Three Dimensional Modeling of the Plasma Spray Process

Results of simulations of three-dimensional (3D) temperature and flow fields inside and outside of a DC arc plasma torch in steady state are presented with transverse particle and carrier gas injection into the plasma jet. The results show that an increase of the gas flow rate at constant current moves the anode arc root further downstream leading to higher enthalpy and velocity at the exit of the torch anode, and stronger mixing effects in the jet region. An increase of the arc current with constant gas flow rate shortens the arc, but increases the enthalpy and velocity at the exit of the torch nozzle, and leads to longer jets. 3D features of the plasma jet due to the 3D starting conditions at the torch exit and, in particular, due to the transverse carrier gas and particle injection, as well as 3D trajectories and heating histories of sprayed particles are also discussed.     

低压条件下的等离子射流特性分析

低压等离子喷涂由于具有特殊的射流特性,以及可以沉积组织均匀的特殊结构涂层而备受关注.涂层的形成受到等离子射流特性的影响,比如焓值、温度、速度等.研究中利用热焓探针技术在环境压力为3 kPa的条件下,测量了Ar-H₂等离子体不同轴向位置射流中心的焓值和压力,并且进一步计算了射流的温度和速度,以及表征等离子体对粉体加热能力的努森数.结果表明,等离子射流在距离喷嘴出口12.5 mm处的温度为11 000 K;400 mm处降为7 000 K;等离子射流速度在喷嘴出口处25 mm左右达到最大值,约为2 000 m/s;喷嘴外部等离子射流的努森数处于过渡区,对粉体的加热能力较低.     

弧电压对等离子射流温度脉动的影响

电弧等离子体射流中的湍流是等离子体射流的典型物理现象之一。射流的脉动将直接影响射流温度的脉动,而以往的研究认为射流存在一个处于稳定状态的核心区域,本文采用电弧等离子体光谱诊断及数字高速摄影的方法对常压电弧等离子体射流进行了研究,采用了傅立叶变换的方法分析弧电压和射流光谱强度信号,发现电源的交流分量和阳极弧点运动对整个射流的脉动特性都有影响。射流并不存在一个处于稳定状态的核心区域,相反从谱线强度脉动与弧电压脉动的FFT分析图中可以看到,射流的脉动主要是由电弧电压的脉动造成的。     

Entrainaient and demixing in subsonic argon/helium thermal plasma jets

The velocity, temperature, entrained air fraction, and Ar/He concentration profiles were measured in a subsonic thermal plasma jet using an enthalpy probe and mass spectrometer. Through interaction with the surrounding atmosphere, air is quickly entrained into the jet, resulting in rapidly decreasing velocities and temperatures. Due to the difference in ionization potential, a significant diffusive separation or demixing of Ar and He is also observed in the large temperature gradients present. Near the exit of the torch, in the jet center, the relative He concentration is enhanced by approximately 50% over that of the premixed feed gases. Demixing occurs primarily in the discharge region and torch nozzle. As jet mixing progresses in the downstream direction, the Ar to He ratio approaches the initial input ratio.     

高速电弧喷枪中电弧场的数值模拟

运用CFD软件，对高速电弧喷枪的流场进行了模拟。通过分析计算结果，将射流分为电弧区、核心区、卷吸区和湍流区4个部分。电弧区内部速度和温度起伏剧烈，但对其外3个区域的流场速度分布影响不大，对轴线温度影响为20-40K。电弧区内部最高温度为6800K，分布在阴极表面上，阴极的平均温度为4250K，从阴极到阳极，电流密度和电弧温度都逐渐降低，阳极的平均温度为538K。     

单孔皮托管快速扫描法测量超音速等离子体射流的速度

针对工业用的超音速等离子体发生器的特点,采用无水冷单孔皮托管快速扫描法测量超音速等离子体射流的动压,用喷管出口平面埋静压管的方法测量静压,从而得到超音速等离子体射流的马赫数。为了对此测量方法进行验证、比较,利用一些可以方便测得的量,通过一些普通定律计算出超音速等离子体射流的马赫数,二者比较,相对误差小于10%。     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria stabilized ZrO2: Part 2. modeling

Yttria stabilized ZrO₂ particle in-flight characteristics in an Ar-H₂ atmospheric plasma jet have been studied using analytical and experimental techniques. In the previous article,^[1] the primary gas flow, plasma composition, current, and powder feed rate were systematically varied and particle surface temperatures, velocities, and size distributions measured and statistically analyzed. In this paper, a mathematical model for the plasma flow and particle characteristics is presented. Model predictions are compared with the experimental results in Ref 1 and a reasonable correlation is found. A statistical investigation (composite cubic face (CCF)) is performed on the particle predictions, giving fast and simple relationships between gun parameters and particle in-flight properties. The statistical and theoretical models that are presented here combine to form a powerful and cost-effective tool, which can be used in the evaluation and optimization of spray parameters off-line.     

On the gas dynamics of HVOF thermal sprays

An experimental study of the gas-dynamic aspects of the high-velocity oxyfuel (HVOF) thermal spray process has been performed using commercially available HVOF equipment (Hobart-Tafa JP-5000, Ho-bart-Tafa Technologies, Inc., Concord, NH). Optical diagnostic techniques, including microsecond-expo-sure schlieren and shadowgraph imaging, were applied to visualize the hot supersonic jet produced by this equipment without particle injection. Rapid turbulent mixing of the jet with the surrounding atmos-phere was observed, which is an issue of concern in coating quality due to the possibility of oxidation of sprayed particles. This mixing appears to be a function of the ratio of densities of the hot jet and the cold atmosphere as well as a function of the velocity of the jet, rather than one of combustion-chamber pres-sure or barrel length. The supersonic core of the HVOF jet dissipates rapidly due to the mixing, so that the jet is no longer supersonic when it impinges on the target surface being sprayed. Secondary issues also observed in this study include strong jet-noise radiation from the HVOF plume and the entrainment and induced bulk motion of the surrounding air.     

Spraying of coatings with the supply of aqueous solutions of various compounds into the plasma jet

Coat obtaining techniques using industrial plasma spraying systems of transformed powder material feed system into plasma jet are considered.     

Influence of gas flow on argon microwave plasma jet at atmospheric pressure

Many kinds of an atmospheric-pressure plasma jet have been developed and used for widespread applications such as a surface treatment and modified. This study focused on the argon atmospheric-pressure microplasma jet generated by discharging of RF power of 2.45 GHz microwave. The plasma jet shows sensitivity to surrounding environment: pressure, temperature and gaseous species. It is therefore absolutely imperative that a nature of atmospheric-pressure plasma jet should be understood from a point of fluid dynamics. This study, therefore, focused on the interrelationship between the plasma jet and the working gas. Motion of the plasma jet and the working gas was evaluated by velocity measurement and fast photography. As a result, the unsteady sinusoidal waving motion in the radial direction of a torch was observed. Advection velocity of the plasma in just downstream region of the torch exit increases with the supplying flow rate, and the velocity ratio is in the range of 0.75-0.87.     

Numerical and Experimental Investigation of Cold Spray Gas Dynamic Effects for Polymer Coating

Low melting temperature materials such as polymers are known to be difficult to deposit using traditional cold spray techniques. Computational fluid dynamics (CFD) models were created for various nozzle geometries and flow conditions. A schlieren optical system was used to visualize the density gradients and flow characteristics in the free jet impingement region. Based on the CFD models, it was determined that a diffuser placed into the carrier gas flow near the nozzle exit not only leads to lower particle impact velocity required for polymer deposition, but also provides for appropriate application of compression heating of the particles to produce the conditions necessary at impact for successful coating adhesion of these materials. Experiments subsequently confirmed the successful deposition of polyethylene powder onto a 7075-T6 aluminum substrate. Using air as the carrier gas, polyethylene particles of 53-75???m diameter and 0.94?g/cm³ density_, were cold spray deposited onto the aluminum substrate, with a critical impact velocity of 191?m/s. No apparent melting of the polymer particles was observed. Refinements to these concepts are currently under investigation and a patent disclosure for the idea is pending.     

Transient Flowfield Characteristics of Polycarbonate Plasma Discharge from Pulse-powered Electrothermal Gun Operation

An electrothermal gun is the device that produces high-temperature and high-velocity plasma vapor using high current pulsed power and has a potential to be an efficient method for producing a variety of nanomaterials. Pulsed plasma discharge from the electrothermal gun into the open air has been investigated numerically, and the time-dependent inviscid gas dynamics equations are solved for the two-dimensional computational domain including electrothermal gun and the open-air space using flux-corrected transport (FCT) scheme. The modeling of the Joule heating and the mass ablation from the bore wall are incorporated in the computation. The computational results yield the details of the plasma discharge behavior inside and outside the capillary bore including choked condition at the bore exit and complex shock structure of external plasma discharge. The flow structure of freely expanding plasma discharge in the open air is essentially the highly underexpanded supersonic jet featuring Mach disk, barrel shock, contact surface, and spherical blast wave. Compared to the experiments, the numerical simulation agrees well with the experimental data such as the capillary mass ablation and shock structure of the plasma jet.     

The characteristics of a low-power-consumption plasma jet and ceramic coatings

The effects of the composition of plasma gases (Ar-N₂, Ar-H₂), arc current, and voltage on the temperature and velocity of a low-power (5 kW) plasma torch in the arc field free region has been investigated using an enthalpy probe. Coatings of Al₂O₃-13TiO₂ were deposited under different conditions. The results show that in the Ar-N₂ plasma, the enthalpy, temperature, and velocity change little with arc current and voltage when regulating the nitrogen proportion in the plasma gas. The hardness of the resulting coatings is 800 to 900 kg/mm² HV.300. For Ar-H₂ plasma, however, increases in the H₂ content in the mixture of the gases remarkably enhanced the velocity and heat transfer ability of the plasma jet, with the result that the coatings showed high hardness up to 1200 HV.     

等离子熔射过程中射流与粒子流的加热效应

采用CCD图像采集系统与图像处理技术提取等离子射流长度;以红外测温仪检测的单位时间内基体温度变化来衡量加热效应,研究不同熔射距离与射流长度条件下射流和粉末粒子流对基体的加热效应特点.结果表明,当熔射距离不大于射流长度时,基体温升主要来至于射流加热效应;随着熔射距离增大,射流对基体的加热效应迅速减弱;当熔射距离大于射流长度时,粒子流加热效应比较明显.提出射流长度可以作为合理选择熔射距离的特征评价指标,并通过不同熔射距离条件下熔射皮膜的截面尺寸以及形貌进行验证.     

Behavior of Ni-Al particles in argon: Helium plasma jets

To better understand the plasma spray coating process, an experimental study of the interaction between a subsonic thermal plasma jet and injected nickel- aluminum particles was performed. The velocity, temperature, and composition of the argon/helium gas flow field was mapped using an enthalpy probe/mass spectrometer system. The sprayed particle flow field was examined by simultaneously measuring the size, velocity, and temperature of individual particles. Particle and gas temperatures were compared at the nominal substrate stand- off distance and axially along the median particle trajectory. Temperature and velocity differences between the particle and the gas surrounding it are shown to vary substantially depending on the trajectory of the particles. On the median trajectory, the average particle is transferring heat and momentum back to the plasma by the time it reaches the substrate. Because the exchange of heat and momentum is highly dependent on the particle residence time in the core of the plasma, the condition of particles at the substrate can be optimized by controlling the particle trajectory through the plasma.     

气流辅助增强匙孔激光焊熔深增加机理

采用直径为0.8 mm的气流喷嘴直吹匙孔,开展了不同吹气方向、气流入射点位置及流量下的激光焊试验,为获得增强的匙孔效应和增加的熔深.通过等值线图分析获得了优化的气流参数,最大熔深较传统激光焊增加了约38%.合适入射点位置和流量的增强匙孔气流,不仅压制了等离子体,还将匙孔口部的液态金属向下压,使得匙孔口部明显扩大、熔深增加、焊缝成形良好,匙孔内等离子体的流向发生了改变,因而熔池内液态金属的流向也发生了变化;入射点位置偏后时,其作用区域为匙孔后方熔池,将液态金属向熔池后方推,会导致驼峰焊道的产生.     

In-flight oxidation of stainless steel particles in plasma spraying

Air engulfment by the plasma jet in air plasma spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classic diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on the convective oxidation. Two different types of austenitic stainless steel particles were air plasma sprayed using a direct current plasma gun and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. The mass transfer from surface to interior of particle occurred, forming oxide nodules within particles. The mass transfer is governed by convective movements inside liquid particles within the plasma jet core due to the plasma-particle kinematic viscosity ratio greater than 50 and particle Reynolds number (Re) higher than 20. The nodules were composed of metastable phases consisting of mixed oxide of Fe and Cr. Convective movements within particles ceased roughly outside of the plasma jet core, and classic surface oxidation was found to be the dominant phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core may become entrained toward the tail of the particle if plasma conditions promote a higher particle Re number. The major oxide phase in collected particles was FeCr₂O₄, in a nonstoichiometric form of Fe_3−x Cr _x O₄. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

大气压层流等离子体射流特性及其用于材料表面处理的效果

在大气压条件下产生出能量衰减慢且分布均匀、噪音小的氩和氮氩混合气的层流等离子体射流。与湍流等离子体射流相比,由于对周围冷气体卷吸减少,层流射流在喷射方向的长度可增长６倍。试验表明,只有采用结构合理的等离子枪,综合调控供气和电参数条件,抑制气流的脉动和抖动,才能形成层流等离子体射流。结合数值计算对等离了体气流温度分布的定性分析,说明层流情况下射流径向能量分布集中,轴向温度变化平缓,有利于材料表面处理     

Experimental study of magnetic arc blow for plasma arc cutting

Oxygen plasma arc cutting (PAC) is widely used in various industrial fields. In the case of cutting magnetized plates, the magnetic field is concentrated around the cutting front according to the progress of the cutting, and the electromagnetic force induced by leakage magnetic field deflects the plasma jet. The deflected plasma jet causes poor cutting quality and sometimes causes fatal damage on the electrode and the nozzle by a double arc as abnormal discharge. This phenomenon is called magnetic arc blow, and it is a critical issue of applying plasma cutting on magnetized plates. The purpose of this study is to investigate the arc blow behaviour and to devise a method to prevent it. We examined the relationship between operating conditions and double arc with external magnetic fields on the plasma jet. We found criteria of operation conditions that induce the arc blow. In addition, we have succeeded in suppressing the double arc generation attributed to the external magnetic field with a mild steel shield cap attachment around the nozzle tip.