A Study On Semi-solid Forming Technique Of High Strength Magnesium Alloys

The magnesium alloys have attracted great research interests and became the ideal lightweight metal structural materials due to their high specific strength, specific stiffness, well damping vibration and abundant reserves, in modern society which has the issue of resources exhaustion. The research and development of magnesium alloy has become one of the hot spot of the world industry. Semi-solid forming technology of metal materials with its unique advantages, is considered to be a new generation of short and efficient process processing forming technology, gained great importance in the eighties of the last century once availableIn this paper, hot extrusion AZ91magnesium alloy and quasicrystal reinforced Mg-3Zn-0.5Y alloy for the study, activating and making the semi-solid billet by isothermal treatment method, and in the determined time and temperature, chilling the sample to room temperature in order to facilitate tissue analysis,study the changing regular pattern of organization. The study found that the thick branches crystal structure of magnesium alloy is disappearing after hot extrusion, replaced by a small equiaxial recrystallization organization, which provides a good microstructure for semi-solid isothermal transformation.In the isothermal treatment process of a certain temperature, low melting point of the second phase and the areas where contain enough solute elements are priority to melt, then penetrated and extended along the grain boundaries of higher free energy, forming separated a-Mg solid-phase grain, that the formation of the semi-solid organization which is liquid phase surrounded by solid-phase grain.When the holding time is the same, with the increasing of isothermal temperature, the liquid increased, solid-phase grain is melted, separation faster, solid-phase grain’s obtuse angle constantly rounding, when reaching high temperature, grain size began to increase.When the isothermal temperature is constant, as the isothermal time extended, a-Mg solid-phase grain separates and gradually melts, liquid phase constantly increased, solid-phase grains tend to be constantly round, the melting separation and spheroidization mechanism are the main influencing factors in this period. When the isothermal treatment long enough, alloy reaches the solid-liquid state of dynamic equilibrium, the Ostwald ripening mechanism began to play a control role and grain growth become the main mechanism, the solid fraction is no longer decreases with timeOn this basis, through the research of microstructure of chilled semi-solid organization of AZ91magnesium alloy, found the hardness of the unmelted a-Mg and the quench zone of liquid phase reduced first, followed by a small increase. Under different heating rate, for Mg-3Zn-0.5Y alloy, semi-solid organization research showed that under the fast heating speed, it can obtain semi-solid organization of smaller size of solid phase grain.