In the course of homogenizing annealing of aluminium alloys being cast continually or semi-continually it appears that chemical inhomogenity takes off within separate dendritic cells (crystal segregation). It is about a diffusion process that takes place at the temperature which approaches the liquid temperature of the material. In that process the transition of soluble intermetallic compounds and eutectic to solid solution occurs and it suppresses crystal segregation significantly . The temperature, homogenization time, the size of dendritic cells and diffusion length influences homogenizing process. The article explores the optimization of homogenizing process in terms of its time and homogenizing annealing temperature which influence mechanical properties of AlZn5,5Mg2,5Cu1,5 alloy.
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.