1 State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China 2 Key Laboratory of Metal High Performance Additive Manufacturing and Innovative Design, Ministry of Industry and Information Technology, Northwestern Polytechnical University, Xi'an 710072, China 3 School of Materials Science and Engineering, Luoyang Institute of Science and Technology, Luoyang 471000, China
Cite this article:
Lei WEI, Yongqing CAO, Haiou YANG, Xin LIN, Meng WANG, Weidong HUANG. Numerical Simulation of Anomalous Eutectic Growth of Ni-Sn Alloy Under Laser Remelting of Powder Bed. Acta Metall Sin, 2018, 54(12): 1801-1808.
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.
Fund: Supported by National Key Research and Development Program of China (No.2016YFB1100100), National Natural Science Foundation of China (Nos.51604227, 51323008, 51475380 and 51271213), National Basic Research Program of China (No.2011CB610402), National High Technology Research and Development Program of China (No.2013AA031103) and Research Fund of the State Key Laboratory of Solidification Processing (NWPU) (No.128-QP-2015)
Table 1 Thermal physical parameters of CBr4-C2Cl6 and Ni-Ni3Sn eutectic alloys[17,28]
Fig.1 CA simulation of 1λO to 1λO-2λO to 2λO pattern for CBr4-C2Cl6 eutectic growth pattern in dimensionless lamellar spacing Λ=2.6, temperature gradient G=8000 K/m, pulling velocity V=2 μm/s, where the blue phase is α, the red phase is β
Fig.2 Transformation of fine lamellar eutectic to anomalous eutectic and coarse lamellar eutectic for Ni-32.5%Sn alloy calculated by CA model, where the blue phase is α-Ni, the red phase is β-Ni3Sn, the yellow green phase is liquid, under pulling velocity changed from 2 μm/s to 800 μm/s in 0.007 s, and kept 800 μm/s (a) t=0.001 s, V=114 μm/s (b) t=0.002 s, V=228 μm/s (c) t=0.006 s, V=684 μm/s (d) t=0.007 s, V=800 μm/s (e) t=0.008 s, V=800 μm/s (f) t=0.009 s, V=800 μm/s (g) t=0.01 s, V=800 μm/s (h) t=0.11 s, V=800 μm/s (i) t=0.013 s, V=800 μm/s
Fig.3 Epitaxial growth of fine lamellar eutectic for Ni-32.5%Sn alloy calculated by CA model under constant pulling velocity of V=2000 μm/s
Fig.4 Anomalous eutectic observed at the bottom of melt pool during laser remelting
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