Defect Susceptibility of Tensile Properties to Microporosity Variation in High-Pressure Die-Cast Aluminium Alloy Controlled by Gas Bubbling Flotation Treatment

摘要

This study aims to investigate the variability in the tensile properties of high-pressure die-cast aluminium alloy controlled by the gas bubbling filtration or flotation (GBF) process in terms of the defect susceptibility of tensile properties to the variation in the overall microporosity level, including bifilm oxide and pre-existing microvoids. Test specimens were fabricated by adopting a high-pressure die-cast process of the cold chamber type at the production scale using A384.0 aluminium alloy. The volumetric porosity of cast specimens gradually decreased with GBF treatment, even though the fractographic porosity tended to decrease only under a specific GBF condition, with a remarkable microstructural refinement. The degassing treatment of 5 min could achieve a remarkable grain refinement, and further, this optimal condition could induce a practical improvement in the defect susceptibility of tensile properties to microporosity variation as well as the nominal level of tensile properties. The main reason for grain refinement in GBF treatment is the dispersion of small-sized pro-eutectic solid particles by fragmentation of abnormally big-sized dendrites, which are preferentially formed by an undercooling effect in a region around the impeller and gas inlet. Additionally, the improvement in melt quality by degassing action for dissolved hydrogen gas is practically nullified through the coarsening of pro-eutectic solid particles as well as the additional inclusion of bifilm oxide by excessive GBF treatment.