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1.
《International Journal of Cast Metals Research》2013,26(1-4):216-219
AbstractThere 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. 相似文献
2.
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, 相似文献
3.
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. 相似文献
4.
Xi Li Annie Gagnoud Yves Fautrelle Zhongming Ren Rene Moreau Yudong Zhang Claude Esling 《Acta Materialia》2012,60(8):3321-3332
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. 相似文献
5.
G. Zimmermann L. Sturz H. Nguyen-Thi N. Mangelinck-Noel Y. Z. Li C.-A. Gandin R. Fleurisson G. Guillemot S. McFadden R. P. Mooney P. Voorhees A. Roosz A. Ronaföldi C. Beckermann A. Karma C.-H. Chen N. Warnken A. Saad G.-U. Grün M. Grohn I. Poitrault T. Pehl I. Nagy D. Todt O. Minster W. Sillekens 《JOM Journal of the Minerals, Metals and Materials Society》2017,69(8):1269-1279
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. 相似文献
6.
柱状晶向等轴晶转变(CET)是在一定的凝固过程中必须控制的一种显微组织转变.本文针对铝合金连铸坯凝固过程中柱状晶向等轴晶的转变,综合运用了正交试验研究、数值模拟计算、数学拟合等方法,分析了连铸过程主要工艺因素拉坯速度、一冷水量、二冷水量和浇注温度对连铸坯凝固过程中柱状晶向等轴晶转变位置的影响,提出了柱状晶向等轴晶转变的转变位置判据,即当连铸坯某一位置处的固相率等于0.3时,温度梯度G与冷却速度R满足函数关系G0.8072/R=0.469时,在该位置处将发生柱状晶向等轴晶的转变. 相似文献
7.
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. 相似文献
8.
We use phase-field models to characterize the microstructure present in magnesium AZ31 alloy solidified under welding conditions. We focus our attention on the study of the conditions under which a columnar-to-equiaxed transition (CET) is observed in resistance spot welds. Our simulations show how the size and shape of the columnar and equiaxed regions depend on factors such as cooling rate, temperature gradient and the nature of inoculant particles. Our results are compared with experimental observations. In addition, we contrast our findings with predictions from a previously developed steady-state model for the CET. 相似文献
9.
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. 相似文献
10.
H. Jung N. Mangelinck-Noël H. Nguyen-Thi B. Billia G. Reinhart A. Buffet 《Metals and Materials International》2009,15(1):21-26
The control of the transition from columnar to equiaxed (CET) dendrite microstructure is an important point to obtain desired
final properties of industrial products. The objective is to understand how the formation and the evolution of the CET are
influenced by the processing parameters and natural convection with Al - 3.5 wt.% Ni and Al - 7.0 wt.% Si alloys. Various
experiments are carried out in a Bridgman furnace for which the thermal gradient and pulling velocity can be independently
controlled. We concentrate our interest on the CET tendency, added particle effects and the evolution of dendrite grain structures
under different test conditions. On the other hand, in-situ and real time observation of the solid-liquid interface is used
to reveal the dynamics of the phenomena that occur, thus deepening our understanding. To achieve this objective, Synchrotron
X-ray Radiography has been designed and performed at the European Synchrotron Radiation Facility. 相似文献
11.
《Intermetallics》2015
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. 相似文献
12.
13.
Modeling of Microporosity Formation in a Vertical Upward Unidirectional Solidification Al-Cu Casting
基于枝晶间流动的Darcy定律, 考虑了凝固过程氢的宏观扩散与传输, 建立了耦合氢宏观偏析的铝合金铸件微观孔洞形成的数学模型, 并进行了实时熔炼的Al-4.5%Cu(质量分数)铸件的垂直向上凝固实验, 铸件微观组织和孔洞的分析结果表明: 铸件沿着高度方向包括了柱状晶、柱状晶向等轴晶转变(CET)和等轴晶3个区域; 柱状晶区域内微观孔洞体积分数存在递减的分布规律, 且邻近底部冷模的试样微观孔洞体积分数最大.采用所建立的模型对实验铸件微观孔洞形成进行了模拟, 模拟结果与实验结果吻合较好; 而当忽略氢宏观偏析时, 模拟结果与实验结果存在较大误差.研究表明, 氢的宏观偏析对微观孔洞的形核与分布具有重要影响. 相似文献
14.
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. 相似文献
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17.
《International Journal of Cast Metals Research》2013,26(1-4):30-33
AbstractCapabilities 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). 相似文献
18.
《International Journal of Cast Metals Research》2013,26(1-4):323-326
AbstractTo treat mixed columnar–equiaxed solidification with dendritic morphology, five phase regions have been distinguished: extradendritic melt, interdendritic melt and solid dendrites in equiaxed grains, interdendritic melt and solid dendrites in columnar arrays of dendrites. These five phases are quantified by their volume fractions, and characterized by different volume-averaged solute concentrations. The equiaxed grains and columnar dendrites are confined by their envelopes, whose shapes are described by morphological parameters. The evolution of the envelopes is derived based on recent growth theories: the growth of primary columnar dendrite tips by the Kurz–Giovanola–Trivedi (KGT) model, the growth of secondary dendrite tips in radial direction of columnar trunk and the equiaxed dendrite tips by the Lipton–Glicksman–Kurz (LGK) model. The solidification of the interdendritic melt is governed by diffusion in the interdendritic melt region. Preliminary modelling results on a benchmark casting (Al–4·7wt-% Cu) show the potentials of the model. 相似文献
19.
《International Journal of Cast Metals Research》2013,26(2):124-129
AbstractThe solidification of grey cast iron is controlled by the addition of inoculants. This is done in order to provide nucleation sites and hence facilitate the formation of eutectic cells and decrease the degree of undercooling. The number of eutectic cells and the graphite morphology affect the final properties of the casting. Preceding the nucleation of graphite and the eutectic cells is the nucleation of the primary austenite. It was found that the addition of inoculants also influences the primary solidification. The largest effect on the primary dendrites is obtained by inoculation using pure iron powder. It was also shown how the columnar to equiaxed transition (CET) depends on the number of equiaxed dendrites per unit volume. In addition, the primary structure was found to influence the eutectic solidification. The relationship between the secondary dendrite arm spacing and the eutectic cell size was found to correlate well with the work of others. 相似文献
20.
A deterministic mathematical model for steady-state unidirectional solidification is proposed to predict the columnar-to-equiaxed transition. In the model, which is an extension to the classic model proposed by Hunt [Hunt JD. Mater Sci Eng 1984;65:75], equiaxed grains nucleate according to either a normal or a log-normal distribution of nucleation undercoolings. Growth maps are constructed, indicating either columnar or equiaxed solidification as a function of the velocity of isotherms and temperature gradient. The fields of columnar and equiaxed growth change significantly with the spread of the nucleation undercooling distribution. Increasing the spread favors columnar solidification if the dimensionless velocity of the isotherms is larger than 1. For a velocity less than 1, however, equiaxed solidification is initially favored, but columnar solidification is enhanced for a larger increase in the spread. This behavior was confirmed by a stochastic model, which showed that an increase in the distribution spread could change the grain structure from completely columnar to 50% columnar grains. 相似文献