Rejoinability, strength recovery, and rejoining behavior of metal–thermoplastic direct joints

Direct joining technologies for metal–polymer bonding are becoming increasingly mainstream in the automotive and electronics industries. Injection molded direct joining (IMDJ) is a promising approach for mass production due to its high processing speed and compatibility with existing manufacturing systems. Beyond initial joining performance, the reusability of fractured joints has emerged as a critical issue from the perspectives of life cycle assessment and sustainability. However, systematic evaluations of the rejoinability of metal–polymer direct joints remain limited. In this study, the rejoinability of metal–polymer joints fabricated by IMDJ was investigated. Aluminum alloy A5052 and the engineering thermoplastic polybutylene terephthalate (PBT) were employed. The original joints achieved a shear strength of 25 MPa. The results demonstrated that fractured IMDJ joints could be rejoined using the same IMDJ process; however, substantial reductions in joint strength of 56% and 80% were observed for different metal surface treatments. In contrast, rejoining by induction heating direct joining (IHDJ) restored the joint strength to 25 MPa, comparable to that of the original joints. The rejoining mechanism in IMDJ was attributed to the infiltration of newly injected polymer into the residual microstructured fracture surface. The superior rejoining performance of IHDJ was considered to result from enhanced interfacial fusion between the residual polymer and the newly introduced polymer. This study provides a systematic evaluation of rejoinability in metal–thermoplastic direct joining and offers insights into the underlying interfacial mechanisms.