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Microstructure Characterization and Corrosion Properties of Two Recycled Aluminium Alloys AA5050 and AA6011
[Thesis]. Manchester, UK: The University of Manchester; 2017.
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Abstract
The influence of recycling on aluminium alloys and subsequent influence on the microstructure and corrosion performances have been investigated. The investigation was commenced by taking two block cast, recycled aluminium alloys (AA5050 and AA6011) and rolling them into 1mm gauge plate. In the case of AA6011, the plate was subjected to subsequent solution heat treatment and artificial aging steps, in order to attain certain temper specifications. To replicate the automotive paint bake industrial practice, a sample was subjected to a 2% tensile stretch followed by heat treatment for 30 minutes at 180˚C. Microstructural observations revealed Al-Fe-Mn-Si intermetallics to be the dominant secondary phase in both alloys. The size, distribution and composition of these were unaffected by artificial aging. Mg2Si was found in a coarse, localised form in both alloys also, albeit in much less amounts in AA5050. The presence of this phase was likely due to poor homogenisation during thermomechanical processing. HR-TEM of AA6011 revealed needle/rod shaped precipitates, aligning in the [001]Al lattice direction. This is consistent with β’’/β’ hardening precipitates consisting of magnesium and silicon. Circumstantial evidence was found for the copper-containing Q phase precipitate also. An additional, unidentified precipitate was observed, nucleating on the {111} habit plane of the aluminium matrix. The high iron content of AA6011 retarded the precipitation hardening response by capturing elements associated with hardening precipitates in the Al-Fe-Mn-Si intermetallics. Electrochemical corrosion experiments revealed the materials had a high susceptibility to localised corrosions, with the open circuit potential and breakdown potential possessing similar values. Atmospheric corrosion experiments showed that artificial aging had a large influence on the preferred corrosion mechanism. Non-heat treated samples showed susceptibility for pitting corrosion. This was particularly true for the –T4P temper, which showed large scale pitting. Heat treated samples saw an introduced susceptibility to intergranular corrosion. This was attributed to precipitation at grain boundaries, which would then form a microgalvanic couple with adjacent depleted zones. In the case of the –T8P temper, tensile stretching introduced defects into the sub-grain microstructure. This resulted in intergranular corrosion fronts of increased width, where grains with higher stored energy undergo preferential dissolution alongside the grain boundary attack. Overall, the detrimental effects of high iron content need to be overcome before AA5050 and AA6011 can be seriously considered for use in the automotive industry. However, the corrosion performance of AA6011–T8P is encouraging.