Rare earth (RE) additions are capable of refining the α-Al phase as well as modifying the eutectic Si particles of alloys. The cooling rate in casting process should be carefully concerned when the Al-Si alloys are refined and modified by adding RE elements. In this study, the effect of cooling rates on the microstructure and mechanical properties of La modified Al-7.0Si-0.3Mg alloys was studied in gravity die casting and semi-solid die casting. It is found that in La modified Al-7.0Si-0.3Mg alloys, with increasing the cooling rate from 0.2 to 9 K/s in gravity die casting, the α-Al grains are greatly refined and the Si particles are modified to branching morphology, which evidently increases the UTS and elongation of alloys. In addition, when increasing the cooling rate from 30 to 130 K/s in semi-solid die casting, the α-Al grains are refined from 140 to 47 μm, and the Si particles are modified to fibrous morphology, which increases the UTS from 190 to 230 MPa and elongation from 10% to 11%. However, the 0.4 wt.% La addition results to La-rich phases formed in microstructure, which impairs the mechanical properties of Al-7.0Si-0.3Mg alloys in semi-solid die casting.

The distribution homogeneity of grain size affects the fluidity of the semi-solid slurry, which in turn affects the properties of the casting. One key factor affecting grain size uniformity resides in the nucleation number, which has been studied thoroughly, while the other factor is temperature gradient which has not been investigated yet. In this study, the microstructure evolutions under certain temperature gradients are investigated by experiment and simulation using a two-dimensional quantitative phase-field (PF) model. A parallel and adaptive mesh refinement algorithm is adopted to solve the nonlinear phase-field equations. The results indicate that temperature gradient can affect the size distribution of microstructure in the semi-solid slurry prepared by the SEED process. A higher temperature gradient (in the range of 0.230~0.657 °C/mm) along the radial direction is beneficial to the homogeneity of the grain size in a slurry.

Application of a coating on a mold surface is widely used in the foundry industry. Changes in coating change the heat transfer at the mold–melt interface, which influences the microstructure of the casting. In this study, the effect of boron nitride coating thickness on the interfacial heat transfer and slug microstructure in the Swirled Enthalpy Equilibration Device (SEED) process was investigated. The temperatures of the semi-solid slug and mold were measured, and the interfacial heat transfer coefficient and heat flux of the mold–slug interface was estimated based on these data. Microstructures of the quenched slugs were also examined. The results indicated that the interfacial heat transfer coefficient decreased with an increase in coating thickness and was sensitive to a coating thickness of less than 0.1 mm. The interfacial heat flux decreased sharply at the early stage, and then slowed down as the swirling time increased and the coating thickened. The coating thickness affected the temperature evolution of the slug at the early stage of the SEED process. As the coating thickness increased from near zero to 1.0 mm, the grain size of the slug increased by ~20 µm and the globular structure of the slug transformed into a dendritic structure.

The biggest challenge in semisolid processing of high-performance aluminum alloys is the narrow temperature processing windows of these alloys, and as a result, the preparation of qualified semisolid slurries is very important. High solid fraction slurries of high-strength A201 alloy were prepared by the Swirled Enthalpy Equilibration Device (SEED) process. The cooling behavior and microstructures of the A201 slurries produced by the standard, as well as a modified, SEED process were investigated. The results show that qualified A201 slurry can be produced by decreasing the pouring temperature and controlling the processing time in the SEED process. The modified SEED process significantly reduced the radial temperature gradient of the melt, due to the slow cooling rates involved, with the resulting slurries being more uniform, with more spherical microstructures, as compared to those produced by the standard SEED process. The formation of the nondendritic grain structure in the SEED process is attributed to the uniformly distributed large number of nuclei within the melt and the slow cooling of the melt in the containing crucible.

The microstructure and mechanical properties of semisolid rheo-diecasting Al-xZn-2Mg-1.5Cu alloys with different Zn contents were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness testing (HV) and room temperature tensile testing. Results show that the as-cast microstructure mainly consists of spherical α-Al and Mg(Al, Cu, Zn)₂ phases. Furthermore, a small amounts of Al₇Cu₂Fe phases were also detected along the grain boundary. Increasing the Zn contents from 8–12%, the volume fraction of the Mg(Al, Cu, Zn)₂ phases increases from 4.9–7.4%. After solution heat treatment at 470 °C for 8 h, most of the Mg(Al, Cu, Zn)₂ dissolves into the α-Al matrix, while the Al₇Cu₂Fe phase keeps with remains. The yield strength linearly increases from 482 ± 5 MPa of 8% Zn to 529 ± 5 MPa of 12% Zn. While, the ultimate strength of 10% Zn is 584 ± 2 MPa, which is higher than that of the other two alloys. Moreover, the average elongation dramatically decreases from 13% for the 8% Zn alloy to 2% for the 12% Zn alloy.