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Particle induced visible light absorption enhancement on Al–Cu–Mg alloy fatigue fracture surfaces has been studied. By the calculations of three possible structures in optical dark areas (ODAs) in Al–Cu–Mg alloys, the light absorption abilities of Cu nanoparticles and Cu/Al2O3 particles are clearly revealed. Based on the calculations, the formation mechanism of ODA can be mainly attributed to the formation of Cu nanoparticles and Cu/Al2O3 particles, which lead to strong light absorption enhancement in the visible spectrum. In detail, Cu nanoparticles with larger radii exhibit higher light absorption. In addition, the formation of Cu/Al2O3 particles can further enhance the light absorption of Cu nanoparticles in ODA, which coincide well with the experimental results. The cathodic protection method is proved as an efficient way for avoiding the formation of ODA, and thus, the fatigue life of Al–Cu–Mg alloys is also greatly improved. This work should provide deeper insights into the formation mechanism of ODA in Al–Cu–Mg and other Cu contained Al alloys with high-cycle loadings.

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.