液态亚共晶 Al-4.5%Si 合金中定向枝晶生长与组织调控

Liquid hypoeutectic Al-4.5%Si alloy was directionally solidified with a novel Bridgman type apparatus which was featured by the upward motion of heating furnace. A broad growth velocity range covering three orders of magnitude from 0.1 μm/s up to 100 μm/s was applied to realize the whole structural evolution process from planar interface through cellular morphology until dendritic growth. In order to explore the dominant influences of growth velocity, the liquid temperature gradient ahead of solid/liquid interface was maintained at the constant level of 200 K/cm during all experiments, which shows that the critical growth velocity to initiate S/L interface instability is 0.43 μm/s, whereas the growth velocity threshold to induce cell-dendrite transition is 3.3 μm/s. The theoretical calculation that the constitutionally supercooled zone extends at 2.6−10 mm in front of S/L interface, where the maximum supercooling varies in the range of 2−193.8 K. In contrast, the actual mushy zone length during directional dendrite growth is in the range of 2.8−6.2 mm. The experimental results validate the theoretical predictions of KGT columnar dendrite growth model for essential parameters as dendrite tip radius, tip temperature and tip compositions. Both the Hunt model for primary dendrite spacing and the Kurz-Fisher model for secondary arm spacing agree well with actual experiments. Nevertheless, the (α(Al) +Si) eutectic growth kinetics within interdendritic spaces of mushy zone only exhibits qualitative consistency with experimental data.