Influence of microstructure and surface finishing on the hard anodizing of diecast Al-Si-Cu alloys

The effect of microstructure and surface finishing on the hard anodizing of diecast Al-Si-Cu alloys is reported. The anodic oxide layer obtained from diecast AlSi9Cu3(Fe), AlSi11Cu2(Fe) and AlSi12Cu1(Fe) alloys has been analysed. Diecast plates have been anodized in as-diecast condition and after milling process. Metallographic and image analysis techniques have been used to quantitatively examine the variations of the oxide layer in terms of thickness and morphology. The final quality of the hard-anodized surfaces, in terms of colouration defects, has been evaluated by means of a surface quality index. The results indicate how the thickness of the anodic layer is strongly influenced both by the initial Cu content in the alloy and the local eutectic fraction in the substrate before anodizing. The anodizing response seems to be more related to the amount of -Al phase than the size of eutectic Si particles when these are relatively fine. The diecast AlSi12Cu1(Fe) alloy shows the best response to anodizing mainly due to the reduced Cu level if compared to the other diecast alloys. The anodic layer obtained on a substrate containing lower eutectic fraction is generally thicker and shows lower colouration defects.