Precipitation of β-Al₅FeSi Phase Platelets in Al-Si Based Casting Alloys

The aim of the present work has been to investigate the factors affecting the precipitation of the β-Al₅FeSi iron intermetallic phase of directionally solidified A356- and 319-type alloys as a function of the iron content, a Sr addition of ~250 ppm, and superheating or cooling rates. The iron levels selected varied from 0.12 to 0.8 wt pct and cover the range of Fe levels in commercial casting alloys. The use of an end-chill mold provided different cooling rates along the height of the same casting, with dendritic arm spacing values that varied from ~15 to 85 μm, corresponding to levels of 5, 10, 30, 50, and 100 mm above the chill end. The microstructure and phase identification were monitored using optical microscopy, scanning electron microscopy (SEM), thermal analysis, and electron probe microanalysis (EPMA) techniques. An image analyzer was used in conjunction with the optical microscope for quantification purposes. The results showed that, for the range of cooling rates covered in the present study, the highest cooling rate (at a 5-mm distance from the chill) is the more significant parameter in controlling the size and distribution of theβ-Al₂FeSi intermetallic phase in the nonmodified 319 and 356 alloys. The addition of strontium leads to fragmentation of coeutectic or posteutectic β platelets. This effect diminishes with an increase in the iron concentration, and further strontium addition leads to the precipitation of Al₂Si₂Sr phase particles, instead.