Friction Stir Processed Al-TiO2 Surface Composites: Anodizing Behaviour and Optical Appearance

Multiple-pass friction stir processing (FSP) was employed to impregnate TiO2 (rutile) particles into the surface of an Aluminium alloy. The surface composites of Al-TiO2 were then anodized in a sulphuric acid electrolyte. The effect of anodizing parameters on the resulting optical appearance was investigated. Microstructural and morphological characterization was performed using scanning (SEM) and transmission electron microscopy (TEM), and X-ray diffraction (XRD). The surface appearance was analysed using an integrating sphere-spectrophotometer setup which measures the diffuse and total reflectance of light from the surface. Compared to samples without TiO2, surface appearance after anodizing of samples with TiO2 changed from dark to greyish white upon increasing the anodizing voltage. This is attributed to the localized microstructural and morphological differences around the TiO2 powder particles incorporated into the anodic alumina matrix. The TiO2 powder particles in the FSP zone were partially or completely amorphized during the anodizing process, and also electrochemically shadowed the anodizing of underlying Al matrix.