Grain refinement of aluminium and its alloys by heterogeneous nucleation has become common industrial practice. The immense technological importance of this field has led to extensive investigations by many industrial and academic researchers during the past 50 years and extensive literature addressing several aspects of the grain refinement has been published.A great majority of the past investigations were primarily focused on the search for grain refiners that act quickly and provide the grain refining effects without fading even on prolonged holding of the molten alloy. Among a number of grain refiners developed, Al-5Ti-1B master alloys are more popular due to their high grain refining efficiency with respect to many aluminium alloys. Al-Ti-C master alloys are also becoming popular as grain refiners for many aluminium alloys, particularly when boron is not desirable in the aluminium alloy.Among a number of techniques that have been developed for the manufacture of Al-Ti-B master alloys, that involving the reaction of molten aluminium with the K_2TiF_6 and KBF_4 salts has become the most popular.The grain refining behaviour of the master alloy appears to be sensitive to its microstructure, particularly the morphology and size distribution of TiAl_3 particles, which are in turn influenced by the processing parameters used in the preparation of the master alloy, such as reaction temperature, reaction time, and thermomechanical treatment.The alloying elements as well as the impurities present in the aluminium alloys play significant roles in deciding their grain refining behaviour. While a large number of alloying elements and some impurities in the aluminium alloys enhance their grain refining capabilities, elements such as Cr, Si, Zr, Li lead to poisoning when aluminium is grain refined by the conventional addition of 0·01 percent Ti of the Al-5Ti-1B master alloy. Master alloys Al-B and boron rich Al-Ti-B appear to have better grain refining capability in the case of aluminium alloys containing poisoning elements. These alloys can, however, be grain refined successfully by higher addition levels of Al-5Ti-1B master alloys.Though the interaction of poisoning elements with the grain refining constituents of the grain refiner is thought to be the cause of poisoning, the exact mechanism of poisoning is not clear in a number of alloys. Similarly, the reasons for fading after long holding periods are also not clearly understood. A full understanding of the mechanisms of the poisoning and fading phenomena can help in the development of these new generation grain refiners.Much work has been focused n attempts to understand the mechanisms of grain refinement and several theories have been proposed. However, none of these theories can explain all the observations made in the grain refining experiments. Perhaps more than one mechanism operates depending on the grain refiner used, the alloy being cast, and the casting process involved. Further work is necessary in this direction to develop a unified description of grain refinement, poisoning, and fading.
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