Aluminium-based alloys are due to their diverse range of properties classified among the most important materials of modern times. A special place among light materials in industrial use is occupied by alloys of the 6xxx group, which, in addition to aluminium, also contain significant amounts of silicon and magnesium. It is a group of alloys of medium strength, good corrosion resistance and an excellent combination of mechanical properties, which are mostly used in extrusion. Alloys of group 6xxx are produced with continuous or by semi-continuous casting in rods. Due to the unequilibrium solidification process, which is a consequence of high casting speeds, alloys are subjected to inhomogeneous distribution of alloying elements, formation of undesired metastable phases and crystal segregation. Irregularities in the material negatively affect many properties. Homogenization processes are used to achieve an even distribution of added elements, melting of low-melting eutectics and crystal segregations.
In the frame of this work, we analyzed the influence of the cooling rate on solidification and the formation of the microstructure of the alloy EN AW 6008, which belongs to the group 6xxx. Alloy samples were obtained from three different parts of a cast rod slice, which were analyzed for the purpose of investigating homogeneity by DSC differential thermal analysis and energy dispersion spectrometry, which was used to analyze the chemical composition. Based on DSC curves and thermodynamic calculations, we predicted the melting / solidification process and predicted which equilibrium phases are present in the solidified microstructure and which phases are present in the microstructure considering the conditions of nonequilibrium solidification and the degree of diffusivity of the elements. The cast rod sample was also melted and poured into a croning measuring cell and into a steel cone mould to monitor the solidification of the investigated alloy at different cooling rates. The type, size, and distribution of microstructural components were analyzed using an optical microscope.
From the comparison of DSC curves, we found significant differences in the chemical composition between the individual sites of the cast slice of the rod and confirmed our findings with the results of chemical analysis. In the microstructure analysis, a decrease in primary grains with higher cooling rates was observed. The eutectic phases, however, became more rounded and diffuse as the cooling rate increased.
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