Failure Mechanism during Hygrothermal Aging of 30 wt % GF/PET-A2024 Joints Strengthened by Cu2+ Ion Surface Treatment of A2024

Abstract

Metal/thermoplastic composite structures are increasingly used in lightweight engineering, yet their interfacial stability under service conditions remains a concern. In this study, joints composed of 30 wt % glass fiber-reinforced polyethylene terephthalate and A2024 aluminum alloy (30 wt % GF/PET-A2024 joints) were fabricated by direct injection molding after CuCl2-induced corrosion pretreatment of the A2024 surface. Their interfacial aging behavior and failure mechanisms were investigated under hygrothermal conditions of 60 °C and 85% relative humidity (RH). CuCl2 treatment markedly improved the initial tensile-shear strength, whereas hygrothermal aging caused continuous degradation, characterized by rapid deterioration during the early stage followed by a later plateau. The tensile-shear strength decreased from 54.45 MPa and subsequently remained within a relatively narrow range of approximately 38–40 MPa during the later aging stage. Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), molecular dynamics (MD) simulations, and density functional theory (DFT) calculations collectively support the interpretation that strength degradation was mainly associated with the accumulation of interfacial −OH and H2O-related species. These species altered the chemical environment of active Cu2+ sites and reduced the number of Cu–O coordination bonds formed between Cu2+ ions and PET carbonyl oxygen atoms. These findings clarify the interfacial aging mechanism through chemical-bond evolution and provide a theoretical basis for understanding the degradation behavior of metal/thermoplastic composite joints under hygrothermal conditions.