Spontaneous nucleation in hypoeutectic Al–Cu system by controlled diffusion solidification process

Abstract

Controlled diffusion solidification (CDS) is a promising casting process. It involves the mixing of two precursor alloys that have different masses and temperatures. Small-sized globular grains appear in the microstructure formed in the process, where a grain refiner does not need to be added. Heterogeneous nucleation and spontaneous nucleation occur during the mixing step. The heterogeneous nucleation occurs at the crucible wall, while the spontaneous nucleation takes place in a supersaturated solution exposed to high undercooling and high cooling rates established during the mixing step. A one-dimensional model was built based on the computational fluid dynamics technique to numerically predict the redistribution of the temperature and solute in the model. The two-step nucleation theory was employed to theoretically provide a better understanding of the probability of copious spontaneous nucleation taking place in the supersaturated solution. The results indicate that the spontaneous nucleation strongly depends on the concentration of the alloys, thermal gradient between the two precursor alloys, the undercooling forming during the mixing, and the supersaturation parameter for the supersaturated solution. The microstructure of the CDS process was compared with that for the conventional casting process; the results show that the small size and non-dendritic microstructure formed with a low superheat for mixed alloys happens from copious nucleation taking place during the mixing step in the CDS process. The numerical results were validated with experimental observations and scanning electron microscopy.