Correlation between optical appearance and orientation of aluminium

The correlation of the optical appearance (= gloss and colour) of aluminium (and also of anodised aluminium) and its microstructure – mainly the orientation - was studied with different aluminium samples (Al single crystals, AlMg1 and AlMgSi0.5 sheets or profiles). Initially, we studied the influence of the anodisation process (etching, anodic oxidation and sealing) on the microstructure and the optical appearance. Next, we investigated the influence of the orientation of aluminium on its optical appearance. Finally, we verified our results with practical examples of anodised aluminium with inhomogeneity of the optical appearance. On the basis of these, we conclude that the optical appearance of aluminium (and also of anodised aluminium) corresponds mainly to the roughness of the aluminium surface and the roughness again corresponds mainly to the etching time and the orientation of the aluminium grains. In particular, the roughness of aluminium grains after the same etching time increases in the sequence: (111) - (100) - (110) - higher index orientations. We also suppose that a dark appearance and a high gloss correlate with small roughness. The inhomogeneity of the optical appearance of the practical examples can be attributed to corresponding inhomogeneity of the texture, which in turn can be attributed to inhomogeneity in the manufacturing conditions.