Beta Al5FeSi phase platelets-porosity formation relationship in A319.2 type alloys

Abstract

The effects of several variables on the precipitation of the β-Al5FeSi phase, and porosity formation in A319.2 alloys were examined in detail. Various techniques were used for microstructural characterization and phase identification, including optical and scanning electron microscopy, electron probe microanalysis coupled with energy-dispersive X-ray and wavelength-dispersive spectroscopy facilities, and thermal analysis. An image analyzer was used in conjunction with the optical microscope for quantification purposes. The results obtained showed that the amount of iron present in the alloy affects the size of the β-Al5FeSi platelets and their distribution, particularly at low solidification rates. Addition of strontium leads to fragmentation of co-eutectic or post-eutectic β-platelets. This effect diminishes with the increase in iron concentration, and further strontium addition leads to the precipitation of Al2Si2Sr phase particles, instead. The porosity observed at any given iron level is the result of the competition between fluidity and β-platelet size and depends on the permeability of the interdendritic regions as solidification proceeds. Although branching of the β-platelets leads to porosity formation, the platelets, on the other hand, also limit pore growth. In general, percentage porosity, maximum pore area and maximum pore length increase with an increase in the average maximum β-Al5FeSi platelet length. In the Sr-modified alloys, porosity formation is frequently associated with strontium oxides (particles or films), as well as the β-Al5FeSi platelets. These oxides (with a stoichiometric composition close to Al2SrO3) are formed during melting, due to the high oxygen affinity of strontium, and are difficult to remove via degassing.