On Porosity Formation in Al–Si–Cu Cast Alloys

The aim of the present work was to investigate the metallurgical parameters controlling porosity formation in Al–Si–Cu alloys, through a study of the microstructural characteristics of directionally solidified 319.2 alloys as a function of iron content, Sr addition (250 ppm), and solidification rate. The iron levels selected varied from 0.12 to 0.8 wt%, and cover the range of Fe levels found in commercial casting alloys. The use of an end-chilled mold provided different solidification rates along the height of the same casting, with DAS values that varied from ~15 to 85 μm, corresponding to levels of 5, 10, 30, 50 and 100 mm above the chill end. The amount of iron present in the alloy affects the size of the beta-iron Al5FeSi platelets and their distribution, particularly at low solidification rates. Addition of strontium leads to fragmentation of co-eutectic or post-eutectic beta-iron platelets.