Numerical Research on Magnetic Field,Temperature Field and Flow Field During Melting and Directionally Solidifying TiAl Alloys by Electromagnetic Cold Crucible

Metallurgical and Materials Transactions B - The electromagnetic cold crucible (EMCC) technique is an effective method to melt and directionally solidify reactive and high-temperature materials...     

Microstructure Evolution in Directionally Solidified Fe-Ni Alloys in Diffusive Regime

Directional solidification experiments have been carried out in Fe-Ni peritectic alloys to study microstructure evolution in diffusive regime. A numerical modeling of melt convection was developed and discussed to investigate the convective velocity in samples of different diameters. The simulation results show that convection effects can be reduced by decreasing the sample diameter, and diffusion-controlled growth can be achieved if the sample diameter is smaller than about 1 mm. Based on the simulation results, experiments were performed in thin samples of 1 mm diameter to obtain microstructures in diffusive regime. Different kinds of microstructure evolutions were observed in the directionally solidified Fe-Ni alloys. The time-dependent microstructure evolution implies that the solidification process seemed to be in non-steady state rather than in steady state. Based on the transient model, solute distributions in the liquid ahead of the primary and peritectic phases were discussed to reveal the microstructure evolution. Since the solute partition coefficients of the primary and peritectic phases are different, the magnitudes of solute element rejected by the two solid phases are different in two-phase growth conditions. And within the boundary layer, the solute fields ahead of the primary and peritectic phases are different owing to the weak effect of solute mixing in diffusion-controlled regime. Furthermore, microstructure evolution in peritectic alloys was discussed based on the analysis of solute redistribution.     

The Influence of Fluid Flow on the Microstructure of Directionally Solidified AlSi-Base Alloys

To obtain a quantitative understanding of the effect of fluid flow on the microstructure of cast alloys, a technical Al-7 wt pct Si-0.6 wt pct Mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions. The solidification was studied in an aerogel-based furnace, which established flat isotherms and allowed the direct optical observation of the solidification process. A coil system around the sample induces a homogeneous rotating magnetic field (RMF) and, hence, a well-defined flow field close to the growing solid-liquid interface. The application of RMFs during directional solidification results in pronounced segregation effects: a change to pure eutectic solidification at the axis of the sample at high magnetic field strengths is observed. The investigations show that with increasing magnetic induction and, therefore, fluid flow, the primary dendrite spacing decreases, whereas the secondary dendrite arm spacing increases. An apparent flow effect on the eutectic spacing is observed. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: in Honor of Prof. John Hunt,” which occurred March 13–15, 2006 during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

定向凝固Fe-6.5%Si合金显微组织的EBSD分析

采用定向凝固方法制备了Fe-6.5%Si实验合金板,并进行了金相观察、X射线衍射分析,EBSD分析和磁性测定。结果表明,合金板由沿定向凝固方向（100）生长的粗大柱状晶所组成,柱状晶的平均截面直径约为3mm,柱状晶之间绝大部分为大角晶界,其余为少量小角晶界和重位点阵晶界。合金板具有强的立主织构并显示出优良的磁性能,其矫顽力只有3．45A／m,小于其他方法制备的Fe-6.5%Si合金。     

NiAl-Cr(Mo)-Hf共晶合金定向凝固组织的特性及力学性能

采用高温度梯度定向凝固装置制备NiAl-Cr(Mo)-Hf共晶合金,系统地研究了凝固速率对合金凝固组织和力学性能的影响.随凝固速率的增大,固液界面依次呈现平、胞、枝的形貌,而且共晶胞的尺寸和层片间距随之减小.同时发现当凝固速率在3.33μm/s到16.7μm/s范围内变化时,合金的力学性能随凝固速率的增大而增大.(Mo)-Hf;定向凝固;显微组织;力学性能     

Microstructure Formation in AlSi7Mg Alloys Directionally Solidified in a Rotating Magnetic Field

To produce high stressed automotive components like engine frames and cylinder heads in foundry industry often AlSi7Mg alloys are used. During mould filling and casting melt flow affects the development of the microstructure, which defines the mechanical properties. In this paper the microstructure formation in AlSi7Mg0.3 and AlSi7Mg0.6 alloys during directional solidification is investigated. To induce a forced melt flow a rotating magnetic field is applied. For that purpose a Bridgman‐type gradient furnace is equipped with a rotary ring magnet. For detailed investigation of the shape of the solid‐liquid interface and the primary dendrite spacing a decanting device is used. As a result, the forced melt flow substantially changes the dendritic solidification microstructure. The rotating magnetic field generates a radial secondary flow in and ahead of the mushy zone, which causes an enrichment of eutectics in the centre of the samples. At lower solidification velocities this locally leads to the transition to mixed columnar‐equiaxed or even to equiaxed growth. In that case the solid‐liquid interfaces of the decanted samples show a significant depression in the centre part. In the out‐of‐centre region columnar growth still exists and the primary dendrite spacing decreases with increasing melt flow.     

An Evaluation of Primary Dendrite Trunk Diameters in Directionally Solidified Al-Si Alloys

The primary dendrite trunk diameters of Al-Si alloys that were directionally solidified over a range of processing conditions have been evaluated. The empirical data are analyzed with a model that is an extension of one used to describe the ripening of dendrite arms. The analysis, based primarily on an assessment of secondary dendrite arm dissolution in the mushy zone, fits well with the experimental data. It is suggested that the primary dendrite trunk diameter is a useful metric that correlates well with the actual solidification processing parameters, and complements the conventionally used primary, secondary, and tertiary arm spacings.     

Macrosegregation of Impurities in Directionally Solidified Silicon

Directional solidification of molten metallurgical-grade Si was carried out in a vertical Bridgman furnace. The effects of changing the mold velocity from 5 to 110 μm seconds^–1 on the macrosegregation of impurities during solidification were investigated. The macrostructures of the cylindrical Si ingots obtained in the experiments consist mostly of columnar grains parallel to the ingot axis. Because neither cells nor dendrites can be observed on ingot samples, the absence of precipitated particles and the fulfillment of the constitutional supercooling criterion suggest a planar solid–liquid interface for mold velocities ≤10 μm seconds^–1. Concentration profiles of several impurities were measured along the ingots, showing that their bottom and middle are purer than the metallurgical Si from which they solidified. At the ingot top, however, impurities accumulated, indicating the typical normal macrosegregation. When the mold velocity decreases, the macrosegregation and ingot purity increase, changing abruptly for a velocity variation from 20 to 10 μm seconds^–1. A mathematical model of solute transport during solidification shows that, for mold velocities ≥20 μm seconds^–1, macrosegregation is caused mainly by diffusion in a stagnant liquid layer assumed at the solid–liquid interface, whereas for lower velocities, macrosegregation increases as a result of more intense convective solute transport.     

Dynamic Model for Metal Cleanness Evaluation by Melting in a Cold Crucible

Melting of metallic samples in a cold crucible causes inclusions to concentrate on the surface owing to the action of the electromagnetic force in the skin layer. This process is dynamic, involving the melting stage, then quasi-stationary particle separation, and finally the solidification in the cold crucible. The proposed modeling technique is based on the pseudospectral solution method for coupled turbulent fluid flow, thermal and electromagnetic fields within the time varying fluid volume contained by the free surface, and partially the solid crucible wall. The model uses two methods for particle tracking: (1) a direct Lagrangian particle path computation and (2) a drifting concentration model. Lagrangian tracking is implemented for arbitrary unsteady flow. A specific numerical time integration scheme is implemented using implicit advancement that permits relatively large time-steps in the Lagrangian model. The drifting concentration model is based on a local equilibrium drift velocity assumption. Both methods are compared and demonstrated to give qualitatively similar results for stationary flow situations. The particular results presented are obtained for iron alloys. Small size particles of the order of 1 μm are shown to be less prone to separation by electromagnetic field action. In contrast, larger particles, 10 to 100 μm, are easily “trapped” by the electromagnetic field and stay on the sample surface at predetermined locations depending on their size and properties. The model allows optimization for melting power, geometry, and solidification rate. This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France.     

电子束悬浮区熔定向凝固ЖC36的组织分析

以第二代单晶高温合金ЖC36为研究对象,利用高梯度电子束悬浮区熔装置,采用籽晶法制取具有不同凝固组织的单晶高温合金试样.研究了加热功率和区熔速率等凝固过程控制参数对凝固组织和固-液界面形态的影响,考察了各合金元素的偏析规律和γ/γ'共晶组织的演化规律,测定了不同组织形态单晶的晶体取向特征.     

On the Characterization of Directionally Solidified Dendritic Microstructures

A novel method for the stereological assessment of arrays of directionally solidified dendrites is presented. It is suitable particularly for the evaluation of the distribution of primary dendrite arm spacings and the dendrite coordination number. The method involves (1) determination of the center of gravity of each dendrite in the array under consideration, (2) calculation of the distances to all neighboring dendrites in the data set (3) sorting of the distances determined into ascending order (4) under the assumption of a minimum number of nearest neighbors—typically three—calculation of the average distance to these and associated standard deviation, (5) an assessment of whether the remaining neighbors are within the range of proposed nearest-neighbor distances and (6) repetition of the previous to determine a set of values for the entire data set and, thus, the distributions of dendrite arm spacings and coordination numbers. By application of the method to experimental microstructures, it is demonstrated that a detailed statistical assessment of directionally solidified dendritic arrays can be accomplished.     

Effect of Ag Content on the Microstructure and Crystallization of Coupled Eutectic Growth in Directionally Solidified Al-Cu-Ag Alloys

A series of coupled eutectic growths along the univariant eutectic groove in the ternary Al-Cu-Ag alloy was studied to investigate the effect of Ag on the microstructure and crystallization of directionally solidified Al-Cu-Ag alloys. The results indicated that the eutectic morphology and orientation relationship (OR) between eutectic phases were modified as the Ag content in the Al-Cu-Ag alloys increased. At a lower growth velocity (R ≤ 1 μm/s), a banded structure formed and the interlamellar spacing decreased with the increasing Ag content. At a higher growth velocity (R ≥ 3 μm/s), the eutectic cell spacing decreased with increasing Ag content. Increasing the Ag content in the Al-Cu-Ag alloys enhanced the enrichment of the Ag solute in the liquid ahead of the quenched liquid/solid interface. In addition, increasing the Ag content in the Al-Cu-Ag alloys promoted the transformation from a “Beta 6” OR to an “Alpha 4” OR between eutectic phases. Modifications of the eutectic morphology and the OR during directional solidification were attributed to the enrichment of Ag content at the solid/liquid interface and the changes in the interfacial energy due to the increase in Ag solubility in the α-Al phase.

     

快凝Nd-Fe-B厚带的制备与组织结构

采用单辊快凝法取代传统的铸锭法制备出了厚度为0.1～0.4mm的NdFeB厚带.通过对制备快凝厚带过程中不同的工艺参数的探索,获得了工艺参数、带片厚度及显微组织间的关系.结果表明:制备0.25～0.35mm厚带的最佳工艺参数为:辊轮转速在10m/s左右,喷射压力0.08～0.10MPa,辊嘴间距(2±0.5)mm.当带片厚度为0.3mm时,带片中以Nd₂Fe₁₄B相为主,沿着(410)方向Nd₂Fe₁₄B含量比例较大;其显微结构主要是Nd₂Fe₁₄B片状晶,富Nd薄层相之间的间距约为5 μm.带片厚度为0.4mm时,厚带试样中α-Fe含量明显大于Nd₂Fe₁₄B含量,并且择优取向变成了(008).厚度0.1mm的厚带的显微结构中是细小的急冷等轴晶,厚度0.4mm的厚带中有较大区域的等轴晶.     

The Effect of Long-Term Thermal Exposure on the Microstructure and Stress Rupture Property of a Directionally Solidified Ni-Based Superalloy

Microstructural degradation and microstructure-property relationship during long-term thermal exposure in a directionally solidified Ni-based superalloy are systematically studied. The coarsening kinetics of γ′ precipitation conforms well to the LSW model during the long-term thermal exposure. The detailed time dependence of MC decomposition during the long-term thermal exposure is revealed. Grain boundary coarsening was mainly facilitated by γ′ and M₂₃C₆ precipitates coarsening in GBs region, and the GB coarsening kinetics conforms well to the JMAK theory. During different stages of the thermal exposure, dominant factors for the decrease of stress-rupture lifetime vary due to the evolution of multiple microstructures (γ′ coarsening, MC decomposition, and grain boundary coarsening).