Evaluation of Joint Characteristics in Self-Piercing Resistance Element Welding of Thick Al6451 and GA980 Sheets Using a High-Resistance Ni Interlayer

In self-piercing resistance element welding (SPREW) of a thick aluminum sheet to a steel sheet, heat is generated predominantly at the contact interfaces along the current path, as in resistance spot welding, and tends to concentrate at the rivet-upper sheet interface. Excessive heat accumulation at this interface can cause the rivet to soften and collapse before it fully pierces the upper sheet. Therefore, conventional process optimization based solely on controlling the amount of heat input makes it difficult to simultaneously achieve sufficient joint strength, a short process time, and rivet integrity. In this study, the spatial distribution of heat generation was introduced as a new design variable. During SPREW of a 2.4 mm-thick Al6451 sheet and a 1.0 mm-thick GA980 steel sheet, a high-resistivity Ni foil was inserted at the aluminum-steel faying interface to shift heat generation from the rivet side toward the faying interface, while the main welding current was fixed at 9 kA, the level at which the rivet geometry was preserved. Thermal imaging under identical measurement conditions showed that, with a 0.1 mm-thick Ni interlayer, the maximum apparent surface temperature adjacent to the faying interface rose from 185.3 °C to 219.9 °C (18.7%), whereas that of the rivet shank decreased by 2.2%. High-speed imaging showed that rivet insertion was completed approximately 29% earlier, at 4.230 s instead of 5.958 s, within the same 6 s piercing current duration. In tensile-shear tests, joints without an interlayer fractured at the interface at an average peak load of 4.62 kN, whereas joints with the 0.1 mm-thick Ni interlayer reached 6.89 kN, approximately 50% higher, with button fracture. Post-test cross-sectional SEM/EDS showed that, with the 0.1 mm interlayer, neither a continuous Ni layer nor a residual aluminum layer remained at the rivet-steel joint (Fe ≥ 95 wt%, Ni ≤ 5 wt%), whereas with the 0.2 mm interlayer a continuous Ni-rich layer (locally up to 98 wt% Ni) remained along the fracture plane, the peak load decreased to 6.61 kN, and the fracture mode reverted to interfacial fracture, indicating that an excessive interlayer thickness is detrimental; within the range examined in this study, 0.1 mm gave the best result. These results indicate that controlling interfacial heat generation with a high-resistance interlayer is an effective means of securing joint strength in the dissimilar joining of thick aluminum and high-strength steel sheets while preserving rivet integrity.