Deformation and failure mechanisms of a modified commercial aluminum alloy at elevated temperatures

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Green, William Paul

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Superplastic aluminum alloys have been the subject of recent research in the automotive industry for several reasons. First, the formability of such alloys under superplastic forming (SPF) conditions is quite good; second, the potential weight reduction associated with formed aluminum sheet as compared to traditional steel sheet is particularly important for automotive applications. Unfortunately, SPF-grade aluminum is relatively expensive, and the typical SPF process is expensive and time-consuming because of the high temperatures and slow strain rates involved. The present study investigates the potential for forming AA5083, the most common commercial alloy for SPF operations, at reduced temperatures and increased strain-rates. The deformation and failure mechanisms under these conditions are examined in a modified AA5083 material and are compared with data from unmodified AA5083. A new method of presenting creep transient data is presented and is used to determine deformation mechanisms at elevated temperatures. A modified AA5083 alloy produced by continuous casting (CC), and containing a small addition of Cu, was studied. This small addition of Cu causes the formation of Mg2Cu and MgCu2 intermetallic particles, which produce incipient melting. It is proposed that these low-melting-temperature phases may reside at grain boundaries and enhance grain-boundary sliding at low temperatures, which may explain the improved ductility of the Cu-containing alloy. Ductility variations for the Cu-containing alloy are also explored. The differences in tensile ductility between the Cu-containing AA5083 and unmodified AA5083 materials, produced by both CC and direct-chill (DC) casting, are related to differences in cavitation behavior. The final part of the present work is a study on the effect of various heat treatment schedules on the constituent particle distribution in AA5083. It is shown that a simple one-day aging treatment at a moderate temperature can effectively reduce the interparticle spacing. This may lead to a finer recrystallized grain size [1] and, thus, improve superplastic properties.


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