Effect of trace Cr alloying on centerline segregations in sub-rapid solidified Al–Mg–Si (AA6061) alloys fabricated by twin-roll casting

Abstract Serious centerline segregations limited the application of twin-roll casting (TRC) to produce Al–Mg–Si alloys. The formation of centerline segregations was closely associated with solidification behaviors especially the flow of residual liquid. In the present study, we chose 0.07 wt% Cr as grain refiner of Al–1.2Mg–0.7Si–0.1Fe (AA6061) alloys, and we found the secondary dendrite arm spacing around the segregations sharply decreased from ~25 to ~9 μm. Grain refinement was probably attributed to the Cr addition induced formation of Al45Cr7 phases acting as heterogeneous nucleation sites for fcc-Al, which decreased the flowing tendency of residual liquid by providing extra flowing resistance, and heterogeneous nucleation for fcc-Al depleted the residual liquid. As a result, the quantity of residual liquid at the centerline was significantly reduced, and then the centerline segregations of TRC were significantly restrained by alloying with 0.07 wt% Cr. Tensile properties of solution treated (T4) and peak aged (T6) alloys were synchronously optimized. In particular, the uniform elongation of peak aged alloys increased from ~9% to ~12%, and the fracture elongation increased from ~15% to ~18%. Hence, this study may provide a simple and economic method to restrain the centerline segregations of TRC, and also improve the tensile properties.