Fabrication of a porous material with a porosity gradient by a pulsed electric current sintering process

A porous structure with a porosity gradient can be applied to the preparation of continuous FGM, where liquid or chemical vapor of the second phase is infiltrated into the graded pores. It also has applications in skeletal implant materials and ultrafiltration media. An attempt was made to fabricate a porous material with a porosity gradient by means of a pulsed electric current sintering (PECS) process. The present work describes not only the measured value of the temperature difference between the upper and lower part of the specimen, which brings about a gradual change in pore distribution, but also the sintering characteristics of the porous structure obtained by the pressureless PECS process.     

单轴拉伸与压缩加载下AZ31镁合金单晶的各向异性塑性行为(英文)

为研究HCP结构单晶在塑性变形中的变形孪晶和塑性各向异性,采用基于晶体塑性本构理论的有限单元法,建立包含滑移与孪生变形机制的晶体塑性本构关系,发展了以应力作为自变量的牛顿-拉普森迭代方法,通过已有文献的试验数据验证模型的有效性,并利用此模型模拟AZ31单晶体在4种(即沿〈2110〉,〈0110〉,〈0001〉和〈0111〉方向)拉伸与压缩变形路径下的塑性变形行为,并获得了相应加载路径下的应力-应变关系曲线。数值计算结果表明,在不同加载路径下该模型可用于预测滑移系或孪生系的活动情况,以及描述孪生变体的活动数量、主要孪生变体和孪生交叉类型。由于机械孪晶具有的极性性质及其在材料非弹性变形中的重要作用,单晶材料表现出显著的各向异性与非对称性。     

Analytical model for the expansion of tubes under tension

This paper focuses on the expansion of metallic tubes subjected to large radial and circumferential plastic deformations. This process can be achieved by driving rigid conical mandrels of various diameters through them either mechanically or hydraulically in order to obtain desirable expansion ratios. A mathematical model was developed to predict the stress field in the expanded zone, the drawing force required for expansion, and the resulting dissipated energy from which optimum mandrel shapes were obtained. A finite element analysis was used to validate the theoretical results. A good agreement was obtained in terms of drawing force and dissipated energy for different geometric constraints and friction coefficients. The study showed that the optimum mandrel angle ranges between 22 and 25 degrees for low friction and increases non-linearly when friction increases.     

Highly thermal conductive metal/carbon composites by pulsed electric current sintering

Various composites with metals such as aluminum and copper, and carbons such as pitch based carbon fiber and carbon nanotube, and scale-like graphite have been prepared to realize high thermal conductive materials utilizing pulsed electric current sintering method. The composites have high thermal conductivity and one- or two-dimensional low thermal expansion coefficient.     

Experimental study and simulation of plastic deformation of zirconia-based ceramics in a pulsed electric current apparatus

Plastic deformation by compression of cylindrically shaped zirconia (ZrO₂)-based ceramics in a pulsed electrical current apparatus was studied using a combined experimental and theoretical approach. Both fully dense electrically insulating 3Y–ZrO₂ and electrically conductive 3Y–ZrO₂–TiCN 60/40 (vol.%) ceramics were subjected to a compressive load at temperatures above 1200 °C. Deformed non-conductive 3Y–ZrO₂ samples were concave shaped, whereas the composite samples exhibited a different behaviour depending on the electrical current path within the set-up. A convex shape was obtained when the current was freely flowing through them, while they started to become concave shaped when the samples were separated from the graphite pressing punches by relatively low conductive silicon carbide disks. The secondary titanium carbonitride (TiCN) phase in the composite materials exhibited a grain boundary pinning effect, which limited coarsening of their microstructure. The influence of current flow on the shape of the deformed ceramic samples was interpreted in terms of the temperature distribution generated during hot deformation. Finite-element simulations, coupling thermal, electrical and mechanical fields, were used to explain the deformation behaviour of the different samples. A subsequently coupled thermal–electrical and mechanical analysis procedure was developed for this aim. Special attention was paid to the materials and interactions properties used during modelling. The modelling results are in good agreement with the experimental data, so that the developed finite-element approach and code can be used for the analysis of near net shaping of ceramic parts assisted by an electrical field.     

Microstructure and texture evolution in a twinning-induced-plasticity steel during uniaxial tension

A combination of electron back-scattering diffraction and X-ray diffraction was used to track the evolution of the microstructure and texture of a fully recrystallized Fe–24 Mn–3 Al–2 Si–1 Ni–0.06 C twinning-induced-plasticity steel during interrupted uniaxial tensile testing. Texture measurements returned the characteristic double fibre texture for face-centred cubic materials, with a relatively stronger 〈1 1 1〉 and a weaker 〈1 0 0〉 partial fibre parallel to the tensile axis. The interaction with the stable 〈1 1 1〉 oriented grains results in preferential plastic flow in the unstable 〈1 1 0〉 oriented grains. Consequently, the grains oriented along the 〈1 1 0〉 and 〈1 0 0〉 fibres record the highest and lowest values of intragranular local misorientation, respectively. The viscoplastic self-consistent model was used to simulate the macroscopic stress–strain response as well as track the evolution of bulk crystallographic texture by detailing the contributions of perfect and/or partial slip, twinning and latent hardening. The simulations revealed the dominant role of perfect slip and the limited volume effect of twinning on the texture development. The effects of initial orientation and grain interaction on the overall orientation stability during uniaxial tension showed that while the 〈1 0 0〉 fibre remains stable and does not affect the unstable orientations along the 〈1 1 0〉 fibre, the orientations along the stable 〈1 1 1〉 fibre strongly affect the unstable 〈1 1 0〉 orientations.     

Defect classification using a new feature for pulsed eddy current sensors

The objective of this study is to identify defects such as surface cracks, subsurface defects and metal losses using feature based pulsed eddy current sensors. A new feature, termed as the rising point, related to the propagation time of electromagnetic waves in metallic targets is proposed for defect classification. Experimental studies of the validation, robustness of the new feature of rising time are reported. In addition to other features, defects can be detected and quantified robustly and lift-off can also be derived from the rising time. Conclusion and further work are derived on the basis of the findings.     

Heating of high current electric contacts under short-circuit shock currents

The article deals with experimental research and the numerical simulation of pulse heating of high-current electrical contacts using short-circuit currents in a wide temperature range up to the melting point. The electrode welding is found to start at temperatures that are much lower than the melting point of the material. The peculiarities of the softening and melting processes of the contact area are described.     

Equipment for electric arc metallizing with pulsed discharge of the air-spraying jet

The results of a review of literature data show that electric arc metallizing systems are characterized by high consumption of compressed air. Spraying is accompanied by the high-intensity interaction of the spraying air jet with the liquid metal of the molten ends of the electrodes, leading to rapid burnout of the alloying elements. The intensity of the oxidation reaction depends on the heat resistance of the sprayed material, the dispersion of the particles, the affinity of the components for oxygen and the technological parameters of the coatings. In order to reduce the oxidation effect, it is proposed to use a pulsed air-spraying jet. To solve this problem, experiments were carried out to develop appropriate equipment for the EN-17 stationary electric arc metallizing system characterized by the pulsed discharge of the air-spraying jet in the range 0–130 Hz. The experimental results show that the spraying jet is pulsed with temporary breaks. When using circular cross sections, the pulse smoothly becomes larger. The application of the right-angled section results in the pulse shape increasing at a higher rate. The experimental results show that the oxidation effect of the air-spraying jet is reduced. It is also shown that the optimum frequency resulting in improved properties of the coating is in the range 40–80 Hz.     

Fast analytical modelling for pulsed eddy current evaluation

Numerical simulations of electromagnetic non-destructive evaluation (ENDE) can be time-consuming in comparison to analytical methods which provide fast closed-form solutions to the ENDE problems. In this paper, the Truncated Region Eigenfunction Expansion (TREE) modelling is extended to solve problems of pulsed eddy current (PEC) evaluation from the traditional multifrequency eddy current. The Fourier transform is employed to make the TREE feasible for solutions to PEC problems in both time and frequency domains. Moreover, because PEC employs magnetic field sensors/arrays to quantify magnetic field, the magnetic field signals from solid-state magnetic field sensors have been simulated using the extended TREE. It has been found that the predicted signals using the extended TREE has good agreement with the experimental results. Consequently, the established model can not only offer an effective solution in terms of faster simulation time and higher computational accuracy, but also be used for PEC evaluation in industry and in the inverse process for exploring the structural and electrical information of stratified conductive specimens during real-time monitoring.     

The influence of percolation during pulsed electric current sintering of ZrO2–TiN powder compacts with varying TiN content

A series of ZrO₂–TiN composite powder compacts with varying TiN content was densified using the field assisted sintering technique, also known as spark plasma sintering or pulsed electric current sintering (PECS). The TiN content was varied between 35 and 90 vol.% in order to obtain an electrical conductive composite material that can be shaped by electrical discharge machining. The influence of the TiN content on the densification behaviour was investigated experimentally, whereas its influence on the temperature and current distribution in the PECS tool set-up was simulated using a previously developed finite element model. The predicted temperature distribution was confirmed experimentally using a double pyrometer set-up, one focusing on the outer die wall surface and one on the bottom of a borehole in the upper punch. The changing thermal and electrical properties of the sintering ZrO₂–TiN powder compacts were calculated using mixture rules. Using a double pyrometer set-up, a clear relationship could be verified experimentally between the changing electrical properties of the sintering compact and the temperature redistribution in the punch/die/sample set-up during the PECS process. The homogeneity of sintering inside the PECS equipment is discussed in detail and suggestions are made in order to promote a more homogeneous sintering process. Carbon felt, acting as a thermal insulator, was placed around the die in order to minimize the radiation heat losses and to minimize the thermal gradients during heating and the dwell period at maximum temperature. The mechanical and electrical properties of the different composite materials are discussed as functions of the TiN content.     

剪切修正GTN模型在拉剪变路径加载问题中的适用性

为评估含剪切修正项GTN模型在非比例加载条件下的适用性,以一组缺口圆棒试样的拉剪变路径加载试验结果为依据,运用有限元软件ABAQUS/Explicit模块及用户子程序VUMAT进行平行仿真分析。研究结果表明:在大预变形下的拉剪变路径加载条件下,增长的预应变会加快后续拉伸过程中孔洞的增长率,导致材料提前断裂。同时,结合断口宏观与微观形貌分析,研究了非比例加载过程材料的损伤演化机理。AA2024铝合金在拉剪变路径加载条件下的断裂机制为拉-剪混合断裂,随着预扭作用的增强,剪切断裂逐渐取代拉伸断裂成为主导的断裂模式。     

Combined analytical-numerical approach to the voltage of a cylindrical coil with pulsed current

This paper presents a combined analytical-numerical approach to solving the pulsed eddy current problem accurately and quickly. Considering the displacement current, the analytical solution to the voltage of a cylindrical coil above a laminated conductor in the complex-frequency domain is deduced by Laplace transform. The time-domain induction voltage values of a cylindrical coil with a pulsed current are calculated by the fourth-order integro-differential FFT-based numerical inversion of Laplace transform. At the same time, the time-domain analytical solution to the induced voltage of a cylindrical coil with a pulsed current above a half-infinite non-ferromagnetic conductor is derived, and has been verified by comparison with Finite Element Method (FEM) simulation results. The calculation results prove that the adopted numerical inversion method of applying Laplace transforms to the pulsed eddy current problem has a high accuracy and fast convergence. The transient voltages produced by a square-wave current excitation when considering the displacement current in the vacuum area are higher than those when ignoring the displacement current, by as much as 27.7% at certain times. The higher the lift-off is, the smaller the voltage peak is and the faster the voltage drops. As the application of this method, the induced voltages are computed in the measurements of metal's thickness and metal coating thickness.     

脉冲大电流扩散焊接Al-Li合金1420

采用脉冲大电流热加工技术焊接Al-Li合金1420.通过比较不同焊接工艺下的焊接产品,在最优焊接条件下获得了接头抗拉强度为母材强度75%的焊接接头.分析了接头微观结构及其对接头性能的影响,通过金相图片和扫描图片分析了接头处焊接点的形成过程,认为焊接过程实际可以分为由于微区熔化现象引起的焊接点高速形成阶段和围绕焊接点的原子扩散过程.脉冲大电流焊接过程由于焊接时间短,因而晶粒并没有发生二次生长.接头断口SEM照片显示,接头断裂方式为沿晶/穿晶混合断裂方式.     

A dynamic welding heat source model in pulsed current gas tungsten arc welding

A time-dependent welding heat source model, which is defined as the dynamic model, was established according to the characteristic of PCGTAW. The parabolic model was proposed to describe the heat flux distribution at the background times. The recommended Gaussian model was used at the peak times due to the bell-shaped temperature contour. The dynamic welding heat source was composed of these two models with a function of time.     

Reduction of lift-off effects for pulsed eddy current NDT

The lift-off effect is commonly known to be one of the main obstacles for effective eddy current NDT testing as it can easily mask defect signals. Pulsed eddy current techniques, which are believed to be potentially rich of information, are also sensitive to the effect. An approach using normalisation and two reference signals to reduce the lift-off problem with pulsed eddy current techniques is proposed. Experimental testing on the proposed technique and results are presented in this report. Results show that significant reduction in the effect has been achieved mainly in metal loss and sub-surface slot inspection. The technique can also be applied for measurement of metal thickness beneath non-conductive coatings, microstructure, strain/stress measurement, where the output is sensitive to the lift-off effect.     

Phase field study of precipitate rafting under a uniaxial stress

We examine rafting of two-phase microstructures under a uniaxial applied stress, a process in which a mismatch in elastic moduli (elastic inhomogeneity) plays a central role. For this purpose, we have used a phase field model of an elastically inhomogeneous alloy; elastic stress and strain fields are calculated using a method adapted from the homogenization literature. We have characterized the efficiency of the resulting iterative algorithm based on Fourier transforms. Our simulations of rafting in two-dimensional systems show that rafting (unidirectionally elongated microstructures) is promoted when the precipitate phase is softer than the matrix and when the applied stress has the same sign as the eigenstrain. They also show that migration (for both hard and soft precipitates) and coalescence (for soft precipitates) have significant contributions to rafting.     

Surface tension of a solid with adsorption on its surface with its deformation and surface electric charge considered

The surface tension of a solid at its interface with a liquid phase containing a surfactant is considered. The deformation of the solid surface and its electric charge are taken into account. In terms of the previously proposed theory on equilibrium one-component adsorption on the solid surface with deformation of this surface taken into account, for the model involving two parallel capacitors, the criteria for the “positive” and “negative” Rehbinder effects are obtained. The conditions under which the “positive” Rehbinder effect disappears in moving from tensile deformation to compressive deformation are given. An expression for the derivative of the surface tension with respect to the electric potential of the solid is derived.