Investigations into the coefficient of thermal expansion of porous films prepared on AA7175 T7351 by anodizing in sulphuric acid electrolyte

The aim of this study was to investigate the Coefficient of Thermal Expansion (CTE) of anodic films on 7175 T7351 aluminium alloy and to evaluate the influence of the film characteristics on this value. In particular, effects of porosity and post-treatments, such as coloring and sealing, were studied. Beam bending analysis was used as the experimental method and a numerical finite element model was developed to verify theoretical relationships hypotheses. In particular, the errors induced by the use of theoretical relationships between the curvature of the sample and stresses in the film were not negligible. A relation based on a finite element model was then developed and used to calculate stresses. The experimental value of CTE obtained by beam bending test was then validated by comparing the experimental cracking temperature of anodic films with a theoretical value directly depending on the previously determined CTE. The CTE of anodic films was found to be 13.0±1.010−6K−1 independent from the porosity of the films and from the post-treatment (inorganic coloring and sealing).