Effect of fly ash particles on the mechanical properties and microstructure of aluminium casting alloy A535
Fly ash is a lightweight coal combustion by-product (CCB) separated from the exhaust gases of power generating plants using suspension-fired furnaces in which pulverized coal is used as the fuel. Its physical and chemical properties make it useful in construction and industrial materials, especially in cement manufacturing, concrete, liquid waste stabilization, and hydraulic mine backfill. The addition of fly ash into aluminum alloys has the potential to reduce the cost and density of aluminum castings while improving other physical and mechanical properties of the resulting metal matrix composites (MMCs). This study investigated the effect of fly ash addition on the mechanical properties and microstructural behaviour of aluminum casting alloy A535. The unreinforced A535 alloy and its MMCs containing a mixture of 5 wt.% fly ash and 5 wt.% silicon carbide, 10 wt.% fly ash and 15 wt.% fly ash were investigated in the as-cast and solution heat treated conditions. Microhardness measurements, Charpy impact testing, tensile testing, optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), inductively coupled plasma/mass spectrometry (ICP/MS), X-ray diffractometry (XRD), and X-ray fluorescence spectroscopy (XRF) were used to evaluate these effects. The results of this study show that increasing the fly ash content of the melt increased the porosity of the castings, which ultimately affected the density, tensile and impact properties of the MMCs. The density, microhardness, tensile strength and Charpy impact energy of the composites decreased with increasing fly ash content. The decline in density of the MMCs was due to extensive porosity developed with fly ash addition. Depletion of solid solution strengthening magnesium in the matrix was the reason observed for the decline in hardness. The loss in Charpy impact energy and tensile properties of the MMCs are also attributed partly to the depletion of solid solution strengthening magnesium atoms from the matrix and partly to porosity. Microstructural studies revealed non-uniform distribution of reinforcement particles in the composites. The fly ash particles were found to congregate at the boundaries of a-aluminium dendrites in the castings. Mg content of A535 alloy decreased with increasing weight fraction of fly ash. Mg was found to be tied up in a complex network of Mg2Si thereby reducing its availability in the matrix for solid solution strengthening.
DegreeMaster of Science (M.Sc.)
SupervisorOguocha, Ikechukwuka N.
Copyright DateNovember 2004
Aluminium Casting Alloy A535
Fly Ash-Reinforced Aluminium Composites