采用低网格各向异性元胞自动机(cellular automaton,CA)模型研究了激光重熔条件下的反常共晶生长机制。为了验证模型的可靠性,建立了二维层片规则共晶CA 模型,针对CBr₄-C₂Cl₆共晶合金,模拟了1λO失稳形态向2λO失稳形态转变过程,计算结果与实验、相场模拟结果吻合。模型通过设定含三相(α、β和液相)的界面元胞,使CA模型中α和β相体积分数能够连续变化,从而更易于捕捉二维层片共晶的失稳过程。与相场模拟结果相比,本工作计算得到1λO-2λO失稳形态,即1λO和2λO失稳的中间状态,并与实验结果吻合。在上述二元共晶CA模型基础上,对Ni-Sn合金粉末床激光重熔条件下,熔池底部出现的从规则层片状向非规则反常共晶组织的转变过程进行模拟研究,发现在初始低冷却速率条件下,细小的层片共晶发生失稳,即β-Ni₃Sn相超越α-Ni相,形成β-Ni₃Sn单相定向生长,在后续加速冷却过程中,固/液界面前沿液相中α-Ni相形核,并发生β-Ni₃Sn相包裹α-Ni相生长形成反常共晶组织。激光重熔过程中,由熔池底部到顶部的凝固过程中确实存在一个由凝固速率为零到接近扫描速率的快速变化过程,因此与CA模拟采用的变抽拉速率的凝固条件吻合。

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 two-dimensional lamellar eutectic CA model of CBr₄-C₂Cl₆ 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, β and 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-Ni₃Sn phase. During the subsequent accelerated cooling process, α-Ni nucleated in the liquid phase at the front of the solid/liquid interface, and the β-Ni₃Sn phase wrapped around the α-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.