Chemical interaction at the interface of metal–plastic direct joints fabricated via injection molded direct joining

[Display omitted] •The existence of hydrogen bond was verified for injection molded direct joining.•Hydrogen bond was formed between –OH on the aluminum surface and –CONH in PA6.•The hydrogen bond between aluminum and PA6 increased the joining strength.•The chemisorbed water undermined the formation of the hydrogen bond. Injection molded direct joining (IMDJ) is a promising metal–plastic direct joining method to make products lighter. IMDJ first treats the metal surface and then injects melted plastic onto the surface via injection insert molding. In this study, the chemical interaction between metal and plastic was discussed with aluminum plates whose surface textures and chemical conditions were carefully controlled. The different joining strengths for these different chemical conditioned plates indicated that the chemical interaction existed between aluminum and polyamide 6 (PA6). We characterized the joint interface with AFM-IR. The IR spectrums change indicated that –CONH in PA6 formed the hydrogen bond with the hydroxyl groups on the aluminum surface. The chemisorbed water on the surface was suspected to undermine the formation of the hydrogen bond. Our study showed that the hydrogen bond existed and contributed to the joining strength, which provided a better understanding of the joining mechanisms.