Analysis of Free Surface Behavior of AlSiCu (ADC12) Molten Alloy as a Function of Slow-Shot Speed Using a Transparent Quartz Sleeve

The slow-shot speed phase in high-pressure die casting critically governs molten metal flow within the shot sleeve, directly influencing air entrapment and casting defect formation. Conventional water modeling has been widely used to analyze this stage, but it cannot reproduce the intrinsic characteristics of the actual melt, such as oxide film formation and the appearance of solid-phase crystals that increase the apparent viscosity. In this study, a transparent quartz sleeve apparatus was developed to directly observe the free surface behavior of AlSiCu (ADC12) molten alloy during slow-shot at speeds of 0.10, 0.20 and 0.30 m/s. Water and silicone oil were also tested as conventional modeling fluids; both showed behavior similar to the actual melt in the early shot stage but deviated significantly in the mid-to-late stage owing to the increase in apparent viscosity caused by oxide film formation and solid-phase crystals, confirming the fundamental limitations of water modeling. For the actual ADC12 melt, free surface stability decreased progressively as the slow-shot speed increased, transitioning from laminar-like flow at 0.10 m/s to turbulent flow with surface folding at 0.30 m/s. The gas content of castings increased with slow-shot speed, and the H2/N2 ratio decreased from 1.07 to 0.53, confirming that air entrapment becomes the dominant source of blow hole defects at higher speeds. Microstructural analysis revealed that at 0.10 m/s, the extended sleeve residence time promoted coarse externally solidified crystal (ESC) growth, whereas at 0.30 m/s these phenomena were suppressed, but porosity increased owing to greater air entrapment.