Solidification, structure and mechanical properties of A357 aluminium alloy

Hashemi-Ahmady, Seyed-Modjtaba (1987) Solidification, structure and mechanical properties of A357 aluminium alloy. University of Southampton, Doctoral Thesis.

Abstract

The microstructural variations in the A357 alloy have been characterised as functions of : casting thickness (Z), casting process, modification and heat treatment. The growth rates (R), and temperature gradients (G), have been obtained through a series of experiments and theoretical analysis of the casting process in sand and in graphite moulds. Subsequently, the total solidification times (t_F), and heat flow rates (.Q) through each Z have also been obtained. It is found that the (t_F-Z) relationship in sand castings obeys the Chvorinov's rule of conduction. The (t_F-Z) relationship in graphite castings is found to obey an interface controlled condition up to the limit (Z = 20 mm), after which the process is found to be greatly influenced by conduction. The superimposition of R and G on standard data reveals excellent agreement with the experimentally observed morphologies of the microstructures. A complete survey of hardness, tensile and fracture toughness properties of the modified and unmodified A357 alloy has been made by using very simple test techniques. These included tests on the A357 and a number of other Al-Si-Mg alloys in the as-cast and in various heat treated conditions. The tensile and fracture toughness properties are found to be dominated by the size and morphology of the interdendritic Si particles. The secondary dendrite arm spacing (SDAS), has been found to have little influence on the properties. The crack path during the fracture of a hypo-entectic alloy has been found to be largely through the interdendritic regions. Some volume fraction of Al dendrites have also been found to be directly involved (sheared) in the fracture process. This is particularly prominent in the heat treated conditions. Consequently, the total fracture toughness of the alloy was found to follow a relationship in the form: K_{Ic = V}_f(dend) K_Ic(dend) + V_f(eut) K_Ic(eut) A series of tests on unidirectionally solidified (UDS) Al-Si entectic alloys - modified and unmodified - have established a strong Si orientation effect on the tensile and fracture properties. Increased Si volume fractions have been found to increase proof stress and UTS, but at the expense of ductility and toughness. Increased Mg contents have also been found to increase proof stress and UTS in the heat treated condition, but also at the significant expense of toughness. Coarse and/or flake Si is found to be more detrimental on tensile and fracture properties than porosity. It is found that pore sizes in the order of ≈10 x SDAS have no real effect on the mechanical properties. Specimens containing a modified Si morphology have been found to exhibit much higher mechanical properties than unmodified alloys, even if pore sizes within them are much larger than those in unmodified alloys. (D82530)

This record has no associated files available for download.

Contributors

Download statistics