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.     

夹杂物在钢液凝固前沿行为的原位动态观察

通过激光扫描共焦显微镜原位动态观察了304奥氏体不锈钢液凝固过程中,钢中氧化铝夹杂被凝固前沿推动/捕捉,最后分布于奥氏体晶界/晶内的情况.在实验中发现,当冷却速率为5 K/s时,直径为7 μm的夹杂物和直径为10 μm的夹杂物被凝固前沿捕捉进入奥氏体晶粒内部.当冷却速率为0.5 K/s时,直径为5 μm的夹杂物被凝固前沿推动,凝固结束时位于奥氏体晶界.最后通过SAS Ⅱ模型,计算了不同冷却速率下夹杂物被凝固前沿推动/捕捉的临界半径,计算结果与观察到的结果吻合较好,说明根据力学平衡原理建立的SAS Ⅱ模型也可以很好地预测钢液凝固过程中夹杂物被凝固前沿推动/捕捉的临界半径.     

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.     

Simultaneous Observation of Melt Flow and Motion of Equiaxed Crystals During Solidification Using a Dual Phase Particle Image Velocimetry Technique. Part II: Relative Velocities

A two-camera Particle Image Velocimetry (PIV) technique is applied to study the flow pattern and the equiaxed crystal motion during an equiaxed/columnar solidification process of Ammonium Chloride in a die cast cell. This technique is able to measure simultaneously the liquid and the equiaxed grain velocity pattern as already shown in Part I of this paper. The interaction between the equiaxed grains and the melt flow was explored by means of relative velocities. In single isolated configurations, the settling velocity of equiaxed crystal was found to be 41 times smaller than spheres of equivalent size. The coupling between the fluid flow and the equiaxed crystals was found to be important in areas of high crystal density. Chaotic and turbulent behaviors are found to be damped in regions of high equiaxed crystal density.     

Simultaneous Observation of Melt Flow and Motion of Equiaxed Crystals During Solidification Using a Dual Phase Particle Image Velocimetry Technique. Part I: Stage Characterization of Melt Flow and Equiaxed Crystal Motion

A dual phase particle image velocimetry technique is applied to study the flow pattern during a combined equiaxed-columnar solidification process. This technique is able to measure simultaneously the liquid and the equiaxed grain velocity pattern within an academic Ammonium Chloride water ingot. After the formation of a steady convection pattern, solutal buoyancy together with falling crystals destabilize and break the steady convection flow into multiple chaotic cells. In the beginning of the solidification process, the flow transitioned from 2D to a 3D turbulent regime. The kinetic energy for the flow was calculated during the solidification process.     

Effect of cooling rate on the solidification Behavior of Al-7 Pct Si-SiCp Metal-Matrix Composites

The solidification behavior of two composites based on Al-Si alloy has been investigated as a function of cooling rate. Thermal analysis techniques have been used to establish the relationship between solidification history and the microstructure developed. The results of thermal analysis show that the characteristic parameters are influenced by the cooling rate. A marked difference in these parameters is observed between the reinforced and the unreinforced materials at all cooling rates studied. The cooling rates used in the present study range from 0.3 to 20 K/s. Increasing the cooling rate is shown to affect the undercooling parameters both in the liquidus and eutectic solidification region. The eutectic growth temperature of the composites is observed to be higher than that of the base alloy at all cooling rates. The depression in eutectic temperature ΔT is found to decrease by 27 K for the unreinforced alloy (A356) and by 17 K for the com- posites (A356 + 10, 20 vol pct SiC) at a higher cooling rate of ≃16 K/s. The presence of SiC reinforcement is observed to suppress the Mg₂Si precipitate formation and decrease the amount of heat liberated during both primary and eutectic phase formation. Dendrite arm spacing (DAS) is correlated to the cooling rate by a relationship of the form DAS =A(T) ^-n, wheren is found to be of the order of 0.33.

     

Dynamic Modeling of Critical Velocities for the Pushing/Engulfment Transition in the Si-SiC System Under Gravity Conditions

An extended non-steady-state model for the interaction between a solid particle and an advancing solid/liquid interface based on the dynamic model of Catalina et al. (Metall Mater Trans A 31:2559–2568, 2000) is used to calculate the critical velocities for the pushing/engulfment transition in Si-SiC system under microgravity and under normal gravity conditions. The aim of this study was to explain the abnormal behavior of the critical velocity in experiments. The simulations were carried out for two cases of the drag force formulation. The effects of the non-spherical form of the particles as well as the cluster formation were also taken into account. It is found that in the presence of the gravity force, the particles will be engulfed when the particle size exceeds a certain limit which does not depend on the choice of the drag force formulation.

     

颗粒增强复合材料超塑性中颗粒作用的研究

本文介绍了颗粒增强复合材料超塑必事颗粒的作用；与基体材料相比超塑行为被改变；有利于形核及阻碍晶粒长大；可省去固溶与过时效处理工序。     

Effects of Porous Structure on the Electrical Conductivity of Highly Porous Metal-Matrix Materials

It is shown that high-porosity materials based on tungsten and nickel that have been made with the use of pore-forming agents show an increase in specific conductivity as the ratio between the dimensions of the metal particles and those of the pore-forming ones increases. This result is explained from the viewpoint of the fractal dimensions and the percolation threshold.     

Solidification Characteristics and Forgeability of Aluminium Alloy Metal Matrix Composites

The solidification behavior and the forgeability of aluminum alloy (Al 413)/SiCp composites at different sections of a three-stepped composite casting at different weight fraction of SiC particles are investigated. The temperature of the cast composites during solidification has been measured by putting K-type thermocouples, from which the solidification curves were constructed. The forgeability of the as cast MMCs were also measured at different sections (having different modulus) of the casting. The results show that the forgeability of cast metal matrix composites decreases on increasing the weight fraction of SiCp. Experiments have been carried out over a range of particle weight percentage of 5?C12.5?wt% in steps of 2.5?wt%. The solidification curves of aluminum alloy composites have been compared with the unreinforced Al alloy and the results reveal that significant increase in solidification time and decrease in liquidus temperature with the addition of SiCp. The curves also show that the rate of cooling and the solidification time are different at different section of the castings.     

Transformation of Inclusions in Pipeline Steels During Solidification and Cooling

Experimental and thermodynamic considerations on the transformation of inclusions during cooling process of pipeline steel were carried out. In plant trials, CaO-Al₂O₃ type inclusions in molten steel were fully or partially transformed into MgO-CaO-Al₂O₃-CaS type in a slab depending on the size. The transformation details were revealed by thermodynamic calculations. The deviations between experimental and calculated results are discussed.     

A Study for Initial Solidification of Sn-Pb Alloy During Continuous Casting: Part II. Effects of Casting Parameters on Initial Solidification and Shell Surface

The initial shell solidification of liquid steel in the mold has significant influence on both surface and internal quality of the final slab, and it is mainly determined by the high transient high temperature thermodynamics occurring in the mold. This study investigated the effects of casting parameters like casting temperature, mold oscillation frequency, and stroke on the initial solidification of a Sn-Pb alloy through the use of a mold simulator to allow the clear understanding of the inter-relationship between irregular shell solidification, heat transfer, negative strip time (NST), and casting conditions. Results suggested that the shell surface oscillation marks (OMs) are strongly depending upon the fluctuations of meniscus responding temperatures and heat flux. An abrupt sudden fluctuation of high frequency temperature and heat flux at the meniscus during the NST would deteriorate the shell surface and leads to deep OMs. The fluctuations of responding temperature and heat flux are determined by the NST, meniscus solidification, and oil infiltration, which in turn are influenced by casting conditions, like casting temperature, oscillation frequency, stroke, etc.     

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.     

颗粒形状及基体热处理对SiCp/LD2断裂韧性的影响

对普通ＳｉＣ颗粒和钝化处理过的ＳｉＣ颗粒增强ＬＤ２铝复合材料的研究表明,颗粒经钝化处理后,几乎去除了很尖锐的部分,使颗粒呈近等轴状,但颗粒的形状对两种热处理态的复合材料的断裂性ＫＱ均无影响,而热挤出态的复合材料ＫＱ低于Ｔ６态。     

Study on Rheological Behavior of Semisolid A356 Alloy During Solidification

In the present work, a model is developed to predict the rheological behavior of an Al-alloy (A356) in semisolid state where the alloy is sheared between two parallel plates during continuous cooling. The flow field is represented by the momentum conservation equation where the non-Newtonian behavior of the semisolid alloy is incorporated considering the Herschel?CBulkley model. In the slurry, the agglomeration and de-agglomeration phenomena of the suspended particles under shear are represented using a time dependent structural parameter. The temperature field during cooling is predicted considering the transient energy conservation equation, and hence the fraction of solid and the yield stress of the semisolid alloy are continuously updated. Considering an apparent viscosity of the semisolid alloy as a function of structural parameter, shear stress and shear rate, the governing equations are solved analytically. Finally, the work predicts the variation of the apparent viscosity of the semisolid A356 alloy with fraction of solid. At first, the present prediction is validated against an available experimental data and, thereafter, the work predicts the effect of process parameters such as shear rate and cooling rate on the apparent viscosity.     

Experimental Study of Particle Motion on a Smooth Bed under Shoaling Waves Using Particle Image Velocimetry

The motion of spherical particles (diameter 1.58 mm, specific gravity 2.5) on 2 and 3% plane slope was studied in a laboratory wave flume for shoaling wave conditions. The range of wave-height-to-water-depth ratio was 0.24相似文献   

增强颗粒对颗粒增强铝基复合材料强度的影响

通过ASHALBY等效夹杂理论分析复合材料受载时作用在增强体上的应力,并假设增强体的断裂符合WEIBULL分布,在综合考虑复合材料各种强化机制的基础上引入增强体断裂对材料屈服强度的影响,建立了一个复合材料的屈服强度模型,将其应用于SiC颗粒增强AJ基复合材料,发现在屈服状态下复合材料的颗粒断裂分数随着增强体的体积含量和粒度的增加而增加,但增强体粒度变化对颗粒断裂影响更大。同时发现WEIBULL常数m取3时模型预测强度值与实测强度吻合得很好。     

红柱石的粒度对莫来石-刚玉材料性能的影响

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