Microstructure Evolution Of Semi-Solid AZ91 Magnesium Alloy Based On SIMA Process

The lightening tendency of mobile industry and 3C industries provides a widely application space for Mg alloys. As a near-forming technique, SIMA semi-solid forming is particularly suitable for those manufacturing parts, which are easy to vaporize and oxidize. In this paper, as-cast AZ91 alloy is processed by heat extrusion using a 5007 extruder at first, and then the alloy is re-melted into a semi-solid state, and rapidly quenched in water to obtain the final samples. The microstructure characteristics of extrusion deformation have been analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD). Then using a quantimet-500 apparatus, the solid phase fraction and particle diameter d in semi-solid-state microstructures are measured in details. Finally, the microstructure evolution of semi-solid AZ91 Magnesium alloy based on SIMA process has been studied.The results show that the coarse dendrites are eliminated and the fine equiaxed microstructure resulting from recrystallization has been obtained via heat extruded deformation. Therefore, the extruded microstructure is of a high stress state and hence provides a mass and energy condition for subsequent holding temperature treatment based on SIMA semi-solid-state technique. With the increase of deformation degree, the extrusion bends become compact and the recrystallized equiaxed grains become fine. And with the increase of extrusion temperature, the extrusion microstructure has a remarkable tendency of coarsening. Experimentally, the optimal extrusion parameters are determined as: T=380℃, λ=10The microstructures of AZ91 Magnesium alloy at semi-solid-state depend not only on the heating temperature and holding time, but also on the initial stress states at a great extent. With the increase of the heating temperature, the solid phase fraction decreases steadily and the average diameter of particles increases in some extent. With the increase of the holding time,some small particles tend to dissolve and disappear and the others coalesce into slightly larger particles, whose size reaches a steady state within 7 to 9 minutes.The semi-solid-state microstructure evolution under extrusion, cast and forge processing conditions is compared. For as-extrusion alloy, it is found that fine spherical particles are distributed uniformly in the continuous liquid phase of the slurry with a solid fraction of fs= 56%, held at the temperature of 565℃ for 5 minutes. Whereas the microstructures of as-cast alloy do not particularize until the holding time attains 12 minutes at 565℃ and irregular and large particles are distributed inhomogeneously in the slurry, for lacking of the necessary S1MA. The as-forge semi-solid-state microstructure is finer and more uniform than the as-cast one, but is not so homogeneous as the as-extrusion microstructure.The optimal holding temperature T and holding time t at semi-solid-state for as-extrusion AZ91 magnesium alloy are 565℃ and 5~7min, respectively.The relationship between the holding temperature and the solid phase fraction under SIMA semi-solid-state condition for extruded AZ91 Magnesium alloy is determined as:fs =-2568.86917+ 10.58326T-0.01052T2...