Baykov Metallurgy and Materials Institute of the Russian Academy of Sciences, Moscow, Russia:

S. G. Bochvar, Senior Researcher, e-mail: sgbochvar@yandex.ru

It is a well-known fact that the quality of ingots and the resultant properties of semi-finished and final products are, to a large extent, governed by how fine the structure of the ingots is. It is for this reason that a lot of attention is given to identifying the inoculant and the inoculation process that would work best as microaddition to aluminium alloys cast in ingots. Various compositions of the Al – Ti – B alloy have recently been used widely as inoculants. However, when casting zirconium containing alloys, a “poisoning” effect occurs, i. e. when titanium diborides become inert as nucleation centres and drop out of the doping process. That is why in the case of high strength alloys of the Al – Zn – Mg – Cu – Zr system it is important to find a similar addition alloy that could replace the Al – Ti – B alloy while producing a strong doping effect. The author managed to obtain a non-dendritic structure with no Zr compounds exceeding 1 micron separated during crystallization of the Al – Zn – Mg – Cu – Zr alloy. Liquid metal in ingots of 40 mm in diameter were exposed to ultrasound and a rod was used with the microaddition of 0.02% Zr (Zr concentration in the burden was 0.12%). The microaddition of 0.01%Zr (Zr concentration in the burden was 0.14%) was used when two ultrasonic sources were in action. The paper shows a difference in the eutectic morphology in the structures of high strength alloys of the Al – Zn – Mg – Cu – Zr system in the case of dendritic versus non-dendritic crystallization, which can be explained by differing eutectic crystallization sequences of the casting techniques compared. Thus, during crystallization of non-dendritic grains the eutectic precipitation occurs immediately and the precipitates, which have a continuous plate-like shape, form between the grains, whereas during crystallization of dendritic grains the eutectic precipitates form between the dendrite branches and have a spherical shape. There is a constant amount of eutectic in both cases. So a greater border area would equate to a finer eutectic, which is characteristic of non-dendritic crystallization. This suggests a possibility of using a shorter homogenization time in the case of nondendritic crystallization.

This research was carried out under the Governmental Assignment No. 007-00129-18-00.

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