Achieving the adhesion improvement between heating elements and thermoplastics in resistance welding of composites via anodizing treatment

Resistance welding technology is an effective solution for joining carbon fiber reinforced thermoplastic composites (CFRTP). However, the resistance welding of CFRTP usually suffers from quality issues due to adhesion failure of heating element (HE). This study focused on enhancing the joint performance of resistance-welded CFRTP by applying anodization technique to form a porous oxide film on the aluminum HE surface. The lap shear strength of joints welded with anodized HE increased to 21.4 MPa, representing a 63.4 % improvement compared to the untreated condition. Surface characterization tests indicated that the porous alumina structure formed on the HE surfaces enhanced the interlocking effect within the weld and facilitated elemental diffusion at the HE interfaces. The improved wettability of the HE enhanced resin impregnation and led to better weld formation. Fractographic analysis revealed a shift in the primary failure mode from HE/CFRTP interfacial damage to mixed failure, suggesting that anodization treatment significantly enhanced HE/matrix interfacial adhesion. The digital image correlation (DIC) results revealed the anodized HE could decrease the strain and suppress crack propagation. Experimental methods and density functional theory (DFT) calculations both identified the formation of a new C[sbnd]O-Al chemical bond between the oxide film and CFRTP. The joining mechanism demonstrated that the joint strength was improved through both mechanical interlocking and chemical bonding. This research offered a reliable approach to creating lightweight CFRTP structures with high strength.