Grain Refinement of Al-Si Hypoeutectic Alloys by Al3Ti1B Master Alloy and Ultrasonic Treatment

Abstract

The concept of constitutional supercooling (CS) including the term itself was first described and discussed qualitatively by in order to understand the formation of cellular structures during the solidification of tin, and then quantified by Tiller, Jackson, Rutter, and Chalmers (1953). On that basis, Winegard and Chalmers (1954) further considered 'supercooling and dendritic freezing of alloys' where they described how CS promotes the heterogeneous nucleation of new crystals and the formation of an equiaxed zone. Since then the importance of CS in promoting the formation of equiaxed microstructures in both grain refined and unrefined alloys has been clearly revealed and quantified. This paper describes our current understanding of the role of CS in promoting nucleation and grain formation. It also highlights that CS, on the one hand, develops a nucleation-free zone surrounding each nucleated and growing grain and, on the other hand, protects this grain from readily remelting when temperature fluctuations occur due to convection. Further, due to the importance of the diffusion field that generates CS recent analytical models are evaluated and compared with a numerical model. A comprehensive description of the mechanisms affecting nucleation and grain formation and the prediction of grain size is presented with reference to the influence of the casting conditions applied during the practical casting of an alloy.

Identifying the Stages during Ultrasonic Processing that Reduce the Grain Size of Aluminum with Added Al3Ti1B Master Alloy

“…TiB2 particles by producing a flat temperature gradient in the melt [14,21] so that the nucleationfree zone that forms around each grain [22] is effectively eliminated. Thus, the nucleated grains are able to survive remelting in the uniformly undercooled melt while being transported throughout the melt.…”

“…Thus, the nucleated grains are able to survive remelting in the uniformly undercooled melt while being transported throughout the melt. [6,22] In addition, cavitation-induced dendrite fragmentation can occur. [11,23,24] It is also possible that grains are generated by a continuous process of nucleation, growth and detachment of crystals from the radiating surface of the sonotrode.…”