A novel rheological high pressure die-casting process for preparing large thin-walled Al–Si–Fe–Mg–Sr alloy with high heat conductivity, high plasticity and medium strength

An air-cooled stirring rod (ACSR) process for efficiently preparing large volumes of semisolid slurry was introduced. A new low-cost casting Al–Si–Fe–Mg–Sr alloy was used to prepare large thin-walled heat-dissipating shells using both conventional HPDC and ACSR rheological high pressure die-casting (Rheo-HPDC) technologies. Their microstructures, thermal conductivities, elevated temperature and room temperature mechanical properties, and corrosion behaviors were compared. Furthermore, this work analyzed the mechanisms responsible for the microstructure refinement and performance enhancement in ACSR Rheo-HPDC alloys. The results showed that only 25 s was needed to prepare 32 kg of the semisolid slurry using ACSR process. The alloy prepared by ACSR Rheo-HPDC generated a large number of fine spherical primary α-Al particles with a volume fraction greater than 40%. The Rheo-HPDC alloy showed a moderate strength, high plasticity, and high heat conductivity. The heat conductivity, ultimate tensile strength and elongation of the Rheo-HPDC alloy were 184 W/(m.K), 264 MPa and 12.2% respectively, while those of the HPDC alloy were 167 W/(m.K), 228 MPa and 5.8% respectively. The improvement of heat conductivity of the alloy formed via ACSR Rheo-HPDC was mainly ascribed to the decrease of electron scattering by the refinement of eutectic silicons and Fe-rich intermetallics. This allowed the electron flow to pass through the eutectic Si region more easily. The refined and uniformly-distributed heat-resistant β-Al₅FeSi intermetallics effectively prevented grain boundary sliding, so the Rheo-HPDC alloy showed superior high-temperature mechanical properties. The Rheo-HPDC alloy presented a more excellent corrosion resistance to the conventional HPDC alloy due to the refinement of iron-rich phase and eutectic silicons and the reduction in the potential difference between the matrix and the iron-rich phase.