Effect of Agglomeration on Nucleation Potency of Inoculant Particles in the Al-Nb-B Master Alloy: Modeling and Experiments

It is a practically significant issue to overcome the low particle efficiency of the Al-Nb-B master alloy for the grain refinement of hypoeutectic Al-Si casting alloys. The relationship between the microstructure and the grain refining potency of the Al-Nb-B master alloy is investigated. Agglomeration of the grain refiner as an essential factor representing the dispersion degree of inoculant particles is quantitatively analyzed. Experimental results show that the decreased mean size and increased number density of agglomerations improve refining potency and antifading performance of as-extruded Al-Nb-B master alloys. Meanwhile, a modified free growth model that includes the effect of agglomeration on nucleation potency of inoculants has been developed. With the role of agglomeration and the subsequent gravitational sedimentation, the critical diameter and the number density of inoculants during various holding times are changed. The quantitative agreement in grain size between the modeling results and experimental results, combined with the predicted maximum undercooling and grain density of inoculants, confirm the influence of the extent of agglomeration on the grain refining potency and particle efficiency of the master alloy. Furthermore, an optimum range of particle-to-agglomeration ratio (PAR) can serve as a guide for the development of the grain refiner.