Study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition

Cathode spot motion and macroparticles (MPs) reduction on related films are the two main issues in the application of the vacuum arc deposition (VAD). In the present work, an axisymmetric magnetic field (AMF) was applied to the cathode surface to investigate the influence of the AMF on the cathode spot motion and the MPs reduction on TiN films. The results show that the AMF affected the cathode spot motion by redistributing the dense plasma connected with the initiation of the new spot. With increasing AMF, there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. Based on the results of FEM simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion in the AMF was discussed. The morphology, detailed size distribution, and roughness of the resultant TiN films were systematically investigated. Fewer and smaller MPs ejection is observed with an increase in the transverse component of AMF. The effect of the AMF on the MPs reduction on TiN films was discussed, and the results were compared with the theoretical predictions.     

On the mechanism of the external magnetic field action on the electron temperature and ion charge state distribution in a vacuum arc plasma

When an external axial magnetic field is applied to a vacuum arc, the radial expansion of plasma from cathode spots transforms into a plasma flow along the magnetic field, provided that the electron-ion collision frequency is smaller than the Larmor frequency. As the magnetic field strength increases, the diameter of the resulting cylindrical channel decreases. This leads to an increase in the electron temperature and the ion charge due to enhanced Joule heating of the plasma. Unlike the intrinsic azimuthal magnetic field, the external axial field only restricts expansion of the plasma, rather than compressing the plasma jet.     

磁场辅助电弧焊接旋转等离子体行为的数值模拟

目的 研究外加纵向磁场对倾斜电极TIG焊接的电弧温度分布、流动模式和工件所受热力作用的影响.方法 建立磁场-电弧复合焊接热、电、磁、流动的三维数学模型.通过数值模拟和高速摄像实验,揭示倾斜电极电弧在外加磁场作用下的流动、形貌及温度演化机制.结果 外加纵向磁场后,电弧流动速度明显增加,流动模式由沿电极方向喷射变为近似沿竖直方向旋转向下的流动模式;电弧对工件的热作用均匀性提高,热作用中心向电极正下方靠近,但在焊接横向方向上存在偏离;工件受到表面的电弧旋转拖拽力和内部的旋转洛伦兹力作用,最大洛伦兹力可达50000 N/m3.结论 基于所建立数学模型的模拟结果与实验电弧形貌吻合良好,结果表明,外加纵向磁场能够显著改变电弧的形态及流动模式,提高电弧热流密度的均匀性,并能够对熔池产生有效的搅拌作用.     

Critical flow of high-speed ions in a high-current vacuum-arc discharge

A physical model of vacuum arc is developed, which is used to construct a quasi-one-dimensional model and a two-dimensional two-fluid mathematical model. The two-dimensional magnetohydrodynamic model is based on the method of trajectories, according to which a set of partial equations is reduced to a set of ordinary differential equations written for derivatives along the lines of current. The quasi-one-dimensional model is based on the principle of compensation of radial magnetic forces, which is valid for a short high-current vacuum arc in axial magnetic field. A criterion is obtained, which defines the range of validity of the quasi-one-dimensional model in external axial magnetic field B ₀. Calculations are performed of the dependence of critical current on B ₀ for discharge gaps of different geometries. The calculation results agree with experiment.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

水中悬浮隧道在冲击载荷作用下的计算模型与数值模拟

建立了水中悬浮隧道在冲击载荷作用下的简化计算模型。用等效质量法将圆柱壳分布质量折算成冲击点处的集中质量,模型中考虑流体附加质量和系统阻尼的影响。根据碰撞过程中的动量守恒、变形过程中的能量守恒以及结构的位移与内力关系,得到问题的解析解。为验证解析解,在ANSYS/LS-DYNA中建立了动态冲击有限元分析模型。通过算例分别考察了在忽略和考虑流体附加质量两种情况下,冲击点位置和冲击速度对冲击点处最大径向位移的影响,将解析解与数值解进行对比,结果吻合较好。然后采用数值模拟方法得到了系统阻尼对计算结果的影响规律。数值模拟过程中还可以得到冲击点处的最大Mises应力。     

Streamlining of a cylinder with an electric arc rotating in a magnetic field

Streamlining of a circular cylinder with a localized heat source modeling an MHD actuator in which the plasma arc channel moves along the cylinder surface under the action of the Lorentz force in a radial magnetic field is studied experimentally and simulated numerically. It is shown that the presence of a moving heat release region leads to a break in the symmetry in cylinder streamlining by the external flow and the appearance of a nonzero lift force and circulation.     

Numerical investigations of arc behaviour in gas metal arc welding using ANSYS CFX

Current numerical models of gas metal arc welding (GMAW) are trying to combine magnetohydrodynamics (MHD) models of the arc and volume of fluid (VoF) models of metal transfer. They neglect vaporization and assume an argon atmosphere for the arc region, as it is common practice for models of gas tungsten arc welding. These models predict temperatures above 20 000 K and a temperature distribution similar to tungsten inert gas (TIG) arcs. However, current spectroscopic temperature measurements in GMAW arcs demonstrate much lower arc temperatures. In contrast to TIG arcs they found a central local minimum of the radial temperature distribution. The paper presents a GMAW arc model that considers metal vapour and which is in a very good agreement with experimentally observed temperatures. Furthermore, the model is able to predict the local central minimum in the radial temperature and the radial electric current density distributions for the first time. The axially symmetric model of the welding torch, the work piece, the wire and the arc (fluid domain) implements MHD as well as turbulent mixing and thermal demixing of metal vapour in argon. The mass fraction of iron vapour obtained from the simulation shows an accumulation in the arc core and another accumulation on the fringes of the arc at 2000 to 5000 K. The demixing effects lead to very low concentrations of iron between these two regions. Sensitive analyses demonstrate the influence of the transport and radiation properties of metal vapour, and the evaporation rate relative to the wire feed. Finally the model predictions are compared with the measuring results of Zielińska et al.     

Heat fluxes in plasmatron channel

Numerical simulation of a longitudinally blown electric arc by two-dimensional gas-dynamic equations made it possible to determine temperature and velocity fields, axial and radial components of convective, conductive and radiative heat fluxes, heat losses through a channel wall. The model takes into account radiation transfer in the actual spectrum of an electric-arc plasma. Using the results of calculations for argon and air, approximation expressions are obtained for the temperature dependences of the divergences of radiation fluxes.     

Metal plasma jet passage through plasma-optical transport system

The passage of a plasma jet of steady-state vacuum arc discharge through a plasma-optical transport (POT) system based on a curvilinear magnetic field has been studied. Special features of the system are the absence of a metallic duct and the misfit of axes of the guiding magnetic field and the plasma jet. It is demonstrated that the position and shape of the plasma jet at the POT system output can be varied within broad limits by changing the currents in solenoids and the positive bias on a control electrode introduced into the plasma jet. For fixed parameters of the POT system, the jet position depends on the mass to charge ratio of plasma ions.     

DC vacuum arc in a nonuniform axisymmetric magnetic field

An experimental investigation is performed of the characteristics of DC vacuum arc during the opening of contacts in an axisymmetric magnetic field with different ratios between the axial and radial components of magnetic field induction. A study is made of the main features of the dynamics of plasma glow in the contact gap during the stable and unstable stages of arcing until the arc extinction in the current range of 50 to 150 A with the voltage limited at the instant of breaking of current to a level of 8 kV. Possible mechanisms of disturbance of the vacuum arc stability in an axisymmetric magnetic field are discussed.     

金属蜂窝平板加热过程的数值模拟及试验研究

对某种金属蜂窝平板进行了模拟气动加热试验，测定了加热表面和非加热表面的瞬态温度。在试验结果的基础上，考虑传导和辐射传热的偶合作用，建立了该金属蜂窝平板传热的有限元分析模型。计算结果与试验结果吻合较好，表明该数值模拟方法可用于计算传导和辐射偶合问题。对有限元计算结果和试验结果的进一步分析表明，温度沿厚度方向呈规则的线性分布，该规律对于金属蜂窝的设计和使用有重要的指导意义。     

Numerical simulation of argon twin torch plasma arc for high heating efficiency

Plasma arc heating technology has been applied for volume reduction through melting of bottom and fly ash, and for producing slag. Recently, a twin torch plasma arc, which has two torches at the cathode and anode, has been anticipated for application to disposal of medical waste because it can treat a wide area and can treat non-conductive materials. For this study, a numerical simulation model of a twin torch plasma arc at opposite electrodes was developed to elucidate high-efficiency heating using a twin torch plasma arc. It is defined as Local Thermal Equilibrium (LTE) and calculated in Magneto-Hydro-Dynamic (MHD) equations. Furthermore, the temperature distribution and conditions of high heating efficiency with the radiation loss were addressed and compared to those of a single torch. The heating efficiency decreases with increasing radiation efficiency because of the temperature increment caused by the current and input power. The radiation efficiency of a twin torch is about 5% higher than that of single torch.     

Calculation of Gas and Electronic Temperatures in the Channel of the Direct Current Arc

The results of calculations of gas and electronic temperatures in the channel of an arc plasma generator are presented. The calculations were carried out within the framework of a self-consistent two-temperature channel model of an arc discharge. The given method can be used with good precision to determine the radial distribution of gas and electronic temperatures in conducting and non-conducting zones of a constant current arc at designated parameters of the discharge (current intensity and power).     

A systematic AMF–FEM coupled method for the thermo-elasto-plastic contact analysis of the plasma sprayed HA-coated biocomposite

Hydroxyapatite (HA) coated Ti-6Al-4V alloy biocomposite has been accepted as one of the most promising implant materials for orthopaedic and dental applications because of its favorable biocompatibility and mechanical properties. After the plasma sprayed HA composite coating on titanium alloy substrate biocomposite was prepared, a novel meshless numerical analysis method of the coupled adaptive meshfree method and finite element method (AMF–FEM) is developed for the simulation of the thermo-elasto-plastic contact problems of the biocomposites in this paper. The adaptive meshfree method based on strain energy gradient is used in the concerned contact domain, and FEM is used in the non-contact domain to overcome the difficulties of the meshfree method and improve the calculation efficiency. The thermo-elasto-plastic contact model using the incremental-initial stiffness method, error estimation and the local adaptive refinement strategy for the AMF–FEM method are combined. The AMF–FEM thermo-elasto-plastic model takes into account the temperature variation, micro plastic flow, the thermo-elasto-plastic coupling behavior and the strain-hardening property of the materials. The examples of the elastic/thermal-elastic contact of real HA-coated rough surfaces using the AMF–FEM is studied for two biomaterial models, respectively. The results all show that the AMF–FEM solutions are accurate, efficient, and can be widely applied to different thermo-elasto-plastic contact multi-layer biomaterial models considering different geometric parameter, material parameter, thermal and friction properties.     

Exploiting Unique Alignment of Cobalt Ferrite Nanoparticles,Mild Hyperthermia,and Controlled Intrinsic Cobalt Toxicity for Cancer Therapy

Nanoparticle-based magnetic hyperthermia is a well-known thermal therapy platform studied to treat solid tumors, but its use for monotherapy is limited due to incomplete tumor eradication at hyperthermia temperature (45 °C). It is often combined with chemotherapy for obtaining a more effective therapeutic outcome. Cubic-shaped cobalt ferrite nanoparticles (Co–Fe NCs) serve as magnetic hyperthermia agents and as a cytotoxic agent due to the known cobalt ion toxicity, allowing the achievement of both heat and cytotoxic effects from a single platform. In addition to this advantage, Co–Fe NCs have the unique ability to form growing chains under an alternating magnetic field (AMF). This unique chain formation, along with the mild hyperthermia and intrinsic cobalt toxicity, leads to complete tumor regression and improved overall survival in an in vivo murine xenograft model, all under clinically approved AMF conditions. Numerical calculations identify magnetic anisotropy as the main Co–Fe NCs’ feature to generate such chain formations. This novel combination therapy can improve the effects of magnetic hyperthermia, inaugurating investigation of mechanical behaviors of nanoparticles under AMF, as a new avenue for cancer therapy.     

Radial Distribution of the Position of the Arc in a Plasmatron

The radial-distribution function of the position of the arc in a plasmatron of an axial scheme with gas-vortex stabilization has been determined from the results of standard measurements of the radial pulsations of the projection of the arc. A model allowing explanation of the displacement of the distribution maximum from the axis of the discharge chamber with increase in the current has been proposed. Based on this model, the condition for the discharge-channel radius with which gas-vortex stabilization is efficient has been obtained.     

Focusing experiments in plasma coaxial railguns

The "projectile" being accelerated in a plasma coaxial railgun is a complex mixture: besides plasma, it contains solid particles of fuse, liquid and vapor metal. Materials accelerated are not limited to the fuse metal, the rail material through ablation also contributes to them. For such a situation "plasma focusing" is a simplified term, borrowed from electron beams in vacuum technology (and electron lenses) where the Busch's theorem describes the tendency of electrons to approach the axis (or to focus) in the presence of radial and axial components of magnetic fields and under the assumption that on the time scale of interest, the ions remain essentially motionless. The present work is the first in a series of papers trying to classify and to explain a larger spectrum of problems affecting a mixture of metals in different states (solid, liquid, and vapor) in which magnetic fields, transient diffusion, and other phenomena may be used to contain or focus a flowing stream under thermal expansion. The main variable used in experiments to affect the focusing was the topology of magnetic fields, temporal and spatial. Emphasis was given to the effects of penetration of magnetic field in the bulk of the "projectile" being accelerated. Experiments were conducted on a 0.5-m coaxial railgun with inner and outer coaxial radii of 0.25 and 0.31 in., respectively. The rails were constructed from stock sizes of copper pipe with a CVD tungsten coating on the inner rail surface to minimize arc damage during the shots. External magnetic fields were applied to a 3-mg metallic vapor arc accelerated with a 250 kA capacitive discharge. Plasma characteristics were measured with magnetic pick-up coils and langmuir probes. Confinement and focusing of the arc was examined with break screens and metal deposition analysis.     

横向稳态磁场作用下微束等离子电弧数值分析

为分析外加横向稳态磁场作用下电弧特性与电弧对工件热、力输入的变化规律,本文建立了微束等离子电弧三维模型,将外加磁场简化为背景场添加至模型,使用有限元分析软件COMSOL进行求解计算。结果表明:外加横向稳态磁场作用下,喷嘴内部各项特性均未发生较大变化,喷嘴下方电弧等离子体在洛伦兹力的作用下向x负方向移动,电弧温度、等离子体速度与电流密度等均发生相应的偏转,因此造成电弧对工件的热输入减小、压力降低。最后在薄壁件上进行了焊接试验,试验过程中拍摄的电弧轮廓与仿真电弧形态基本一致,加磁区域堆焊层余高增加、熔宽减小,焊缝向两侧"下塌"现象消失。外加横向稳态磁场可减小电弧对熔池的热、力输入,进而改变焊缝形貌,抑制薄壁件堆焊过程中由于热、力输入过大而产生的"下塌"现象,有利于薄壁件的堆焊成形。