Numerical Investigation Of Dendrite Structure Refinement Of Cast Mg-4Y-3Nd-1.2Al Alloy

Adding rare earth elements to magnesium alloys can improve strength,heat resistance and creep resistance.As one of the main methods of alloy strengthening,grain refinement can improve the strength and plasticity of materials.Adding Al element to refine Mg-RE alloy has been a research hotspot in recent years.In this paper,Mg-4Y-3Nd-1.2Al alloy is selected,and the refinement process is studied by numerical simulation method.Based on the sharp interface theory,a two-dimensional cellular automaton model(CA)is developed.The model is used to simulate?-Mg dendrites with close-packed hexagonal structure.The shape of the cell mesh is square for simulation calculation.Compared with the hexagonal mesh,it can reproduce the growth of?-Mg dendrites at any angle.First,for the?-Mg dendrites precipitated during the solidification of the Mg-4Y-3Nd-1.2Al alloy,the effects of changes in the degree of undercooling,mesh size,Gibbs-Thomson coefficient and surface energy anisotropy coefficient on the growth results are investigated.The simulation results show that as the degree of subcooling increases,the growth rate of dendrites increases significantly and the branching becomes more obvious.The simulation results are consistent with the theory of dendrite growth.The mesh size and Gibbs-Thomson coefficient have less influence on the final growth result of dendrite,while the selection of the surface energy anisotropy coefficient has a greater influence on the growth result.The larger the surface energy anisotropy coefficient,the faster the dendrite growth rate.Improper selection of the surface energy anisotropy coefficient will seriously deviate from the sixfold symmetry of magnesium dendrite growth.Using the developed CA model,the refinement process of sand casting Mg-4Y-3Nd-1.2Al alloy is further simulated.Aiming at the feature that the Al₂RE particles generated by the in-situ reaction are used as effective heterogeneous nucleation particles,the grain structure refinement mechanism of the alloy is simulated.During the simulation,the macroscopic temperature field is calculated,and input the obtained cooling rate and temperature gradient into the microscopic CA structure simulation program.The effects of cooling rate,whether Al₂RE particle size changes with cooling rate,nucleation density on microstructure refinement,and the characteristics of grain refinement when the migration of Al₂RE particles are considered at different cooling rates.The simulation results show that when the Al₂RE particle size does not change with the cooling rate,increasing the cooling rate can refine the structure.The particle size of Al₂RE decreases with the increase of cooling rate.Compared with the slow cooling rate(1.8?s^-1),the grain structure becomes coarser at the fast cooling rate(4.1?s^-1).The reason is that the small-sized Al₂RE particles require larger nucleation undercooling.Due to the redistribution and diffusion of the solute during the solidification process,some small-sized nucleating particles do not obtain sufficient constitutional undercooling,and therefore fail to become effective nucleation particles.The simulation results are consistent with experimental results.When Al₂RE particles migrate,the nucleation particles that migrate to the dendrite under slow cooling rate are more likely to grow into a crystal grain.The numerical simulation results in this paper provide a valuable theoretical reference for the improvement and development of Mg-RE alloy structure refinement technology.For example,developing new Mg-RE alloys in which large-sized nucleation particles will be achieved can help to obtain fine grains under different cooling rate regions.