Grain Refinement of Al-Si Alloys – Scientific and Industrial Aspects

Abstract

Casting of aluminum alloys is a common industrial practice and widely used in automotive industry in the entire world. This practice is used to promote the formation of a fine, uniform and –equiaxed grain structure. To understand the mechanism of grain refinement is very important since a refined microstructure lead to improve uniform mechanical properties and reduced hot tearing tendency of the alloy by improving feeding ability of liquid metal. Sufficient amount of liquid fraction at last stage of solidification ensure the less shrinkage porosity, and finer distribution of second phases. Now a day in casting industry, finer grain structure is achieved by addition of grain refiner in the melt. The most commonly used inoculants are Al-Ti, Al–Ti–B, and Al-Ti-C master alloys. Usually Al–5Ti–1B, is added to molten aluminum at a typical level of .01 to .05 by weight of titanium. After addition of grain refiner, the number of active nucleation sites during solidification start to dispersed in the melt and nucleate solid particles. There are two mechanism of grain refinement; (i) nucleant effect – it promotes suitable sites for heterogeneous nucleation of grains; and, - (ii) solute effect – it restricts the growth of grains through segregation of solute elements. It has been seen that TiAl₃ and TiB₂ particles are the maximum common heterogeneous nucleation particles for aluminium crystals. The required driving force for grain refinement during solidification is provided by cooling rates in the form of constitutional undercooling. Many researcher has given a great contribute to understand the mechanism of grain refinement in melt. However, the exact mechanisms by which grain refinement occurs using the grain refiner have not been fully understood yet. Experimental and theoretical studies on the mechanisms have mainly focused on (a) the role of the titanium solute causing a growth restriction effect, (b) the thermodynamics of Al–Ti-based alloy systems, and (c) the heterogeneous nuclei of  TiAl₃ and TiB₂ particles. In the present paper a review is given of the studies of grain refinement taking into consideration both the industrial practice and scientific aspects of grain refinement. The paper will be useful in understanding the performance of grain refiners for the scientific and industrial community.