Multi-scale simulation of columnar-to-equiaxed transition during laser selective melting of rare earth magnesium alloy

Abstract

• A model for studying the columnar-to-equiaxed transition during selective laser melting of rare earth magnesium alloy is developed. • The liquid undercooled layer under the combined action of forced cooling, the temperature gradient and the liquid solute concentration gradient leads to columnar-to-equiaxed transition. • The effects of Zr solubility in magnesium matrix and preheating temperature on columnar-to-equiaxed transition are obtained. A model of coupling macro finite volume method (FVM) and cellular automata (CA) is proposed in this paper to explore the columnar-to-equiaxed transition (CET) during selective laser melting (SLM) of rare earth magnesium alloy. Taking into account the impact of recoil pressure and Marangoni convection on the molten pool temperature field, the grain structure is simulated. As suggested by the simulation results, with the undissolved Zr serving as heterogeneous nucleation sites, the liquid undercooled layer under the combined action of forced cooling, the temperature gradient and the liquid solute concentration gradient leads to CET. While considering the dissolution of Zr in magnesium matrix, the results demonstrate that the dissolution of element Zr is effective in significantly inhibiting the growth of columnar crystals and ensuring the sufficient constitutional supercooling (CS) required for nucleation. In addition, to raise the preheating temperature contributes to enhancing the outcome of nucleation and incresing the grain size. Invoking the interdependence model (IM), with the cooling rate gradually increasing in the SLM process of magnesium alloy, the nucleation-free zone (NFZ) reduces by decreasing the solute diffusion layer in the front of the solid/liquid (SL) interface and the temperature gradient. The reduction in temperature gradient can promote undercooling for nucleation and facilitate the development of equiaxed crystals. The simulation results are qualitatively verified as highly consistent through experimentation.