The present research was conducted on thin-walled castings with 5 mm wall thicknesses. This study addresses the effect of the influence of

different master alloys, namely: (1) Al-5%Ti-1%B, (2) Al-5%Ti and (3) Al-3%B, respectively on the structure and the degree of

undercooling (ΔTα = Tα-Tmin, where Tα - the equilibrium solidification temperature, Tmin - the minimum temperature at the beginning of

α(Al) solidification) of an Al-Cu alloy. The process of fading has been investigated at different times spent on the refinement treatment ie.

from 3, 20, 45 and 90 minutes respectively, from the dissolution of master alloys. A thermal analysis was performed (using a type-S

thermocouple) to determine cooling curves. The degree of undercooling and recalescence were determined from cooling and solidification

curves, whereas macrostructure characteristics were conducted based on a metallographic examination. The fading effect of the refinement

of the primary structure is accompanied by a significant change in the number (dimension) of primary grains, which is strongly correlated

to solidification parameters, determined by thermal analysis. In addition to that, the analysis of grain refinement stability has been shown

with relation to different grain refinements and initial titanium concentration in Al-Cu base alloy. Finally, it has been shown that the

refinement process of the primary structure is unstable and requires strict metallurgical control.