Hot-top direct chill casting assisted by a twin-cooling field: Improving the ingot quality of a large-size 2024 Al alloy

Abstract

Microstructure inhomogeneity and negative segregation have long been challenges for large-size alloy ingots, directly affecting the downstream processing and final performance of products. Here, we used 2024 aluminum alloy as a model alloy to propose a technique, named double-cooling field casting, i.e., one 2024 Al alloy rod (Φ20 mm) at room temperature was introduced into the melt along the central axis of the hot-top with the protection of a thermal-insulation tube during the direct chill (DC) casting process of a Φ300 mm 2024 Al alloy ingot. The results show that the introduction of the same alloy solid insert has a remarkable influence on refining grains in the center region of the ingot, reducing negative centerline segregation and decreasing the depth of the center part of the sump. With the application of the 2024 Al insert, the mean size of equiaxed grains at the center part of the ingot decreased from 1204 ± 132 μm to 721 ± 69 μm. The relative deviation of the Cu and Mg main solutes reduced from -0.062 and -0.054 to -0.03 and -0.024, respectively, and the sump depth decreased from 280 mm to 242 mm. Moreover, the shape of the solidification front was changed from ‘V’-shaped to ‘W’-shaped. The ingot quality was thus improved, mainly arising from the dissolution of the cold 2024 Al insert at a proper position of the hot-top counteracting some latent heat of solidification of the ingot, dissipating the heat of the central part of the hot-top by conducting the 2024 Al insert to the outside, and providing extra-nuclei from the unmolten α-Al particles of the insert.