In Al-Mg alloys with Mg content 0 ≤ X_Mg ≤ 6 wt. %, the effects of X_Mg on dissolved hydrogen content ([H]) and melt quality were investigated. [H] was measured using the Closed-Loop Recirculation method, and the melt quality was quantified using the density index (DI), bifilm index (BI), and porosity measurement. [H] in the molten alloys increased with increasing X_Mg and melt temperature T_Melt; these trends agree with theoretical calculations for hydrogen solubility. The tendency of melt quality vs X_Mg was similar in DI, BI, and porosity measurements, and the poorest melt quality was observed in the Al-4Mg alloy that had X_Mg = 4 wt. %, whereas the highest [H] was obtained in the Al-6Mg alloy melt that had X_Mg = 6 wt. % Mg. During thermogravimetric/differential thermal analysis, rapid oxidation occurred in the Al-4Mg alloy melt during the holding time between 45 and 60 min at 800 °C, i.e., just before the molten metal was cast. The inferior melt quality of Al-4Mg alloy may have been caused by high-temperature oxidation.

In this study, the optimal conditions of gas bubbling filtration (GBF) treatment for securing highly-clean molten Al-Si-Mg-Cu alloy were identified. The effects of GBF treatment time and stabilization time on the degree of molten metal cleanliness were examined by measuring melt quality parameters such as density index, bifilm index, porosity, and the amount of dissolved hydrogen [H]. A high melt quality was achieved when GBF treatment was performed on 10 kg melt for more than 10 min (i.e., 1 L gas/kg melt). However, as the stabilization holding time after GBF treatment increased to 10, 20, and 30 min, the melt quality degraded. GBF treatment for 30 min had a similar effect to treatment for 10 min, and the degree of deterioration of melt quality during the stabilization time was also similar. Considering the economics, 10 min GBF treatment and short holding time are required. Observations of the shape and volume of the largest pore suggested the cause of defect formation and confirmed that the volume of the largest pore can be used as an index of the melt quality.

With the rise in the demand for eco-friendly and electric vehicles, welding and heat treatment are becoming very important to meet the necessary weight reduction, complexity, and high functionality of die castings. Pore-free (PF) die casting is an effective process that enables heat treatment and welding due to low gas porosities. Indeed, this process affords castings of low gas porosity, similar to those attained by high-vacuum die casting. In this study, we compared the gas porosities of different castings fabricated by PF die casting using varied injected oxygen amounts. The castings were all subjected to T6 heat treatment and analyzed by computed tomography (CT) to compare their microstructure and mechanical properties before and after T6 heat treatment. The results revealed that with the increasing injected oxygen amount, the gas porosity of the specimens decreased while their mechanical properties increased. In particular, the gas porosity was the lowest at 1.26 L. Moreover, the 1.26 L specimen displayed the best tensile strength, yield strength, and elongation results. Finally, Weibull distribution analysis revealed that the tensile strength and elongation repeatability and reproducibility increased with increasing injected oxygen amount.

A383 Al-Si-Cu alloy matrix composites were reinforced with different amounts (0.5, 1.0, 1.5 and 2.0 wt%) of chopped multiwalled carbon nanotubes (MWCNTs) and fabricated using the oxygen-replacing die casting (ORDC) process to reduce gas porosities via the reaction of molten Al and O₂ replaced in the mold cavity. MWCNTs were added to the mold cavity by supplying O₂ and using a poly gate in the ORDC mold to improve CNT dispersity in the matrix of the composite. Microstructure studies of the composites showed a uniform CNT distribution within the matrix and grain refinement. X-ray computed tomography images showed that the internal porosities were affected by the CNT addition amount and gate type used in the mold, and Raman spectroscopy analysis indicated that CNTs in the matrix were free of significant defects. The 1.0 wt% CNT-added composite cast using the poly gate showed the highest ultimate tensile strength of 258.5 ± 5.2 MPa and hardness of 157.9 ± 3.0 Hv; these values were, respectively, 21% and 30% higher than those of the monolithic A383 alloy, confirming the feasibility of fabricating the MWCNT-added A383 alloy composite with a poly gate using the ORDC process.

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.