Effect of Foil Degreasing on Adhesion and Defect Formation in Aluminum Honeycomb Cores: A Wettability-Based Analysis

Defect formation during foil degreasing represents a critical but insufficiently quantified factor affecting the mechanical performance of aluminum honeycomb cores. In particular, deviations in degreasing conditions may not manifest directly but lead to defect initiation at subsequent stages of manufacturing, making the problem inherently indirect and challenging to control. In this study, a physically grounded approach to assessing and controlling degreasing quality is proposed based on the relationship between substrate wettability and adhesion-related performance. A mathematical model describing droplet spreading on the foil surface is developed, enabling the formulation of a practical tolerance criterion that relates the spreading diameter of a liquid droplet to its initial size. This criterion provides a simple and robust tool for indirect evaluation of surface cleanliness under industrial conditions. The results demonstrate a high sensitivity of adhesion characteristics to variations in wettability, while also revealing that strict optimization of spreading parameters is not required. Instead, a threshold-based criterion ensures process stability. Furthermore, the study establishes a clear link between local surface heterogeneities and macroscale defect formation, including incomplete bonding, cell distortion, and degradation of mechanical properties. An alternative combined anodic oxidation and degreasing process is proposed and experimentally validated. The developed approach leads to a twofold increase in peel strength (from 0.75·10³ N/m to 1.44·10³ N/m), demonstrating its effectiveness for improving adhesion performance. Overall, the proposed methodology provides a practical framework for defect prevention and quality control in the manufacturing of aluminum honeycomb cores, bridging the gap between process parameters, surface condition, and structural performance.