Papers by Author: George E. Thompson

Abstract: The present work studied the microstructure of near-surface deformed layers and their evolution from the transfer slab to the cold rolled final gauge sheet of an AA3104 aluminium alloy. Electron microscopy of ultramicrotomed cross-sections revealed two types of near-surface deformed layers, i.e. type A and type B, both with different microstructures to the underlying bulk alloy. A typical feature of the deformed layers is the nano-sized ultrafine grains, with diameters ≤ 200 nm for the type A and ≤ 500 nm for the type B deformed layer. Electron energy loss spectroscopy (EELS) indicated that oxide particles are present along grain boundaries within the type A deformed layer, while the type B deformed layer is free of oxide particles. The type A deformed layer is mainly generated at elevated temperatures during the early stages of hot rolling. Its thickness is non-uniform across the surface, with a maximum of ~4 µm on the transfer slab, ~1 µm on the re-roll gauge sheet and ~0.8 µm on the final gauge sheet. While the surface of the hot rolled sheet is mainly covered by the type A deformed layer, the surface undergone cold rolling is alternately covered by the type A and the type B deformed layers.

Abstract: In the present study, the microstructure and corrosion behaviour of an extruded Mg-10Gd-2Y-0.5Zr alloy (noted as GW102) in 3.5 wt.% NaCl for different times have been investigated using a combination of computed phase diagrams, optical microscopy, high resolution scanning electron microscopy (SEM) and scanning Kelvin probe force microscopy (SKPFM). It was revealed that the extruded alloy is composed of fine recrystallized, equiaxed Mg solid solution grains. Additionally different second phases of varying sizes and shapes are present, including a large square-shaped Mg₅(Gd,Y) particles which solidified from the melt and are located within the Mg grains and/or at grain boundaries, fine spherical zirconium-rich particles which are located in the Mg grains and fine needle-like precipitates of Mg₅(Gd,Y) and Mg₂₄(Gd,Y)₅ which are uniformly distributed within the Mg grain interior. The latter was formed during thermal extrusion processing. Further, a number of deformation bands or twins were found existing in the magnesium grains. SKPFM potential maps showed that Zr-rich particles and Gd(Y)-rich particles have positive potentials relative to the Mg matrix and the highest potential was recorded on Zr-rich particles. Immersion testing revealed that microgalvanic corrosion initiated at the periphery of Zr-rich particles due to their strong cathodic activity, thereby forming microgalvanic couples with the adjacent Mg grains.

Abstract: In the present study, microstructure and its influence on corrosion behaviour of friction stir welded aluminium airframe alloy have been investigated. Further, the effect of laser surface treatment on the microstructure and the corrosion behaviour was also assessed. The welded AA2024 alloy showed the expected zones associated with friction stir welding. Corrosion testing showed that modified microstructure reduced its corrosion resistance to localized corrosion, with the regions immediately outside TMAZ being most susceptible to corrosion. Laser treatment resulted in a melted near-surface layer, up to 5 (m thick, where normal constituent particles are absent. Scrutiny of the melted near-surface layer revealed continuous segregation bands, approximately 10 nm thick, containing mainly copper. Corrosion testing showed that laser treatment increases the short term resistance to localized corrosion due to the removal constituent particles. However, prolonged testing revealed corrosion within the melted near-surface layer and delamination of the melted near-surface layer from underlying bulk alloy. The corrosion is associated with copper rich segregation bands that promote local microgalvanic action.

Abstract: High strength aluminium alloys are widely used in the civil and military aerospace industry due to their low weight and high mechanical properties, achieved by selected alloying elements and heat treatments. The resulting multiphase alloy system, a solid solution of alloying elements in the aluminium matrix and a variety of second phase material, requires specific anticorrosion measures in order to prevent localized corrosion, which is promoted by microgalvanic coupling between the different metallographic phases. Traditionally, the anticorrosion performances are achieved by chromic acid anodizing (CAA), followed by painting. However, environmental issues and associated costs for the disposal of chromate wastes, require the development of new approaches for anodizing of aluminium alloys. In this work, the potential for tailoring the porous anodic film morphology through the film thickness by controlled variations of the anodizing potential is inspected. The procedure developed is, in principle, applicable to any aluminium alloy in any anodizing electrolyte and results in the generation of innovative graded porous anodic film morphologies which promise improvement of anticorrosion properties and replacement of CAA .

Abstract: The influence of alloying elements, deliberate or otherwise, and material processing on the performance of aluminium during surface treatment and finishing are considered. Thus, with a significant focus on copper, but with consideration of other elemental additions, the behaviour of aluminium during growth of oxide at ambient temperature, etching or pickling, conversion coating and anodizing, essential processes for generation of fit-for-purpose products, is highlighted. Further, such processes generate, modify or transform the initially present air-formed alumina film. Consequently, with knowledge of the phenomena proceeding at the alloy/film and film/environment interfaces and those within anodic or other films, the possibility of controlling features of nanoscale dimensions for improved performance arises. For example, deliberate selection of alloying elements enables control of nanotextures formed at treated surfaces, and formation of compositionally and morphologically modified films as well as generation of nanoparticles with various functional properties.

Abstract: Over the past ten years a detailed understanding of the dominant factors controlling the corrosion susceptibility of painted aluminium alloy sheet used for architectural and automotive applications has been developed. Work carried out in this field will be reviewed and the different modes of cosmetic corrosion that can occur-, the mechanisms of surface activation, and the role of different thermo-mechanical processing steps on controlling corrosion susceptibility for different alloy systems, including Al-Mn, Al-Mg and Al-Mg- Si-(Cu) will be discussed. The critical role of chemical or electro-chemical cleaning prior to pre-treatment and coating will be highlighted and prospects for meaningful accelerated testing will be discussed.