Friction stir processed Al-TiO₂ surface composites: DC vs. High frequency pulse and pulse reverse anodising

High Frequency Pulse and Pulse Reverse Pulse anodising was applied to AI-TiO₂ surface composites and compared with conventional DC anodising. Friction stir processing (FSP) was employed to impregnate TiO₂ powders into the surface of an Aluminium alloy. The aim was to understand the anodising behaviour of the Aluminium composites and to develop optical effects in the anodised layer owing to the presence of incorporated oxide particles that influence the scattering of light. The microstructure, morphology and resulting optical appearance of the anodised surface are investigated. The effect of anodising parameters on the optical appearance of the anodised surface especially the difference between conventional DC, high frequency pulse and high frequency pulse reverse pulse anodising was studied. Characterization was performed using SEM, FIB-SEM, TEM and GI-XRD. The surface appearance was analysed using photospectrometry technique which measures the diffuse and total reflectance of the surface. The appearance of the anodised surface changed from dark to bright upon increasing the anodising voltage. TiO₂ particles in the FSP zone were partially or completely modified during the anodising process. Anodic pore morphology was modified extensively around the TiO₂ particles and cavities were observed at the TiO₂ particle-Al matrix interface depending on the anodising process used. Clear influence of the process parameters like anodic and cathodic cycle voltage is observed and the mechanism of light interaction like scattering and absorption from the anodised surface are modified accordingly changing the anodised surface appearance.