Surface defects of anodized HPDC Al alloy components

Aluminum high-pressure die casting (HPDC) technology is particularly suitable for making complex part shapes in the automotive industry in order to reduce the cost and weight of various components. The automotive industry uses for anodized aluminum are endless. On the other side, diecastings pose some of the most challenging problems in anodizing. The main problem with HPDC components is related to inherent flaws like porosity, segregations and oxide films that are difficult to avoid, which cause a non-uniform thickness and an unfavourable appearance of the anodic oxide film. This work is aimed to understand some defects that may occur in the anodic oxide film due to different alloying elements and different surface finishing of various diecast Al alloys. Anodized HPDC components made of hypoeutectic and eutectic Al-Si-Cu alloys, AlSi9Cu3(Fe) (EN AC-46000), AlSi11Cu2(Fe) (EN AC-46100) and AlSi12Cu1(Fe) (EN AC-47100) were considered. These components were studied at locations where different surface finishing and aesthetic defects were detected after visual testing. Microstructural investigations were carried out using an optical microscope and a field emission gun scanning electron microscope (FEG-SEM) equipped with an energy-dispersive Xray spectroscopy (EDS) detector. A casting substrate with a reduced eutectic Si segregation due to surface finishing operations showed greater anodizability and a thicker anodic oxide layer. On the contrary, asdiecast substrates show a poor anodizability and, sometimes, filling-related defects which induce a nonuniform oxide layer. Cu and Fe-rich intermetallics are detrimental to the compactness and uniformity of anodic oxide layer.