Bangyikhan, Kittiphan (2005)
Ph.D. thesis, University of Birmingham.
Since mechanical properties of cast Al-Si-Mg alloys are directly influenced by microstructural defects, understanding the characteristics of these defects and any interactions between them is important for improving the properties of castings. This research studied the effect of the most common defects normally found in cast aluminium alloys namely, double oxide films, Fe-rich phase, and porosity. A different level of each defect was introduced into the castings to investigate their effects on tensile properties with the results analysed by Yate’s algorithm and Weibull statistical analysis to determine their relative effects. The most important defect in this research was the oxide film that had an effect on UTS and elongation of about 21 MPa and 0.9% respectively and on the UTS Weibull modulus and elongation Weibull modulus of about 37 and 3 respectively. Increasing Fe content from 0.1 to 0.5 wt% influenced the tensile properties of the castings by decreasing the UTS and elongation by about 28 MPa and 1.7 % respectively. Although Fe-rich phases produced the greatest effect on tensile properties, their relatively high Weibull modulus showed that the reliability of the castings was at least predictable compared to the effects associated with oxide films. The porosity defects caused by an increase in hydrogen content from 0.1 to 0.45 ml/100g metal were the least detrimental to tensile properties. The most important interaction found in this research was the interaction between oxide films and porosity suggesting a mechanism for porosity formation in which entrained oxide film acted as initiation sites for pore formation in the castings. The main factor in the formation of porosity was hydrogen and shrinkage, since both could encourage the expansion of the oxide film defects to become gas porosity or shrinkage porosity in the castings. The other interaction between the microstructural defects observed in this research was that oxide films were found to be substrates for the nucleation and growth of Fe-rich phases, particularly the β-Al5FeSi phase. An interaction between all three defects were also observed and it further influenced the tensile properties of the cast Al-Si-Mg alloys by decreasing UTS and elongation by about 5 MPa and 0.5% respectively and the UTS Weibull modulus and elongation Weibull modulus of about 8 and 0.6 respectively.
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