Effect of Strontium Modification on Porosity Formation in A356 Alloy

Abstract

Sr-modified Al–Si alloys are promising for automotive and aerospace industrial applications. However, Sr modifier increases the porosity level and deteriorates the performance of the castings, which has confused foundrymen for several decades. Many researchers have studied the phenomenon, but there is no unified explanation. In this study, two types of Sr modifier (50 ppm and 250 ppm) are applied to explore the effect of Sr modification on the surface oxide film and the porosity of the castings. The results of optical emission spectrometer and X-ray photoelectron spectroscopy (XPS) reveal that the surface oxide film has some degree of Sr segregation. Combining the high-resolution XPS spectrums with electron probe microanalysis–wavelength-dispersive spectroscopic (EPMA–WDS), it can be found that a small amount of SrO is distributed on the Al2O3 film. Further analyzing the surface oxide film by scanning electron microscopy and EPMA mapping, it can be concluded that the compact Al2O3 film is fractured by congregated SrO and then some new Al2O3 films and oxide inclusions are formed in the cracks. Those can cause the increase in the porosity. And the result of reduced pressure test (RPT) shows that the densities of RPT samples are decreasing with the increase in Sr content. It means that Sr modification dose increase the porosity of the A356 alloy castings.