Settling behaviour of different grain refiners in aluminium

Grain refiner is added to the melt in order to introduce particles which are effective nucleant substrates for the primary aluminium phase. The most widely used grain refiners are based on the Al-Ti-B system. Depending on the composition of the master alloy, TiB2, (Al,Ti)B2 and Al3Ti particles can be introduced. Ideally, the nucleant particles should be homogeneously distributed throughout the melt to ensure a large number of nucleation events and a uniform microstructure. However, a difference in density exists between the grain refining particles (TiB2 density ∼4.5 g/cm3) and the melt (pure aluminium density ∼2.3 g/cm3) and therefore the particles sink, making it difficult to maintain a homogeneous distribution of grain refining particles. Particle characteristics (size and density) strongly influence the settling behaviour when the master alloy is diluted in the melt. This paper reports on the behaviour of three grain refiner master alloys containing particles of different size distribution and composition. In all three cases, it was found that the particles settled quite quickly and agglomeration of the refining particles also played an important role in the rate of settling.