Elucidating the Influence of Polymer Structure on the Joining Strength of Polyamide 6–Aluminum Direct Joints

Polymer–metal direct joining has become increasingly important in lightweight structural applications. However, the factors that govern joint strength, particularly the role of the interfacial crystalline structure, remain poorly understood. In this study, the interfacial mechanisms influencing joint strength in polymer–metal direct joining were investigated using anodized aluminum and polyamide 6 (PA6) as a representative system. Emphasis was placed on the extent of PA6 penetration into the anodized layer and the crystalline structure formed at the joining interface. Mechanical testing of specimens molded at different temperatures revealed that higher mold temperatures enhanced joint strength. Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy revealed that elevated mold temperatures promoted deeper PA6 penetration into the anodized layer. TEM, wide-angle X-ray scattering, and optical photothermal infrared spectroscopy indicated the formation of transcrystals at the interface in the joints molded at 120 and 160 °C, with the crystalline phases being the γ phase and a mixture of γ and α phases, respectively. Nanoindentation measurements revealed a positive correlation between the composite modulus at the interface and joint strength. The presence of α phase crystals with a high crystal modulus alleviated stress concentration at the interface, thereby contributing to improved joint strength. These findings demonstrate that both mechanical interlocking and interfacial crystalline structure play critical roles in determining the strength of polymer–metal joints. This insight offers a promising strategy for enhancing joint strength through interfacial engineering, with potential applications in advanced lightweight structures for the automotive and aerospace industries.