Corrosion Protection and Microstructure of Dissimilar Materials. A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy by Uyime, Donatus on the 30th of July, 2015.Investigations on the micro- and macro-galvanic corrosion mechanisms in un-coupled AA2024-T3 alloys, AA2024-T3 coupled with mild steel (with and without the influence of cadmium and under varying solution temperatures), dissimilar friction stir welds of AA5083-O and AA6082-T6 alloys and a friction stir welded AA7018 alloy have been carried out. Selected methods of preventing and / or minimising the investigated corrosion phenomena were also investigated. The investigation of the corrosion behaviour of the uncoupled AA2024-T3 alloy revealed that there are two distinct stages of polarization during the galvanostatic polarization of AA2024T3 alloy in de-aerated 3.5% NaCl solution. From the first stage, the relationships between the pitting incubation time, pitting potential and applied current density for AA2024T3 alloy in the de-aerated condition were established. Whilst studying the in situ corrosion phenomena on the uncoupled AA2024-T3 alloy using the scanning vibrating electrode technique (SVET),three distinct stages in the variation of the recorded current density values with time were revealed. Attempts were made to correlate these stages with the corrosion behaviour of the alloy. The study of the galvanic interactions between AA2024-T3 and mild steel revealed that AA2024-T3 is anodic to mild steel at room temperature, but polarity reversal of the couple starts (from a temperature as low as 35 oC upwards) when the couple is introduced into the solution above ambient temperature. Importantly, AA2024-T3 is clearly cathodic to mild steel at 60 oC, although with very low measured galvanic current values. Cadmium coating (at ambient temperature) on the mild steel reduced the galvanic corrosion of the couple by as much as 20 µA/cm2 because of the formation of a CdO/Cd(OH)2 layer on mild steel. In the study of the dissimilar friction stir welds of AA5083-O and AA6082-T6 alloys, it was observed that material flows (pushes but does not mix) more from the advancing side into the retreating side and that the mixture of materials is far from complete. Two welding speeds were compared; the welding speeds have no clear influence on the microhardness, but affected the mixing proportions in the flow arm and in the nugget stem. The faster welding speed resulted in increased susceptibility to corrosion because of the reduced tool rotation per weld length for heat generation and the mixing of materials. The heat affected zones of both alloys and the transition regions between the AA5083-O alloy and the AA6082-T6 alloy rich zones have been identified to be the regions that are most susceptible to corrosion. Anodizing the weld in order to minimise corrosion showed that the AA5083-O alloy rich zones materials, in the weld, oxidizes more during anodizing compared with the AA6082-T6 alloy rich zones. Sputtering deposition prior to anodizing, promotes the formation of a uniform oxide film across the entire weld zones and prevents the boundary dissolution that occurs when the dissimilar weld of AA5083-O and AA6082-T6 alloys is anodized in 4 M H2SO4 solution at 15 V at ambient temperature. The investigation of the corrosion susceptible regions in friction stir welded AA7018 alloy, which was based on the use of ISO 11846 immersion test and the potentiodynamic polarization technique in naturally aerated 3.5 % NaCl solution, revealed intergranular, crystallographic and second phase particle influenced mode of attack. The heat affected zone was found to be the most susceptible to corrosion.

abstract: AA 7XXX alloys are used extensively in aircraft and naval structures due to their excellent strength to weight ratio. These alloys are often exposed to harsh corrosive environments and mechanical stresses that can compromise their reliability in service. They are also coupled with fasteners that are composed of different materials such as Titanium alloys. Such dissimilar metal contact facilitates galvanic and crevice corrosion, which can further reduce their lifetimes. Despite decades of research in the area, the confluence of mechanical, microstructural, and electrochemical aspects of damage is still unclear. Traditionally, 2D and destructive methods have often been employed to study the corrosion and cracking behavior in these systems which can be severely limiting and lead to inaccurate conclusions. This dissertation is aimed at comprehensively studying the corrosion and cracking behavior of these systems using time-dependent 3D microstructural characterization, as well as correlative microscopy. The microstructural evolution of corrosion in AA 7075 was studied using a combination of potentiodynamic polarization, X-ray Computed Tomography (XCT) and Transmission X-ray Microscopy (TXM). In both experiments, a strong emphasis was placed on studying localized corrosion attack at constituent particles and intergranular corrosion. With an understanding of the alloy’s corrosion behavior, a dissimilar alloy couple comprising AA 7075 / Ti-6Al-4V was then investigated. Ex situ and in situ x-ray microtomography was used extensively to investigate the evolution of pitting corrosion and corrosion fatigue in AA 7075 plates fastened separately with Ti-6Al-4V screws and rivets. The 4D tomography combined with the extensive fractography yielded valuable information pertaining the preferred sites of pit initiation, crack initiation and growth in these complex geometries. The use of correlative microscopy-based methodologies yielded multimodal characterization results that provided a unique and seminal insight on corrosion mechanisms in these materials. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Combinations of different materials are increasingly used in the modern engineering structures. The driving forces of this trend are rising fuel costs, global warming, customer demands and strict emission standards. Engineers and industries are forced to improve fuel economy and cut emissions by introducing newly design engines and lightweighting of structural components. The use of lightweight materials in the structures has proved successful to solve these problems in many industries especially automobile and aerospace. However, industry still lacks knowledge how to manufacture components from polymeric materials in combination with metals where significant differences exist in properties. This thesis aims to demonstrate and generate the methodology and guidelines for hybrid joining of aluminium alloys to thermoplastics using friction stir welding. The developed technique was identified, optimized and evaluated from experimental data, metallography and mechanical characterization. The success of the technique is assessed by benchmarking with recent literatures. In this work, lap joints between aluminium alloys (AA5754, AA6111) and thermoplastics (PP, PPS) were produced by the friction stir welding technique. The specimens were joined with the friction stir welding tools under as-received conditions. Metallic chips were generated and merged with the molten thermoplastic to form a joint under the influence of the rotating and translating tool. The effects of process parameters such as rotational speed, translational speed and distance to backing were analyzed and discussed. The investigation found joint strength was dominated by mechanical interlocking between the stir zone and the aluminium sheet. The results also show that the joint strength is of the same order of magnitude as for other alternative joining techniques in the literature. / <p>QC 20150908</p>

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No / Successful joining of dissimilar metals such as Al and Mg can provide significant advantages to the automotive industry in the fabrication of vehicle bodies and other important components. This study explores dissimilar joining of Al–Mg using a resistance spot welding process to produce microstructurally sound lap joints and evaluates the impact of interlayer composition on microstructural evolution and the formation of intermetallic compounds within the weld nugget. The results indicated that mechanically sound joints can be produced, with fine equiaxed and columnar dendrites within the weld nugget. The presence of intermetallic compounds was also confirmed by the variation in the microhardness values recorded across the weld zone.

To address the concerns of simultaneously improving crash performance and fuel efficiency in the automotive industry, multi-material car bodies are becoming increasingly popular. Aluminium and steel are two materials whose properties complement each other well for this application. However, ultra-high strength steel (UHSS) and aluminium alloy is a hard-to-join material combination. In the last decades different solutions to this problem have been proposed, one of these being friction element welding (FEW). In this work, different UHSS have been joined to different aluminium alloys through the FEW equipment EJOWELD developed by EJOT. The joints have been evaluated though visual inspection, cross-section analysis, and mechanical tests. All materials could successfully be joined by the EJOWELD process.

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