Effect of shot speeds on the microstructural framework and abnormal eutectic bands in a high pressure die casting hypoeutectic AlSi10MnMg alloy

The microstructural framework of a high-pressure die-cast hypoeutectic AlSi10MnMg alloy and the formation mechanism of two type abnormal eutectic bands under varied die casting shot speeds are proposed. Before the eutectic band zone solidifies, the central external solidified crystals (ESCs) dendrite-enriched zone and surface chilling zone have been developed. The residual Si-rich melt stars to solidify from surface to center along the formed feeding path of framework. In addition, liquid filling speed in high pressure die casting (HPDC) will not alter the sequential solidification sequence but affect the formation of eutectic band. The high-speed liquid flow causes relatively dispersive ESCs from surface to center while low-speed liquid flow accelerates the migration of ESCs to center and thus causes the formation of a larger dendritic network which is decisive to the different formation mechanism of eutectic bands. Eutectic band position before solidification serving as an interface between solidified zone and central dendritic network exhibits the characteristics of a higher velocity melt flow, a lower solid fraction and a fewer ESCs content. This study provides insights into near-rapid solidified behavior combining liquid filling process and it is expected to expand the conception to other die cast alloys.