Research On Homogenization And Mass Transfer In AZ61 Magnesium Alloy Ingot

With many advantages, magnesium alloys are considered as eco-structural metal material in the 21st century and interest many researchers. The advantages of magnesium alloys for industrial and structural applications are their low specific gravity, high specific strength and specific rigidity, damping capacity, processability, excellent thermal conductivity and electromagnetic shielding,and recyclability. It's said to be that the inhomogeneity of structure and chemical constitution is unavoidable in ingot of as cast magnesium. This inhomogeneity incorporates in some aspects following: Too much non-equilibrium second-phase and excess-phase exists, further worse, they become dendritic network; The supersaturated solid solution is formed when the dissolvability of alloying agent is more than equilibrium concentration in Mg; The coring segregation or dendritic segregation will begin to form if solid solution of Mg's constituent is not homogeneous. The cast structure not only make the plasticity of ingot worse, but also decrease the hot-working character and it's strong heredity will do bad to intensity and plasticity of product. The ingot should be sufficiently treated to avoid the inhomogeneity.In the thesis, the variation of micro structure of AZ61 was discussed under different heat treatment, then we get the curve of micro hardness with temperature and time, also the parameter of heat process were carefully studied; Homogeneous level of alloy which had been treated was innovatively judged by the variance of micro hardness distribution; The dendritic spacing was measured to determine how it changed while the alloy was under heat process, then further study of its influence toward to homogeneity was taken over combined with diffusional kinetics; The regularities of distribution of alloying agent were analyzed to provide the attestation for studying on mass transfer and diffusional mathematical model in secession by SEM; The x-ray diffraction was used to analyzed the variation regularity of phase, which was also a good method to judge the homogenous of the alloy.With metallurgy quality control moving to microstructure control, the accurate description of mass transfer in dendrite segregation become the key to control microstructure. In order to more deeply represent the process of mass transfer and diffusional of dendrite segregation, the mathematical solution of Fick-II Law was researched and proved to make it easier to describe and calculate the homogenizing process, then the mathematical model was used to demonstrate the heat process which was recommended by the essay.