Microstructures, mechanical properties, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys prepared by rheological high pressure die-casting and high pressure die-casting

The microstructures, mechanical properties, thermal conductivities, and corrosion behavior of novel high-thermal-conductivity hypoeutectic Al-Si alloys, prepared by rheological high-pressure die-casting (Rheo-HPDC) and HPDC, were investigated and compared. The microstructure of the Rheo-HPDC alloy, including the primary α-Al (α1), secondary α-Al (α2-Al), β-Al5FeSi, and Si phases, was significantly more refined than that of the HPDC alloy. Compared to the HPDC alloy, the Rheo-HPDC alloy exhibited superior mechanical properties and thermal conductivity, and its ultimate tensile strength (UTS), yield strength (YS), elongation, hardness, and thermal conductivity were improved by 27%, 21%, 72%, 10% and 8%, respectively. Pitting corrosion of the Rheo-HPDC alloy was observed to originate at the interfaces between the Fe-rich intermetallics and the α2-Al particles. Corrosion was observed to propagate in the eutectic areas upon prolonged immersion in 3.5 wt% NaCl solution. Electrochemical testing, scanning Kelvin probe (SKP) measurements, and corrosion-morphology analyses reveal that the Rheo-HPDC alloy has better corrosion resistance, primarily due to the decrease in the potential difference between the Fe-rich intermetallics and the matrix, the refinement of the eutectic Si and intermetallics, and the decrease in the areal ratio of eutectic Si to α2-Al.