Effects of Sc and Y on the As-Cast Microstructure of AA6086

AA6086 is a novel Al–Mg–Si alloy that contains more Si and Cu than AA6082, as well as 0.15–0.25 wt% Zr. This new alloy is distinguished by its improved tensile strength and hardness, with a convenient ductility (max. UTS = 490 MPa, Brinell hardness 140 HBW, elongation at fracture 10%). Additions of rare-earth elements can further improve these properties. In order to clarify their influence, we prepared several alloys containing either individual or combined additions of Sc and Y in the range 0.2–1.0% Sc and 0.1–1.0% Y. The alloys were melted and then cast into copper moulds. They were investigated by optical microscopy and SEM with EDS. There are several microstructural constituents in the as-cast state. The additions of Sc and Y resulted in the appearance of scandium- and yttrium-rich phases. The effects of Sc and Y on the grain size were determined using the ASTM E112 standard. With small additions of Sc (0.2 wt%) we already detected a difference in the grain size for the range of 1.5 G in comparison with AA6086. Larger additions of Sc (1 wt%) greatly increased the grain-size number, up to 4.5 G. In the centre of the grains were Al₃(Sc, Zr) particles, which are effective grain refiners. However, the grain-refinement effects were minor when we used Y. The additions of Sc and Y also affected the formation of α-AlMnSi and the solidification sequence, which will be explained in detail.