Quality Index of Tensile Property on Porosity Variation in A356 Casting Alloys upon T6 Treatment

Abstract

The contribution of porosity variation affecting the tensile properties of A356 aluminium alloys was investigated in terms of the variations in the quality index of tensile properties as well as the strength coefficient and strain-hardening exponent associated with the T6-treatment. The test specimens were prepared using a low-pressure die-casting and subsequent T6-treatment, and the contribution of microporosity, microstructural features and strain-relating factors to quality index was quantitatively evaluated using a modified constitutive model. The quality index of A356 alloys increases gradually with the lapse of ageing time upon T6-treatment, which increases significantly at the initial ageing stage with rise in the ageing temperature. Additionally, the quality index of A356 alloy primarily depends on the porosity variation with power law relationship, irrespective of its state, i.e. solutionised or artificially aged. Theoretically, the strength coefficient directly determines the nominal level of quality index. The overall dependence of quality index on porosity variation is weakened with increasing tensile strain, but it significantly depends upon the porosity variation for lower values of the strain-hardening exponent. The age hardening arises an explicit transition on fracture path by the damage evolution of eutectic Si particles, which can influence the overall level of quality index through a change in the effective void area fraction, with the microporosity variation by pre-existing micro-voids in casting alloys. A wide deviation on quality index of A356 casting alloy depends practically upon the microporosity variation. Additionally, the nominal level of quality index is influenced by the damage evolution of eutectic Si particles as well as the variation of strength coefficient and strain-hardening exponent by age hardening on T6-treatment. The theoretical approach by constitutive model can quantitatively describe the individual contributions of the strength coefficient and strain-hardening exponent to the overall level of quality index, including the microstructural features such as the area fraction and distribution aspect of micro-voids and eutectic Si particles.