Green and cost-effective anodizing approach for enhanced aluminium-polyurethane adhesive joints

Umut Bakhbergen, Madina Moldabayeva, Murat Demiral, Qingshi Meng, Sherif Araby

Smart Materials in Manufacturing, 2025

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DOI 10.1016/j.smmf.2025.100100
Data confidence 85 high
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Abstract

Acid-based anodizing treatment for metals is a traditional approach to improve the interfacial strength in adhesive joints. However, this raises handling risk and environmental concerns due to the use of hazardous/toxic chemicals in addition to the high expenses. This study proposes and systematically examines a cost-effective and green KCl/DMF anodizing process as an alternative surface treatment for aluminium (Al) alloy (Al2024-T3). A full factorial design of experiments was employed to quantify the influence of electrolyte concentration (0.05–0.1M), voltage (8–12 V) and time (5–15 min) on the surface characteristics of Al substrates. The treated Al sheets were adhesively bonded using polyurethane (PU) in the form of single lap joints. The formation of a porous oxide layer led to high surface roughness and surface free energy which facilitated strong mechanical interlocking at Al-PU interface. Consequently, the lap shear strength and fracture energy of Al-PU joints exhibited 3.5 and 67-fold increments after anodizing, respectively. Statistical analysis including main effects, Pareto chart and ANOVA identified time and concentration as dominant factors for both surface characteristics and strength. Strong interfacial bonding between KCl/DMF-anodized Al and PU was observed by SEM fracture analysis, shifting the failure mode to fully cohesive. In addition, finite element (FE) model was developed to investigate the role of surface texture in Al-PU adhesive joint behaviour. The FE models confirmed that optimized surface texture is needed in order to achieve the maximum lap joint strength. This study establishes KCl/DMF anodizing process as a novel, cost-effective, acid-free and readily compatible with the existing industry for adhesively joining polymer-metal composites. • A green anodizing method using potassium chloride (KCl) was developed for surface treatment of Al2024. • A full factorial experimental design was employed to investigate the effects of concentration, voltage, and time. • Lap shear strength and fracture energy of Al-PU joints increased by up to 3.5 and 67 times, respectively. • Optimized KCl anodizing at(0.1M KCl, 12 V, 5 min) achieved highest strength, minimal cost and adverse environmental impact. • Surface morphology, free energy and FEA confirmed that controlled anodized topography improves interfacial adhesion.