气雾化过程中合金熔滴的凝固行为研究进展

气雾化制粉以其粉末球形度高、氧含量低等优点已经成为现在一种重要的粉末制备方法,制粉过程可粗略分为破碎和凝固两步,但其工艺本身是多相流相互耦合的复杂过程,因此人们对其机理尚在进一步研究中。本文主要描述了凝固阶段熔滴的热交换与凝固行为,并对凝固过程做了详细的介绍。     

Solidification of Highly Undercooled Hypereutectic Ni-Ni3B Alloy Melt

The solidification of undercooled Ni-4.5 wt pct B alloy melt was investigated by using the glass fluxing technique. The alloy melt was undercooled up to ΔT _p ~ 245 K (245 °C), where a mixture of α-Ni dendrite, Ni₃B dendrite, rod eutectic, and precipitates was obtained. If ΔT _p < 175 K ± 10 K (175 °C ± 10 °C), the solidification pathway was found as primary transformation and eutectic transformation (L → Ni₃B and L → Ni/Ni₃B); if ΔT _p ≥ 175 K ± 10 K (175 °C ± 10 °C), the pathway was found as metastable eutectic transformation, metastable phase decomposition, and residual liquid solidification (L → Ni/Ni₂₃B₆, Ni₂₃B₆ → Ni/Ni₃B, and L_r → Ni/Ni₃B). A high-speed video system was adopted to observe the solidification front of each transformation. It showed that for residual liquid solidification, the solidification front velocity is the same magnitude as that for eutectic transformation, but is an order of magnitude larger than for metastable eutectic transformation, which confirms the reaction as L_r → Ni/Ni₃B; it also showed that this velocity decreases with increasing ΔT _r, which can be explained by reduction of the residual liquid fraction and decrease of Ni₂₃B₆ decomposition rate.     

急冷水雾化工艺对金属粉末性能的影响

为了开发制粉新工艺和制备用于MIM的微细合金粉末，设计了组合雾化。在常规水雾化喷嘴的下方附加了冷却喷嘴，并以锡青铜粉为试验对象，研究了工艺条件对水雾化金属粉末性能的影响。结果表明，使用同样的设备，急冷组合雾化与单一雾化相比，能使粉末更加细化。同时，由于粉末冷却速度的提高，使粉末氧含量得到降低，颗粒外形变得更加不规则。     

Heat Flow Model for Surface Melting and Solidification of an Alloy

The heat flow model previously developed for a pure metal is extended to the solidification of an alloy over a range of temperatures. The equations are then applied to rapid surface melting and solidification of an alloy substrate. The substrate is subjected to a pulse of stationary high intensity heat flux over a circular region on its bounding surface. The finite difference form of the heat transfer equation is written in terms of dimensionless nodal temperature and enthalpy in an oblate spheroidal coordinate system. A numerical solution technique is developed for an alloy which precipitates a eutectic at the end of solidification. Generalized solutions are presented for an Al-4.5 wt pct Cu alloy subjected to a uniform heat flux distribution over the circular region. Dimensionless temperature distributions, size and location of the “mushy” zone, and average cooling rate during solidification are calculated as a function of the product of absorbed heat flux,q, the radius of the circular region, a, and time. General trends established show that for a given product ofqa all isotherms are located at the same dimensionless distance for identical Fourier numbers. The results show that loss of superheat and shallower temperature gradients during solidification result in significantly larger “mushy” zone sizes than during melting. Furthermore, for a given set of process parameters, the average cooling rate increases with distance solidified from the bottom to the top of the melt pool.     

反向凝固结晶器中流动与传热的数值模拟

文中根据连续统一模型建立了描述反向凝固结晶器中的流动与传热的数学模型。研究结果表明了反向凝固工艺母带增长的三阶段规律。即快速增长。平衡相持和快速回熔^[1,3,5]。通过母带厚度的实验值和模拟值的比较。验证了模型的正确性。     

Solidification of the Undercooled Al-Si Alloy Containing 1.0 PctRE

Al-80 pctSi-1.0 pctRE alloy was levitated and melted using the electromagnetic levitation facility in combination with a laser heating unit. The growth morphologies of primary silicon were observed using a high-speed video, and the microstructure was analyzed by the scanning electron microscopy. The morphologies of primary silicon at low, intermediate, and high undercooling are dendrites, fragmented bulks and granular grains, and equiaxed grains, respectively. In addition, the growth velocities of primary silicon were measured, which were consistent with the theoretical prediction. The microstructure refinements of primary silicon played a dominant role in its large microhardness, which increased with the increase of undercooling. Moreover, the hardening effect of dendritic structure was stronger than that of equiaxed grain.

     

Computational Modelling of Heat/Mass Transfer Near the Liquid-Solid Interface During Rapid Solidification of Binary Metal Alloys Under Laser Treatment

Abstract

This paper presents the results of an investigation into the problem of planar solid-liquid interface stability during rapid solidification of binary metal alloys under laser treatment. A new quantitative model is proposed. This model describes the self-organized development of stable spatially-periodic vortices in the melt near the solid-liquid interface due to concentration- (or thermal) capillary effects) together with effects due to the influence of normal concentration or temperature gradients directed from the interface towards the melt. These vortices give rise to a cellular structure at the solid-liquid interface of rapidly frozen melts.

A computer code was developed to solve the set of second-order linear differential equations which describe heat and mass transfer at the liquid-solid interface. This model allows calculation of the liquid phase velocity field, the second component concentration field in the melt, as well as the temperature field in the liquid and solid phases near the solid-liquid interface at a given solidification rate. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Résumé

Ce document presenté les résultats de l'etude du problème de stabilité de l'interface solide-liquide plane lors de la solidification rapide d'alliages metalliques binaires par traitement au laser. On propose un nouveau modèle quantitatif. Ce modele decrit le développement auto-organisé de tourbillons stables,périodiques dans l'espace, dans le bain fondu près de l'interface solide-liquide. Ces tourbillons sont dus aux effets1 capillaires de concentration (ou thermiques) ainsi qu'à l'influence de la concentration normale ou des gradients de températures diriges de l'interface vers le bain fondu. Ces tourbillons donnent naissance à une structure cellulaire a l'interface solide-liquide des bains fondus rapidément solidifiés.

On a développé un code d'ordinateur pour resoudre l'ensemble d'equations differentielles lineaires de second ordre qui decrivent le transfert de chaleur et de masse à l'interface liquide-solide. Ce modèle permet le calcul du champ de vitesse de la phase liquide, du champ de concentration de la seconde composante dans le bain fondu, ainsi que du champ de temperature dans la phase liquide et la phase solide près de l'interface solide-liquide, à un taux donné de solidification. © 1998 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.     

Heat Flow and Solidification Modeling of Industrial Scale,Ingot Casting Operation

A model, based on the concept of effective thermal conductivity, was developed to study thermal fields and the resultant solidification behavior of large, round, industrial size ingots. In this, flow and turbulence phenomena during mold filling as well as subsequent solidification were not modeled explicitly but their influence was accounted for by artificially raising the thermal conductivity of solidifying steel. Thus, a conduction like equation embodying a conjugate approach was applied to simultaneously predict the evolution of temperature fields in the mold as well as in the solidifying ingot following teeming. Prior to comparing model predictions against industrial scale measurements, sensitivity of calculations to grid size, time step height, convergence criterion etc. were rigorously assessed. Similarly, modeling of interfacial resistance, chemical reactions and heat effects in the hot top as well as their influence on predicted results were evaluated computationally. Embodying mixed thermal boundary conditions (free convection + radiation) at the mold wall, temperature fields during solidification of two different industrial large ingots were predicted numerically. Parallely, mold wall temperature was monitored as a function of time and surface temperature of ingot was measured at the instant of mold stripping using hand held, radiation pyrometers. Incorporating relevant operating conditions (viz., mold dimensions and size, ingot and hot top dimensions and material, initial mold and liquid temperature etc.) into the calculation scheme, predictions were made via a computational procedure developed in-house and results thus obtained were compared against equivalent industrial scale measurements. Very reasonable agreement between the two was demonstrated.     

金属连续定向凝固技术

针对日趋活跃的金属定向凝固技术,阐述定向凝固技术的基本原理,以及其特点.简要说明了金属定向凝固技术的应用.介绍了目前金属定向凝固技术在国内外的发展状况,存在的问题及未来的前景.     

快速凝固过程分析

从快速凝固的基本原理着手,详细总结了快速凝固技术的冷却方式判定方法,从物质的导热原理出发讨论了单辊熔体急冷法、双辊熔体急冷法的冷却方式,分别概括了它们的界面导热系数的计算,分析了冷却速度的计算方法。对快速凝固的过程分析具有参考价值。     

金属凝固过程中的形核技术

在金属凝固过程中,对金属液施加各种物理场可以提高铸坯的等轴晶比例,细化凝固组织,从而提高铸坯的力学和物理性能.文章介绍了近年来一些机械法凝固技术的研究现状,并简要指出这些技术的优缺点所在.     

On the Formation of Dispersoids during Rapid Solidification of an AI-Fe-Ni Alloy

Examination of the effect of rapid solidification velocity on the microstructure of Al-3.7 wt pct Ni-1.5 wt pct Fe has revealed a new mechanism for the formation of discrete second phase particles in rapidly solidified alloys. Cellular growth of α-Al occurs with the intercellular phase, Al₉(Fe, Ni₂ in two distinct morphologies. At low velocity (<50 cm/sec) the phase is continuous in the growth direction while at higher velocity discrete rounded particles are observed. Analysis of the orientation relationship and the number of variants which exists between phases leads to the proposal of a mechanism where liquid droplets are deposited by the grooves of a moving cellular interface. These droplets solidify subsequently to form the rounded second phase particles.     

An Electron Microscope Study of the Featureless Zone Obtained during Rapid Solidification

The microstructure observed on the rapid solidification of dilute alloys of Cu in Al has been examined. A high intensity heat source, namely, a 6 kW CO₂ laser beam was passed over the surface of the alloy, and on regrowth a featureless zone is seen prior to a cellular morphology. Scanning transmission electron microscopy in conjunction with X-ray microanalysis was used to show that a concentration gradient exists across this region. The results support the theory that it is a plane front zone formed during the resolidification process. Formerly with the Department of Metallurgy, University of Illinois, is Scientist, Defence Metallurgical Research Laboratory, P. O. Kanchanbagh, Hyderabad, India 500258.     

Rapid Solidification of Sm-Co Permanent Magnets

In this article, we report the microstructural evolution of rapidly solidified Sm-Co alloys from 4 to 16 at. pct Sm. We have observed a wide variety of phase formation and microstructures, ranging from primary Co dendrite formation to eutectic structures to the formation of the metastable SmCo₇ compound. In particular, we observed nonequilibrium formation of Co along with SmCo₇, whose presence caused a decrease in coercivity from ∼10 kOe to 500 Oe. Alloying elements reduced the scale of the microstructure, effectively offsetting the detrimental effects of the Co phase formation and leading to a recovery of the coercivity. The eutectic structure with Co rods surrounded by SmCo₇ provides a natural path to nanoscale hard/soft magnetic nanocomposites, where control of scale and phase content is critical. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials”, which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS/MPMD and ASMI-MSCTS Phase Transformations Committee.

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软接触电磁连铸过程中结晶器及初生坯壳的传热

为了解高频磁场对软接触电磁连铸结晶器及初生坯壳传热行为的影响,用有限元二维数值模拟方法计算了软接触连铸过程中结晶器及初生坯壳的传热.得知在高频磁场（f=20 kHz）作用下,电磁场的感应加热会减少连铸初生坯壳的厚度、提高连铸坯的表面温度,并大幅度提高分瓣结晶器铜壁的温度.     

圆坯连铸凝固传热数学模型及应用

建立了34Mn5V钢Φ400 mm圆坯连铸过程中的凝固传热数学模型,运用该模型计算得到的二冷各区控制点的表面温度与现场实测结果一致。用该模型计算得出,钢水过热度对铸坯表面温度影响较小,拉速和二冷区冷却强度对铸坯温度影响较大。生产实践表明,当生产Φ400 mm铸坯的拉速达到0.4～0.6 m/min时,自动控制的二次冷却制度能满足工业连铸要求,可得到优质铸坯。     

The Decarburization Kinetics of Metal Droplets in Emulsion Zone

Metallurgical and Materials Transactions B - A mathematical model has been developed to predict the decarburization rate within individual droplets in the emulsion zone. All the chronological...     

Heat Flow during the Autogenous GTA Welding of Pipes

A theoretical and experimental study of heat flow during the welding of pipes was carried out. The theoretical part of the study involves the development of two finite difference computer models: one for describing steady state, 3-dimensional heat flow during seam welding, the other for describing unsteady state, 3-dimensional heat flow during girth welding. The experimental part of the study, on the other hand, includes: measurement of the thermal response of the pipe with a high speed data acquisition system, determination of the arc efficiency with a calorimeter, and examination of the fusion boundary of the resultant weld. The experimental results were compared with the calculated ones, and the agreement was excellent in the case of seam welding and reasonably good in the case of girth welding. Both the computer models and experiments confirmed that, under a constant heat input and welding speed, the size of the fusion zone remains unchanged in seam welding but continues to increase in girth welding of pipes of small diameters. It is expected that the unsteady state model developed can be used to provide optimum conditions for girth welding, so that uniform weld beads can be obtained and weld defects such as lack of fusion and sagging can be avoided.