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To study the grain morphology of Mg–Al alloy semisolid structure during the process of mechanical stirring, a multiple grains PF-LBM model (phase-field-lattice Boltzmann model) coupled with a rigid body rotation model was developed. In this study, an orientational field model was introduced to describe the orientation of different grains and it was determined that the growth of multiple grains with different orientations can be computed by only one-phase field function which improves the computing efficiency. The simulation results of the growth of two semisolid grains showed that the growth and coalescence representation can be simulated by the proposed model. Then, multiple grains growth of a Mg–Al alloy semisolid structure was successfully simulated using the developed model and the result is in good agreement with the experimental result.

In order to investigate the effects of stirring rate on semi-solid structure of Mg-Al alloy, a PF-LBM model (Phase Field Lattice Boltzmann model) coupled with a rigid body rotation model were adopted in this work. Grain morphology, grain growth rate and grain rotation at different stirring rates were systematically studied. The results indicated that by driving force of melt flow velocity difference around the grain, the grain rotated during the stirring process, and then solute concentration would present to the low flow areas around the grain, which was caused by the forced convection. The solutes are uniformly distributed around the grain as the grain rotates. As a result, the fine and spherical grain can be obtained owing to the inhibition of grain growth.

The effect of La/Nd ratio on microstructure and mechanical properties of as-cast AZ91D alloys were investigated. The alloys were produced by casting the molten metal into a preheated steel mould. Microstructures were investigated by XRD and SEM (equipped with EDS). The hardness measurement was carried out by Brinell hardness (HB) digital display tester. Compression tests were conducted at both room and high temperatures (20 °C and 150 °C). Microstructural characterizations reveal that the area fraction of the β-Mg17Al12 phase changed with the variation of La/Nd ratio. The area fraction of the β-Mg17Al12 phase decrease significantly and then increased slightly with the increase of La/Nd ratio. Besides, the results show that the addition of La and Nd contributed to the formation of needle-like phase Al11RE3. Meanwhile, the Brinell hardness and high-temperature compression properties of this alloy reach the maximum values (67 HB and 330 MPa, respectively) when the La/Nd ratio is 2/3. In this alloy, the amount of Al11RE3 phase is 51/TA and the average length is 69.55 μm. Moreover, a generation model of Al11RE3 is established and the growth mechanism of Al11RE3 is analyzed. This work thus has proved that La/Nd ratio has a great effect on the microstructure and mechanical properties of AZ91.