Analysis of variability in tensile ductility of a semi-solid metal cast A356 Al-alloy

Variability in the ductility of tensile test specimens of semi solid process cast A356 Al-alloy (Al-7 wt.% Si-0.5 wt.% Mg-base alloy) is examined. The variability in the ductility does not correlate to the global average microstructural parameters such as dendrite cell size, Si particle size, and amount of porosity in the three-dimensional microstructure. Tensile fracture surfaces contain micro-defects that are essentially residues of modifiers, fluxes, grain refiners, and mold release agents: the energy dispersive analysis shows presence of elements such as O, Na, K, C, Cl, Ca, Fe, Ti, and S in these defects. The fracture path preferentially goes through the regions containing the defects. Scanning electron microscopy, quantitative fractography was used to estimate the total area fraction of such defects in the fracture surfaces of the tensile test specimens. The percent tensile ductility e shows strong quantitative correlation with the area fraction of the defects f in the corresponding fracture surfaces, which can be represented by the following simple equation. 0 [ 1 − f ] n s equation, e0 and n are empirical constants. For the present data set, e0 is equal to 11.5% and n is equal to 41.66. Thus, the variability in the ductility can be decreased through a reduction in the amount of processing defects via better process control.