Effect of Casting Conditions and Composition on Microstructural Gradients in Roll Cast Aluminum Alloys

Abstract

Twin Roll Casting (TRC) is a process by which thin strip is produced directly from molten metal. This process has many advantages over traditional Direct Chill (DC) casting where metal is cast into large slabs or ingots and subsequently hot and cold rolled into strips. While the DC casting involves large capital investment and energy consumption, TRC produces strip with thickness in the range of 10 mm to 2.5 mm in one operation without hot rolling, with less cost, energy usage and equipment requirements. In TRC AA3105, high solidification and post-solidification cooling rates lead to high manganese supersaturation throughout the as-cast strip and very fine constituent particles near strip surface. After 90% cold rolling, the TRC material shows sluggish recrystallization kinetics and a very coarse grain structure, due to concurrent forming dispersoids and/or fine constituent particles. These particles interfere with the nucleation process by preventing migration of the boundaries associated with potential nuclei, resulting in fewer recrystallization nuclei and slower nucleation rate. In addition, the fine particles can impede grain growth by exerting a pinning pressure on moving boundaries. The current study investigated the microstructure evolution in TRC AA3105 during different thermo-mechanical treatments. A systematic study of the effects of different homogenization treatments on second phase particles was carried out. The heating rate effect on recrystallization was studies using an infrared heating furnace. The effect of second phase particles on recrystallization behavior was summarized and experimental results were presented. It was shown that a desirable, fine recrystallized grain size could be obtained by either of two means- rapid heating rate ot the annealing tempeature, or by a pre-cold work homogenization heat treatment.