface layers with variation of the extrusion ratio has no difference in the titanium–copper clad materials. It is concluded that the fabrication of high conductivity and high bonding strength titanium–copper clad materials without surface crack could be obtained to control the extrusion conditions such as extrusion temperature up to 800 ◦ C, extrusion ratio below 38, initial titanium thickness 2.0 mm and dies angle over 40, respectively. 4. Conclusions Fabrication and characterization of titanium–copper clad materials by indirect extrusion have been carried out. The obtained results are as follow: (1) The extrusion pressure decreased with increasing extrusion temperature owing to the reduction of flow stress. However, increment of extrusion temperature resulted in the drawback of lubricant effect between dies and billet. (2) The thickness of interface layer between titanium and copper was increased with increasing extrusion temperature. Resulting from the XRD analysis on interface layer, the interface layer composed of hard inter-metallic phases which may act a reducer of bonding strength depending upon its thickness.
(3) The difference of thickness on the interface layer with variation of extrusion temperature was attributed to the phase transformation of titanium from HCP structure (␣-Ti) to BCC structure (-Ti) at 882 ◦ C. (4) The optimum fabrication conditions of high conductivity and high bonding strength titanium–copper clad materials without surface crack could be obtained to control the extrusion conditions such as extrusion temperature up to 800 ◦ C, extrusion ratio below 38.0, initial titanium thickness 2.0 mm and dies angle over 40, respectively. References [1] N.H. Kim, C.G. Kang, H.C. Kwon, Extrusion process analysis of Al/Cu clad composite materials by finite element method, J. Korean Soc. Compos. Mater. 43 (1999) 1507–1520. [2] J.M. Story, B. Avitzur, W.C. Hahn Jr., Criterion for the prevention of core fracture during extrusion of bimetal rods, J. Eng. Ind. 104 (1982) 293– 299. [3] H.J. Park, K.H. Na, N.S. Cho, Y.S. Lee, Hydrostatic extrusion of copperclad aluminum rod, J. Korean Soc. Technol. Plast. 4 (1994) 123–130. [4] Y. Yamaguchi, M. Noguchi, T. Matsushita, M. Nishihara, Hydrostatic extrusion of clad materials, J. JSTP 15 (1974) 723–729. [5] T. Matsushita, M. Noguchi, K. Arimura, Hot hydrostatic extrusion of Ti–Cu alloy, J. Mater. Sci. 37 (1988) 413–417. [6] C.G. Kang, H.C. Kwon, Finite element analysis considering fracture strain of sheath material and die lubricant in extrusion process of Al/Cu clad composites and its experimental investigation, Int. J. Mech. Sci. 44 (2002) 247–267. [7] S. Ikeda, S. Saito, Manufacturing process and properties of clad metals by plastic working, J. Mater. Process. Technol. 45 (1994) 395–400. [8] M. Nakamura, S. Miki, N. Takahashi, Cladding of rod or pipe by drawing making use of interfacial slide, J. JSTP 30 (1987) 1051–1057. [9] T. Kawanami, Fabrication technique of clad sheet material, J. JSTP 32 (1991) 360. [10] M. Kiuchi, M. Hoshino, Two-billets extrusion with modified shape for bonding interface, J. JSTP 36 (1995) 410. [11] T.S. Lee, I.K. Kim, C.H. Lee, Effect of bonding temperature of Ti/Al bond interface and bonding strength, J. Korean Inst. Met. Mater. 36 (1998) 1678. [12] F. Yoshida, H. Iwagaki, Analysis of stretch bending and unbending for clad sheet metal, J. JSTP 35 (1994) 399.