Floating crystals are unfragmented crystalline grains that form in the central region of EN AW-8011A aluminum alloy slabs. As they cannot be removed through homogenization annealing, their formation must be prevented during the casting process. The aim of this master's thesis was to investigate the formation of floating crystals, determine the conditions under which they develop, and propose an effective method for their elimination.
Simulations of semi-continuous casting revealed that extending the combobag—a filtering mash made of fibre glass—reduces the size of the dead zone in the mold, which may lower the likelihood of floating crystal formation. Semi-continuous casting of 1450×510 mm slabs was performed using both the basic (450×120×100 mm) and newly designed (525×120×100 mm) combobags. Melt sampling between the casting table and the holding furnace confirmed that floating crystals do not form in the casting trough.
Analysis of samples taken at 300 mm from the start and 100 mm from the end of the casting showed that floating crystals form during solidificaton in the liquid sump. Chemical analysis revealed that the slab center was enriched with aluminum and titanium, while the D/4 region contained higher concentrations of silicon and iron. The newly dimensioned combobag enabled a more homogeneous distribution of alloying elements. Microstructural examination confirmed the presence of floating crystals only in the center part of slabs casted with the original combobag. Spot analysis on the scanning electron microscope revealed that the crystals were rich in aluminum and titanium. Thermodynamic modeling and differential scanning calorimetry confirmed the formation of the microstructure and differences in melting and solidification enthalpies between samples with and without floating crystals.
The results demonstrate that reducing the dead zone by optimizing combobag geometry effectively prevents the formation of floating crystals during the casting of EN AW-8011A alloy slabs.
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