Grain count in castings: theoretical background and experimental verification

In this work, a theoretical model is proposed for heterogeneous nucleation on substrates whose size distributions can be described by the Weibull statistics. In particular, the proposed model suggests that the size distribution for the various nucleation sites is exponential in nature. Measurements of grain count were carried out on experimental Al–1·3Si and Al–5·0Cu single phase alloys inoculated using an Al–5Ti–1B master alloy. In addition, experimental nodular and flake graphite iron castings were processed under various metallurgical conditions. In single phase alloys, the area of the equiaxed dendritic grain count was estimated from the electron backscattering diffraction analysis, whereas the graphite nodule and graphite eutectic cell count were estimated on polished cast iron surface sections by stereological means. In addition, maximum undercoolings were determined by thermal analysis. The experimental outcome indicates that the grain count can be properly described by a proposed exponential function of the maximum undercooling at the onset of alloy solidification. Finally, the magnitudes of the nucleation parameters were experimentally determined in this work.