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In Al-Mg alloys with Mg content 0 ≤ XMg ≤ 6 wt. %, the effects of XMg 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 XMg and melt temperature TMelt; these trends agree with theoretical calculations for hydrogen solubility. The tendency of melt quality vs XMg was similar in DI, BI, and porosity measurements, and the poorest melt quality was observed in the Al-4Mg alloy that had XMg = 4 wt. %, whereas the highest [H] was obtained in the Al-6Mg alloy melt that had XMg = 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.

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.

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