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DSC examinations of the Al-20 wt. % Zn sand-cast alloy inoculated with Ti-containing grain-refiners

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Master alloys (MAs) are added to the melt during casting in order to achieve as fine and uniform equiaxed grain structures as possible upon solidification. Their action is enhanced by the presence of higher amounts of solute in the base alloy [1]. The main concern for grain refinement is to avoid the formation of columnar or mixed columnar and equiaxed grains in the cross-section of the cast ingot. The sole restriction on the grain refinement ability of the master alloys is imposed by the ingot dimensions and the cooling rate; a large ingot size and a low cooling rate lead to larger differences in grain morphology and size between the surface and centre of the ingot [2]. There are several benefits for a finer cast structure after solidification. These are, namely, fewer casting defects, easier processing in the subsequent fabrication stages, higher ductility of the ingot surface shell, which reduces surface cracking susceptibility, and higher yield stress of the final wrought products [2]. However, the grain refinement process is affected by several parameters, such as the quantity of the master alloy added in the melt, the distribution characteristics of the nucleating particles in the master alloy and in the melt, the exact compositions of the master alloy and of the base alloy, the casting speed and temperature, the ingot dimensions and the cooling rate [1]. Regarding the four master alloys of the present work, the (Al, Zn)3Ti has not been studied before, while the ZnTi4 alloy was reported to yield a larger grain size compared to the typical AlTiB and AlTiC additions [3, 4]. The latter two MAs constitute the most common refiner additions, with the AlTiB-type reported to be superior in terms of grain refinement potential compared to AlTiC-type [2÷5]. This is said to occur due to the lower number of nuclei in the latter MA-type [2]. Li et al. [6] reported that increasing the amount of Ti and C beyond the standard contents in [...]

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