Dendritic grain growth simulation in weld molten pool based on CA‐FD model

The finite difference model is coupled with a cellular automaton (CA) model to simulate dendrite growth in TIG (tungsten inert‐gas) weld molten pool of nickel‐based alloys. Based on the macro finite difference computation of the thermal field of the whole weldment, the micro CA model is used to simulate grain formation process in 2‐dimension area within weld molten pool. The results illustrate that the complicated thermal field and solute field can lead to complex grain morphologies in weld molten pool. The primary dendrite spacing changes during welding solidification process and the final primary dendrite spacing is decided by the thermal field, solute field and other parameters. And it is indicated that the grain boundary segregation has important relationship with welding speed. The grain boundary segregations become more severe as the welding speed increases when the other parameters are kept unchanged. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electroless deposition of Cu dendrites decorated with ZnO rods

In this paper, copper dendrites decorated with ZnO rods have been electrolessly deposited on brass substrate by a simple galvanic replacement method. SEM images show that these copper dendrites possess a pronounced trunk and highly ordered branches distributed on both sides of the trunk. Meanwhile, both the trunk and branches are decorated with ZnO rods. The diffusion‐limited aggregation (DLA) model has been used to explain the fractal growth of Cu dendritic structures. This method provides a facile route to the synthesis of copper dendrites with ZnO, which can be extended to the preparation of other forms three‐dimensional (3‐D) metal structures or metal/ZnO composites by modifying electrolyte parameters such as composition, concentration, pH and temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

枝晶生长和气泡形成的数值模拟

本文建立了二维的格子玻尔兹曼方法-元胞自动机(lattice Boltzmann method-cellular automaton, LBM-CA)耦合模型, 对凝固过程中枝晶生长和气泡形成进行模拟研究. 本模型采用CA方法模拟枝晶的生长, 根据界面溶质平衡法计算枝晶生长的驱动力. 采用基于Shan-Chen多相流的LBM模拟气泡在液相中的生长和运动. 在LBM-CA的耦合模型中包含了固-液-气三相之间的相互作用. 应用Laplace定理和模拟气-液-固三相之间的润湿现象对模型进行了验证. 应用所建立的LBM-CA耦合模型模拟研究了气-液相互作用系数对单气泡生长的影响. 发现单气泡的生长速度和平衡半径随气-液相互作用系数的增大而增大. 定向凝固过程中枝晶和气泡生长的模拟结果再现了枝晶的择优生长、 气泡的优先形核位置、气泡的长大、合并、在枝晶间受挤变形以及在液相通道中的运动等物理现象, 与实验结果符合良好. 此外, 初始气体含量越高, 凝固结束时气泡的体积分数也相对较高. 本模型的模拟结果可以揭示在凝固过程中气泡形核、 生长和运动演化以及与枝晶生长相互作用的物理机理.     

Dendritic and eutectic growth in Sb<Subscript>60</Subscript>Ag<Subscript>20</Subscript>Cu<Subscript>20</Subscript> ternary alloy

The rapid solidification of Sb60Ag20Cu20 ternary alloy was realized by high undercooling method, and the maximum undercooling is up to 142 K (0.18TL). Within the wide undercooling range of 40-142 K, the solidified microstructures are composed of (Sb), θand ε phases. High undercooling enlarges the solute solubility of (Sb) phase, which causes its crystal lattice to expand and its crystal lattice constants to increase. Primary (Sb) phase grows in two modes at small undercoolings non-faceted dendrite growth is the main growth form; whereas at large undercoolings faceted dendrite growth takes the dominant place. The remarkable difference of crystal structures between (Sb) and θphases leads to (θ Sb) pseudobinary eutectic hard to form, whereas strips of θform when the alloy melt reaches the (θ Sb) pseudobinary eutectic line. The cooperative growth of θand ε phases contributes to the formation of (ε θ) pseudobinary eutectic easily. In addition, the crystallization route has been determined via microstructural characteristic analysis and DSC experiment.     

定向凝固过程中枝晶侧向分枝生长行为与强制调控规律

采用相场法数值模拟研究了定向凝固过程中随机噪声条件下枝晶侧向分枝生成行为与强制扰动条件下侧向分枝调控规律. 模拟结果表明: 随机噪声条件下, 侧向分枝整体上并无规则性, 但产生频率存在一定分布范围, 且在一定时间段内会出现生成频率一致且具有极强相关性的一组侧向分枝, 即波包; 不同波包之间不具有相关性, 但不同波包内部的侧枝生成频率基本相同, 且与侧枝整体频谱曲线峰值位置处的频率基本相当; 强制周期扰动条件下, 当扰动频率处于侧向分枝整体生成频率范围内时, 可激发枝晶产生规则侧向分枝, 且扰动频率与波包内侧枝生成频率一致时侧向分枝最发达. 研究结果可为向定向凝固枝晶形态的调控提供理论指导.     

Growth of tertiary dendritic arms during the transient directional solidification of hypoeutectic Pb–Sb alloys

Despite the importance of a complete characterization of dendritic patterns in castings, the availability of studies on the development of tertiary dendrite arms is scarce in the literature. In the present study, the tip cooling rate, local solidification time, primary and tertiary dendrite arm spacings have been determined in Pb–Sb alloys castings directionally solidified under unsteady-state heat flow conditions. The alloys compositions experimentally examined are widely used in the as-cast condition for the manufacture of positive and negative grids of lead-acid batteries. The initial growth of tertiary dendritic arms from the secondary branches was found to occur only for a Pb–3.5 wt% Sb alloy at cooling rates in the range 0.4–0.2?K/s, with no evidence of this spacing pattern for Pb–Sb alloys having lower solute content. Tertiary dendritic branches have been observed along the entire casting lengths for alloys of the Pb–Sb hypoeutectic range having compositions higher than 4.0 wt% Sb. It is shown that a power function experimental law with a characteristic ?0.55 exponent is able to characterize the tertiary spacing evolution with the solidification cooling rate for alloys compositions ≥4.0 wt% Sb. The only exception was the Pb–3.5 wt% Sb alloy for which λ ₃ exhibited significant lower values when compared with the experimental values obtained for the other Pb–Sb alloys for a same solidification cooling rate.     

Phase field simulation of the columnar dendritic growth and microsegregation in a binary alloy

This paper applies a phase field model for polycrystalline solidification in binary alloys to simulate the formation and growth of the columnar dendritic array under the isothermal and constant cooling conditions. The solidification process and microsegregation in the mushy zone are analysed in detail. It is shown that under the isothermal condition solidification will stop after the formation of the mushy zone, but dendritic coarsening will progress continuously, which results in the decrease of the total interface area. Under the constant cooling condition the mushy zone will solidify and coarsen simultaneously. For the constant cooling solidification, microsegregation predicted by a modified Brody- Flemings model is compared with the simulation results. It is found that the Fourier number which characterizes microsegregation is different for regions with different microstructures. Dendritic coarsening and the larger area of interface should account for the enhanced Fourier number in the region with well developed second dendritic arms.     

In situ observation of NaBi(WO4)2 dendrite growth and the study on microstructure of solidification

The dendrite growth process of transparent NaBi(WO4)2 with small prandtl and high melting point was studied by using the in-situ observation system. According to the dynamic images and detailed information, there are two kinds of restriction effect on the dendrite growth, the competition between arms and branches and the convection in the melt. The dendrite growth rate was time dependent, and the rate of arm growth reached the maximum 5.8 mm/s in the diffusive-advective region and rapidly decreased in the diffusive-convective region. The growth rate of branch had the same change trends as the arm's. Based on the EPMA-EDS data of solidification structure of quenched NaBi(WO4)2 melt, it was found that there were component differences from stoichiometric concentration in the melt near the interface during the growth process.     

液态三元Fe-Cr-Ni合金中快速枝晶生长与溶质分布规律

采用落管方法实现了液态三元Fe-Cr-Ni合金的深过冷与快速凝固,合金液滴的冷却速率和过冷度均随液滴直径的减小而迅速增大.两种成分合金近平衡凝固组织均为粗大板条状α相.在快速凝固过程中,不同直径Fe_(81.4)Cr_(13.9)Ni_(4.7)合金液滴凝固组织均为板条状α相,其固态相变特征很明显,随着过冷度增大,初生δ相由具有发达主干的粗大枝晶转变为等轴晶.Fe_(81.4)Cr_(4.7)Ni_(13.9)合金液滴凝固组织由α相晶粒组成,随着过冷度增大,初生γ相由具有发达主干的粗大枝晶转变为等轴晶,其枝晶主干长度和二次分枝间距均显著下降,晶粒内溶质的相对偏析度也明显减小,溶质Ni的相对偏析度始终大于溶质Cr.理论计算表明,与γ相相比,δ相枝晶生长速度更大.在实验获得的过冷度范围内,两种Fe-Cr-Ni合金枝晶生长过程均由热扩散控制.     

In situ observation of NaBi(WO4)2 dendrite growth and the study on microstructure of solidification

The dendrite growth process of transparent NaBi(WO₄)₂ with small prandtl and high melting point was studied by using the in-situ observation system. According to the dynamic images and detailed information, there are two kinds of restriction effect on the dendrite growth, the competition between arms and branches and the convection in the melt. The dendrite growth rate was time dependent, and the rate of arm growth reached the maximum 5.8 mm/s in the diffusive-advective region and rapidly decreased in the diffusive-convective region. The growth rate of branch had the same change trends as the arm’s. Based on the EPMA-EDS data of solidification structure of quenched NaBi(WO₄)₂ melt, it was found that there were component differences from stoichiometric concentration in the melt near the interface during the growth process. Supported by the National Natural Science Foundation of China (Grant No. 50331040) and the Innovation Funds from Shanghai Institute of Ceramics, Chinese Academy of Sciences (Grant No. SCX0623)