稳恒磁场在金属相变过程中的应用

稳恒磁场在材料科学中的应用越来越受到关注。本文从稳恒磁场作用下生成的磁化力和洛伦兹力两个角度归纳了稳恒磁场对金属材料的固态相变和液态相变的影响。其中磁化力可以影响固态相变过程,形成取向,引起磁对流,进行磁性分离。而洛伦兹力可以抑制熔体的流动和波动,提高溶质离子在基体中的固溶度,另外,利用洛伦兹力还可以在电磁离心铸造中获得均匀的凝固组织。     

非常规情况下金属磁记忆检测方法的研究

介绍了铁磁构件自身通入稳恒电流，外部存在稳恒干扰磁场以及当铁磁构件被交流磁化这三种非常规情况对铁磁构件金属磁记忆检测的影响。实验发现，1A以下稳恒电流的存在对于管道的金属磁记忆检测几乎没有影响，稳恒外磁场的存在对金属磁记忆检测影响是很大的，可能造成金属磁记忆信号整体的改变，而交流磁化则会完全破坏构件的自有漏磁场信息。     

Al-Si合金熔体中磁性杂质颗粒的电磁分离

分析了处于交变磁场中的磁性杂质颗粒的受力情况，比较了由交变磁场和稳恒磁场产生的磁场力。结果表明这两种力的本质相同，可以统一起来，而且磁性杂质颗粒在其驱动下的运动规律也相同。Al-Si合金中的富铁相杂质是弱磁性，电磁力和磁场力都可以促使其运动，电磁分离试验结果表明，当磁感应强度大于145mT时，电磁力起主要作用，杂质颗粒向分离器内侧运动；当磁感应强度小于120mT时，磁场力起主要作用，杂质颗粒向分离器侧面运动，据此可以指导分离器的结构设计。     

成膜时外加稳恒磁场对Fe-Ga-Al-Y磁致伸缩薄膜磁性能的影响

借助自行设计的磁场发生器,以铍青铜为衬底,采用离子束溅射沉积法在不同稳恒磁场环境下制备Fe-Ga-Al-Y磁致伸缩薄膜(MF)样品,其成分为Fe_(74.34~79.33)Ga_(11.45~13.73)Al_(5.33~6.95)Y_(3.27~4.36)。采用原子力显微镜(AFM)与扫描电镜(SEM)观测Fe-Ga-Al-Y MF表面形貌,通过激光微位移传感器和交变梯度磁强计分别对薄膜样品的悬臂梁自由端偏转量与磁滞回线进行测定,研究Fe-Ga-Al-Y MF表面形貌以及成膜时外加稳恒磁场方向及大小对Fe-Ga-Al-Y MF磁性能的影响。研究结果表明:通过IBSD法制备的薄膜样品表面光滑平整,无明显的缺陷,薄膜组织结构均匀致密,具有优异的成膜质量。成膜时施加的稳恒磁场能调控薄膜的易磁化轴方向,Fe-Ga-Al-Y MF的易磁化轴方向与成膜时所施加的稳恒磁场方向一致;成膜时施加与衬底短轴方向平行的稳恒磁场,可以显著提高Fe-Ga-Al-Y MF的饱和磁致伸缩性能,且随着磁场的增大,薄膜的饱和磁致伸缩性能逐渐增加。薄膜面内(沿着x轴方向、y轴方向)的饱和磁化强度、矫顽力也随着磁场的增大而逐渐增大。     

稳恒磁场对铁素体转变的影响

用自制的磁场热处理设备对低碳钢进行施加磁场与不施加磁场的淬火处理,研究了奥氏体在饱和肥皂水及1．67mol／L的NaCl溶液冷却介质中的组织转变,观察了相变后的组织变化。结果表明：在低碳钢发生奥氏体向铁素体转变时,稳恒磁场可以明显提高铁素体的体积分数,并能促进铁素体形核,使铁素体晶粒细化和均匀化。由于奥氏体和铁素体的磁导率不同,磁场能使相变后的铁素体和珠光体的析出形成链状组织。     

脉冲磁通量密度和Lorentz力的数值模拟

用ANSYS5．5有限元软件对在脉冲磁场中金属熔体凝固条件下，直螺线管线圈及其内部凝固样品中的瞬态脉冲磁通密度、洛伦兹力做了数值模拟。模拟计算结果表明，在脉冲磁场中凝固的条件下，直螺线管线圈内及其凝固样品中脉冲磁通密度和洛伦兹力分布具有轴向、径向分布不对称性和不均匀性。脉冲磁通量和洛伦兹力的这种分布特性对了解脉冲磁场凝固条件下，瞬态的脉冲磁通量和洛伦兹力的特性及其对金属熔体的凝固行为和凝固组织的作用具有重要意义。     

磁场在材料制备中的应用

总结近几年来国内外有关磁场在材料制备中的应用现状。分别讨论和分析了稳恒磁场、旋转磁场、交变磁场、脉冲磁场和强磁场在金属凝固过程中对金属凝固组织的影响。由于稳恒磁场、旋转磁场、交变磁场、脉冲磁场和强磁场在金属凝固过程中改善凝固组织的机理各不相同,在电磁学、流体力学和金属凝固原理的理论基础上,对磁场影响金属凝固组织的主要机理分别进行了讨论。另外,根据磁场发展技术的局限性对磁场在凝固过程中的应用和进一步研究进行了展望。     

稳恒磁场凝固过程中液相流动及对微观组织的影响

对比研究了Pb-61.9Sn共晶合金和Pb-80Sn过共晶合金在常规条件和稳恒磁场作用下的凝固组织形貌.发现稳恒磁场使得Pb-61.9Sn共晶组织的层片结构更加明显,共晶间距变小,并且有着不同区域的差别和弯曲;Pb-80Sn过共晶凝固组织在稳恒磁场下变得粗大,枝晶间距变大,规则程度降低,枝晶臂有所碎断.这主要是因为稳恒磁场产生的Lorentz力抑制了熔体的流动,同时,在固、液界面处产生了热电磁流体效应.     

H_2S—H_2O系统中奥氏体不锈钢破裂机理的研究附录

<正> 磁称法原理 附录:磁称法原理 磁称法对弱磁性的测量在原理上归结为测量弱磁性物质在不均匀磁场中所受的力。置于不均匀磁场中的样品所受的力与磁化系数x之间的关系可以由以下分析得到。在磁场中被磁化物体的位能为     

Landau理论研究TiNi顺磁合金热/强磁场耦合下的马氏体相变

利用改进后的Landau理论模型研究了顺磁TiNi合金在热/强磁场耦合作用下的马氏体相变行为.利用第一原理计算了TiNi合金在不同相变剪切应变(序参量)下的Flermi面态密度,得到相变过程中磁化率和磁自由能的变化,把磁场效应引入到Landau模型中.结果表明,稳恒强磁场可使顺磁材料的相变温度(M6和T0)出现突变性的升高,这主要是因为相变驱动力随磁场增强而呈二次曲线上升规律.另外,因强磁场下马氏体变体之间的自由能差急剧增大,导致磁场对变体的促进和抑制作用变得明显,出现取向生长现象.在5 T稳恒强磁场下的TiNi合金相变实验中,利用TEM观察到了一些变体以相互垂直的二变体形式出现(即取向生长),证实了模型的结果.     

Effect of electromagnetic force on melt induced by traveling magnetic field

A new apparatus was designed to measure the electromagnetic force and a computational study of the traveling magnetic field(TMF)and its application to the Ga-In-Sn melt(with low melting point),then the forces on Al,Mg,and Li melt,were simulated. The result show that the electromagnetic force on the melt increases linearly with the increasing length of the melt in the TMF.The TMF-induced Lorentz force increases with increasing frequency,and then decreases.The maximum value is obtained when the current frequency is 160 Hz,over that frequency the force decreases rapidly.When the iron-core is activated,the force increases when the melt closes to the iron-core.The Lorentz forces have inversely-proportional relationships with the electrical resistivity,the dfx/dρdecreases and the dfy/dρincreases with the increasing electrical resistivity(df/dρis the slope of the Lorentz force profile).     

Effect of Transverse Static Magnetic Field on Microstructure of Al-12% Si Alloys Fabricated by Powder-Blow Additive ManufacturingEI北大核心CSCD

Due to the great advantage in manufacturing component with complex structures, additive manufacturing (3D print), essentially the rapid solidification of tiny metallic molten pool (hemisphere like with diameter ranging from dozens of microns to several millimeters) has become an important formation technique. Using powder laser melting, the effect of transverse static magnetic field on the solidified structure of additive manufactured Al-12% Si alloy was studied. The macrostructure was formed by white band (mainly primary alpha-Al phase) and dark grey area (mainly eutectic phase) and no obvious influence was presented with or without static transverse magnetic field of 0.35 T. However, for the microstructure, the primary alpha-Al in dark grey area formed as columnar structure without magnetic field was found to transform to dendritic like with developed dendrite arms when under a static transverse magnetic field. The analysis on thermoelectricity and dimensionless Hartman parameter which used to characterize the restriction of static magnetic field on molten flows show that under a static transverse magnetic field of 0.35 T, the thermoelectric magnetic force can be as high as a magnitude of 10(5) N/m(3), and Hartman values is far more than 10. The results indicate that the Marigoni and thermosolutal convection in laser melting pool was restricted. The transform from columnar to equiaxed dendrite of primary alpha-Al in dark grey area under static magnetic field was attributed to the fragmentation by thermoelectric magnetic force (10(5) N/m(3)) in solid phase. In addition, the formation of high order dendrite arms was supposed to be caused by the restriction of static magnetic field on the melt.     

Effect of harmonic magnetic field and pulse magnetic field on microstructure of high purity Cu during electromagnetic direct chill casting

The effects of two types of magnetic fields, namely harmonic magnetic field (HMF) and pulse magnetic field (PMF) on magnetic flux density, Lorentz force, temperature field, and microstructure of high purity Cu were studied by numerical simulation and experiment during electromagnetic direct chill casting. The magnetic field is induced by a magnetic generation system including an electromagnetic control system and a cylindrical crystallizer of 300 mm in diameter equipped with excitation coils. A comprehensive mathematical model for high purity Cu electromagnetic casting was established in finite element method. The distributions of magnetic flux density and Lorentz force generated by the two magnetic fields were acquired by simulation and experimental measurement. The microstructure of billets produced by HMF and PMF casting was compared. Results show that the magnetic flux density and penetrability of PMF are significantly higher than those of HMF, due to its faster variation in transient current and higher peak value of magnetic flux density. In addition, PMF drives a stronger Lorentz force and deeper penetration depth than HMF does, because HMF creates higher eddy current and reverse electromagnetic field which weakens the original electromagnetic field. The microstructure of a billet by HMF is composed of columnar structure regions and central fine grain regions. By contrast, the billet by PMF has a uniform microstructure which is characterized by ultra-refined and uniform grains because PMF drives a strong dual convection, which increases the uniformity of the temperature field, enhances the impact of the liquid flow on the edge of the liquid pool and reduces the curvature radius of liquid pool. Eventually, PMF shows a good prospect for industrialization.

     

Effect of Magnetic Suspension Force in Electromagnetic Stirring on Microstructure of Al-Si Alloy

Based on Maxwell equations, the Lorentz force in Al-Si alloy melt was calculated during the process of electromagnetic stirring using analytic approximation method. The effects of magnetic suspension force on the microstructure of Al-Si alloy were given under different electromagnetic stirring frequency. The surface layer for alloy melt (thickness is about skin depth) were under a larger magnetic suspension force in intermediate frequency electromagnetic stirring. As a result, it was found that Si phase ten...     

近强磁场作用对Sn-9Zn钎料合金组织性能的影响

运用X射线衍射仪、扫描电镜、显微硬度计等仪器设备,研究了0.8 T近强磁场作用对Sn-9Zn钎料合金组织和性能影响。研究结果表明：近强磁场可促进Sn-9Zn钎料合金显微组织显著细化,使熔化温度下降2℃,显微硬度提高11%。分析认为,由于外加磁场可促进Sn-9Zn钎料合金在特定晶面迅速结晶形核以及作用于液态钎料合金的的洛仑兹力对钎料作负功,从而促进Sn-9Zn钎料合金组织和性能的改变。     

Effect of high magnetic field on solidification microstructure evolution of a Cu-Fe immiscible alloy

The liquid phase separation behavior and the evolution of the solidification microstructure of a binary Cu₅₀Fe₅₀ alloy were investigated under the conditions of without and with a 10 T magnetic field, with different undercooling during the solidification process. Results show that the combined effect of Stokes motion and Marangoni convection leads to the formation of the core-shell structure under the condition without the magnetic field. In addition, specific gravity segregation is reinforced by increasing the undercooling, resulting in Fe-rich phase drifts towards the sample edge. In the 10 T magnetic field, the Fe-rich phase is elongated in the parallel direction of the magnetic field under the action of demagnetization energy due to the difference of static magnetic energy and surface energy. In the vertical direction, through the action of Lorentz force, the convection in the melt is inhibited and Fe-rich phase becomes more dispersed. Meanwhile, the diffusion of the two phases and the coagulation of the Fe-rich phases are also restrained under the magnetic field, therefore, the phase volume fraction of the Fe-rich phase decreases at the same undercooling in the 10 T magnetic field. The magnetic field inhibits the segregation behavior in the vertical direction of the magnetic field, and at the same time, improves the gravitational segregation to a certain extent, which has a very important impact on microstructure regulation.

     

铝合金半固态浆料制备过程的电磁-流体数值模拟

利用ANSYS软件时旋转型电磁搅拌作用下铝舍金半固态浆料内的电磁场、不同温度下浆料内流场进行了数值模拟。分析了励磁电流强度和频率的变化对半固态浆料内的磁感应强度、电磁力及流动状态的影响。结果表明：在相同频率下，半固态浆料内的磁感应强度、电磁力的大小随电流强度的增加呈线性增加的关系；在相同电流强度下，磁感应强度是随着频率的增大而减小的，而电磁力是增大的，但数值变化不大。随浆料中固相体积分数的增加，使半固态浆料的温度降低．粘度增大，流动速度降低。     

Morphology and distribution of Al3Fe phase in hypereutectic Al-Fe alloy solidified under magnetic field

In this study,the (low) DC and AC magnetic fields and the high magnetic field were applied separately during the solidification process of Al-2.89％Fe alloy.The influences of these magnetic fields on the morphology and distribution of Al3Fe phase in Al-2.89％Fe alloy were investigated.The microstructure and macrostructure of the samples were observed using an optical microscope.The results show that the majority of the primary Al3Fe phase particles in the hypereutectic Al-2.89％Fe alloy is gathered at the bottom of the sample under DC and AC magnetic fields or without magnetic field.The primary Al3Fe phase becomes coarse when the alloy solidifies under DC magnetic field,while it are refined and accumulated towards the center of the sample under the AC magnetic field.When the high magnetic field of 12 T is applied,the primary Al3Fe phase distributes throughout the sample homogeneously because the magnetic force acting on the primary Al3Fe phase balances with the gravity force; and the long axis of the Al3Fe phase aligns perpendicularly to the magnetic field direction.Also,the mechanism of the effect of magnetic fields is discussed.