# Self-Piercing Riveted Joint of Vibration-Damping Steel and Aluminum Alloy
## 1 Introduction  
A vibration-damping steel plate is a sandwich-structured steel panel with a viscoelastic adhesive layer between two steel sheets. This plate is mainly considered for car cowl and dash panels, since it not only reduces noise and vibration, but also provides proper mechanical strength. In response to the environmental regulations and market forces for improved fuel economy, the automotive industry has made an effort to enhance fuel efficiency and reduce carbon emissions. One of the key solutions to meet current regulations is weight reduction by application of lightweight materials, including advanced high-strength steel, aluminum alloy, and composites [1; 2]. Consequently, dissimilar joining of the vibration-damping steel to lightweight materials is inevitable to realize a variety of lightweight designs for cowl and dash panels. These days, aluminum alloy, in particular, is being increasingly adopted to replace conventional steels as the lightweight material in automobiles. Accordingly, it is necessary to develop dissimilar joining methods of aluminum alloys to the vibration-damping steel sheets. However, it is challenging to join the vibration-damping steels to aluminum alloys, since conventional fusion joining methods for metals, such as resistance spot welding [3; 4; 5] and laser welding [6; 7; 8], are not applicable to aluminum alloys and steel dissimilar joints due to their largedifference in thermo-mechanical properties and Fe-Al-based brittle intermetallic compounds (IMC) in the weld area [9; 10; 11]. Diffusion bonding can be used to join dissimilar metals; however, the suitability of diffusion bonding has been limited to highly reactive materials (titanium, beryllium, or zirconium) so far [12]. Therefore, non-fusion and non-diffusion joining methods, including adhesive bonders or mechanical fasteners, have been mainly considered for such dissimilar joints. However, the toxic chemicals comprising the bonders pose environmental and health problems. In addition, the relatively long processing time, along with the need for temporary fixtures, has made the chemical joining process inefficient [13]. The mechanical joining methods usually require careful alignment of holes between the parts to be joined, and this is considered a challenging task in automation. Alternatively, mechanical fasteners without requiring a pre-hole, such as self-piercing rivet (SPR) [14; 15], clinching [16; 17], and flow drill screw (FDS) [18], are being increasingly adopted in mass production of multi-material lightweight vehicles. In this study, the performance of SPR is investigated for the dissimilar joining of vibration-damping steel to aluminum alloy. The SPR is widely adopted for steel-aluminum lap joints because of its simplicity and high joining strength [14; 15; 19; 20; 21; 22]. In the SPR process, a semi-tubular river is pushed by a punch to drill the top sheet of the joint. A rivet shank subsequently enters into a bottom sheet, and then the rivet and bottom sheet are plastically deformed into a die, which is placed beneath the bottom sheet by a continued pushing force. This plastic deformation finally induces mechanical interlock in the joint. Depending on the material and joint configuration, the types of rivet and dye should be properly selected. [PERSON] et al. [19] reported the effect of rivet and die on SPR joints of aluminum alloy (AA6061) and mild steel (CR 4). Various combinations of rivet and die were selected to investigate the influence of rivet hardness, rivet shank length, die width, and pip height on the rivetability and mechanical performance of the joint. The SPR joint between aluminum alloy (A5052-H34) and mild steel (SPCC) was also experimentally and numerically studied by [PERSON] et al. [20]. It was found that the joinability for the combination of steel top and aluminum bottom was better than that for the reverse combination. [PERSON] et al.