Numerical Simulation Of Grain Refinement Of Cast Mg-4Y-0.5Zr Alloy

Under the urgent requirements of environmental protection and energy conservation and emission reduction,the application and development of ultra light and high-strength magnesium alloys have attracted extensive attention.Casting is the main forming method of magnesium alloys.However,due to the hexagonal closepacked crystal structure of magnesium alloys,and the wide range of crystallization temperature,low thermal conductivity,large volume shrinkage,magnesium alloys will have coarse grains,uneven distribution of the second phase,shrinkage porosity and other defects.The grain refinement is helpful to improve the properties of magnesium alloys,so grain refinement is the key to improve the mechanical properties of magnesium alloys.A two-dimensional cellular automata(CA)model is adopted,based on the sharp interface theory.Selected Mg-4Y-0.5(0)Zr alloy as the research object,the effects of cooling rate,nucleation density,initial alloy composition and other factors on grain refinement were studied.First of all,by solving the LGK analytical model,the variation of dendritic tip growth rate with undercooling under the conditions of adding Zr and not adding Zr is obtained.The results show that the growth rate of the dendritic tip after adding 0.5Zr is about one sixth of that without Zr,and the growth inhibition factor of the dendritic tip after adding Zr is determined.Single α-Mg dendrite growth is simulated,and the results of simulation are compared with Scheil model.It is concluded that the two-dimensional CA model adopted in this paper can be used to predict the dendritic growth and microstructure formation of Mg-4Y-0.5(0)Zr alloy during solidification.The grain refinement of Mg-4Y-0Zr and Mg-4Y-0.5Zr alloys during solidification was studied by experiment and numerical simulation.The experimental characterization shows that the addition of Zr and high cooling rate resulted in a significant grain refinement.The cooling rate and temperature gradient obtained from the experiment are taken as input conditions and substituted into the two-dimensional cellular automata model.The results of numerical simulation show that high cooling rate has an important effect on grain refinement.With the increase of cooling rate,the grain structure will be refined with or without Zr,because high cooling rate improves the effective undercooling.However,with the increase of nucleation density,solute enrichment inhibits the formation of nucleation,the effect of cooling rate on the component undercooling becomes smaller.For Mg-4Y-0Zr alloy,with the increase of initial nucleation density,the solute suppressed nucleation effect(SSN)is also amplified.Because the solute diffusion layer overlaps rapidly,most of nucleus are suppressed,and the grain refinement cannot achieve the desired effect.For Mg-4Y-0.5Zr,the grain refinement mechanism of adding Zr is to inhibit the dendrite growth and slow down the latent heat release rate.Without too much interference from the temperature of neighboring dendrites,more nucleus can obtain enough undercooling,nucleate and become grains.At the same time,at high cooling rate,Zr element will promote multi-step nucleation and significantly refine grains,which cannot be achieved at low cooling rate or without adding Zr.In order to predict the dendritic morphology more intuitively,a three-dimensional cellular automaton model is adopted in this paper.This model is applicable to magnesium alloys with hcp crystal structure,and is firstly proposed the anisotropy function for calculating the curvature of α-Mg dendrite solid-liquid interface,and the surface energy anisotropy coefficient in the function is adjusted to reproduce α-Mg dendrite morphology.By introducing a new isotropic finite difference method,the anisotropy brought by discrete calculation is overcome,the prediction results of the model are more stable,and the dendritic morphology can be reproduced better.