Hard anodizing of diecast Al-Si-Cu alloys

High-pressure die casting (HPDC) for aluminium castings is particularly suitable for high production rates and it is applied in several industrial fields. HPDC is used for making complex part shapes, especially in the automotive industry, in order to reduce the cost and weight of components. Hard anodizing Al alloy components is a way to improve the surface properties in terms of hardness, wear and corrosion resistance. On the other side, diecastings result in the most challenging problems during anodizing. One of the main problems with HPDC components is related to segregations which are difficult to avoid and cause a non-uniform thickness and an unfavourable appearance of the anodic oxide film. This work is aimed to understand the impact of the die cast substrate characteristics on the hard anodizing process. The anodic oxide obtained on HPDC plates made of AlSi9Cu3(Fe), AlSi11Cu2(Fe) and AlSi12Cu1(Fe) alloys was analyzed. In particular, the components were hard anodized both in as diecast conditions and after surface milling operation, and the oxide thickness was investigated by means of optical microscopy. Milling operations were performed in order to remove the surface eutectic macrosegregations. In addition, the surface quality of the plates has been evaluated after hard anodizing process by means of a quality index.The results indicate that the thickness of the anodic layer is influenced both by the initial Cu concentration and the amount of eutectic Si. In particular, AlSi12Cu1(Fe) alloy shows the best anodizability mainly due to the reduced Cu amount. The anodic layer obtained on substrates showing lower amounts of eutectic Si is generally thicker and shows a better aesthetic quality. These improvements are more noticeable on AlSi9Cu3(Fe) alloy.