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 Al-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

Interface strength and degradation of adhesively bonded porous aluminum oxides

For more than six decades, chromic acid anodizing has been the main step in the surface treatment of aluminum for adhesively bonded aircraft structures. Soon this process, known for producing a readily adherent oxide with an excellent corrosion resistance, will be banned by strict international environmental and health regulations. Replacing this traditional process in a high-demanding and high-risk industry such as aircraft construction requires an in-depth understanding of the underlying adhesion and degradation mechanisms at the oxide/resin interface resulting from alternative processes. The relationship between the anodizing conditions in sulfuric and mixtures of sulfuric and phosphoric acid electrolytes and the formation and durability of bonding under various environmental conditions was investigated. Scanning electron microscopy was used to characterize the oxide features. Selected specimens were studied with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy to measure resin concentration within structurally different porous anodic oxide layers as a function of depth. Results show that there are two critical morphological aspects for strong and durable bonding. First, a minimum pore size is pivotal for the formation of a stable interface, as reflected by the initial peel strengths. Second, the increased surface roughness of the oxide/resin interface caused by extended chemical dissolution at higher temperature and higher phosphoric acid concentration is crucial to assure bond durability under water ingress. There is, however, an upper limit to the beneficial amount of anodic dissolution above which bonds are prone for corrosive degradation. Morphology is, however, not the only prerequisite for good bonding and bond performance also depends on the oxides’ chemical composition