电磁搅拌频率对Pb-Sn过共晶合金凝固组织的影响

对比分析了磁场和电磁搅拌频率对Pb-80%Sn过共晶合金铸锭凝固组织和宏观成分偏析的影响。结果表明:螺旋磁场可在合金熔体的更大区域内形成流动,使熔体内溶质场、温度场分布更均匀。励磁电流一定时,初生相晶粒尺寸随频率的增大而减小,旋转磁场和螺旋磁场在频率分别为13 Hz和10 Hz时对铸锭凝固组织的改善效果最好,晶粒尺寸由无磁场时的247.3μm分别减小到142.5μm、140.6μm,继续增大频率,凝固组织反而出现粗化和不均匀化。旋转磁场和螺旋磁场均在频率为13 Hz时对成分偏析的改善效果最好,由无磁场时的9.71%分别减小到1.14%、0.34%。     

磁场作用下偏晶合金凝固组织演变的数值分析

建立偏晶合金难混溶区凝固过程的两相数学模型,模拟研究了有、无磁场Al-10%Bi过偏晶合金微观组织演变,研究了温度、速度、第二相体积分数等物性参数对凝固组织宏观偏析的影响。结果表明,在磁场作用下温度场为中心对称分布,更有利于第二相液滴的均匀分布;电磁力抵消了部分重力和Marangoni力,使无磁场时外环流的速度场变为有磁场时斜向下的速度场,且速度明显降低,从而减轻了强对流导致的重力偏析;在磁场的作用下,试样底部第二相的体积分数减小,凝固组织宏观偏析得到改善。     

凝固方式对Sn-Bi钎料组织和性能的影响

采用炉冷、空冷、水冷和液氮冷却方式以及外加磁场的方法研究不同的凝固方式对Sn-Bi钎料的冲击韧性和显微组织的影响.研究结果表明,快速冷却与旋转磁场均能细化钎料合金的微观组织,抑制粗大树枝晶的生长,但快速冷却会造成Bi的偏析,旋转磁场会造成组织不均.同时快速冷却与旋转磁场都会破坏含Ge合金的塑性改善机制,造成含Ag合金中Ag3Sn相粗大,而旋转磁场的离心力作用还会造成Ag3Sn相和Bi相的宏观偏析.在组织细化以及成分偏析的共同作用下,Sn-57Bi共晶钎料的冲击韧性随冷却速率的增大呈现先增加后减少的趋势.     

交变电磁场在晶体生长与凝固方面的应用

在合金凝固过程中施加时变电磁场,导电熔体在洛仑兹力的驱动下产生的强迫对流可有效控制合金溶质的再分配过程。在材料凝固与晶体生长过程中施加电磁场有助于深入研究其热物理性质、相平衡、亚稳态、组织形成、成分过冷和形核。人们对重力环境下合金的凝固过程已进行了大量深入详尽的实验探究,利用电磁场手段对凝固前沿进行有效干预并在晶体组织结构、成分偏析等方面取得了一定的进展。而微重力环境下浮力对流减少,为在宏观和微观尺度上获得成分更为均匀的半导体或合金材料提供了一个独特的平台,目前的研究表明在微重力环境下合金的凝固特征与在重力环境下的有所不同。从理论与实验的角度阐释了在微重力及重力环境下电磁场引起的强迫对流强度及方向对晶体生长过程中的宏观偏析、微观偏析、晶体形态以及金属间化合物的生长模式及空间分布的影响规律。     

磁场作用下的金属凝固研究进展

磁场作用下的金属凝固已成为金属材料基础研究和开发制备新技术的重要领域.综述了交变/旋转磁场、直流磁场作用下金属凝固的研究历史及现状,运用电磁学及金属凝固原理,揭示了磁场对凝固组织形貌及凝固过程产生的影响及主要机制,并对以后的理论研究工作提出了建议.     

等轴晶移动对宏观偏析影响的数值模拟

建立了自由等轴晶移动对宏观偏析影响的数学模型,对铸锭凝固过程中的对流和溶质分布进行了数值模拟.在模型中按照临界固相分数将糊状区分为紧密树枝晶和自由等轴晶两个不同的区域.对带冒口铸钢锭的宏观偏析进行了数值模拟,并同实验结果进行了比较.与假设糊状区内固相静止的模型相比,考虑等轴晶移动的模型得到的溶质分布结果与实验结果更接近.在凝固过程中,等轴晶随液体流动并在铸锭的底部中心聚集,在凝固后的铸锭中形成锥形的负偏析.还发现,在铸锭的中心靠上的区域形成正偏析,在铸锭的外部区域形成负偏析.     

等轴晶移动条件下金属凝固温度场、流场及溶质分布数值模拟

本文利用作者提出的固相移动条件下金属凝固传热、传质及动量传输数学模型对砂型铸造 Al-4.5%Cu 合金铸锭凝固过程进行了数值模拟研究与试验验证。模拟冷却曲线及宏观偏析与试验结果基本相符。     

旋转磁场对Pb-Sn-Sb合金组织及性能的影响

为了改善Pb-Sn-Sb三元轴承合金液态成型过程中的比重偏析,用旋转磁场控制Pb-Sn-Sb合金液态成型过程.采用光学显微镜、扫描电镜能谱分析研究了旋转磁场对Pb-Sn-Sb合金显微组织及成分分布的影响.用布氏硬度计、摩擦磨损试验机测试Pb-Sn-Sb合金的硬度及摩擦磨损性能.实验结果表明:旋转磁场能有效改善Pb-Sn-Sb合金的比重偏析,且能细化晶粒.激磁电压为45V时,SbSn块状化合物在试样上下截面分布最均匀,块度变小,比重偏析改善效果最好;试样上下截面Pb、Sn、Sb三元素含量基本趋于一致.随着激磁电压增加,试样上下截面的硬度及耐磨性有一定程度提高,并明显趋于一致.     

旋转磁场作用下Pb-Sn合金的凝固行为

采用永磁体旋转装置实现了Pb-50wt%Sn亚共晶、Pb-61.9wt%Sn共晶合金旋转磁场条件下的凝固,并对其凝固规律进行了研究.发现与常规条件下相比,旋转磁场导致的合金熔体内部强迫对流加快了合金熔体的冷却速率,提高熔体初始形核温度,促发熔体提前形核并能使凝固时间缩短.Pb-61.9wt%Sn共晶合金的宏观偏析被有效抑制,在整个样品中得到了二元共晶组织,共晶层片粗化,局域地区出现了不规则共晶组织.对于Pb-50wt%Sn亚共晶,初生相(Pb)的生长方式由粗大枝晶转变为细小的球状颗粒,且晶粒显著细化.     

电磁场对铅黄铜水平连铸坯组织及偏析的影响

研究了铅黄铜在无磁场、低频交变电磁场下水平连铸得到的铸坯横截面宏观组织、微观组织形貌及合金元素Pb的分布,并对其机理进行了分析。试验结果表明,在铅黄铜水平连铸的过程中施加低频交变电磁场能有效地细化晶粒及影响合金元素在横截面上的分布,并且电磁场有效地抑制了在常规水平连铸过程中出现的铸坯横截面上中下部位组织分布不均匀的现象。表现在铸坯横截面的凝固组织由上方主要为细小柱状晶、下方为粗大柱状晶变为均为细小的等轴晶,铸锭上部凝固滞后的现象消除;合金元素Pb的宏观偏析得到抑制,并且电流为30Hz、100A的组织优于30Hz、80A的组织。     

Remote Field Eddy Current Control Using Rotating Magnetic Field Transducer: Application to Pressure Tubes Examination

The remote field eddy current (RFEC) technique is used to investigate the possibility of detecting the discontinuities practiced on pressure tubes samples from nuclear reactors, pressurized heavy water reactors (PHWR) type. In this article, we propose to develop the RFEC using the technique of rotating magnetic field (RMF). A method for calculating the field generated by the eddy current transducer with RMF using propagator matrix was developed. The experimental measurements are realized for artificial discontinuities practiced in pressure tubes samples.     

Grain refinement of horizontal continuous casting of the CuNi10Fe1Mn alloy hollow billets by rotating magnetic field (RMF)

Rotating magnetic field (RMF) is used in horizontal continuous casting of CuNi10Fe1Mn alloy hollow billets. The result shows that the formerly inhomogeneous columnar grain macrostructure turns into homogeneous equiaxed grain with the application of RMF. The microstructure without RMF transforms from coarse and disordered dendrites to dense dendrites which have obvious orientation, while the microstructure with RMF from center to edge displays the evolution from spherical grains to disordered dendrites without orientation. The mechanical properties are improved remarkably.     

Effect of Low Frequency Electromagnetic Field on Macrosegregation of Horizontal Direct Chill Casting Aluminum Alloys

The horizontal direct chill (HDC) casting process is a well-established production route for aluminum alloy ingot but the ingot may suffer from macrosegregation sometimes. In order to control the defect, a low frequency electromagnetic field has been applied in HDC casting process and the relevant influence has been studied. The results show that application of low frequency electromagnetic field can reduce macrosegregation in HDC casting process; and two main parameters of electromagnetic field density and frequency, have great influences on the solution distribution along the diameter of ingot. Moreover, the mechanisms of reduction of macrosegregation by electromagnetic field have been discussed.     

Effect of combined magnetic field on the eliminating inclusions from liquid aluminum alloy

The effect of electromagnetic field on the removal of inclusions in the aluminum alloy was investigated. Primary silicon particles precipitating from the solidification of Al-Si hypereutectic alloy were regarded as inclusions. An experimental apparatus applied with both rotating magnetic field (RMF) and traveling magnetic field (TMF) was employed to study the distribution of silicon particles in Al-Si alloy under magnetic field. The results showed that combined magnetic field (CMF) consisting of RMF and TMF eliminated the silicon particles from the molten alloy. Compared with TMF or RMF, CMF increased the separating effectivity substantially. It was proposed that CMF provided a highly effective approach for metal purification.     

定向凝固法多晶硅杂质控制数值模拟概述

在传统能源日渐消耗及可再生能源开发利用日趋受到重视的形势下,太阳能光伏发电逐渐成为最具潜力的可再生能源技术之一。多晶硅凭借高效率、低成本的优势成为最主要的光伏材料,其铸锭的品质和成本将直接影响太阳能电池的成本和光电转换效率。定向凝固法是制备多晶硅铸锭的重要方法,该方法晶硅生长过程中存在很多问题,包括熔体流动、杂质传输、固液界面的形状和结构以及缺陷。定向凝固过程中引入的有害杂质严重影响晶硅的机械和电学性能,是限制多晶硅光电转换效率的关键因素。长晶过程中定向凝固炉处于高温环境中,内部的传热传质极其复杂,不具备单一的线性关系和可推断性,且难于进行实验测量,因而数值模拟是研究定向凝固过程中传热传质现象的重要方式。降低长晶过程中杂质的含量可从两方面入手:(1)杂质的来源——原材料本身所携带的杂质和长晶过程中生成的杂质;(2)杂质的输运——找到杂质在熔体和氩气中的输运规律,并利用该规律控制杂质的分凝与输运。近年来,从控制杂质产生和输运的角度考虑,国内外对降低多晶硅中有害杂质的研究主要采用以下手段:(1)控制杂质产生,包括减缓坩埚与挡板之间的化学反应、优化顶部坩埚盖板、增设碳化硅涂层等;(2)优化氩气流动,例如使用导流系统、调整炉膛压力和氩气流量;(3)优化熔体对流形式,包括控制熔体流动方式及分凝、调整加热器功率大小及其排布、采用可旋转坩埚和调整石墨碳毡位置等。为进一步降低定向凝固法多晶硅铸锭成本,坩埚尺寸和投料量不断增大,熔体对流、杂质输运和界面形状也更加难于控制,外加磁场则成为控制熔体对流的一种强有力工具,并能进一步控制杂质输运。目前利用外加磁场控制定向凝固法多晶硅杂质的研究仍刚刚起步,具有很大的研究价值,其中电磁场(EMF)和行波磁场(TMF)在控制搅拌熔体对流方面具有巨大潜力,逐渐被用于多晶硅长晶过程。本文在深入分析杂质来源和输运机理的基础上,综述了国内外对多晶硅定向凝固过程中有害杂质的产生、分布、输运以及排出等问题的研究现状,总结了数值模拟中氩气导流系统、加热器以及外加磁场等因素对杂质的影响。     

Exposure to inhomogeneous static magnetic field beneficially affects allergic inflammation in a murine model

Previous observations suggest that static magnetic field (SMF)-exposure acts on living organisms partly through reactive oxygen species (ROS) reactions. In this study, we aimed to define the impact of SMF-exposure on ragweed pollen extract (RWPE)-induced allergic inflammation closely associated with oxidative stress. Inhomogeneous SMF was generated with an apparatus validated previously providing a peak-to-peak magnetic induction of the dominant SMF component 389 mT by 39 T m⁻¹ lateral gradient in the in vivo and in vitro experiments, and 192 mT by 19 T m⁻¹ in the human study at the 3 mm target distance. Effects of SMF-exposure were studied in a murine model of allergic inflammation and also in human provoked skin allergy. We found that even a single 30-min exposure of mice to SMF immediately following intranasal RWPE challenge significantly lowered the increase in the total antioxidant capacity of the airways and decreased allergic inflammation. Repeated (on 3 consecutive days) or prolonged (60 min) exposure to SMF after RWPE challenge decreased the severity of allergic responses more efficiently than a single 30-min treatment. SMF-exposure did not alter ROS production by RWPE under cell-free conditions, while diminished RWPE-induced increase in the ROS levels in A549 epithelial cells. Results of the human skin prick tests indicated that SMF-exposure had no significant direct effect on provoked mast cell degranulation. The observed beneficial effects of SMF are likely owing to the mobilization of cellular ROS-eliminating mechanisms rather than direct modulation of ROS production by pollen NAD(P)H oxidases.     

Structure refinement of pure Mg under pulsed magnetic field

Abstract

A pulsed magnetic field (PMF) was introduced into the solidification of pure Mg. Fine uniform equiaxed grains are acquired in the whole ingot from the PMF treatment, in contrast with the coarse columnar grains observed in conventional casting, and the average grain size is refined to 260 μm with a 200 V PMF treatment. Pulsed magnetic field increases melt convection during solidification, and the violent agitation causes warmer liquid to fracture the tip of columnar dendrites or to break off dendrite branches to promote the formation of an equiaxed structure, with the broken pieces transported into the bulk liquid acting as nuclei. In addition, the uniform temperature field resulting from the stirring increases the likelihood of nuclei survival. The Joule heat effect also participates in the structure refinement. The pure Mg produced with a 200 V PMF treatment exhibits improved mechanical properties, such as the ultimate compressive strength (227 MPa) and fracture strain (33·2%).     

Effect of the simultaneous application of a high static magnetic field and a low alternating current on grain structure and grain boundary of pure aluminum

Effect of the simultaneous application of a high static magnetic field and a low alternating electric current on the solidification structure of pure aluminum has been investigated. Results show that the refinement of the solidification structure is enhanced by the electric current under a certain magnetic field. However, when the magnetic field intensity exceeds a certain value, the refinement is impaired under a certain electric current. The observation by electron backscattered diffraction (EBSD) shows the complex fields have led to the increase of the low angle boundaries with the refinement. Moreover, the application of the static gradient magnetic field is capable of modifying the distribution of the refined grains. The above results may be attributed to the formation of the cavities during the electromagnetic vibration process and the high magnetic field.