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The replacement of copperlbrass radiators in the automotive industry with radiators made from aluminium components provided the basis of this research. Since aluminium is more susceptible to corrosion than either copper or brass, factors that contribute to its corrosion are of major interest and importance, and have been investigated. Three different aluminium alloys were selected for study because of a special interest in their corrosive behaviour by the automotive industry. These are the aluminium alloy AA 3003 (samples A and B) and two supplier specific alloys (sample D containing Zn and sample E containing Cu and Mg). The various joining operations used in the automotive manufacturing process dictated the preparation of the aluminium alloys used for corrosion studies. Mechanically Assembled (MA) aluminium radiators use alloy samples as supplied by the aluminium industry and hence suitable experiments were carried out on the 'as-supplied' (AS) samples used for both finstock and tubestock material. The development of Composite Deposition (CD) Technology to braze together finstock and tubestock material introduced new challenges to corrosion research. To gain an insight into the corrosion of a Brazed aluminium radiator, all samples were subjected to a thermal profile identical to that experienced industrially under a Controlled Atmosphere Brazing (CAB) furnace. Two cases of interest emerged. Firstly the 'heat-treated' (HT) samples were used to evaluate the effect ofheat treatment on the alloy's resistance to corrosion. Secondly, alloy samples treated with a Composite Powder Coating (CPC) and then subjected to the thermal profile provided a surface of an AI-Si melt which represented the brazed joint. Experiments on these samples yielded information on the AI-Si melt and the likely corrosion in a brazed joint. The resulting corrosion of the AS, HT and CPC samples immersed in various corrosive electrolyte solutions for 60 minutes was examined using two microscopic techniques. Firstly, the actual surface pitting was examined using a Scanning Electron Microscope (SEM), and secondly, cross-sections of the samples mounted in a resin, then suitably polished and etched were examined using an optical microscope to further reveal the nature of corrosion of the samples. The nature of corrosion was best revealed in an acidified chloride solution. The AS samples showed delocalised crystallographic pitting consisting of coalesced pits at localised regions of the surface. The HT samples showed IV localised crystallographic pIttIng consIstIng of many individual pits and intergranular corrosion both at and below the surface. Intergranular corrosion was most severe for HT sample E containing Cu and Mg. The CPC samples showed total corrosion of the surface layer and eutectic AI-Si melt, some crystallographic pitting of the a-AI filler metal, and crystallographic pitting including intergranular corrosion of the base alloy. The extent of corrosion was found to depend on the chemical composition of the aluminium alloys, the presence of Zn, Cu and Mg causing more severe corrosion of the aluminium alloys, with the effect ofZn being most severe. The electrochemical investigation involved the measurement of two fundamentally important parameters. Firstly, the open circuit potentials (OCP) of the alloy samples immersed in the various corrosive electrolyte solutions were measured as a function of time. Secondly, the pitting potentials (Bp) of the alloy samples were measured using anodic polarisation techniques by extrapolation of the resulting log i vs E plots. The OCP and Bp of the AS samples were found to be influenced by the chemical composition of the aluminium alloys. Heat treatment of the AS samples was found to change their microstructure and solid solution composition which in turn affected the electrochemical results. The effect of the Composite melt layer on the electrochemistry of the CPC samples is discussed. Micrographic and electrochemical results were used to assess the best combination of finstock and tubestock material that would yield an aluminium radiator most resistant to corrosion. The likely corrosion of the components in these combinations was assessed and these results were compared with the actual results obtained industrially using the SWAAT exposure test. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.

Novel casting alloys that retain their mechanical strength at temperatures up to 300 °C are needed in order to improve automotive engine efficiency. The addition of Zr and V is a promising combination as a means of introducing the thermally stable cubic Al3ZrxV1-x nano-precipitates. In this project, the evolution of the Al3ZrxV1-x precipitates in multi-component cast aluminium alloys was characterised in a statistically significant way using small-angle X-ray scattering (SAXS). Unlike established metallurgical techniques, such as transmission electron microscopy (TEM), SAXS measures billions of precipitates in a single experiment. Two Al-6.8Si (wt.%) alloys were employed. One contained 0.30 wt.% Zr and 0.30 wt.% V and the other 0.37 wt.% Zr and 0.34 wt.% V. The alloys were cast and isothermally aged at 400 °C for times ranging from 3 to 50 h. Synchrotron SAXS was undertaken at the Diamond Light Source (DLS). Direct evaluation methods of SAXS spectrum were employed in order to extract information about the average precipitate size, precipitate size distribution and their evolution with ageing time. These results were compared with the results provided via TEM, both validating the methodology, and demonstrating the benefits and drawbacks of SAXS.From the SAXS analysis, the Guinier radius, Rg, was found to increase with ageing time from around 2 nm at 3 h to approximately 9 nm at 50 h. Precipitate volume fraction also increased from 0.11 vol.% to 0.24 vol.% for the lower solute alloy and from 0.14 vol.% to 0.33 vol.% for the higher solute one. In both alloys, the precipitate number density was observed to continuously decrease with ageing time. For the higher solute alloy, the average precipitate radius from the TEM measurements was found to be consistently smaller than Rg. Such a study is important because it extends the experimental SAXS methodology previously used only in simplified alloys to multi-component cast alloys of industrial importance. Moreover, the findings from this investigation suggest that both alloys quickly enter a stage where the precipitate growth and coarsening overlap, and the precipitate number density continuously decreases with time. Finally, this study underlines the importance of preliminary SAXS data treatment and provides an analytical guide in order to successfully characterize low volume fraction of precipitates in multi-component cast alloys.

The corrosion protection of aluminium alloys is of high importance, particularly in the aerospace industry. The most widely used technologies utilise Chromium(VI) compounds for conversion coatings and primer additives in paint systems to provide corrosion protection to these alloys. These compounds are highly toxic, carcinogenic and detrimental to the environment, therefore the identification of alternative systems that are safe and environmentally benign, that meet or exceed the current levels of corrosion protection is vital. This research program examines the corrosion inhibition effectiveness of selected rare earth diphenyl phosphates (RE(dpp)3). These compounds incorporate known inhibitor species, namely rare earth metals, organics and phosphates into a single complex, with the aim of achieving synergistic inhibition in corrosive environments. A screening study utilising immersion and weight loss experiments identified Cerium diphenyl phosphate (Ce(dpp)3) and Mischmetal diphenyl phosphate (Mm(dpp)3) as the most effective inhibitors of corrosion for AA2024-T3. The inhibiting efficiency, mechanism of inhibition and surface interaction of these complexes on aluminium alloy AA2024-T3 was characterised using a range of electrochemical and surface techniques. A similar study was carried out using AA7075-T6 to assess the adaptability of the RE(dpp)3 compounds to protect different alloy compositions. The complexes were effective in significantly reducing the corrosion rate of the alloys, with both the cathodic and anodic corrosion processes being suppressed. This mixed inhibition was not attained with the constituent rare earth and diphenyl phosphate ions individually, indicating the need for the complex to remain intact in solution to achieve the high level of corrosion protection observed. The initiation and propagation of surface pits was effectively suppressed by the RE(dpp)3 complexes. The combination of electrochemical and surface characterisation techniques has for the first time allowed insights into the mechanism of action of these compounds on aluminium alloys, and indicated deposition was initiated at electrochemically active intermetallic particles. The mixed rare earth phases present in Mm(dpp)3 produced a synergistic effect, providing a greater degree of corrosion protection compared with Ce(dpp)3, particularly on AA2024-T3. In the final phase of this research project the RE(dpp)3 inhibitor compounds were incorporated into an epoxy coating system, and demonstrated that the initiation of filiform corrosion on AA2024-T3 could be reduced by up to a factor of 3 by their addition. The growth rate of filaments was also impeded.

Multilayered aluminium alloy composites were produced by accumulative roll bonding (ARB) to very high strain to generate sheet materials consisting of either 32 or 64 alternating layers of Al and Al-0.3w.%Sc alloy. Based on the starting heat treatment condition of the Al(Sc) alloy and the roll bonding temperature, several different Al/Al(Sc) combinations were produced: (i) SSSS-ARB (Al(Sc) in the supersaturated condition; Tdef = 200 ???C; 32 layers); (ii) Aged-ARB (Al(Sc) in the artificially aged condition; Tdef = 200 ???C; 32 layers), and (iii) SSSS-ARB-HT (Al(Sc) in the SSSS condition; Tdef = 350 ???C; 64 layers). Regardless of the roll bonding conditions, Al(Sc) in the form of a dispersion of ultrafine Al3Sc particles strongly impedes structural changes during thermomechanical processing whereas Al readily undergoes extensive dynamic and static restoration. The major aim of the thesis is to understand the effect of initial microstructure and processing conditions on microstructural development in these multilayered Al/Al(Sc) composites. The microstructures were investigated mainly by backscatter electron (BSE) and ion channeling contrast (ICC) imaging in the DualBeam Platform and transmission electron microscopy (TEM) whereas the crystallographic nature of the microstructures were investigated by electron backscatter diffraction (EBSD) and the various diffraction techniques available in the TEM. The mechanical properties of the materials were investigated by hardness and tensile testing. The deformation microstructure and texture of these two alloy combinations were strongly influenced by both the initial heat treatment condition of the Al(Sc) alloy whereby large-scale shear bands are generated during rolling when a dispersion of fine Al3Sc particles is present in the Al(Sc) layers. The deformation mechanism of both SSSS-ARB and Aged-ARB was strongly controlled by the relative hardening behaviour of adjacent layers. In Aged-ARB, a higher magnitude of in-plane shear stress, exceeding the flow stress of Al(Sc), was operative at the interfaces between layers; this was shown to cause the shear banding in this material. All materials were annealed for up to 6h at 350 ??C. This extended annealing generated alternating layers of coarse grains (Al layers) and a recovered substructure (Al(Sc) layers) with the substantial waviness of the layers in both Aged-ARB and SSSS-ARB-HT being inherited from the as-deformed material. While the Al(Sc) layers remain unrecrystallized in all materials due to particle pinning effects, the Al layers underwent continuous and discontinuous recrystallization after low and high temperature roll bonding, respectively. Shear banding in Aged-ARB also resulted in a reduction in intensity of the rolling texture components and had a randomizing effect on the recrystallization texture of the Al layers. The Al/A(Sc) multilayered composites were found to conform to the classic inverse strength/ductility relationship and no significant improvement in ductility (for a given strength) was evident. The barriers to achieving an excellent combination of ductility and strength (i.e. toughness) in these materials were identified to be delamination of the layers, which can be largely reduced (or eliminated) by careful control of starting materials (heat treatment condition and thickness) as well as the processing parameters during ARB.

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