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Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590
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Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590 Ming Lou b , YongBing Li a,b,∗ , Yuan Wang b , Bin Wang c , Xinmin Lai b a b c
State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, PR China Shanghai Key Laboratory of Digital Manufacture for Thin-Walled Structures, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Engineering, Zhejiang Normal University, Jinhua 321004, PR China
a r t i c l e
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Article history: Received 14 July 2013 Received in revised form 2 January 2014 Accepted 1 March 2014 Available online xxx Keywords: Advanced high strength steel (AHSS) Rivet-welding (RW) Micro-hardness distribution Inter-metallic compounds (IMC)
a b s t r a c t The hybrid use of aluminum alloy and advanced high strength steel (AHSS) has become an inevitable trend for fabricating a lightweight auto-body. Self-piercing riveting (SPR) as a preferred cold-forming fastening method is facing problem like weak interlocking when joining dissimilar combinations with considerably unequal thickness. In this study, a hybrid joining method, named rivet-welding (RW) was proposed to improve the robustness and strength of the SPR joint, by applying an electric current to it. For better evaluating the new process, the effects of heating time and electrode design on the microstructure, micro-hardness distribution, and mechanical performance of the RW joints were studied and compared systematically with the traditional SPR ones. The results showed that the electric current could improve the microstructure of the steel rivet and bottom DP590, and under long heating time, the inter-metallic compounds (IMC) could be formed at the interface of trapped AA6061-T6 and bottom DP590. Meanwhile, the electric current could increase the micro-hardness of the rivet and bottom DP590, and soften the AA6061-T6 around the rivet leg. In addition, the RW process using lower annular electrode A (LAE A) could obtain 12.1% higher tensile-shear strength compared with the traditional SPR process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of light metals, such as aluminum alloy could significantly reduce the weight of vehicle body. However, the aluminum alloy cannot completely replace the advanced high strength steel (AHSS) yet, considering the cost and performance. Therefore, the hybrid use of both aluminum and AHSS structures in bodyin-white (BIW) has become a more practical method to realize lightweighting as mentioned by Sun et al. (2007), for most of the automobile manufacturers. Abe et al. (2009) indicated that it is of great difficulty to join aluminum and steel directly by conventional resistance spot welding (RSW) process, since there are large differences in physical and chemical properties between aluminum and steel, and the hard and brittle intermetallic compounds easily form at the interface of the dissimilar metals. Recent researches showed that to obtain a sound joint by RSW, either a cover plate placed between the aluminum
∗ Corresponding author at: School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 21 34206304. E-mail address: [email protected] (Y. Li).
and electrode showed by Qiu et al. (2009) or aluminum-cladded steel sheet showed by Sun et al. (2004) should be introduced, which makes the process too complicated and costly to industrialize. Friction stir spot welding (FSSW) is thought to be a potential method to join aluminum to steel for the significantly reduced heat input, but Feng et al. (2005) indicated that the residual process hole could weaken the joint strength by decreasing bonding widths. New methods have been invented to fill the process hole, however, the cost is much higher and the extended cycle time cannot meet the requirements of manufacturing takt time. For clinching, even though the running cost is quite low, the clinched joint is not strong enough, and thus not suitable for joining force-bearing components. Nowadays, self-piercing riveting (SPR) as a cold forming process is preferred for joining dissimilar materials, and has been successfully applied to join multi-material components of BMW series 5 and Audi TT etc. As reported by Porcaro et al. (2010), different rivet and die should be used to achieve optimal joint strength for different sheet combination. However it is impractical to on-line replace the rivet and die with the variation of the sheet combinations in production line. Therefore, to guarantee the joint quality, more SPR equipments should be used in each assembly station, which would
http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006 0924-0136/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 1. Overview of the rivet-welding process.
greatly increase the cost of vehicle body manufacturing. On the other hand, the steel sheets used in auto-body become stronger and thinner so as to reduce the weight, and it is a consensus that for a better SPR result, the bottom sheet material should be no less than one-third of the total stackup thickness, as specified by Henrob Co. (2011). Thus, when joining dissimilar combinations of thick top aluminum and thin bottom sheet steel, the mechanical interlocking may not be adequate for designed joint strength. In order to improve the performance of the SPR joints with considerably unequal sheet thickness combination, a novel joint reinforcement method, named rivet-welding (RW) is proposed in this study, in which an electric current is applied to the SPR joint via specially designed electrodes, to produce resistance heat, and the effects of heating time and electrode design on the joint characteristics were studied systematically. For better evaluating the influence of resistance heat, the RW joints were compared with the traditional SPR ones in both tensile-shear and fatigue tests.
2. Experimental procedure 2.1. Process overview The RW process involves two phases: SPR phase and welding phase, as shown in Fig. 1. In the SPR phase, a conventional SPR joint is made to form the initial mechanical connection firstly. Then, an electric current is applied to the riveted sample during the welding phase via two specially designed annular electrodes to produce resistance heat in the joint. Zn/Sn coated medium carbon steel rivets with a head diameter of 7.8 mm and a total length of 6.5 mm, and the tool steel die (DZ0902000H1R1) with a middle diameter of 9 mm were used, which were supplied by Henrob Co. Specially designed annular electrodes made from Cu–Cr–Zr alloy are used in this study, as shown in Fig. 2. The inner diameter of the upper annular electrode (UAE) is 8 mm (Fig. 2(a)), which is slightly larger than the rivet’s head diameter, to prevent overheating it. Meanwhile, the middle
Fig. 2. Drawings of the sectional view of (a) upper annular electrode (UAE) and (b) lower annular electrode A (LAE A), and top view of (c) LAE A and (d) LAE B. (unit: mm).
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 3. Rivet-welding setups: (a) SPR machine and (b) MFDC welding system.
diameter of the lower annular electrode A (LAE A) is designed as 8.8 mm (Fig. 2(b)), which is also smaller than the middle diameter of the die used in SPR phase to make sure electric current flow through deformed steel. Furthermore, lower annular electrode B (LAE B), as shown in Fig. 2(d), is designed by removing partial material above the red line from LAE A every 60◦ , so as to locally increase the electric current flowing through the rivet tip and lower steel sheet. 2.2. Materials For a dissimilar sheet combination, especially the combination of aluminum and steel, the corrosion problem is critical because of the great discrepancy in electric potential. Thus, most of the automobile manufacturers prefer to use galvanized steel sheet in Al/Steel dissimilar sheet combinations. In this study, 3 mm thick aluminum AA6061-T6 and 1.2 mm thick hot-dipped galvanized dual-phase steel DP590 were used for the propose of anti-corrosion, and sheared into 130 mm × 38 mm coupons, to make up dissimilar sheet combination of AA6061-T6 + DP590. The chemical compositions, physical and mechanical properties of these materials are listed in Table 1. The aluminum coupons were polished by abrasive paper to remove the surface oxide layer, and acetone was used to clean up the surface greases on the DP590 coupons.
splashes through the bottom steel due to the excessive heat input. In present study, the trapped aluminum of RW joints using LAE A and LAE B splashed under welding time of 450 ms and 300 ms, respectively. Therefore, 400 ms and 250 ms were set as the respective maximum welding time here for RW process using LAE A and LAE B.
2.4. Microscopic analysis For the metallographic examination, all the obtained samples were cross-sectioned along the center of the button carefully, and mechanically ground and polished by conventional metallographic techniques. The cross-sections were etched using 3% nitric acid alcohol solution for 5 s and Keller reagent (the mixed aqueous solution of 2 ml HF + 3 ml HCl + 5 ml HNO3 + 190 ml H2 O) for 10 s, to observe the microstructures of steel and aluminum alloy, respectively. A Leica DM2500 M optical microscope and a SMT HV-1000 hardness testing machine were used to investigate the changes of the microstructure and micro-hardness distributions of the joints, respectively. In addition, scanning electron microscope (SEM) was used to verify the formation of the metallurgical bonding at interfaces between AA6061-T6 and DP590, and energy-dispersive spectroscope (EDS) was used to analyze the thickness and corresponding chemical compositions of the bonded regions.
2.3. Experimental setups and conditions 2.5. Mechanical tests The SPR phase is executed by a servo powered Henrob SPR device, as shown in Fig. 3(a). For each sample, the punch speed is chosen to make sure the surface of the rivet head is flush with the top material’s upper surface. After that, the riveted joints are heated by a Medar 5000s MFDC welding machine, which is equipped to Fanuc R2000-Ib210f robot with six degrees of freedom for accurate positioning, as shown in Fig. 3(b). In this study, 13 kA welding current (maximum welding current provided by the welder) and 5 kN electrode force are adopted for all the joints. Besides, to investigate the effect of heat input on the properties of RW joints, the welding time is chosen to begin with 50 ms and increases every 50 ms, until the trapped AA6061-T6
The strength of RW joints was evaluated by tensile-shear test. The tensile-shear specimens were prepared in accordance with the standard of GB/T2651-81, named “Test Methods of Tensile-Shear of Spot-Welded Joints”. During all the tensile-shear testing, a spacer was placed at the gripped region and clamped together with the specimen to make sure the alignment of the specimen and tensile direction. For this test, the joint was made at the center of the overlapped 38 mm × 38 mm square area precisely, as shown in Fig. 4. The tensile-shear tests were conducted at a constant speed of 2.5 mm/min on a SUNS testing machine. At least three replicates were made in the tests.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Table 1 Chemical compositions and physical and mechanical properties of the materials used. Material
Chemical composition (wt.%)
AA6061-T6 DP590
Si 0.69 C 0.18
Typical physical & mechanical properties Melting point (◦ C)
Resistivity (m × 10−8 )
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Fe 0.46 Mn 2.2
Cu 0.33 P 0.035
Mg 1.06 S 0.03
650
4
288
334
13.3
1521
18.2
430.6
600
18
Fig. 4. Drawing of tensile-shear testing sample (unit: mm).
3. Results and discussion 3.1. Flow path analyses of electric current in RW process During a typical resistance welding process, the electric current flow is affected significantly by the contact condition of the diverse interfaces in the sheet combinations to be welded. Different from the traditional RSW, there are at least five main interfaces among the rivet and sheets in a typical SPR joint, as shown in Fig. 5. When the electric current was applied to the SPR joint using the aforementioned electrodes (referring to Fig. 2), the possible electric current paths are complicated and hard to be predicted. Thus, to get further knowledge about the influence of electric current on the SPR joint, it is necessary to study the initial contact condition of the SPR joint, and then analyze the current flow path in the SPR joint. The cross section of a typical SPR joint and the corresponding locally magnified interfaces among the rivet, aluminum and sheet steel are shown in Fig. 5. From the micrographs of the interfaces a and b, it was found that even though the newly generated shear surface in AA6061-T6 was rough, the high pressure generated from extruding AA6061-T6 still made a sound contact between the rivet
and aluminum, especially at interface b. For the interface c, it was clear that the surfaces of the top aluminum AA6061-T6 and bottom steel DP590 separated from each other, which can be explained by the springback of the deformed bottom DP590. For the similar reason, the contact between the rivet tip and bottom DP590 (interface d) was also insufficient. For the interface e, under the effect of the great extruding force produced by the rivet, the surfaces of AA6061T6 and bottom DP590 contacted with each other intimately. The great contact condition of the interfaces a, b and e indicated that they would be the potential electric current flow paths for the RW process. Based on the above initial contact condition analysis in a typical SPR joint and the electrode design, the electric current will flow into the joint from the top AA6061-T6 outside the rivet firstly, and then most of electric current is expected to go through the well contacted aluminum/rivet interfaces (interfaces a and b) into the trapped AA6061-T6, instead of going through the aluminum/steel sheet interface (interface c), for its bad contact condition. Afterwards, the current will pass through the trapped AA6061-T6 and interface e centralizedly, then converges at the deformed region of the bottom DP590, and finally outflows from the lateral of the joint bulging into the lower electrode, as illustrated in Fig. 6. Thus, under the effect of resistance heat, the heat treatment strengthening is expected to occur on local regions of the rivet and the bottom steel, and meanwhile the brazing bonding is also possible to be formed at the aluminum/steel interfaces.
3.2. Microstructure of RW joints The typical cross-sectional views and corresponding microstructures of the SPR joint and RW joints using LAE A and LAE B under 150 ms are presented in Fig. 7. Generally, the cross sections of RW joints (Fig. 7(e) and (i)) are quite similar in macro view to the traditional SPR one (Fig. 7(a)). However, via
Fig. 5. Initial contact conditions of a typical SPR joint.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 6. Schematic diagram of the current flow path.
microscopic examination of the cross sections, it was found that the microstructure of the SPR joints was affected by the heating current. As shown in Fig. 7(b), the in situ microstructure of the rivet in the SPR joint mainly consists of fine acicular tempered martensite, and less residual austenite created by tempering also should be contained in it. During the RW process using LAE A, the rivet was heated rapidly and then cooled by the adjacent aluminum. As a result, the residual austenite will transform into martensite, and finally some lath martensite was regenerated in this process, as shown in Fig. 7(f). It is noted that in Fig. 7(j), with the increase of the current density using LAE B, the lath martensite of the rivet in the RW joint becomes much coarser, accompanied by the precipitation of a spot of carbides. The microstructure of the deformed DP590 steel, as shown in Fig. 7(c), had a ferritic matrix with dispersed martensite islands at the grain boundaries. It is worth noting that during SPR phase, DP590 was stretched significantly around the rivet leg, which
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resulted in a strong grain orientation there. During the heating process, the temperature at the lateral of the joint bulging rose quickly since electric current converged there, as shown in Fig. 6. Therefore, the martensite dissolved and transformed into ferrite and austenite, which resulted in the further growth of the ferrite. Then, most of the newly generated austenite transformed into martensite owing to the subsequent rapid cooling process, similar to the microstructure at the intercritical heat-affected-zone observed in RSW process reported by Khan et al. (2008), as shown in Fig. 7(g). Furthermore, since more heat was produced in local regions beside the contact positions of LAE B, the peak temperature here was higher than that of using LAE A, which increased the volume fraction of transformed austenite significantly. Then, the subsequent cooling process led to the formation of uniformly mixed structure of massive fine-grained martensite and ferrite (Fig. 7(k)) as reported by Zhao et al. (2013). The microstructure of the trapped aluminum alloy (Fig. 7(d)), mainly consisted of ␣-Al solid solution matrix with a small amount of undissolved strengthening phases, such as Mg2 Si and Al2 CuMg distributed in it. Since most of the current passed through the trapped AA6061-T6 during the welding phase, the precipitation phases underwent a process of thermal cycling, which resulted in the precipitation of strengthening phases with larger size, as presented in Fig. 7(h). Furthermore, because part of the trapped AA6061-T6 was overheated while using LAE B, the aluminum alloy here was melted and columnar crystals grew along the cooling direction, as presented in Fig. 7(l).
3.3. IMC formation in RW joints The metallurgical bonding was only found at interface e (Fig. 5) for the RW joints using both LAE A and LAE B under certain welding time. Figure 8(a) presents the details of a typical bonding at the interface e produced using LAE B under 200 ms via SEM, and it was found that metallurgical connection was achieved between top AA6061-T6 and bottom DP590 steel. From the linear scanning of EDS analysis given in Fig. 8(b), it was found that the transition between the aluminum and steel was rather gradual, which was similar to the phenomenon found by Miles et al. (2010), and
Fig. 7. Typical metallographic structures of SPR joint and RW joints using LAE A and LAE B under 150 ms: (a) cross-sectioned SPR joint, and corresponding amplified views of (b) rivet, (c) DP590 and (d) AA6061-T6; (e) cross-sectioned RW joint using LAE A, and corresponding amplified views of (f) rivet, (g) DP590 and (h) AA6061-T6; (i) cross-sectioned RW joint using LAE B, and corresponding amplified views of (j) rivet, (k) DP590 and (l) AA6061-T6.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 8. EDS analysis of interface e in a typical RW joint using LAE B under 200 ms.
indicated that the formation and composition of the intermetallic compounds (IMC) at interface e were unstable. Moreover, the thickness of the IMC bonding here was quite thin that made the IMC bonding free of brittleness, according to the work by Zhang et al. (2007). The influence of heating time on the thickness of IMC at the interface e was also measured by the linear scanning of EDS analysis. Three replicates were made for each case to reduce the experimental errors, as presented in Fig. 9. It could be seen that under the available heating time, IMC layer was only formed under heating time of 400 ms using LAE A. In contrast, by using LAE B the IMC was first found under a heating time of 150 ms, because the current density at the electrode contact local region increased. Moreover, it was clear that the IMC thickness increased linearly with the increased heating time. 3.4. Micro-hardness distribution in RW joints Fig. 10 shows the effect of the electrode design on the microhardness distributions of the RW joints under various heating time. It should be noted that for the joints using LAE B, the hardness measuring position locates at the cross section along the middle of the electrode contact region, as shown by the yellow line in Fig. 10(a). Here four different measuring regions were selected for micro-hardness evaluation: deformed region of the rivet (region
R), deformed DP590 at the lateral of the joint bulging (region S), deformed AA6061-T6 outside the rivet (region A1), and deformed AA6061-T6 inside the rivet (region A2). The micro-hardness of region R versus heating time is presented in Fig. 10(b). As shown, when the heating time is less than 150 ms, the hardness of region R increased owing to the increased volume of martensite and precipitation of carbides, but when the heating time is longer than 150 ms, the increased heat input and subsequent cooling process may result in the mixed structures of coarse martensite and ferrite, which makes the micro-hardness decrease. Meanwhile it is worth noting that the micro-hardness of the deformed DP590 (region S) using LAE B, shown in Fig. 10(c), is much higher than that of DP590 using LAE A, due to the generation of the fine-grained martensite and ferrite (referring to Fig. 7(k)). Moreover, the aluminum of region A1 and A2 becomes soft rapidly; owing to the excessive heat input induced softening, as shown in Fig. 10(d). Moreover, the contradiction between formation of metallurgical bonding and heat softening presented in Figs. 9 and 10 was worth noting. In this research, to form intermetallic compounds at interface e, the power-on time should be longer than or equal to 400 ms and 150 ms by using LAE A and LAE B, respectively. However, the power-on time of 150 ms seemed to be an inflection point of the hardness of rivet steel (Fig. 10(b)) and aluminum (Fig. 10(d)) that when the power-on time was longer than 150 ms, the hardness decreased significantly. 3.5. Quasi-static tensile-shear tests
Fig. 9. Effect of heating time on the IMC thickness of RW joints.
In order to quantify the quality of both RW joints using LAE A and LAE B, they were compared with SPR ones on tensile-shear strength, as shown in Fig. 11. The failure mode of all the tensileshear tests is the separation of the bottom DP590 from the rivet and the top AA6061-T6, with serious aluminum and rivet bending against the tensile direction, and limited deformation at structure reinforced bottom DP590, which indicated that the hardness variation of regions R, A1 and A2 mainly determined the tensile-shear strength of the joints here. It is clear that the tensile strength for most of the RW joints is greater than that of SPR ones due to the significant increase of hardness in rivet leg. Meanwhile, under relatively short heating time, the static strength of RW joints either using LAE A or LAE B increases with heating time, because the hardness elevation of rivet leg is more significant than the hardness reduction of aluminum, as shown in Fig. 10. Under relatively longer heating time, the reduction in hardness of both rivet and aluminum results in the decrease of
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 10. Effects of the electrode design and heating time on the micro-hardness distributions of RW joints.
the joint strength, even though the IMC bonding forms at interface e. As shown in Fig. 10, the hardness discrepancy between rivets of joints using LAE A and LAE B is limit. On the contrary, resulting from more heat input, the hardness reduction of aluminum (region A1
and A2) in joints using LAE B is larger than that joints using LAE A, which makes most of the RW joints using LAE A stronger than the joints using LAE B. 4. Conclusions In this study, a novel fastening method, named RW was proposed to improve the robustness and strength of the SPR joints with considerably unequal thickness, e.g. 3 mm AA6061-T6 + 1.2 mm DP590 used here, by applying an electric current to the SPR joint, using specially designed upper and lower annular electrodes. The effects of both electrode design and heating time on the characteristics of RW joints were systematically studied, and the following conclusions could be drawn:
Fig. 11. Effects of the electrode design and heating time on the tensile-shear strength of RW joints.
(1) The electric current could change the original microstructure of the medium carbon steel rivet, bottom DP590 and AA6061-T6 to some extent. On the other hand, the relatively longer heating time contributed to the formation of IMC bonding between the trapped AA6061-T6 and zinc coated bottom DP590. (2) The micro-hardness of the rivet and bottom DP590 could be improved a lot due to the heat treatment, by using annular electrode under certain heating time, no matter what kind of structure it is. However, the micro-hardness of AA6061-T6 was decreased with increase of heating time, due to the effect of heat softening.
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(3) Comparing with the traditional SPR joints, the RW ones could obtain 12.1% and 6.7% higher tensile-shear strength by using LAE A and LAE B, respectively. Nevertheless, the excessive heat input might have negative effect on the joint performance, even though the IMC bonding was generated as the heat input added. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51275300 and 51322504), Program for New Century Excellent Talents in University (NCET-12-0361), and the Natural Science Foundation of Shanghai City (Grant No. 12ZR1415500). References Abe, Y., Kato, T., Mori, K., 2009. Self-piercing riveting of high tensile strength steel and aluminum alloy sheets using conventional rivet and die. J. Mater. Process. Technol. 209, 3914–3922. Feng, Z., Santella, M.L., David, S.A., Steel, R.J., Packer, S.M., Pan, T., Kuo, M., Bhatnagar, R.S., SAE paper 2005-01-1248 2005. Friction stir spot welding of advanced highstrength steels – a feasibility study.
Henrob Technology. 2011. http://www.henrob.com/GB/downloads.php Khan, M.I., Kuntz, M.L., Biro, E., Zhou, Y., 2008. Microstructure and mechanical properties of resistance spot welded advanced high strength steels. Mater. Trans. 49, 1629–1637. Miles, M.P., Feng, Z., Kohkonen, K., Weickum, B., Steel, R., Lev, L., 2010. Spot joining of AA 5754 and high strength steel sheets by consumable bit. Sci. Technol. Weld. Join. 15 (4), 325–330. Porcaro, R., Hanssen, A.G., Langseth, M., Aalberg, A., 2010. An experimental investigation on the behaviour of self-piercing riveted connections in aluminium alloy AA6060. Int. J. Crashworthiness 11 (5), 397–417. Qiu, R.F., Iwamoto, C., Satonaka, S., 2009. Interfacial microstructure and strength of steel/aluminum alloy joints welded by resistance spot welding with cover plate. J. Mater. Process. Technol. 209, 4186–4193. Sun, X., Stephens, E.V., Khaleel, M.A., 2007. Fatigue behaviors of self-piercing rivets joining similar and dissimilar sheet metals. Int. J. Fatigue 29, 370–386. Sun, X., Stephens, E.V., Khaleel, M.A., Shao, H., Kimchi, M., 2004. Resistance spot welding of aluminum alloy to steel with transition material – from process to performance. Part I. Experimental study. Weld. J. 83, 188–195. Zhang, H.T., Feng, J.C., He, P., Hackl, H., 2007. Interfacial microstructure and mechanical properties of aluminium–zinc-coated steel joints made by a modified metal inert gas welding–brazing process. Mater. Charact. 58, 588–592. Zhao, D.W., Wang, Y.X., Zhang, L., Zhang, P., 2013. Effects of electrode force on microstructure and mechanical behavior of the resistance spot welded DP600 joint. Mater. Des. 50, 72–77.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590 Ming Lou b , YongBing Li a,b,∗ , Yuan Wang b , Bin Wang c , Xinmin Lai b a b c
State Key Laboratory of Mechanical System and Vibration, Shanghai Jiao Tong University, Shanghai 200240, PR China Shanghai Key Laboratory of Digital Manufacture for Thin-Walled Structures, Shanghai Jiao Tong University, Shanghai 200240, PR China School of Engineering, Zhejiang Normal University, Jinhua 321004, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2013 Received in revised form 2 January 2014 Accepted 1 March 2014 Available online xxx Keywords: Advanced high strength steel (AHSS) Rivet-welding (RW) Micro-hardness distribution Inter-metallic compounds (IMC)
a b s t r a c t The hybrid use of aluminum alloy and advanced high strength steel (AHSS) has become an inevitable trend for fabricating a lightweight auto-body. Self-piercing riveting (SPR) as a preferred cold-forming fastening method is facing problem like weak interlocking when joining dissimilar combinations with considerably unequal thickness. In this study, a hybrid joining method, named rivet-welding (RW) was proposed to improve the robustness and strength of the SPR joint, by applying an electric current to it. For better evaluating the new process, the effects of heating time and electrode design on the microstructure, micro-hardness distribution, and mechanical performance of the RW joints were studied and compared systematically with the traditional SPR ones. The results showed that the electric current could improve the microstructure of the steel rivet and bottom DP590, and under long heating time, the inter-metallic compounds (IMC) could be formed at the interface of trapped AA6061-T6 and bottom DP590. Meanwhile, the electric current could increase the micro-hardness of the rivet and bottom DP590, and soften the AA6061-T6 around the rivet leg. In addition, the RW process using lower annular electrode A (LAE A) could obtain 12.1% higher tensile-shear strength compared with the traditional SPR process. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The extensive use of light metals, such as aluminum alloy could significantly reduce the weight of vehicle body. However, the aluminum alloy cannot completely replace the advanced high strength steel (AHSS) yet, considering the cost and performance. Therefore, the hybrid use of both aluminum and AHSS structures in bodyin-white (BIW) has become a more practical method to realize lightweighting as mentioned by Sun et al. (2007), for most of the automobile manufacturers. Abe et al. (2009) indicated that it is of great difficulty to join aluminum and steel directly by conventional resistance spot welding (RSW) process, since there are large differences in physical and chemical properties between aluminum and steel, and the hard and brittle intermetallic compounds easily form at the interface of the dissimilar metals. Recent researches showed that to obtain a sound joint by RSW, either a cover plate placed between the aluminum
∗ Corresponding author at: School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. Tel.: +86 21 34206304. E-mail address: [email protected] (Y. Li).
and electrode showed by Qiu et al. (2009) or aluminum-cladded steel sheet showed by Sun et al. (2004) should be introduced, which makes the process too complicated and costly to industrialize. Friction stir spot welding (FSSW) is thought to be a potential method to join aluminum to steel for the significantly reduced heat input, but Feng et al. (2005) indicated that the residual process hole could weaken the joint strength by decreasing bonding widths. New methods have been invented to fill the process hole, however, the cost is much higher and the extended cycle time cannot meet the requirements of manufacturing takt time. For clinching, even though the running cost is quite low, the clinched joint is not strong enough, and thus not suitable for joining force-bearing components. Nowadays, self-piercing riveting (SPR) as a cold forming process is preferred for joining dissimilar materials, and has been successfully applied to join multi-material components of BMW series 5 and Audi TT etc. As reported by Porcaro et al. (2010), different rivet and die should be used to achieve optimal joint strength for different sheet combination. However it is impractical to on-line replace the rivet and die with the variation of the sheet combinations in production line. Therefore, to guarantee the joint quality, more SPR equipments should be used in each assembly station, which would
http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006 0924-0136/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 1. Overview of the rivet-welding process.
greatly increase the cost of vehicle body manufacturing. On the other hand, the steel sheets used in auto-body become stronger and thinner so as to reduce the weight, and it is a consensus that for a better SPR result, the bottom sheet material should be no less than one-third of the total stackup thickness, as specified by Henrob Co. (2011). Thus, when joining dissimilar combinations of thick top aluminum and thin bottom sheet steel, the mechanical interlocking may not be adequate for designed joint strength. In order to improve the performance of the SPR joints with considerably unequal sheet thickness combination, a novel joint reinforcement method, named rivet-welding (RW) is proposed in this study, in which an electric current is applied to the SPR joint via specially designed electrodes, to produce resistance heat, and the effects of heating time and electrode design on the joint characteristics were studied systematically. For better evaluating the influence of resistance heat, the RW joints were compared with the traditional SPR ones in both tensile-shear and fatigue tests.
2. Experimental procedure 2.1. Process overview The RW process involves two phases: SPR phase and welding phase, as shown in Fig. 1. In the SPR phase, a conventional SPR joint is made to form the initial mechanical connection firstly. Then, an electric current is applied to the riveted sample during the welding phase via two specially designed annular electrodes to produce resistance heat in the joint. Zn/Sn coated medium carbon steel rivets with a head diameter of 7.8 mm and a total length of 6.5 mm, and the tool steel die (DZ0902000H1R1) with a middle diameter of 9 mm were used, which were supplied by Henrob Co. Specially designed annular electrodes made from Cu–Cr–Zr alloy are used in this study, as shown in Fig. 2. The inner diameter of the upper annular electrode (UAE) is 8 mm (Fig. 2(a)), which is slightly larger than the rivet’s head diameter, to prevent overheating it. Meanwhile, the middle
Fig. 2. Drawings of the sectional view of (a) upper annular electrode (UAE) and (b) lower annular electrode A (LAE A), and top view of (c) LAE A and (d) LAE B. (unit: mm).
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 3. Rivet-welding setups: (a) SPR machine and (b) MFDC welding system.
diameter of the lower annular electrode A (LAE A) is designed as 8.8 mm (Fig. 2(b)), which is also smaller than the middle diameter of the die used in SPR phase to make sure electric current flow through deformed steel. Furthermore, lower annular electrode B (LAE B), as shown in Fig. 2(d), is designed by removing partial material above the red line from LAE A every 60◦ , so as to locally increase the electric current flowing through the rivet tip and lower steel sheet. 2.2. Materials For a dissimilar sheet combination, especially the combination of aluminum and steel, the corrosion problem is critical because of the great discrepancy in electric potential. Thus, most of the automobile manufacturers prefer to use galvanized steel sheet in Al/Steel dissimilar sheet combinations. In this study, 3 mm thick aluminum AA6061-T6 and 1.2 mm thick hot-dipped galvanized dual-phase steel DP590 were used for the propose of anti-corrosion, and sheared into 130 mm × 38 mm coupons, to make up dissimilar sheet combination of AA6061-T6 + DP590. The chemical compositions, physical and mechanical properties of these materials are listed in Table 1. The aluminum coupons were polished by abrasive paper to remove the surface oxide layer, and acetone was used to clean up the surface greases on the DP590 coupons.
splashes through the bottom steel due to the excessive heat input. In present study, the trapped aluminum of RW joints using LAE A and LAE B splashed under welding time of 450 ms and 300 ms, respectively. Therefore, 400 ms and 250 ms were set as the respective maximum welding time here for RW process using LAE A and LAE B.
2.4. Microscopic analysis For the metallographic examination, all the obtained samples were cross-sectioned along the center of the button carefully, and mechanically ground and polished by conventional metallographic techniques. The cross-sections were etched using 3% nitric acid alcohol solution for 5 s and Keller reagent (the mixed aqueous solution of 2 ml HF + 3 ml HCl + 5 ml HNO3 + 190 ml H2 O) for 10 s, to observe the microstructures of steel and aluminum alloy, respectively. A Leica DM2500 M optical microscope and a SMT HV-1000 hardness testing machine were used to investigate the changes of the microstructure and micro-hardness distributions of the joints, respectively. In addition, scanning electron microscope (SEM) was used to verify the formation of the metallurgical bonding at interfaces between AA6061-T6 and DP590, and energy-dispersive spectroscope (EDS) was used to analyze the thickness and corresponding chemical compositions of the bonded regions.
2.3. Experimental setups and conditions 2.5. Mechanical tests The SPR phase is executed by a servo powered Henrob SPR device, as shown in Fig. 3(a). For each sample, the punch speed is chosen to make sure the surface of the rivet head is flush with the top material’s upper surface. After that, the riveted joints are heated by a Medar 5000s MFDC welding machine, which is equipped to Fanuc R2000-Ib210f robot with six degrees of freedom for accurate positioning, as shown in Fig. 3(b). In this study, 13 kA welding current (maximum welding current provided by the welder) and 5 kN electrode force are adopted for all the joints. Besides, to investigate the effect of heat input on the properties of RW joints, the welding time is chosen to begin with 50 ms and increases every 50 ms, until the trapped AA6061-T6
The strength of RW joints was evaluated by tensile-shear test. The tensile-shear specimens were prepared in accordance with the standard of GB/T2651-81, named “Test Methods of Tensile-Shear of Spot-Welded Joints”. During all the tensile-shear testing, a spacer was placed at the gripped region and clamped together with the specimen to make sure the alignment of the specimen and tensile direction. For this test, the joint was made at the center of the overlapped 38 mm × 38 mm square area precisely, as shown in Fig. 4. The tensile-shear tests were conducted at a constant speed of 2.5 mm/min on a SUNS testing machine. At least three replicates were made in the tests.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Table 1 Chemical compositions and physical and mechanical properties of the materials used. Material
Chemical composition (wt.%)
AA6061-T6 DP590
Si 0.69 C 0.18
Typical physical & mechanical properties Melting point (◦ C)
Resistivity (m × 10−8 )
Yield strength (MPa)
Ultimate tensile strength (MPa)
Elongation (%)
Fe 0.46 Mn 2.2
Cu 0.33 P 0.035
Mg 1.06 S 0.03
650
4
288
334
13.3
1521
18.2
430.6
600
18
Fig. 4. Drawing of tensile-shear testing sample (unit: mm).
3. Results and discussion 3.1. Flow path analyses of electric current in RW process During a typical resistance welding process, the electric current flow is affected significantly by the contact condition of the diverse interfaces in the sheet combinations to be welded. Different from the traditional RSW, there are at least five main interfaces among the rivet and sheets in a typical SPR joint, as shown in Fig. 5. When the electric current was applied to the SPR joint using the aforementioned electrodes (referring to Fig. 2), the possible electric current paths are complicated and hard to be predicted. Thus, to get further knowledge about the influence of electric current on the SPR joint, it is necessary to study the initial contact condition of the SPR joint, and then analyze the current flow path in the SPR joint. The cross section of a typical SPR joint and the corresponding locally magnified interfaces among the rivet, aluminum and sheet steel are shown in Fig. 5. From the micrographs of the interfaces a and b, it was found that even though the newly generated shear surface in AA6061-T6 was rough, the high pressure generated from extruding AA6061-T6 still made a sound contact between the rivet
and aluminum, especially at interface b. For the interface c, it was clear that the surfaces of the top aluminum AA6061-T6 and bottom steel DP590 separated from each other, which can be explained by the springback of the deformed bottom DP590. For the similar reason, the contact between the rivet tip and bottom DP590 (interface d) was also insufficient. For the interface e, under the effect of the great extruding force produced by the rivet, the surfaces of AA6061T6 and bottom DP590 contacted with each other intimately. The great contact condition of the interfaces a, b and e indicated that they would be the potential electric current flow paths for the RW process. Based on the above initial contact condition analysis in a typical SPR joint and the electrode design, the electric current will flow into the joint from the top AA6061-T6 outside the rivet firstly, and then most of electric current is expected to go through the well contacted aluminum/rivet interfaces (interfaces a and b) into the trapped AA6061-T6, instead of going through the aluminum/steel sheet interface (interface c), for its bad contact condition. Afterwards, the current will pass through the trapped AA6061-T6 and interface e centralizedly, then converges at the deformed region of the bottom DP590, and finally outflows from the lateral of the joint bulging into the lower electrode, as illustrated in Fig. 6. Thus, under the effect of resistance heat, the heat treatment strengthening is expected to occur on local regions of the rivet and the bottom steel, and meanwhile the brazing bonding is also possible to be formed at the aluminum/steel interfaces.
3.2. Microstructure of RW joints The typical cross-sectional views and corresponding microstructures of the SPR joint and RW joints using LAE A and LAE B under 150 ms are presented in Fig. 7. Generally, the cross sections of RW joints (Fig. 7(e) and (i)) are quite similar in macro view to the traditional SPR one (Fig. 7(a)). However, via
Fig. 5. Initial contact conditions of a typical SPR joint.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 6. Schematic diagram of the current flow path.
microscopic examination of the cross sections, it was found that the microstructure of the SPR joints was affected by the heating current. As shown in Fig. 7(b), the in situ microstructure of the rivet in the SPR joint mainly consists of fine acicular tempered martensite, and less residual austenite created by tempering also should be contained in it. During the RW process using LAE A, the rivet was heated rapidly and then cooled by the adjacent aluminum. As a result, the residual austenite will transform into martensite, and finally some lath martensite was regenerated in this process, as shown in Fig. 7(f). It is noted that in Fig. 7(j), with the increase of the current density using LAE B, the lath martensite of the rivet in the RW joint becomes much coarser, accompanied by the precipitation of a spot of carbides. The microstructure of the deformed DP590 steel, as shown in Fig. 7(c), had a ferritic matrix with dispersed martensite islands at the grain boundaries. It is worth noting that during SPR phase, DP590 was stretched significantly around the rivet leg, which
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resulted in a strong grain orientation there. During the heating process, the temperature at the lateral of the joint bulging rose quickly since electric current converged there, as shown in Fig. 6. Therefore, the martensite dissolved and transformed into ferrite and austenite, which resulted in the further growth of the ferrite. Then, most of the newly generated austenite transformed into martensite owing to the subsequent rapid cooling process, similar to the microstructure at the intercritical heat-affected-zone observed in RSW process reported by Khan et al. (2008), as shown in Fig. 7(g). Furthermore, since more heat was produced in local regions beside the contact positions of LAE B, the peak temperature here was higher than that of using LAE A, which increased the volume fraction of transformed austenite significantly. Then, the subsequent cooling process led to the formation of uniformly mixed structure of massive fine-grained martensite and ferrite (Fig. 7(k)) as reported by Zhao et al. (2013). The microstructure of the trapped aluminum alloy (Fig. 7(d)), mainly consisted of ␣-Al solid solution matrix with a small amount of undissolved strengthening phases, such as Mg2 Si and Al2 CuMg distributed in it. Since most of the current passed through the trapped AA6061-T6 during the welding phase, the precipitation phases underwent a process of thermal cycling, which resulted in the precipitation of strengthening phases with larger size, as presented in Fig. 7(h). Furthermore, because part of the trapped AA6061-T6 was overheated while using LAE B, the aluminum alloy here was melted and columnar crystals grew along the cooling direction, as presented in Fig. 7(l).
3.3. IMC formation in RW joints The metallurgical bonding was only found at interface e (Fig. 5) for the RW joints using both LAE A and LAE B under certain welding time. Figure 8(a) presents the details of a typical bonding at the interface e produced using LAE B under 200 ms via SEM, and it was found that metallurgical connection was achieved between top AA6061-T6 and bottom DP590 steel. From the linear scanning of EDS analysis given in Fig. 8(b), it was found that the transition between the aluminum and steel was rather gradual, which was similar to the phenomenon found by Miles et al. (2010), and
Fig. 7. Typical metallographic structures of SPR joint and RW joints using LAE A and LAE B under 150 ms: (a) cross-sectioned SPR joint, and corresponding amplified views of (b) rivet, (c) DP590 and (d) AA6061-T6; (e) cross-sectioned RW joint using LAE A, and corresponding amplified views of (f) rivet, (g) DP590 and (h) AA6061-T6; (i) cross-sectioned RW joint using LAE B, and corresponding amplified views of (j) rivet, (k) DP590 and (l) AA6061-T6.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 8. EDS analysis of interface e in a typical RW joint using LAE B under 200 ms.
indicated that the formation and composition of the intermetallic compounds (IMC) at interface e were unstable. Moreover, the thickness of the IMC bonding here was quite thin that made the IMC bonding free of brittleness, according to the work by Zhang et al. (2007). The influence of heating time on the thickness of IMC at the interface e was also measured by the linear scanning of EDS analysis. Three replicates were made for each case to reduce the experimental errors, as presented in Fig. 9. It could be seen that under the available heating time, IMC layer was only formed under heating time of 400 ms using LAE A. In contrast, by using LAE B the IMC was first found under a heating time of 150 ms, because the current density at the electrode contact local region increased. Moreover, it was clear that the IMC thickness increased linearly with the increased heating time. 3.4. Micro-hardness distribution in RW joints Fig. 10 shows the effect of the electrode design on the microhardness distributions of the RW joints under various heating time. It should be noted that for the joints using LAE B, the hardness measuring position locates at the cross section along the middle of the electrode contact region, as shown by the yellow line in Fig. 10(a). Here four different measuring regions were selected for micro-hardness evaluation: deformed region of the rivet (region
R), deformed DP590 at the lateral of the joint bulging (region S), deformed AA6061-T6 outside the rivet (region A1), and deformed AA6061-T6 inside the rivet (region A2). The micro-hardness of region R versus heating time is presented in Fig. 10(b). As shown, when the heating time is less than 150 ms, the hardness of region R increased owing to the increased volume of martensite and precipitation of carbides, but when the heating time is longer than 150 ms, the increased heat input and subsequent cooling process may result in the mixed structures of coarse martensite and ferrite, which makes the micro-hardness decrease. Meanwhile it is worth noting that the micro-hardness of the deformed DP590 (region S) using LAE B, shown in Fig. 10(c), is much higher than that of DP590 using LAE A, due to the generation of the fine-grained martensite and ferrite (referring to Fig. 7(k)). Moreover, the aluminum of region A1 and A2 becomes soft rapidly; owing to the excessive heat input induced softening, as shown in Fig. 10(d). Moreover, the contradiction between formation of metallurgical bonding and heat softening presented in Figs. 9 and 10 was worth noting. In this research, to form intermetallic compounds at interface e, the power-on time should be longer than or equal to 400 ms and 150 ms by using LAE A and LAE B, respectively. However, the power-on time of 150 ms seemed to be an inflection point of the hardness of rivet steel (Fig. 10(b)) and aluminum (Fig. 10(d)) that when the power-on time was longer than 150 ms, the hardness decreased significantly. 3.5. Quasi-static tensile-shear tests
Fig. 9. Effect of heating time on the IMC thickness of RW joints.
In order to quantify the quality of both RW joints using LAE A and LAE B, they were compared with SPR ones on tensile-shear strength, as shown in Fig. 11. The failure mode of all the tensileshear tests is the separation of the bottom DP590 from the rivet and the top AA6061-T6, with serious aluminum and rivet bending against the tensile direction, and limited deformation at structure reinforced bottom DP590, which indicated that the hardness variation of regions R, A1 and A2 mainly determined the tensile-shear strength of the joints here. It is clear that the tensile strength for most of the RW joints is greater than that of SPR ones due to the significant increase of hardness in rivet leg. Meanwhile, under relatively short heating time, the static strength of RW joints either using LAE A or LAE B increases with heating time, because the hardness elevation of rivet leg is more significant than the hardness reduction of aluminum, as shown in Fig. 10. Under relatively longer heating time, the reduction in hardness of both rivet and aluminum results in the decrease of
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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Fig. 10. Effects of the electrode design and heating time on the micro-hardness distributions of RW joints.
the joint strength, even though the IMC bonding forms at interface e. As shown in Fig. 10, the hardness discrepancy between rivets of joints using LAE A and LAE B is limit. On the contrary, resulting from more heat input, the hardness reduction of aluminum (region A1
and A2) in joints using LAE B is larger than that joints using LAE A, which makes most of the RW joints using LAE A stronger than the joints using LAE B. 4. Conclusions In this study, a novel fastening method, named RW was proposed to improve the robustness and strength of the SPR joints with considerably unequal thickness, e.g. 3 mm AA6061-T6 + 1.2 mm DP590 used here, by applying an electric current to the SPR joint, using specially designed upper and lower annular electrodes. The effects of both electrode design and heating time on the characteristics of RW joints were systematically studied, and the following conclusions could be drawn:
Fig. 11. Effects of the electrode design and heating time on the tensile-shear strength of RW joints.
(1) The electric current could change the original microstructure of the medium carbon steel rivet, bottom DP590 and AA6061-T6 to some extent. On the other hand, the relatively longer heating time contributed to the formation of IMC bonding between the trapped AA6061-T6 and zinc coated bottom DP590. (2) The micro-hardness of the rivet and bottom DP590 could be improved a lot due to the heat treatment, by using annular electrode under certain heating time, no matter what kind of structure it is. However, the micro-hardness of AA6061-T6 was decreased with increase of heating time, due to the effect of heat softening.
Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006
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(3) Comparing with the traditional SPR joints, the RW ones could obtain 12.1% and 6.7% higher tensile-shear strength by using LAE A and LAE B, respectively. Nevertheless, the excessive heat input might have negative effect on the joint performance, even though the IMC bonding was generated as the heat input added. Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (Grant Nos. 51275300 and 51322504), Program for New Century Excellent Talents in University (NCET-12-0361), and the Natural Science Foundation of Shanghai City (Grant No. 12ZR1415500). References Abe, Y., Kato, T., Mori, K., 2009. Self-piercing riveting of high tensile strength steel and aluminum alloy sheets using conventional rivet and die. J. Mater. Process. Technol. 209, 3914–3922. Feng, Z., Santella, M.L., David, S.A., Steel, R.J., Packer, S.M., Pan, T., Kuo, M., Bhatnagar, R.S., SAE paper 2005-01-1248 2005. Friction stir spot welding of advanced highstrength steels – a feasibility study.
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Please cite this article in press as: Lou, M., et al., Influence of resistance heating on self-piercing riveted dissimilar joints of AA6061-T6 and galvanized DP590. J. Mater. Process. Tech. (2014), http://dx.doi.org/10.1016/j.jmatprotec.2014.03.006