Research On Microstructures And Properties Of Microalloying Mg-Zn-Si Alloy Prepaired By Electromagnetic Stirring

Compared with other structural materials, magnesium alloys have many excellent properties and have been widely applied in the automobile, IT and aerospace industries. Fabrication of high performance magnesium alloys by means of electromagnetic stirring combined with alloying is an important process in commercial application in the future.In the thesis, a set of rotating permanent magnet electromagnetic stirring device has been used, which is of manipulation of temperature and magnetic field distribution and intension. The evolution process of non-dendritic structure Mg-6%Zn-2%Si slurry at nonisothermal and isothermal stirring temperature under electromagnetic stirring has been investigated with the device, the results indicate that primary Mg2Si evolves firstly from fine dendrite to dendrite and becomes spherical grains at last. Furthermore, effects of process parameters (e.g. rotating speed of magnet barrel, alternation of rotating direction and stirring temperature) on the microstructure of semisolid Mg-6%Zn-2%Si alloy have been also investigated, which indicate that higher rotating speed of the magnet barrel makes the morphology of primary Mg2Si more spherical, alternation of rotating direction has no obvious effect on the the microstructure of semisolid Mg-6%Zn-2%Si alloy and stirring temperature reducing in stirring progress makes the primary Mg2Si more spherical while some of the grains may combined with each other. The mechanical properties of solidified Mg-6%Zn-2%Si ingot stirred by different stirring process have been studied, the results of which showed that isothermal stirring at 670℃70 Hz 10 min and solidifying with the most high cooling rate is the optimal process.Microstructure and properties of Mg-6%Zn-2%Si alloy modified with different content of Ce-rich mish metals, Ca, and B have been investigated. It is found that the alloys modified with 0.3wt.% content Ce-rich mish metals have the best tensile stength in the same series alloy modified with same alloy element, while 0.6wt.% and 1wt.% are the optimal content to alloys containing Ca and B. Those alloys which possess highest properties were stirred with the optimal process mentioned above, it is found that electromagnetic stirring has obvious effect on the semisolid microstructure of the alloys: the primary Mg2Si is refined in the alloy which containing Ce-rich mish metals, and exist in form of short-rod and pellet in the stirred alloy containing Ca. B make the primary Mg2Si to become coarse after stirred. Tensile strength and elongation at all test temperature (20℃and 150℃) are enhanced in all the three kinds of alloys after stirred, while the hardness has no obvious progress.