Nanoindentation Measurement of Interfacial Reaction Layers in 6000 Series Aluminum Alloys and Steel Dissimilar Metal Joints with Alloying Elements* 1

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1 Materials Transactions, Vol. 5, No. 5 (11) pp. 979 to 9 Special Issue on Aluminium Alloys 1 #11 The Japan Institute of Light Metals Nanoindentation Measurement of Interfacial Reaction Layers in Series Aluminum Alloys and Steel Dissimilar Metal Joints with Alloying Elements* 1 Tomo Ogura, Keisuke Ueda*, Yuichi Saito* 3 and Akio Hirose Division of Materials and Manufacturing Science, Osaka University, Suita 55-71, Japan Nanoindentation measurements were successfully applied to the interfacial reaction layers in dissimilar metal joints of series aluminum alloys containing alloying elements to steel in order to characterize their mechanical properties. The nanoindentation hardness of the reaction layer formed at the aluminum side was lower than that formed at the low carbon steel () side of the investigated joints. At the aluminum side, the nanoindentation hardness changed by the addition of alloying elements. The hardness of the resulting Al 1 Fe 3 Si intermetallic compound (IMC) (and the same IMC containing Cu) was lower than that of Al 3 Fe. In comparison with the hardness values obtained from bulk Al-Fe binary series IMCs, it is considered that hardness changes of interfacial reaction layers are derived from the crystal structural changes produced by the alloying elements. The result of micro-testing of Al-Fe series IMCs indicates that the modification of the interfacial reaction layer by alloying elements contributes to higher ductility and the improvement of joint strength through crystal structural change. [doi:1.3/matertrans.l-mz111] (Received October, 1; Accepted December 7, 1; Published April, 11) Keywords: dissimilar metal joints of aluminum alloys and steels, interfacial reaction, nanoindentation, diffusion bonding, micro-tensile test 1. Introduction Reduction of fuel consumption and curbing carbon dioxide (CO ) emissions are key issues being tackled by automotive industries to resolve energy problems and global warming. To achieve weight reduction in automobiles at lower cost, hybrid car bodies made from aluminum alloys and steels are feasible structures, and involve the joining of these dissimilar metals. 1 5) The dissimilar metal joining of series aluminum alloys and steels has been considerably researched using several joining techniques, such as spot welding, ) laser welding 7) and friction stir welding (FSW). ) However, it has been a common problem that the formation of a brittle aluminum-rich Al-Fe intermetallic compound (IMC) layer at the bonded interface causes low strength in aluminum/steel dissimilar metal joints. 1) Therefore, microstructual control of the interfacial reaction layers is essential to obtain highreliability dissimilar metal joints. Our previous work reported that the interfacial reaction layer was modified by the addition of silicon and/or copper to series aluminum alloy through diffusion bonding, and this leads to a higher strength of an aluminum alloy/low carbon steel () dissimilar metal joint for the same thickness of the reaction layer. 9,1) This result suggests that the characteristics of the interfacial reaction layer would be changed by the addition of these alloying elements. Therefore, characterization, such as the mechanical properties, of IMCs formed at the interface during bonding becomes quite important. Because nanoindentation techniques enable us to examine the local deformation and hardness of alloys at the nanometer-scale, they are a powerful tool for the direct examination of the mechanical properties of micro-scale structures. 11,1) Nanoindentation is expected to clarify the * 1 The Paper Contains Partial Overlap with the ICAA1 Proceedings by USB under the Permission of the Editorial Committee. * Graduate Student, Osaka University * 3 Graduate Student, Osaka University. Present address: Nippon Kaiji Kyokai, Tokyo 1-57, Japan characteristics of the interfacial reaction layers in aluminum/steel dissimilar joints. In the present work, nanoindentation measurements were therefore applied to the interfacial reaction layers in dissimilar aluminum/steel joints containing alloying elements. From the obtained experimental results, nanoindentation hardness changes are discussed based on the crystal structures of interfacial reaction layers. Micro-tensile tests were also carried out to evaluate the mechanical properties of Al-Fe binary series IMCs.. Experimental Procedure The chemical compositions of the series aluminum alloys used in the present work are listed in Table 1. For simplicity, the Al-.%Mg-.%Si alloy (in mass%) is designated as the base alloy, whereas the Al-.%Mg-1.5%Si, Al-.%Mg-.%Si-1.%Cu and Al-.%Mg-1.5%Si-1.%Cu alloys are designated as the Si-containing, Cu-containing and (Si+Cu)-containing alloys, respectively. A low carbon steel including.1%c,.15%mn and.1%si (in mass%) () was also prepared. After polishing the surface of the samples using emery paper (#), the surfaces were cleaned by acetone. Diffusion bonding was carried out in a vacuum of <1: 1 1 Pa at a temperature at 75 K for a holding time of 1. ks with a pressure of.5 MPa, which forms the reaction layers with larger thickness regardless of the alloying elements. 9,1) The heating rate was 3 K/s. The bonding apparatus and a specimen used in the present work are shown in Fig. 1. The steel plate was sandwiched between Table 1 Chemical compositions of the alloys used in this work (in mass%). Alloy Mg Si Cu Fe Mn Ti Al Bal. Si-containing Bal. Cu-containing.. 1. Bal. (Si+Cu)-containing Bal.

2 9 T. Ogura, K. Ueda, Y. Saito and A. Hirose Pressure control system Gas Air cylinder (b) Pressure rod Vacuum Chamber Work piece Infrared Oven Thermocouple ϕ1 15 Aluminum alloy ϕ Steel Vacuum Pump and Evacuation System Heat Controller Aluminum alloy ϕ9 ϕ Unit:mm Fig. 1 Schematic illustration of bonding apparatus and (b) dimensions of the specimen used for diffusion bonding. Fig two aluminum alloy cylinders and put into a vacuum chamber. The resulting joint sample was embedded in conductive resin, and then cut vertically through the bonded surface for nanoindentation measurements. Nanoindentation measurements were performed along the interface of the joints at mm intervals using an ENT-11a (Elionix Inc.) instrument. The applied load was 5 mgf and the load and release times were 1 and 1 s, respectively. Microstructual observation and composition analysis after nanoindentation measurement were performed using electron probe microanalysis (EPMA; JXA-7, JEOL Ltd.). Microtensile tests were also carried out with a cross-head speed of 1 mm/s at R:T:, in order to evaluate the mechanical properties of Al-Fe binary series IMCs. The specimen for a microtensile test was prepared by wire-cutting, not from the reaction layer but from another bulk Al-Fe IMC specimen, as shown in Fig.. Elongation is estimated by measuring directly the change of gage length during deformation of a specimen using a microscope. 3. Results and Discussion 3.1 Nanoindentation hardness changes of the reaction layers in aluminum alloys/ dissimilar metal joints Figure 3 shows a typical SEM image at the interface of.7 1. t =.3 Unit: mm Dimensions of the specimen used for micro-tensile test. the (Si+Cu)-containing alloy/steel joint after nanoindentation measurements. Although no indentations were seen in the interfacial reaction layer due to the very low indentation depth (< 5 nm), it can be confirmed that some of the indentations were successfully located within the reaction layer from the trace of indentations in the matrix (because they were located at regular mm intervals). Figures 3(b) and (c) show EPMA mapping results of silicon and copper, respectively. It was found that both silicon and copper were enriched within the interfacial reaction layer, especially on the aluminum side, indicating the presence of a coppercontaining Al 1 Fe 3 Si IMC. The interfacial reaction layers in this bonding condition are summarized in Table, which was obtained from TEM observations and EDX analyses in our previous work. 9,1) At the initial stage of bonding, Al 3 Fe IMC is formed at the aluminum side in the base alloy joint, whereas Al 1 Fe 3 Si (containing copper) IMC is formed when alloying elements are present. Then, at a later stage of bonding, an Al 5 Fe IMC is additionally formed at the side of the joint. The nanoindentation hardness was estimated as a function of the distance from the interface of all the investigated joints, as shown in Fig.. The center of the interface is defined as the boundary between the and the reaction layers. The corresponding SEM images are also shown. The hardness in each interfacial reaction layer was classified into different categories from the results of EPMA mapping. In all the investigated joints, the hardness of reaction layers is much higher than that of the aluminum alloy or the matrix, as normally estimated using a conventional hardness test. In addition, within the interfacial reaction layers, the hardness of the reaction layer at the aluminum side (black symbol) was found to be lower than that at the side (gray symbol). Nanoindentation hardness results for the interfacial reaction layers are summarized in Fig. 5, together with the compositions of the corresponding IMCs. At the side, the hardness value of the interfacial reaction layer is the

3 Nanoindentation Measurement of Interfacial Reaction Layers in Series Aluminum Alloys and Steel Dissimilar Metal Joints 91 Table Effects of alloying elements on the reaction layers in aluminum alloy/ joints. 9;1Þ Reaction layer formed at initial stage of bonding Reaction layers formed at later stage of bonding Al 3 Fe Al 3 Fe + Al 5 Fe (Si+Cu)-containing Si-containing Al 1 Fe 3 Si Al 1 Fe 3 Si + Al 5 Fe Cu-containing (Si+Cu)-containing Al 1 Fe 3 Si containing lower Cu (<:5%) Al 1 Fe 3 Si containing higher Cu (%) Al 1 Fe 3 Si containing lower Cu (<:5%) +Al 5 Fe Al 1 Fe 3 Si containing higher Cu (%) +Al 5 Fe (b) Si (c) Cu 5µm Fig. 3 An SEM image of nanoindentation marks around the interface and (b) and (c) corresponding EPMA mappings in the (Si+Cu)-containing alloy/ joint bonded at 75 K for 1. ks. Nanoindentation marks are indicated by the arrow. same, regardless of the alloying elements, showing that Al 5 Fe IMC is formed in this area, as found in the earlier work shown in Table. On the other hand, the hardness of the reaction layer at the aluminum side was changed by the presence of alloying elements. The hardness in the reaction layer in the base alloy joint (Al 3 Fe IMC) is lower than that of Al 5 Fe as normally measured using micro-vickers hardness. 13) It should be noted that the hardness in the reaction layer of the Al 1 Fe 3 Si IMC is lower than that of Al 3 Fe IMC in the Si-containing alloy. The effect becomes larger by incorporating % copper to Al 1 F 3 Si IMC in the (Si+Cu)- containing alloy joint. 3. Nanoindentation hardness and tensile properties changes in Al-Fe binary series IMCs From the nanoindentation measurement around the interface in aluminum alloys/ joints, it was recognized that the hardness of the reaction layers were much higher than that of aluminum and matrix (Fig. ). Moreover, the hardness in the reaction layer of Al 1 Fe 3 Si IMC was lower than that of Al 3 Fe IMC for the Si-containing alloy and the effect was increased by incorporating % copper in Al 1 F 3 Si (Fig. 5). To better understand the hardness changes found in the present work, nanoindentation measurements were also performed on Al-Fe binary series IMCs. From the phase diagram, it is well known that there are five types of Al-Fe IMCs (AlFe 3, AlFe, AlFe, Al 5 Fe, and Al 3 Fe). 1) Bulk specimens of these Al-Fe IMCs were therefore prepared. Nanoindentation hardness results for these Al-Fe series IMCs are shown in Fig.. The hardness of Al 5 Fe was the highest in Al-Fe IMCs. With the exception of Al 3 Fe, the hardness monotonously decreased with an increasing atomic ratio of iron. The hardness values of the Al 5 Fe IMC and Al 3 Fe IMC are in good agreement with the hardness of reaction layers of these materials reported in Fig. 5. The observed hardness changes are considered to be derived from the crystal structures of the IMCs involved. The crystal structures of Al-Fe binary series IMCs are shown in Table 3. 15) Aluminum rich-imcs (Al Fe, Al 5 Fe and Al 3 Fe) have a small number of slip planes, causing them to be brittle. In contrast, equivalent or iron-rich IMCs (AlFe and AlFe 3 ) have cubic structures, having a comparatively larger number of slip planes, which leads them to be ductile. Therefore, it is considered that the hardness decrease produced by alloying elements in the reaction layer at the aluminum side of the joints (Fig. 5) originates in a change of crystal structure. To evaluate the effect of these crystal changes on the mechanical properties of Al-Fe series IMCs, micro-tensile tests were carried out. Although specimens of AlFe 3 and AlFe (%Al) IMCs were succesfully wire-cut, Al Fe, Al 5 Fe and Al 3 Fe IMCs were too brittle to cut. The obtained stress-elongation curves to fracture are shown in Fig. 7. Some of the noise in the curves was caused not by the materials, but by electrical signals from the equipment. It is seen that AlFe 3 IMC has higher strength and elongation than AlFe IMC. In particular, the average elongation of AlFe 3 IMC is 3.5(%) is more than twice that of AlFe IMC (1.%), showing good deformability. We recognize that tensile properties of these Al-Fe IMCs are indirect evidence of those of Al 1 Fe 3 Si (and containing Cu) IMCs and that more

4 9 T. Ogura, K. Ueda, Y. Saito and A. Hirose (b) 1 5µm Si-containing 5µm 1 Hardness, H /GPa Sncontaining Distance from an interface of the joint, l /µm (c) (d) Cu-containing 5µm (Si+Cu)-containing 5µm 1 1 ess, H /GPa Hardn Cucontaining (Si+Cu)- containing Distance from an interface of the joint, l /µm Fig. SEM images and distributions of hardness around the interface in the base alloy, (b) Si-containing alloy, (c) Cu-containing alloy and (d) (Si+Cu)-containing alloy/ joints bonded at 75 K for 1. ks. The hardness of the reaction layer in the aluminum side and the side is plotted in black and gray symbols, respectively. study is needed. However, the tensile properties of a copper containing Al 1 Fe 3 Si IMC is thought to be very similar to those of the Al-Fe IMCs shown in Fig. 7 based on the results from nanoindentation hardness measurements and their crystal structures. Therefore, the high ductility of a copper containing Al 1 Fe 3 Si IMC would lead to a higher strength of the (Sn+Cu)-containing joint. 9) The modification of the interfacial reaction layer by alloying elements is, therefore, considered to contribute the improvement of joint strength through changes in the crystal structure.. Conclusions Nanoindentation measurements were successfully applied to the interfacial reaction layers formed in dissimilar metal joints of series aluminum alloys to steel with alloying elements in order to characterize their mechanical properties. Micro-tensile tests were also carried out to evaluate the mechanical properties of Al-Fe binary series IMCs. The results obtained in the present work are summarized as follows.

5 Nanoindentation Measurement of Interfacial Reaction Layers in Series Aluminum Alloys and Steel Dissimilar Metal Joints 93 9 (b) Hardness, H /GPa 7 5 Al 5 Fe Al3Fe Al 1Fe 3 Si Si- Cucontaining (Si+Cu)- containing containing Al 1 Fe 3 Si + lower Cu (<.5%) Al 1 Fe 3 Si + higher Cu (~%) Si- Cucontaining (Si+Cu)- containing containing Aluminum alloy Fig. 5 Nanoindentation hardness in the reaction layer at side and (b) aluminum side in aluminum alloys/ joints. 9 Fig. Hardness, H /GPa AlFe 3 AlFe Al Fe Al5Fe Al 3 Fe (%Al) Al-Fe binary series IMC Nanoindentation hardness of bulk Al-Fe binary series IMCs. /MPa Stress, σ Fig. 7 AlFe 3 AlFe(%Al) 1 Elongation, e (%) Stress-elongation curves of bulk Al-Fe binary series IMCs. Table 3 Crystal structures of Al-Fe binary series IMCs. 15Þ Acknowledgements Al-Fe IMCs AlFe 3 AlFe Al Fe Al 5 Fe Al 3 Fe Cristal structure Cubic(D 3) Cubic(B) Triclinic Orthorhombic Monoclinic The nanoindentation hardness of the reaction layer formed at aluminum side was lower than that formed at side in all joints. Moreover, the hardness of Al 1 Fe 3 Si containing % copper IMC was lower than that of Al 3 Fe at the aluminum side. The hardness of bulk IMC specimens monotonously decreased with increasing atomic ratio of iron from Al 5 Fe to AlFe 3. In comparison with the hardness result of Al-Fe binary series IMCs, it is considered that hardness changes of interfacial reaction layers are derived from the crystal structural change produced by alloying elements. The result of micro-testing of Al-Fe series IMCs indicates the modification of the interfacial reaction layer by alloying elements contributes to higher ductility and the improvement of joint strength through crystal structural change. The authors would like to thank Prof. T. Nakano and Dr. T. Ishimoto of Osaka University for their significant help with nanoindentation measurement, and Prof. F. Minami, Prof. M. Ohata and Dr. Y. Takashima of Osaka University for their significant help with micro-tensile test. We deeply acknowledge Prof. H. Y. Yasuda of Osaka University for providing material for this research. We are also grateful to Mr. K. Ohmitsu of Osaka University for operating EPMA. This work was supported by a Grant-in-Aid for Young Scientists (B) No. 1755, Japan, and Priority Assistance for the Formation of Worldwide Renowned Centers of Research The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. REFERENCES 1) H. Oikawa, T. Saito, T. Nagase and T. Kiriyama: Tetsu-to-Hagane 3 (1997) 37. ) A. Hirose and K. F. Kobayashi: J. Japan Inst. Light Metals 5 () ) K. Mori: J. JSAE 1 (7) ) A. Hirose: J. JSAE 1 (7) 1 3.

6 9 T. Ogura, K. Ueda, Y. Saito and A. Hirose 5) S. Sasabe: J. JSAE 1 (7) 9. ) K. Miyamoto, S. Nakagawa, C. Sugi, H. Sakurai and A. Hirose: SAE Int. J. Mater. Manuf. (9) ) K. J. Lee, S. Kumai, N. Kawamura, N. Ishikawa and K. Furuya: Mater. Trans. (7) ) A. Elrefaey, M. Takahashi and K. Ikeuchi: Q. J. Jpn. Weld. Soc. 3 (5) ) T. Ogura, H. Umeshita, Y. Saito and A. Hirose: Q. J. Jpn. Weld. Soc. 7 (9) ) Y. Saito, H. Umeshita, T. Ogura and A. Hirose: Q. J. Jpn. Weld. Soc. 7 (9) ) T. Ogura, S. Hirosawa and T. Sato: Sci. Tech. Adv. Mater. 5 () ) M. Watanabe, T. Ookawa and S. Kumai: J. Jpn. Inst. Light Metals 57 (7) ) M. Yasuyama, K. Ogawa and T. Taka: Q. J. Jpn. Weld. Soc. 1 (199) ) Japan Institute of Metals (ed.): Metals data book, (Maruzen, Tokyo, ) p ) O. A. Bannykh, K. Emani, S. Nagasaki and A. Nishiwaki: Handbook of phase diagram of iron alloys, (Agune Engineering Center, Tokyo, 7) pp. 3 5.