Influence of iron on castability and properties of aluminium silicon alloys: literature review

The castability and properties of AI-Si based foundry alloys are affected by the inevitable introduction of impurity elements during melting and casting. Iron has consistently emerged as the most common impurity element that generally imparts negative influences on the soundness and mechanical properties of these alloys. It leads to the formation of iron-bearing intermetallics whose type, size, morphology and amount depend on the alloy composition, melt thermal history, cooling rate and post-casting heat treatments. These compounds, particularly their size and morphology, are the main culprits responsible for the significant influence of iron on the castability and mechanical properties. The most common of these compounds are beta-Al5FeSi with a needle/platelet morphology, alpha-Al15Fe3Si2 with a Chinese script or polyhedron morphology and pi-Al8Mg3FeSi6 also with a Chinese script morphology. The iron in the a phase can be substituted by Mn or Cr whereas the pi phase forms when Mg is present. The beta phase (the most detrimental phase) causes an increase in the hardness accompanied by a reduction in ductility, fracture toughness, impact energy, fatigue resistance and perhaps tensile and yield strengths of the alloy. The beta-Al5FeSi phase also reduces the alloy's feedability by physically blocking the interdendritic flow channels during the last stages of solidification. Furthermore, it is said to serve as an active pore nucleation site besides restricting pore growth and expansion. Iron also forms primary intermetallic compounds under certain conditions of melt processing and composition, particularly in the presence of Mn and Cr. These primary phases reduce the alloy's fluidity and machinability. Iron, however, reduces the tendency of the alloy to hot tear and to solder the dies during high-pressure die-casting. It can also grain refine aluminium and its alloys besides aiding the efficiency of Al-5Ti-1B grain refiners. (C) 2003 W. S. Maney Son Ltd.