Studies on the mechanisms of heterogeneous nucleation of grains and pores in aluminum castings

In the present study, a fundamental theoretical and experimental investigation has been carried out on the mechanisms of heterogeneous nucleation of grains and pores in aluminum castings. A direct addition technique has been developed to introduce known types and quantities of inoculants into liquid aluminum alloys, irrespective of their wettability and chemical reactivity while preserving the surface characteristics and melt chemistry. Many different types of inoculants such as: ;The commercial grain refining practice of Al and its alloys has been experimentally simulated by introducing synthetic TiB;Particles which do not nucleate the solid phase and/or do not get engulfed by the growing solid, are continuously rejected by the solid/liquid (S/L) interface until the end of local solidification. These substrates act as a barrier to the fluid flow as well as to the diffusion field at the S/L interface, giving rise to enhanced gas segregation and viscous pressure drop. A novel theoretical mechanism for the heterogeneous nucleation of pores has been proposed, based on this behaviour of foreign particles at the advancing S/L interface. Mathematical analyses have been employed to predict the gas segregation and pressure drop in the gap between the particle and the S/L interface. An order of magnitude analysis is done, and it is shown that pressures in the range of the activation barrier can be obtained in normal castings. To substantiate the mechanism further, experimental studies were carried out by introducing various possible inclusions into liquid aluminum. The experimental findings are in line with the theoretical predictions.