Characterization of joint between titanium and aluminum alloy welded by resistance spot welding with cover plate *

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1 [ 溶接学会論文集第 27 巻第 2 号 p. 109s -113s(2009)] Characterization of joint between titanium and aluminum alloy welded by resistance spot welding with cover plate * by Ranfeng Qiu**, Katsuya Higuchi***, Shinobu Satonaka**** and Chihiro Iwamoto**** We welded commercially pure titanium sheet to aluminum alloy A5052 sheet using a method of resistance spot welding with a cover plate, and investigated the mechanical properties and interfacial microstructure of the joint. The interfacial microstructure was observed using transmission electron microscopy. An approximately 160 nm thick layer of Al solid solution supersaturated with Ti was observed at the welding interface, in which contain the precipitates TiAl 3. Mechanical property analysis suggests that the reaction layer has no effect on the tensile shear load of the joint. Key Words: Resistance spot welding; Titanium; Aluminum alloy; Cover plate; Tensile shear load; Interface; Reaction layer 1. Introduction Titanium and its alloys are one of the most attractive engineering metals in industrial applications, because they exhibit superior properties such as high specific strength, high fatigue life, toughness, excellent resistance to corrosion and low density. However, the applications of titanium and its alloys are restricted by their high material cost. On the other hand, sound joints between dissimilar materials enable multi material design methodologies and low cost fabrication process to be employed. Therefore, the joining between titanium (its alloys) and other materials such as aluminum alloy is indispensable to effectively utilize titanium and its alloys with low fabrication cost and to extend their application. However, the joining between titanium and aluminum alloy accompanies some difficulties, because of the large difference in physical and thermal properties between the two kinds of materials, and the formation of brittle reaction products at the welding interface. In order to avoid the reaction between titanium and aluminum alloy at the welding interface, most of the previous studies have focused on solid-state bonding, such as, friction welding 1), vacuum roll bonding 2) and diffusion welding 3). On the other hand, resistance spot welding (RSW) is a widely used and important welding process for thin sheet products; nevertheless, only a few of studies have been reported on the RSW between titanium and aluminum alloy to date. In the previous study 4, 5), we have welded steel to aluminum alloy sheets and obtained stronger joints under the welding condition of low welding current using **** Received: **** Student Member, Henan University of Science and Technology, Luoyang, China (Now) **** Student Member, Graduate School of Science and Technology, Kumamoto University **** Member, Graduate School of Science and Technology, Kumamoto University the technique of RSW with cover plate. It is well known that there are also problems in the process of RSW between steel and aluminum alloy, which caused from the large difference in physical and thermal properties between the base materials and the brittle reaction products formation at the welding interface. The same problems occur in the process of the RSW of Ti/Al. In the present study, therefore, we also welded titanium to aluminum alloy using this method, investigated the interfacial microstructure and strength of joints, and analyzed the effect of reaction layer on the mechanical properties of joints. 2. RSW with cover plate As well known, RSW is a joining process based on the heat source obtained from Joule s effect of the resistance and electric current flow through the sheets which are held together by the electrode force. In this process, the coalescence occurs at the spot area in the faying surfaces. Therefore, in the process of RSW between titanium and aluminum alloy, enormously high electric current is required because of low heat generation and high heat conduction of aluminum alloy. However, enormously high welding current would reduce electrode tip life and require installing larger capacity RSW machine. In order to enable the RSW between titanium and aluminum alloy under relatively low welding current condition, we proposed a technique of RSW with cover plate. Figure 1 shows the schematic diagram of this welding process, in which a cover plate was placed on the aluminum alloy sheet. Here, it was required that the cover plate is a metal sheet with relatively lower electrical conductivity than aluminum alloy, so the higher heat generated in the cover plate as to be conducted from the cover plate to aluminum alloy sheet. Taking low cost and availability into account, we chose cold-rolled steel sheet SPCC as the cover plate.

2 110s 研究論文 Ranfeng Qiu et al.:characterization of joint between titanium and aluminum alloy welded by resistance spot welding with cover plate In the present study, the similar material joints of aluminum alloy sheets were also prepared to compare the strength with the Fig. 1 Schematic diagram of spot welding with cover plate dissimilar material joints between titanium and aluminum alloy. In such a case, the aluminum alloy sheets were placed between two cover plates. 3. Experimental procedure In this study, 1.0 mm thick commercially pure titanium (Ti) sheet and aluminum alloy A5052 (A5052) sheet were used as the base materials. Their chemical compositions are listed in Table 1. Spot welding was carried out by an AC spot welding machine. Figure 2 shows the configuration and dimension of the joint, in which mm SPCC cover plate was used. Welding conditions are given in Table 2, in which the welding current and electrode force were varied separately. The interfacial microstructure of the joint was investigated using a transmission electron microscope (TEM; TECNAI F20, acceleration voltage: 200 kv) equipped with a scanning unit and an energy dispersive X-ray spectroscopy (EDX) detector. The TEM observation was performed with thin foil, which was prepared with ion milling (3keV Ar + ) after mechanical polishing of sliced piece from the welded sample. Besides, the chemical compositions of the interfacial region were examined using an EDX with an electron beam probe of approximately 10 nm in diameter. Fig. 2 Configuration and dimension of the joint In order to examine the mechanical properties of the joints, the tensile shear test was performed under a cross-head velocity of ms -1 at room temperature. The weld diameter was measured from the fractured surface after the tensile shear testing. Table 1 Chemical composition of materials Materials Elements (mass %) Mg Fe Cr Si Mn Cu Al A Bal. H O N Fe Ti Ti Bal. Table 2 Welding conditions Set 1 Set 2 Welding Current 6-12 ka 10 ka Welding Time 10 cycles 10 cycles Electrode Force 4 kn 1-6 kn Electrode Cu-Cr Alloy; Conical Electrode Tip (ø6) Pre-treatment Degreasing with Acetone 4. Experimental results and discussions 4.1 Interfacial microstructure Figure 3 shows the optical micrograph of the cross-section of the joint, which was welded under the condition of welding current of 8 ka. In the picture, a drum-shaped nugget was observed in the A5052, the thickness of which was thinner than the original thickness at the center. In contrast, no fusion zone was observed in the Ti adjacent to the welding interface of Ti/A5052. Similar morphology was also observed in the other joints welded under different welding conditions. Fig. 3 Optical micrograph of the cross-section In addition, some blowholes were observed at the center of the nugget as shown in Fig.3. In recent literature 6), Gean A. et al. claimed that excessive porosity ( up to 40% of the nugget diameter) do not affect the static performance of the welds in shear when maintaining a constant 6.3 mm weld diameter, who investigated the effect of discontinuities in welds on the static and fatigue properties of resistance spot welded aluminum alloy joint. In this study, therefore, the blowholes formed in the nugget would not affect the tensile shear load of the joint according to their results. Figure 4a shows a scanning transmission electron microscope

3 溶接学会論文集第 27 巻 (2009) 第 2 号 111s (STEM) image taken from the interfacial region of the joint welded under the condition of 10 ka welding current. It can be seen that there were five layers (U-Y). Figure 4b shows the results of EDX analysis from the line MN shown in Fig.4a. Table 3 summarizes the concentration of elements Ti, Al and Mg in the typical points A-G shown in Fig.4a. It is visible that the layer U was the base metal Ti, the layer Y to W was the base metal A5052 according to the results of EDX analysis as given in Fig.4b and Table 3. some compound formed in there. In order to clarify the detailed structure of the diffusion reaction layer, we observed the welding interfaces using TEM. Figure 5a shows a typical bright field image of the reaction layer. In the image, some precipitated particles were observed. Figures 5b shows the electron diffraction patterns of the selected particle. According to the analyses of electron diffraction patterns, it was identified that the precipitated particles were TiAl 3. Similar results were obtained by Nishio K. et al., who have investigated the interfacial microstructures of Al/Ti clad material produced by a vacuum roll bonding method 2). They also observed that the precipitates TiAl 3 (D0 22 type) within the ultra fine-crystal zone having Al solid solution (FCC crystal) supersaturated with Ti. Ti Fig. 4 STEM micrograph of analysis line across the welded zone (a) and results of EDX analysis (b) Table 3 Results of EDX analysis at the points shown in Fig.4a (at. %) A B C D E F G Ti Al Mg As shown in Fig.4a, a layer (Layer V) of approximately 160 nm in thickness between the base metal Ti (Layer U) and the A5052 (Layer W) was observed. The EDX analysis showed that it was a diffusion reaction layer. However, element Ti content was more than the solubility of element Ti within Al 7). The formation of this supersaturated solid solution is considered to be due to rapid cooling rate during RSW. Moreover, it should be noted that there was a region (marked by a black arrow in Fig.4b) having steady compositions of Al and Ti (Al:Ti 3:1). This suggests that Al Mg Fig. 5 TEM image observed in the welding interface (a) and the electron diffraction patterns taken from the selected particle (b) which incident electron beam direction is parallel to [131] In addition, an Mg-rich layer (Layer X), in which element Mg content exceed the solubility limit of element Mg within Al (1.9 at.%, 100 C) 7), was also observed in the A5052 near the welding interface. Although the origin of the formation of the layer remains unclear, rapid cooling rate during RSW is a possible reason for the production of the Mg-rich layer. Ozaki H. et al. have welded commercially pure titanium sheet to aluminum alloy A5052 sheet using a laser roll welding, during which the interfacial reaction also took place between molten aluminum and solid titanium 8). They observed 1-10 μm thick reaction layer at the welding interface. Compared with their results, the reaction layer (Layer V), which formed in the welding interface of the resistance spot welded joint obtained in this study, was thinner. This is considered to be responsible for relatively short RSW time, which exerts influence on the reaction layer thickness 9). 4.2 Mechanical property of the joint

4 112s 研究論文 Ranfeng Qiu et al.:characterization of joint between titanium and aluminum alloy welded by resistance spot welding with cover plate Generally, welding current and electrode force have influences on the nugget diameter and tensile shear load of joints. In this section, we investigated the relationships between these welding parameters and the nugget diameter, the tensile shear load of joints welded by the RSW with cover plate. initial sheet-sheet, electrodes-sheets contact area increased with the increasing of the electrode force. For the fracture mode of the joints, the fracture type of joints was plug fracture through all electrode force. The maximum tensile shear load of 6.4 kn was obtained from the tensile testing of the joint welded under the welding conditions of 10 ka welding current and 1 kn electrode force as shown in Fig.7. Fig. 6 Effects of welding current on the tensile shear load and weld diameter of the joints Figure 6 shows the effects of welding current on the tensile shear load and nugget diameter of the joints welded under the welding conditions of set 1. As shown, the nugget diameter and tensile shear load of joints increased with the increasing of the welding current, although rise in the tensile shear load of the joints was slight at the welding current above of 10 ka. Moreover, the fracture type of the joints varied depending on the welding current. The fracture type of joints was shear fracture at the welding current of 6 ka, and above the welding current of 6 ka it became to plug fracture. In RSW, heat input increases with the increasing of welding current, result in the increasing of nugget diameter. As shown in Fig.6, the tensile shear load of joints increased with the increasing of the welding current. It is attributed to increasing of the nugget diameter, which is a major influence factor on joint strength in both cases of shear fracture and plug fracture 10). However, with the increasing of welding current, the joint thickness decreases as shown in Fig.3, which is another essential influence factor on plug fracture strength 11). Thus, at the welding current above 10 ka, the increasing of tensile shear load of the joint was slight. Figure 7 shows the effects of electrode force on the nugget diameter and tensile shear load of joints welded under the welding conditions of set 2. The nugget diameter and tensile shear load of the joints decreased with the increasing of the electrode force. This is considered to be due to the following two factors. Firstly, superior sheet separation, which was caused by higher electrode force, suppressed the nugget growth. Secondly, the decrease in the energy density of the welding region, which resulted from the initial contact resistance between sheet-sheet decline and the radiation via electrodes increase because the Fig. 7 Effects of electrode force on the tensile shear load and weld diameter of the joints Ichikawa R. and Ohashi T. welded 1.0 mm thick commercially pure titanium sheet to aluminum alloy 5052-H34 sheet using a conventional RSW, and obtained the maximum tensile shear load of 320 kg ( kn) and nugget diameter of 5.5 mm from joints welded under the condition of welding current of 5.7 ka 11). In comparison with their results, the joints obtained in this study revealed higher tensile shear load and larger nugget as shown in Fig.6 and 7. According to American National Standard 12), the nugget diameter >4t 1/2 (t represents the thickness of specimen) is required for resistance spot welded joint; from this viewpoint, the nugget diameter of joints welded by the RSW with cover plate was also large enough. This is attributed to the effect of the cover plate on the formation of nugget. That is, large heat generated in the cover plate due to its low electrical conductivity, transferred to the welded region in the aluminum alloy, enhanced heat of welded region, and resulted in the formation of larger nugget. In this study, the optimized welding condition is the condition of 1.0 kn electrode force and 10 ka welding current, since the joint with the maximum tensile shear load was obtained under this welding condition. 4.3 Influence of interfacial reaction layer on the tensile shear load of the joint Generally, nugget diameter and interfacial reaction layer have influences on the strength of resistance spot welded dissimilar material joints. In this section, we discussed the effect of the interfacial reaction layer on the tensile shear load of the Ti/A5052

5 溶接学会論文集第 27 巻 (2009) 第 2 号 113s joints by making a comparison between the Ti/A5052 joint and the similar material joint A5052/A5052. Figure 8 shows the relationship between the nugget diameter and tensile shear load for both the Ti/A5052 joint and the A5052/A5052 joint. Both types of joints, the A5052/A5052 and Ti/A5052, revealed that the tensile shear load increased with the increasing of the nugget diameter. Under the same nugget diameter, the dissimilar material joints Ti/A5052 revealed almost the same tensile shear load in comparison with the A5052/A5052 joints where no reaction layer formed. Fig. 8 Relationship between the weld diameter and tensile shear load of the joints The fracture observation after the tensile testing of the joints also revealed that the fracture occurred in the base metal A5052. Therefore, the results described above suggest that the tensile shear strength of the Ti/A5052 joint was not affected by the reaction layer formed the welding interface. 5. Conclusions In this study, we welded commercially pure titanium sheet to aluminum alloy A5052 sheet using resistance spot welding with a cover plate. The joint performance was evaluated by the interfacial microstructure and mechanical properties of the joint. Main results obtained from this study are as follows: 1. It is feasible to weld titanium to aluminum alloy sheet via a RSW method using a cover plate. Larger nugget and high tensile shear strength were obtained under relatively low welding current condition. 2. The layer of Al solid solution supersatulated with Ti was observed at the welding interface of the Ti/A5052 joint, which contain the precipitates TiAl The reaction layer has no effect on the tensile shear strength of the Ti/A5052 joint welded by RSW with cover plate. Acknowledgements The authors would like to express their thanks to Associate Professor Y. Morizono of Graduate School of Science and Technology, Kumamoto University for helpful discussions. References 1) Katoh K. and Tokisue H.: Friction welded 5052 aluminum alloy to pure titanium joint, Journal of Japan Institute of Light Metals (2004), (in Japanese) 2) Nishio K., Katoh M., Yamaguchi T., Era H. and Sakamoto K.: Observation of bond interface of Al/Ti clad material by transmission electron microscopy-development of clad materials by vacuum roll bonding and its characteristics (Report 4), Quarterly Journal of the Japan Welding Society, 22-2 (2004), (in Japanese) 3) Enjyo Toshio, Ikeuchi Kenji and Kanai Masahito: Diffusion welding of titanium to aluminum, Journal of the Japan Welding Society, 46-2(1977), (in Japanese) 4) Qiu R., Iwamoto C. and Satonaka S.: Interfacial microstructure, strength of steel/aluminum alloy joints welded by resistance spot welding with cover plate, Journal of Materials Processing Technology, 209(2009), ) Qiu R., Iwamoto C. and Satonaka S.: The influence of reaction layer on the strength of aluminum/steel joint welded by resistance spot welding, Materials Characterization, 60(2009), ) Gean A., Westgate S.A., Kucza J.C. and Ehrstrom J.C.: Static and fatigue behavior of spot-welded 5182-O aluminum alloy sheet, Welding Journal, 78-3 (1999), 80s-86s. 7) Nagasaki S. and Hirabayashi M.: Binary Alloy Equilirium Diagrams Volume, Agune Technique Center, (2001), 44, 33. 8) Ozaki H., Hayashi S. and Kutsuna M.: Laser roll welding of dissimilar metal joint of titanium to aluminum alloy, Quarterly Journal of the Japan Welding Society, 26-1 (2008), (in Japanese) 9) Takeshita Kunimasa and Matsui Kazuhiko: Tensile strength of titanium joint brazed with aluminum, Journal of the Japan Institute of Metals, 57-11(1993), (in Japanese) 10) Satonaka S, Kaieda K. and Okamoto S.: Prediction of tensile shear strength of spot welds based on fracture modes, Welding in the world, 48-5/6 (2004), ) Ichikawa Riei and Ohashi Teruo: Resistance spot welding of dissimilar metals-commercial pure titanium to some aluminum alloys, Journal of the Japan Welding Society, (1979), (in Japanese) 12) American National Standard: Weld button criteria, recommended practices for test methods for evaluating the resistance spot welding behavior of automotive sheet steel materials. ANSI/AWS/SAE/D (1997), Section 5.7.