Observation of equiaxed dendrite growth and motion in Al-Cu alloy using 4D-CT

Time-resolved computed tomography (4D-CT) was performed to observe the formation process of equiaxed grain structure in Al-5mass%Cu alloy refined by inoculant TiB 2 particles. The temporal resolution of as short as 1 s was achieved using a high-brilliance X-ray beam at a beamline BL20B2 of SPring-8 (Hyogo, Japan). A filtering method based on a phase field model was used to improve the quality of reconstructed images, evaluating equiaxed grain motion in conjunction with their dendritic growth. Nucleation events of α-Al was followed by the settlement of equiaxed grains. The moving velocity in the vertical direction was first −100 μm/s and then reduced to zero due to sedimentation at the bottom of the specimen as solidification proceeded. The solid fraction was 0.15 when the observation region was filled with stacked equiaxed grains. No nucleation events occurred in the remaining liquid phase between stacked equiaxed grains. 4D-CT observation demonstrates that the number and arrangement of equiaxed grains in the early stage of solidification determines the grain size distribution in the as-cast microstructure.