电磁搅拌技术在铜管水平连铸生产中的应用研究

在传统铜管水平连铸的基础上应用电磁搅拌原理开发了电磁连铸技术,采用结晶器外施加电磁场的方法来提高铜管铸坯的质量。当铜液在交变电磁场中运动时产生感应电流,感应电流和交变磁场相互作用,产生作用于初始坯壳、指向液芯的电磁力。其径向分力减小了铜液与结晶器内壁之间的接触压力,继而减小了拉坯摩擦阻力,有利于提高铸造管坯的表面质量;其水平方向的分力则促使铜液强迫对流,减小了液穴内的温度梯度,破坏了枝状晶的生长,增加了非自发晶核,细化了晶粒,减少铸坯的内部的疏松、偏析和裂纹等缺陷,提高了凝固组织分布的均匀性,从而达到改善铜管铸坯后续加工性能和提高产品成品率的目的。     

冷坩埚电磁冶金数值模拟研究

冷坩埚电磁冶金技术是制备特殊新材料的重要技术,其特点为凝壳参数可通过分瓣感应加热和坩埚水冷控温的方法控制。采用数值模拟,从磁通密度和电磁力方面,研究熔融精炼使用的冷坩埚结构和电磁场。研究结果表明：在坩埚强度允许范围内,分瓣数增加有利于提高透磁性能,熔体产生的磁通密度和所受的电磁力主要集中于切缝附近。     

A numerical investigation of gas flow effects on high-pressure gas atomization due to melt tip geometry variation

A parametric numerical study is presented on how melt feed tube geometry influences the gas flow field of a high-pressure gas atomizer (HPGA). The axisymmetric, turbulent, compressible Navier-Stokes equations are solved for the gas-only flow in the vicinity of the melt tip for a confined-feed, annular-slit atomizer. The numerical results indicate that a fully retracted melt tip can develop a high overambient base pressure over a wide range of operating pressures, while an extended melt tip can develop a subambient pressure, called an aspirating effect, which encourages melt to flow. Adding a taper angle to an extended melt tip decreases the aspirating effect. However, a melt tip can develop a radial pressure gradient along its base that forces the melt to move radially outward into a high-shear region of the flow, thus encouraging droplet formation. A fully retracted melt tip develops the highest such radial force, but this tends to decrease in magnitude with tip extension. Hence, a compromise must be established whereby the good aspiration character of a longer tip is balanced against the good pressure-driving force along the base of a shorter tip. The results provide details of the flow field and the effects due to variation of melt tip taper and extension.     

Application of electromagnetic separation of phases in alloy melt to produce <Emphasis Type="Italic">In-situ</Emphasis> surface and functionally gradient composites

The principle of electromagnetic separation of phases (primary phase) in alloy melt is that the electromagnetic force scarcely acts on the primary phases due to its low electric conductivity as compared to the melt. As a result, a repulsive force acts on the primary iron-rich phases to push them to move in the direction opposite to that of the electromagnetic force. The in-situ surface composite and the functionally gradient composite reinforced by primary Si are produced when the hypereutectic Al-Si alloy solidifies under electromagnetic force induced by static magnetic field and DC current. Similarly, the Al-Si-1.20 pct Fe-1.60 pct Mn alloy in-situ surface composite reinforced by primary iron-rich phase is produced. Based on this, a new method for production of in-situ multigradient composite with several layers, by electromagnetic separation of phases and directional solidification technique, is proposed.     

线材水平电磁连铸过程中磁场分布的数值模拟

针对线材电磁水平连铸过程中磁场分布问题，建立了二维轴对称有限元模型。利用ANSYS软件包进行划分网格及计算，模拟出了磁场在连铸结晶器区域的分布。研究了磁场频率对金属熔体内磁感应强度及洛仑兹力的影响。计算结果表明：随着频率的增大，磁感应强度减小，洛仑兹力先增大后减小且其作用范围逐渐集中于熔体表面区域。     

Effect of interactions between bubbles and graphite particles in copper alloy melts on microstructure formed during centrifugal casting: Part I. Theoretical analysis

Frequently, particles get associated with gas bubbles in a melt and their interaction influences the final distribution of particles and porosity in the casting. An analytical model for the separation of a particle from a bubble in melts containing dispersed particles and bubbles is proposed. During centrifugal casting of alloys containing dispersed particles, both the particles and gas bubbles present in the melt move with the centrifugal forces. Using the force balance between surface tension and net centrifugal forces (centrifugal force minus buoyancy force), the critical rotational speed of the mold for the separation of the particles and the bubbles during centrifugal casting is calculated. The critical rotational speed of the mold to separate the particle from the bubble is lower for a small particle attached to a larger bubble, as compared to the case when a large particle is attached to a smaller bubble. For a given bubble size, the critical rotational speed of the mold to separate the bubble from the particle decreases with increasing particle size. For the specific case of spherical 5-μm radius graphite particles dispersed in copper alloy melt, it was found that even at a low semiapical angle of about 9 deg, the critical rotational speed needs to be around 5000 rpm for a bubble size of 500-μm radius and 0.09-m-diameter mold. The rotational speed decreases to 1000 rpm when the graphite particle radius increases to 100 μm for the same bubble size in copper alloy melt.     

A two-phase flow model of the stirring of Al-SiC composite melt

A two-phase flow, three-dimensional, steady-state model is developed to study the flow field and volume fraction distribution in a stirred tank used in the processing of silicon carbide-reinforced aluminum composites in the melt state. The aim is to optimize the stirring to obtain a good mixing of SiC particles. The model is based on the general-purpose code PHOENICS. In addition to the liquid-aluminum phase, the SiC particles are treated as a nonviscous second phase. Interphase momentum transfer occurs through a drag force. Sedimentation is simulated by assigning a high viscosity to the second phase and removing the gravity force when particle concentration reaches a critical value. The stirrers' blades impart a momentum on both phases, proportional to their respective volume fractions. A water model is simulated first, followed by the real Al-SiC melt. The study reveals the importance of particle size that affects the drag force applied on the particles and hence their motion and distribution. The model can be used to study the effect on mixing of tank geometry and the stirrers' operation.     

Electromagnetic and fluid flow phenomena in a mercury model system of the electromagnetic casting of aluminum

A mathematical formulation has been developed to represent the electromagnetic force field and the velocity field in the melt for the electromagnetic casting of aluminum. The theoretical predictions based on fundamental considerations are compared with experimental measurements obtained on a physical model system. The measurements and predictions were found to be in good agreement, regarding both the velocity fields and the electromagnetic force fields. The principal conclusion emerging from this work is of critical importance in achieving the dual objective, that is providing a restraining force, while minimizing the melt velocity perpendicular to the free surface. The mathematical formulation presented in the paper provides the theoretical framework for quantitatively defining these conditions in terms of the coil and the shield parameters. J.L. Meyer, formerly with the Department of Materials Science and Engineering at Massachusetts Institute of Technology N. EL-KADDAH, formerly with the Department of Materials Science and Engineering at MIT     

Redistribution of particles during casting of composite melts: Effects of buoyancy and particle pushing

When a melt containing a dispersion of second phase particles is solidified, the initial distribution of the particles can change due to three phenomena, namely, buoyant motion of the particles, pushing of the particles by the moving solidification front, and by convection currents in the melt. This paper presents a computer simulation model using which, the net redistribution due to the combined effect of the first two phenomena can be predicted. the existing theory for calculating the critical velocity for particle pushing is extended to include the effect of the buoyancy force and a numerical correlation is developed for easy calculation of the critical velocity. This correlation is incorporated into a computer programme which tracks the position, velocity and direction of the solidification front as well as the position of each particle in the melt as a function of time. The final positions of the particles describe the distribution of the particles in the solidified material. Predicted distributions for various heat extraction rates and particle sizes are presented for a system of silicon carbide particles in a pure aluminium melt solidifying unidirectionally as well as multidirectionally in cylindrical moulds. It is shown that for any heat extraction rate there is an optimum particle size which gives the maximum uniformity of distribution in the solidified material.     

离心铸造高速钢轧辊偏析控制技术研究

偏析严重影响高速钢轧辊组织和性能,离心铸造高速钢轧辊偏析的主要原因是高速钢熔液中存在密度不同的原子簇团。首次采用单辊激冷制带设备制备了高速钢薄带,并对激冷薄带进行X射线衍射分析,发现高速钢熔液中存在原子簇团,提出了改变原子簇团在离心力场的移动规律和提高凝固冷却速度,有利于减轻离心铸造高速钢轧辊偏析,改善高速钢轧辊性能。     

圆坯凝固末端电磁搅拌作用下的流动与传热行为

以特殊钢圆坯连铸为研究对象, 建立了研究凝固末端电磁搅拌作用效果的三维耦合数值模型.利用分段计算模型获得末端电磁搅拌区域钢液流动与凝固的实际状态, 并采用达西源项法处理凝固末端钢液在糊状区的流动, 研究了不同电磁搅拌工艺参数下的电磁场分布及钢液的流动与传热特征.通过测量搅拌器中心线磁感应强度和铸坯表面温度验证了模型的准确性.研究结果表明: 电流强度每增加100 A, 搅拌器中心磁感应强度增加19.05 mT, 电磁力随着电流强度的增加显著增大.在20~40 Hz范围, 随着电流频率的提高, 中心磁感应强度略微下降, 但电磁力仍有所增加.在搅拌器区域, 液相穴内的钢液在切向电磁力的作用下旋转流动, 其切向速度随着电流强度和频率的增加而变大.末端电磁搅拌可促进钢液在圆坯径向的换热, 随着电流强度和频率的提高, 铸坯中心轴线上的钢液温度降低, 同时末端搅拌位置处的中心固相分率增加.      

直流电弧炉钢棒式底电极热状态数值模拟

 采用Fluent软件和文献建立的二维传热模型,对底电极稳态和非稳态的传热过程进行了数值模拟。计算中考虑了底电极物性随温度的变化、空气隙的等效对流换热系数、底电极内部电磁力和焦耳热的影响等,并采用UDF函数加以实现。通过数值模拟,研究不同底电极结构、电流强度以及绝热/绝缘材料熔损对底电极热状态的影响。计算得到的冷却水进出口温差随冶炼时间的变化与现场实测结果吻合,进一步验证了所建模型和参数选择的合理性。计算结果表明,底电极周围绝热/绝缘材料熔损和电磁力对底电极的热状态和熔化深度起着重要的作用。     

Mathematical modeling of particle segregation during centrifugal casting of metal matrix composites

A one-dimensional transient heat-transfer model coupled with an equation for force balance on particles is developed to predict the particle segregation pattern in a centrifugally cast product, temperature distribution in the casting and the mold, and time for complete solidification. The force balance equation contains a repulsive force term for the particles that are in the vicinity of the solid/liquid interface. The solution of the model equations has been obtained by the pure implicit finite volume technique with modified variable time-step approach. It is seen that for a given set of operating conditions, the thickness of the particle-rich region in the composite decreases with an increase in rotational speed, particle size, relative density difference between particles and melt, initial pouring temperature, and initial mold temperature. With reduced heat-transfer coefficient at the casting/mold interface, the solidification time increases, which, in turn, results in more intense segregation of solid particulates. Again, with increased initial volume fraction of the solid particulates in the melt, both the solidification time and the final thickness of the particulate-rich region increase. It is noted that for Al-Al₂O₃ and Al-SiC systems, in castings produced using finer particles, lower rotational speeds, and an enhanced heat-transfer coefficient at the casting/mold interface, the volume fraction of particles in the outer layer of the casting remains more or less the same as in the initial melt. However, for castings produced with coarser particles at higher rotational speeds and reduced heat-transfer coefficients at the casting/mold interface, intense segregation is predicted even at the outer periphery of the casting. In the case of the Al-Gr system, however, intense segregation is predicted at the innermost layers.     

Suppression of channel convection in solidifying Pb-Sn alloys via an applied magnetic field

Channel convection through the porous, dendritic mushy zone in solidifying alloys results from a nonlinear focusing mechanism, whereby liquid enriched in the solute melts dendrites as it convects away from the solid. The local melting reduces the parameterized (Darcy) viscous force and increases the flow speed to form a convective channel. However, it has been predicted that an applied magnetic field might prevent channels from forming because, as the Lorentz force replaces the Darcy force as the primary resistance to flow, the retarding force becomes less sensitive to the lengthscale of the flow, so that the focusing mechanism no longer operates. In this study, it is found experimentally that, as predicted, an applied horizontal magnetic field can suppress channel convection when Q _m , the Chandrasekhar number appropriate to a mushy zone, exceeds an order of one. The nondimensional number Q _m is a measure of the strength of the Lorentz force relative to the Darcy force in the mushy zone and, for a given magnetic field, is much smaller than the analogous Chandrasekhar number (Q) for the fluid melt, since the Darcy force in the mushy zone far exceeds the viscous force in the fluid. Previous experimental work failed to find that magnetic fields could suppress channel convection because, although Q exceeded an order of one, Q _m did not. For experiments with a smaller cooling rate, and, thus, a larger permeability and larger mushy zone Rayleigh number (Ra _m ) a stronger magnetic field is necessary to suppress channel convection. The longitudinal macrosegregation is not affected by the absence of channel convection, suggesting that such channels are not always primarily responsible for the mass flux between the mushy zone and the melt.     

The effect of curvature on the grain boundary migration induced by diffusional coherency strain in mo-ni alloy

When a liquid phase sintered Mo-Ni alloy is heat-treated at 1400°C after replacing the liquid matrix with a Cu melt, the grain boundaries between some grains migrate, producing a Ni depleted Mo-Ni solid solution behind them. The phenomenon is same as those commonly referred to as DIGM with the Cu melt acting as the sink for Ni atoms. When Fe of 1% by weight is added to the Cu melt, the grain boundaries do not migrate, because the compressive coherency strain produced by Ni diffusion from the lattice is exactly compensated by the tensile strain due to the Fe diffusion into it. The diffusional coherency strain energy is thus shown to be the driving force for the grain boundary migration. Because Mo is insoluble in liquid Cu, the grain boundaries are pinned at the grooved ends. The grain boundary curvature thus increases during the migration, causing a migration reversal and consequently an oscillatory motion. The observed critical curvature for the migration reversal falls closely into the range predicted on the basis of the generation of misfit dislocations when the migration velocity decreases to a critical value because of the curvature. The reversal of the grain boundary migration resulting in an oscillatory motion is thus shown to be a natural consequence of the coherency strain hypothesis for the driving force if the inhibiting effect of the grain boundary curvature is taken into account.     

氟钽酸钾及其混合熔盐体系表面张力的测定

推导了柱体最大拉力法的一个经验公式用于测定熔盐体系的表面张力，并验证了该公式的正确和实用性，并用该最大拉力法测定了氟钽酸钾及其和氯化钠，氯化钾，氟化钠，氟化钾等混合熔盐体系在熔融状态下的表面张力。     

Liquid Metal Flow in Horizontal Rods

The flow velocity and flow pattern of liquid tin contained in a long horizontal boat have been determined by radioactive tracer techniques. The major flow resulted from buoyancy forces generated by an imposed temperature gradient along the melt. The flow pattern in the longitudinal direction was observed to be unicellular. Flow in planes transverse to the longitudinal direction was also observed. A small adverse vertical temperature gradient was detected in the melt and is believed to be the driving force for the transverse flow. The results indicate that the flow velocity increases linearly with the average temperature gradient between the hot and cold ends of the melt, in the temperature range examined. The velocity is reproducible and is not particularly sensitive to slight variations in experimental procedure. The velocity is not dependent on the temperature distribution (linear or nonlinear) along the melt, providing there are no sections of the melt with a zero temperature gradient. In this case, fluid does not flow through these sections. The flow velocity increases with increasing average temperature of the melt. The results are in general agreement with the results predicted by a modification of Batchelor’s solution of fluid flow in a rectangular cavity. L. C. MacAULAY, Formerly a Graduate Student, Department of Metallurgy, University of British Columbia     

Structure and properties of melt-quenched manganese- and nickel-diamagnetic element systems

The choice of melt quenching temperatures from a single-phase region is justified by theoretical calculations of the positions of maxima of the immiscibility regions in liquid Mn-Pb, Ni-Pb, and Mn-Bi alloys, and the influence of an alloy composition and a high cooling rate on the phase formation in these alloys and their magnetic properties is studied. The coercive force of an Mn-Bi alloy is shown to be increased by increasing the melt quenching temperature to 1923 K to form nanocrystals of the ??-MnBi phase.     

Capillary interaction between inclusion particles on the 16Cr stainless steel melt surface

Both in situ observational and theoretical analyses were carried out for inclusion particle behavior on a 16Cr stainless steel melt surface by paying special attention to the phase classification of inclusions and to the differences in interaction due to the type of phase (solid, liquid, or complex). The interaction was attractive between pairs of particles of the same kind, such as between solid-solid, complex-complex, liquid-liquid, or solid-complex particles, but it was repulsive for pairs of particles of different kinds, such as between solid-liquid or complex-liquid particles. As a result, this reverse phenomenon leads to selective interaction among various inclusions. The origin of attraction or repulsion between inclusion particles is the capillary force. This capillary interaction is strongly influenced by the particle size and shape, and by the contact angle of a particle with steel melt; it is less influenced by the particle density and shape, and by the interfacial tension. In particular, the degree of attractive or repulsive force strongly depends on the contact angle of a particle. Thus, some chalcogen elements should strongly affect the interaction of particles.