Metal casting is one of the oldest metal shaping processes and it used to be an art that was kept as a secret among families. Events in the second half of past century elevated the casting process from art to science. In order to remain competitive in today's market, modern foundries must take full advantage of all of the opportunities that modern technology has to offer. It enables a more detailed and a faster analysis of design problems, thus enabling better decision-making to improve the design and quality itself. Therefore, costs and risks are minimized. In this thesis, the influence of modifications of the eutectic of AlSi12 alloy by the addition of sodium was investigated as well as the influence of the cooling rate of the casting on the mechanical and metallurgical properties. In the second part of the thesis, an experiment in the Ljevaonica Bujan d.o.o. was conducted. The influence of modifications of eutectic AlSi12 alloys by the addition of sodium was investigated and the weakening of sodium action with time was observed. Melting samples from the furnace were taken at intervals of approximately ten minutes. Thermal analysis of each melt was performed, followed by an analysis of chemical composition, microstructure and and a static tensile strenght experiment. According to the obtained results, it was concluded that the influence of sodium disappeared 23 minutes after the melt treatment, and accordingly the disappearance of fibrous silicon and the reappearance of needle silicon in the microstructure of eutectic can be seen.