Metal−thermoplastic hybrid structures have proven their effectiveness to achieve lightweight design concepts in both primary and secondary structural components of advanced aircraft. However, the drastic differences in physical and chemical properties between metal and thermoplastic make it challenging to fabricate high-reliability hybrid structures. Here, a simple and universal strategy to obtain strong hybrid structures thermoplastics is reported by regulating the bonding behavior at metal/ thermoplastic interfaces. To achieve such, we first researched and uncovered the bonding mechanism at metal/thermoplastic interfaces by experimental methods and density functional theory (DFT) calculations. The results suggest that the interfacial covalency, which is formed due to the interfacial reaction between high-electronegativity elements of thermoplastics and metallic elements at metal surfaces, dominates the interfacial bonding interaction of metal−thermoplastic hybrid structures. The differences in electronegativity and atomic size between bonding atoms influence the covalent-bond strength and finally control the interfacial reliability of hybrid structures. Based on our covalent-bonding mechanism, the carboxyl functional group (COOH) is specifically grafted on polyetheretherketone (PEEK) by plasma polymerization to increase the density and strength of interfacial covalency and thus fabricate high-reliability hybrid structures between PEEK and A6061-T6 aluminum alloy. Current work provides an in-depth understanding of the bonding mechanism at metal−thermoplastics interfaces, which opens a fascinating direction toward highreliability metal−thermoplastic hybrid structures.
Interfacial structures govern the reliability of metal−thermoplastic hybrid joints used in the aerospace industry. The current work demonstrated by experimental methods and density functional theory (DFT) calculations that introduction of carbon fibers (CFs) enhanced the mechanical properties and weakened the corrosion resistance of polyamide 6 (PA6)/A6061-T6 (6061) joints. The bonding strength of typical PA6/6061 joints was increased by 33.70% with the introduction of CF. However, the differences in intrinsic work functions of the CF and various phases within 6061 led to the formation of serious cracks at CFRPA6/ 6061 interfaces and heavy corrosion on 6061 surfaces, corresponding to the decreased corrosion resistance of PA6/6061 joints.Herein, we present a potential solution to adjust the welding heat input to enhance metal/thermoplastic interfacial reliability. With a rotation speed of 400 mm/min during friction lap joining (FLJ), the fabricated CFRPA6/6061 joint could achieve a strong interface with high strength (bonding strength = 1.730 kN) and relative corrosion resistance (corrosion rate < 0.1 mm/a). The results provide a reliable explanation for the effect of CF on mechanical properties and corrosion resistance of CFRPA6/6061 joints. Furthermore, the knowledge gained in this work will benefit future research in the optimization of processes to improve the reliability of metal− thermoplastic hybrid structures.
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