Laser transmission welding (LTW) of high-temperature thermoplastic films to carbon fiber–reinforced polymer (CFRP) laminates is a promising approach for integrating film-based, pre-manufactured sensor networks into aerospace structures. To enable welding of thermoset composites, a thermoplastic surface layer can be applied during laminate manufacturing, forming a semi-interpenetrating network (SIPN) that provides a weldable interface.

Welding thin polyetherimide (PEI) films to CFRP remains challenging due to narrow thermal processing windows, limited interfacial wetting, and insufficient understanding of fracture behavior at modified composite surfaces. In particular, the effect of SIPN surface layers on interfacial strength during LTW has not been systematically investigated.

This study examines LTW of 200 µm thick PEI films to CFRP laminates using a near-infrared diode laser. Laser power (20–30 W), energy per unit length (0.16–10 J/mm), and welding strategy (contour and quasi-simultaneous) were varied within a structured design of experiments to identify a stable process window. Interfacial temperatures were monitored by pyrometry, while joint performance was evaluated using lap-shear testing.

A defined process window enabled reproducible weld seams without thermal degradation of the PEI films, achieving lap-shear strengths of 8–11 MPa. For SIPN-coated CFRP laminates, floating roller peel tests revealed predominantly cohesive or semi-cohesive failure within the weld interface.

These results demonstrate that LTW of PEI films to SIPN-modified CFRP provides a scalable joining approach for integrating sensor networks in aerospace and advanced lightweight structures.