Influence of metal surface temperature on polymer morphology and adhesion of metal-composite joints produced by fused filament fabrication

This study investigates how aluminium substrate temperature governs polymer crystallinity and interfacial adhesion in continuous carbon-fibre-reinforced polyamide-6 (PA6) joints produced by fused filament fabrication (FFF). A custom heated build plate enabled controlled deposition of the composite onto aluminium surfaces at temperatures between 160 to 210°C. Three-point bending and single-lap shear tests were used to quantify adhesion, while differential Scanning Calorimetry (DSC) and microscopy provided complementary insight into interfacial morphology. Increasing substrate temperature up to the PA6 melting point (200°C) improved wetting behaviour, promoting complete filling of the aluminium surface topography, and increased interfacial crystallinity from 29.7% to 42.3%, resulting in nearly a fourfold increase in delamination load. At 210°C (10°C above polymer melting temperature), an adhesion reduction was attributed to altered solidification history caused by prolonged melt residence and delayed solidification. Fractography revealed a transition from adhesive failure to mixed and cohesive modes at elevated temperatures, with metallic asperity fracture and polymer entrapment evidencing strong mechanical interlocking. The maximum lap shear strength of 12.8 MPa compares favourably with established metal–thermoplastic joining techniques, demonstrating that substrate-temperature-controlled crystallinity is a critical design parameter for optimising interfacial performance in additively manufactured metal–polymer hybrids.