<p>This study investigates the filtration behavior of Al–Si alloy melts through pressure filtration and correlates the findings with inclusion analysis on solidified, unfiltered samples, as well as with the mechanical performance of high-pressure die-cast specimens. The filtration characteristics of the original ingot melt and the melt treated by combined gas–bubbling–filtration and flux addition processes were compared. Filtration time versus filtrate mass measurements revealed that melt-treated samples exhibited a 650% increase in filtration time and an 8000% faster cake buildup rate compared to original ingots, indicating significant inclusion deposition and reduced filtration efficiency. Resistance analysis showed that original ingot formed predominantly incompressible cakes, whereas melt-treated sample transitioned to compressible cakes, leading to a 140% to 380% higher steady-state resistance and a 2,030% increase in the final stage. Microstructural characterization confirmed substantial inclusion accumulation on the filter surface, with a 760% increase in inclusion area, and identified large oxide agglomerates (≈100 µm) capable of blocking filter pores. Tensile testing of die-cast specimens revealed minimal variation in strength (2.4%) but large fluctuations in elongation (57.8%), due to residual oxides and gas-induced porosity. Although GBF and flux refining were conducted under melt treatment conditions that reduced the density index (DI) to below 1, indicating effective hydrogen degassing, these treatments were insufficient for complete inclusion removal. These findings demonstrate that DI alone is inadequate for evaluating melt quality and highlight the importance of adopting more comprehensive assessment strategies, with filtration behavior analysis emerging as a particularly valuable approach.</p>