Enhancing connection strength and repair efficiency in stainless steel −CFRP joints via autonomous temperature-regulated vitrimer composite films

Stainless steel and carbon fiber reinforced polymer hybrid structures demand precise temperature regulation during resistance welding. This study develops a self-regulating heating unit using a sandwich Vitrimer composite film. The upper and lower layers incorporate multi-walled carbon nanotubes with negative temperature coefficients for rapid heating, while the middle layer utilizes vapor-grown carbon fibers exhibiting positive temperature coefficients to autonomously stabilize temperatures. By optimizing layer thickness ratios, the welded joint strength increases by 90.3% and 253.5% respectively compared to conventional methods using Vitrimer resin and commercial adhesives. Thicker temperature-control layers enhance fracture resistance via nanofiller pull-out mechanisms, whereas balanced heating-layer thickness promotes resin flow and avoids thermal degradation. The system enables electrothermal joint repair with 21.5% higher efficiency than hot pressing, facilitated by dynamic Vitrimer network reorganization. This design achieves autonomous temperature regulation, multi-cycle repairability, and mechanical robustness, offering a sustainable solution for lightweight hybrid structures requiring adaptive welding processes.