Effective combined treatments of micro-arc oxidation and resin pre-coating on Ti-6Al-4V substrate: constructing epoxy micro-anchor bolts for bonding strength improvement of Ti-6Al-4V/CFRP joint

Robustly bonding Ti-6Al-4V and carbon fiber reinforced polymer (CFRP) into composites for aerospace engineering applications is highly challenging due to poor compatibility and thermal expansion mismatch. This study systematically investigated the synergistic effects of micro-arc oxidation (MAO) and resin pre-coating (RPC) to enhance the Ti-6Al-4V/CFRP interfacial performance. Ti-6Al-4V was subjected to MAO using different phosphate-based electrolytes at 20 g/L or NaAlO₂ solutions at 20 to 70 g/L, and RPC was conducted to reduce pore defects at the bottoms of the micro/nano channels. The epoxy resin infiltrating the micro/nano channels constituted reinforced mechanical interlocking structures as through-the-thickness “epoxy micro-anchor bolts”. Electrolyte types and concentrations significantly altered film/substrate interfacial structures. Surface characterization revealed that MAO with the 50 g/L NaAlO₂ solution produced a dense oxide film with uniformly distributed vertical pores, achieving 433.5 HV₁ hardness, optimizing epoxy infiltration and interfacial adhesion. Single lap shear tests demonstrated that the 50 g/L NaAlO₂ MAO and RPC combination achieved a maximum shear strength of 23.6 MPa, a 103.9 % improvement over the baseline. Failure mode analysis revealed a shift from adhesive debonding to CFRP interlaminar fracture, demonstrating the efficacy of the optimized interface design. This research provided critical insights into electrolyte-based pore morphology regulation and established a framework for designing high-strength hybrid composites in aerospace applications.