Experiments and LBM Simulations of Grain Refining in Al-Zn Alloys by Solid–Liquid Mixing

Grain refinement in Al-Zn alloys treated by solid–liquid mixing (SLM) process was investigated experimentally and using numerical simulations. In the present SLM, a solid alloy block (SAB) was used to cool the molten alloy internally, and mechanical rotation was employed to enhance the mixing. A numerical model based on the lattice Boltzmann method was developed to describe the SLM. The obtained results show that SLM could effectively refine the solidified microstructure. The introduction of concentration fluctuations or the addition of refiners in the treated molten alloy resulted in further grain refinement. SLM consisted of three stages, the formation of a solidified shell, the remelting of the solidified shell, and the melting of the SAB. The cold rotating SAB acted as a large heat sink that absorbed heat, and it initiated a large thermal undercooling region and created a uniform temperature field throughout the molten alloy. At least two mechanisms are responsible for the grain refinement in the SLM process. The most important is a wall mechanism, where the crystals nucleate on or near the SAB surface due to a large thermal undercooling and are then detached and fragmented by mixing-induced convection.