Research status of slurry preparation for magnesium alloy semi solid forming casting

At present, scholars at home and abroad have invented different preparation processes for semi-solid slurry / ingot preparation. The purpose is to obtain the ideal semi-solid structure, and then combined with different forming processes to give full play to the advantages of semi-solid forming.

Microstructures of AZ91D alloy solidified under different conditions (a) conventional condition; (b) At slow cooling rates, (c), (d) Stirred at 100 Hz, 100 V and 50 Hz and 60 V, respectivel and quenched in water at 585℃

At present, Thixoforming of magnesium alloys focuses on mg Al Zn system (such as AZ91D, AZ80, etc.), mg Al Mn system (such as AM50, AM60, etc.), mg Al Si system, etc. Alloy ingots are usually prepared by chemical grain refinement and semi-solid isothermal heat treatment. Compared with thixoforming, rheo forming saves solidification and remelting steps of ingot, and has the advantages of high efficiency and low cost. Therefore, in recent years, more and more scholars focus on the rheological forming of magnesium alloys. The preparation of non dendritic semi-solid slurry of magnesium alloy is the key of rheological forming technology. Different external fields are usually used, such as double helix mechanical stirring, ultrasonic vibration, bubble stirring and electromagnetic stirring.

Microstructures of castings containing a solid fraction of primary solid (a) 7%, (b) 16%, (c) 36% and (d) 40%

In addition to the traditional mg Al series and mg Zn alloy series, in recent years, more research has focused on Mg Re alloys with higher properties, such as Mg Gd, Mg CE / LA and so on. Among the methods for preparing semi-solid slurry / ingot invented at present, the electromagnetic stirring method is the most successful one, which is widely used in the continuous casting production of ferrous metals and the continuous preparation of semi-solid aluminum alloy billets. It also has a wide application prospect in the production of magnesium alloy products. Therefore, this paper intends to use electromagnetic stirring to prepare magnesium alloy slurry. The following focuses on the research status of semi-solid non dendritic slurry structure of magnesium alloy prepared by electromagnetic stirring.

Optical micrographs of semisolid slurries prepared by LFEMS with different rotational frequencies: (a) 10 Hz, (b) 15 Hz, (c) 20 Hz and (d) 30 Hz

Mao et al. Show that the primary phase of AZ91D alloy is coarse dendrite under traditional solidification and slow solidification, and rose like primary particles can be obtained after low frequency electromagnetic stirring (Fig. 1). In addition, EBSD analysis shows that a rosette like primary particle evolves from the same particle, and polygonal particles come from different particles with higher frequency. Zhang et al. Prepared AZ91 alloy slurries with different solid ratio by electromagnetic stirring. It was found that the primary phase of the alloy changed from dendrite morphology to non dendrite primary particles with the effect of electromagnetic stirring. However, with the increase of solid fraction, the primary phase segregation becomes more and more serious, and the β phase also increases (Fig. 2). Yao et al. Conducted electromagnetic stirring on Mg Li Al Zn alloy melt. The results show that the α phase changes from long strip to near spherical, and the β phase is more dispersed around the α phase. After electromagnetic stirring, the tensile strength of the alloy increases from 172mpa to 195mpa, and the elongation increases from 10.65% to 25.75%. Liu et al. Found that the primary α – Mg phase of az91-0.8% Ce alloy melt changed from coarse dendrite to rosette like and near spherical particles after electromagnetic stirring, the alloy grains were refined, and the amount of β phase increased significantly. It is also found that the grains of AZ91 alloy are refined and the β phase is reduced by adding a small amount of SR. The increase of hardness of AZ91 is mainly due to the solid solution of Al and Zn, which makes the element distribution uniform and the grain refinement plays a strengthening role. In addition, DTA analysis shows that the refinement of primary α phase is mainly due to the increase of undercooling and nucleation temperature. Wang et al. Prepared mg-2.5gd-zn (at.%) alloy semi-solid slurry by electromagnetic stirring. The results show that the mg-2.5gd-zn (at.%) alloy can be refined by electromagnetic stirring treatment, and the grain size can be refined from ~ 680 μ m to ~ 150 μ M. finally, the non dendritic semi-solid structure with round primary particles uniformly suspended in liquid phase is obtained. Fig. 3 shows the microstructure of semi-solid slurry under different stirring frequencies. It can be seen from the figure that if the frequency is too high or too low, there are dendrites in the slurry structure. The results show that the slurry structure is composed of rose like crystal and fine primary particles. The optimum technological parameters for preparing the semi-solid slurry are as follows: voltage 300-350 V, frequency 15-20 Hz, cooling rate 1.4 K / min. In addition, according to the existing equipment and research, it is proposed that the dendrite arm may fuse under the following conditions:

Among them, λ 2 is the secondary dendrite arm spacing, the dendrite length in the mushy region should be greater than 8 λ 2, Δ t is the temperature gradient, t is the cooling rate, FS is the solid fraction, η is the dynamic viscosity of the alloy melt, and dind is the distance from the inductor to the front of the liquid surface.

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