Understanding mechanisms of grain refinement of aluminium alloys by inoculation

For over half a century, grain refinement of aluminium alloys has been achieved by chemical inoculation; current grain-refinement practice involves the addition of master alloys (e.g. Al-Ti-B, Al-Ti-C) before casting, introducing inoculant particles to the melt. These particles act as nucleation points for alpha-Al grains, resulting in a uniformly fine, equiaxed as cast microstructure. Despite the ubiquity of this process, its underpinning science was not fully understood, hindering development of the area. From the 1950s onwards, the phase responsible for nucleation in alloys refined by Al-Ti-B was fiercely disputed. The debate focused so closely on this issue that other important factors were frequently ignored. During the 1990s, this debate was resolved through careful thermodynamic reasoning and novel experiments that derived their inspiration far from the foundry. This review focuses on subsequent experimental work and modelling: the expansion of the current understanding of grain refinement to include effects relating to the release of latent heat, the size distribution of inoculant particles and the alloying elements present in the melt (through poisoning and growth-restriction effects). The current contention regarding the nature of the interface between the nucleant phase (TiB2) and the melt is also discussed. These recent advancements have lead to improvements in grain refining practice resulting in savings in the foundry and development of alternative master alloys for aluminium systems. The ideas have also been successfully applied to other metallic systems, notably magnesium and zinc alloys.