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.     

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.     

Macrosegregation During Re-melting and Holding of Directionally Solidified Al-7 wt.% Si Alloy in Microgravity

As-cast aluminum-7 wt.% ailicon alloy sample rods were re-melted and directionally solidified on Earth which resulted in uniform dendritically aligned arrays. These arrays were then partially back-melted through an imposed, and constant, temperature gradient in the microgravity environment aboard the International Space Station. The mushy zones that developed in the seed crystals were held for different periods prior to initiating directional solidification. Upon return, examination of the initial mushy-zone regions exhibited significant macrosegregation in terms of a solute-depleted zone that increased as a function of the holding time. The silicon (solute) content in these regions was measured on prepared longitudinal sections by electron microprobe analysis as well as by determining the fraction eutectic on several transverse sections. The silicon content was found to increase up the temperature gradient resulting in significant silicon concentration immediately ahead of the mushy-zone tips. The measured macrosegregation agrees well with calculations from a mathematical model developed to simulate the re-melting and holding process. The results, due to processing in a microgravity environment where buoyancy and thermosolutal convection are minimized, serve as benchmark solidification data.     

Phase-field simulation of solidification morphology in laser powder deposition of Ti-Nb alloys

A phase-field model of alloy solidification is coupled to a new heat transfer finite element model of the laser powder deposition process. The robustness and accuracy of the coupled model is validated by studying spacing evolution under the directional solidification conditions in laser powder deposition of Ti-Nb alloys. Experimental Ti-Nb samples reveal the microstructure on a longitudinal section with significant change in the size of the dendrites across the sample. Quantitative phase-field simulations of directional solidification under local steady-state conditions extracted from the results of the finite element thermal model confirmed this behavior. Specifically, the phase-field simulations agree with the results of the analytical model of Hunt in predicting a minimum spacing value, which is due to the mutual effects of the increasing temperature gradient and decreasing solidification velocity towards the bottom of the microstructure. This work demonstrates the potential of coupling the phase-field method to complex heat transfer conditions necessary to simulate topologically complex microstructure morphologies present in laser powder deposition and other industrially relevant casting conditions.     

新型真空熔化定向凝固装置的研制

对真空感应熔炼与双区加热结合在一起的定向凝固装置的可行性进行了研究，并研制出一套新型真空熔化定向凝固装置。该装置的主要特点是：（１）可定向凝固出较大尺寸的样品；（２）兼有熔炼和定向凝固的功能，熔炼好的合金液能自动底注到加热器内的型壳中；（３）定向凝固部分采用了先进的双区加热技术，既提高了温度梯度又减少了合金元素的烧损；（４）抽拉速度能在很大范围内调节。     

电磁搅拌对激光熔凝TA15钛合金熔池凝固研究

为了研究电磁搅拌作用对激光熔凝熔池凝固过程的影响,采用有限体积法对施加磁场前后激光单道动态熔凝TA15钛合金过程进行三维磁-热耦合数值模拟。研究了磁场对激光熔池流场、熔凝单道及其周边基材温度分布、固液界面处温度梯度和凝固速度的影响,并采用试验手段对模拟结果进行了验证。模拟结果表明:电磁搅拌作用使激光熔池最大流速增加了约20%,对流加剧促进了熔池热交换作用,使其最高温度下降,固液分界面处温度梯度大幅降低,凝固速度小幅增大,从而有利于熔池顶部组织发生柱状晶-等轴晶转变(CET)。试验结果显示施加磁场后熔凝层顶部有等轴晶组织生成,且随着远离磁场中心,电磁力增大,等轴晶区有扩大趋势。试验结果和模拟结果一致性较好。     

采用温度回升法对任意结晶区间的铸件凝固结晶潜热的数值计算

铸件凝固结晶潜热的释放行为与凝固传热、溶质传输及铸造合金种类与成分等诸多因素有关，合金凝固过程中其温度、固相体积分数与液相成分(T-fs—CL)三者之间一般存在着非线性强耦合关系．采用合金凝固传输统一模型及温度回升(补偿)法提出的处理任意结晶温度区间(包括零结晶区间)凝固潜热释放问题的数值迭代计算方法，对不同合金成分与不同固相反扩散效应的二维Al—Cu合金铸件定向凝固传输过程进行了数值计算，表明该方法对于从纯金属到共晶成分的不同成分合金及从Scheil模型到Lever-Rule模型之间的任意凝固模式均是有效的．将该方法推广应用于叶片铸件三维凝固传输过程的T-fs—CL耦合数值模拟仍显示出高的计算效率．通过对计算结果进行的三维图像数据处理，展示了铸件几何形状对凝固传输行为的重要影响．     

Secondary dendrite arm migration caused by temperature gradient zone melting during peritectic solidification

During dendritic solidification of Al–Ni and Sn–Ni peritectic alloys in a temperature gradient, it is observed that a thick peritectic layer forms on the front edge of the secondary dendrite arm of the primary phase, while there is almost no peritectic phase on the back edge. This observation is explained satisfactorily by a new version of secondary dendrite arm migration caused by temperature gradient zone melting during peritectic solidification, which involves both primary and peritectic phases. Experimental and theoretical analysis in the present work demonstrates that temperature gradient zone melting can cause extensive melting/solidification of the primary and peritectic phase during peritectic solidification.     

静磁场下热电磁效应及其对凝固组织的影响

由于塞贝克效应,当施加一个温度梯度时,在凝固界面将会产生一个热电流。在磁场下定向凝固过程中热电流和静磁场相互作用将会产生一个显著的热电磁力。此磁力将会诱发各种现象,比如液体搅拌、固相运动以及固相受力。由于在金属的凝固过程中常常存在温度梯度,这些效应将会普遍存在。在较小和适度的磁场下热电磁力将促进液体的流动,在较强的磁场下其将抑制液体的流动。另外,热电磁流有多尺度效应,即尺度越小,抑制液体流动所需的磁场越高。至今,已经完成了大量涉及多种合金的实验,所有的实验结果均表明在凝固前沿和糊状区均存在热电磁流。热电磁流动显著地影响凝固过程中微观和宏观偏析、凝固组织以及糊状区晶界结构。热电磁流动的方向以及相应的偏析可以通过改变磁场的方向而被控制。另一方面,作用于固相的热电磁力也将显著地影响固相凝固组织,表现为固相所受的热电磁力导致枝晶的断裂、枝晶碎片的运动和柱状晶-等轴晶转变等。近年来,同步辐射X射线成像技术被应用于实时观测在横向磁场下定向凝固过程中枝晶的生长行为,观察在磁场下枝晶发生断裂和枝晶碎片沿一定方向运动的行为,这进一步证实了磁场下热电磁效应在凝固过程中显著地影响凝固组织。     

Experimental and numerical analysis of effect of cooling rate on thermal–microstructural response of spheroidal graphite cast iron solidification

This work presents an experimental and numerical study of the solidification process of an eutectic spheroidal graphite cast iron (SGI). The effect of the cooling rate on the thermal–microstructural response is particularly analysed. To this end, experiments as well as numerical simulations were carried out. The experiments consisted in a solidification test in a wedge-like casting such that different cooling rates were measured at specific positions along the part. A metallographic analysis was also performed in five locations of the sample with the aim of obtaining the number and size of graphite nodules at the end of the process. The numerical simulations were made using multinodular based and uninodular based models. These two models predicted similar results in terms of cooling curves and nodule counts. Besides, good experimental–numerical agreements were obtained for both the cooling curves and the graphite nodule counts.     

Splat shapes in a thermal spray coating process: Simulations and experiments

We studied the deposition of nickel particles in a plasma spray on a stainless steel surface using both experiments and numerical simulations. We developed a three-dimensional computational model of free-surface fluid flow that includes heat transfer and solidification and used it to simulate the impact of nickel partcles. In our experiments, particles landing on a polished stainless steel surface at a temperature below 300 °C splashed and formed irregular splats, whereas those deposited on substrates heated above 400 °C formed round disk splats. Simulations showed that formation of fingers around the periphery of a spreading drop is caused by the presence of a solid layer. Droplets that spread completely before the onset of solidification will not splash. To sufficiently delay the instant at which solidification started in our simulations to obtain disk splats, we had to increase the thermal contact resistance between the droplet and the substrate by an order of magnitude. We measured the thickness of the oxide layer on the test surfaces used in our experiments and confirmed that heating them creates an oxide layer on the surface that increases the thermal contact resistance. We demonstrated that the numerical model could be used to simulate the deposition of multiple droplets on a surface to build up a coating.     

Pb-Bi包晶合金定向凝固过程中带状组织的形成(英文)

采用Bridgman定向凝固技术对Pb-Bi包晶合金进行定向凝固实验,生长速度为0.5μm/s,温度梯度为35K/mm。在亚包晶和过包晶成分的Pb-Bi合金中(Pb-xBi,x=26%,28%,30%和 34%)均观察到带状组织。由于熔体对流的影响,在试样中心形成树状初生α相,周围被包晶β相基体包围。带状组织出现在树状组织后,并且该带状组织为过渡性的。测定带状组织中的成分分布,从而确定α和β两相的形核过冷度。在有限长试样中,熔体对流是形成过渡性带状组织的主要原因,该过渡性带状组织表现为有限带数,带宽不恒定,且在Pb-Bi合金的两相包晶区很宽的成分范围内出现。     

On macrosegregation in ternary Al–Cu–Si alloys: numerical and experimental analysis

Macrosegregation phenomena in ternary Al–Cu–Si alloys is investigated by a numerical modeling technique and by upward unidirectional solidification experiments. In particular, we consider a vertically aligned casting experiment of a ternary Al8.1wt%Cu3wt%Si alloy sample. The experimental measurements are used for validation of numerical simulations.     

Influence of natural convection during upward directional solidification: A comparison between in situ X-ray radiography and direct simulation of the grain structure

This paper presents a comparison between the in situ and real time observation of a directional solidification experiment carried out at the European Synchrotron Radiation Facility and a direct simulation of grain structure formation in the sample using a two-dimensional cellular automaton–finite element (CA-FE) model. In situ characterization of the columnar-to-equiaxed transition in a refined Al–3.5 wt.% Ni alloy is achieved by synchrotron X-ray radiography. Two main characteristics are derived from the radiographs for each nucleated grain, namely the nucleation position in the sample and the orientation of the main trunk relative to the vertical temperature gradient. These data are then used as input for the CA-FE simulations. The CA-FE model takes into account the effects of macroscopic transport of heat, liquid momentum and solute mass on the development of the dendritic grain structure and vice versa. The influence of convection on macroscopic shape of the growth front, the grain structure, the microstructure distribution and macro-segregation is determined by a comparison between the experimental observations and results from the numerical simulations with and without fluid flow. Good qualitative agreement is obtained and limitations that are linked to the two-dimensional approximation and the need for direct tracking of the eutectic grain structure are pointed out.     

连铸Ag-28Cu温度场非稳态过程的数值模拟

采用Procast软件中的Mile算法对Ag-28Cu合金连铸凝固过程中温度场的非稳态变化进行了模拟,研究了不同拉速、过热度和换热系数对温度场分布、凝固速率和凝固前沿温度梯度变化的影响。模拟结果表明:随拉速的增大,铸件中心区域的凝固速率加快,凝固前沿温度梯度变化范围减小。换热系数的改变在靠近铸件表面区域对凝固速率基本没有影响,但靠近铸件中心区域后,其凝固速率随换热系数开始大幅度增大。在整个凝固过程中,换热系数越大,凝固前沿温度梯度越大。随浇注温度的提高,其凝固速率呈振荡式增大,凝固前沿温度梯度呈振荡式减小。     

单晶高温合金HRS定向凝固过程凝固参数的计算机检测与分析

建立了一套炉前计算机数据自动采集和控制系统，该系统可对HRS定向凝固过程的温度数据进行自动采集。对单昌高温合金DD98定向凝固过程湿度场的现场测定和分析的结果表明：该系统现场采集的数据平稳，较好地解决了干扰问题；在恒定的加热温度和抽拉速率下，温度梯度、糊状区宽度等凝固参数是不断变化的，起始端的温度梯度最高，凝固后期温度梯度趋于平稳，糊状区的宽度随着凝固的进行呈增加的趋势。     

重燃叶片定向凝固宏/微观数值模拟及实验研究

通过模拟和实验的方法对比研究了重燃叶片定向凝固过程宏观温度场及微观组织的变化规律。建立了非均匀网格的求解模型,提高了计算效率。基于温度场的模拟结果分析了糊状区的演化规律。采用线性插值算法结合元胞自动机有限差分(cellular automaton finite difference,CAFD)模型模拟了叶片的微观组织,并和实验进行了对比,模拟和实验结果吻合良好。讨论了几种常见晶粒缺陷产生的原因,提出了预防措施。采用电子背散射衍射(electron backscattered diffraction,EBSD)技术进一步探讨了晶粒的竞争生长行为。建立了枝晶臂间距的计算模型,模拟了叶片的枝晶臂间距分布,并进行实验观察,分析了枝晶臂间距的变化规律。从宏、微观的角度解释了叶片的凝固特征,为实际生产提供帮助。     

A multi-phase micro-segregation model

Abstract

A multi-component multi-phase micro-segregation model was developed to cope with eutectic transformations of ductile iron, assuming mixed dendritic–globular solidification morphology. Solute partitioning is calculated by an equilibrium assumption at the interfaces using commercial CALPHAD software. Time-sensitive micro-segregation patterns and phase fractions are solved by the micro-segregation model. The development of fraction liquid over temperature with time was compared to Gulliver–Scheil simulations for a model with and without cross-diffusion. The micro-segregation model was coupled to a commercial process simulations tool to deal with interaction effects between material and process scale during solidification of a benchmark test casting made of EN-GJSA-X NiCr 20–2. The precipitation kinetics of phases, especially of graphite, is of particular interest during solidification of ductile iron, due to feeding effects. The coupling procedure as well as important aspects during solidification of ductile iron will be explained.     

Evaluation of weld metal hot cracking susceptibility in superaustenitic stainless steel

Solidification cracking susceptibilities of two types of superaustenitic stainless steel, 254SMO and SR50A, were evaluated by transverse Varestraint tests. The susceptibilities were compared with those of conventional austenitic stainless steel 316L, and factors influencing the difference of susceptibility were discussed. The comparison showed that 254SMO and SR50A are more sensitive to solidification cracking than 316L. In the transverse Varestraint tests, both total and maximum crack lengths are longer in the superaustenitic stainless steel. Because of the longer maximum crack length, the superaustenitic stainless steel also has a wider brittleness temperature range of cracking than 316L: about 178 °C for the superaustenitic stainless steel and 43 °C for 316L. It is believed that straight subgrain boundaries owing to the cellular dendritic solidification and segregations of sulfur and phosphorus in the subgrain boundaries of superaustenitic stainless steel make it more sensitive to solidification cracking. In addition to the solidification cracking, reheat cracking is also observed within the previous weld bead in the superaustenitic stainless steel because of fully austenitic solidification with significant segregations. This suggests that caution should be given to the occurrence of reheat cracking when superaustenitic stainless steel is multi pass welded.     

Numerical simulation on directional solidification of Al-Ni-Co alloy based on FEM

The ratio,of the temperature gradient at the solidifiationfront to the solidification rate of solid-liquid interface,plays a large part in columnar grain growth.The transient temperature fields of directional solidification of Al-Ni-Co alloy were studied by employing a finite element method.The temperature gradient at the solidification front and the solidification rate were analyzed for molten steels pouring at different temperatures.The results show that with different initial pouring temperatures,the individual ratio of the temperature gradient at solidification front to the solidification rate soars up in the initial stage of solidification,then varies within 2,000-6,000℃.s.cm-2,and finally goes down rapidly and even tend to be closed to each other when the solidification thickness reaches 5-6 cm.The simulation result is consistent with the practical production which can provide an available reference for process optimization of directional solidified Al-Ni-Co alloy.