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.     

Effect of thermal convection on columnar-to-equiaxed transition during solidification of Al-Cu alloy

To investigate the effect of three-dimension(3 D) thermal convection on columnar-to-equiaxed transition(CET), the CET transition during the solidification of an Al-Cu alloy was simulated by 3 D cellular automaton model coupled with the finite element method(CAFE). The thermal convection in the liquid phase was considered. The results show that the thermal convection in the liquid phase promotes the CET. When the convection is present, the temperature gradient at the start position of CET increases and the growth velocity of columnar dendrite decreases. The convection influences the formation of elongated equiaxed grain through changing the local temperature gradient and dendritic growth velocity.     

Numerical Simulation of Columnar to Equiaxed Transition for Directionally Solidified Ti-44Al Alloy

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition （CET） during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt,     

NUMERICAL SIMULATION OF COLUMNAR TO EQUIAXED TRANSITION FOR DIRECTIONALLY SOLIDIFIED Ti-44Al ALLOY

Solute diffusion controlled solidification model was applied to simulate the columnar to equiaxed transition (CET) during directional solidification of Ti-44Al alloy. The simulation results show that the solutal interactions from growing equiaxed grains play an important role on CET. The effects of the applied thermal gradient and pulling velocity, the equiaxed seed spacing and nucleation undercooling on the CET are investigated in the present simulation. The simulated results indicated that the columnar branch spacing depends not only on the thermal gradient and the pulling velocity, but also on number of the seeds. A spacing adjustment can occur through initiation of seeds that develop into new columnar grains. The dependence of the CET on the thermal gradient and pulling velocity, qualitatively agrees with the analytical CET model of Hunt.     

激光金属成形定向凝固显微组织及成分偏析研究

利用高温合金Rene95粉末在镍基高温合金基材上进行激光多层涂覆。研究熔覆层中凝固显微组织的生长特性。基于对柱状晶向等轴晶转化理论的分析，证实通过控制工艺参数组合，可获得具有良好取向的单道多层、多道搭接多层定向凝固涂层和圆环的定向凝固试样，涂层内部的定向凝固柱状枝晶组织细密，枝晶一次间距为5－30μm，二次臂很小或者完全退化。涂层内无明显的成分偏析现象。     

Dendrite fragmentation and columnar-to-equiaxed transition during directional solidification at lower growth speed under a strong magnetic field

The effects of strong magnetic fields on the columnar-to-equiaxed transition (CET) have been investigated experimentally. Six alloys have been directionally solidified at low growth speeds (1–10 μm s^?1) under magnetic fields up to 10 T. Experimental results show that the application of a strong magnetic field causes a dendrite fragmentation and then the CET. The thermoelectric magnetic force acting on cells/dendrites and equiaxed grains in the mushy zone has been studied numerically. Numerical results reveal that the value of the thermoelectric magnetic force increases as the magnetic field intensity and the temperature gradient increase. A torque is created on cells/dendrites and equiaxed grains. This torque breaks cells/dendrites and drives the rotation of equiaxed grains. The rotation of equiaxed grains in the mushy zone will further destroy cells/dendrites. Thus, with the increase of the magnetic field intensity and the temperature gradient, the volume fraction of equiaxed grains in front of columnar dendrites increases. When the magnetic field intensity and the temperature gradient reach a critical value, the growth of columnar dendrites is blocked and the CET then occurs. The present work may initiate a new method of inducing the CET via an applied strong magnetic field during directional solidification.     

Industrial effects associated with the columnar to equiaxed transition in remelt ingots

Abstract

There have been many studies of the mechanisms involved in causing the solidification mode of an alloy to change from a columnar dendritic to an equiaxed structure, the columnar to equiaxed transition (CET). The parameters which cause the change are alloy segregation characteristics, freezing rate, undercooling and the presence or absence of nucleating sites for the equiaxial crystals. These parameters have been collected into theories which describe the CET and which generally satisfy the results found in laboratory or model systems. Although the effect has considerable scientific interest, the impact of a CET on the structure of an industrial ingot or casting is less widely recognised. In this report, we comment on the CET in relation to the structure of superalloy castings and ingots and on the formation of segregation defects in titanium alloy ingots. We conclude that one of the more important results of accurately modelling the solidification of these systems is that we may predict the casting conditions leading to the CET and also assess its impact on cast structure.     

The solidification path related columnar-to-equiaxed transition in Ti–Al alloys

Columnar-to-Equiaxed Transition (CET) of binary Ti–Al alloys and multi-component Ti–48Al–2Cr–2Nb alloys is studied using Bridgman solidification technique. The effect of aluminum concentration and growth rate on CET is determined. It is found in Ti–46Al and Ti–50Al alloy ingots equiaxed grains develop ahead of the moving solid–liquid interface with a growth rate of 500 μm/s; microstructures in Ti–49Al alloy stay columnar dendrites with the same growth rate. CET in Ti–Al alloys are not only influenced by growth rate, but also by the solidification path that is related to alloying composition. CET in Ti–Al alloys is predicted using the dendritic growth model based on the criterion of growth at marginal stability. According to the calculated results and directionally solidified microstructures, values of the nucleation undercooling for α and β phases are given. The growth rates to avoid CET in Ti–48Al–2Cr–2Nb alloy are suggested.     

Ti-Al合金定向凝固过程中胞/枝晶间距选择的计算机模拟

利用溶质扩散控制模型对Ti-Al合金定向凝固初始阶段变速冷却过程中胞/枝晶转变过程进行了数值模拟。在给定的冷却速率下,枝晶臂间距大于胞晶臂间距,而在过渡区,枝晶间距达到最大。另外,模拟结果也显示,晶核数量对柱状晶间距产生影响,随着植入晶核数量的增加,柱状晶间距非均匀化程度明显减小。出现过渡区的原因与枝晶生长所引起固/液界面前沿成分波动有关。模拟与试验结果吻合较好。     

Effect of Gravity on Directionally Solidified Structure of SuperalloysEI北大核心CSCD

分别利用常规下抽拉法与新型上提拉法进行不同方向的高温合金定向凝固实验,对比研究重力对单晶铸件凝固组织的影响。结果表明,在常规下抽拉法实验的向上凝固过程中,容易出现雀斑、γ/γ’共晶上聚和籽晶回熔紊乱等问题。原因是糊状区内液体由于元素偏析引起密度减小,在重力作用下形成了上重下轻的失稳状态并引起对流。而通过新型上提拉法实现的顺重力凝固过程中,密度减小的液体处于糊状区上端,形成上轻下重的稳定状态,使重力的作用由失稳因素转化为维持稳定的因素,抑制了液体对流的产生与发展。采用新型上提拉法制备的单晶铸件中彻底消除了雀斑缺陷,抑制了γ/γ’共晶组织的向上聚集,也保证了低密度籽晶稳定的回熔和外延生长。顺重力定向凝固技术从根本上消除了重力对高温合金定向凝固的不良影响,有希望发展成为新一代的先进单晶叶片成型技术。     

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.     

Solidification and segregation behaviors in 6061 aluminum alloy

The characteristics of a solid/liquid interface established during directional solidification were investigated in a 6061 aluminum alloy (Al6061 alloy) in which interface patterns, such as planar, cellular and dendritic, form in the mushy zone. Directional solidification experiments were carried out in alumina tubes with an inner diameter of 5 mm to obtain the different microstructures for varying growth rates under an imposed thermal gradient of 9.45 K/mm. As a consequence, primary arm spacing, secondary arm spacing and the length of primary arm were measured for steady-state growth conditions. Furthermore, the solute concentration profiles for the final solidification region were obtained using a SEM-EDS system in order to examine the segregation behaviors of alloying elements. Therefore, it was found that the degree of segregation in the Al6061 alloy was dependent on the growth rate under given solidification conditions, that is, the elemental segregation became more severe with increasing growth rate in the interdendritic region.     

同步辐射X射线成像法原位研究Al-15％Cu合金柱状晶一等轴晶转变以及无轴柱状枝晶生长

利用上海光源同步辐射装置（SSRF）,通过原位及实时探测研究Al—15％Cu合金定向凝固。结果表明：外部热扰动激发柱状晶一等轴晶转变（CET）。当固一液界面前沿的溶质边界层较薄时,枝晶尖端碎片的分离和漂浮是转变的前奏。而枝晶的三角尖端是断裂敏感区域。只要条件合适,一种新的枝晶形貌将孕育和长大。这种枝晶没有明显的主枝臂,称为无轴柱状枝晶。     

Microstructure evolution of Cu-Pb monotectic alloys during directional solidification

1 Introduction Monotectic alloy is an important class of alloy whose binary phase diagram has a miscibility gap, in which the original single liquid will decompose into two distinct immiscible liquids within a few seconds. In the normal gravity field, a …     

纵向磁场对不同尺寸定向凝固高温合金DZ417G组织的影响

进行了外加纵向静磁场下高温合金DZ417G的定向凝固实验,考察了纵向磁场对不同尺寸试样凝固组织形貌的影响.结果显示,在温度悌度为70℃/cm,抽拉速率为5μm/s时,施加磁场后一次枝晶间距减小,并在试样边缘出现等轴晶组织;随着试样尺寸的增大,在试样边缘和中心的柱状枝晶组织遭到破坏,形成等轴晶组织,且出现"斑状"偏析.这些现象可归结为磁场在固/液界面前沿合金熔体中诱发的热电磁对流(TEMC)所致.     

Observation of dendritic growth and fragmentation in Ga–In alloys by X-ray radioscopy

Abstract

Capabilities of the X-ray attenuation contrast radioscopy were utilised to provide a real time diagnostic technique for observations of dendritic growth and fragmentation during solidification of a Ga–30In (wt-%) alloy. The solidification process was visualised by means of a microfocus X-ray tube providing shadow radiographs at spatial resolutions of about 10 μm. Experiments have been carried out to solidify the Ga–In alloy unidirectionally either starting from the bottom or the top of the specimen. The first case is significantly affected by solutal convection, which governs a redistribution of solute concentration. A detachment of dendrite side arms, which is unambiguously caused by melt flow, was not observed. Dendritic fragmentation occurs during the solidification in the reverse top down direction. Variations of the applied cooling rate excited a transition from a columnar to an equiaxed dendritic growth (CET).     

TGZM效应下Co-87.9wt%Sb合金糊状区的组织演化及成分变化

以TGZM效应为理论基础,将Co-87.9wt%Sb合金置于定向凝固设备中加热熔化,在一定温度梯度下分别保温20min,2h,4h后淬火,对其糊状区的凝固组织和成分分布进行了研究。结果表明：在一定温度梯度下,介乎于完全液相区和未熔固相区之间存在着固液相共存的糊状区,Co-87.9wt%Sb合金的糊状区沿温度梯度方向分为(CoSb3+L),(CoSb2+L)以及(CoSb+L)三层;在TGZM效应作用下,随着保温时间的增加,糊状区内液相体积分数逐渐减少,糊状区与完全液相区界面向低温区移动;经保温处理后,由EDS获得的糊状区成分分布,糊状区内的溶质浓度明显偏离合金初始浓度,完全液相区溶质浓度高于初始值;通过理论分析与计算解释了上述现象。经过4h保温处理后,糊状区(CoSb3+L)部分中的CoSb3含量大幅增加,在该部分高温端甚至可达98.8%,表明借助TGZM效应经过长时间保温制备CoSb3热电材料的方法是可行的。     

涡轮叶片柱状晶制备工艺过程仿真及参数控制

依据涡轮叶片柱晶制备工艺，自主开发了温度场模拟软件HRSCAE。通过正交设计及模拟试验，研究了不同抽拉速度、加热及冷却条件对柱晶涡轮叶片生产过程温度场分布、固液界面形态的影响。研究表明，抽拉速度是影响铸件质量的主要因素，通过控制抽拉速度，使固液界面尽量保持在绝热区附近，可以使固液界面在较长时间内保持平坦状态。固液界面位置越靠近绝热区，越有利于柱晶的生长。模拟试验为准确控制工艺参数和解决定向凝固生产实践中发散度问题提供理论依据，也为获得表面光洁的高质量柱状晶提供了技术途径。     

Role of fragmentation in as-cast structure: numerical study and experimental validation

A volume average solidification model is extended to incorporate fragmentation as the source of equiaxed crystals during mixed columnar-equiaxed solidification. This study is to use this model to analyze the role of fragmentation in the formation of as-cast structure. Test simulations are made for the solidification of a model alloy(Sn-10wt.%Pb) with two different geometries. The first one is a 2D rectangular domain(50 × 60 mm~2) as cooled from the top boundary. Solidification starts unidirectionally as columnar structure from the top. The solute(Pb) enriched interdendritic melt is heavier than the bulk melt, and sinks downwards, hence leads to solutal convection. Fragmentation phenomenon occurs near the columnar tip front. The fragments are transported out of the columnar region, and they continue to grow and sink, and finally settle down and pile up at the bottom. The growing columnar structure from the top and pile-up of equiaxed crystals from the bottom finally lead to a mixed columnar-equiaxed structure, in turn leading to a columnar-to-equiaxed transition(CET). The second geometry is a 3D plate, 100 × 60 ×10 mm~3, as cooled laterally from one side. It was cast experimentally and analyzed for the as-cast structure. The equiaxed fragments are produced in the solidification front and transported into the bulk melt, leading to a special pattern of as-cast structure: columnar structure in the cool wall side and equiaxed structure in the upper left corner near the hot wall side, extending downwards to the middle bottom region. Numerically calculated as-cast structures agree with the experiment results.     

Effects of hot top pulsed magneto-oscillation on solidification structure of steel ingot

Achieving a uniform structure with few defects in heavy steel ingot is of high commercial importance. In this present work, in order to verify the potential of pulsed magneto-oscillation(PMO) applied in the production of heavy ingot, an induction coil was located at the hot top of the steel ingot to develop a novel technique, named hot top pulsed magneto oscillation(HPMO). The influences of HPMO on the solidification structure, macro segregation and compactness of a cylindrical medium carbon steel ingot with the weight of 160 kg were systematically investigated by optical microscope(OM) and laser induced breakdown spectroscopy original position metal analyzer(LIBSOPA-100). The results show that HPMO not only causes significant grain refinement and promotes the occurrence of columnar to equiaxed transition(CET) but also can homogenize the carbon distribution and enhance the compactness of the steel ingot. Therefore, HPMO technique has the potential to be applied in the production of heavy steel ingots on an industrial scale.