Abstract

HAMED BASUNY, F.  and  EL-SAYED, M.A.. The effect of holding time before solidification on the properties of aluminium castings. J. S. Afr. Inst. Min. Metall. [online]. 2020, vol.120, n.8, pp.461-466. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/1102/2020.

The properties of aluminium castings are strongly affected by their inclusion content, particularly double oxide film defects, or bifilms. Such defects have been reported not only to decrease the tensile and fatigue properties of Al casting, but also to increase their variability, making the properties of such alloys unreliable and unreproducible. Earlier research suggested that the bifilm atmosphere might be consumed by reaction with the surrounding melt, which could improve the mechanical properties of the castings. In this work, the effect of holding an Al casting in the liquid state for up to 20 minutes before solidification was studied using a three-level general factorial design of experiments. Two responses were considered, the ultimate tensile strength (UTS) and elongation of the resulting castings. The results showed that the holding treatment had a significant effect on the elongation of the castings produced. In addition, the UTS and elongation peaked at a holding time of 10 minutes. Scanning electron microscopy (SEM) investigation detected many oxide fragments inside pores on the fracture surfaces, reflecting the role of entrained defects in the formation of porosity. The results suggest that two opposing phenomena may take place during the holding treatment. Thus, the consumption of air inside the entrained defects due to reaction with the surrounding molten metal may lead to improvements in mechanical properties, but this may be accompanied by hydrogen passing into the defects, which has a deleterious effect on properties.

Keywords : oxide film; aluminium casting; porosity; defects; hydrogen; design of experiments.

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