| With the entrance of 21st century the environment protecting and sustainable development have become the common concerns of the world. In order to realize the conversion of traditional high consuming pattern to sustainable developing pattern, reasonable using, saving and protecting of the resource have received the recognition. Magnesium and magnesium alloys are kinds of the lightest structural metal alloys without pollution. The exploitation and application of them are very important to the development of the materials science field and society. Magnesium alloys have many excellent properties, such as low weight, high specific strength and stiffness, high heat and electrical conductivity, good damping and shock absorption, excellent electromagnetic shielding capability and easy processing, as well as a good recovery capability. In recent years, the application of magnesium alloys has become the important research item for the countries with the developing automobile industry. Magnesium resource is abundant in our country. The production of original magnesium is sencond in the word, which is only lower than that of the America. However, the total amounts of excellent magnesium alloys products are very small. It is mainly because that the magnesium alloys processing technology of our country is far drop behind the developing countries. It is urgent affairs for our country to exert the resource predominance and tranform the original to the excellent products under the condition of resource absence. So the study and extending applization of the magnesium alloy processing techonology have extensive foreground and high significance. In present, more than 90% magnesium alloy components are produced by die-casting. However, the die-casting processing has many problems which cannot be overcomed. Semisolid casting can solve the problems above and decrease macrosegregations, porosity and forming efforts. The non-dendritic microstructure is the key of the semisolid casting. Several processes can generate the non-dendritic globular microstructure. Among which,Magnetohydrodynamic Stirring and Strain-induced Melt Activation ( SIMA) are the better processes applied in the industry. However, the electricity energy consumption of the former is too large, and the equipment and processing of which are too complex,and the stirring efficiency of which is also low. SIMA has been the subject of many studies. Comparing with other starting semisolid material producing methods SIMA has several advantages. It omits the procedure of molten metal treatment, cleans the metals, increases the production and is applicable for both low and high melting alloys. Moreover, oxidation and burning can be avoided for the magnesium alloy produced by SIMA process. However, for the last few decades, most attention has been focused on the investigation into thixotropic properties and forming of aluminum alloys and Pb-Sn alloys. However, attention to the non-dendritic microstructure and microstructure evolution mechanism of magnesium alloy is very limited, which retards the development and application of the magnesium alloy semisolid technology. The AZ91D magnesium alloy semisolid billets are fabricated by SIMA processing in this paper. The main researches are following. (1) The semisolid microstructure evolution mechanism of AZ91D magnesium alloy produced by SIMA process. The microstructures of AZ91D magnesium alloy after different heat-treatment were investigated. The results indicate that there are three microstructure evolution mechanisms for AZ91D magnesium alloy: dendrites melting, dendrites combing and recrystallization. However, therecrystallization is the main mechanism. In general, the recrystallization mechanism will be dominant gradually with the increasing of compression ratios. (2) Effect of heat-treatment processing on the semisolid microstructure of AZ91D magnesium alloy produced by SIMA process. The effects of heat-treatment processing such as heat-treatment temperature, heat-treatment time, heating velocity and predeformation were investigated in this paper. The results real that the preferred heat-treatment was suggested at 570℃for20min. The microstructure will coarsen when the temperature is higher 570 ℃or the time is longer. The degree of microstructure spherdization will worse when the temperature is lower or the time is shorter. Moreover, the heating velocity of 6.4℃/min was also suggested because the larger velocity will accelerate the microstructure fining. The size and spherdization degree of the semisolid globules and the liquid island are mainly determined by predeformation during the SIMA processing. In general, the fine and globule semisolid microstructure can be obtained for the alloy with the 20.6% compression ratio. First, the semisolid microstructure becomes finer and globular with the increasing of the compression ratio rapidly. Then, the fining and spherdization become slowly when the compression reaches 10%. Finally, the size and spherdization degree keep invariable when the compression higher than 20%. (3)Effect of as-cast microstructure on the semisolid microstructure of AZ91D magnesium alloy produced by SIMA process. The effects of initial microstructure produced by different casting moulds on the semisolid microstructure are slight for the predeformed alloys. However, the effects of initial microstructure on the semisolid microstructure are obvious when the compositions of the alloys are different. The amount of liquid ratio increases with the aluminium content increases. Addition of La2O3 to AZ91D alloy results in the formation of LaAl4 in the semisolid microstructure. The new intermetallic phases show needle-like or particle-shape morphology and distribute dispersedly. With the increasing of... |
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