Abstract
The research was conducted for castings with 3-25-mm wall thicknesses and using AlTi5B1 master alloy as well as AlTi75 additions. A thermal analysis was performed to determine the real cooling rate and the degree of undercooling (ΔT α = T α − T min, where T α—the equilibrium solidification temperature, T min—the minimum temperature at the beginning of α(Al) solidification) of an Al-Cu alloy. It was shown that the Ti-based grain-refining process does not influence the cooling rate. Metallographic examinations were carried out to determine the primary grains (the number of primary α(Al) grains per unit volume). It was found that the grain count can be properly described by the exponential function of the undercooling for castings processed under various cooling and metallurgical conditions. Combining the cooling rate and the efficient grain refiner increased the tensile ductility by up to 150%.
Highlights
Primary grain count is one of the most important factors for determining the quality of castings
The recorded cooling rate varies within a broad range (23.7-1.2 °CsÀ1) when the wall thickness changes from 3 to 25 mm for all alloys
The cooling rate varies within a wide range (23.7-1.2 °CsÀ1) when the wall thickness is changed from 3 to 25 mm, and this is accompanied by a significant variation in the maximum undercooling
Summary
Primary grain count is one of the most important factors for determining the quality of castings. The final number of primary a(Al) grains in the casting depends on the process of nucleation at the beginning of solidification because each nucleus gives rise to one single grain. It represents the nucleation potential which depends on the following factors: the type and quantity of the grain refiner addition, the time and liquid metal temperature, slag and furnace atmosphere, molten metal mixing, or the chemical composition of the alloy (Ref 1-4). The cooling rate represents the thermal conditions (of heat exchange) at the beginning of solidification which in turn determine the final number of a(Al) phase grains for the given nucleation potential. The benefits of grain refinement are improved fluidity, improved feeding, better distribution of porosity, and secondary phases, and it
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