Effect of Solidification Parameters on the Microstructure and Tensile Properties of 319-Type Alloys

Abstract

The present study was performed on an Al–Si–Cu 319-type alloy using an end-chill mold, while varying the cooling rate along the solidification axis. Good fits for response parameters of percent porosity, maximum pore area, average pore area, maximum pore length and average pore length were obtained from the statistical analysis of the porosity data for the end-chill mold samples. These results strongly support the porosity formation observed in an aluminum A319.2 alloy reported in an earlier study. Tensile samples were T6-heat-treated, and the data obtained from these samples were analyzed statistically. The results show that the ultimate tensile strength (UTS) is sensitive to variations in the porosity and solidification conditions, and varies in a nonlinear fashion with respect to both, while the yield strength is practically unaffected. Ductility is also sensitive to variations in porosity and cooling rate, following a trend similar to that shown by the UTS. Increasing the porosity volume fraction above 0.5 % in the base alloy (unmodified and non-grain-refined) reduces the ductility to negligible levels. Strontium modification and grain refining allow for increases in porosity before the same level of degradation in ductility is observed. The cooling rate significantly influences the ductility at low hydrogen levels. At higher levels, however, hydrogen has a more pronounced effect (porosity related) on the drop in ductility.