Stir cast and extruded Al-7Si-0.3Mg alloy containing iron and beryllium

Abstract

Stirring an Al-7Si-0.3Mg casting alloy during solidification inhibits the creation of the dendritic structure such that the primary α-Al forms as spheroids and other compact irregular shapes owing to thixotropic behaviour. The extruded stir cast alloy exhibits improved mechanical properties, particularly high elongation (15%) and impact energy (14.5 J) but with slightly lower ultimate tensile strength (270 MPa). It is suggested that the superior ductility and impact properties of the extruded stir cast alloy compared with the standard product is a result of the presence of alternately arranged bands of α-Al and eutectic silicon, such that cracks nucleated preferentially on silicon particles are arrested as they approach the adjacent α-Al. The presence of iron in the alloy results in the formation of coarse β-FeSiAl₅ phase associated with poor mechanical properties. However, trace additions of beryllium lead to formation of a new (Be-Fe)-BeSiFe₂Al₈ phase which mitigates some of the deleterious effects of the iron.