粉末床激光重熔条件下Ni-Sn 反常共晶微观组织的数值模拟

Eutectic is one of the most commonly observed solidification patterns, the growth morphology of which is important to materials properties. Anomalous eutectic is typically coarser and globular than lamellar eutectic, which is commonly observed during solidification of binary eutectic alloy, including deep undercooled melt and laser remelting process. The morphological evolution mechanism of anomalous growth is still unknown due to the lack of simulation evidence. During laser remelting process, the anomalous eutectic is sandwiched between lamellar eutectic at the bottom of melt pool. Comparing to deep undercooled melt, laser remelting has simpler temperature field distribution, which can be simplified into directional solidification. Thus, simulations of anomalous eutectic growth in laser remelting process are feasible. In the present work, the anomalous eutectic growth mechanism under laser remelting conditions was simulated using a low mesh induced anisotropy cellular automaton (CA) model. Firstly, a twodimensional lamellar eutectic CA model of CBr ₄ -C ₂ Cl ₆ alloy was established, and the morphological transition from 1λO to 2λO was simulated. The calculated results are in good agreement with experiments and phase field simulations. By setting the interface cells containing three phases (α, βand liquid phases), the model can continuously change the α and β phase volume fractions in the CA model, making it easier for the model to capture the instability of lamellar eutectic. Compared with the results of the phase field model, the intermediate 1λO-2λO state of oscillation instability of 1λO and 2λO which is consistent with the experimental results was calculated. Based on the above-mentioned binary eutectic CA model, the lamellar eutectic to anomalous eutectic transition at the bottom of the molten pool was simulated. Under the condition of initial low cooling rate, the fine lamellar eutectic is decoupled, it leads to the overgrowth of β-Ni ₃ Sn phase. During the subsequent accelerated cooling process, α-Ni nucleated in the liquid phase at the front of the solid/liquid interface, and theβ-Ni ₃ Sn phase wrapped around theα-Ni phase forming anomalous eutectic morphology. During the laser remelting process, there is indeed a rapid change of solidification rate from zero to scanning speed rate from the bottom to the top of the melt pool, and therefore coincides with the solidification conditions of the variable pulling velocity used in the CA simulations.