Preparation Of Magnesium Alloy AZ91D Semisolid Slurry By Electromagnetic Stirring

Magnesium alloys are known as the "green" metal material in the 21 st century. Because it has light-weight, high specific strength, good shock absorption and electromagnetic shielding performance, it has applied widely in the aerospace, military industry, electronic communication, automobile and other fields. Recently, accompanying with the emphasis of environmental protection, there is an inevitable developing trend of lightweight in automotive industry. The application of magnesium alloy will be payed more attention.Its forming technology includes liquid, solid and semi-solid forming. Pressure casting is the main liquid forming method. However, the magnesium alloys would be oxidation and combustion, that brings to the magnesium alloy molding difficult.And casting defects are resulted from the high seed and high pressure casting production application. Thus, its application has been greatly limited.Semi-solid forming technology, a new generation of metal forming technology in the 21 st century, has obvious advantages compared with the traditional casting process. Its outstanding characteristic is near-net forming, which improves material utilization and saves material and energy. In addittion, it can effectively reduce macrosegregation, internal porosity and inclusions. Moreover, it is beneficial for improving the mould life due to the reduction of thermal shock impact on the mould.The slurry preparation is the key and premise of semi-solid forming technology. The primary phase morphology should be fine, spherical or near-spherical nondendritic structures, and uniformly distribute in the liquid metal melt.Electromagnetic stirring method is recommended in the semi-solid slurry preparation of magnesium alloys. The electromagnetic effect transfers energy to the semi-solid magnesium alloy without direct contact. It results in more pure metal slurry, low gas entrapment as well as the easily-controlled electromagnetic stirring process parameters. However, the different melting devices, electromagnetic stirring devices, casting processes and semi-solid forming methods have different quality requirements, which directly affect the immanent quality of semi-solid forming parts. Therefore, it is important to explore the appropriate electromagnetic stirring process for high quality semi-solid magnesium alloy slurry.In this study, we use Adel DMS-05-SMMG electromagnetic stirring device to electromagnetic stirred AZ91 D magnesium alloy. We study the influence of different frequency of electromagnetic stirring, electromagnetic stirring power, stirring temperature, stirring time and others for the primary phase morphology and size of semi-solid slurry. This work is the experimental basis for AD91 D semi-solid forming by squeeze casting.The main results are as follows:The primary phase morphology of semi-solid AZ91 D magnesium alloy slurry is influenced by the electromagnetic stirring frequency and intensity. The primary phase is more spherical with the electromagnetic stirring frequency and stirring power increased. Moreover, the best experiment electromagnetic stirring frequency is 50 Hz, and stirring intensity is 90%.The solid phase volume fraction of semi-solid slurry is decided by the electromagnetic stirring temperature. The best electromagnetic stirring temperature range is 590 ~ 595 ? for AZ91 D magnesium alloy. The evolution of primary phase morphology of semi-solid slurry is influenced by the electromagnetic stirring time. It has no spherical morphology with too short time, and too long time makes primary phase “welded”, which is harmful for semi-solid forming. The ideal electromagnetic stirring time is 10~ 15 minutes.The mechanism of the spherical primary phase in AZ91 D magnesium alloy by electromagnetic stirring is that the dendritic arms of a-Mg is broken by the effect of Lorentz force, and then free to form isolated particles. Maybe, the dendrites produce plastic deformation because of the Lorentz force, which results in grain boundary at the root of dendrites. When the grain boundary is completely infiltrated by liquid phase, the roots of fusing occur, and the isolated particles are formed. The morphology and size of isolated primary phase constantly changes in the following isothermal stirring process, the primary phase becomes smaller and more spherical at the beginning, and then it becomes bigger and reduces roundness subsequently.