A numerical and experimental study of the rate of transformation in three directionally grown peritectic systems

Three systems, Ag-Sn, Cu-Sn, and Cd-Ag, with very different peritectic phase diagrams, have been investigated. Specimens were directionally grown then quenched. The volume fraction of the two solid phases and the quenched liquid were measured using image analysis of cross sections. A numerical model was developed to calculate the quantity of the phases as a function of temperature and the results were compared with experiment. It is concluded that the quantities of the different phases can be predicted reasonably well with a simple diffusion model. The shape of the phase diagram, the size of the primary phase, and the solid-state diffusion coefficients play a crucial role in determining the volume fraction of the final products.     

Rheological behavior of Al-Cu alloys during solidification constitutive modeling,experimental identification,and numerical study

The rheological behavior of a solidifying alloy is modeled by considering the deforming material as a viscoplastic porous medium saturated with liquid. Since the solid grains in the mush do not form a fully cohesive skeleton, an internal variable that represents the partial cohesion of this porous material is introduced. The model parameters are identified using shear and compressive stress states under isothermal conditions on an Al-Cu model alloy. The model is partially validated with non-isothermal conditions and we complete this study with tensile conditions. Such conditions, when applied on the mush, may lead to severe defects in many casting processes. The model has been implemented into a commercial finite-element code to simulate a tensile test. Comparison with experimental data shows that the model is able to reproduce the main features of a solidifying alloy under tension, although fracture is not directly addressed here. We show that two critical solid fractions must be introduced in the model to account for the rheology: the coherency solid fraction at which the mush acquires significant strength and the coalescence solid fraction at which solid grains start to form solid bridges.     

An experimental and analytical study of the solidification of a binary dendritic system

Experimental measurements are reported on the controlled solidification of a 30 pct aqueous solution of ammonium chloride in a two-dimensional slot. The actual measurements taken include the transient temperature profiles, the transient velocity fields (using tracers) and photographic observations. Through the statement of the differential energy balance and the laminar Navier-Stokes equations, written for both the liquid and the two phase regions, a mathematical model has been developed for the system. The theoretically predicted temperature and velocity profiles were found to be in good agreement with the experimentally measured values for both the two phase regions and the liquid region. For the experimental conditions the velocities in the liquid region were of the order of 0.4 to 0.8 cm/s, while the corresponding values for the two phase region were at least an order of magnitude smaller.     

An experimental and analytical study of the solidification of a binary dendritic system

Experimental measurements are reported on the controlled solidification of a 30 pct aqueous solution of ammonium chloride in a two-dimensional slot. The actual measurements taken include the transient temperature profiles, the transient velocity fields (using tracers) and photographic observations. Through the statement of the differential energy balance and the laminar Navier-Stokes equations, written for both the liquid and the two phase regions, a mathematical model has been developed for the system. The theoretically predicted temperature and velocity profiles were found to be in good agreement with the experimentally measured values for both the two phase regions and the liquid region. For the experimental conditions the velocities in the liquid region were of the order of 0.4 to 0.8 cm/s, while the corresponding values for the two phase region were at least an order of magnitude smaller.     

Computational and experimental study of turbulent flow in a 0.4-scale water model of a continuous steel caster

Single-phase turbulent flow in a 0.4-scale water model of a continuous steel caster is investigated using large eddy simulations (LES) and particle image velocimetry (PIV). The computational domain includes the entire submerged entry nozzle (SEN) starting from the tundish exit and the complete mold region. The results show a large, elongated recirculation zone in the SEN below the slide gate. The simulation also shows that the flow exiting the nozzle ports has a complex time-evolving pattern with strong cross-stream velocities, which is also seen in the experiments. With a few exceptions, which are probably due to uncertainties in the measurements, the computed flow field agrees with the measurements. The instantaneous jet is seen to have two typical patterns: a wobbling “stair-step” downward jet and a jet that bends upward midway between the SEN and the narrow face. A 51-second time average suppressed the asymmetries between the two halves of the upper mold region. However, the instantaneous velocity fields can be very different in the two halves. Long-term flow asymmetry is observed in the lower region. Interactions between the two halves cause large velocity fluctuations near the top surface. The effects of simplifying the computational domain and approximating the inlet conditions are presented.     

板坯连铸结晶器钢液行为的水模拟试验

以某钢厂250mm×2 100mm断面板坯连铸结晶器为研究对象,采用水力模拟的方法研究了拉速在0.88m/min条件下不同水口结构参数对钢液行为的影响。研究表明:1号水口(原方案)在浸入深度为57mm时,液面波峰谷差值达到3.67mm,卷渣严重,冲击深度为205mm;优化设计的5号水口在浸入深度为57mm时,液面波峰谷差值减小为2.74mm,卷渣现象基本消失,冲击深度为177mm。确定5号水口为最优水口结构参数,浸入深度以57~67mm为宜。     

仿真计算在板坯连铸机工程设计中的应用

针对板坯连铸机设计,采用数值仿真技术（基于板坯连铸浇铸过程静态仿真软件CCPS OFFLINE）对二次冷却水量进行了优化计算,并对结晶器锥度、扇形段辊缝和铸坯应变分布等相关的重要工艺／结构参数进行了理论预测。部分研究成果已投入到某钢厂板坯连铸机的实际生产运用中,取得了较好的效果,并在其新建板坯连铸机的设计工作中进行了充分应用。     

Analytical,numerical, and experimental analysis of inverse macrosegregation during upward unidirectional solidification of Al-Cu alloys

The present work focuses on the influence of alloy solute content, melt superheat, and metal/mold heat transfer on inverse segregation during upward solidification of Al-Cu alloys. The experimental segregation profiles of Al 4.5 wt pct Cu, 6.2 wt pct Cu, and 8.1 wt pct Cu alloys are compared with theoretical predictions furnished by analytical and numerical models, with transient h _i profiles being determined in each experiment. The analytical model is based on an analytical heat-transfer model coupled with the classical local solute redistribution equation proposed by Flemings and Nereo. The numerical model is that proposed by Voller, with some changes introduced to take into account different thermophysical properties for the liquid and solid phases, time variable metal/mold interface heat-transfer coefficient, and a variable space grid to assure the accuracy of results without raising the number of nodes. It was observed that the numerical predictions generally conform with the experimental segregation measurements and that the predicted analytical segregation, despite its simplicity, also compares favorably with the experimental scatter except for high melt superheat.     

A numerical and experimental study of deformation characteristics of the plane strain punch stretching test

Recently, a simple new test method called the plane strain stretching (PSS) test has been developed to evaluate the stamping formability of sheet materials. The PSS test has been proven to have good reproducibility and show good correlation with press performance. In order to clarify the deformation characteristics of the PSS test and investigate the effect of material and process variables on the performance of the PSS test, three-dimensional finite element simulations for the PSS test were performed and the results compared with experiments.     

Heat transport and solidification in the electromagnetic casting of aluminum alloys: Part I. Experimental measurements on a pilot-scale caster

While many investigators have examined electromagnetic and magnetohydrodynamics phenomena in electromagnetic casting (EMC) of aluminum, there appears to be no published work on heat transport and solidification in such casters. This two-part series is an attempt to remedy this deficiency. The first part describes two experimental campaigns, carried out on a pilot-scale electromagnetic caster at Reynolds Metals Company, in which sacrificial thermocouples were used to obtain many data on temperature distributions within the aluminum of a pilot-scale caster and thereby to obtain the shape of the liquid metal pool (“sump”). The data reveal a strong dependence of temperature distribution and sump depth on casting speed but a relatively weak dependence on the flow rate of the quenching water striking the outside of the ingot.     

Effect of in-mould electromagnetic stirring on the flow,initial solidification and level fluctuation in a slab mould: a numerical simulation study

In-mould electromagnetic stirring (M-EMS) is a widely used technique during slab continuous casting. To investigate the effect of M-EMS on the flow and initial solidification in a slab mould, a three-dimensional model coupling electromagnetic field, flow and solidification was developed. To track the steel/slag interface, the volume of fraction (VOF) model was coupled in the model. The electromagnetic force presents a centrally symmetrical distribution on the cross-section. The influences of stirring current and stirrer position on flow, initial solidification and level fluctuation have been discussed. M-EMS enhances the transversal flow and shrinks the lower recirculation in mould. The results indicate that both a higher stirring current and a lower stirrer position produce a reduction of the temperature in the mould centre. Furthermore, a higher stirrer position and a lower stirring current are favourable to the solidified shell growth. However, M-EMS aggravates the level fluctuation, which may lead to slag entrainment.     

An experimental and numerical study of cyclic deformation in metal-matrix composites

The cyclic stress-strain characteristics of discontinuously reinforced metal-matrix composites are studied both experimentally and numerically. The model systems used for investigation are aluminum alloys reinforced with SiC particulates and whiskers. Finite element analyses of the fatigue deformation of the composite are performed within the context of axisymmetric unit cell formulations. Two constitutive relations are used to characterize the matrix of the composite: the fully dense Mises model of an isotropically hardening elastic-viscoplastic solid and the Gurson model of a progressively cavitating elastic-viscoplastic solid (to simulate ductile matrix failure by the nucleation and growth of voids). The brittle reinforcement phase is modeled as elastic, and the interface between the ductile matrix and the reinforcement is taken to be perfectly bonded. The analyses provide insights into the effects of reinforcement shape and concentration on (1) constrained matrix deformation under cyclic loading conditions, (2) cyclic hardening and saturation, (3) the onset and progression of plastic flow and cavitation within the matrix, and (4) cyclic ductility. The numerical predictions of flow strength, strain hardening, evolution of matrix field quantities, and ductility under cyclic loading conditions are compared with those predicted for monotonic tensile deformation and with experimental observations. formerly Visiting Scientist, Division of Engineering, Brown University     

A melting and solidification study of alloy 625

The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M₆C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M₆C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m _L , m _S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.     

Relation between the solidification rate of a binary alloy and the alloying-element distribution coefficient

The change in the rate of equilibrium and nonequilibrium (according to Petrov–Scheil) solidification in binary model and some real solid-solution alloys with a distribution coefficient 0.2–2.5 is comprehensively analyzed. A relation between the solidification rate, on the one hand, and growth restriction factor Q and supercooling parameter P, which are used to determine the grain sizes in as-cast alloys, is revealed.     

Effect of the casting rate of a billet on the resistance of the mold shell in a high-capacity section continuous caster

Numerical experiments demonstrate the presence of a certain casting-rate range in which the resistance of the mold increases. To determine the boundaries of the optimum casting-rate range in any particular case, one has to analyze a calculated curve that describes the force interaction intensity in the bottom portion of the mold as a function of the casting rate.     

A water model and numerical study of the spout height in a gas-stirred vessel

The height and width of spouts in a water model of a gas-stirred ladle were measured photographically. The shapes were found to be Gaussian, and correlations were developed for the height and width. To mathematically model the free surface effects, a novel combined staggered-grid and volume-of-fluid computational code was developed. It was tested successfully against an established code in the “broken dam” validation test. The new free surface code was added to an existing code for a gasstirred plume to model the entire vessel. It was found that the code under-predicted the actual spout height. The model handled the momentum of the plume accurately, but did not account for the phenomena associated with the bubbles breaking the surface. The importance of these inter-related phenomena had not been appreciated previously.     

The effect of solidification rate on microsegregation

An X-ray diffraction technique has been utilized to determine the second phase content and average composition of the primary phase in aluminum-copper and aluminum-silicon alloys solidified at a cooling rate in the range of 0.06 to l0⁵ K/s. For the Al-Cu samples, the normalized Θ phase content (ratio of the 6 content to the value predicted by the Scheil model) was found to be 0.71 at 0.1 K/s (solidification rate = 0.001 cm/s) and to increase with increasing cooling rate to 0.96 at about 180 K/s (1 cm/s). Beyond this cooling rate, it decreased with increasing the cooling rate to 0.44 at about 3.7 x lO4 K/s. The same trend was observed in the Al-Si samples, except that the normalized silicon content was much lower. Also, for both systems the normalized average composition of the primary phase was found to decrease progressively with increasing the solidification rate until it reached a minimum at 1 cm/s, beyond which it increased with higher solidification rates. The results are discussed with respect to the prevailing segregation models that include back-diffusion in the solid, dendrite tip undercooling, and the eutectic temperature depression. An equation which combines these effects at all cooling rates is given.     

大型圆台钢锭定向凝固的数值模拟

利用大型软件CFX建立大型圆台钢锭凝固过程温度场的数学模型，分析讨论了不同换热条件对钢锭凝固进程的影响。