Effect of post-joining annealing on crystallinity and mechanical performance of CF-PA6/ steel joints fabricated by induction heating direct joining

Induction heating direct joining (IHDJ) employs electromagnetic induction heating combined with applied force to achieve localized bonding between polymers and metals and is well suited for continuous fiber-reinforced thermoplastic composites. While most studies have focused on the joining process, the influence of post-joining thermal history remains underexplored. The rapid, localized thermal evolution inherent to IHDJ can significantly affect the crystallization behavior and mechanical properties of the polymer matrix, thereby limiting joint performance. Therefore, effective post-joining thermal treatments are essential to regulate thermal history and polymer crystallinity, thereby enhancing load-bearing capacity and improving the mechanical reliability of IHDJ joints. This study aims to enhance the strength of IHDJ-fabricated continuous carbon fiber–reinforced PA6 (CF–PA6)/steel joints through post-joining annealing and to elucidate the underlying strengthening mechanisms. The results show that annealing promotes crystallization and structural homogenization of the PA6 matrix in the joint region, as evidenced by an increased fraction of the thermodynamically stable α phase and a reduced amorphous content. In addition, annealing improves matrix mechanical properties, enhances fiber–matrix interfacial constraint, and suppresses crack initiation and propagation. Consequently, the tensile shear strength of CF–PA6/steel joints increased from 35.5 MPa to 42.8 MPa after annealing. These findings demonstrate that regulating polymer crystallization via post-joining thermal treatment is an effective strategy for improving the reliability of IHDJ-fabricated joints.