Microstructural and tribological investigations of diecast and hard anodized AlSiCu alloys

This work is aimed to understand the impact of the surface characteristics of diecast Al-Si-Cu alloy on the hard-anodizing process. High-pressure die-cast plates made of AlSi9Cu3(Fe), AlSi11Cu2(Fe) and AlSi12Cu1(Fe) alloys were hard anodized in as-diecast condition and after sandblasting and surface milling operations. The thickness and morphology of the anodic layer were investigated using optical and electron microscopy, whereas the tribological properties were evaluated using a reciprocating sliding testing machine under ball-on-flat configuration. The removal of both external eutectic Si segregation and oxide passivating layer due to milling operation is beneficial to increase the thickness of the anodic film, independently of the investigated alloy. However, this leads to a decrease of the abrasion resistance due to the presence of coarser Si particles. Sandblasting removes lower amounts of material and its effect on the anodizing response is more affected by the initial chemical composition of the alloy. The thickness of the oxide layer over the as-diecast surfaces increases by increasing the eutectic fraction and decreasing the Cu content. The greater the amount of α-Al phase in the surface microstructure, the thicker the anodic layer will be. Fine and globular eutectic Si particles lead to a thicker hard anodic layer respect to coarser and plate-like ones. The anodizing response seems in any case to be more related to the amount of α-Al matrix than the shape of Si particles. Higher eutectic fraction with globular eutectic Si particles is useful to increase the tribological properties of the anodic film. Lower eutectic amounts lead to increase the thickness of the anodic film but also to decrease the abrasion resistance.