Effect of electrodes and thermal insulators on grain refinement by electric current pulse

The application of electric current pulse（ECP） to a solidification process refers to the immersion of electrodes into the liquid metal and the employment of thermal insulators on the upper surface of metal.In order to ascertain the effects of these two factors on the structure refinement by the ECP technique,three groups of experiments were performed with different types of electrodes or various thermal insulators.By the comparison between solidification structures under different conditions,it is followed that the electrode and the thermal insulator have an obvious influence on the grain refinement under an applied ECP,and further analysis demonstrates that the thermal conditions of the liquid surface play a vital role in the modification of solidification structure.Also,the results support the viewpoint that most of the equiaxed grains originate from the liquid surface subjected to an ECP.

Migration behavior of solidification nuclei in pure Al melt under effect of electric current pulse

A mathematical model considering free nuclei was developed to reveal the migration behavior of the free nuclei. Numerical simulation results show that most of the nuclei on the top surface of the melt move downwards and distribute randomly inside the Al melt, which induces more nucleation sites resulting in grain refinement. At the same time, the effect of nuclei size on the nuclei distribution and refinement employing electric current pulse （ECP） was also investigated. The smaller nuclei migrate a short distance with the Al melt at lower speed. But for the larger nuclei, the migration downwards with higher speed benefits the refinement of interior grains of the melt. The research results help to better understand the refinement process and provide a more reasonable explanation of the grain refinement mechanism using ECP.

Influences of pulse electric current treatment on solidification microstructures and mechanical properties of Al-Si piston alloys

Three kinds of AI-Si piston alloys were prepared and subjected to pulse electric current treatment （PECT） at different pouring temperatures. Some aspects of the solidification microstructures were examined including the morphology and the distribution of the matrix and the secondary phases by using of optical microscopy （OM）, SEM and EDS methods. Results indicate that PECT can refine the grains of α-AI in the alloys as effectively as chemical modification by sodium salt. The processing parameters of PECT on the multi-component AI-Si alloys were then optimized through the testing of tensile strength, elongation and microhardness of the prepared alloys. A new theory was put forward to explain the mechanism of PECT.

Microstructure and Temperature Distribution in ZnAl_2O_4 Sintered Body by Pulse Electric Current

Microstructure of reaction sintering of ZnAl2O4 at 1500℃ by hot-pressing(HP) and pulse electric current was investigated. The results indicated that the existed cracks in sintered body were caused by structure mismatch. It is the evidence that periodical temperature field existed during pulse electric current sintering of nonconductive materials. The distance between high temperature areas was related to die diameter.

EFFECT OF ELECTRIC CURRENT PULSE ON SUPERPLASTICITY OF ALUMINIUM ALLOY 7475

The superplastic deformation of aluminium alloy 7475 applied electric current pulse has been studied.The results presented that applying a high density current pulse raises both the elongation rate and the m value,and makes supperior superplastic properties at high strain rate of 10^(-2)s^(-1),the optimum deformation temperature could move from 530℃to 480~510℃,TEM observation showed the superplastic deformation of the alloy is the results of grain boundary slips and dislocation slips in grains under function of the electron wind;the intergranular tear is a main behavior of fracture of the alloy.

Effect of melt pulse electric current and thermal treatment on A356 alloy

Effects of the melt pulse electric current and thermal treatment on solidification structures of A356 alloy were investigated. In the experiments, the low temperature melt(953 K and 903 K) treated by pulse electric current was mixed with high temperature melt(1 223 K). By the control experiments, the results show that the solidification structure of A356 alloy is refined apparently by the pulse electric current together with melt thermal treatment process, and the mechanical properties, especially the elongation ratio of the specimen treated is improved greatly. The structure change of the melt by pulse electric current and melt thermal treatment is the main reason for the refinement of the solidification structure of A356 alloy.

Equivalent Resistance in Pulse Electric Current Sintering

The sintering resistance for conductive TiB2 and non-conductive A12O3 as well as empty die during pulse current sintering were investigated in this paper. Equivalent resistances were measured by current and valtage during sintering the conductive and non-conductive materials in the same conditions. It is found that the current paths for conductive are different from those for non-conductive materials. For non-conductive materials, sintering resistances are influenced by powder sizes and heating rates, which indicates that pulse current has some interaction with non-conductive powders. For conductive TiB2 , sintering resistances are influenced by heating rates and ball-milling time, which indicates the effect of powders activated by spark.

Evolution of melt convection in a liquid metal driven by a pulsed electric current

Gain refinement in metal alloy can be achieved by applying an electric current pulse(ECP)in solidification process.Forced flow inside the melt has been proved to be a key role in grain refinement.In this paper,the fluid flow inside Ga 20 wt%-In 12 wt%-Sn alloy induced by a damping sinusoidal ECP flowing through two parallel electrodes into the cylindrical melt was investigated by both experimental measurements and numerical simulations.Experimental results showed that a strong descending jet was induced beneath the bottom of electrodes under the application of ECP.Besides,it was found that flow intensity increases with the increase of amplitude,frequency,and pulse width,respectively.In order to unlock the formation mechanism of flow pattern and the relevance of flow intensity varied with electrical parameters,a three-dimensional numerical model under the application of ECP was established.Meanwhile,a comparative study was conducted by numerical simulations to reveal the distributions of electromagnetic fields and forced flow.Numerical results showed that the downward Lorentz force induced by ECP was concentrated beneath the bottom of electrodes.This downward Lorentz force induces a descending jet and provokes a global forced flow.According to numerical simulations,the evolution of flow intensity with electrical parameters under the application of ECP can be understood by the time averaged impulse of Lorentz force.

Effect of high density pulse electric current on solidification structure of low temperature melt of A356 alloy

The effect of high density pulse electric current (HDPEC) on the solidification structure of the low temperature melt(LTM) of commercial A356 alloy was investigated. In the experiments, the HDPEC was discharged in the LTM (953?K, 903?K and 873?K). By the control experiments, the results showed that the solidification structure of the LTM of A356 alloy is refined apparently when the HDPEC is discharged in low temperature melt. However, the holding time of melt treated has an adverse effect on the solidification structure. The longer the holding time of the melt treated with HDPEC, the coarser the microstructure. With the same discharge voltage, the lower the temperature of LTM, the more obscure the refinement of solidification structure. Finally, the mechanism of microstructure refining by HDPEC was analyzed.

Migration Behavior of Impurity Iron in Silicon Melt Under Pulsed Electric Current

The impurity iron in silicon material will seriously affect the photoelectric conversion efficiency of silicon solar cells.However,the traditional silicon purification method has the disadvantages of long cycle,high energy consumption and serious pollution.In this study,an efficient and green pulsed electric current purification technology is proposed.The electromigration effect of iron elements,the current density gradient driving of iron phase,and the gravity of iron phase all affect the migration behavior of iron phase in silicon melt under pulsed electric current.Regardless of the depth of electrode insertion into the silicon melt,the solubility of iron in silicon decreases under the pulsed electric current,which helps to form the iron phase.At the same time,the iron phase tends to sink toward the bottom under the influence of gravity.When the electrode is shallowly inserted,a non-uniform electric field is formed in the silicon melt,and the iron phase is mainly driven by the current density gradient to accelerate sink toward the bottom.When the electrode is fully inserted,an approximately uniform electric field is formed in the silicon melt,and iron elements are preferentially migrated to the cathode by electromigration,forming iron phase sinking at the cathode.The study of impurity iron migration behavior in silicon melt under pulsed electric current provides a new approach for the purification of polycrystalline silicon.

Development of a high current short pulse electron gun

Dielectric wall accelerator(DWA), towards high gradient acceleration field(30 MeV/m–100 MeV/m), is under development at Institute of Modern Physics. A prototype was designed and constructed to prove the principle. This needs a short pulse high current electron source to match the acceleration field generated by the Blumlein-type pulse forming lines(PFLs). In this paper, we report the design and test of a new type short pulse high current electron gun based on principle of vacuum arc discharge. Electron beams of 100 mA with pulse width of 10 ns were obtained.

不同电学参数和水流条件下电栅对鲤(Cyprinus carpio)的阻拦效果

脉冲直流电导阻鱼技术已被广泛应用于阻止入侵物种扩散、驱赶鱼类远离危险区域并将其引导至有利区域,如鱼道。但是脉冲直流电的有效性受多方面因素的影响,其主导因素目前尚未定论。以鲤为研究对象,在静水条件下采用4因素3水平正交实验方法分析了脉冲电压、脉冲宽度、脉冲频率和电极摆放方式的电栅拦鱼效果及其对鱼类受伤尾数的主次影响,然后选择最优组合开展不同流速下(0,0.3,0.5 m/s)的驱鱼实验,分析流速对拦鱼效果的影响。结果表明:在静水工况下,影响拦鱼电栅效率的主要因素为电极布置方式>脉冲宽度>脉冲频率>脉冲电压,电极布置为方式1(垂直水流间距80 cm)、脉冲电压140 V、脉冲频率10 Hz、脉冲宽度10 ms是拦鱼效果最佳的组合。电学参数对鲤的伤害无显著性,平均受伤尾数不超过实验鱼总数的15%。流水条件下,三种流速工况下拦鱼电栅的阻拦率无显著差异,但是随着流速的增高阻拦率呈现先降低后增加的趋势。该研究将为保护鱼类资源和防止入侵鱼类跨区域传播提供依据。

人工耳蜗植入术后电诱发听性脑干反应的电生理特征与参数优化研究

目的研究不同测试参数对电诱发听性脑干反应(electrical auditory brainstem response,EABR)波形的影响,选择优化的人工耳蜗术后EABR测试参数,探讨EABR与人工耳蜗术后行为测试结果之间的关系。方法选取17名人工耳蜗植入者,记录和分析不同刺激电极位置、脉冲间隔和交替/非交替双相脉冲电流下EABR波V的引出率、阈值和潜伏期等特征,分析不同刺激电极位置下EABR阈值与行为学T、C值的相关性。结果(1)蜗顶处EABR的V波阈值(90.58±27.39 CU)显著小于蜗中处(106.98±15.66 CU);(2)蜗顶处的V波潜伏期(4.47±0.357 ms)显著短于蜗中处(4.72±0.335 ms);(3)采用交替双相脉冲电流的电伪迹幅值(9.40±12.35μV)显著小于非交替脉冲(19.49±15.12μV);(4)蜗顶处EABR阈值与行为测试的T值和C值均有显著相关性。结论EABR的刺激电极位置和脉冲电流类型均影响波形结果,蜗顶电刺激比蜗中更易引出理想的EABR波形,使用交替双相脉冲电流刺激可有效减少电伪迹的影响。EABR阈值可为人工耳蜗术后开机和调试提供参考。

A/B位计量比对脉冲电流促进钙钛矿溶出行为的影响

以(La_(0.7)Sr_(0.3))_(x)Ti_(0.93)Co_(0.07)O_(3)钙钛矿氧化物为基体活性材料,使用PEC(脉冲电流)处理技术,在600 V-3 Hz的脉冲参数下90 s内制得溶出型钙钛矿氧化物。正常化学计量下的ABO_(3)(钙钛矿型氧化物)中A位与B位摩尔比为n_(A)/n_(B)=1,结合A位缺位(x=0.8、0.9,即n_(A)/n_(B)<1)和A位过位(x=1.2,即n_(A)/n_(B)>1)2种不同计量比情况,研究脉冲电流对上述3大类钙钛矿氧化物溶出行为的影响。利用XRD(X射线衍射仪)分析脉冲电流处理后的A位缺位钙钛矿(即x=0.8、0.9)时存在新的Co单质衍射峰;SEM(扫描电子显微镜)、TEM(透射电镜)观察发现,脉冲电流处理后A位缺位时有明显的Co纳米颗粒在表面溶出;电化学测试表明脉冲处理后的A位缺位钙钛矿氧化物x=0.9时,面积最大且CV(循环伏安)曲线更接近矩形,有着更优秀的可逆性能和良好的电荷传递能力。综合分析发现,当A位缺位时对于脉冲电流处理后的钙钛矿氧化物有助于形成氧空位,更有利于触发钙钛矿B位Co还原成金属纳米粒子溶出,提高电子传导性和催化活性。

电脉冲轧制6061Al电热耦合模型及电弧加热机理分析

通过COMSOL Multiphysics仿真软件模拟高频脉冲电流激励时两板间界面处电弧的产生。讨论了在高频脉冲电流激励下,两块具有一定角度的金属板在电脉冲辅助轧制过程中界面处产生电弧的条件,并结合电磁学分析了电弧的形成机理。结果表明,在电脉冲辅助轧制过程中,除了焦耳加热外,还存在电弧加热现象,当加载300 A电流和10 kHz频率时,可以产生电弧并使两块铝板结合形成焊点。随着电流的增加,电弧温度和电势差也相应增加。在800 A电流下,电弧温度可达4 210 K,电势差可达54.5 V,已达到电弧形成的条件,但产生的电弧并不稳定。电弧的存在不仅瞬间加热了板材,提高了界面结合强度,而且在电弧力的作用下更容易形成波纹界面。模拟结果显示,加载800 A电流下可以产生超过4 000 K的电弧,并且在第一个脉冲周期内形成;并拟合了电弧产生的热源方程;为电脉冲辅助轧制研究提供了新思路。

电场处理对YG8硬质合金及其刀具性能的影响

为探索一种高效的硬质合金强化处理技术,研究了电场处理对YG8硬质合金显微组织及性能的影响,采用TEM、SEM等对电场处理前后的YG8硬质合金试样及其刀具进行微观组织分析,采用维氏硬度计进行力学性能测试,数控车床进行切削加工实验。结果表明:经电场处理后,YG8硬质合金试样断裂韧性有所增大,其刀具切削性能得到提升,且电场处理刀具的被加工工件表面质量明显得到提高。在脉冲电流作用下,材料会发生电塑性效应,为位错运动提供驱动力,使位错更容易进行增殖和滑移,位错密度增加,从而有利于材料性能的提高。当脉冲电流为15 A/mm^(2)时,断裂韧性达到最大值9.41 MPa·m^(1/2),相比于未使用电场处理的断裂韧性提高了近10%。经电场处理的YG8硬质合金刀具在切削过程中切削温度降低,可有效防止积屑瘤产生,改善切削过程的摩擦状态,保证了切削质量,使得被加工工件表面质量明显提高。

热成型钢氢脆行为研究进展

提高汽车用钢的强度是实现汽车轻量化设计的有效途径。然而,随着热成型钢强度不断提高,其氢脆敏感性也相应增大,氢致开裂已成为阻碍超高强度热成型钢发展与应用的重要问题。对热成型钢氢脆的影响因素、研究方法、机理和防护方法这4个方面的最新研究进展进行了综述,为相关领域的研究人员提供参考。

Altering the Residual Stress in High-Carbon Steel through Promoted Dislocation Movement and Accelerated Carbon Diffusion by Pulsed Electric Current

Residual stress in high-carbon steel affects the dimensional accuracy, structural stability, and integrity of components. Although the evolution of residual stress under an electric field has received extensive attention, its elimination mechanism has not been fully clarified. In this study, it was found that the residual stress of high-carbon steel could be effectively relieved within a few minutes through the application of a low density pulse current. The difference between the current pulse treatment and traditional heat treatment in reducing residual stress is that the electric pulse provides additional Gibbs free energy for the system, which promotes dislocation annihilation and carbon atom diffusion to form carbides, thus reducing the free energy of the system. The electroplastic and thermal effects of the pulse current promoted the movement of dislocations under the electric field, thus eliminating the internal stress caused by dislocation entanglement. The precipitation of carbides reduced the carbon content of the steel matrix and lattice shrinkage, thereby reducing the residual tensile stress. Considering that a pulsed current has the advantages of small size, small power requirement, continuous output, and continuously controllable parameters, it has broad application prospects for eliminating residual stress.

电脉冲红外成像技术在变电站直流电源系统故障预测中的应用

随着变电站自动化技术的不断进步,直流电源系统作为保障变电站正常运行的关键组成部分,其可靠性和安全性受到极大关注。电脉冲红外成像技术作为一种非接触式的高精度诊断技术,近年来在电力系统中的应用越来越广泛。文章通过深入分析电脉冲红外成像技术的基本原理和特性,结合变电站直流电源系统的运行机制和故障特性,提出一种新的故障预测模型。

脉冲电流作用下核反应堆压力容器修复延寿技术

核反应堆压力容器钢的使用年限直接决定了核电站的服役寿命。压力容器钢在中子辐照环境下长期服役会逐渐产生大量位错环和纳米团簇缺陷,导致材料脆化甚至失效,严重威胁核反应堆的安全运行。传统退火处理可使材料性能部分恢复,但需对压力容器进行整体拆卸,导致反应堆长时间停堆,造成巨大经济损失。作为一项绿色高效的技术,脉冲电流“原位”修复处理为实现核反应堆压力容器服役延寿提供了新途径。简要综述了脉冲电流修复机理、服役寿命预测方程构建以及实用案例,对未来核反应堆关键构件延寿技术的发展具有重大意义。