Influence of Curing-Induced Adhesive Behavior on Joint Formation and Mechanical Performance in CFRP/Al Hybrid Joints

This study investigates how the adhesive curing state before riveting influences material flow during riveting, joint formation, and the mechanical performance of CFRP/aluminum hybrid joints. Hybrid joints were fabricated in a single-lap configuration using electromagnetic self-piercing riveting (E-SPR) at curing times of 0, 20, 40, 60, and 80 min, and the adhesive distribution, joint geometry, load-displacement behavior, energy absorption, and failure mode were examined. As curing time increased, adhesive squeeze-out decreased and adhesive displacement during riveting was progressively restricted, leaving more adhesive near the contact point. Consequently, the head height increased from 0.12 to 0.21 mm, whereas the interlock distance decreased from 0.67 to 0.54 mm. In the bonded region, the peak load increased with curing time, and a peak load of 11.15 kN was observed at 40 min, indicating an increased contribution of the adhesive layer. In contrast, the load in the riveted region decreased at 60 and 80 min because the increased resistance of the adhesive interlayer limited the rivet deformation and mechanical interlocking. A maximum energy absorption of 32.13 J was observed at 40 min, where the joint exhibited relative contributions of the adhesive and the rivet. Failure analysis showed bearing failure at 40 min, whereas rivet pull-out was observed at 60 min, consistent with the curing-dependent changes in joint formation. These results indicate that curing-induced changes in adhesive behavior govern the interaction between adhesive flow and rivet deformation, thereby influencing joint formation and mechanical performance.