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Corrosion Behavior of 2024 Aluminum Alloys Structure produced by Wire Arc Additive Manufacture
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
Wire arc additive manufacture (WAAM) is a novel manufacturing method for producing Aluminium alloy structures. Processing of high strength Aluminium Alloy is very difficult owing to their good thermal conductivity, reflectivity and poor weldability. In this work, a detailed investigation on microstructural modification and corrosion behaviour of the WAAM produced AA2024 alloy structure, which is supplied by The University of Cranfield, is conducted. The grain structure of the WAAM produced AA2024 was analysed by etching and microscopy, revealing a unique layered structure within WAAM structure. Later, energy dispersive X-ray (EDX) analysis was conducted on the layered structure to determine the distribution of second phases and the associated elemental segregation. The corrosion behaviour of the WAAM structure was assessed by immersion testing and anodic polarization in 3.5% NaCl solution. Laterally, characterisation of the surface topographical feature and the internal microstructure of the original structure and the corroded structure were conducted by scanning electron microscopy (SEM) to determine the key microstructural features responsible for corrosion initiation and propagation and to understand the corrosion mechanism in WAAM structure. It is found that the unique banding structure of the WAAM produced AA2024 alloy structure is separated as heat affected zone (HAZ), deposited zone, columnar dendrite zone and equiaxed dendrite zone, characterized by grain size, the shape of grain, and dendrite distribution in each zone. By comparing the change of surface topography and the second phase morphology with different corrosion testing time, it was found that the heat affected zone (HAZ) has the highest localized corrosion susceptibility compared to other zones in the WAAM 2024 alloy. Localized corrosion in the WAAM 2024 alloy structure develops in the form of both pitting and intergranular attack. The high intergranular corrosion susceptibility of WAAM 2024 alloy structure is associated with the S-phase and θ-phase interdendritic intermetallics.