1种新的模拟铝合金铸态组织的随机性方法

采用1种新的随机性模拟方法并与宏观传热过程相耦合，对铝合金的微观组织进行了模拟研究。计算中采用了简化的枝晶形状。建立了简化的枝晶形状的物理与数学模型，并提出了1种形状函数来描述晶粒的外部轮廓。基于简化的晶粒形状，采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程。连续形核模型被用来处理异质形核现象，在生长模型中则考虑了枝晶尖端生长动力学和择优生长方向。开发了等轴晶生长的模拟程序，并进行了二维计算。进行了浇注金属型和砂型试样的模拟验证实验。结果表明，对两种不同的工艺，所得到的晶粒组织不同，金属型铸造时得到的晶粒尺寸较小，砂型铸造的较大。模拟结果与金相观察结果相符。     

铝合金枝晶生长的数值模拟

提出一种新的模拟铝合金枝晶生长的随机性方法，建立了简化的枝晶形状的物理与数学模型，并提出了一种形状函数来描述晶粒的外部轮廓，基于简化的晶粒形状，采用坐标变换技术来描述过冷液相中晶粒的生长过程及其对周围节点的捕获过程，根据上述思想，开发了等轴晶生长的模拟程度，并进行了二维计算，为了验证模拟结果，浇注了金属型和砂型试样，模拟结果表明，金属型铸造晶粒尺寸较小，砂型铸造的较大，模拟结果与金相观察结果相符。     

焊接熔池凝固过程组织演变模拟

以枝晶生长动力学和晶粒生长能量最小原理为基础,建立了宏观传热与微观传质、形核、生长相耦合的焊接熔池CAMC晶粒生长二维数学模型.模型以CA方法模拟晶粒生长主干,MC在内部辅助晶粒生长以体现枝晶分枝机制,同时考虑非均匀形核对熔池结晶的影响因素,模拟了焊接熔池组织形成过程.结果表明,CAMC模型能够定量地描述熔池晶粒数目、尺寸和形貌演变,可以较准确地反映焊接熔池微观组织结构和熔池凝固过程中晶粒择优生长、柱状晶向等轴晶转换等物理机制.     

Modeling equiaxed solidification with melt convection and grain sedimentation—I: Model description

Part I of this two part investigation presents a modified volume-averaged equiaxed solidification model which accounts for nucleation, globular grain growth, globular-to-dendritic transition, dendritic growth, formation of extra- and interdendritic eutectic, grain transport and melt convection, and their influence on microstructure and macrosegregation. Globular grain growth is governed by diffusion around a spherical grain. For the dendritic growth, a “natural” grain envelope smoothly enclosing the primary and secondary dendrite tips is assumed to separate the interdendritic melt from the extradendritic melt. The solid dendrites and interdendritic melt, confined in the “natural” grain envelope, combine to form a dendritic grain. Two “hydrodynamic” phases are considered: the extradendritic melt and the equiaxed grains; and three thermodynamic phase regions are distinguished: the solid dendrites, the interdendritic melt and the extradendritic melt. The velocities of the hydrodynamic phases are solved with a two-phase Eulerian approach, and transport of the mass and solute species of each thermodynamic phase region are considered individually. Growth kinetics for the grain envelope and the interdendritic melt solidification are implemented separately. Simplification of the grain dendritic morphology and treatment of the non-uniform solute distribution in the interdendritic melt region are detailed. Illustrative modeling results and model verification are presented in Part II.     

Three-dimensional modeling of equiaxed dendritic growth on a mesoscopic scale

A novel mesoscopic modeling technique has been developed to simulate the unsteady growth of multiple equiaxed dendritic grains into a supercooled melt of a pure substance. In the model, the numerical calculation of the temperature field in the supercooled melt between the grains is coupled with a stagnant-film model for dendrite tip growth, such that without resolving individual dendrite arms the evolution of the grain envelope and the internal solid fraction can be predicted. The simulations are in good agreement with experiments for the growth of a single dendritic grain of the model substance succinonitrile. The model is then applied to simulate the growth of various configurations of up to 14 strongly interacting grains. The results indicate that the use of local analytical solutions in numerical calculations is a viable technique for simulating large-scale dendritic growth phenomena.     

焊接熔池快速凝固过程的微观组织演化数值模拟

基于晶粒形成原理和枝晶生长动力学特点,建立了焊接熔池凝固过程中的形核、枝晶生长、溶质再分配及扩散的二维数学物理模型,对焊接熔池快速凝固过程中柱状晶向等轴晶的转变以及不同的冷却速度对这一转变过程的影响进行了模拟.结果表明,焊接熔池在快速冷却凝固过程中,溶质再分配与扩散明显;柱状晶向等轴晶转变时,熔池中心等轴晶凝固排出的溶质使柱状晶尖端浓度急剧升高,抑制了柱状晶的生长;冷却速度越大,柱状晶越容易向等轴晶转变,且转变所需时间越短.     

镁合金等轴及柱状树枝晶模拟(英文)

基于CA方法,开发了模拟具有HCP晶体结构的镁合金的枝晶生长模型,生长动力学通过求解传输方程获得。通过定义特殊的正方网格CA邻居单元及采用由BELTRAN-SANCHEZ和STEFANESCU提出的具有立方晶体结构的金属凝固过程中枝晶生长的捕获规则,实现了具有不同生长取向的镁合金枝晶生长的建模及仿真。对镁合金等轴树枝晶和定向凝固条件下柱状树枝晶的生长进行模拟。对模拟结果与实验结果及发表的实验结果进行对比,从而验证了模型的可靠性。     

Multiscale modelling and simulation of single crystal superalloy turbine blade casting during directional solidification process

As the key parts of an aero-engine,single crystal（SX）superalloy turbine blades have been the focus of much attention.However,casting defects often occur during the manufacturing process of the SX turbine blades.Modeling and simulation technology can help to optimize the manufacturing process of SX blades.Multiscale coupled models were proposed and used to simulate the physical phenomena occurring during the directional solidification（DS）process.Coupled with heat transfer（macroscale）and grain growth（meso-scale）,3D dendritic grain growth was calculated to show the competitive grain growth at micro-scale.SX grain selection behavior was studied by the simulation and experiments.The results show that the geometrical structure and technical parameters had strong influences on the grain selection effectiveness.Based on the coupled models,heat transfer,grain growth and microstructure evolution of a complex hollow SX blade were simulated.Both the simulated and experimental results show that the stray grain occurred at the platform of the SX blade when a constant withdrawal rate was used in manufacturing process.In order to avoid the formation of the stray crystal,the multi-scale coupled models and the withdrawal rate optimized technique were applied to the same SX turbine blade.The modeling results indicated that the optimized variable withdrawal rate can achieve SX blade castings with no stray grains,which was also proved by the experiments.     

Columnar and Equiaxed Solidification of Al-7 wt.% Si Alloys in Reduced Gravity in the Framework of the CETSOL Project

During casting, often a dendritic microstructure is formed, resulting in a columnar or an equiaxed grain structure, or leading to a transition from columnar to equiaxed growth (CET). The detailed knowledge of the critical parameters for the CET is important because the microstructure affects materials properties. To provide unique data for testing of fundamental theories of grain and microstructure formation, solidification experiments in microgravity environment were performed within the European Space Agency Microgravity Application Promotion (ESA MAP) project Columnar-to-Equiaxed Transition in SOLidification Processing (CETSOL). Reduced gravity allows for purely diffusive solidification conditions, i.e., suppressing melt flow and sedimentation and floatation effects. On-board the International Space Station, Al-7 wt.% Si alloys with and without grain refiners were solidified in different temperature gradients and with different cooling conditions. Detailed analysis of the microstructure and the grain structure showed purely columnar growth for nonrefined alloys. The CET was detected only for refined alloys, either as a sharp CET in the case of a sudden increase in the solidification velocity or as a progressive CET in the case of a continuous decrease of the temperature gradient. The present experimental data were used for numerical modeling of the CET with three different approaches: (1) a front tracking model using an equiaxed growth model, (2) a three-dimensional (3D) cellular automaton–finite element model, and (3) a 3D dendrite needle network method. Each model allows for predicting the columnar dendrite tip undercooling and the growth rate with respect to time. Furthermore, the positions of CET and the spatial extent of the CET, being sharp or progressive, are in reasonably good quantitative agreement with experimental measurements.     

Numerical simulation of the effect of fluid flow on solute distribution and dendritic morphology

Abstract

The presence of bulk and interdendritic flow during solidification can alter the microstructure, potentially leading to the formation of defects. In this paper, a numerical model is presented for the direct simulation of dendritic growth in the presence of fluid flow in both liquid and mushy zones. The Navier–Stokes equations are solved for multiphase flow using a projection method. The energy conservation and solute diffusion equations are solved via a combined stochastic nucleation approach and finite difference solution to simulate dendritic growth. The predicted microstructures illustrate typical asymmetric dendritic growth behaviour under forced convection, which is consistent with prior similar simulations of a single dendrite during unconstrained growth (both 2D and 3D). The micromodel was coupled with a macromodel to investigate the effects of forced fluid flow on equiaxed dendritic growth and micro-segregation during vacuum arc remelting.     

Refinement mechanism of low voltage pulsed magnetic field on solidification structure of silicon steel

The grain refinement of a silicon steel solidified with a low voltage pulsed magnetic field (LVPMF) was investigated by experiments and modeling. The experiment results show that complete fine equiaxed grains are acquired by applying the LVPMF. The effects of process parameters, such as the melt cooling rate and superheating on the solidification structure, were also studied. Complete fine equiaxed grains can be obtained under a larger range of cooling rate or superheating when the LVPMF is applied. The magnetic force and the melt flow during solidification were modeled and simulated to reveal the grain refinement mechanism. The simulation results show that the LVPMF has a double role in electromagnetic convection and electromagnetic vibration on the alloy melt. We propose the refining mechanism: The melt vibration and convection can promote nucleus multiplication, which contributes to heterogeneous nucleation enhancement and leads to a high nucleation rate and grain refinement.     

基于一种改进CA模型的微观组织模拟

基于热传输、溶质传输等基本传输过程及晶粒生长物理过程,建立了单相合金凝固过程微观组织及枝晶形貌演化的三维数学模型,并提出了新的生长模型和单元捕获规则,新规则在三维立方有限差分单元的基础上保证了晶粒在各个随机的优先生长方向生长时的枝晶连续性,并较好的在三维立方网格中体现出了晶粒形貌.此外,模型中考虑了成分过冷、曲率过冷和晶粒随机各向异性生长等重要因素.数值模拟结果表明,所建立的数学模型能够合理描述晶粒沿任意角度生长的过程,温度场、溶质场和微观组织形貌的模拟计算结果合理.多晶核定向生长模拟结果表明,多晶核定向竞争生长达到稳定状态时,枝晶间的一次枝晶臂间距与初始形核数目无关.所建立的模型和晶粒捕获规则适用于进一步定向凝固微观组织演化的研究.     

Shape control and solidification microstructure of Al-Cu alloys produced by Fused Spinning Deposition method

Using a process called Fused Spinning Deposition (FSD) Al-Cu alloy melt flowing out through a small nozzle located at the bottom of the quartz tube container was rapidly solidified in a tube form on a rotating copper substrate. The fusion weldability between layers, shape stability and microstructures of the tube specimens were investigated as a function of alloy composition, superheat and rotation speed of the substrate. Good weldability between the layers and steady shape forming were achieved with smaller superheat and higher rotation speed. Better shape stability was obtained for hypo-eutectic alloys. The microstructure of the tube specimens subjected to the fused spinning deposition process was found to consist of fine equiaxed grains and the specimen exhibited tensile strength 1.5 times higher than metal-mould cast alloys with a fine columnar structure. The solidification analysis showed that the equiaxed grain structure is formed as a result of the cyclic remelting and solidification process.     

Microstructure of aluminum twin-roll casting based on Cellular Automation

Nucleation and growth model based on Cellular Automation（CA） incorporated with macro heat transfer calculation was presented to simulate the microstructure of aluminum twin-roll casting. The dynamics model of dendrite tip （KGT model） was amended in view of characteristics of aluminum twin-roll casting. Through the numerical simulation on solidification structure under different casting speeds, it can be seen that when the casting speed is 1.3 m/min, that is, under conditions of conventional roll casting, coarse columnar grains dominate the solidification structure, and equiaxed grains exist in the center of aluminum strip. When the casting speed continuously increases to 8 m/min, that is, under the conditions of thin-gauge high-speed casting, columnar grains in solidification structure all convert into equiaxed grains. Experimental and numerical results agree well.     

Application of cellular automaton–finite element model to the grain refinement of directionally solidified Al–4.15 wt% Mg alloys

Cellular automaton–finite element simulations and Bridgman experiments are used to study the grain refinement of directionally solidified Al–4.15 wt% Mg. The simulations can successfully map the conditions (velocity and temperature gradient) for columnar or equiaxed growth and account for variations in grain size in equiaxed structures. The simulations show that the latent heat released by dendritic solidification gives a quasi-isothermal zone interrupting the temperature gradient. This zone is important in limiting the degree of grain refinement which can be achieved, but also facilitates growth of equiaxed rather than elongated grains. The columnar-to-equiaxed transition is gradual, and its dependence on the addition level of refiner has been characterized. Hunt's model of the transition is supported by microstructural studies of quenched Bridgman samples.     

A three-dimensional cellular automaton model for simulation of dendritic growth of magnesium alloy

A numerical model based on the cellular automaton method for the three-dimensional simulation of dendritic growth of magnesium alloy was developed. The growth kinetics was calculated from the complete solution of the transport equations. By constructing a three-dimensional anisotropy model with the cubic CA cells, simulation of dendritic growth of magnesium alloy with six-fold symmetry in the basal plane was achieved. The model was applied to simulate the equiaxed dendritic growth and columnar dendritic growth under directional solidification, and its capability was addressed by comparing the simulated results to experimental results and those in the previously published works. Meanwhile, the three-dimensional simulated results were also compared with that of in two dimensions, offering a deep insight into the microstructure formation of magnesium alloy during solidification.     

Fe-C合金焊接熔池凝固过程CET转变的数值模拟

基于元胞自动机方法建立了枝晶生长的数值模型,应用该模型模拟了Fe-C合金焊接熔池凝固期间柱状晶向等轴晶(CET)转变过程中枝晶生长形貌与溶质浓度分布状况. 模拟过程主要考虑不同扰动振幅和冷却速率对柱状晶向等轴晶转变的影响. 结果表明,随着时间步长增大,晶粒数逐渐增多,最终趋于稳定值;当冷却速率增加时,生长速度增大,枝晶生长的越充分,显微偏析越严重,而CET转变所需时间越短;当扰动振幅增大时,一次枝晶生长的越细,二次、三次枝晶竞争越激烈.     

枝晶生长的数值模拟

对朱、洪已发展的一种微观尺度cellular automaton(CA)模型做了进一步改进．在改进的模型中用二元合金的Gibbs-Thomson方程建立固／液界面的平衡关系．考虑了动力学和表面能各向异性对枝晶择优生长方向的影响．应用改进的模型模拟了不同择优取向的单枝晶在过冷熔体中的自由生长、定向凝固过程中柱状晶的竞争生长以及等轴晶的演变过程．模拟结果表明,改进后的模型成功地模拟出各种不同择优取向的单枝晶和多枝晶的生长形貌。     

Simulation of international space station microgravity directional solidification experiments on columnar-to-equiaxed transition

It is well known that gravity affects solidification of alloys due to the convective effects it induces. As a result, different outcomes are expected if solidification experiments are carried out in near-zero gravity conditions achievable in space. Directional solidification experiments were conducted on board the Material Science Lab (MSL) in the International Space Station (ISS). The experiments, on Al–7 wt.% Si alloys, were carried out with a low gradient furnace (LGF). The LGF is a Bridgman-type furnace insert for the MSL. Numerical simulations for two such microgravity directional solidification experiments are presented and compared with experimental results. A front tracking algorithm to follow the growing columnar dendritic front, and a volume averaging model to simulate equiaxed solidification, were employed simultaneously in a common thermal simulation framework. The thermal boundary conditions for the simulation domain were computed via the temperature readings which were recorded during the experiments. The simulation results include the prediction of columnar-to-equiaxed transition (CET) and average as-cast equiaxed grain diameters, and agreed with the experimental results reasonably. The simulations predict that although an undercooled zone forms ahead of the growing columnar front, thermal conditions in the diffusion-controlled experiments were inadequate to trigger an entirely equiaxed zone without grain refiners.     

Al-Si7-Mg消失模振动凝固组织半固态热处理

利用消失模铸造振动凝固技术,得到Al-Si7-Mg合金的近等轴晶组织。在此基础上研究了580℃不同时间保温半固态等温热处理(SSIT)组织中α-Al晶体的形貌与尺寸变化。结果表明:合金580℃保温1h,得到α晶粒的等积圆直径在50~150μm的分布比例达85%,α晶粒圆度均值1.41。通过凝固动力学和热力学分析,得出半固态热处理过程中,低熔点的三元共晶和二元共晶相首先熔化,并随着溶质元素扩散速度的增加,液相不断增多,α晶体形状大小不断改变;当液相率达到平衡之后,α晶粒在界面张力的作用下,不断圆整球化,合并长大。半固态热处理α枝晶演变形式明显不同于等轴晶演变过程。