Study On The Thixomolded Process And Heat Treatment Of AZ91D Magnesium Alloy

Magnesium alloy ,the lightest metal material in engineering application, is considered the environmental engineering material used more and more widely in the 21st century ,and the research on it has been the global subject. Semi-solid processing technology is well-known as the most promising material fabrication method in the 21st century for many advantages. Thixomolding is the only SSF applied to Magnesium alloy industrial manufacture, which is suitable for the complex-shaped and compact products. For the great application potential, this technology is introduced to our country in 2002.However, due to the protection from foreign patent, thixomolding has not been known very well in China till now.The 220 Tons thixomolding machine made by Japan Steel Works (JSW) was used to fabricate the samples in this research. According to the requirement to thixomolding materials and the characteristic of this forming method, AZ91D, the most popular magnesium alloy in the present industrial field, was selected to fabricate the mobile phone house. The mobile phone house has complicated structure and thin wall about 1mm, and this kind of product can adequately show the advantages of thixomolding. In this article, the characteristic and evolution mechanism of semi-solid microstructure, thixomolding process and heat treatment are researched systermatically.Firstly, the microstructrue evolution was analysed in detail from the chips in the beginning to the products in the end. Besides, the microstructure and elements distribution in solid and liquid phase of thixomolding process were investigated. The typical features of thixomolded microstructure of the AZ91D alloy can be described as a suspension of solid particlesα-Mg dispersed in a liquid matrix with the lack of dendrites. In general view, three types of morphology of the solid particles may be found in the micrographs, one is the particle with entrapped liquid, and one is with pure solid, the last is with rosette shape. The rest is eutectic microstructure. The microstructure and elements distribution of entrapped liquid in solid particles are the same as that of liquid phase around solid particles, but solute segregation happened obviously in the entrapped liquid.Secondly, evolution mechanism of semi-solid microstructure is discussed theoretically .To the area deformed severely, the mechanism can be divided into two stages where, at first, in a solid state, an equiaxed structure would form due to recrystallization of chips controlled by the plastically deformation induced during the mechanical fragmentation of chips during the very initial stages of heating. At the same time, Al diffuses from Mg17Al12 intoα-Mg, which makes the enrichment of the solute along the primary grains boundary. Along with the increase of the barrel temperature, the high concentration region of solute will begin to melt at the eutectic temperature and the liquid phase appears at the primary and recrystallized grains boundaries. In addition, chemical segregation of Al and Zn within the particles would cause some small entrapped liquids inside the particles. Furthermore, some secondary dendrite arms can detach at their roots because of melting resulted from solute enrichment and thermosolutal convection. Then the grain boundaries created by recrystallization are disintegrated by melting during the stirring above the solidus temperature. After the first stage, in a semisolid state, Ostwald ripening and coalescence, driven by the reduction of the interfacial energy between particles and liquid, and controlled by solutes diffusion, are superimposed on the alloy melting. Coalescence usually brings the instantaneously formation of larger particle by contact of smaller particles. It appears that the existence of the particles with entrapped liquid or rosette shape is caused by the coalescence coarsening of the particles, especially coalescence of dendritic arms in complex shaped particles, rather resulted from a mechanical fragmentation. However, Ostwald ripening, resulted from the Gibbs-Thomson effect, and limited here due to the very short processing times, would lead to a loss of entrapped liquid, because the small particles with entrapped liquid will dissolve and the entrapped liquid will join the bulk liquid. In addition, some particles would be broken down into smaller particles under the influence of forced convection, coupled with the spheroidization of the solid particles due to the shearing and abrasion among particles . Finally, the typical semisolid microstructure forms after the series of processes above. However, the severe-deformed area is so small and the injection time is so short , as a result, the mechanism above does not play the main role. To most undeformed area, the solid particles are obtained by breaking dendrites with the screw.Thirdly, the effect of process parameters on the mechanical properties is researched and the optimization principle is found out. The experimental results can be summarized as follows: semi-solid microstructure mainly depends on barrel temperature and small solid fraction under high temperature results in good mechanical properties. There are so many defects and air pores under high injection velocity that the mechanical properties are weakened. When the balance between the solid phase change and pore rate is achieved, the property is the best. Although the increase of screw rotating speed can sphere the solid particles, the mechanical properties is not good enough for the large number of air pores. Mold temperature decided the cooling rate after the semi-solid slurry entered into the mold. When the other process conditions are constant, lower temperature of mold means quicker cooling rate, and the products with finer microstructure and better mechanical properties could be acquired. If the mold temperature is too low, the products would have some defects, such as the crack, the warp, etc. In general, mold temperature should be set higher to thinner products than thicker ones.Finally, heat treatment of both chips and products is carried on so that the whole process of thixomolding and microstructure evolution are perfected. According to the experiment results, the most suitable parameters are confirmed.The results obtained in this dissertation will play an important guidance role for thixomolding of magnesium alloy in the further industrial application.