Abstract
Entrapped double oxide film defects are known to be the most detrimental defects during the casting of aluminium alloys. In addition, hydrogen dissolved in the aluminium melt was suggested to pass into the defects to expand them and cause hydrogen porosity. In this work, the effect of two important casting parameters (the filtration and hydrogen content) on the properties of Al–7 Si–0.3 Mg alloy castings was studied using a full factorial design of experiments approach. Casting properties such as the Weibull modulus and position parameter of the elongation and the tensile strength were considered as response parameters. The results suggested that adopting 10 PPI filters in the gating system resulted in a considerable boost of the Weibull moduli of the tensile strength and elongation due to the enhanced mould filling conditions that minimised the possibility of oxide film entrainment. In addition, the results showed that reducing the hydrogen content in the castings samples from 0.257 to 0.132 cm3/100 g Al was associated with a noticeable decrease in the size of bifilm defects with a corresponding improvement in the mechanical properties. Such significant effect of the process parameters studied on the casting properties suggests that the more careful and quiescent mould filling practice and the lower the hydrogen level of the casting, the higher the quality and reliability of the castings produced.
Highlights
Aluminium alloys are the most used materials in the automotive industry due to their excellent properties such as strength, durability, safety and low density, resulting in reduced emissions and an increase in fuel efficiency of the produced vehicles
These process parameters or casting conditions are the amount of hydrogen in the solidified casting and the filtration
The results showed a significant effect of the degassing treatment as well as the holding of the sand mould under reduced pressure, for a given time before pouring in, on the casting hydrogen content
Summary
Aluminium alloys are the most used materials in the automotive industry due to their excellent properties such as strength, durability, safety and low density, resulting in reduced emissions and an increase in fuel efficiency of the produced vehicles. Oxide film defects (bifilms) are typically created due to surface disturbance of the Al melt during pouring and/or transfer processes This causes the oxidised surface to be folded upon itself entrapping an air layer within it and be entrained in the bulk liquid Al [11,12,13,14]. The current research was carried out to cover the research gap and utilizes statistical techniques by means of Full Factorial Design of Experiments (DoE) and Analysis of Variance (ANOVA) to identify the significance of casting process parameters and study their effect on the UTS and % elongation of the castings produced. The effect of the hydrogen content of aluminium casting and the use of filters on the amount and size of bifilm defects, and by implication on the properties of Al–7 Si–0.3 Mg alloy castings was studied. The aim of such study is to provide a better understanding of the factors dominating the quality and reproducibility of light metal cast alloys
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