Abstract:

An ultrasonic vibration-assisted air-cooled stirring rod process (ACSR + UV) was used to efficiently prepare a large-volume semisolid slurry with a mass of more than 40 kg. A low-cost Al-Si-Fe-Mg-Cu-Zn die-casted alloy with high thermal conductivity, high plasticity and medium strength was developed. The alloy was used to manufacture large, thin-walled parts for 5G base stations by using the ACSR + UV rheological die-casting (ACSR + UV R-DC) process. Investigations were performed on the microstructure, porosity, mechanical properties, fracture behaviour and thermal conductivity of the ACSR + UV R-DC alloy, which was then compared to traditionally die-casted (T-DC) and ACSR R-DC alloys. The mechanisms for the microstructural refinement and enhancement of the mechanical and thermal conductivity performances of the ACSR + UV R-DC alloy were also analysed. The results showed that the ACSR + UV process increased the nucleation rate of the melt due to the increase in the nucleation area and the generation of cavitation bubbles. A radial- and an axial-forced convection was also generated inside the melt under the combined effects of acoustic flow and mechanical stirring, thereby homogenising the melt composition field and the temperature field. Therefore, the ACSR + UV R-DC process not only refined the primary α-Al (α₁-Al), the eutectic silicon and the secondary α-Al (α₂-Al), but also greatly improved the morphology and the distribution of the β-Al₅FeSi phase. The mechanical properties of the ACSR + UV R-DC alloy were higher than those of the T-DC and the ACSR R-DC alloys. Compared to the T-DC alloy, the ultimate tensile strength, elongation and yield strength of the ACSR + UV R-DC alloy were increased by 34%, 122% and 19%, respectively. This was because the ACSR + UV R-DC technique gave the alloy the characteristics of high density, fine spherical α₁-Al grain and a fine and uniform β-phase, which improved the fracture behaviour of the alloy. The thermal conductivity of the ACSR + UV R-DC alloy was 184 W/(m K), which was 10.2% and 3.4% higher than that of T-DC and ACSR R-DC alloys, respectively. This was because the refined eutectic silicon and β phases in the ACSR + UV R-DC alloy facilitated an easier electron flow through the eutectic region, and the decrease in porosity increased the effective area of heat conduction.

Key words: Rheological die-casting, Ultrasonic vibration, Al-Si-Fe-Mg-Cu-Zn alloy, Mechanical properties, Thermal conductivity