STIR CAST AND EXTRUDED AL-7SI-0.3MG ALLOY CONTAINING IRON AND BERYLLIUM

Stirring an Al-7Si-0.3Mg casting alloy during solidification inhibits the creation of the dendritic structure such that the primary alpha-Al forms as spheroids and other compact irregular shapes owing to thixot ropic behaviour. The extruded stir cast alloy exhibits improved mechan ical properties, particularly high elongation (15%) and impact energy (14.5 J) but with slightly lower ultimate tensile strength (270 MPa). Ir is suggested that the superior ductility and impact properties of t he extruded stir cast alloy compared with the standard product is a re sult of the presence of alternately arranged bands of alpha-Al and eut ectic silicon, such that cracks nucleated preferentially on silicon pa rticles are arrested as they approach the adjacent alpha-Al. The prese nce of iron in the alloy results in the formation of coarse beta-FeSiA l5 phase associated with poor mechanical properties. However, trace ad ditions of beryllium lead to formation of a new (Be-Fe)-BeSiFe2Al8 pha se which mitigates solve of the deleterious effects of the iron. (C) 1 997 The Institute of Materials.