Quantifying the effects of cooling rates and alloying additions on the microporosity formation in Al alloys

Microporosity in aluminum alloy castings is detrimental to the final mechanical properties such as fatigue life. Previous studies have shown that alloying additions in sand casting have a large impact on the percentage of porosity but no systematic studies on the effects of cooling rates and alloying additions have been reported. In this study, we investigate the porosity formation in three alloys with different cooling conditions using X-ray computed tomography (XCT). The microporosities of the three aluminum alloys at different cooling rates were visualized and statistically analyzed. When the cooling rate increased from 0.7 to 4.2 degrees C/s, the average equivalent diameter of microporosity in Al-7.3Si-1.5Cu decreased from 149.8 mu m to 61.3 mu m, with a size reduction of about 60 %; At a cooling rate of 0.7 degrees C/s, the average equivalent diameter of microporosity increased nearly three times with the increase of Si content to 7.3 wt.%. Comparing to three-dimensional (3D) cellular automata (CA) model predictions of porosity evolution in Al-Cu and Al-Si-Cu alloys, it was found that high cooling rates can increase the nucleation rate of microporosity and adding Cu and Si will give rise to the low solubility of hydrogen in the interdendritic liquid region during solidification. Therefore, robust casting components can be achieved by controlling the chemistry and applying non-equilibrium solidification conditions which change the interdendritic liquid flow and hydrogen supersaturations.