Effect of pouring temperature on distribution characteristics and formation mechanism of eutectic-rich band in vacuum-assisted high pressure die casting AlSi9MnMg alloy

Abstract

The distribution characteristics and formation mechanism of the microstructure in vacuum-assisted high pressure die casting (HPDC) AlSi9MnMg alloy were elucidated by varying pouring temperature. The results show that the casting sample was divided into five distinct regions from the surface to the center, namely the chilling layer, the skin layer, the near-skin layer, the eutectic-rich band, and the core region. Externally solidified crystals (ESCs) play a critical role in the formation of heterogeneous microstructures within the casting. At excessively low pouring temperatures, a large number of ESCs form in the sleeve and enter the die cavity rapidly through the ingate, promoting uniform mixing with the liquid phase. The concentration of ESCs increases the melt viscosity, preventing them from migrating to the center of the casting. Consequently, the solute-rich liquid phase is dispersed within the dendritic network formed by the interconnected ESCs. The eutectic structure is dispersed by the primary α-Al dendrite network, and no distinct eutectic-rich band is formed within the casting. With increasing pouring temperature, the eutectic-rich phenomenon becomes more pronounced, the eutectic-rich band shifts toward the center of the casting. At middle pouring temperatures (650 ℃, 690 ℃, and 730 ℃), an increase in pouring temperature leads to a reduction in the number of ESCs, causing the ESCs dendritic network front to shift closer to the casting center, thereby driving the eutectic-rich band toward the center as well. These conclusions enabled the establishment of the relationship between the spatial distribution of ESCs and eutectic-rich band, thereby revealing the formation mechanism of the eutectic-rich band. This study sheds light on the formation mechanism of eutectic-rich band in HPDC Al-Si alloy and offers guidance for improving HPDC process design, not limited to such alloys.