Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys

We analyzed the columnar solidification of a binary alloy under the influence of an electromagnetic forced convection of various types and investigated the influence of a rotating magnetic field on segregation during directional solidification of Al-Si alloy as well as the influence of a travelling magnetic field on segregation during solidification of Al-Ni alloy through directional solidification experiments and numerical modeling of macrosegregation. The numerical model is capable of predicting fluid flow, heat transfer, solute concentration field, and columnar solidification and takes into account the existence of a mushy zone. Fluid flows are created by both natural convection as well as electromagnetic body forces. Both the experiments and the numerical modeling, which were achieved in axisymmetric geometry, show that the forced-flow configuration changes the segregation pattern. The change is a result of the coupling between the liquid flow and the top of the mushy zone via the pressure distribution along the solidification front. In a forced flow, the pressure difference along the front drives a mush flow that transports the solute within the mushy region. The channel forms at the junction of two meridional vortices in the liquid zone where the fluid leaves the front. The latter phenomenon is observed for both the rotating magnetic field (RMF) and traveling magnetic field (TMF) cases. The liquid enrichment in the segregated channel is strong enough that the local solute concentration may reach the eutectic composition.     

Effects of Thermoelectric Magnetic Convection on the Solidification Structure During Directional Solidification under Lower Transverse Magnetic Field

This work investigated the thermoelectric magnetic convection (TEMC) during directional solidification under a transverse magnetic field numerically and experimentally. Numerical results show that the TEMC will form in liquid near the liquid/solid interface and in the dendritic network. The value of the TEMC mainly depends on the crucible diameter, the temperature gradient, and the magnetic field intensity. The value of the TEMC increases as the crucible diameter and the temperature gradient are increased. The value of the TEMC on the sample scale increases to a maximum when the magnetic field is of the order of 0.1 T, and then decreases as the magnetic field still increases. However, the value of the TEMC on the cell/dendrite scale continues to increase with the increase of the magnetic field intensity when the applied magnetic field is less then 1 T. Two alloys are solidified directionally in the vertical configuration under a transverse magnetic field, and results show that the application of a lower transverse magnetic field (B < 1 T) modified the liquid/solid interface shape and the cellular/dendritic array significantly. Indeed, it was observed that, along with the refinement of the cell/dendrite, the magnetic field caused the deformation of the liquid/solid interfaces and the extensive segregations (i.e., channel and freckle) in the mushy zone. Comparison of the numerical and experimental results shows that the modification amplitude of the liquid/solid interface and the cellular/dendritic morphology is in good agreement with the value of the TEMC at the liquid/solid interface and in the dendritic network. This implies that changes of the interface shape and the cellular/dendritic morphology should be attributed, respectively, to the TEMC on the sample and the cell/dendrite scales.     

定向凝固过程中固液界面稳定性的研究现状

界面稳定性是金属定向凝固过程中一个很重要的问题,关于它的研究也层出不穷。综述了这一领域的主要进展,着重讨论了基于MS理论研究的各种界面稳定性理论,并指出电磁场对界面稳定性的影响,同时分析了需进一步研究的主要方向。     

扰动对单相合金定向凝固固液界面生长形态的影响

研究了扰动对单相合金在定向凝固过程中平面晶和胞晶固液界面生长形态的影响规律,给出了单相合金由平面晶向胞晶转变的临界条件及胞晶稳定生长的条件.首次用非线性理论中的线性稳定性理论,证明了单相合金在定向凝固过程中,固液界面由平面晶向胞晶转变和胞晶生长的分叉机制是亚临界分叉。利用有机物模拟合金BrC₄,在固液界面前沿有扰动的情况下进行实验,研究了固液界面前沿扰动对固液界面平胞转变和胞晶生长的分叉机制的影响规律,证明了其分叉机制是正确的.     

Motion of Solid Grains During Magnetic Field-Assisted Directional Solidification

In this paper, we report the visible evidence for thermoelectric magnetic forces (TEMFs) during magnetic field-assisted directional solidification, and their potential to control the motion of solid grains(dendrite fragments or equiaxed grains). These motions are observed by means of synchrotron X-ray radiography and compared with analytic calculations for a spherical particle’s motion driven only by TEMFs, which confirms that the observed solid grain motions are the combined result of the TEMFs and gravity. We also carried out corresponding 3D numerical simulations to validate the calculations and further prove our conclusion that TEMF acts on the solid grain and affects its motion trajectory.     

Influence of Pressure Field in Melts on the Primary Nucleation in Solidification Processing

It is well known that external fields applied to melts can cause nucleation at lower supercoolings, fragmentation of growing dendrites, and forced convection around the solidification front. All these effects contribute to a finer microstructure of solidified material. In this article, we analyze how the pressure field created with ultrasonic vibrations influences structure refinement in terms of supercooling. It is shown that only high cavitation pressures of the order of 10⁴ atmospheres are capable of nucleating crystals at minimal supercoolings. We demonstrate the possibility of sononucleation even in superheated liquid. Simulation and experiments with water samples show that very high cavitation pressures occur in a relatively narrow zone where the drive acoustic field has an appropriate combination of pressure amplitude and frequency. In order to accurately predict the microstructure formed by ultrasonically assisted solidification of metals, this article calls for the development of equations of state that would describe the pressure-dependent behavior of molten metals.     

抽拉速率对高温合金叶片定向凝固组织的影响

采用以液态金属锡作为冷却介质的定向凝固设备,研究了五种不同的抽拉速率对叶片定向结晶组织的影响。结果表明抽拉速率对高温合金叶片的定向结晶组织具有重要的影响,抽拉速率为6mm/min时,可保证叶片的良好成形,定向柱晶挺直,晶粒沿竖直方向排列整齐,晶粒大小均匀。抽拉速率增大,合金组织细化,枝晶间距变小。     

Reaction of Ni-Based Superalloy with Liquid Sn During Liquid-Metal-Cooled Directional Solidification

The liquid metal cooling (LMC) process has attracted increasing attention in the investment casting industry in recent years. Liquid Sn is generally used as the cooling medium in state-of-the-art LMC processes even though Sn is known to be a detrimental element in Ni-based superalloys. Therefore, Sn contamination in superalloys has become one of the top concerns for the LMC process. In this work, the reaction between liquid Sn and a Ni-based superalloy was investigated. The detectable reaction between superalloy and liquid Sn began at approximately 500 °C, and the reaction products became complex with increasing temperature. At high temperatures beyond 750 °C, a very short contact period of less than 1 minute led to a severe surface reaction. The results were compared to the surface reaction zone of the large blade. The critical time when the superalloy casting contacted liquid Sn is obtained based on experimental observations and numerical simulations. The surface reaction will occur if the ceramic mold cracked at this point or previously. The surface contamination during LMC solidification is associated with the volume of the casting. The present results indicate that surface reaction would be avoided if the volume of the large blade is reduced to ~?30 pct of the original size.     

Influence of Rare Earths on the Directional Solidification Microstructure of Tin-Lead Eutectic Alloy

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Effects of a High Magnetic Field on the Microstructure of Ni-Based Single-Crystal Superalloys During Directional Solidification

High magnetic fields are widely used to improve the microstructure and properties of materials during the solidification process. During the preparation of single-crystal turbine blades, the microstructure of the superalloy is the main factor that determines its mechanical properties. In this work, the effects of a high magnetic field on the microstructure of Ni-based single-crystal superalloys PWA1483 and CMSX-4 during directional solidification were investigated experimentally. The results showed that the magnetic field modified the primary dendrite arm spacing, γ′ phase size, and microsegregation of the superalloys. In addition, the size and volume fractions of γ/γ′ eutectic and the microporosity were decreased in a high magnetic field. Analysis of variance (ANOVA) results showed that the effect of a high magnetic field on the microstructure during directional solidification was significant (p < 0.05). Based on both experimental results and theoretical analysis, the modification of microstructure was attributed to thermoelectric magnetic convection occurring in the interdendritic regions under a high magnetic field. The present work provides a new method to optimize the microstructure of Ni-based single-crystal superalloy blades by applying a high magnetic field.     

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.     

Dependence of Competitive Grain Growth on Secondary Dendrite Orientation During Directional Solidification of a Ni-based Superalloy

The competitive grain growth in bicrystal samples during unidirectional solidification of a Ni-based superalloy was found to depend on secondary dendrites perpendicular to the grain boundary of bicrystal samples, rather than primary dendrites parallel to the thermal gradient as generally recognized. The primary dendrite orientation, however, had significance for the dendrite blocking in overgrowth processes and the resultant overgrowth rate during competitive grain growth.     

电子束诱导定向凝固过程中晶体形貌对杂质分布的影响

产业条件下,利用电子束诱导定向凝固技术提纯多晶硅实现晶硅尾料的循环再利用,在硅锭中包括类单晶和柱状晶两种晶体形貌。与多晶区域相比,类单晶区域电阻率和少子寿命等电学性能分布比较均匀,铁杂质的含量分布也较均匀,其质量百分数平均值为0.000031%。电子束诱导定向凝固过程中类单晶的出现,不仅可以保证铸锭提纯区金属杂质成分均匀,而且可以进一步促进杂质向铸锭顶部富集,铸锭顶部的铁杂质含量高达0.101%。因此,利用电子束诱导类单晶生长成为可能,促进金属杂质的去除,为循环硅料的再生提供途径。     

On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidification

Metallurgical and Materials Transactions A - The influence of the melt flow on the solidification structure is bilateral. The flow plays an important role in the solidification pattern, via the...     

Influence of Electropulsing Treatment on the Initial Solidification of Molten Steel During Continuous Casting

Metallurgical and Materials Transactions B - An electropulsing-assisted mold simulator (EPMS) technique was developed to investigate the effect of pulsed electric current on the initial...     

The Effect of Processing Conditions on Heat Transfer During Directional Solidification via the Bridgman and Liquid Metal Cooling Processes

Finite-element-based solidification modeling was used to investigate the thermal characteristics of the Bridgman and liquid metal cooling (LMC) directional solidification (DS) processes. Physically representative boundary conditions were implemented within a finite-element model to test its applicability to a broad range of processing conditions. The dominant heat-transfer step for each case was identified. Relationships between the thermal gradient and the solid–liquid interface position relative to the transition region of the furnace were developed. The solidification rate, the local velocity of the solid–liquid interface, and the cooling rate as a function of withdrawal rate were analyzed. The curvature of the solid–liquid interface varies with the processing conditions and influences the local thermal condition and, therefore, the morphological development of dendritic structure during solidification. An extensive sensitivity analysis of process conditions was conducted for both the Bridgman and LMC techniques. The relative importance of process parameters on the resulting thermal conditions during solidification was identified. A protocol for determination of preferred process conditions was defined. The maximum axial thermal gradient at the surface of the casting occurs when the solid–liquid interface is just above the baffle for both the Bridgman and LMC DS processes, independent of casting geometry or mold-heater temperature.