强交变磁场对冷塑性变形铝合金性能影响的研究

为揭示强交变磁场对金属塑性成形后力学性能的影响规律,采用对比实验的方法,研究了冷塑性变形后LY12铝合金试件在强交变磁场作用前后的残余应力和微观组织变化情况,残余应力测试采用超声波无损检测方法。研究结果表明,在足够强度的交变磁场作用下,可以使经过冷塑性变形后的LY12铝合金发生有利于提高其力学性能的变化,均匀化了由于塑性变形引起的组织缠结,细化了晶粒,并降低了残余应力。强交变磁场可以作为改善冷塑性变形后金属材料力学性能的有效方法。     

Residual magnetic field variation induced by applied magnetic field and cyclic tensile stress

Magnetic memory testing (MMT) method is a novel non-destructive testing technique due to its unique advantages of stress concentration identification and early damage detection for ferromagnetic materials. However, a thorough understanding of the impact of exciting magnetic source and cyclic stress on the residual magnetic field variation has not been clearly addressed. The surface magnetic memory signal H_p(y) induced by applied magnetic field and cyclic tensile stress was measured throughout the fatigue process. The correlation of H_p(y) and its gradient K changes with loading cycles and applied magnetic field intensity H reported. The results show that applied magnetic field can only change the magnitude of MMT signal instead of changing the H_p(y) curve׳s profile. The H_p(y) value increases with the increase of the H, and the K value is approximately linear to the H. The maximum gradient K_max indicating the degree of stress concentration increases with the increase of either stress cycles or H. The phenomenon was also discussed from the view of the magnetic dipole in a ferromagnet.     

Evaluation of residual stress in ferromagnetic steels based on residual magnetic field measurements

The paper presents a residual stress evaluation method using the gradients of the residual magnetic field (RMF) components. Distributions of the RMF components were measured on the surface of samples with a various degree of plastic strain. The finite element method was used to model residual stress in samples. The impact of residual stress on changes in the residual magnetic field was shown. A very good qualitative correlation was found between places with residual stress and areas with increased values of the gradients of the RMF components. An algorithm was developed and verified for steel T/P24 to make a quantitative evaluation of residual equivalent (von Mises) stress based on the gradients of tangential component dH_T,Y/dx and field gradient dH/dx. Directions of further research were formulated, which included the validation of the method and which took into consideration the factors affecting its accuracy. The developed algorithm can be a significant complement to the Metal Magnetic Memory (MMM) method.     

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).     

电解加工中磁场对电场分布的影响

探讨如何将磁场叠加到电解加工装置上，使间隙电场按有利于提高加工精度的方式分布，分别进行了静态和动态磁场的试验。测量、分析后发现，只有在叠加磁场方向垂直于电场方向且N极指向电场叠加磁场时，对电场均布有较明显的作用。     

一种降低残余应力的新方法——脉冲磁处理法

脉冲磁处理是一种崭新的非热处理型降低焊接结构中残余应力的方法,本工作在国内外属首次开发开展定量研究。由试验表明,无论是定拘束单向拉伸试样,还是焊接试样,适当的强脉冲磁处理后,残余应力水平均发生了明显的下降。在大刚度拘束焊接试样上的结果表明,在距焊缝４０ｍｍ附近处的纵向与横向残余应力下降了１００－１５０ＭＰａ,下降率达４０％以上,原有不同水平的残余应力值都呈下降趋势,表现为纵向与横向应力分布曲线呈一     

NDE of a 3-D surface crack using magnetic field induced by DC current flow

A method is proposed for nondestructive evaluation of a 3-D surface crack by means of measurement of a magnetic field induced in the air by DC current flow in a material. A semielliptical crack is considered. The crack length is assumed to be known in advance and the crack depth is to be found. The current is applied between two close points on both sides of the crack and the change in magnetic: flux density in the air due to the crack is measured in non-contact with the surface of the material. An approximate equation relating the magnetic flux density to the depth and length of the crack is derived numerically and some experimental results show that the evaluated crack depth is in good agreement with the actual one.     

Study on traveling magnetic field casting of sheet components

The distribution of a magnetic field over an inductor used for sheet castings has been investigated experimentally. With an increase in height from the surface, the magnetic field density decreases according to the exponential law. In the transverse direction, the magnetic field density is approximately uniform except near the edges. The magnetic field density is direct proportional to the number of turns multiplied by the current in amperes. Ferromagnetic material in the upper mould can enhance the magnetic field density. Under the electromagnetic force, the mould-filling process of sheet casings has been studied by physical simulation method. The difference in filling capability between gravity casting and travelling magnetic field casting has been studied. The electromagnetic force can enhance the filling process, but it also brings a problem. With an increase in the magnetic field density, the surface quality of the aluminum alloy sheet castings becomes poor. The reason is discussed.     

一种定性判断三维残余应力场测量值的新思路

提出了一种定性判断工件表面残余应力变化趋势的新思路并利用前人实验进行了验证。对于已获得表面残余压应力场的表面强化试样,该理论表明:累计应力松弛量导致剥层工件剩余部分的残余压应力场强度与残余拉应力场强度大幅下降;在表层一定深度范围内残余应力的测量值略大于实际值;表面残余压应力场的深度相比实际残余压应力场的值较大。     

冷塑性成形后铝合金交变磁处理及耦合场分析

利用自行研制的QJC-1磁场发生器产生强交变磁场,通过对比实验研究了LY12硬铝合金磁处理前后组织的变化,研究发现,强交变磁处理可以使冷塑性成形后的硬铝合金组织变得均匀、致密;采用数值分析的方法,应用电-磁-结构耦合场理论,对处于电磁场中的LY12铝合金的磁致应力场分布状态进行了分析,给出了涡旋电流矢量图、磁感应强度云图和磁场应力云图,数值分析表明,在交变磁场作用下,LY12铝合金构件内部产生很大的涡旋电流,引起明显的磁致震荡,有利于微观组织脱离缠结,使冷塑性成形后的金属组织趋于均匀致密。数值分析结果直观地揭示了,强交变磁处理改善冷塑性成形后金属组织结构的力学机理。     

低频交变磁处理降低钢材内应力的微观机理

磁场处理被许多研究者用于提高材料的力学性能并取得了一定效果。作者首次运用磁处理方法进行降低材料内应力的研究。结果表明，不同的处理方法，如高强磁脉冲处理和低频交变磁场处理都能够不同程度地降低材料内部存在的残余应力，而且其降低残余应力的方式与常规的热处理有本质的不同。本文定量研究了低频交变磁处理方法降低残余应力的程度、特点和工艺参数。通过透射电镜分析，着重观察了对30CrMnSiA进行磁处理时处理前后材料内部的位错运动规律。在经过磁处理后的试样中，其明显的胞壁结构消失，位错分布均匀化；整体位错密度与处理前相比变化不大。根据上述观察和宏观试验结果，对磁处理降低残余应力的特点和微观结构进行了分析了讨论，认为磁场作用下的位错均匀化是磁处理降低残余应力的主要原因。     

有限元法在旋转磁场磁粒研磨加工技术中的应用

磁场分布是影响磁粒研磨加工的重要因素之一，研究磁场分布的主要方法是数值计算法，本文介绍了旋转磁场磁粒研磨加工的基本原理，并用有限元法对旋转电磁场进行了计算，得到了一些重要结论。     

Modelling the transverse magnetic field generated by equipment in arc welding

The design of equipment for generating the transverse magnetic field in arc welding can be optimised by modelling the direct magnetic field produced by the device for generating (GD) the transverse magnetic field (TMF) by the electric field of the current flowing in flat models made of electrically conducting materials. The lines of force of the electric field in the flow of the current in the modelling medium correspond to the lines of force (induction lines) of the magnetic field generated by GD TMF. Using these modelling methods, it is shown that to obtain the maximum values of the transverse component of the induction of the magnetic field in the zone of the welding arc of the electrode droplet and liquid metal of the weld pool, the optimum design of GD TMF is the one in which the angle of inclination of the bars to the vertical is equal to 45° and the end surfaces have chamfers parallel to the plane of the welded sheets.     

堆焊金属残余应力场的计算机模拟

采用60CrMnMo钢为基体材料,用AST型X射线应力分析仪对堆焊残余应力场进行了测定。发现在堆焊金属中心部位和熔合区附近的热影响区出现了两个残余拉应力峰值。用ANSYS计算软件,模拟了堆焊过程中温度场和残余应力场的分布,在模拟中考虑了马氏体相变、杨氏模量与其它参数对计算结果的影响,并将计算值与残余应力场的测量值进行了比较。结果表明,残余应力场的计算结果与实测结果吻合较好,说明本文建立的有限元计算模型是有效的。采用这一模型,模拟了不同试样尺寸对堆焊金属残余应力场分布的影响。计算结果表明,随着试样尺寸的增大,残余应力峰值增大,且峰值益发生变化。     

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.     

外加磁场下GMAW熔池电磁力的有限元计算

针对外加各种磁场作用下的GMAW堆焊过程,开发了基于电弧简化模型的熔池电磁力工程化有限元计算方法. 利用此方法,系统研究了无外加磁场、外加纵向磁场和横向磁场三种工况下熔池内电磁力的分布情况,并预测了电磁力对熔池对流的影响. 结果表明,焊接自有磁场驱动了熔池中心收缩下沉流;外加纵向磁场导致周向搅拌流,形成电磁搅拌细化晶粒;外加横向磁场可以驱动熔池流动演变为"单涡"对流状态,有利于熔池接纳熔滴减少飞溅. 基于模拟结果的预测与试验结果吻合,因此验证了所用计算方法的正确性.     

Monel-400合金环焊温度场和残余应力场的数值模拟

采用ANSYS大型通用有限元分析软件,利用APDL语言模拟计算了Monel-400合金环焊温度场.绘制出焊缝熔合线及其附近的热循环曲线,将热分析得到的节点温度作为体载荷进行应力场的数值模拟计算.结果表明:外表面节点x方向的应力刚开始时为拉应力,12s后转变为压应力,而中间节点和内表面节点x方向的应力始终为压应力,并大于...     

Measurement of residual stress in steel using the magnetic Barkhausen noise technique

Surface residual stresses on a structural beam steel sample were evaluated using a non-destructive technique based on the measurement of surface magnetic Barkhausen noise (MBN). MBN measurements were performed using a high resolution probe consisting of a small magnetic read head mounted between the poles of a ferrite U-core magnet. Applied stress magnitudes were correlated to MBN energy levels for calibration purposes. MBN energy measurements were done at various locations on the steel sample. The magnitude of the residual stress component along the sample beam axis was evaluated across the width of the beam using these calibration curves. The range of sensitivity of the MBN signal to stress as defined by the calibration curves was limited by the direction of easy magnetization. The effect of stress on the MBN signal was interpreted in terms of the active 180° domain wall population. The validity of the residual stress results was confirmed experimentally using conventional methods: cutting and sectioning and hole drilling. Possible sources of residual stresses and their distribution on the structural steel specimen are discussed.     

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.

     

稳恒磁场对多晶硅定向凝固结晶阶段流速场的影响

采用有限元Comsol 4.3a软件对多晶硅定向凝固结晶阶段3个时期（前期、中期、后期）坩埚内硅熔体的流速场进行数值模拟,并与不同磁场强度下的硅熔体流速场进行了对比分析。结果表明,硅熔体的平均流速随着结晶时间的延长而减小,从初期的38 μm/s减小到后期的16 μm/s,下降了57.89%。施加磁场后,硅熔体的平均流速随着磁场强度的增大而减小：结晶初期、中期和后期,硅熔体的平均流速分别减小了42.11%、58.59%和45.16%。结晶阶段3个时期分别施加磁场强度为0.6、0.4、0.2 T时,磁场对硅熔体的对流抑制作用最为明显。