Effect of melt flow control via runner design on microstructure and defect formation in high-pressure die cast Al-Si alloy

This study investigates the influence of runner design on the formation of externally solidified crystals (ESCs), porosity and eutectic bands in a non-heat-treated high-pressure die cast (HPDC) AlSi9MnVZr alloy. Three runner configurations: simple runner, pinched runner and pinched runner with an ESCs collector were employed to understand how melt flow dynamics affect microstructural evolution and defect distribution. Microstructure and porosity were analyzed using optical microscopy, scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS) and synchrotron radiation-based 3D tomography. The results showed that the simple runner, due to undisturbed melt flow, promoted the accumulation of coarse ESCs in the central regions, resulting in higher porosity and the formation of wide and well-defined eutectic band. The pinched runner, by introducing flow acceleration and shear at the gate, effectively fragmented and dispersed ESCs, reduced porosity and suppressed eutectic band formation by promoting shear-induced dendrite disruption and solute redistribution. The runner design incorporating an ESCs collector further reduced the ESCs content and porosity by filtering large crystals before cavity entry. However, the accompanying reduction in melt velocity and shear intensity limited solute redistribution, leading to the appearance of more visible eutectic bands compared to the pinched runner alone.