Solidification Structure Formation of Undercooled Single Phase Alloys

Solidification structure variation of single phase alloy with undercooling prior to nucleation has been widely studied.The progress,especially during the last decade,is reviewed so as to give a comprehensive knowledge for it ,in which the emphases are laid on the structure evolution mechanism and the potential application.Lastly, the future interesting subjects are presented.     

Lamellar Spacing Selection in Regular Eutectic Solidification at Low Velocity

Based on the Jackson and Hunt＇s analysis, the selection of lamellar spacing in regular eutectics was reexamined at low velocity. The isothermal assumption was released and the effective interface undercooling was determined by the weighted average of the eutectic phases. It is found that the lamellar spacing minimized the effective interface undercooling depends only on the intrinsic characteristics of a given system at a fixed velocity. In addition, the selection of lamellar spacing is related to the relationship between the average interracial undercoolings of the eutectic phases and the lamellar spacing. The selected lamellar spacing obviously deviated from that predicted by the Jackson and Hunt＇s analysis if the variation of the average interracial undercoolings of the solid phases with the lamellar spacing was markedly different at a constant growth rate.     

Microstructure Evolution during Solidification of AZ91D Magnesium Alloy in Semisolid

The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.     

The surface tension of liquid silicon at high temperature

The measurement of the surface tension of liquid silicon has a long history with many results but no general agreement between them. Two values at the melting temperature are cited in reviews (749 and 827 mN/m [N. Eustathopoulos, E. Ricci, B. Drevet, Note Technique DEM No. 97/58, CEA, 1997]) but there are few arguments to determine the correct one. In the present study, new data for the surface tension obtained with the analysis of characteristic frequencies of a levitating drop are presented. The effect of oxygen and nitrogen are also considered. These data are compared with former data obtained with contactless techniques. The most recent surface tension values obtained with drop weights ranging on two orders of magnitude and environments of different natures (argon, hydrogen and vacuum) show excellent agreement (within a 1.5% margin) at temperatures between 1350 K and 2400 K. The comparison of these data to others obtained with different techniques, reveal a good agreement, except those obtained with the sessile drop technique on some supports like BN, SiO₂ and MgO. However, these special cases may be connected with the reactivity of silicon with these supports.     

Parameter determination of critical nucleation frequency in solidification of undercooled metallic melts

Abstract

A simple method was proposed to calculate the essential parameters correlated with the critical nucleation frequency of undercooled metals and alloy melt. Numerical results show that the calculation accuracy from this method can be improved using the experimental data either with high undercooling or with low undercooling range (the difference of undercoolings between two solidification events). The calculations of the interfacial energy for high undercooling of silver and of the catalytic factor f(θ) for high undercooling of Al, Cu and Al–30 wt-%Cu alloy indicate that the results are consistent with the experimental measurements and with the results of Jian’s model [Metall. Trans. A, 2001, 32A, 391–395]. In addition, by analysing the differential scanning calorimetry data of pure Sn subjected to different cooling rates, similar values of catalytic factor f(θ) are obtained. This further indicates the validity of the current method.     

Selective synthesis of metallic nickel particles with control of shape via wet chemical process

Metallic nickel particles with shapes varying from sea urchin-like to spherical have been selectively synthesized via aqueous chemical reduction. The phase structure and morphology of particles have been investigated by means of X-ray diffraction and scanning electron microscopy. It was found that sodium carbonate greatly accelerated the reduction process to form needle-like dendrites under atmospheric pressure. The probable formation mechanism of the sea urchin-like particles is also discussed. In the mild reduction process, the particle size of nickel nanospheres could be easily controlled by adjusting the experimental parameters.     

Directional Solidification Assisted by Liquid Metal Cooling

An overview of the development and current status of the directional solidification process assisted by liquid metal cooling （LMC） has been presented in this paper. The driving force of the rapid development of the LMC process has been analyzed by considering the demands of （1） newer technologies that can provide higher thermal gradients for alleviated segregation in advanced alloy systems, and （2） better production yield of the large directionally solidified superalloy components. The brief history of the industrialization of the LMC process has been reviewed, followed by the discussion on the LMC parameters including selection of the cooling media, using of the dynamic baffle, and the influence of withdrawal rates and so on. The microstructure and mechanical properties of the traditional superalloys processed by LMC, as well as the new alloys particularly developed for LMC process were then described. Finally, future aspects concerning the LMC process have been summarized.     

深过冷熔体激发快速定向凝固

阐述了深过冷的方法，深过冷的遗传性，深过冷度的理论极限以及深过冷熔体激发发快速定向凝固的试验方法及其凝固过程，给出了深过冷熔体（Ｃｕ－５ｗｔ％）激发快速定向凝固与铸态试验激发快速定向凝固的初步试验结果。     

Rapid Growth of TiNi intermetallic compound within undercooled Ti50Ni50 alloy under electrostatic levitation condition

The undercooling dependence of the solidification mechanism was systematically explored by the elec-trostatic levitation(ESL)facility.During the experiments,the maximum undercooling reached up to 406 K(0.26 TL)and the growth velocity of the primary TiNi phase was in-situ determined at various undercool-ings.At the initial increase of alloy undercooling,the value of growth velocity sluggishly rose followed by a power function.In this case,the primary TiNi phase preferentially developed as the equiaxed dendrite,then the remnant liquid participated as Ti2Ni and α-Ti phases on the grain boundary.Once the under-cooling exceeded the critical value of 350 K,the growth velocity of the primary phase displayed a sharply increase tendency.Meanwhile,the TEM results demonstrated that the precipitation of the intermetallic Ti2Ni compound was gradually restrained during the rapid solidification and the R-phase existing in the TiNi matrix at large undercooling implied that the martensitic transformation was incomplete.     

喷射雾滴烟气流动蒸发特性

电站锅炉烟气脱硫废水喷入烟道蒸发是电厂废水零排放最经济可行的技术途径之一。针对该技术实际应用中存在的在烟道壁面上结垢腐蚀问题,以单台300 MW机组为对象,将针对连续相烟气的湍流流动传热欧拉方法,以及离散相雾化液滴群流动蒸发的拉格朗日方法相结合,建立物理数学模型。通过数值模拟方法,研究脱硫废水喷射雾滴在烟气中的流动蒸发特性及其影响因素,获得不同运行条件下喷雾的扩散范围和液滴在烟气内的运动轨迹。结果表明：烟气温度越高、雾化液滴群的直径越小,其完全蒸发所需的时间和距离越短;采用多喷嘴小流量的布置方式可以提高雾化液滴群的蒸发质量;喷嘴喷射方向的选择应该保证雾化液滴群与烟气相对运动增强的同时,保证液滴能在规定距离和时间内完全扩散并与烟气进行充分接触和换热,实现雾化液滴群蒸发质量的最大化;而液滴初速度、喷嘴的喷射全锥角、烟气速度对蒸发率的影响不大。研究结果可为火电厂脱硫废水烟气蒸发的工艺设计及性能调控提供依据。     

Solidification microstructure development

In the present article, evolution of microstructure during solidification, as a function of various parameters, is discussed. Macrosegregation is described as being due to insufficient diffusivity of solute in the solid. Pattern formation is discussed in the light of instabilities at the solidification growth front. An overview of the scaling relations for various microstructures is given. Metastable extensions to equilibrium phase diagrams and corrections to equilibrium quantities are described.     

金属的凝固和凝固系数

本文根据耗散结构理论所建立的凝固模型,推导出的过冷度方程,指出金属的凝固与结晶凝固系数β是密切相联系的。其中结晶凝固系数为并且讨论了ρ值在不同情况下的物理意义以及β值在金属凝固中的作用。     

Vibrating Breakup of Jet for Uniform Metal Droplets

Uniform droplet formation from capillary stream breakup provides promising opportunities for many applications such as solder balls manufacturing, circuit board printing and rapid prototype manufacturing. In this study an apparatus capable of making monosize metal spheres by vibrating breakup has been developed. The droplets were electrically charged to avoid collision and merging with one another during flight. As a result, uniformly sized tin powders （180 μm in diameter） were obtained after cooling and solidification.     

雾化液滴对湿法采集纳米TiO_2气溶胶的影响研究

为掌握雾化液滴对湿法采集纳米气溶胶的影响作用,考察不同喷雾流量和雾化室尺寸对采集效率的影响,并通过Fluent软件仿真模拟不同尺寸雾化室内雾化液滴的运动状况,了解液滴对湿法采集的影响机理。结果表明,雾化室中的液滴作用以润湿包覆为主,而储液腔中的液滴主要以碰撞凝并实现对气溶胶粒子的捕获,润湿包覆对纳米气溶胶粒子的采集效果大于碰撞凝并的捕获效果。当雾化室和储液腔的喷雾流量均控制为40 mL/min时,湿法采样器的采集效率可达到最高(92%)。仿真和实验结论均表明,雾化室的最佳尺寸是高度为170 mm,内径为60 mm。     

Modeling of Liquid Metal Infiltration of Porous Fiber Preform During Squeeze Casting

The present work adopts a new approach to the analytical modeling of infiltration of porous fiber preforms by liquid metal in the squeeze casting of metal matrix composites, with the assumption that the process is adiabatic and that the flow is unidirectional. Fluid dynamics is described on the basis of Darcy's law, while separate equations are derived to explain the thermal behavior of the liquid metal and the fiber, assuming that the thermal interactions between the two are interfacial. Unlike earlier models, this approach does not consider the thermal behavior of a “composite,” but instead studies the behavior of the liquid metal and the fiber preform separately. In addition to the conventional application of heat balance techniques and development of partial differential equations involving temperatures, this work introduces supplementary conditions for temperature calculations, specifically at the entry and front points during infiltration. Differential equations are solved by a method of finite differences, and the problem of additional unknowns (preform temperature) at the infiltration front position is overcome using the “virtual point” concept. Simple expressions are derived for the calculation of process parameters like total time for complete infiltration and time for solidification, on the basis of which the occurrence of complete infiltration is predicted. A novel attempt in generating the profiles of the preform and liquid temperatures at specific instants during infiltration has also been made. The relative influence of the liquid superheat temperature, the preform preheat temperature, and the squeeze pressure on the infiltration mechanism is analyzed by studying the infiltration characteristics for various squeeze conditions.     

Solidification Simulation of Single Crystal Investment Castings

The three dimensional solidification simulation of the single crystal investment castings at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min was performed with the finite element thermal analysis method.The calculated result were in accordance with the experimental ones.The results showed that with the increase of with-drawal rate the concave curvature of the liquidus isotherm was bigger and bigger and the temperature gradient of the castings decreased.No effects of withdrawal rate on the distribution of the temperature gradient of the starter and helical grain selector of the castings were observed at withdrawal rates of 2 mm/min ,4.5 mm/min and 7 mm/min.The relatively high temperature gradient between 500℃/cm and 100℃/cm in the starter and helical grain selector was obtained at three withdrawal rates.The study indicates the three dimensional solidification simulation by finite element method is a powerful tool for understanding solidification and predicting defects in single crystal investment castings.     

Solidification modelling

The modelling of solidification of a metal/alloy in a mold cavity is increasingly becoming popular with numerous attempts being made to understand the phenomena that occur at the level of the casting (macro level) and that which occur at the microscopic level (micro level). In this paper, an attempt has been made to describe the phenomena occurring at both the macro and the micro levels. At the macro level, the effect of fluid flow on various thermal and solidification parameters has been studied. The results were compared with simulations carried out considering conduction alone and with experimental results. The relative importance of including fluid flow on solidification simulation of a casting has been brought out. At the micro level, an algorithm based on the macro-micro model to take the melt superheat into account while numerically predicting the grain size and dendritic arm spacing at different locations of an Al-7% Si alloy sand casting has been developed. The results are compared with the experimental values.     

一种高压缩性水雾化铁粉生产工艺的探讨

高压缩性水雾化钢铁粉末市场需求日益扩大,研发高压缩性水雾化生产工艺势在必行,根据实际生产情况,理论结合实践,改善提高生产条件,在生产过程中不断摸索合适生产因素,在不影响粉末整体使用性能的前提下,逐步提高水雾化粉压缩性能,为广大制品用户提供优质可靠的原料。