Modification-related porosity formation in hypoeutectic aluminum-silicon alloys

Abstract

A series of aluminum-10 wt pct silicon castings were produced in sand molds to investigate the effect of modification on porosity formation. Modification with individual additions of either strontium or sodium resulted in a statistically significant increase in the level of porosity compared to unmodified castings. The increase in porosity with modification is due to the presence of numerous dispersed pores, which were absent in the unmodified casting. It is proposed that these pores form as a result of differences in size of the aluminum-silicon eutectic grains between unmodified and modified alloys. A geometric model is developed to show how the size of eutectic grains can influence the amount and distribution of porosity. Unlike traditional feeding-based models, which incorporate the effect of microstructure on permeability, this model considers what happens when liquid is isolated from the riser and can no longer flow. This simple "isolation" model complements rather than contradicts existing theories on modification-related porosity formation and should be considered in the development of future comprehensive models.

Eutectic grain size and strontium concentration in hypoeutectic aluminium–silicon alloys

“…It is well accepted that strontium additions have a detrimental effect on porosity formation [3][4][5][6][7][8][9][10][11][12][13]. The change in the eutectic interface morphology is one variable that has been associated with the increased porosity formation [12,[14][15][16] however the Fig.…”

“…Adapted from [44]. spatial distribution and evolution of eutectic grains is likely to play a more important role [11,13,[17][18][19]. Variations in eutectic grain size may also affect the mechanical properties as well as influence the formation of casting defects such as segregation, hot-tearing and porosity formation thus indicating these problems may be minimised by controlling the eutectic grain size.…”

Three-dimensional characteristics of Fe-rich intermetallics in gravity-cast Al-6Si alloys

“…Terzi et al [3] observed the growth of β-Fe intermetallic in the Al-8Si-4Cu-0.8Fe alloy, and found the β-Fe intermetallic branching during growth. In addition, researchers found that Fe-rich intermetallics have an important effect on micropores [4][5][6][7][8][9] . In this study, with the high-resolution 3D X-ray computed tomography, the Fe-rich intermetallics in the gravity-cast Al-6Si alloys with different Fe contents were investigated.…”

The influence of Cu, Mg and Ni on the solidification and microstructure of Al-Si alloys

“…Cu and Mg are often added to achieve strengthening [1,2]. Recent research has shown that changes in the nucleation frequency and macroscopic evolution of the Al-Si eutectic influence the final microstructure and formation of casting defects [3][4][5][6]. Impurity modification, alloy purity, the presence of solute and the solidification conditions are some of the factors that can influence eutectic nucleation, macroscopic growth patterns and the scale and morphology of the microstructure [7][8][9].…”