Research On Fabrication Of Semi-Solid Magnesium Alloy By SIMA

Magnesium alloy, as one of the green industry in the 21st century, had caught more and more international researchers' attention. The technology of the magnesium alloy's semi-solid processing was studying. Moreover the fabrication of the semi-solid magnesium alloy by SIMA had the unique advantage.The reheating parameters of the fabrication and the mechanism of the globalization were investigated systematically in this article. The extruded AZ91D alloy was heated continuously and soaked at constant temperature .Then it was quenched in the water and the instant texture at high temperature was acquired. The equal-grain-area-circle diameter and the roundness were defined to scale the form factor and quantified the degree of the grain's globalization. Metallographic analysis, XRD, TEM and EDAX were expired to observe the evolution of the microstructure, the changing of the phase and the changing of the component. Through the analysis of the evolution of the microstructure, the honey comb at the grain boundary and the pool in the grain, furthermore according to the phase change thermodynamic and kinetics, the conclusions of the globalization mechanism and the reheating parameters were drawn as follows.1. When the preformed Magnesium alloy's texture was globalizing, there were two important control factors, which were holding temperature and holding time, affecting the globalization processing. With the holding temperature going up, the globalization velocity accelerated gradually. At the same heating velocity, the result of holding 5min at 560 X? was the same as holding 4min at 570 癈 and as holding 2.5min at 580"C.Ill2,XI'AN UNIVERSITY OF TECHNOLOGY2. At the same holding temperature, when the holding time increased, the equal-area-circle diameter trended to uniform, and the distribution of the roundness approached to Gaussian distribution. The average of the equal-area-circle diameter was changed from long to short and then to long, to short at last, as a wave. The average of the roundness was changed from low to high and to low again, as para-curve.3. At the same holding time, when the holding temperature changed, the number of the long equal-area-circle diameter grain increased, and the average of the equal-area-circle diameter lengthened, the average of the roundness heightened. The relationship between the holding temperature and the average of the equal-area-circle diameter or the average of the roundness was parabolic.4. After continuously heated and isothermally soaked, the equiaxed grain, as the result of the recrystallization, was the necessary texture condition of the semi-solid grain's globalization processing. The semi-solid recrystallization velocity was run-up when the holding temperature risen and the heating velocity accelerated.5. When it was soaked in the semi-solid temperature, the preformed texture was accompanied with the part high-energy-area melting. Its presentations were the honey combs at the grain boundary and the pools in the grain. It was the essence condition that the semi-solid metal maintained the solid character and had the thixotropy.6. The melting of the grain boundary is the reason of the semi-solid grain globalization, and its control factors were the atomic diffusion velocity and the liquid-solid interface curvature. The globalization course was that the preformed texture recrystallized and became the granular equiaxed grain, when the temperature rose and the holding time prolonged, the granular grain melted at some area and at the liquid-solid interface the grain globalized at the role of the curvature overheated.