Significant refinement and spheroidization of harmful Fe-rich phases in Al–Cu–Mg alloys by small Sn addition

Solidification phases of conventional cast aerospace Al–Cu–Mg alloys and their evolution during homogenization were comprehensively studied. Significant refinement and spheroidization of solidification phases were found by the addition of 0.04, 0.15, and 1.00 mass % Sn to Al–Cu–Mg alloys. The soluble Sn component is dissolved in the Al matrix as solute atoms, while Sn in excess of the solid solubility segregates in the intradendrites in the form of Sn pools with a size (diameter) of one-half to several micrometers. These solute Sn atoms and Sn pools can capture Cu, Mg, and Fe atoms or hinder their diffusion into the residual liquid phase in the interdendritic region during solidification. Owing to the Sn addition, the contents of the Al2Cu, Al2CuMg, and Al7Cu2Fe phases in the interdendritic region are greatly reduced, thereby leading to significant refinement and spheroidization of these solidification phases. The formation of proeutectoid Mg2Sn phases is also found to be effective for the refinement of solidification phases.